Models for the Anionic Polymerization of P═C Bonds: Cyclization of <i>n</i> ‐BuLi‐Initiated MesP═CPh ₂ and Related Phosphaalkenes with H ₂ C═CPh ₂

To model the first propagation step in the anionic polymerization of MesP═CPh₂ we studied the addition of Li[MesP(Bu)-CPh₂] (and related species) to nonpolymerizable H₂C═CPh₂. Addition proceeds via the o-CH₃ of the P-Mes followed by unprecedented cyclization to C₅P-rings with release of Li[CHPh₂]. Further investigation of the aforementioned reaction using phosphaalkenes, RP═CAr₂ (R ═ Mes, m-Xyl; R' ═ Ph, 4-FC₆H₄, 4-MeC₆H₄, 4-MeOC₆H₄), resulted in the observation of a relatively long-lived intermediate in two instances (R ═ Mes, R' = 4-MeC₆H₄, 4-MeOC₆H₄). For the latter, the intermediate was identified as n-BuP(CH(4-MeOC₆H₄)₂)[C₆(4,6-Me₂)H₂-(2-CH₂CH₂CPh₂Li) by isolation of the oxidized, H⁺-quenched product. These observations provide intriguing clues into the complex mechanism of polymerization of P-Mes phosphaalkenes and the chiral cyclophosphane products are of interest as ligands for catalytic applications.