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Aurell, MJ.; González-Cardenete, MA.; Zaragoza, RJ. (2018). A new mechanism for internal
nucleophilic substitution reactions. Organic & Biomolecular Chemistry. 16(7):1101-1112.
doi:10.1039/c7ob02994b

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New mechanism for internal nucleophilic substitution reactions.

María J. Aurell, a Miguel A. González-Cardenete, b and Ramón J. Zaragozá*,a
Received 00th January 20xx,
Accepted 00th January 20xx A new mechanism of the classic internal nucleophilic substitution reactions SNi by means of computational studies
in gas-phase, DCM and acetonitrile is reported. Despite the importance of the SNi mechanism, since the mid-
DOI: 10.1039/x0xx00000x
1990s this mechanism has remained unexplored. The study has focused mainly on the comparison between the
www.rsc.org/ mechanisms postulated to date for the SNi reactions and a new mechanism suggested by us that fits better the
experimental observations. The comparative study has been applied to the conversion of ethyl, neopentyl,
isopropyl and tert-butyl chlorosulfites into the corresponding alkyl chlorides. This new mechanism occurs through
two transition structures. For primary and secondary substrates, the first transition structure is a 6-center syn-
rearrangement of the alkanesulfonyl chloride that produces the corresponding olefin by simultaneous expulsion
of HCl and SO2. The olefin, HCl and SO2 form a molecular complex. The final syn addition of HCl to the olefin leads
to the alkyl chloride with retention of configuration. For tertiary substrates a variation of the previous mechanism
is postulated with the intervention of contact ionic pairs. It is of great importance to emphasize that the new
mechanism is able to explain some experimental observations such as the presence of olefins in this type of
reactions and the low reactivity of some systems such as neopentyl chlorosulfite. Our results pave the way to a
new mechanistic perspective in similar reactions which will need further studies and validation.

1-Introduction O H O
A general method for converting alcohols to halides involves R OH + Cl S Cl R O S Cl
reactions with various halides of nonmetallic elements. Thionyl Cl
-
chloride, phosphorus trichloride, and phosphorus tribromide Cl
are the most common examples of this group of reagents.1-5
H O
Thionyl chloride is often preferable to other reagents due to its
HCl + R OSOCl R O S Cl
gaseous byproduct (SO2) for simplified purification.4The excess
of thionyl chloride can also be readily removed by distillation Path I
-
or evaporation. Organic bases such as pyridine are often added Cl ,SO2 Path II Path III Path IV
to the reaction mixture because they provide a substantial + -
concentration of chloride ion needed for the final reaction of
- R Cl
Cl HCl,SO2
the chlorosulfite intermediate. Recently, the use of catalysts SN1 SN 2 SN i E
such as titanium tetrachloride has been introduced for the
stereoretentive chlorination of cyclic alcohols with thionyl R Cl R Cl R Cl
chloride.6 However, the reaction also proceeds in the absence racemization inversion retention elimination
of either bases or catalysts and under heating conditions.
The reaction of alcohols with thionyl chloride initially results in Scheme 1 Simplified mechanism for reaction of alcohol with SOCl2
the formation of a chlorosulfite ester which is transformed
subsequently into the chloride (Scheme 1).1-6In the first stage, the alcohol attacks SOCl2 and after expulsion of Cl- and
deprotonation, a chlorosulfite is formed. The mechanism for
a. Departamento
this stage is widely accepted and proceeds with retention of
de Química Orgánica, Universidad de Valencia, Dr. Moliner 50,
46100 Burjassot, Valencia, Spain. configuration at the carbon bearing the hydroxyl group.
b. Instituto de Tecnología Química (UPV-CSIC), Universitat Politècnica de Valencia-
The further course of the reaction will depend on both the
Consejo Superior de Investigaciones Científicas, Avda. de los Naranjos s/n, 46022
Valencia, Spain. structure of the chlorosulfite and the reaction conditions. The
E-mail: Ramon.J.Zaragoza@uv.es transformation of the chlorosulfite ester into a chloride,
through a nucleophilic substitution, proceeds with
† Electronic Supplementary Information (ESI) available. See
DOI: 10.1039/x0xx00000x
racemization (Path I), inversion (Path II) or retention (Path III)

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of configuration at the carbon bearing the sulfite group this mechanism which indeed has not received much
(Scheme 1).1-12The decomposition of the chlorosulfite may also attention. For example, the SNi mechanism is not explicitly
produce an alkene via an elimination mechanism (Path IV) considered in some well-known textbooks. It is also ironic that
(Scheme 1).4-7, 11Thus, several mechanisms might account for chlorination with SOCl2 is a technique that is covered in most
the variety of products formed under different reaction undergraduate organic chemistry textbooks, and is even
conditions. performed in some undergraduate teaching laboratories,
For example, if racemization occurs, a SN1 mechanism is without mentioning the SNi designation.
normally accepted.7In the cases of inversion of configuration, The present paper reports the results of a detailed study, by
the suggested mechanism is of SN2 type7, 9, 11which may need means of computational studies, of the main mechanistic
the assistance of added base such as pyridine.7, 12 options suggested to date for the SNi reaction (Scheme 2, paths
On the other hand, the retention of configuration is achieved A/A’ and B), comparing them with a new postulated
mechanistically either through a double inversion in the mechanism (Scheme 2, path C). The study has been applied to
presence of nucleophilic solvents9 or via an internal the conversion of a chlorosulfite ester into an alkyl chloride.
nucleophilic substitution (SNi).6, 7, 13The reaction of alcohols The new mechanism is able to explain some experimental
with thionyl chloride, with retention of configuration at the observations such as the presence of olefins in this type of
carbon bearing the hydroxyl group, is not the only case where reactions and the low reactivity of some systems such as
a possible SNi mechanism is postulated. For example, the neopentyl chlorosulfite.
reactions of certain secondary alcohols with phosgene, PCl5 in
liquid SO2, or dry HBr to give halides, whose configurations are
the same as those of the starting alcohols, have been classified 2-Results and Discussion
as examples of the SNi reaction.7 In Scheme 2, the three studied routes for the conversion of
A recent work of synthesis14 allowed us to perform a review on alcohol 1 into the corresponding alkyl chloride 3, through

