Mechanism of the Gold(I)-Catalyzed Rearrangement of Alkynyl Sulfoxides: A DFT Study

The mechanisms of the gold(I)-catalyzed rearrangement of homopropargyl sulfoxides have been investigated using density functional theory calculations done at the B3LYP/6-31G(d, p) (SDD for Au) level of theory. Solvent effects on these reactions have been explored by calculations that included a polarizable continuum model (PCM) for the solvent (CH2Cl2). Two plausible pathways which lead to the formation of benzothiepinones or benzothiopines via an a-carbonyl Au carbenoid through 5-exo-dig cyclization or 6-endo-dig cyclization were proposed. Our calculation results suggested the following. (1) The first step of the cycle is nucleophilic addition of the sulfoxide oxygen onto the triple bond to form an alkenyl gold intermediate through 5-exo-dig cyclization or 6-endo-dig cyclization. The alkenyl gold species is then capable of pushing out the sulfide moiety, forming an a-carbonyl Au carbenoid. Finally, a-carbonyl Au carbenoids undergo intramolecular Friedel-Crafts alkylation to produce the observed products and liberate the cationic gold(I) catalyst. (2) When the alkyne is substituted with an electron-withdrawing group, 5-exo-dig cyclization of the nucleophile onto the internal carbon of the alkyne is favored. On the other hand, when the alkyne is substituted with an alkyl group, 6-endo-dig cyclization of the nucleophile onto the internal carbon of the alkyne is favored. (3) For 6-endo-dig cyclization, an intramolecular reaction of the a-carbonyl Au carbenoid with the benzene ring was the rate-determining step. However, migration of the hydrogen atom resulting in the formation of the final product was the rate-determining step for 5-exo-dig cyclization. (4) In the presence of water, the direct [1,5]-hydride shift has been changed into a deprotonation/protonation process. A very easy deprotonation process was found in the case of water. The higher activation free energies for the protonation process indicate that this step became the rate-determining one.