Studies of the Thiocarbonyl Compounds. III. The Mechanism of the Thermal Rearrangement of Aryl Thionocarboxylates

The thermal rearrangement of aryl thionocarboxylates was kinetically investigated in diphenyl ether. The rate constants of the rearrangement of aryl N,N-dimethylthionocarbamates were well correlated with the σ− values, but the plots of the electron-releasing para-substituents deviated slightly on the lower side of the meta correlation line. A good linear free-energy relationship existed between the rearrangement of aryl N,N-dimethylthionocarbamates and the bimolecular nucleophilic reaction of 4- or 5-substituted 1-chloro-2-nitrobenzenes with piperidine. By the use of the substituent constants obtained from the latter reaction for the electron-releasing para-substituent constants, a fairly good ρ–σ− relationship was obtained. Linear free-energy relationships also existed between the rearrangements of aryl N,N-dimethylthionocarbamates and aryl thionobenzoates, and between the rearrangements of aryl N,N-dimethylthionocarbamates and O-aryl S-phenyl dithiocarbonates. The order of the reaction constants was in accord with that of the inductive effects of the α-substituents of the thiocarbonyl group, so the electron-releasing conjugative effects of the α-substituents of the thiocarbonyl group did not play important roles in the rate-determining step. The present results indicate that the thermal rearrangement of aryl thionocarboxylates is an intramolecular SN–Ar which involves a four-membered cyclic transition state formed by a nucleophilic attack of the lone-pair electrons of the thiocarbonyl sulfur atom on the migrating aromatic ring.