New Lithium Phosphonium Diylides: A Methylene and a Cyclopentadienyl Moiety as Ylidic Coordination Sites

Abstract

A set of lithium phosphonium diylides Li[CH 2 -PR 2 -Cp X ] ( 9 – 12; Cp X = C 5 Me 4, C 5 H 3 t Bu, R = Ph, Me) is presented. Two of the lithium complexes were characterized by means of single-crystal X-ray analysis, revealing a dimeric head-to-tail arrangement in the solid state. The coordination behavior of 9 – 12 in the liquid phase is solvent dependent. These lithium phosphonium diylides exist as contact ion pairs in benzene and as solvent-separated ion pairs in THF solutions. Phosphonium salts [H 3 C-PR 2 -Cp X H)] + I – ( 1 – 4 ) are starting materials for the syntheses of the title compounds and exist as mixtures of isomers due to [1,5]-prototropic rearrangements. The dynamic behavior in solution has been investigated. Two different routes allow access to title compounds 9 – 12 . Reactions of 1 – 4 with 2 equiv of n BuLi give 9 – 12 in a one-pot synthesis. In an alternative two-step route, dehydrodehalogenation of 1 – 4 with KH gives the corresponding phosphonium ylides 5 – 8 . Two of these phosphonium ylides were characterized by single-crystal X-ray analysis. In one case two different conformers were obtained.