Solvent-Assisted Ketone Reduction by a Homogeneous Mn Catalyst

The choice of a solvent
and the reaction conditions often defines
the overall behavior of a homogeneous catalytic system by affecting
the preferred reaction mechanism and thus the activity and selectivity
of the catalytic process. Here, we explore the role of solvation in
the mechanism of ketone reduction using a model representative of
a bifunctional Mn-diamine catalyst through density functional theory
calculations in a microsolvated environment by considering explicit
solvent and fully solvated ab initio molecular dynamics simulations
for the key elementary steps. Our computational analysis reveals the
possibility of a Meerwein–Ponndorf–Verley (MPV) type
mechanism in this system, which does not involve the participation
of the N–H moiety and the formation of a transition-metal hydride
species in ketone conversion. This path was not previously considered
for Mn-based metal–ligand cooperative transfer hydrogenation
homogeneous catalysis. The MPV mechanism is strongly facilitated by
the solvent molecules present in the reaction environment and can
potentially contribute to the catalytic performance of other related
catalyst systems. Calculations indicate that, despite proceeding effectively
in the second coordination sphere of the transition-metal center,
the MPV reaction path retains the enantioselectivity preference induced
by the presence of the small chiral N,N′-dimethyl-1,2-cyclohexanediamine ligand within the catalytic
Mn­(I) complex.