Mechanism and origin of cyclization selectivity for Ru(ii)-catalyzed gem-hydrogenation of 1,3-enynes: a DFT study

Abstract

The mechanism of Ru(II)-catalyzed gem-hydrogenation of 1,3-enynes was studied with the aid of DFT calculations. The origins of cyclization selectivity involved in the two Ru(II)-catalyzed hydrogenative cyclization reactions, where reaction A bears an –OMe group and reaction B bears an –OSiMe₃ group, were explored explicitly. For reaction A, the thermodynamically unfavorable chair-to-twist boat isomerization is found to be involved in the process of forming a five-membered carbocycle product (P′), thus leading to the formation of the preferred five-membered heterocycle product (P). In contrast, for reaction B, the low electronegativity of the silicon atom in –OSiMe₃ makes the proton transfer from the methyl group to the carbene atom more difficult to form a six-membered heterocycle product (P1′), thus leading to the preferred five-membered carbocycle product (P1). Additionally, the influence of a series of heteroatoms on the cyclization selectivity was predicted theoretically (reaction A with –XMe (X = O, S, Se and Te) and reaction B with –OYMe₃ (X = C, Si, Ge and Sn)).