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 (<b>1</b>) with benzyl((<i>E</i>)-buta-1,3-dien-1-yl)dimethylsilane (<b>2c</b>) catalyzed by [Ru(η⁶-naphthalene)(η⁴-1,5-cycloocatdiene)] produces (1<i>E</i>,3<i>E</i>)-(6-aryl-5-silylhexa-1,3,5-trien-1-yl)benzyl(dimethyl)silane (<b>3</b>), where the dominant stereochemistry at the 5-position in <b>3</b> is the <i>E</i>-form. Subsequent Hiyama cross-coupling of <b>3</b> with aryl iodide catalyzed by [Pd₂(dibenzylideneacetone)₃]·C₆H₆ in the presence of TBAF·3H₂O gives a series of (1<i>E</i>,3<i>E</i>,5<i>E</i>)-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 <b>3</b> takes place first, and then protodesilylation of the internal silyl group occurs to give (1<i>E</i>,3<i>E</i>,5<i>E</i>)-DAHs. An X-ray structure analysis reveals the molecular structure of (1<i>E</i>,3<i>E</i>,5<i>E</i>)-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.