J . Am. Chem. Soc.

1987, 109, 8025-8032 8025

Selective Catalytic Dehydrogenation of Alkanes to Alkenes

Mark J. Burk and Robert H. Crabtree*
Contribution from Sterling Chemistry Laboratory, Yale University,
225 Prospect Street, New Haven, Connecticut 06520. Received May 8, 1987

Abstract: Linear and cyclic alkanes can be selectively dehydrogenated to the corresponding alkenes both thermally and
photochemically (254 nm) with iridium complexes as catalysts. In the photochemical case, a sacrificial hydrogen acceptor
is not required and H2 is evolved directly. Preferential initial formation of the least stable alkene (e.g., methylenecyclohexane
from methylcyclohexane) is explained by attack at unhindered C-H bonds by an oxidative addition to the metal. A subsequent
@-eliminationgives the alkene. A key feature of the catalyst, [IrH2(~2-02CCF3)(PR3)2] (4), is that the chelating acetate group
can open up to allow p-elimination to take place in the alkyl hydride intermediate [IrH(R)($-02CCF3)(PR3)2]. Prolonged
reaction times lead to progressive isomerization of the alkene to give the thermodynamic product (e.g., I-methylcyclohexene
from methylcyclohexane). Up to 32 turnovers of dehydrogenation are seen. Deactivation of the catalyst takes place by P-C
hydrogenolysis of the PAr, ligand in the thermal case; the rise in the amount of ArH formed parallels the fall-off in activity
of the catalyst. P-C cleavage does not take place in the photochemical system (R = cyclohexyl). A reversible cyclometalation
of the catalyst 4a (R = p-FC,H,) is observed after removal of the hydride ligands with t-BuCH=CH2. In the presence of
C6H6,a C-H oxidative addition product, [IrH(Ph)(~2-0,CCF3)(PR3)2], is formed instead (kH/kD = 4.5) and can be isolated
from the reaction mixtures; this supports the oxidative addition pathway proposed for the alkanes. Equilibrium constants for
the reaction of 4a with alkenes to give [IrH2(alkene)($-02CCF3)(PR3)2] are reported.

Alkane C-H bond activation by transition-metal complexes is the two H ligands of 2, in contrast, is 4e, and so a 14e fragment
a topic of current interest. Of the many possible approaches,’ (or its equivalent) would be required to dissect an alkane to give
we will consider here only the oxidation addition pathway (eq 1). 2. We do not necessarily imply that any 14e fragment need have
an independent existence. We merely require that the catalyst
precursor be capable of rearrangement or ligand dissociation to
liberate vacant sites at the metal. These sites must be capable
of holding the C=C and the two H ligands derived from the
alkane and therefore must be capable of accepting 4e.
We have previously used [IrH2(Me2CO),L2]+(3) (L = PPh,)
in this connection and have shown that this species dehydrogenates
As indicated in the equation, the reaction seems usually to be alkanes beyond the alkene stage to arenes? c y c l o p e n t a d i e n y l ~ , ~ ~ ~ ~
thermodynamically unfavorable for the d-block metals, the M-C and related species (eq 4 and 5). Since the H 2 and both M e 2 C 0
bond being substantially weaker than the M-H bond.’,2 This
situation contrasts to that for H2 addition (eq 2), which is ther-
modynamically favorable, and many cases of H2addition to metal
complexes are known. We have always been interested in catalytic
applications of C-H bond-breaking reactions, and so the instability
of the alkyl hydride (1) in eq 1 and 3 is not a disadvantage. We
only need to channel the decomposition in the direction of p-
elimination to an alkene dihydride (2), rather than reductive
elimination to alkane (eq 3).
H H
\ / H,C=CH,

M
HzC-CH,
T M /CH‘cHz
I I
-
-
I
I
M-H (3)
U

ligands of 3 are lost in eq 4 and 5, this complex can be considered

H H H as equivalent to a 12e reagent. The dehydrogenation products,
1 2 benzene and cyclopentadienyl, are too tightly bound to be released
electron count of
alkane-derived lragmenls
4e thermally under mild enough conditions to avoid destruction of
the IrL2+ fragment, and so these reactions are not catalytic. We
As indicated in the equation, when the alkane is dissected to wondered whether we could divert the process to give alkenes
give 1, the two l e donor fragments, R and H, together constitute selectively as products by using a reagent equivalent to a 14e
a net 2e donor system, and so a 16e starting metal fragment can fragment and so stopping the dehydrogenation at the alkene stage.
in principle give this reaction (eq 1). Several photochemically Since alkenes often dissociate from metal complexes relatively
generated 16e fragments have been observed to give adducts of easily, we believed that the overall process might then be catalytic.
type l.3 The corresponding net electron count for the C=C and We thought that a neutral catalyst might be useful, because it
should be alkane soluble, and so we aimed for a 14e “IrL2X”
(1) Crabtree, R. H. Chem. Reu. 1985, 85, 245-269. Shilov, A. E. The system ( X = anionic ligand). This paper describes the develop-
Activation of Saturated Hydrocarbons by Transition Metal Complexes; D. ment of such a ~ a t a l y s t . ~
Reidel: Dordrecht, The Netherlands, 1984.
(2) Halpern, J. Acc. Chem. Res. 1982, 15, 238-244.
(3) (a) Janowicz, A. H.; Bergman, R. G. J . Am. Chem. SOC.1982, 104,
352; 1983, 105, 3929. Wax, M. J.; Stryker, J. M.; Buchanan, J. M.; Kovac, (4) (a) Crabtree, R. H.; Parnell, C. P.; Uriarte, R. J. Organometallics
C. A.; Bergman, R.G. J . A m . Chem. SOC.1984, 106, 1121, and references 1987, 7, 696-699. (b) Crabtree, R. H.; Mihelcic, J. M.; Quirk, J. M. J . Am.
therein. (b) Hoyano, J. K.; McMaster, A. D.; Graham, W. A. G. J . Am. Chem. SOC.1979, 101, 7738-7739. (c) Crabtree, R. H.; Mihelcic, J. M.;
Chem. SOC.1983, 105, 7190. (c) Jones, W. D.; Feher, F. J. J . Am. Chem. Mellea, M. F.; Quirk, J. M. J . Am. Chem. SOC.1982, 104, 107-113.
SOC.1982, 104, 4240; 1984, 106, 1650; 1985, 107, 620-631; 1986, 108, ( 5 ) Burk, M. J.; Crabtree, R. H.; McGrath, D. V. J . Chem. Soc., Chem.
48 14-48 19. Commun. 1985, 1829-1830.

