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 NN 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 ).