The reduction of
carboxylic acids to the respective alcohols, in
mild conditions, was achieved using [MnBr(CO)
5
] as the
catalyst and bench stable PhSiH
3
as the reducing agent.
It was shown that the reaction with the earth-abundant metal catalyst
could be performed either with a catalyst loading as low as 0.5 mol
%, rare with the use of [MnBr(CO)
5
], or on a gram scale
employing only 1.5 equiv of PhSiH
3
, the lowest amount of
silane reported to date for this transformation. Kinetic data and
control experiments have provided initial insight into the mechanism
of the catalytic process, suggesting that it proceeds via the formation
of silyl ester intermediates and ligand dissociation to generate a
coordinatively unsaturated Mn(I) complex as the active species.
Recent developments in manganese-catalyzed reducing transformations—hydrosilylation, hydroboration, hydrogenation, and transfer hydrogenation—are reviewed herein. Over the past half a decade (i.e., 2016-present), more than 115 research publications have been reported in these fields. Novel organometallic compounds and new reduction transformations have been discovered and further developed. Significant challenges that had historically acted as barriers for the use of manganese catalysts in reduction reactions are slowly being broken down. This review will hopefully assist in developing this research with a clear and concise overview of the catalyst structures and substrate transformations published so far.
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