Abstract and subjects
For
years, following the ideas of Shvo and Noyori, the core assumption
of metal–ligand bifunctional molecular catalysis has relied
on the direct involvement of the chelating ligand in the catalytic
reaction via a reversible proton (H+) transfer through
cleavage/formation of one of its X–H bonds (X = O, N, C). A
recently revised mechanism of the Noyori asymmetric hydrogenation
reaction (Dub, P. A. et al. J. Am. Chem. Soc. 2014, 136, 3505) suggests that the ligand
is rather involved in the catalytic reaction via the stabilization
of determining transition states through N–H···O
hydrogen-bonding interactions (HBIs) and not via a reversible H+ transfer, behaving in a chemically intact manner within the
productive cycle or predominantly in a chemically
intact manner within productive cycles. By reexamining selected examples
of computational mechanistic studies involving bifunctional catalysts
from the literature in the years between 2012–2017, the purpose
of this work is to point out common misconceptions in modeling concerted
reactions and show that the actual stepwise nature of key transition
states unveils a more complicated catalytic reaction pool (all conceivable catalytic pathways and their crossovers). Such
a realization can not only potentially result in a reconsideration
of the “accepted” mechanism but also lead us to a new
conceptual understanding of the role that the ligand plays in the
reaction. The ultimate goal of this paper is, therefore, to encourage
the reader to reconsider the function of the ligand in catalytic cycles
of hydrogenation/dehydrogenation with bifunctional catalysts, which
until recently has relied almost exclusively on a chemically noninnocent
ligand.