McMurry Coupling of 2-Acylphosphacymantrenes: <i>E-</i> vs <i>Z-</i>Stereochemistry of the 1,2-Bis(phosphacymantrenyl) Alkenes

The McMurry coupling of a 2-acetylphosphacymantrene with the TiCl4/Zn couple preferentially provides the Z-alkene as a mixture of the rac and meso diastereomers. The coupling of the 2-benzoylphosphacymantrene mainly gives the E-alkene together with the 2-benzyl reduction product. The alkene stereochemistries have been established by X-ray crystal structure analyses. A titanium−carbene intermediate is suggested in the second case.