Ambiphilic Alcohol Dehydrogenation by BICAAC Mimicking Metal–Ligand Cooperativity

In this report, an unusual bond activation strategy has
been demonstrated
by BICAAC, which essentially emulates the behavior of a transition
metal. The ambiphilic nature of this specific carbene has facilitated
a simultaneous proton and hydride capture from an alcohol molecule
to carry out smooth dehydrogenation under mild conditions. The activation
route closely follows the traditional metal–ligand bifunctional
activation of a substrate. The hydrogen molecule extracted from the
substrate alcohol becomes stored in the carbene carbon, which has
been unambiguously ascertained by the isolation of this intermediate
and its X-ray crystallographic characterization. Such an event has
further been interrogated in detail by the deuterium-labeling experiment
and DFT computations to substantiate the critical role of carbene’s
ambiphilicity. Additionally, the stored hydrogen in the carbene molecule
has been delivered to an in situ-generated olefinic bond to completely
mimic a borrowing hydrogen reaction in an organocatalytic fashion.
Both dehydrogenation and rehydrogenation reactions have been conducted
in a single pot using BICAAC as the catalyst that alkylates fluorene
at its 9-position using a series of alcohols as the alkyl source.
A thorough mechanistic sketch describes the involvement of a radical
for the latter part of the reaction, overall bringing a different
outlook to carbene-promoted small-molecule activation reactions.