0002-7863/87/1509-8025$01.50/0 0 1987 American Chemical Society

8026 J . Am. Chem. SOC.,Vol. 109, No. 26, 1987 Burk and Crabtree

Felkin et aL6 have observed catalytic alkane dehydrogenation Table I. Thermal Alkane Dehvdroaenation with 4a"
with a variety of transition-metal polyhydrides, notably ReH7L2
time, total
and IrH5(P(i-Pr)3)2,and with t-BuCH=CH, (tbe), a reagent we alkane davs Droducts (turnovers) turnovers
originally introduced as a hydrogen a c ~ e p t o r . ~The
~ - ~presence
of tbe is required to make the overall process thermodynamically cyclopentane 2 cyclopentene (1.4) 1.4b
cyclohexane 2 cyclohexene (3) 3
favorable. U p to 40-70 turnovers of cyclooctene (coe) can be 14 cyclohexene (8) 8
obtained in this way from cyclooctane. In the presence of tbe, methyl- 14 methylenecyclohexane (0.46)
IrH5(P(i-Pr)3)2dehydrogenates pentane to 1-pentene (0.3 turn- cyclohexane 1-methylcyclohexene ( 2 . 6 )
over), but this is subsequently largely isomerized to 2-pentene. 3-methylcyclohexene (0.6)
Methylcyclohexane gives up to 3.6 turnovers of alkenes, most of 4-methylcyclohexene (0.8) 4.46
which consists of methylenecyclohexene." We hoped in our work cyclooctane 2 cyclooctene (9) 9
to make a catalyst that would be generally useful for the dehy- 5 cyclooctene (20) 20c
drogenation of a variety of alkanes and also to explore ways of 14 cyclooctene (34) 34
avoiding the apparent necessity for having a hydrogen acceptor cyclooctane/ 14 cyclooctene (1 7) 17
present. Some of this work has appeared in a preliminary form.* cyclohexane cyclohexene (3) 3
n-hexane 14 1-hexene (0.2)
Results and Discussion trans-2-hexene (2.83)
cis-2-hexene (0.92)
Early Attempts To Obtain "IrXL," Fragments. Our first step trans-3-hexene (0.7)
was to try X = H ; IrHL, has now been proposed as the active cis-3-hexene (0.4) ' 5.05d
species in IrH,L, catalysts.6c For this we felt we need only de- "Conditions: 4a (7.1 mM), tbe (50 mol equiv) in the neat alkane at
protonate the [IrH,(Me2C0)2L2]+complex. This would have the 150 ',c; see also the Experimental Section. bReH7(P(P-FC6H4)3)2
additional merit of labilizing the acetone groups, because loss of gives 1.1 turnovers after 30 min at 80 O C b a CIrHS(P@-FC6H4)3)2
the net cationic charge should make the metal less oxophilic. gives 30-35 turnovers after 5 days at 150 oC.6b dIrH5(P(i-Pr)3)2gives
Proton Sponge (1,8-bis(dimethylamino)naphthalene) in cy- 9 turnovers after 30 min at 80 O C b d
clooctane/tbe proved to be a suitable base. Heating the mixture
resulted in the dissolution of l a ([IrH,(Me,CO),[(p- catalytically inactive, however. The red intermediate [Ir(cod)-
FC6H4),P],]SbF6), the precipitation of the protonated form of ( p O C O R ) ] , shown in eq 7 is known for R = CH3.I0
the base, and in the formation of 8 turnovers of cyclooctene/mol
of l a after 24 h at 140 "C. Full activity required 1 equiv of Proton
Sponge, but the presence of excess base partially inhibited the
[Ir(cod)Ci], -
AgOCOR
[ir(~od)(p-OCOR)]~ -
I) 4L

ii) H,

reaction.
W e felt we could obtain a more convenient system by starting
with a neutral complex. We first considered a formate complex [IrH2(OCOR)L21 (7)

[IrH2($-O2CH)LZ] because we thought this might lose C 0 2 under 4

the conditions of the reaction! Unfortunately, the formate proved
Molecular weight determinations, together with IR and N M R
to be very labile, losing CO, near room temperature. C 0 2 loss
is probably reversible, however, because the complex could be
isolated by treatment of 3 with N a O C H O under C 0 2 . Two
spectral studies, confirmed that 4 has the structure shown in eq
6. In particular, the Ir-H protons appear as a triplet (JPH 16
Hz) a t -28.5 to -33.5 ppm, appropriate1Ia for an Ir(II1)-H trans
-
turnovers of coe were formed with the formate complex as catalyst
to an 0-donor ligand, and the I R shows bands at ca. 1450 and
a t 140 "C for 24 h.
1625 cm-' as expected for this structure.'Ib While the carboxylates
In the course of this work, we found that formate ion reacts
are chelating in 4, it was anticipated that these ligands could open
(25 OC, 1 h) with [(cod)IrL,]+ to give [(cod)IrHL,] and CO,.
to the q1 form, during the catalytic cycle. Evidence for this view
Evidently formate acts as a selective, mild H--equivalent reagent
is presented in a later section.
in this case. Other similar examples are known.*
Catalytic Studies with 4. 4a proved to be an effective catalyst
The best catalyst was found through exploration of other
for thermal alkane dehydrogenation. Thirty-four turnovers of coe
carboxylates. Two synthetic routes proved useful. The first, which
were obtained from coa and tbe after 14 days of reaction in a glass
was most useful for arylphosphine complexes, involves treatment
vessel sealed with a Teflon stopcock (7.1 m M catalyst, 355 m M
of 3 with the sodium salt of the carboxylic acid (eq 6) to give 4a
tbe, 150 "C).
(L = @-FC6&)$, R = CF3). On the other hand, the alkyl- Table I shows that alkanes other than coa can also be dehy-
L L drogenated. 2a is therefore a generally applicable catalyst.
H, 1 +,OCMe, NaoCoR The identification and determination of the mixture of product
H. 'r
I 'OCMe, alkenes in the presence of a large excess of the corresponding
L L alkanes is a challenging problem for which we were able to develop
3 4 a general solution. As described in detail elsewhere,', we bro-
phosphine species such as 4b ( L = P(C6H11)3,R = CF,) were best minated the alkenes with [C6H5NH][Br,] and then determined
prepared by eq 7. These species are analogous to the known the resulting alkene dibromides by capillary GC.
complex [IrH2(~2-0,CCH,)(PPh3)2],9 which in our hands is All the possible linear hexenes were formed from n-hexane
under these conditions. These alkenes could have been formed
either by direct methods or by isomerization of an initial kinetic
(6) (a) Baudry, D.; Ephritikine, M.; Felkin, H. J . Chem. SOC.,Chem. product mixture. Only in the first case would the product dis-
Commun. 1980, 1243; 1982, 606. Baudry, D.; Ephritikine, M.; Felkin, H.; tribution give information about the site of attack. Subjecting
Holmes-Smith, R. J . Chem. SOC.,Chem. Commun. 1983, 788-789. (b) 1-hexene in cyclohexane to the reaction conditions for 2 days gave
Felkin, H.; Fillebeen-Khan, T.; Gault, Y.; Holmes-Smith, R.; Zakrzewski, J.
Tetrahedron Lett. 1984, 25, 1279. (c) Felkin, H.; Fillebeen-Khan, T.; a very similar product mixture to that shown in Table I (hexenes
Holmes-Smith, R.; Lin, Y. Tetrahedron Lett. 1985, 26, 1999-2000. (d) from n-hexane shown first, hexenes from 1-hexene in parentheses):
Baudry, D.; Ephritikine, M.; Felkin, H.; Fillebeen-Khan, T.; Gault, Y.;
Holmes-Smith, R.; Lin, Y.; Zakrzewski, J. In Organic Synthesis: An Inter-
disciplinary Challenge; Streith, J., Prinzbach, H., Schill, G., Eds.; Blackwell: (10) Haszeldine, R. N.; Lunt, L. J.; Parish, R. V . J . Chem. SOC.A 1971,
Oxford, 1985, pp 23-34. ( e ) Cameron, C. J.; Felkin, H.; Fillebeen-Khan, T.; 3696-3698.
Forrow, N. J.; Guittet, E. J . Chem. SOC.,Chem. Commun. 1986, 801-802. (1 1) (a) Crabtree, R. H.; Demou, P. C.; Eden, D.; Mihelcic, J. M.; Parnell,
(7) Crabtree, R. H. Ann. N.Y. Acad. Sci. 1984, 415, 268-270. C. P.; Quirk, J. M.; Morris, G. E. J . Am. Chem. SOC.1982, 104, 6994-7001.
(8) Laing, K. R.; Roper, W. R. J. Chem. SOC.A 1969, 1889-1891. Roper, (b) Robinson, S . D.; Uttley, M. F. J . Chem. Soc., Dalton Trans. 1973,
W. R.; Wright, A. J . J . Organomet. Chem. 1982, 234, C5-C8. 1912-1920.
(9) Araneo, A,; Martinengo, S.; Pasquale, P. Rend.-1st. Lomb. Accad. Sci. (12) Burk, M. J.; Crabtree, R. H.; McGrath, D. V . Anal. Chem. 1986, 58,
Lett., A : Sci. Mat., Fis., Chim. Geol. 1965, 99, 795. 977.
Catalytic Dehydrogenation of Alkanes to Alkenes J . Am. Chem. SOC.,Vol. 109, No. 26, 1987 8027
0.7 7 Table 11. Photochemical Alkane Dehydrogenation

