Functional carbo ‐Butadienes: Nonaromatic Conjugation Effects through a 14‐Carbon, 24‐π‐Electron Backbone

A systematic study of carbo ‐butadiene motifs not embedded in an aromatic carbo ‐benzene ring is described. Dibutatrienylacetylene (DBA) targets R 1 C(R)CCC(Ph)C≡CC(Ph)CCC(R)R 2 are devised, in which R is C≡CSi i Pr 3 and R 1 and R 2 are R, H, or 4‐X‐C 6 H 4 , with the latter including three known representatives (X: H, NMe 2 , or NH 2 ). The synthesis method is based on the SnCl 2 ‐mediated reduction of pentaynediols prepared by early or late divergent strategies; the latter allows access to a OMe–NO 2 push–pull diaryl‐DBA. If R 1 and R 2 are H, an over‐reduced dialkynylbutatriene (DAB) with two allenyl caps was isolated instead of the unsubstituted DBA. If R 1 =R 2 =R, the tetraalkynyl‐DBA target was obtained, along with an over‐reduced DBA product with a 12‐membered 1,2‐alkylidene‐1 H 2 ,2 H 2 ‐ carbo ‐cyclobutadiene ring. X‐ray crystallography shows that all of the acyclic DBAs adopt a planar trans – transoid – trans configuration. The maximum UV/Vis absorption wavelength is found to vary consistently with the overall π‐conjugation extent and, more intriguingly, with the π‐donor character of the aryl X substituents, which varies consistently with the first (reversible) reduction potential and first (irreversible) oxidation peak, as determined by voltammetry.