a R1,R2=H
R1 R2
b R1=CH3,R2=H Cl2SO R1 R2 + HCl
c R1,R2=CH3 OH OSOCl
1 2 Path C

Path A Path A' Path B R1 R2

O
R1 R2 R1 R2
R1 R2 H S
O Cl O
OSOCl OSO Cl
Cl S
TS1 TS1' O TS3
TS2
R1 R1
R1 H 2C C
- + -
+
CH3C + ClSOO CH3C SO2 + Cl + R2
R2 R2 HCl SO2
SO2
IN1 (ion pair) IN1' (ion pair) IN2 Molecular complex
SO2
-
Cl R1
R2
R1 C SO2
SO2 SO2 H 2C
CH3C Cl Cl
R2 H
3 TS4 syn-addition

Scheme 2 Transformation of an alcohol to the corresponding chloroalkane via a SNi mechanism.

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chlorosulfite ester 2, by a SNi mechanism with retention of studies using ethyl chlorosulfite (2a) as a primary substrate,
configuration at the carbon bearing the hydroxyl group, are isopropyl chlorosulfite (2b) as a secondary substrate, and tert-
shown. Path A/A' and B describe current accepted reaction butyl chlorosulfite (2c) as a tertiary substrate. To facilitate
mechanisms. Path C is a new alternative suggested by us. reading this article, we use the letters a, b and c when
Here, our work demonstrates that path C is a superior referring to primary substrate, secondary substrate and
energetically favorable reaction path. tertiary substrate, respectively.
In path A/A’, the chlorosulfite ester is converted into the ion Firstly, we make a brief description of the calculations
pair IN1/IN1’. This IN1/IN1’ undergoes front-side attack of the previously made in similar systems and then, for each of the
Cl- and loss of SO2 to give the alkyl chloride 3 with retention of substrates, the calculations made by us. To this end, the
configuration. Lewis and Boozer1,8 proposed a multistep energy and geometry discussion is made initially on the basis
ionization mechanism, with the ion-pairs IN1 and IN1’, to of data in gas-phase, and subsequently the effect of solvent is
explain the formation of alkyl chloride 3. Cram7 also suggested introduced. To study this effect, the calculations have been
that the SNi mechanism involves ion-pair intermediates and carried out in a solvent of high polarity such as acetonitrile
that the SNi reaction differs from the SN1 reaction only in the (CH3CN) and in a solvent of medium polarity such as
sense that the departing group is complex, and that the anion dichloromethane (DCM) (see 2.2 Computational methods).
of the first ion-pair can decompose internally faster than an 2.1 Previous calculations.
anion can react at the rear of the carbon undergoing Schreiner, Schleyer and Hill,16 using semiempirical and ab initio
substitution. calculations, concluded that primary alkanesulfonyl chlorides
Subsequently King et al.15 proposed that the hydrolysis of tert- in polar solvents, e.g., CH3CN should ionize to give
butyl chlorosulfite (2c in Scheme 2), in water or methanol- preferentially an alkyl sulfinyl cation (ROSO+) and Cl- (path A’ in
chloroform mixture, is an ionization to the tert-butyl cation Scheme 2). The formation of the ion pair ROSO+Cl- precedes
and the chlorosulfite anion (path A in Scheme 2) followed by the loss of SO2 and is a key step in the SNi reaction. This
further reaction of these species. Moreover, Schreiner, mechanism has been named by the authors as SN2i. They
Schleyer and Hill16, 17 concluded that the ionization of exclude the possible ionization to give the carbocation R+ and
alkanesulfonyl chlorides in polar solvents, e.g., acetonitrile, the anion SO2Cl- (path A in Scheme 2). Their conclusions are
yields ROSO+ + Cl- for R = primary alkyl group, and R+ + OSOCl- based on the relative energies calculated for the ions ROSO+
for R = secondary or tertiary alkyl group. The formation of ion- and Cl- versus the ions R+ and SO2Cl-.
pairs always involves the competition between the SNi (front- Subsequently, the same authors17 examined the transition
side attack), SN2 (back-side attack) and SN1 (front- and back- structures for the front-side (SNi) and back-side (SN2)
side attack) mechanisms. substitutions of methyl and ethyl chlorosulfite in gas-phase
On the other hand, path B (Scheme 2) corresponds to the and in solution. They conclude that for primary alkyl
direct conversion of alkanesulfonyl chloride 2 into the alkyl chlorosulfites, such as ethyl chlorosulfite, the front-side attack,
chloride 3 through a unique transition structure TS2. This through a transition structure like TS2 (see Scheme 2), is
transformation was depicted as proceeding via a 4-center always preferred in the gas-phase and in solution.
rearrangement of the alkanesulfonyl chloride.7, 16, 17This option These authors also concluded that secondary and tertiary
permits a pure SNi mechanism by producing only the front-side alkanesulfonyl chlorides should ionize to give the carbocation
attack with retention of configuration. R+ and the chlorosulfinyl anion SO2Cl- (path A in Scheme 2).16,17
It is interesting to note that frequently, significant amounts of Subsequently, the anion SO2Cl- is fragmented into SO2 and Cl-
olefin are formed during decomposition of alkanesulfonyl (Scheme 3). The chloride ion, can front-side attack to the R+
chlorides.7, 16 leading to retention of configuration (named by the authors as
Finally, path C (Scheme 2) represents our proposed SNi SN1i) or back-side attack leading to inversion of configuration.
mechanism, which illustrates how olefins are formed and why
neopentyl chlorosulfite displays such low reactivity. In this
case, the conversion of the alkanesulfonyl chloride 2 into the
+ -
alkyl chloride 3 occurs through two transition structures TS3 OSOCl C + OSOCl
R1
and TS4. The transition structure TS3 is a 6-center syn- R2
rearrangement of the alkanesulfonyl chloride that produces
R1 R2
the corresponding olefin by simultaneous expulsion of the HCl
and SO2. This mechanism is similar to the pyrolytic eliminations + + SO
C Cl + Cl
of esters (Ei mechanism) to give olefins.18 The olefin, the HCl 2
R1 R1
and the SO2 form a molecular complex (IN2). The syn addition inversion R1 R2 retention R2 R2
of HCl to the olefin through the transition structure TS4 leads
+ SO2
-
to the alkyl chloride 3 with retention of configuration. Cl
With the aim of comparing the three alternative routes in
Scheme 3 Ionization of secondary and tertiary alkanesulfonyl chlorides.
Scheme 2 (paths A-C), we have carried out computational

