Ambiphilic Alcohol Dehydrogenation by BICAAC Mimicking Metal–Ligand Cooperativity
Surbhi Bansal, Ayanangshu Biswas, Abhishek Kundu, Manu Adhikari, Sanjay Singh, Debashis Adhikari
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.