0.6 - alkene
co- time,
reactant days products (turnovers) total
0.5- -f cyclooctane
cyclooctane
tbe
tbe
7
14
cyclooctene (1 2)
cyclooctene (23)
12
23
0.4 - L cyclooctane none 7 cyclooctene (8) 8

0.3 - -E
Q
methylcyclo-
hexane
tbe 7 methylenecyclohexane (2.77)
1-methylcyclohexene (2.19)
3-methylcyclohexene (0.85)
8 4-methylcyclohexene (1.26) 7.07
02 - none 7 methylenecyclohexane (1.6)
1-methylcyclohexene (3.84)
3-methylcyclohexene (0.32)
4-methylcyclohexene (0.82) 6.58
# O cyclohexane tbe 7 cyclohexene (3.2) 3.2
o 2 4 6 8 1 0 1 2 1 4 cyclohexane none 7 cyclohexene (3.9) 3.9
n-hexane tbe 7 1-hexene ( I . 18)
time (days) trans-2-hexene (2.48)
cis-2-hexene (0.47)
Figure 1. Time course study of the rate of production of fluorobenzene
trans-3-hexene 10.52)
(solid points, left scale) in the deactivation of catalyst 4a compared with ~I

cis-3-hexene (0.2) 4.85

the activity of the catalyst as measured by cyclooctene (coe) production
from cyclooctane (squares dotted, right scale).
that 1 P-C cleavage per metal is sufficient to deactivate the system,
1, 4% (3%); trans-2, 56% (53.5%); cis-2, 18 (17.5%); trans-3, 14% so an “L21r2(pPR2)”species may be the metal-containing deac-
(18.5%); cis-3, 8% (7.5%). In addition, the isomerization system tivation product. Fluorobenzene formation is much more rapid
also gave some cyclohexene (ca. 2 turnovers) from the dehydro- at higher catalyst concentration (0.9 equiv is formed after 2 days
genation of the solvent cyclohexane. The results suggest that the a t a 71 m M catalyst concentration), so the reaction may be
product ratio observed in the thermal alkane dehydrogenation bimolecular. Felkin et aL6 also found that yields of alkene were
results from isomerization of an initial kinetic product. Similarly, reduced upon raising the catalyst concentration and observed the
1methylcyclohexene, the thermodynamically most stable alkene, formation of A r H from ReH7(PAr3)26aand of propane from
is formed from the dehydrogenation of methylcyclohexane with IrH5(P(i-Pr)3)2.6eThe rate of both fluorobenzene and cyclooctene
our catalyst (Table I), probably also as a result of isomerization. formation was essentially independent of the concentration in the
ReH7L26agives mostly 2- and -methylcyclohexene, indicating range 0.35-2.1 M. Tbe therefore does not compete effectively
preferential attack at the least hindered ring methylene groups. with the neat alkane for binding to the metal and is probably not
IrH5(P(i-Pr)3)26C also dehydrogenates methylcyclohexane, but at involved in the turnover limiting step. No alkane dehydrogenation
150 “ C after 65 h, isomerization is essentially complete and the takes place in the absence of tbe however. The observed product
thermodynamic ratio of alkenes has been attained. In contrast, mixture is far from the thermodynamic equilibrium for eq 8, and
the trifluoroacetate catalyst is a poor isomerization catalyst, and so conversion is limited by catalyst deactivation.
the product mixture is still far from the equilibrium ratio of alkenes
even after 14 days at 150 OC. This suggests that the active species
derived from the two catalysts are different. W W
The more efficient dehydrogenation of cyclooctane compared coa tbe coe tba
to the other substrates is probably a result of the transannular
steric repulsions in coa, which are less severe in coe; the heat of We applied the usual Hg testI5 to check for homogeneity. The
hydrogenation of m e is therefore unusually low (ca. 23 kcal/mol). rate of reaction was unaffected, and this supports our view that
Cyclodecane is a good substrate for the same reason. Interestingly, the reaction is authentically homogeneous.
cyclooctane is one of the poorest substrates for reaction with Changing the carboxylate ligand to acetate gave a catalytically
~ ~ is not unreasonable. W e are not so much con-
C P * I ~ L .This inactive complex: neither alkene nor fluorobenzene was observed.
cerned with the initial rate of C-H oxidative addition as with the Similar activity differences were observed in RCHO hydrogenation
rate at which the intermediate alkyl hydride decomposes to alkene with [RuX(O,CR)(CO)(PP~,)~],’~ and the likely origin for the
dihydride, which may well be the turnover limiting step of the difference is the much higher tendency for C F 3 C 0 2to open to
catalytic system. the 9’ form, experimental evidence for which is presented below.
Any catalyst active enough to break alkane C-H bonds is Changing the phosphine to (p-CH3C6H4),Pled to an increased
usually sufficiently reactive to attack its own ligands and so lead rate of P-c cleavage and decreased catalyst lifetime. P ( C 6 H , I ) 3
to catalyst deactivation. This is a major problem for homogeneous and P(i-Pr)3 gave poor catalysts: only small yields of alkenes were
catalytic alkane conversion in general and has led us to explore observed.
the possibility of avoiding the use of ligands altogether, as discussed Photochemical Alkane Dehydrogenation. We considered
below. Catalyst deactivation was evident from the formation of whether we could also dehydrogenate alkanes under photochemical
fluorobenzene in the hydrogenolysis of the P-C bonds of the conditions. If the role of the tbe in the thermal catalyst system
(p-FC6H4)3Pligands, as detected in the alkene dehydrogenation is to remove the H2 from the catalyst precursor and so prepare
products. The corresponding (p-CH3C6H4)3Pcatalyst gave toluene it for alkane oxidative addition, then irradiation might serve the
under the same conditions. Figure 1 shows that the growth of same purpose. Stoichiometric photochemical alkane C-H acti-
the C6H5Fover 14 days correlates with the fall-off in the rate of vation is ~ e l l - k n o w n ,and
~ Shilov17”has observed stoichiometric
coe formation. This reaction is probably an important pathway formation of alkene complexes on irradiation of PtC162- in the
for catalyst deactivation in general, and yet it may often be missed presence of alkane.
because the small amount of arene formed can easily escape While the thermal catalyst 4a proved inactive, 4b did dehy-
detection in the product mixture. GarrouI3 has reviewed the area drogenate alkanes catalytically under photochemical conditions.
in detail. Direct oxidative addition of the P-C bond to the metal 3a (7.7 mM) and tbe (385 mM) in cyclooctane gave 12 turnovers
followed by reductive elimination of the resulting aryliridium
species with a hydride is the likely mechanism.14 Figure 1 suggests (15) Anton, D. R.; Crabtree, R. H. Organometallics 1983, 2, 855-859.
(16) Sanchez-Delgado, R. A,; Valenica, N.; Marquez-Silva, R.-L.; An-
~ ~~

driollo, A.; Medina, M. Inorg. Chem. 1986, 25, 1106-1 11 1.

