Oligosiloles: First synthesis based on a novel endo-endo mode intramolecular reductive cyclization of diethynylsilanes

Article abstract of DOI:10.1021/ja00105a012

A general and versatile synthesis of 2,5-difunctional siloles and their conversion into oligosiloles are described. Diethynylsilanes undergo intramolecular reductive cyclization in an endo-endo mode upon treatment with lithium naphthalenide to form 2,5-dilithiosiloles. The 2,5-dilithiosiloles are converted into various 2,5-difunctional siloles by treatment with electrophiles. The resulting 2,5-dibromosilole is further converted into several highly functionalized siloles via palladium-catalyzed cross-coupling reaction or selective mono-lithiation using n-butyllithium in ether. Oligosiloles, from bisiloles to quatersilole, are prepared from certain functional siloles. Oxidative coupling of 2,5-dilithiosilole by use of an Fe(III) complex affords 2,2′-bisilole as yellow crystals. Difunctional oligosiloles, 5,5′-dibromo-2,2′-bisilole and 5,5?-dibromo-2,2′:5′,2″:5″,2? -quatersilole, are prepared by oxidative coupling via higher order cyanocuprate of 2-bromo-5-lithiosilole and 5-bromo-5′-lithio-2,2′-bisilole, respectively. X-ray crystal structures of these bisiloles show highly twisted arrangements between two silole rings with 62-64° of torsion angle. ¹H NMR studies on bisiloles show a rapid equilibration between non-coplanar conformers in solution. In UV-visible spectra, nevertheless, all of the oligosiloles have unusually long absorption maxima.