Arylsulfonyl azides, cycloaddition

Regioselective ring expansion of alkynyl cyclopropanes to cyclobutenes has been reported via a copper-catalysed cycloaddition of an alkyne with an arylsulfonyl azide and a silver-catalysed carbene formation followed by ring expansion of a cyclopropyl carbene intermediate (Scheme 75)7° ... 

W.G. Dauben, R.A. Bunce, Organic reactions at high pressure. Dipolar cycloaddition-ring contraction reactions of hindered silyl enol ethers and arylsulfonyl azides, J. Org. Chem. 47 (1982) 5042-5044. 

Alkyl azides readily undergo 1,3-dipolar cycloaddition to arylsulfonyl isothiocyanates (375) to yield thiatriazolines (376). Thermolysis of (376) in the presence of isocyanates or carbodiimides produces 1,2,4-thiadiazole derivatives (378) and (379), respectively. The intermediate formation of a thiaziridinimine (377) has been postulated as indicated in Scheme 137 (75JOC1728, 75S52). The use of isothiocyanates as dipolarophiles produces dithiazolidines (380) instead of the thiadiazole derivatives. In these reactions the intermediate thiazirine (377) functions as a 1,3-dipole with the positive charge primarily localized on sulfur. It was recently proposed that the reaction of oxaziridines (381) with isothiocyanates produces a similar thiazirine intermediate (382) which reacts in a different regiospecific manner with isothiocyanates to produce 1,2,4-thiadiazole derivatives (383) and (384 Scheme 138) (74JOC957). 

Organic azides like alkyl azides and TMSA (146) are also able to add to isothiocyanates. Alkyl azides 189, namely -butyl azide or benzyl azide, react with an equimolar amount of arylsulfonyl isothiocyanates 190 at room temperature in carbon tetrachloride or without solvent during 7 hours up to 2 days to exclusively afford 4-alkyl-5-arylsutfonylimino-l,2,3,4-thiatriazolines 191 in 50-75% yield. The product was the result of a [3+2]-cycloaddition to the C=S bond of the isothiocyanate moiety. Upon heating to 45-80 °C in inert solvents, the 4-alkyl-5-arylsulfonylimino-l,2,3,4-thiatriazolines 191 evolve nitrogen and sulfur, decomposing to sulfonylcarbodiimides 192 which are valuable precursors for the synthesis of further heterocycles like 4-amino-thiazolidines or thiazolines (Scheme 37) [139,140]. 

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