Relative reactivities of carbonyl and thiocarbonyl groups toward dimethoxycarbene: two new dimethoxythiiranes.

Reaction of dimethoxycarbene (DMC), generated by thermolysis of a 2,5-dihydro-1,3,4-oxadiazole, with 2,2,4,4-tetramethyl-3-thioxocyclobutanone afforded primarily 2,2-dimethoxy-3,3,5,5-tetramethyl-4-thioxocyclopentanone from ring expansion by overall insertion into the C-CO bond. 4,4-Dimethoxy-2,2,5,5-tetramethyl-3-thioxocyclopentanone, from overall insertion into a C-CS bond, was a minor product. Thus the carbene had reacted preferentially at the carbonyl group, rather than the thiocarbonyl group of the four-membered ring. However, the minor product reacted with DMC at the thiocarbonyl group to afford a dimethoxythiirane. A product from a corresponding reaction at the carbonyl group could not be found. A rationale for the apparent reversal of relative reactivities of the carbonyl and thiocarbonyl groups is offered, with supporting evidence.