1,4-Dialkynylbutatrienes: Synthesis, Stability, and Perspectives in the Chemistry of carbo-Benzenes

The pi-electron-rich C(8)-conjugated sequence of 1,4-dialkynylbutatrienes is identified as a fragile and fascinating motif occurring in carbo-benzene derivatives, and in Diederich's 1,4-bis(arylethynyl)- or 1,4-bis(triisopropylsilylethynyl) butatriene capped representatives, in particular, in tetraalkynylbutatriene. The family of symmetrical 1,4-dialkynylbutatrienes (E-C C)RC=C=C=CR(C C-E) is extended to functional caps (E=H, CH(3), C CPh, CPh=CHBr, or CPh=CBr(2)) with non-alkynyl substituents at the sp(2) vertices (R = Ph or CF(3)). The targets were selected for their potential in appealing retrosynthetic routes to carbo-benzenes, in which the aromatic C(18) macrocycle would be directly generated by sequential metathesis or reductive coupling processes. The functional 1,4-di-alkynylbutrienes were synthesized by either classical methods used for the preparation of generic butatrienes (R'Li/CuX-mediated reductive coupling of gem-dihaloenynes or SnCl(2)/HCl-mediated reduction of 3,6-dioxyocta-1,4,7-triyne precursors). Their spectroscopic and electrochemical properties are compared and analyzed on the basis of the relative extent of total conjugation.