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2.2 Computational methods

All calculations were carried out with the Gaussian 09 suite of
programs.19 Initially, density functional theory20 calculations
(DFT) have carried out using the B3LYP21 exchange-correlation
functionals, together with the standard 6-31G** basis set, in
gas phase.22 The stationary points were characterized by
frequency computations in order to verify that TSs have one
and only one imaginary frequency. Subsequently, since the
mechanism involves ionic species the inclusion of solvent
effects have been considered by using a relatively simple self-
consistent reaction field (SCRF) method23 based on the
polarizable continuum model (PCM) of Tomasi's group.24
Geometries have been fully optimized with PCM. As solvents
we have used DCM and acetonitrile. The intrinsic reaction
coordinate (IRC) paths25 were traced in order to check the
energy profiles connecting each TS to the two associated Fig. 1 Free energy profile (ΔG in kcal mol-1) for the conversion of 2a into
3a in vacuo (in black), DCM (in red) and CH3CN (in blue). The striped
minima of the proposed mechanisms using the second order and solid lines correspond to the path B and path C (Scheme 2),
respectively.
González–Schlegel integration method.26 In the case of
problematic IRCs (flat PES in the vicinity of TS), a relaxed scan form a molecular complex IN2a quite stable. The regio- and
was performed. stereoselective addition of HCl to the olefin through the
Due to the presence of ionic pairs (in 2b and 2c) we performed transition structure TS4a finally leads to chloroethane 3a. It
a more precise study by the introduction of four explicit should be noted that a carbocation is never formed, as it is
solvent molecules of acetonitrile. postulated in the normal mechanism of HCl addition to olefins.
The values of enthalpies, entropies and free energies in The Gibbs free energy of the transition structure TS4a is 45.2
vacuum (gas phase), DCM and acetonitrile were calculated kcal mol-1 higher than that of 2a (but 1.4 kcal mol-1 lower to ΔG
with the standard statistical thermodynamics at 298.15K and 1 of TS2a), being in this case the rate limiting step for the
atm (in gas phase) and 1 M (in solvent).22 conversion of 2a into 3a. Therefore, in terms of energy in gas-
The electronic structures of stationary points were analyzed by phase, the path C (Scheme 2) is more favorable than the path
the natural bond orbital (NBO) method.27 Finally, stationary B (Scheme 2) (see 2.3.1-extended of the ESI for more details).
points involved in these reactions were optimized, in DCM and 2.3.2 Solvent effects in geometries, charges and energies.
acetonitrile, using the MPWB1K hybrid meta functional28 We have selected two different solvent parameters (DCM and
together with the 6-31++G** basis set.22 This level of theory CH3CN) to cover the solvent polarities. In the calculations in
has shown to have good results for combinations of DCM and acetonitrile for the ions CH3CH2OSO+ + Cl-, CH3CH2+ +
thermochemistry and thermochemical kinetics in processes SO2Cl- (Table S1), a preference for ionization through path A’ in
including weak interactions.28 Also single point at B3LYP- Scheme 2 (CH3CH2OSO+ + Cl-) was observed. However, it was
D3/def2TZVP level and M062X/cc-pVTZ level over MPWB1K not possible to locate neither the ion pair CH3CH2OSO+Cl-
geometries (B3LYP-D3/def2TZVP//MPWB1K/6-31++G** and (IN1’a) nor the corresponding transition structure TS1’a in
M062X/cc-pVTZ//MPWB1K/6-31++G**) were performed in either DCM or CH3CN. For example, on using 2a and setting the
acetonitrile. Enthalpies, entropies and free energies were
calculated using the values of the standard statistical
thermodynamics obtained previously at B3LYP/6-31G** level.
2.3 Conversion of ethyl chlorosulfite (2a) into chloroethane (3a)
The relative energies (ΔG) in gas-phase, DCM and CH3CN
associated with the conversion of 2a into 3a are given in Fig. 1
(see also Table S1 and Fig. S1a-1/S3a-3 of the ESI). Fig. 2 shows
the geometries of the more relevant species.
2.3.1 In gas-phase.
Only was possible to locate the transition structure TS2a
corresponding to the path B (Scheme 2), and the transition
structures TS3a and TS4a, and intermediate IN2a
corresponding to the path C suggested by us. The transition
structure TS2a corresponding to the path B is a 4-center syn-
rearrangement (Fig. 2). The TS3a is a 6-center syn-
rearrangement, similar to the Ei pyrolitic eliminations of
Fig. 2 Geometries, in gas-phase, of the species involved in the conversion
esters,18 and leads to the corresponding olefin by of ethyl chlorosulfite 2a into chloroethane 3a. The bond lengths (in black)
are given in angstroms. Values in parentheses and in brackets correspond
simultaneous expulsion of HCl and SO2. The olefin, HCl and SO2 to the bond lengths in DCM and CH3CN, respectively. The bond order (BO),
in DCM, is in red.

