Preparation and [2 + 3] cycloaddition of thienyl isocyanates with trimethylsilyl azide: One-pot synthesis of thienyltetrazolin-5-ones

Efforts to prepare thienyl isocyanates by thermal reactions of thenoyl chlorides with trimethylsilyl azide (TMSA) led preferentially to the formation of 1,4-disubstituted tetrazolin-5-ones, arising by interaction of the initially formed thienyl isocyanate with TMSA. In fact, 2-thenoyl chloride and benzo[6]thiophene-2-carbonyl chloride reacted with 1 equiv. TMSA in refluxing carbon tetrachloride to give 1-(2-thienyl or benzo[b]thienyl)-4-(2-thenoyl or benzo[b]thiophenecarbonyl)tetrazolin-5-one 4d, e, whereas 3-thenoyl chloride led to 1-(3-thienyl)-4-trimethylsilyltetrazolin-5-one adduct II which was converted, after hydrolytic desilylation, to 1-(3-thienyl)tetrazolin-5(4H)-one 4c. Similar reactions, carried out with more than two equiv. of TMSA, led in all cases to the formation of the corresponding 1-heteroaryltetrazolin-5-one 4a-c via the corresponding silylated tetrazolin-5-ones II, whereas at room temp. such reactions gave essentially thenoyl azides which, after elimination of the excess TMSA, were thermally converted to the corresponding thienyl isocyanates 3 in fairly good yields.