OLIGOSILOLES - FIRST SYNTHESIS BASED ON A NOVEL ENDO-ENDO MODE INTRAMOLECULAR REDUCTIVE CYCLIZATION OF DIETHYNYLSILANES

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 ate 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 degrees of torsion angle. H-1 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.