Surmounting Alkoxide Trap Strategy: N-Heterocyclic Carbene Chromium(0)-Catalyzed C-Alkylation between Alcohols

The crucial role of the alkoxide trap problem and the impact of the oxidation state of the chromium center were demonstrated on the catalytic activity due to the d-wall issue. Through the strategy of surmounting the alkoxide trap, a Cr(0) catalyst was presented herein for efficient C-alkylation between alcohols via borrowing hydrogen/hydrogen autotransfer. The synthesized bis-(N-heterocyclic carbene)-Cr(0) system shows an efficient catalytic performance (40 examples, up to 96% yield). Only a catalytic loading of a cheap and readily available base NaOH is effective enough for the reaction. Compared with Cr(III)/Cr(II), Cr(0) can well avoid the d-p p interaction in the key metal-alkoxide intermediate, thus overcoming the thermodynamic sink due to the alkoxide trap problem. It is plausible that the Cr(III) systems need to be reduced to Cr(II) for weakening the alkoxide trap effect and enhancing activity by using reductive strong bases. This surmounting alkoxide trap strategy should be helpful for the development of efficient and nonprecious transition metal catalysts.