Oxidation of Thioketenes by Singlet Oxygen

Oxidation of di-tert-butylthioketene (1) and 1,1,3,3-tetramethyl-2-(thiocarbonyl)cyclohexane (2) by singlet oxygen has been investigated and has been found to exhibit a unique behavior different from that of ketenes and ketenimines. Oxidation of 1 in methylene chloride yielded the corresponding thioketene S-oxide (3), thioketone S-oxide (4), and trans-2,2,7,7-tetramethyl-4-octene-3,6-dione (5), and that in methanol gave 3, 4, methyl 1-tert-butyl-3,3-dimethyl-2-oxobutanesulfinate (6), and 1-methoxy-3,3-dimethylbutanone (7). Similar oxidation of 2 in methylene chloride gave 1,1,3,3-tetramethyl-2-(thiocarbonyl)cyclohexane S-oxide (8), 2,2,6,6-tetramethylcyclohexanethione S-oxide (9), and 2,2,6,6-tetramethylcyclohexanone (10), and in methanol methyl 3,3,7,7-tetramethyl-2-oxocycloheptanesulfinate (11) and 10 were formed. Formation of the above products has been rationalized to arise through the involvement of zwitterionic intermediates (12) resulting from the attack of singlet oxygen on the sulfur lone pair of the thioketene functionality. The difference in behavior between 1 and 2 is suggested to be due to the difference in the nature of cleavage of the suspected intermediate α-peroxy thiolactone. Reasons for the variation in the product distribution with respect to temperature and solvent are yet to be understood.