Towards the synthesis of azoacetylenes

The synthesis of azoacetylenes (=dialkynyldiazenes) 1 and 2 has been investigated. They represent a still elusive class of chromophores with potentially very interesting applications as novel bistable photochemical molecular switches or as antitumor agents (Fig. 1). Our synthetic efforts have led us alongside three different approaches (Scheme 1). In a first route, it was envisioned to generate the azo (=diazene) bond by photolysis of N,N'-dialkynylated 1,3,4-thiadiazolidine-2,5-diones that are themselves challenging targets (Scheme 2). Attempts are described to obtain the latter by alkynylation of the parent heterocycle with substituted alkynyliodonium salts. In a conceptually similar approach, the no-less-challenging dialkynylated 9,10-dihydro-9,10-diazanoanthracene (29) was to be generated by alkynylation of the unsubstituted hydrazine 28 (Scheme 6). In a second route, the generation of the N=N bond from Br-substituted divinylidenehydrazines (ketene-azines) 35 was attempted in a synthetic scheme involving an aza-Wittig reaction between azinobis(phosphorane) 36 and (triisopropylsilyl)ketene 37 (Scheme 7). Finally, a third approach, based on the formation of the central azo bond as the key step, was explored. This route involved the extrapolation of a newly discovered condensation reaction of N,N-disilylated anilines with nitroso compounds (Scheme 11, and Tables 1 and 2) to the transformation of N,N-disilylated ynamine 55 and nitroso-alkyne 54 (Scheme 13).