Chapter 10
Organoboran Chemistry
STABILITY OF ORGANOBORANES
EXCHANGE REACTIONS
According to Ahrland et al., boron is of borderline softness. Borane
derivatives are mildly soft if all the ligands are from Groups V and
VI. However, boron trifluoride is a hard acid.
Boron Trihalides
Boron trifluoride complexes easily with ethers. The complexes
are stabilized by symbiosis of F and O ligands around the boron.
2
Dialkyl ethers are ruptured by BBr3 to furnish alkyl
bromides. This is a consequence of the mutual
weakening of B-Br and O-R bonds (both are hard-soft
pairs) on coordination. The splitting of the bromide ion
from the complexes and its return attack on the alkyl
group of the oxonium intermediates are favored on
HSAB grounds.
3
Thioboranes
Owing to its borderline softness, boron atom may interact equally
well with both hard and soft bases. Thus, the solvolysis of
thioboranes is easily ac-complished. A simple admixture of
thioboranes with carbonyl compounds affords Dithioacetals.
Mixed disulfides are obtained when sulfenate esters are treated
with trialkylthioboranes. This reaction is very efficient because
the sulfenates contain an unstable soft-hard bond.
It is of interest to note that ethers are also cleaved on exposure to
monoalkylthioboranes.
4
An attempt to prepare mixed borates results in
disproportionation product. Symbiosis appears to be
the driving force for the secondary transformation.
5
Thioboranes also undergo bromolysis as shown
below:
6
Oxygenative Degradation of Organoboranes
The treatment of organoboranes with alkaline hydrogen
peroxide leads to alcohols, this involves addition of ΗO2-
to boron followed by migration of a soft alkyl group to the
soft oxy center of the hydroperoxyboranide intermediates.
7
Peracid, alkaline hypochlorite, and tertiary amine oxides may
be employed as alternative reagents as they provide
appropriate interacting loci for the organoboranes.
8
Degradation of benzeneboronic acid with bromine is greatly
facilitated by added hard bases such as water. The findings
suggest a push-pull mechanism in which the aromatic ring
is activated by the hard donor which supplies electrons on
bonding with the boron atom.
Iododeboronation of naphthaleneboronic acids has also been studied.
9
Cleavage of C-B Bond by Other Nucleophiles
Homologous alcohols may be synthesized by treating
trialkylboranes with dimethylsulfonium methylide or
dimethyloxosulfonium methylide , followed by oxidative
workup.
10
Although carbenes are electron-deficient species, they
react, as donors, with boranes. Secondary alcohols are
produced via two successive alkyl migrations from the
boron to carbenium centers of the intermediates when
chlorocarbene and methoxycarbene are employed.
11
By having a carbanionoid center, diazo compounds react with
boranes easily. For example, diazoketones yield ß-oxoboranes
which rearrange in situ to generate enol borates. The use of
organoazides instead of diazo compounds leads to secondary
amines.
12
Indirect alkylation of ketones and esters via their α-bromo
derivatives can be achieved by base-catalyzed reactions with
trialkylboranes.
13
Triallylborane reacts with monosubstituted
acetylenes by an ene-type process. This could be a
reflection of the fact that boron prefers bonding to a
slightly harder sp2 carbon.
14
HYDROBORATION
Borane is a soft Lewis acid, therefore its complexation with
soft olefin linkages is very favorable. Apparently the initial
borane-alkene complexes collapse rapidly to a four-centered
transition state leading to the organoboranes without the
participation of external nucleophiles. This process is called
hydroboration.
The unusual orientation of addition of the B-H elements across
double bonds may be due to the thermodynamic stability of the
transition state which has a certain degree of polar character.
15
It has been found that the central atom of the
allenic bond is the preferred boron-bonding
site during hydroboration. This center is
harder than the terminal sp2 carbons.
16
In the hydroboration of styrene derivatives, the
orientation is influenced to some extent by the
substituent on the benzene ring. The presence of an
electron-withdrawing group stabilizes the
transition state leading to boron attachment to the
benzylic position. This carbon is harder than that
of styrene. On the other hand, an electron-donating
function tends to direct the C-B bonding toward
the terminal carbon.
17
Hydroboration of 1-haloalkenes and enol
acetates places the boron atom predominantly
at the α-carbon.
18
However, enol ethers and enamines, regardless of
the degree of substitution on the original double
bond, give 1,2-disubstituted products.
The halogen and the acetoxy group infuse hardness
in the ipso carbon, rendering it a better partner for
the boron.
19
Although alkoxy [including intracyclic analogs such
as 2,3-dihydrofuran and dihydropyran] and amino
substituents should exert a similar inductive effect,
this is swamped by the resonance inherent in these
systems.
20
Chloroborane (C1BH2) is a more regioselective
hydroborating agent than borane. This property
can be attributed to the heightened hardness of the
boron atom.
The reaction of organoboranes with carbon-
nitrogen multiple bonds always leads to
aminoborane derivatives through a hard-hard
interaction.
21
REACTIONS OF ARYLBORON COMPOUNDS
Arylboron compounds are susceptible to
nucleophilic attack which often results in the
cleavage of the CAr—B bond. On the other hand,
the introduction of a boron substituent into the
aromatic ring may be performed by the
interaction of aryl iodides with boron triiodide.
22
Ortho-disubstitution of aromatics via aryltrialkylboranides has been
studied. Reaction of the boranides with alkyl fluorosulfates affords
dihydroarene derivatives which rearomatize on treatment with
alkaline hydrogen peroxide. The hydroperoxyboranides may
decompose in either of two ways according to the HSAB concept.
23
SOME ASPECTS OF ORGANOALUMINUM REACTIONS
Aluminum and boron display similar chemical
characteristics. One important difference is that boron
is a metalloid, whereas aluminum is a bonafide metal.
As a consequence, the boron center shows soft or hard
characteristics depending on the ligands it carries; the
aluminum center is hard most of the time.
Thioesters are synthesized from methyl esters by
treatment with alkylthiodimethylalanes.
24
Functional reagents transform esters and lactones
into ketene thioacetals and dithio orthoesters,
respectively. In these reactions, the alkoxide moiety
attaches to the hard Al atom. That the carbonyl C is
left to combine with a soft sulfide represents a
compromise as the hard-hard Al-O interaction far
outweighs the loss in stability resultant from the
change of
25
Unsymmetrical epoxides are cleaved differently by lithium
tetraalkylaluminate and trialkylaluminum. Alkyl transfer from
the softer "ate“ complexes takes place at the softer (less
substituted) carbon atom, whereas the harder R3A1 reacts at
the alternative position. It can also be shown that "ate“
complexes effect opening of epoxides with inversion of
configuration, whereas R3A1 gives alcohols in which the
configurations are retained.
26
The conversion of epoxides to allylic alcohols can
be accomplished by dialkylamide anions, but the
treatment with N-diethylalanyl-2,2,6,6-
tetramethylpiperidine is more efficient. A cyclic
syn-elimination pathway is indicated. A perfect
match of hard interactions is provided by such
combinations.
27
