Mechanistic differences between the Ru(ii) and Zn(ii)-catalyzed cross-coupling of cyclopropenes with diazo compounds: a DFT study

Abstract

Transition-metal-catalyzed cross-coupling of two different carbene precursors has been established to construct olefins. In this context, Ru(II) and Zn(II)-catalyzed cross-coupling of cyclopropenes with diazo compounds to synthesize 1,3-butadiene derivatives has been developed. Herein, a detailed computational study was performed to shed light on the mechanistic differences between the Ru(II) and Zn(II)-catalyzed cross-coupling of cyclopropenes with diazo compounds. For the Ru(II)-catalyzed reaction, the formation of an Ru–carbene intermediate with diazo compounds is more feasible. Next, cyclopropenes could undergo a [2 + 2] cycloaddition with the Ru–carbene intermediate to form a four-membered ring intermediate, from which an olefin metathesis mechanistic pathway is suggested. Afterward, an unusual alkenyl 1,2-migration might occur to afford the desired cross-coupling product. Meanwhile, for the Zn(II)-catalyzed reaction, the formation of Zn–carbenoid with cyclopropenes more readily occurs. Then, the nucleophilic C-attack of diazo compounds to the carbene moiety is suggested, leading to the desired product in a concerted manner. The origin of the different chemoselectivities in activating cyclopropenes/diazo compound carbene precursors for the Ru(II) and Zn(II) catalysis was discussed.