Synthesis of Unexpected Bifunctionalized Thiazoles by Nucleophilic Attack on Allenyl Isothiocyanate

Treatment of allenyl isothiocyanate with a variety of nucleophiles leads to 5-methylthiazoles with a functional group at the 2-position. The same pattern of reactivity is also seen with N-aminophthalimide. In the presence of azide salt, hydrazoic acid, or N,N-disubstituted hydroxylamines, however, allenyl isothiocyanate is converted into bifunctionalized thiazoles. We explain the formation of these products by nucleophilic addition at the isothiocyanato moiety followed by ring closure and an N-N or N-O cleavage reaction to generate short-lived 2-imino-5-methylidenethiazole or 5-methylidenethiazol-2-one. Such intermediates are trapped by addition reactions to give the final heterocyclic compounds. In the case of N,N-disubstituted hydroxylamines, the primary addition products with allenyl isothiocyanate can be detected as unstable intermediates by IR and NMR spectroscopy.