(13) Garrou, P. Chem. Reu. 1985, 85, 171-185. (17) (a) Shul’pin, G. B.; Nizova, G. V.; Shilov, A. E. J . Chem. Sot., Chem.
(14) Ortiz, J. V.; Havias, J. Hoffmann, R. Helu. Chim. Acta 1984, 67, Commun. 1983, 671-672. (b) Renneke, R. F.; Hill, C. J. Am. Chem. SOC.
1-17. 1986, 108, 3528-3529.
8028 J . Am. Chem. SOC.,Vol. 109, No. 26, 1987 Burk and Crabtree

of coe after 7 days a t 25 OC (8-W low-pressure Hg lamp, quartz Once again, the system is homogeneous by the Hg test. During
vessel). The 254-nm line appears to be responsible for the pho- these experiments, mercury-photosensitized alkane coupling also
tochemistry, because a Pyrex filter stopped the reaction. Table occurred.19 In following up this result, some interesting new
I1 summarizes the results. This was the first example of the mercury-sensitized alkane functionalization reactions were dis-
transition-metal-mediated photochemical functionalization of an covered.20 These are of practical interest, because multigram
alkane (eq 9).5 Hill17bhas recently reported a photochemical quantities of alcohols, ethers, aldehydes, and silanes are obtained
in this way from any of a wide variety of alkanes.
The electronic absorbtion spectrum of 4b shows three bands,
one at 344 nm ( e = 650 L/cm.mol), one a t 264 nm ( e = 5740),
and the third at 225 nm (t = 12040), of which the 264-nm band
coa coe
is probably responsible for the photochemistry. A Pyrex filter
polyoxometalate system that gives amides, ketones, and alkanes. was sufficient to prevent alkene formation, so the 344-nm band
The products from methylcyclohexane are decidely different from is inactive. The photochemically inactive complex 4a has two
the thermodynamic mixture of alkenes obtained in the thermal UV-visible absorbtion bands, one a t 332 nm ( t = 610) and the
system. To test the isomerization activity of the photochemical other a t 249 nm ( t = 23 100). The latter may be a ligand tran-
system, methylenecyclohexane (12 mol equiv) was subjected to sition (cf. P(p-C6H4F),P: 252 nm ( e = 23 OOO)), and this may
the reaction conditions and gave only 25% conversion to 1- account for the lack of photochemical activity: the energy is
methylcyclohexene, suggesting that only about half of the 1- dissipated in the ligands when the phosphine is aromatic.
methylcyclohexene shown in Table I1 may have come from Some Chemistry of 4 Related to the Mechanism of Alkane
isomerization. Methylenecyclohexane is therefore the preferred Dehydrogenation. A plausible first step in the thermal alkane
kinetic product in this system, as is seen for IrH5(P(i-Pr)3)2.kThe reactions is binding of tbe to 4a followed by transfer of the hydride
preferential attack a t the 1" C-H bonds seen in these systems ligands to the olefin to give tba and a reactive metal fragment.
argues in favor of an oxidative addition pathway and compares W e find that reaction of complexes of type 4 in an N M R tube
with the similar results of Bergman,3a Jones,3f and their with a variety of ligands shows the greater tendency of the tri-
collaborators for stoichiometric oxidative addition of alkane C-H fluoroacetate group to adopt an 7' form. While C O opens the
bonds to Ir, Rh, and Re species. A radical or carbonium ion chelate ring in both acetate and trifluoroacetate complexes, the
mechanism would have shown a preference for 3' C-H activation. weaker ligand MeCN gives an adduct only with the trifluoro-
In contrast, the transition state for oxidative addition is very acetate 4a. In the case of 4b the product seems instead to be
sensitive to steric e f f e c t ~ . ~ s ~This,
J ~ together with the isolation [IrH2(MeCN),[P(C6H11)3]2]02CCF3. Only the trifluoroacetates
of an oxidative addition product from benzene, discussed below, were found to be active in alkane dehydrogenation, and we imagine
makes it difficult to avoid the conclusion that alkane C-H oxi- that this is due to their greater tendency to open the 7' form. Table
dative addition is involved in these systems. By extrapolation, I11 lists the N M R and I R data for these adducts.
the same mechanism is likely both in our original ~ ~ r and k ~ ~ vAdding
~ olefins, including tbe, to 4a allowed the proposed
in the Felkin6 systems. equilibrium (eq 10) to be observed directly a t low temperature.
Such a photochemical system does not necessarily require a Table IV lists the 'H N M R data for these species as well as the
hydrogen acceptor because the 254-nm quantum carries 112 values of Kq for eq 10. Of particular interest is the observation
kcal/einstein, enough to easily overcome the heat of hydrogenation
of any alkane. W e were pleased to find experimentally that the
rate and extent of alkane dehydrogenation were essentially un-
changed in the presence or absence of tbe. As can be seen from
the table, isomerization of the product alkene is more extensive
in the absence of tbe. 1-Methylcyclohexene is now the major (L' = alkenes, CO, MeCN)
product from methylcyclohexane, for example.
Even in the presence of tbe, the tbe does not act as hydrogen that tbe is one of the poorest ligands studied. This is no doubt
acceptor for all the H2 removed from the alkane. For example, a reflection of the steric bulk of the t-Bu group. A low Keq is a
4b and tbe (4 mol equiv) in C6DI2gave 3 turnovers of C6D10after valuable property for a hydrogen acceptor because we do not want
7 days but only 1.2 turnovers of tbe were formed. We conclude the alkene to exclude the alkane from the metal by strong binding.
that some free H 2 is formed even in the presence of tbe. N o Cyclooctene is a very effective ligand, but it is not hydrogenated
reaction took place between 4b and tbe in the absence of 254-nm by the system. Cyclopentene and cyclodecene are very poor
light. ligands, possibly because of steric congestion in the complexes.
The H2 formed in these photochemical systems were detected Me3SiCH=CHz is much less hindered and so is expected to bind
by passing the gases evolved into a solution of [Ir(cod)L2]SbF6 better than the carbon analogue. Steric effects alone do not explain
in acetone-d, a t 0 OC; was produced why this ligand is so much better than 1-hexene, so it may be that
quantitatively and detected by ' H N M R . the electrophilic metal prefers a more strongly electron releasing
The photochemical system degrades very slowly with time. The alkene. Others have reported Kq values for the reaction of alkenes
rate of alkene production is essentially linear (e.g., 12 turnovers with various metal complexes, but tbe and Me,SiCH=CH, were
of coe are formed from coa after 7 days, 23 turnovers after 14 not studied.2'
days), and 70% of the catalyst can be recovered after 7 days. The Free and bound tbe gave distinct resonances a t -80 " C but
system is therefore much more resistant to P-C cleavage than was coalescence took place a t ca. -20 OC. By 25 "C,hydrogenation
the thermal one, although it is not yet clear whether this is because took place slowly, and the hydride peaks of 4a disappeared over
it is photochemical or because P(C6H11)3may be inherently more 15 h (1 equiv of tbe) or 1 h (10 equiv of tbe). The acetate, in
P - C cleavage resistant than PAr,; the latter is expected to be the contrast, did not react with tbe over 5 days, which is probably
case from Hoffmann's arguments.I4 the reason that this complex is not catalytically active for alkane
With C6D12as substrate, the 2H N M R of the recovered catalyst dehydrogenation.
4b showed deuteriation of the Ir-H groups as well as of the In the presence of 10 equiv of tbe, an apparent intermediate
cyclohexyl rings, indicating reversible cyclometalation of this in the dehydrogenation of 4a was detected by ' H N M R . This
ligand. The degree of deuterium incorporation was greater in the intermediate grows until it constitutes ca. 95% of the reaction
absence of tbe, consistent with the stabilizing coordinatively un-
saturated intermediates. (19) Cvetanovic, R. J. Prog. React. Kinet. 1964, 2, 17.
(20) Brown, S. H.; Crabtree, R. H. J . Chem. SOC.,Chem. Commun. 1987,
970.
(18) Crabtree, R. H.; Holt, E. M.; Lavin, M.; Morehouse, S . M. Inorg. (21) Cramer, R. J . Am. Chem. SOC.1967,89,4621-4626. Tolman, C. A.
Chem. 1985, 24, 1986-1992. J . Am. Chem. SOC.1974, 96, 2780-2789.
Catalytic Dehydrogenation of Alkanes to Alkenes J. Am. Chem. SOC.,Vol. 109, No. 26, 1987 8029

Table 111. IR and NMR Data for the Adducts of 4 with Various Ligands, L
compound' L IRb 'H NMR 3'P NMR"
IrH2(02CCF3)(PAr3)2(4a) co 2099c -21.6, dt (4.4," 14.@), IrH +9.25
200Se -7.32, dt (4.4," 18.21), IrH
1686 7.15, 7.52, c, Ar
1rH2(02ccF3)(Pcy3)2 (4b) co 2090. -24.4, dt (5.5," 13.21), IrH +28.1
197SC -7.93, dt (5.5," 18.2/), IrH
1683' 1.6-1.9, C, Cy
14448
1rH2(02CcH3)(PAr3)2 co 2093' -20.6, dt (5.5," 15.2j), IrH +9.6
1996' -7.93, dt (5.5," 18.@), IrH
1603' 1.28, s, OAc
14448 7.15, 7.52, c, Ar
I~H~(O~CCH~)(PCY~)~ co 2076e -23.4, dt (5.5," l d ) , IrH +25.5
197Oe -7.93, dt (5.5", 18.2/), IrH
1602f 1.3-1.9, C, OAC & Cy
13698
1rH2(02CCF3)(PAr3)2 (4a) MeCN 2184k -25.7, dt (6.9,* 16.@), IrH +17.2
1686 -20.1, dt (6.9," 17.133, IrH
1.64, s, MeCN
7.1. 7.6, c, Ar
'Ar = p-FC6H5,Cy = cyclohexyl. CD2C12. CInCD2C12, reported as position (6), multiplicity (coupling constant, hertz), assignment.
CD2C12,relative to external 85% H3P04. .v(Ir-H) or v(C0). fv(C0) symmetric. E v ( C 0 ) asymmetric. h2JHH.. l2JpHcis. "(C-N).

