Abstract
PHOTOINDUCED RING OPENING AND COS ELIMINATION OF MESOIONIC THIADIAZOLES * A. Helm, N. H. Toubro and N. Harrit Department of General and Organic Chemistry, University of Copenhagen The H. C. 0rsted Institute, DK-2100 Copenhagen, Denmark (Reoeived in UK 13 April 1976; accepted for publication 23 April 1976) A variety of photochemical reactions of five membered mesoionic compounds have been reported. Among these a majority describes fragmentation into a heterocumulene and a 1,3-dipole. 1 Moriarty, Chapman and co-workers2 have suggested ring opening to be another primary photochemical process from mesoionic thiadiazoles, exemplified by 4-methyl-5-phenyl-1,3,4-thiadiasolylio-2-sulfide (hf) and 4-methyl-5-phenyl-l,3,4-thiadiazolylio-2-oxide (A). The main photolysis product from both compounds is E-methyl thiobenzamide (3J. 2a They monitored the reactions in acetonitrile 3 and nujol by means of infrared spectroscopy_ 2b The absorptions produced, their assignments and the mechanism suggested are summed in the left hand part of Scheme I. We have reexamined these experiments as part of our current investigations of the photochemistry of mesoionic compounds 4 and suggest new assignments for all the infrared absorptions observed by Moriarty and Chapman according to the following Table:
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