Synthesis and structure of aryl-substituted phospha-alkenes

An improved synthesis of the two triarylsubstituted phospha-alkenes mesityl (diphenylmethylene)phosphine (la) and 2,6dimethylphenyl(diphenylmethylene)phosphine (lb) is described. Of several variants, the preferred route starts with the aryl bromides 4 which are converted to the corresponding Grignard reagents and to the arylphosphonous diamides 7 which with PCl3, yield arylphosphonous amide chlorides 9 thence phosphinous amides 10 and to the phosphinous chlorides 11 which with DBU eliminate Hcl to furnish the title compounds in 60-85%, yield. The chemical and spectral data of 1a and 1b are discussed; they contain essentially localized P=C bonds and. are stabilized mainly by steric protection. These conclusions are confirmed by HFS-calculations on simple model compounds (17, 16', 18, 19 and 20) which identify the phosphorus lone pair as HOMO and the π-orbital as NHOMO; however, both orbitals are close in energy. Furthermore, the calculations reveal the importance of phosphorus d-orbitals in bonding, and the polarization in the P=C bond (P as positive pole) which had earlier been derived from chemical evidence. Finally, it is shown that interaction of the P=C bond with phenyl groups does not dramatically influence the bonding situation, but substitution by a heteroatom (nitrogen substituted on carbon; 20) does. Of particular interest in the crystal and molecular structure of la are the short P=C bond length (1.692(3)AA) and the Mes-P-C bond angle (107.5°); the latter is smaller than expected (and found) for purely sp2-hybridized atoms, but larger than the unsubstituted parent compound HP=CH2 (16).3 It is also noteworthy that the structure of 1a is essentially unchanged on complexation to metal centres.