Conjugated Enyne Synthesis by Rearrangement of Acetylenic Epoxides Mediated by Low-Valence Organotitanium and Organozirconium Reagents

Abstract

The rearrangement of acetylenic epoxides mediated by low-valence organotitanium and organozirconium reagents is ­reported to give conjugated enynes. Moderate to good yields and high selectivities are obtained when using the organozirconium(II) Negishi reagent in toluene at 20 °C; whereas only poor yields and low selectivities are achieved with the organotitanium(II) Sato reagent. The process is stereospecific and involves formation of titana- and zirconacyclopropenes by oxidative insertion of the low-valence titanium and zirconium reagents into the carbon–carbon triple bond of the acetylenic epoxides. These metallacyclopropenes then rearrange to afford stereodefined propargylmetals through the epoxide ring-opening. Conjugated enynes are finally produced by β-elimination of metal oxide.

Key words

elimination - enynes - rearrangement - titanium - zirconium

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