Effect of Substituent on the Mechanism and Chemoselectivity of the Gold(I)-Catalyzed Propargyl Ester Tandem Cyclization

Abstract

This study reports a detailed theoretical analysis of the mechanisms and chemoselectivity for the formation of benzo[b]fluorenes or benzofulvenes from propargyl esters catalyzed by an organometallic Au(I) complex. Three different substitution patterns within the 1,5-diyne ester substrates were explored to rationalize the reaction mechanism and chemoselectivity. DFT calculations reveal that the title reaction proceeds through four main steps: (i) 1,3-acyl-shift, (ii) 6-endo-dig or 5-exo-dig cyclization, (iii) Friedel–Crafts-type, and (iv) proton transfer, with step (ii) being rate-determining in all studied pathways. In the absence of substituents at the aromatic rings of the substrate (R = H), the 6-endo-dig cyclization is favored. In turn, in the presence of one strong electron-donating substituent at the backbone (R = OCH3) of the substrate, the 5-exo-dig cyclization is favored. Besides, a modification of the substrate’s acetyl group by a pivaloyl group leads to an activation barrier difference between ...