Mechanism of the Gold(I)-CatalyzedRearrangement ofAlkynyl Sulfoxides: A DFT Study.

The mechanisms of the gold(I)-catalyzed rearrangementof homopropargylsulfoxides have been investigated using density functional theorycalculations done at the B3LYP/6-31G(d, p) (SDD for Au) level of theory.Solvent effects on these reactions have been explored by calculationsthat included a polarizable continuum model (PCM) for the solvent(CH2Cl2). Two plausible pathways which leadto the formation of benzothiepinones or benzothiopines via an α-carbonylAu carbenoid through 5-exo-dig cyclization or 6-endo-dig cyclization were proposed. Our calculation resultssuggested the following. (1) The first step of the cycle is nucleophilicaddition of the sulfoxide oxygen onto the triple bond to form an alkenylgold intermediate through 5-exo-dig cyclization or6-endo-dig cyclization. The alkenyl gold speciesis then capable of pushing out the sulfide moiety, forming an α-carbonylAu carbenoid. Finally, α-carbonyl Au carbenoids undergo intramolecularFriedel–Crafts alkylation to produce the observed productsand liberate the cationic gold(I) catalyst. (2) When the alkyne issubstituted with an electron-withdrawing group, 5-exo-dig cyclization of the nucleophile onto the internal carbon of thealkyne is favored. On the other hand, when the alkyne is substitutedwith an alkyl group, 6-endo-dig cyclization of thenucleophile onto the internal carbon of the alkyne is favored. (3)For 6-endo-dig cyclization, an intramolecular reactionof the α-carbonyl Au carbenoid with the benzene ring was therate-determining step. However, migration of the hydrogen atom resultingin the formation of the final product was the rate-determining stepfor 5-exo-dig cyclization. (4) In the presence ofwater, the direct [1,5]-hydride shift has been changed into a deprotonation/protonationprocess. A very easy deprotonation process was found in the case ofwater. The higher activation free energies for the protonation processindicate that this step became the rate-determining one. [ABSTRACT FROM AUTHOR]