The reaction of CuCl-catalyzed coupling of thiol ester with organostannane has been theoretically investigated using density functional calculations. This reaction takes place with CuCl as a catalyst, giving the product ketone and an organotin sulfide. The relatively low overall activation barrier calculated (26.0 kcal/mol) supports the experimental fact that this reaction proceeds under mild reaction conditions. The conversion of the resulting Cu(I) thiolate intermediate (D) to the benzoisothiazole (E) was found to proceed via an one-step process. The relatively high overall activation barrier calculated (35.8 kcal/mol) supports the experimental fact that this reaction proceeds under harsher reaction conditions. The influence of hybridization of the carbon atom bonded to sulfur on the reaction was also discussed.