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distance Cl-SO2CH2CH3 in steps of 0.2 Å, the energy profile in the gas-phase to solvents of increasing polarity, a flattening
CH3CN rises to a maximum (distance Cl-S ≈ 6 Å), and then around the transition structures in the corresponding IRCs is
stabilizes (see Fig. S5a of the ESI). When the bond Cl-S was observed (compare Fig. S1a-1/S1a-3, S2a-1/S2a-3 and S3a-
freed, the calculations converged to give the starting 1/S3a-3 of the ESI).
chlorosulfite 2a without passing through any transition With independence of the medium, the transition structure
structure. In order to obtain approximate calculations, the TS4a is about 1.2-1.4 kcal mol-1 more stable than the transition
distance RSO2+---Cl- was set at 6Å. In this situation, the ΔG structure TS2a. Based on these results we can suggest that for
values obtained for the molecular complex CH3CH2OSO+ ---Cl- a primary substrate the preferred mechanism, for the
are 40.7 and 35.6 kcal mol-1 in DCM and acetonitrile, conversion of an alkyl chlorosulfite into the corresponding
respectively, above the energy value of ethyl chlorosulfite 2a. chloride with retention of configuration, would be through the
The ΔG values obtained for the molecular complex two steps (TS3a and TS4a) of path C in comparison with the
CH3CH2OSO+ ---Cl- is similar to the values obtained for the path B in one step (TS2a).
transition structures TS2a, TS3a and TS4a (see Fig. 1), what It should be noted that in the case of ionization of a primary
makes possible the presence of these ions in polar solvents alkyl chlorosulfite (for example in acetonitrile) to give the
such as CH3CN. RSO2+ and Cl- ions, the Cl- ion must subsequently produce the
With regard to the transition structures TS2a, TS3a and TS4a, substitution of the SO2. In order to maintain the retention of
the inclusion of the solvent does not produce significant configuration, the Cl- ion attack would imply a transition
geometric changes. On increasing the polarity (gas-phase < structure similar to TS2a or TS3a (followed by TS4a), and
DCM < CH3CN), it is observed a light increase in the length of therefore these TSs would control the course of the reaction.
the bonds either being formed or broken except the bonds H- It is of great importance to emphasize that none of the
C(2) (TS3a and TS4a) in which the increase in polarity produces previously postulated mechanisms (A’ or B) can explain the
a shorter bond (Fig. 2). low rate constant for the decomposition of neopentyl
The extent of bond formation along a reaction pathway is chlorosulfite. 16 However, it can be explained by the absence,
provided by the concept of bond order (BO). The BO values, in in this case, of the new postulated C mechanism. For the
DCM, of some bonds are summarized in Fig. 2. In TS2a the BO mechanism C to occur, the presence of at least one hydrogen
values of 0.13, 0.28 and 0.20 for the C(1)-OSO, Cl-SO2 and on the carbon adjacent to the reactive center is required. In
C(1)-Cl bonds, respectively, indicate that formation of these the absence of such a proton (for example neopentyl
bonds is delayed. In the transition structure TS3a the C(1)- chlorosulfite), the mechanisms A’ (ionization to give Cl- and
OSO, Cl-SO2 and Cl-H bonds are also delayed (BO values of RSO2+) or B (TS2a) could act. In the case of ionization, a
0.15, 0.19 and 0.27, respectively), while the bond H-C(2) is reasonable pathway involves an ion pair return, Cl- to RSO2+, to
quite advanced (BO value of 0.58). Finally, at TS4a, the BO the front side of the substrate through a transition structure
values of 0.12 (C(1)-Cl), 0.09 (Cl-H) and 0.52 (C(2)-H) confirm similar to TS2a (the pathway through a transition structure
(see 2.3.1-extended of the ESI) the initial transference of the H similar to TS3a is not possible). As can be seen in the Table S2
atom of HCl to the olefinic C(2) before the formation of the (see ESI), the free energy barrier of TS2a1 in acetonitrile is 39.5
bond C(1)-Cl. kcal mol-1. This value is 7.9 kcal mol-1 higher than TS3a which
The natural charges are a valuable tool for the prediction of may explain the low reactivity of the neopentyl chlorosulfite
the ionic character of the involved species in a reaction with respect to a primary substrate such as ethyl chlorosulfite
mechanism. In this case, they helped us to establish the 2a.
carbocationic character of the hydrocarbon fragments of the Another experimental data that indirectly reinforces the
different species, and its variation changing the polarity of the mechanism C suggested by us is related to the presence of the
solvent. All the obtained transition structures (TS2a, TS3a and chiral chlorosulfite. Hudson et al. reported that neopentyl
TS4a) possess an important carbocationic character, which chlorosulfite shows an ABX9 system in the 1H NMR spectra.29
increases with the increase in polarity (Table S8 of the ESI). The Upon raising the temperature of undiluted neopentyl
transition structure TS3a possesses the least carbocationic chlorosulfite to 100 °C, a partial collapse of the AB pattern was
character in any medium, while TS4a has the highest observed. This is thought to be due to the onset of chlorine-
carbocationic character. chlorine exchange between molecules, each exchange being
The inclusion of the solvent produces a strong stabilization of accompanied with inversion of configuration at the sulfur
all species (between 3.8 and 11.0 kcal mol-1 in DCM and tetrahedron and causing loss of asymmetry for the time
between 4.4 and 13.0 kcal mol-1 in CH3CN), being this averaged environment of the protons of the methylene group
stabilization smaller in 2a and 3a with lesser ionic character (see TS-II in Scheme 4). Nevertheless, Schreiner, Schleyer and
(between 3.8 and 4.8 kcal mol-1) and higher in TS3a (between Hill16 prefer to postulate that chlorine exchange is due to an
8.1 and 9.6 kcal mol-1) and TS2a/TS4a (between 10.7 and 13.0 ionization process rather than an inversion. This premise is
kcal mol-1) (see Table S1 of the ESI), in agreement with their used to support the SNi mechanism through an ionization
higher cationic character. The result is that in the studied process with formation of RSO2+ and Cl- ions (see path-I in
solvents (DCM or CH3CN) the free energy barrier (see Fig. 1) for Scheme 4). In order to computational study this chlorine
the conversion of 2a into 3a is lower than in gas-phase, exchange we carried out a study, using methyl chlorosulfite as
regardless of the mechanism. In addition, when passing from a model, of the proposal of Schreiner, Schleyer and Hill (path-I

