First Examples of Reactions of AzoleN-Oxides with Thioketones: A Novel Type of Sulfur-Transfer Reaction

Abstract

The reactions of 1,4,5-trisubstituted imidazole 3-oxides 1a - k with cyclobutanethiones 5a,b in CHCl3 at room temperature give imidazole-2(3H)-thiones 9a - k in high yield. The second product formed in this reaction is 2,2,4,4-tetramethylcyclobutane-1,3-dione (6a; Scheme 2). Similar reactions occur with 1 and adamantanethione (5c) as thiocarbonyl compound, as well as with 1,2,4-triazole-4-oxide derivative 10 and 5a (Scheme 3). A reaction mechanism by a two-step formation of the formal cycloadduct of type 7 via zwitterion 16 is proposed in Scheme 5. Spontaneous decomposition of 7 yields the products of this novel sulfur-transfer reaction. The starting imidazole 3-oxides are conveniently prepared by heating a mixture of 1,3,5-trisubstituted hexahydro-1,3,5-triazines 3 and alpha-(hydroxyimino) ketones 2 in EtOH (cf. Scheme 1). As demonstrated in the case of 9d, a 'one-pot' procedure allows the preparation of 9 without isolation of the imidazole 3-oxides 1. The reaction of 1c with thioketene 12 leads to a mixture of four products (Scheme 4). The minor products, 9c and the ketene 15, result from an analogous sulfur-transfer reaction (Path a in Scheme 5), whereas the parent imidazole 14 and thiiranone 13 are the products of an oxygen-transfer reaction (Path b in Scheme 5).