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

A set of lithium phosphonium diylides Li­[CH2-PR2-CpX] (9–12; CpX = C5Me4, C5H3tBu, 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 [H3C-PR2-CpXH)]+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 nBuLi 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.