Table IV. 'H NMR and Equilibrium Data for Alkene Complexes of related Rh complex. After several hours, 6 decomposes to give
4a free tba and two new species 8 and 9 are formed. The 31PN M R
'H NMRb of 8 shows mutually coupled resonances at + 17.1 and -44.3 ppm.
The latter is appropriate for a cyclometalated ring, and the large
alkene K," Ir-H coordinated vinyl Jpp.of 368 H z is appropriate for a trans arrangement of phos-
PhCMe=CH2 -0 no complex observed phines. 'H N M R shows a doublet of doublets at -27.6 ppm, and
t-BuCH=CH, 0.58 -27.74, br t (16) 3.16, t, (12) irradiation of either the +17.1 or the -44.3 ppm lines in the 31P
-10.8, br t (20.6) 3.47, d (8.5) spectrum led to collapse of the Ir-H peak to a simple doublet,
4.66, dd (8.5, 12) confirming the orthometalated structure for 8. 9 was characterized
cyclopentene 1.9 -28.75, dt (6, 13.7) 4.2, br
-11.1, dt (6, 19.8) as the Ph-H oxidative addition product with the solvent C6D6
cyclodecene 2.3 -27.25, br 4.2, br because it was obtained in a pure form after recrystallization. This
-12.95 br t (23) supports our view that the first step in the reaction of an alkane
EtOCH=CHZ 6.3 -24.3, br t (13) 2.33, 2.98, with the catalyst is oxidative addition. Up to now, no C-H
-10.8, br t (16) 3.56, br oxidative addition product has been observed in an alkane con-
n-BuCH=CH2 17 -27.75, dt (6, 16) 3.1, br d (8) version catalyst, although Baker and Field23bhave observed the
-10.7, dt (6, 19) 3.4, br d (13) photochemical decomposition of a pentane-derived complex,
4.3, br dd (8, 13) (dmpe)2Fe(1-pentyl)H, to give I-pentene stoichiometrically.
PhCH=CH2 32 -27.3, br 3.6, 3.7, br We were able to determine kH/kD for the (thermal) oxidation
-9.94, dt (6, 19) 4.65 br t (11)
Me3SiCH=CH2 1250 -28.2, br t (13) [-0.59, s, SiMe3] addition of benzene by monitoring the reaction between 4a and
-11.1, dt (6, 19.8) 3.17, 3.5, 3.56, br benzene by 31PNMR. The resulting value of 4.5 (f0.1) is normal
cyclooctene 5500 -27.85, dt (6, 17.5) 3.45, br and higher than the values of 1.4 obtained by Feher and Jones3c
-10.9, dt (6, 19) for the oxidative addition of Ph-H to Cp*Rh(PMe3) and of 1.38
CH2=CH2 large -26.45, dt (6, 16) 3.15, br obtained by Bergman et al.3afor cyclohexane and Cp*Ir(PMe,).
-8.4, dt (6, 17) Our intermediate is probably much less reactive than Cp*IrL.
norbornene large -27.5 dt (7.5, 16) 3.55, br The final picture that emerges from this study is shown in
-9.6, dt (7.5, 17.5) Figure 2. Tbe reacts with 4a to give first 5 and then 6. Reductive
"In CD2C12at -80 OC (L mol-'). bReported as in Table 111. elimination from 6 takes place presumably to give the very reactive
I r L 2 ( ~ ' - O C O C F 3 ) .This is capable of oxidatively adding C-H
products after 1 h and it shows a hydride resonance at -28.7 ppm bonds as shown by its subsequent conversion to 8 and then (in
(JPH= 16 Hz) and a new t-Bu group a t 0.52 ppm. It also shows the presence of C6H6) 9. Of course, in the presence of alkane,
a doublet a t +18 ppm in the selective 'H-decoupled 3'P N M R , the products would be 8 and the corresponding "alkyl hydride"
which is consistent with it being a monohydride. The I3C N M R 10, which can follow the sequence shown in Figure 2 to generate
shows new resonances coupled to phosphorus a t +95.4 ppm (Jpc alkene from the alkane. Analogy with 6 suggests that the alkyl
= 8.2 Hz) for C ( l ) and +139.0 ppm (Jpc = 1 Hz) for C(2). These hydride 10 may in fact be agostic, and this would increase its
rather "vinylic" positions suggest that the intermediate is probably thermodynamic stability with respect to the free alkane and so
an agostic22alkyl hydride, e.g., 6 , broadly related to the known perhaps lower the transition-state energy both for the formation
species 7.'* Analogous intermediates are not observed in the case reaction and for its @-elimination to give alkene. A key feature
L R L
of the system, which allows catalysis to proceed, is the facile
opening of the trifluoroacetate chelate ring. This allows the
alkane-derived alkyl hydride complex to become coordinatively
unsaturated so that it can P-eliminate.
kH/kDfor thermal cyclohexane dehydrogenation by 4a was 4.4
t-BU

