Theoretical Elucidation of Ru(II)-Catalyzed <i>Gem</i>-Hydrogenative Cycloisomerization and Sigmatropic Rearrangement of Propargyl Alcohol Derivatives

<i>Gem</i><b>-</b>hydrogenative cycloisomerization and sigmatropic rearrangement reactions of propargyl alcohol derivatives with different Ru(II) catalysts afforded cyclopropanation, cyclopentenylation, cyclohexenylation, and enol acetate, respectively. To understand the mechanism and origin of product selectivity, density functional theory calculations were performed. Calculated results reveal that propargyl alcohol derivatives with the electron-abundant neutral active catalyst [Cp*RuCl] facilitate the formation of a Cl^- counterion-assisted π-back bonding between Ru and σ* of C-H, which decreases the energy barrier of agostic interaction determining the cyclopropane product. Whereas, this enyne with the electron-deficient cationic catalyst [CpRu(MeCN)]⁺ is prone to cyclopentenylation through C-C bond formation and a 1,2-H shift, because the electron-deficient cationic catalyst is beneficial for the 1,2-H shift. With the cationic catalyst [CpRu(MeCN)]⁺, the enyne bearing a methyl ether can facilely undergo ligand exchange to afford cyclohexenylation, which is ascribed to the strong Ru···O interaction between Ru and O (methyl ether). Conversely, the enyne with the bulky silyl ether is unfavorable to proceed with ligand exchange to afford cyclohexenylation due to the steric repulsion of the silyl ether. The propargyl alcohol derivative attached to a nucleophilic substituent is favorable to experience [2,3]-sigmatropic rearrangement rather than cycloisomerization, which is ascribed to the strong electrophilic ruthenium carbene.