Theoretical Studies on Rh-Catalyzed Cycloisomerization of Homopropargylallene-Alkynes through C(sp3)–C(sp) Bond Activation

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 homopropargylallene-alkynes through C­(sp3)–C­(sp) bond activation. The DFT results indicate that the catalytic cycle is divided into four main processes involving oxidative coupling, insertion, 1,3-alkyl migration, and 1,2-alkyl migration or 1,2-H migration. It is found that a Rh­(I) Fischer carbene intermediate, which is derived from the third process 1,3-alkyl migration, selectively determines the formation of the six/five/five tricycle Product 1 or the six/five/four tricycle Product 2, depending on the substitutes on the substrates. The origin of cycloisomerization product selectivity has also been addressed.