6
u 7
(*O.l), although interpretation of this result is not straightforward
in a catalytic system in which several steps involving C-H and
M-H bonds may have comparable transition-state energies.
of any of the other alkenes, but H a l ~ e r n *has
~ ~seen a distantly
(23) (a) Chan, A. S. C.; Pluth, J. J.; Halpern, J. J. Am. Chem. SOC.1982,
~~~

102, 5952. (b) Baker, M. V.; Field, L. D.J. Am. Chem. SOC.1987, 109,
(22) Brookhart, M.; Green, M. L. H. J. Organomet. Chem. 1983,250,355. 2825-2828.
8030 J . Am. Chem. SOC.,Vol. 109, No. 26, 1987 Burk and Crabtree

Values for the photochemical system (4b, cyclohexane) were 5.1 L

(fO.l) (in the presence of tbe, 50 mol equiv) and 7.7 (fO.l) (in
the absence of tbe). Crossover was not observed (<3%) in the
alkenes from C6Hl2/c6Dl2mixture (by GC/MS of the final alkene
dibromides), again consistent with the mechanism proposed. Since
D2 exchanges readily with the Ir-H groups of 4a, the lack of L L
crossover suggests that 4 does not liberate free H2 or D2.The
observations are consistent, with the elements of H 2 or D2 being
transferred to tbe by a hydrogenation reaction.
R 1-BU
The exact nature of the reactive species is unknown; we have hv 5
suggested the Ir(7'-OCOCF3)L2 form because [Ir(v2-O2CCF3)L2]
might be expected to be a relatively unreactive square-planar Ir(1)
species. In the PCy, case, two L groups might not easily be able
11
to become cis. In spite of many attempts, we have not been able
to synthesize [Ir(72-02CCF3)L2]by the standard methods; the
Rh analogue has been reported, however.24
The fact that the rates of dehydrogenation (Table 11) correlate
with the stability of the product alkene suggests that the rate-
determining step may be &elimination from the intermediate alkyl
hydride and not C-H activation, which is known3ato be slower
for cyclooctane than hexane, at least for Cp*Ir(PMe,).
I
Reactions of 9. In the hope of functionalizing benzene, we L
treated 9 with C O at room temperature. A mixture of the two
carbonyls shown in eq 11 was formed. Interestingly, l l a is the
major product even though the phenyl group is normally con- '/
L
sidered to have a lower trans effect than H.25 These gave no
benzaldehyde on heating under CO.

H
>
L
I 0
IrO +
R
lbe, PhH
L
H I 0
ph;lrO +R - co
L
9 ,'> lr: 0
* CF,
L

I '0 1'0 Figure 2. Proposed mechanism for the catalytic thermal dehydrogenation
L L of alkanes with 4a. In the photochemical reaction, the catalyst is the
4 9 closely related complex 4b and the proposed cycle is very similar except
that irradiation expels H2 from 4b to give the reactive intermediate A,
for which the structure shown here is speculative.

118 llb

65% 35%

The reacrtion of I2 with 9 gave only C6H6 like Janowicz and

Bergman3a saw for Cp*Ir(PMe,)(Ph)(H).
D2 reacts rapidly with 4a to give 4a-d and then 4a-d2. The most
reasonable mechanism would involve the dihydrogen dihydride
12 analogous to known species that are known to undergo exchange
L
H,I OCOR
H, lrO
I
L
>9 1
.3J,. , . , . l . , . , . , . , . , . , . , . , I
12
o 2s 5s 8s iio 153 i e o 210 240 270 303 321

rapidly.26 In support of this mechanism, the hydride triplet of tlmo (mlnr)

4a broadens in the presence of H2 and sharpens with an N2purge. Figure 3. Typical rate plot for the reductive elimination of benzene from
Furthermore, 4a, 4a-d, and 4a-d2 have distinct 31Presonances, 9.
with an isotope shift of -0.127 ppm per bound D.
9 loses arene on heating with first-order kinetics a t 65 ( f 2 ) for Cp*Rh(PMe2CH2Ph)(Ph)(H) but that the cyclometalated
OC ( k = 1.1 ( f O . l ) X s-l) to give an equilibrium mixture form is favored, as is reasonable for the more stable 5-membered
of 9 and the orthometalated species 8. A typical plot is shown ring formed in the Rh case. The phenyl hydride, do 9, gives >95%
in Figure 3. Expressing the concentrations in terms of mole do C,H6 on treatment with D,. This may be a ligand-induced
fractions leads to a Kq of 3.64 (k0.05)in favor of the phenyl reductive elimination but must be very rapid to avoid substnatial
hydride (AGO = -0.87 kcal/mol). Jones and Feher3chave found D incorporation.
that benzene is lost from Cp*Rh(PMe,)(Ph)(H) with a rate of In the past, cyclometalation has been viewed (at least by
1.06 X s-l at 60 OC and that an equilibrium between the ourselves) as an obstacle to alkane C-H activation. We now see
oxidative addition product and the cyclometalated form is found that cyclometalation is compatible with alkane C-H activation,
as long as it is reversible. P-C cleavage in the phosphine, however,
remains an irreversible deactivation reaction.
(24) Commerene, D.; Douek, I.; Wilkinson, G. J . Chem. SOC.A , 1970,
171-178. Experimental Section
(25) Hartley, F. R. Chem. SOC.Rev. 1973, 2, 163-179. Syntheses were performed under purified N, or Ar by standard
(26) Crabtree, R. H,; Lavin, M. J . Chem. Soc., Chem. Commun. 1985,
1661-1662. Crabtree, R. H.; Lavin, M.; Bonneviot, L. J . Am. Chem. SOC. Schlenk techniques. NMR spectra were obtained on Bruker WM500,
1986, 108,4032-4037. Crabtree, R. H.; Hamilton, D. G. J . Am. Chem. SOC. HX490, and WM250 or a JEOL FX9OQ instrument and IR spectra on
1986, 108, 3124-3125. a Nicolet 5-SX FT-IR. Photolyses were carried out in a Rayonet Series
Catalytic Dehydrogenation of Alkanes to Alkenes J. Am. Chem. SOC.,Vol. 109, No. 26, 1987 8031

RMR-500 photochemical reactor with four 8-W 254-nm lamps. Reag- Hz, Ir-H), 1.25-2.1 (c, Cy). ,'P NMR: 6 +32.6. IR: 1619 and 1440
ents were obtained from Strem Chemical Co. and Aldrich Chemical Co. cm-' (vas and v,).
Solvents were dried and purified by standard techniques. Hydridophenyl(q2-trifluoroacetato)bis[tris(p-fluorophenyl)phos-
Purification of AlkanesaZ7 The alkanes were stirred over successive phineliridium(II1) (9). [ IrH2[@-FC6H4)3P]2(q2-02CCF3)] (4a; 0.50 g,
portions of concentrated H2S04until no further coloration developed in 0.53 mmol) and 3,3-dimethyl-l-butene (tbe; 0.20 mL, 1.6 mmol) were
the acid layer. The alkane was separated, washed with H 2 0 and then heated in C6H, (7 mL) to 65 "C for 3 h. After the solution was cooled,
four times with saturated NaHCO,, then dried over anhydrous MgS04, heptane (10 mL) was added, and the solution was concentrated to ca. 5
and passed through a column of neutral A1203. Spinning-band distilla- mL during which time the product precipitated as a colorless solid, which
tion gives a pure and rigorously alkene-free product. was then filtered, washed with cold heptane (-20 OC, 2 X 2 mL), and
Dihydridobis(acetone)bis[tris@ -fluorophenyl)phosphine]iridium(III) dried in vacuo; yield 0.382 g (71%). This compound is recrystallized from
Hexafluoroantimonate (3). [ I ~ ( W ~ ) { @ - F C ~ H(sbF6)4
~ ) ~ P (0.50
) ~ ] g, 0.43 a warm benzene/heptane (65 OC, 2 mL/6 mL) solution cooled to 4 OC.
mmol) in acetone (7 mL) was cooled to 0 OC. The solution was then 'H NMR (CD2C1,): 6 -28.9 (t, 16.0 Hz, Ir-H), 6.19 (dd, JHH = 7.75
vigorously stirred under H, (1 atm) for 30 min. With a maintained Hz, m-H of Ph), 6.34 (t, 7.2 Hz,p-H), 6.60 (br d, 8.75 Hz, o-H), 7.02
hydrogen atmosphere, the complex l a was precipitated as a cream-col- (t, phosphine to F), 7.35 (m, phosphine ortho to P). 'HI3'PJ NMR
ored solid by the addition of Et,O (70 mL) and pentane (20 mL). After (C6D6): 6 +17.15. IR (CD2C12): 1682 and 1639 cm-' (vaS(CO2)).Anal.
the solid was filtered and washed with pentane (3 X 5 mL), it was Calcd for C44H3oF,O,P2Ir"/8C6H6: C, 52.91; H, 3.05. Found: c, 52.88;
