Mechanism and Origin of Enantioselectivity in Nickel-Catalyzed
Alkyl–Alkyl Suzuki Coupling Reaction
Posted on 2019-07-24 - 20:45
Enantioselective
Suzuki coupling reactions are a widely used method
in asymmetric synthesis of chiral compounds. In an important extension
of this protocol, 1-bromo-1-fluoroalkanes were coupled with alkyl-9-BBN
using chiral NiCl2L* as the catalyst (where L* = bis(pyrrolidine) ligand) under Suzuki conditions to obtain
a product with a stereogenic center bearing a fluorine. In view of
the current interest in chiral fluorine-containing compounds as well
as lack of clarity on the mechanism of Ni-catalyzed asymmetric Suzuki
coupling reactions, we decided to examine various mechanistic pathways
of the title reaction. The (U)M06 density functional theory computations
have been employed to identify the energetically preferred pathway
first and then to probe the origin of high enantioselectivity. In
particular, we have compared the likely involvement of different redox
couples such as Ni(0)/Ni(II) and Ni(I)/Ni(III) in the catalytic cycle.
For the Ni(0)/Ni(II) pathway, both singlet and triplet spin states
have been considered whereas a doublet spin multiplicity has been
examined in the case of the Ni(I)/Ni(III) system. The most preferred
catalytic pathway is found to proceed through a Ni(I)/Ni(III) redox
cycle with key mechanistic steps such as (a) a transmetalation involving
the transfer of the alkyl group of 9-BBN to the Ni-catalyst, (b) an
oxidative addition of bromo(fluoro) alkane to give a penta-coordinate
Ni(III) intermediate, and (c) an enantio-controlling reductive elimination
(RE) that facilitates the C–C bond formation between the Ni-bound
fluoroalkyl and alkyl moieties to yield the final product. The transmetalation
is found to be the turnover determining transition state (TS) according
to the activation span model. The RE is found to be the enantio-controlling
step, wherein the TS for the addition of the si prochiral
face of the Ni-bound fluoro alkyl moiety to the alkyl group is 4.3
kcal/mol lower than the corresponding re face addition.
Distortion-Interaction analysis suggested that the extent of distortion
in the catalyst Ni(Br)L* fragment in the si face reductive elimination TS is much lower than in the re face addition, thus making a vital contribution to the
energy difference between diastereomeric TS.
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Singh, Sukriti; Sunoj, Raghavan B. (2019). Mechanism and Origin of Enantioselectivity in Nickel-Catalyzed
Alkyl–Alkyl Suzuki Coupling Reaction. ACS Publications. Collection. https://doi.org/10.1021/acs.jpca.9b04284
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AUTHORS (2)
SS
Sukriti Singh
RS
Raghavan B. Sunoj