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O
Me S
O
path-I Cl
O
O
Me S
Me S O Cl
O Cl
O
path-II Me S
O Cl

TS-I

O Cl O
path-III S Me
2 MeOSOCl Me S
O Cl O Fig. 3 Free energy profile (ΔG in Kcal mol-1) for the conversion of 2b into
3b in vacuo (in black), DCM (in red) and CH3CN (in blue). The striped and
solid lines correspond to the path B and path C (Scheme 2), respectively.
TS-II The free energy of the ion-pair IN1b is also shown (in DCM and CH3CN).

2.4.1 In gas-phase.
O OMe
path-IV S It was possible to locate TS2b corresponding to the path B
3 MeOSOCl Cl Cl (Scheme 2), and TS3b, TS4b, IN2b corresponding to the path C
MeO S suggested by us.
S O The Gibbs free energy of the transition structure TS4b is 33.4
Cl OMe
O kcal mol-1 higher than that of 2b (but 1.0 kcal mol-1 lower to ΔG
of TS2b), being in this case the rate limiting step for the
TS-III
conversion of 2b into 3b. Therefore, in terms of energy in gas-
phase, the path C (Scheme 2) is more favorable than the path
Scheme 4 Chlorine exchange in methyl chlorosulfite
B (Scheme 2) (see 2.4.1-extended of the ESI for more details).
2.4.2 Solvent effects in geometries, charges and energies.
in Scheme 4) and Hudson et al. (path-III in Scheme 4). The calculations in DCM and CH3CN for the species
Additionally, we also studied two new possible alternatives: (CH3)2CHOSO+ + Cl-, (CH3)2CH+ + SO2Cl- indicate a higher
Path-II where the chlorine exchange occurs through an stability of ions (CH3)2CH+ + SO2Cl- (see Table S4 of the ESI) and
inversion similar to that of Walden in amines and path-IV therefore, the preference of an ionization process through
similar to Hudson's proposal but with the intervention of three path A in Scheme 2. All attempts to locate the corresponding
methyl chlorosulfite molecules. ion pair IN1b starting from different initial geometries gave
The results in acetonitrile indicate (see Table S3 of the ESI), disappointing results. In all cases, it evolves to the final
that the mechanism suggested by Schreiner, Schleyer and Hill product 3b or the intermediate IN2b. It was only possible to
is the most unfavorable (∆G=35.6 kcal mol-1). The Hudson
mechanism is more reasonable (∆G=22.8 kcal mol-1). But the
mechanism suggested by us, through TS-III, is the most
favorable from an energetic point of view (∆G=16.8 kcal mol-1).
Finally, it should be noted that, depending on the reaction
conditions, other mechanisms (SN2 and E2) to convert a
primary alkanesulfonyl chloride into either an olefin or the
corresponding chloride can compete with the paths A-C herein
studied (see Scheme S1a of the ESI for more details).
2.4 Conversion of isopropyl chlorosulfite (2b) into 2-
chloropropane (3b)
The relative energies (ΔG) in gas-phase, DCM and CH3CN
associated with the conversion of 2b into 3b are given in Fig. 3
(see also Table S4 and Fig. S1b-1/S3b-3 of the ESI). Fig. 4 shows
the geometries of the more relevant species.
Fig. 4 Geometries, in gas-phase, of the species involved in the conversion
of chlorosulfite 2b into chloride 3b. The bond lengths (in black) are given in
angstroms. Values in parentheses and in brackets correspond to the bond
lengths in DCM and CH3CN, respectively. The bond order (BO), in DCM, is
in red.