dissolved in CH,Cl,/acetone (5 mL/0.5 mL) and the resultant mixture H, 3.27. The benzene of crystallization was observed and quantified ( ' / E
stirred under H2for 5 min. Precipitation with Et,O/pentane (30 mL/15 mol of C6H6/mol of 9) by 'H NMR.
mL) afforded 3 as a flocculant white solid, 418 mg (90%). Anal. Calcd The deuterio analogue [Ir(C6D5)(D)[@-FC6H4)3P]2(q2-02CCF~)]
for C4,H,802P2F,2SbIr:C, 42.80; H, 3.22. Found: C, 42.78; H, 3.05. (9-d6)was prepared in an analogous manner at 50 OC for 2 h with C6D.5
'H NMR (acetone-d,): 6 -27.6 (t, JPH = 16 Hz,Ir-H), 1.86 (s, acetone), as solvent. *H NMR (C6H6): 6 -28.7 (br, Ir-D), 6.24, 6.37, and 6.81
7.28 (t, phenyl ortho to F), 7.61 (c, phenyl ortho to P). The analogous (br, Ir-C6D5). 1H(3'P)NMR (C6D6): 6 +17.29. The initial rates of
tri-p-tolylphosphine complex was prepared in a similar manner. 'H formation of 9 and 9 4 , in a competitive experiment were determined by
NMR (acetone-d6): 6 -27.9 (t, 16.0 Hz, Ir-H), 1.78 (s, acetone), 2.39 )'P NMR at 25 OC (202.1 MHz) and gave k H / k D= 4.5. The kinetics
(s, p-MeAr), 7.29-7.35 (c, phenyl). Anal. Calcd for C48H56P+&F6SbIr: of the reductive elimination from 9 were determined by '€4 NMR in C6D6
C, 49.94; H , 4.85. Found: C, 50.20; H, 4.80. at 65 OC over 3 half-lives.
Dihydrido(q2-trifluoroacetato)bis[tris(p -fluorophenyl)phosphine]iridi- The cyclometalated species, 8 was never obtained as a solid in a pure
"111) (4a). To [IrH2(acetone)2(@-FC6H4)3P)2](SbF6) (3; 0.4 g, 0.34 state but was identified from its spectral properties. IH NMR (CD2C12):
mmol) in dry THF (10 mL) was added Na02CCF3(0.23 1 g, 1.7 mmol), 6 -27.6 (dd, J H p = 20, 15 Hz), 6-7.5 (c, Ar). "P NMR (CH2CIJ: 6
and the mixture was stirred for 2 h. After the solvent was removed in +17.1 and -44.3 (d, Jpp,= 368 Hz).
vacuo, the residue was taken up in C6H6(10 mL) and the solution filtered Thermal Alkane Dehydrogenations. All thermal alkane dehydroge-
through dry Celite to remove the insoluble inorganic salts. The colorless nation reactions were performed in resealable glass vessels (capacity
benzene filtrate was then concentrated to ca. 2 mL, and heptane (8 mL) 10-15 mL) equipped with Kontes or Ace glass Teflon stopcocks and
was added to precipitate 4a as a colorless solid (0.24 g, 75%), which was made of triple-thickness Pyrex glass. The reaction mixtures were stirred
recrystallized from benzene/heptaneZ8or CH2C12/heptane(2 mL/8 mL). with small Teflon-coated stirbars. Temperatures were maintained to
Generally, precipitation of 4a was facilitated by concentrating the solu- within f 2 OC with a fully equilibrated GE silicone oil bath.
tion to ca. 5 mL following the addition of heptane. The product 4a was (i) Proton Sponge Reactions. In a typical experiment, the reactor was
filtered, washed with cold heptane (-20 OC, 2 X 2 mL), and dried in charged with [IrH2(acetone)2[(p-FC6H4)3P]2](SbF6) (3; 20.0 mg, 0.017
vacuo. IR: 1647 and 1610 cm-l (s, vas), 'H NMR: 6 -30.3 (t, 16.6 Hz, mmol), tbe (0.1 1 mL, 8.5 mmol), 1,8-bis(dimethylamino)naphthalene
Ir-H), 6.74 and 7.10 (c, Ar). ,'P NMR: 6 +22.1. Anal. Calcd for (4.0 mg, 0.0187 mmol), and degassed cyclooctane (2 mL). The solution
C38H26H902P21r:C, 48.56; H, 2.79. Found: C, 48.58; H, 3.21. The was degassed with three freeze-pumpthaw cycles, and the reactor was
compound was monomeric by vapor pressure lowering measurements in heated to 150 OC for 24 h and then cooled to room temperature. The
CH,CI, (MW Calcd 939, found 900 f 50). orange-yellow solution was then distilled in vacuo, and the volatiles were
The dideuteride 4a-d2 was prepared as follows. A C6H6 (5 mL) analyzed as described in ref 12. The involatile products were analyzed
solution of 4a (0.2 g, 0.21 mmol) was stirred under D2 (1 atm) for 20 spectroscopically.
min. The solvent was then removed in vacuo to give 4a-d2in quantitative (ii) Reactions Involving 4. In a typical procedure, [IrH,[@-
yield. This complex was recrystallized as described for 4a. 'H NMR FC6H4)3P]2(q2-02CCF,)] (4a; 10.0 mg, 0.0106 mmol), the (0.07 mL,
(C&): 6 -30.1 (Ir-D). We were not able to obtain MS data, owing 0.53 mmol), and degassed cyclooctane (1.5 mL) were placed in the
to decomposition. reactor, and the solution was degassed as above. The highest activities
Analogous Complexes. The analogous acetato complex was prepared were obtained when the reactor was filled with an Ar atmosphere prior
in a manner identical with that described above for 4 s except that the to heating. The reaction was then run, and the products were analyzed
corresponding quantity of Na0,CCH3 was used in the place of sodium as above. A homogeneous yellow solution formed after 5 min and per-
trifluoroacetate. The isolated yield of product was 0.25 g (82%). Anal. sisted throughout.
Calcd for C39.75H33F902P21r: C, 52.41; H, 3.65. Found: C, 52.04; H, (iii) Reactions in the Presence of Mercury. The procedure used was
3.44. Heptane (0.25 mol/mol of 2b) was observed by 'H NMR. IR: identical with that described above except that 1 drop of Hg (=2 g) was
1528 and 1452 cm-I (s, Y,, and vS). IH NMR: 6 -28.6 (t, 16 Hz, Ir-H), added to the reaction mixture prior to degassing and heating.
1.09 (s, CH,), 6.76 and 7.52 (c, Ar). Photochemical Alkane Dehydrogenations. All photochemical experi-
[ i~H2~(p~MeC6H4)3P)T(qz-OzCCF3)] was prepared by a procedure ments were carried out in quartz glass Schlenk tubes (capacity 20-50
identical with that described above for 4a except that a corresponding mL) equipped with high vacuum Teflon stopcocks. The reactions were
amount of [IrH,(a~etone)~[(p-Mec~H~),P]~](SbF~) was used as the stirred with small Teflon-coated stirbars. The photolyses were performed
starting iridium complex, 0.23 g (75%). 'H NMR: 6, -30.14 (t, 16 Hz, at 25 OC (air cooling) by a Rayonet Series photoreaction with four
Ir-H), 1.99 (s, ArMe), 6.98 and 7.71 (c, Ar). 254-nm (or 350-nm) UV lamps.
Dihydrido(q2-trifluoroacetato) bis(tricyclohexylphosphine)iridium(III). In a typical run, [IrH2(PCy3),(q2-02CCF3)] (4b; 10.0 mg, 0.01 15
A mixture of [Ir(cod)Cl], (0.30 g, 0.45 mmol) and AgO2CCF, (0.31 g, mmol) and degassed cyclooctane (1.5 mL) were placed in the reactor and
0.90 mmol) in CH2C12(20 mL) was stirred in the dark for 30 min. The degassed as before, and the reactor was filled with Ar. After 7 days of
resulting red solution was filtered through Celite and concentrated to ca. photolysis at 254 nm, the resulting colorless homogeneous solution was
10 mL, and PCy, (0.50 g, 1.8 mmol) was then added. After the resulting distilled, and the volatiles were analyzed as above. The involatile or-
orange solution was stirred for 5 min, it was vigorously stirred under H, ganometallic fraction was analyzed spectroscopically and was found to
(1 atm) for 1 h during which time the solution turned colorless. The consist largely (up to 70%) of starting complex 4b.
addition of heptane (10 mL) precipitated 4b as a white solid, which was Detection of H,. After a typical photolysis reaction was performed
filtered, washed with heptane (2 X 3 mL), and dried in vacuo: yield 0.66 as above, the evolved fractions were passed into a vessel containing [Ir-
g (84%). The compound can be recrystallized from CH2Cl,/heptane or (cod)(PPh,),](SbF,) (20.0 mg, 0.0188 mmol) in acetone-d6 (1 mL) at
bemzene/heptane.28 Anal. Calcd for C38H68F302P21r: C, 52.57; H, 7.89; 0 OC over 1 h with a stream of argon. After the solution was stirred for
