Ambiphilic Alcohol Dehydrogenation by BICAAC Mimicking Metal–Ligand Cooperativity

Abstract

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.