NIR chromophores from small acetylenic building blocks: a Diels-Alder approach to octaalkynylphthalocyanines

Two new routes to cross-conjugated 3,4-dimethylenehexa-1,5-diynes, both sta rting from dialkynyl 1,2-diones, have been devised. Whereas triisopropylsil yl-protected diketones could be diolefinated in a bis-Wittig reaction with methylenetriphenylphosphorane, their aryl-terminated congeners had to be su bjected to the conditions of the Peterson olefination (trimethylsilylmethyl magnesium chloride and subsequent dehydration of the resulting diol with th ionyl chloride). The reactivity of the diethynylbutadienes towards standard dienophiles was found to be low and alternative thermal reactions compete with [4+2] cycloadditions. However, dicyanoacetylene was shown to be an eff ective cycloaddition partner leading, after aromatisation, to the correspon ding dicyanodiethynylbenzenes. These were cyclotetramerised with magnesium butanolate in butanol to furnish octaalkynylphthalocyanines, thereby comple ting a concise, three-step synthesis of these NIR chromophores of relevance to photodynamic forms of therapy.