alpha-Thioxothioamides [4+2] and [2+2] cycloaddition reactions with aryl isothiocyanates and phenyl isocyanate

Aryl and methyl isothiocyanates react with 1,2-dithiocarbonyl compounds 2 to give a mixture of 2,3-dihydro-2-thioxothiazole 3 and 2-iminothiazole 5. The product ratios depend mainly on the nature of the starting (RNCS)-N-1. We show that alpha-iminothioamides 4 are intermediates in the formation of cycloadducts 5. The results are explained by [4+2] and [2+2] additions to the C=N bond of the heterocumulene followed by ring contraction of the 2,3-dihydro-1,4,2-dithiazine 6 (sulfur extrusion) or cycloreversion of the 2-thioxothiazetidine 7 (CS2 elimination). The [4+2] addition of 4 occurs exclusively on the carbon-sulfur bond of isothiocyanates. Some differences are observed for additions of phenyl isocyanate to heterodienes 2 and 4. The regiochemical preferences in all cycloaddition and ring contraction reactions have been demonstrated.