E. Marchand et G. Morel, ALPHA-THIOXOTHIOAMIDES [4+2] AND [2+2] CY CLOADDITION REACTIONS WITH ARYL ISOTHIOCYANATES AND PHENYL ISOCYANATE, Bulletin de la Societe chimique de France, 134(6), 1997, pp. 623-634
Aryl and methyl isothiocyanates react with 1,2-dithiocarbonyl compound
s 2 to give a mixture of 2,3-dihydro-2-thioxothiazole 3 and 2-iminothi
azole 5. The product ratios depend mainly on the nature of the startin
g (RNCS)-N-1. We show that alpha-iminothioamides 4 are intermediates i
n 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 extrusi
on) 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 phe
nyl isocyanate to heterodienes 2 and 4. The regiochemical preferences
in all cycloaddition and ring contraction reactions have been demonstr
ated.