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obtain the intermediate IN1b with the distance C-H and the olefin or the corresponding chloride can compete with the
rotation of the methyl group frozen (see Fig. S4b of the ESI). paths A-C herein studied (see Scheme S1b of the ESI for more
The calculated energies (ΔG) of ion pair IN1b were 20.9 and details).
19.6 kcal mol-1 in DCM and acetonitrile, respectively. This ion 2.4.3 Effect of the solvent including explicit solvent molecules.
pair will evolve by Cl- ion attack, through a TS similar to TS4b, Due to the presence of ionic pairs, we performed a more
to the final product 3b or by abstraction of a proton, through a precise study by the introduction of explicit solvent molecules.
TS similar to TS3b, to the intermediate IN2b. The solvent used was acetonitrile. All calculations were carried
On the other hand, TS2b could only be obtained by keeping out at B3LYP/6-31G** level using the SCRF-PCM method. Four
the rotation of the methyl groups frozen, with a free energy molecules of acetonitrile were used for solvation. The
barrier of 24.3 kcal mol-1 in DCM and 22.6 kcal mol-1 in CH3CN acetonitrile molecules were positioned such that N was near
(not shown in Fig. 3). This energy barrier is higher to that positive charged centers (protons and S) and acetonitrile
obtained for the mechanism C suggested by us through TS3b methyl hydrogens near the negatively charged centers (Cl and
and TS4b in both DCM (ΔG = 19.7 and 23.5 kcal mol-1, O) (see Fig. 5).
respectively) and CH3CN (ΔG = 18.6 and 21.7 kcal mol-1, There are three distinct types of ion pairs, depending on the
respectively). extent of solvation of the two ions: fully solvated, solvent-
Again, the inclusion of the solvent does not produce significant shared and contact. When both ions have a complete primary
geometric changes to the transition structures TS2b, TS3b and solvation sphere, the ion pair may be termed fully solvated.
TS4b (Fig. 4). On increasing the polarity (gas-phase < DCM < When there is about one solvent molecule between cation and
CH3CN), it is observed a light increase in the length of the anion, the ion pair may be termed solvent-shared. Lastly, when
bonds either being formed or broken. On the other hand, the the ions are in contact with each other, the ion pair is termed a
length of these bonds is greater than the corresponding contact ion pair. Only contact and solvent-shared ion pairs
distances in TS2a, TS3a and TS4a. The exception are the bonds were considered for calculation.
H-C(2) in TS3b and TS4b in which the increase in polarity The results of the geometries and energies are shown in Fig. 5
produces a shorter bond (except in TS4b in acetonitrile). The (see also Table S6 of the ESI).
length of these bonds is less than the corresponding distances Species 2b-s, IN1b-contact-s, TS3b-s, IN2b-s and TS4b-s were
in TS2a, TS3a and TS4a. This lengthening and shortening of the minimized keeping the core frozen and acetonitrile molecules
bonds in TS2b, TS3b and TS4b with respect to TS2a, TS3a and aligned with the nearest proton. Only IN1b-shared-s was
TS4a is also reflected in the BO (see Fig. 2 and 4). totally optimized.
All the obtained transition structures were found to possess an The ionic pair IN1b-shared-s is 6.2 kcal mol-1 more stable than
important carbocationic character, which increases with the
increase in polarity (Table S9 of the ESI). In any medium, the
transition structure TS3b possesses the least carbocationic
character while TS4b has the highest carbocationic character.
As expected, there is an increase in the carbocationic character
of TS3b and TS4b with respect to TS3a and TS4a as we move
from primary substrates to secondary substrates.
The inclusion of the solvent produces a moderate stabilization
of 2b, IN2b and 3b with lesser ionic character (between 3.5
and 4.7 kcal mol-1) and a strong stabilization of TS3b and TS4b
(between 11.9 and 16.6 kcal mol-1) in agreement with their
higher cationic character (see Table S4 of the ESI). Overall, the
free energy barrier for the conversion of 2b into 3b, regardless
of the mechanism, is lower that the free energy barrier for the
conversion of 2a into 3a, previously studied.
Again, it is observed a flattening around the transition
structures in the corresponding IRCs when passing from the
gas-phase to solvents of increasing polarity (compare Fig. S2b-
1/S2b-3 and S3b-1/S3b-3 of the ESI).
With independence of the medium, the transition structure
TS4b is about 1 kcal mol-1 more stable than the transition
structure TS2b. Based on these results we can suggest that for
a secondary substrate the preferred mechanism, for the
conversion of an alkyl chlorosulfite into the corresponding
chloride with retention of configuration, would be through the
two steps (TS3b and TS4b) of path C. As in the case of the Fig. 5 Geometries in acetonitrile of the species, solvated with four
conversion of 2a to 3a, other mechanisms (SN2 and E2) to molecules of acetonitrile, involved in the conversion of chlorosulfite 2b into
chloride 3b. ΔG relatives to 2b-s.
convert a secondary alkanesulfonyl chloride into either an

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IN1b-contact-s. Comparing the results with those obtained for

the unsolvated species (Fig. 3), a slight stabilization of TS3b-s
and TS4b-s is observed with free energy values of 17.4 and
19.0 kcal mol-1, respectively. The most interesting fact is that
these last two values are lower than the value of 22.0 kcal mol-
1 obtained for the more favourable ion pair IN1b-shared-s. This

