Cyclization of 1,n-Enynes Initiated by the Addition Reaction of gem-Dichromiomethane Reagents to Alkynes.

Treatment of 1,n-enynes with gem-dichromiomethane species prepared in situ from 1,1-diiodomethanes and CrCl2 gave alkenylbicycloalkanes. Alkenylcarbene species, which was generated via [2 + 2] cycloaddition with a triple bond, were intercepted by intramolecular cyclopropanation of a pendant double bond, and alkene metathesis did not proceed. This is a very rare example of transformation of alkynyl C≡C with gem-dimetalloalkanes. Silyl- and borylcarbene equivalents generated from gem-(dichromio)silyl- and borylmethanes promoted the coupling and cyclization of enediynes as well as 1,n-enynes.