Cross-Dimerization Giving Silyl-Substituted Conjugated Hexatrienes: An Approach to 1,6-Diarylhexa-1,3,5-trienes

A new approach to 1,6-diarylhexa-1,3,5-trienes (DAHs) has been achieved. Cross-dimerization of 1-aryl-2-silylethyne ( 1 ) with benzyl(( E )-buta-1,3-dien-1-yl)dimethylsilane ( 2c ) catalyzed by [Ru(η 6 -naphthalene)(η 4 -1,5-cycloocatdiene)] produces (1 E,3 E )-(6-aryl-5-silylhexa-1,3,5-trien-1-yl)benzyl(dimethyl)silane ( 3 ), where the dominant stereochemistry at the 5-position in 3 is the E -form. Subsequent Hiyama cross-coupling of 3 with aryl iodide catalyzed by [Pd 2 (dibenzylideneacetone) 3 ]·C 6 H 6 in the presence of TBAF·3H 2 O gives a series of (1 E,3 E,5 E )-1,6-diarylhexa-1,3,5-trienes (DAH), showing that the C(5)═C(6) double bond rotates in the event of protodesilylation. The controlled experiments suggest that the Hiyama cross-coupling of the terminal silyl group in 3 takes place first, and then protodesilylation of the internal silyl group occurs to give (1 E,3 E,5 E )-DAHs. An X-ray structure analysis reveals the molecular structure of (1 E,3 E,5 E )-1-(4-acetylphenyl)-6-phenylhexa-1,3,5-triene. The photochemical study of some new DAHs has been conducted, showing bright fluorescence upon irradiation with a large Stokes shift (∼4026 cm –1 ) by π–π* transition according to TD-DFT calculations.