circumstance reinforces our hypothesis of mechanism C

suggested by us against mechanism A through ion pairs.
2.5 Conversion of tert-butyl chlorosulfite (2c) into 2-chloro-2-
methylpropane (3c)
The relative energies (ΔG) in gas-phase, DCM and CH3CN
associated with the conversion of 2c into 3c are given in Fig. 6
(see also Table S5 and Fig. S1c-1/S3c-3 of the ESI). Fig. 7 shows
the geometries of the more relevant species.
2.5.1 In gas-phase.
All attempts to locate TS2c corresponding to the path B
(Scheme 2) were unsuccessful. It was only possible to locate
TS3c, TS4c and intermediate IN2c corresponding to the path C
suggested by us. TS3c and TS4c have free energy barriers of
21.8 and 22.8 kcal mol-1, respectively, appreciably lower than Fig. 7 Geometries, in gas-phase, of the species involved in the conversion
of chlorosulfite 2c into chloride 3c. The geometries of IN1c and TS5c are in
that found for TS3a/TS4a and TS3b/TS4b. Therefore, in terms dichloromethane. The bond lengths (in black) are given in angstroms.
Values in parentheses and in brackets correspond to the bond lengths in
of energy in gas-phase, the path C (Scheme 2) is the only DCM and CH3CN, respectively. The bond order (BO), in DCM, is in red.
possible one (see 2.5.1-extended of the ESI for more details).
2.5.2 Solvent effects in mechanism, geometries, charges and mol-1, in DCM and acetonitrile, respectively.
energies. The geometry of TS1c is practically the same as the geometry
The inclusion of the solvent in both DCM and acetonitrile of TS3c in gas-phase (see Fig. 7). However, as it has been seen
produces an important change in the reaction mechanism. in the corresponding relaxed scans (Fig. S1c-2 and S1c-3 of the
Firstly, it is necessary to emphasize the presence in that ESI), the transition structure TS1c connects the starting
mechanism of the carbocation intermediate t-Bu+ in the form product 2c with the ion pair IN1c and not with the molecular
of ion pair IN1c (t-Bu+ + SO2Cl-). IN1c is stabilized by three complex IN2c.
hydrogen bonds, two O --- H and one Cl --- H (see Fig. 7). Free The intermediate IN1c evolves through TS5c to the molecular
energy values for IN1c in DCM and acetonitrile are 10.8 and complex IN2c. The barrier of this process is very small if we
9.0 kcal mol-1, respectively, above the energy value of tert- consider ∆E (between 0.7 and 1.4 kcal mol-1) or barrierless if
butyl chlorosulfite 2c. we consider ∆G. The molecular complex IN2c is quite stable
The intermediate IN1c is obtained through the transition with a free Gibbs energy of 0.5 kcal mol-1 (in DCM) and 0.9 kcal
structure TS1c with free energy barriers of 12.0 and 11.0 kcal mol-1 (in acetonitrile) higher than 2c.
The final conversion to the chloride 3c is carried out through
TS6c with free energy barriers of 2.4 kcal mol-1 (in DCM) and
2.2 kcal mol-1 (in acetonitrile) from IN1c. The geometry of TS6c
is practically the same of TS4c in gas-phase (see Fig. 7).
However, as it can be seen in the corresponding IRCs (Fig. S2c-
2 and S2c-3 of the ESI), the transition structure TS6c connects
the final product 3c with the ion pair IN1c and not with the
molecular complex IN2c.
The overall process for the transformation of tert-butyl
chlorosulfite 2c into 2-chloro-2-methylpropane 3c is outlined
in Scheme 5. The path can be considered as a hybrid between
path C and A of Scheme 2.
The initial chlorosulfite 2c is transformed into the ion pair IN1c
through TS1c. As a consequence of the small energy barriers
involved, the ion pair IN1c is in equilibrium with the initial
product 2c and the molecular complex IN2c through TS5c.
Finally, through TS6c the final product 3c is obtained. The
driving force of the whole process is the greater stability of the
(
Fig. 6 Free energy profile ΔG in Kcal mol-1) for the conversion of 2c into 3c
in vacuo (in black), DCM (in red) and CH3CN (in blue). The striped lines final product 3c together with the expulsion of SO2.
correspond to a variation of the mechanism in DCM and CH3CN.

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+ Me
Me TS1c C H TS6c
H Me
Me HO Cl
OSOCl - Me
Me Cl S SO2
O
2c Ion pair IN1c 3c
TS5c
Me
H 2C C
+ Me
HCl SO2
Molecular complex IN2c

Scheme 5 Overall process for the conversion of 2c into 3c

Due to the tertiary nature of the substrate, these species

present a high carbocationic character (see 2c, 3c, IN2c,
TS3c/TS1c and TS4c/TS6c in Table S10). The new transition
structure TS5c and the ion pair IN1c show less cationic
character than TS3c/TS1c and TS4c/TS6c. This is possibly due
to the dispersion of the charge produced by the hydrogen
bonds present in both species.
It should be noted that the free energy barrier for the
conversion of 2c into 3c is lower that the free energy barrier
for the conversion of 2a into 3a and 2b into 3b, previously
studied. This free energy barrier so low is in agreement with
the difficulty for the isolation of tertiary chlorosulfites due to
its rapid decomposition.16 However, it has been possible to
identify them by NMR spectroscopy at low temperature.
30There is also a large flattening around the transition
Fig. 8 Geometries in acetonitrile of the species, solvated with four
molecules of acetonitrile, involved in the conversion of chlorosulfite 2c
structures in the corresponding IRCs or relaxed scans (see Fig. into chloride 3c. ΔG relatives to 2c-s.
S1c-2, S1c-3, S2c-2, S2c-3, S3c-2 and S3c-3 of the ESI).
As in the case of the conversion of 2a to 3a and 2b to 3b, other 2.6 Effect of calculation level
mechanisms (E1, E2, SN1) to convert a tertiary alkanesulfonyl
In order to verify that the results obtained using the B3LYP/6-
chloride into either an olefin or the corresponding chloride can
31G** are correct, the main species involved in the studied
compete with the paths herein studied (see Scheme S1c of the
mechanisms were optimized, in DCM and CH3CN, using
ESI for more details).
MPWB1K /6-31++G**. This level of theory has shown to have
2.5.3 Effect of the solvent including explicit solvent molecules.
good results for combinations of thermochemistry and
A study similar to 2b was performed with the explicit inclusion
thermochemical kinetics in processes including weak
of four molecules of acetonitrile. The results of the geometries
interactions.
and energies are shown in Fig. 8 (see also Table S6 of the ESI).
Also single point at B3LYP-D3/def2TZVP level and M062X/cc-
Species 2c-s, TS1c-s, IN2c-s, TS6c-s and TS5c-s were minimized
pVTZ level over MPWB1K geometries (B3LYP-
keeping the core frozen and acetonitrile molecules aligned
D3/def2TZVP//MPWB1K/6-31++G** and M062X/cc-
with the nearest proton. IN1c-contact-s and IN1c-shared-s
pVTZ//MPWB1K/6-31++G**) were performed in acetonitrile.
were totally optimized. In this case a clear difference was
The energies associated with the conversion of 2a into 3a, 2b
observed with respect to the stability of the ion pairs, being
into 3b and 2c into 3c are given in Table S7 of the ESI, while a
much more stable the ion pair IN1c-contact-s (∆G=4.5 kcal
schematic representation of the free energy profile at
mol-1 versus 14.1 kcal mol-1). Comparing the results with those
MPWB1K /6-31++G** level is shown in Fig. 9.
obtained for the unsolvated species (Fig. 6), is observed again
We can summarize the observed changes, with respect to
a stabilization of TS1c-s, TS6c-s and TS5c-s, with free energy
B3LYP/6-31G** level, as follows:
values of 8.1, 9.0 and 6.2 kcal mol-1, respectively. The energy
- At MPWB1K /6-31++G** level there is an increase in free
values obtained do not substantially modify the proposed
energy between 1.3 and 5.9 Kcal mol-1 of all transition
mechanism.
structures and intermediate species in relation to 2 (see Fig. 9
and compare Tables S1, S4 and S5 with Table S7 of the ESI).

