Cycloadditions and Methylene Transfer in Reactions of Substituted Thiocarbonyl S ‐Methylides with Thiobenzophenone: A Computational Study

Regiochemistry and methylene transfer reactions in cycloadditions of aliphatic thiocarbonyl S ‐methylides and thiobenzophenone are analyzed by ab initio [(U)HF/3‐21G*] and DFT calculations [(U)B3LYP/6‐31G*//(U)HF/3‐21G* and (U)B3LYP/6‐31G*]. The formation of regioisomeric 1,3‐dithiolanes is explained by the competition of concerted (2,4‐substituted 1,3‐dithiolane) and stepwise cycloaddition via C , C ‐biradicals (4,5‐substituted 1,3‐dithiolane). Aliphatic thiocarbonyl S ‐methylides with sterically demanding substituents undergo substantial methylene transfer in the reaction with thiobenzophenone. This process involves dissociation of the C , C ‐biradical intermediate with liberation of thiobenzophenone S ‐methylide which, in turn, combines with a second molecule of thiobenzophenone. Calculated activation parameters for the different processes are in agreement with the experimental observations. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)