F, 6.57. Found: C, 52.74; H, 7.60; F, 6.63. 'H NMR: 6 -33.3 (t, 14.8 1 h, it turned colorless, and formation of [IrH,(a~etone-d~)~(PPh~),]-
(SbF6) and coa was observed by 'H NMR. The amount found was
(27) We thank H. Felkin for communicating his finding that alkenes are equivalent to 4.90 mol of H2/mol of 4b and corresponded to the ca. 5 mol
produced by pyrolysis of an as yet unidentified species present after the H2S04 of alkene formed.
treatment, unless removed by A120, before distillation. NMR Observation of Substitution Products from 4. (i) With CO.
(28) All compounds needed for alkane activation were recrystallized for [IrH~@-FC,H4),P]2(~2-02CCF3)] (4a; 20.0 mg, 0.021 mmol) was placed
benzene/heptane to ensure no halocarbons were present in the sample. in an NMR tube under Ar. Degassed C6D6(or CD,CI2) (0.5 mL) was
 8032 J. Am. Chem. SOC.1987, 109, 8032-8041
then added, CO (1 atm) was bubbled through the solution for 5 min, and the olefin was studied.
the resulting solution was analyzed by IH and ,IP NMR and IR spec- (v) With D2.To a solution of 4a as in i above was added D,. Early
troscopy as shown in Table 111. in the conversion, the mixed species [IrHD(02CCF3)L2]was observed,
(ii) With PMe,. To a solution of 4 s as above was added PMe, (2.16 but the final product was 4a-d2 ('H,,H, and ,'P NMR; see text).
fiL, 0.021 mmol). Quantitative conversion to the PMe, adduct was NMR Observation of Reactions of 9. (i) With CO. CO (1 atm) was
observed. bubbled through a solution of 9 (15 mg, 0.015 mmol) in CD2C12(0.5 mL)
(iii) With MeCN. To 4a (40.0 mg, 0.042 "01) in C6D6(0.5 mL) to give l l a and l l b . 'H NMR (resonances for l l a given first; 1 l a : l l b
in an NMR tube was added MeCN (2.2 pL, 0.042 mmol). Quantitative intensity ratio 2:l): 6 -20.15 (t, 14.5 Hz), -6.03 (t, 16 Hz),7.02 and
conversion to the MeCN adduct was observed. 7.14 (t, aryl CH ortho to F), 7.31 and 7.51 (c, aryl CH ortho to P). IR:
(iv) With Alkenes. Low-temperature 'H NMR experiments were 2038 and 2012 cm-' (s, CO).
carried out on a Bruker WM 250 (250-MHz) instrument with a probe (ii) With 12. Free C6H6was observed when excess I, was added to a
precooled to 193 K (-80 "C). The temperature was maintained to within solution of 9 identical with that used above. The organometallic products
=t1 O C . In a typical experiment, [IrH2[@-FC6H4)3P]2(~2-02CCF3)] (4a; were not characterized.
12.2 mg, 0.013 mmol) dissolved in CD2CI2at -80 " C (0.5 mL) was (iii) With H2and D,. A solution of 9 as above was treated with H2
placed in an NMR tube under Ar, and tert-butylethylene (8.3 pL, 0.065 (1 atm, 5 min), and free C6H6was observed in the 'H NMR. When D2
mmol) was then added at -80 OC. The 'H NMR data indicated that an was used, the C6H6formed contained no D (GC/MS).
equilibrium between 4a and the tbe adduct 5 had been established (Table
IV). Kq was determined by integration, or for more strongly binding Acknowledgment. W e thank the National Science Foundation
olefins, which gave =loo% of 5, the equilibrium between cyclooctene and (R.H.C.) and the Department of Energy (M.J.B.) for funding.

Relative Reactivities and Mechanistic Aspects of the Reactions

of Organic Halides with Alkali Metals in Alcohol
Environments
J. L. Reynolds, D. Doshi, and H. Shechter*
Contribution from the Department of Chemistry, State University of New York,
Potsdam, New York 13697, and the Department of Chemistry, The Ohio State University,
Columbus, Ohio 43210. Received June 4, 1986. Revised Manuscript Received August 17, I987

Abstract: The relative reactivities of organic halides over wide concentration ranges have been determined with limited amounts
of lithium, sodium, and potassium in 2-ethoxyethanol (1) at 0 OC. Under these conditions the organometallics formed protonate
to their hydrocarbons rather than undergo exchange, elimination, and simple or crossed coupling. In dilute solution in 1 the
relative reactivities ( r l / r 2 of
) varied halides with lithium are essentially structure independent. However, as the concentrations
of the halides increase, their relative reactivities become significantly different and depend on the total concentrations (C,
(M) = [R,X] + [R2X]) of the organic halides. With lithium at increased halide concentrations (1) the reactivities are iodides
> bromides > chlorides, (2) halides of lower molecular weight react more rapidly than their higher homologues, and (3) the
reactivity orders of chlorides are (a) allyl > primary > secondary > tertiary > neopentyl, (b) 2-buten-1-yl > I-buten-3-yl,
(c) benzyl > phenyl, and (d) p-chlorotolyl > o-chlorotolyl > m-chlorotolyl. As examples, the relative reactivities of 1-
chlorobutane/2-chloro-2-methylpropane(C, = 5.83 M), 3-chloropropene/ I-bromobutane (C, = 4.60 M), bromobenzene/
p-chlorotoluene (C, = 4.37 M), and benzyl chloride/chlorobenzene (C,= 4.02 M) are 6.71, 5.43, 24.1, and 22.1, respectively.
Additions of aprotic solvents to I-chlorobutane and 2-chloro-2-methylpropane in 1 decrease the relative reactivities of the halides.
The effectiveness of cosolvents in lowering the relative reactivities of lithium with 1-chlorobutane and 2-chloro-2-methylbutane
=
is tetrahydrofuran > dioxane z 2-ethoxyethanol (1) > cyclohexene benzene. The relative reactivities of halides with sodium
and with potassium in 1 a t 0 O C are also total halide concentration (C,) dependent. Under comparable concentrations the
relative reactivity differences of halides are greater with lithium than sodium than potassium. The reactivities of halides under
conditions of chemical control can be correlated with the ionization potentials of the alkali metals, and the kinetically controlling
features of these systems are different from those with magnesium. The behavior of the alkali metals, the effects of concentration,
and the roles of solvents on the reactivities of halides are discussed on the basis of (1) the active sites on the metal surfaces
as modified by induction and (2) steric and electronic factors in the organic substrates. The kinetically controlled reactions
of lithium with sp3 halides may be interpreted to involve formation of lithio organohalide radical anions (R'-X-,Li+), electron
transfer to the lithio radical anions on the metal surface, or unsymmetrical four-center carbanionic processes on the metal.
In addition to incorporating an electron into the lowest unoccupied u level of its C-X bond, an sp2 halide offers the possibility
for kinetically controlling electron transfer into the T system of its carbon-carbon double bond(s).

Varied organic halides react with alkali metals (Li, N a , and Scheme I
K) to yield alkali metal derivatives.' Although such organo- R*--5-
metallics are widely used, knowledge of their structures2 and their
mechanisms of formation is limited.3 Of particular significance (Li-Li);
with respect to reaction mechanism is that lithium reacts with
chiral l-halo-2,2-diphenylcyclopropaneswith major retention R-X t (Li-Li)"
Ywl y RLi t LIX + (Li-Lih-2
(68-85%) of stereochemistry and the stereospecificities of the
conversions are related to the halide (C1 > Br > I) and to the
sodium content and the particle size of the l i t h i ~ m . ~ ~ - T" *h e~
+y+ ( L i b - 1 Li

*To whom correspondence should be addressed at The Ohio State Univ- reactions of lithium and halides have been d i s c ~ s s e d ~
(Scheme
~J
ersity. I, processes 1-5) on the basis of transfer of a single electron to

0002-7863/87/1509-8032$01.50/00 1987 American Chemical Society