Insights into the mechanism and selectivity of the Rh(i)-catalyzed cycloisomerization reaction of benzylallene-alkynes involving C–H bond activation

Abstract

A systematic theoretical study has been carried out with the aid of density functional theory (DFT) calculations on the mechanism of the Rh-catalyzed cycloisomerization of benzylallene-alkynes involving C_sp2–H bond activation on the benzyl phenyl ring. Experimentally, it was found that the substituent on the allene moiety has a drastic effect on the cycloisomerization selectivity and the terminal substituent on the alkyne moiety also shows an effect on the product selectivity. In this work, through our detailed mechanistic calculations, we found that when the terminal substituent on the alkyne moiety is a hydrogen, regardless of the substituent Y on the allene moiety is an EDG or EWG, the pathway involving β-elimination of this hydrogen in an intermediate (derived from an olefin insertion step following oxidative coupling and σ-bond metathesis) is always favorable. When the terminal substituent on the alkyne moiety is an alkyl, two scenarios were found depending on whether the substituent Y on the allene moiety is an EWG or EDG.