Abstract

The reactions of several thioketones containing a conjugated C=C bond with diazo compounds were investigated. All of the selected compounds reacted via a 1,3-dipolar cycloaddition with the C=S group and subsequent N2 elimination to yield thiocarbonyl ylides as intermediates, which underwent a 1,3-dipolar electrocyclization to give the corresponding thiirane 25, or, by a subsequent desulfurization, to give the olefins 33a and 33b. None of the intermediate thiocarbonyl ylides reacted via 1,5-dipolar electrocyclization.
If the a,b-unsaturated thiocarbonyl compound bears an amino group in the b-position, the reactions with diazo compounds led to the 2,5-dihydrothiophenes 40a–40d. In these cases, the proposed mechanism of the reactions led once more to the thiocarbonyl ylides 36 and thiiranes 38, respectively. The thiiranes reacted via an SNi’-like mechanism to give the corresponding thiolate/ammonium zwitterion 39, which underwent a ring closure to yield the 2,5-dihydrothiophenes 40. Also in these cases, no 1,5-
dipolar electrocyclization could be observed. The structures of several key products were established by X-ray crystallography.