Mechanism of NHC-Catalyzed Conjugate Additions of Diboron and Borosilane Reagents to α,β-Unsaturated Carbonyl Compounds

Broadly
applicable enantioselective C–B and C–Si
bond-forming processes catalyzed by an N-heterocyclic carbene (NHC)
were recently introduced; these boryl and silyl conjugate addition
reactions (BCA and SCA, respectively), which proceed without the need
for a transition-metal complex, represent reaction pathways that are
distinct from those facilitated by transition-metal-containing species
(e.g., Cu, Ni, Pt, Pd, or Rh based). The Lewis-base-catalyzed (NHC)
transformations are valuable to chemical synthesis, as they can generate
high enantioselectivities and possess unique chemoselectivity profiles.
Here, the results of investigations that elucidate the principal features
of the NHC-catalyzed BCA and SCA processes are detailed. Spectroscopic
evidence is provided illustrating why the presence of excess base
and MeOH or H2O is required for efficient and enantioselective
boryl and silyl addition reactions. It is demonstrated that the proton
sources influence the efficiency and/or enantioselectivity of NHC-catalyzed
enantioselective transformations in several ways. The positive, and
at times adverse, impact of water (biphasic conditions) on catalytic
enantioselective silyl addition reactions is analyzed. It is shown
that a proton source can facilitate nonenantioselective background
reactions and NHC decomposition, requiring the catalyst to surpass
such complications. Stereochemical models are presented that account
for the identity of the observed major enantiomers, providing a rationale
for the differences in selectivity profiles of BCA and SCA processes.