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chloride. Finally, the path C occurs through two transition

structures TS3 and TS4. The transition structure TS3 is a 6-
center syn-rearrangement of the alkanesulfonyl chloride that
produces the corresponding olefin by simultaneous expulsion
of the HCl and SO2. The olefin, HCl and SO2 form a molecular
complex (IN2). The syn addition of HCl to the olefin through
the transition structure TS4 leads to the alkyl chloride 3 with
retention of configuration.
The study led to three main conclusions:
- For a primary substrate, the mechanism suggested by us
through two transition structures (path C) is more favorable
than the mechanism through one transition structure (path B),
both in gas phase and in solvent. Only in very polar solvents
such as acetonitrile the ionization mechanism can co-occur
(ionization to ROSO+ and Cl-, path A’).
- For a secondary substrate, the mechanism suggested by us
through two transition structures (path C) is more favorable
than the mechanism through one transition structure (path B)
(
Fig. 9. Free energy profile ΔG in Kcal mol-1) for the conversion of 2a into
and the ionization mechanism (path A), both in gas phase and
3a (hashed lines), 2b into 3b (striped lines) and 2c into 3c (solid lines) in
DCM (in red) and CH3CN (in blue) at MPWB1K /6-31++G** level.
in solvent.
In acetonitrile, by the introduction of explicit solvent
- A similar situation is observed at M062X/cc-pVTZ level. In this
molecules, the ionization mechanism through a solvent-shared
case the increase in free energy is higher (between 2.5 and 8.3
ion pair (R+--acetonitrile--SO2Cl-) is disfavoured against the
Kcal mol-1; compare Tables S1, S4 and S5 with Table S7 of the
mechanism C.
ESI).
- For a tertiary substrate the scenario changes remarkably. In
- At B3LYP-D3/def2TZVP level the situation changes and there
gas phase, the only reasonable mechanism is the two-stage
is a decrease in free energy between 1.7 and 5.2 Kcal mol-1 of
mechanism (path C). In solvent, the most likely mechanism is a
all transition structures and intermediate species in relation to
variation of the mechanism in two stages (path C) with the
2 (compare Tables S1, S4 and S5 with Table S7 of the ESI).
intervention of contact ionic pairs. In this case, the initial
However, these variations do not significantly affect the
chlorosulfite is transformed into an ion pair R+SO2Cl- in
reaction profile and we can therefore conclude that the results
equilibrium with a more stable molecular complex R’2C=CH2-
obtained at B3LYP/6-31G** level can be considered adequate.
HCl-SO2, which undergoes regio- and stereoselective addition
of HCl.
3. Summary and Conclusions Finally, it should be noted that depending on the reaction
conditions and the nature of the substrate other mechanisms
In summary, a study of the classic internal nucleophilic (E1, E2, SN1, SN2,) to convert an alkanesulfonyl chloride into
substitution mechanism SNi by means of computational studies either an olefin or the corresponding chloride can compete
at B3LYP/6-31G**, MPWB1K /6-31++G**, B3LYP-D3/def2TZVP with the paths studied.
and M062X/cc-pVTZ level in gas-phase, DCM and acetonitrile From an experimental point of view, our proposed mechanism
has been carried out. The study focused mainly on the explains a) the presence of olefins in this reaction type, b) the
comparison between the mechanisms postulated to date for low reactivity of systems without hydrogens on the carbon
the SNi mechanism (path A/A’ and path B) and the new adjacent to the reactive center, a key fact inexplicable by
mechanism suggested by us (path C). The comparative study current accepted mechanisms. For the mechanism C to occur,
has been applied to the conversion of a chlorosulfite ester into the presence of at least one hydrogen on the carbon adjacent
an alkyl chloride. We have carried out computational studies to the reactive center is required. In the absence of such a
using ethyl chlorosulfite (2a) (and neopentyl chlorosulfite) as a proton (for example neopentyl chlorosulfite), the mechanisms
primary substrate, isopropyl chlorosulfite (2b) as a secondary A/A’ or B with greater energy barrier could act, thus lowering
substrate, and tert-butyl chlorosulfite (2c) as a tertiary the reaction rate.
substrate. Due to the presence of ionic pairs, we performed a
more precise study, for 2b and 2c, by the introduction of
explicit solvent molecules (acetonitrile). Conflicts of interest
In the path A/A’, the chlorosulfite ester is converted, through
There are no conflicts to declare.
an ionization process, into an ion pair. This ion pair undergoes
front-side attack of the Cl- and loss of SO2 to give the alkyl
chloride with retention of configuration. The path B
Acknowledgements
corresponds to the direct conversion, via a 4-center
rearrangement (TS2), of the chlorosulfite ester into the alkyl

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