Reactivity of aryl and heteroaryl azides with vinylsilane and alkynylsilane. Formation of C-silylated 1,2,3-triazolines and 1,2,3-triazoles

Abstract

The reactions of the silylated dipolarophiles trimethyl(vinyl)silane, trimethoxy(vinyl)silane and (trimethylsilyl)acetylene, with some para-substituted phenyl azides, 2-azidobenzo[b]thiophene (2-BTA) and 3-azidobenzo[b]thiophene (3-BTA) have been examined. At room temperature these reactions give the respective 1-substituted 4-silylated-1,2,3-triazoline. However, only the reactions of 4-nitrophenyl and 4-cyanophenyl azide occur smoothly, furnishing triazolines in high yields; similar reactions with phenyl, 4-tolyl, 4-methoxyphenyl and 4-chlorophenyl azide are slower, affording mixtures of a 3-(N-arylamino)methyl-3,5-bis(trimethylsilyl)-1-pyrazoline, arising from the primary triazoline adduct through ring-opening to diazo compounds 2, and the corresponding 1-aryl-2-(trimethylsilyl)aziridine, resulting from nitrogen extrusion from the same elusive triazoline. On the other hand reactions of 2-BTA and 3-BTA involve rapid extrusion of nitrogen to afford exclusively the corresponding 1-(benzo(b)thienyl)-2-silylated aziridine.

Results suggest that all these azides generally undergo 1,3-dipolar cycloadditions to terminally silylated alkenes to give triazoline adducts with the same geometrical orientation and whose thermal behaviour was found to be strongly dependent on the activating or deactivating effects exerted by N-1 substituents.

Suitable support for electronic and steric factors acting in the same direction in the orientations of the former cyclo-A_NA_E adducts has been provided by the observation that the same aryl azides, 2-BTA and 3-BTA exhibit analogous 1,3-dipolar cycloadditions with (trimethylsilyl) acetylene from which stable 1-aryl(or heteroaryl)-4-trimethylsilyl-1,2,3-triazoles are obtained in quantitative yields.