Synthesis of New Vinyl Thiocyanates by [3,3] Sigmatropic Rearrangement of Isothiocyanates

Propargyl isothiocyanates 3 and buta-2,3-dienyl isothiocyanates 20 were prepared conventionally from amines and thiophosgene, or by a new, one-pot procedure using nucleophilic substitution to generate azides, which in turn served as the starting materials for a Staudinger reaction, followed by treatment of the resulting iminophosphoranes or iminophosphates with CS2. Equilibration, through a [3,3] sigmatropic rearrangement, of 3 and the allenyl thiocyanates 4 was established by flash vacuum pyrolysis or by thermolysis in solution. Even the reversible isomerization of the parent compounds 3f and 4f favors the allenyl thiocyanate. In the case of 20, an irreversible rearrangement reaction gave high yields of 2-thiocyanatobuta-1,3-dienes 21. A sequence of two [3,3] migration steps transformed 1,4-diisothiocyanatobut-2-ynes 3m and 3n into 2,3-dithiocyanatobuta-1,3-dienes 21m and 22, respectively. These reactions demonstrate that the [3,3] sigmatropic rearrangement of mustard oils, to afford high yields of thiocyanates bearing the thermodynamically less stable functional group, is possible if the conversion gives rise to a more stable carbon skeleton. The equilibration of 1-thiocyanatopent-2-en-4-ynes 12 and 1-thiocyanatopenta-1,2,4-trienes 14 could be explained by tandem [3,3]−[3,3] sigmatropic rearrangements through short lived 3-isothiocyanatopent-1-en-4-ynes 13. Thiocyanato-substituted vinylallenes 4k and 14 tended to electrocyclize to give cyclobutenes 7 and 15, respectively. Thiocyanates 21a, 21b, 21m and 14a, which exhibit a buta-1,3-diene structure, could be used in Diels−Alder reactions to afford the cycloadducts 25a, 25b, 26a, 26m, 28m, 29m, and 30.