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Keywords:

  • alkynes;
  • allenes;
  • aromaticity;
  • conjugations;
  • UV/Vis spectroscopy

Abstract

The π-electron-rich C8-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[BOND]C[TRIPLE BOND]C)RC[DOUBLE BOND]C[DOUBLE BOND]C[DOUBLE BOND]CR(C[TRIPLE BOND]C[BOND]E) is extended to functional caps (E=H, CH3, C[TRIPLE BOND]CPh, CPh[DOUBLE BOND]CHBr, or CPh[DOUBLE BOND]CBr2) with non-alkynyl substituents at the sp2 vertices (R=Ph or CF3). The targets were selected for their potential in appealing retrosynthetic routes to carbo-benzenes, in which the aromatic C18 macrocycle would be directly generated by sequential metathesis or reductive coupling processes. The functional 1,4-dialkynylbutrienes were synthesized by either classical methods used for the preparation of generic butatrienes (R′Li/CuX-mediated reductive coupling of gem-dihaloenynes or SnCl2/HCl-mediated reduction of 3,6-dioxy-octa-1,4,7-triyne precursors). Their spectroscopic and electrochemical properties are compared and analyzed on the basis of the relative extent of total conjugation.