Thermal fragmentation reactions of 1,4,2-dithiazines and 1,4,2,5-dithiadiazines in the presence of dienophiles: Synthesis of 1,4-dithiine derivatives. X-Ray crystal structures of a 1,4,2-dithiazine 1,1-dioxide and a 1,4,2,5-dithiadiazine derivative

The reaction of 5,6-dimethyl-3-(4-bromophenyl)-1,4,2-dithiazine 1b with dimethyl acetylenedicarboxylate (DMAD) at 180 degrees C in o-dichlorobenzene affords a mixture of thiophene derivative 5 and isothiazole derivative 6: the former probably via the zwitterionic adduct 3 and dithiine derivative 4, neither of which are isolable; the latter by sulfur extrusion from 1b. Reaction of norbornene with 1b affords dithiine derivative 7. Oxidation of 1b with meta-chloroperoxybenzoic acid yields the 1,1-dioxide derivative 8, the X-ray crystal structure of which is reported. The efficient synthesis of 3-aryl-6-methylthio- 1,4,2,5-dithiadiazine derivatives 12a-c by ring expansion of 1,4,2-dithiazolium salts 11a-c with an iodine-ammonia reagent is described. Reaction of 12c with DMAD at 180 degrees C affords the stable dithiine derivative 14, probably via the intermediate 1,4,2-dithiazine 13. Electrochemical oxidation of 12a-c is irreversible, yielding 1,4,2,5-dithiadiazinium cation radicals at potentials [E-ox = 1.55-1.73 V (vs. Ag/AgCl)] which are sensitive to the electronic nature of the para-substituent on the aryl ring. The X-ray crystal structure of 12b is reported.