An Addition–Isomerization Mechanism for the Anionic Polymerization of MesPCPh₂ and <i>m</i>‑XylPCPh₂

We report that the anionic polymerization of P-mesityl and <i>m</i>-xylyl-substituted phosphaalkenes follows an unusual addition–isomerization mechanism. Specifically, the polymerization of ArPCPh₂ [Ar = Mes (<b>1a</b>), <i>m</i>-Xyl (<b>1b</b>)] involves the hindered nucleophilic anion intermediate, Ⓟ–P­(Ar)–CPh₂^–, which undergoes a proton migration from the <i>o</i>-CH₃ of the Mes/<i>m</i>-Xyl moiety to the −CPh₂ moiety to afford a propagating benzylic anion. This mechanism is supported by the preparation of model compounds MeP­(CHPh₂)-4,6-Me₂C₆H₂–2-CH₂–CPh₃ (<b>2a</b>) or MeP­(CHPh₂)-6-MeC₆H₃–2-CH₂–CPh₃ (<b>2b</b>), which were both crystallographically characterized. Polymerization of <b>1a</b> or <b>1b</b> in THF solution using <i>n</i>-BuLi (2 mol %) revealed ¹H and ¹³C NMR signals assigned to −CH₂– and −CHPh₂ groups consistent with an addition–isomerization polymerization mechanism to afford poly­(methylene­phosphine) <b>3a</b> or <b>3b</b>. A large kinetic isotope effect (≤23) was determined for the <i>n</i>-BuLi-initiated polymerization of <b>1a</b>-<i>d</i>₉ compared to <b>1a</b> in THF at 50 °C, consistent with C–H (or C–D) activation as the rate-determining step. This C–H activation step was modeled using DFT computations which revealed that the intramolecular proton transfer from the <i>o</i>-CH₃ of the Mes moiety to the −CPh₂ moiety has an activation energy (<i>E</i>_a = +18.5 kcal mol^–1). For comparison, this computational value was quite close to the experimentally measured activation energy of propagation ArPCPh₂ in THF [<i>E</i>_a = 14.0 ± 0.9 kcal mol^–1 (<b>1a</b>), 15.6 ± 2.8 kcal mol^–1 (<b>1b</b>)]

An Addition–Isomerization Mechanism for the Anionic Polymerization of MesPCPh₂ and <i>m</i>‑XylPCPh₂

We report that the anionic polymerization of P-mesityl and <i>m</i>-xylyl-substituted phosphaalkenes follows an unusual addition–isomerization mechanism. Specifically, the polymerization of ArPCPh₂ [Ar = Mes (<b>1a</b>), <i>m</i>-Xyl (<b>1b</b>)] involves the hindered nucleophilic anion intermediate, Ⓟ–P­(Ar)–CPh₂^–, which undergoes a proton migration from the <i>o</i>-CH₃ of the Mes/<i>m</i>-Xyl moiety to the −CPh₂ moiety to afford a propagating benzylic anion. This mechanism is supported by the preparation of model compounds MeP­(CHPh₂)-4,6-Me₂C₆H₂–2-CH₂–CPh₃ (<b>2a</b>) or MeP­(CHPh₂)-6-MeC₆H₃–2-CH₂–CPh₃ (<b>2b</b>), which were both crystallographically characterized. Polymerization of <b>1a</b> or <b>1b</b> in THF solution using <i>n</i>-BuLi (2 mol %) revealed ¹H and ¹³C NMR signals assigned to −CH₂– and −CHPh₂ groups consistent with an addition–isomerization polymerization mechanism to afford poly­(methylene­phosphine) <b>3a</b> or <b>3b</b>. A large kinetic isotope effect (≤23) was determined for the <i>n</i>-BuLi-initiated polymerization of <b>1a</b>-<i>d</i>₉ compared to <b>1a</b> in THF at 50 °C, consistent with C–H (or C–D) activation as the rate-determining step. This C–H activation step was modeled using DFT computations which revealed that the intramolecular proton transfer from the <i>o</i>-CH₃ of the Mes moiety to the −CPh₂ moiety has an activation energy (<i>E</i>_a = +18.5 kcal mol^–1). For comparison, this computational value was quite close to the experimentally measured activation energy of propagation ArPCPh₂ in THF [<i>E</i>_a = 14.0 ± 0.9 kcal mol^–1 (<b>1a</b>), 15.6 ± 2.8 kcal mol^–1 (<b>1b</b>)]