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

Abstract

A set of lithium phosphonium diylides Li­[CH₂-PR₂-Cp^X] (<b>9</b>–<b>12</b>; Cp^X = C₅Me₄, C₅H₃<i>t</i>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 <b>9</b>–<b>12</b> 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 [H₃C-PR₂-Cp^XH)]⁺I^– (<b>1</b>–<b>4</b>) 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 <b>9</b>–<b>12</b>. Reactions of <b>1</b>–<b>4</b> with 2 equiv of <i>n</i>BuLi give <b>9</b>–<b>12</b> in a one-pot synthesis. In an alternative two-step route, dehydrodehalogenation of <b>1</b>–<b>4</b> with KH gives the corresponding phosphonium ylides <b>5</b>–<b>8</b>. Two of these phosphonium ylides were characterized by single-crystal X-ray analysis. In one case two different conformers were obtained