2 research outputs found
An Addition–Isomerization Mechanism for the Anionic Polymerization of MesPCPh<sub>2</sub> and <i>m</i>‑XylPCPh<sub>2</sub>
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 ArPî—»CPh<sub>2</sub> [Ar = Mes (<b>1a</b>), <i>m</i>-Xyl (<b>1b</b>)] involves the hindered nucleophilic anion intermediate,
Ⓟ–PÂ(Ar)–CPh<sub>2</sub><sup>–</sup>, which
undergoes a proton migration from the <i>o</i>-CH<sub>3</sub> of the Mes/<i>m</i>-Xyl moiety to the −CPh<sub>2</sub> moiety to afford a propagating benzylic anion. This mechanism
is supported by the preparation of model compounds MePÂ(CHPh<sub>2</sub>)-4,6-Me<sub>2</sub>C<sub>6</sub>H<sub>2</sub>–2-CH<sub>2</sub>–CPh<sub>3</sub> (<b>2a</b>) or MePÂ(CHPh<sub>2</sub>)-6-MeC<sub>6</sub>H<sub>3</sub>–2-CH<sub>2</sub>–CPh<sub>3</sub> (<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 <sup>1</sup>H and <sup>13</sup>C NMR signals assigned to −CH<sub>2</sub>–
and −CHPh<sub>2</sub> groups consistent with an addition–isomerization
polymerization mechanism to afford polyÂ(methyleneÂphosphine) <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><sub>9</sub> 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>-CH<sub>3</sub> of the
Mes moiety to the −CPh<sub>2</sub> moiety has an activation
energy (<i>E</i><sub>a</sub> = +18.5 kcal mol<sup>–1</sup>). For comparison, this computational value was quite close to the
experimentally measured activation energy of propagation ArPCPh<sub>2</sub> in THF [<i>E</i><sub>a</sub> = 14.0 ± 0.9
kcal mol<sup>–1</sup> (<b>1a</b>), 15.6 ± 2.8 kcal
mol<sup>–1</sup> (<b>1b</b>)]
An Addition–Isomerization Mechanism for the Anionic Polymerization of MesPCPh<sub>2</sub> and <i>m</i>‑XylPCPh<sub>2</sub>
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 ArPî—»CPh<sub>2</sub> [Ar = Mes (<b>1a</b>), <i>m</i>-Xyl (<b>1b</b>)] involves the hindered nucleophilic anion intermediate,
Ⓟ–PÂ(Ar)–CPh<sub>2</sub><sup>–</sup>, which
undergoes a proton migration from the <i>o</i>-CH<sub>3</sub> of the Mes/<i>m</i>-Xyl moiety to the −CPh<sub>2</sub> moiety to afford a propagating benzylic anion. This mechanism
is supported by the preparation of model compounds MePÂ(CHPh<sub>2</sub>)-4,6-Me<sub>2</sub>C<sub>6</sub>H<sub>2</sub>–2-CH<sub>2</sub>–CPh<sub>3</sub> (<b>2a</b>) or MePÂ(CHPh<sub>2</sub>)-6-MeC<sub>6</sub>H<sub>3</sub>–2-CH<sub>2</sub>–CPh<sub>3</sub> (<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 <sup>1</sup>H and <sup>13</sup>C NMR signals assigned to −CH<sub>2</sub>–
and −CHPh<sub>2</sub> groups consistent with an addition–isomerization
polymerization mechanism to afford polyÂ(methyleneÂphosphine) <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><sub>9</sub> 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>-CH<sub>3</sub> of the
Mes moiety to the −CPh<sub>2</sub> moiety has an activation
energy (<i>E</i><sub>a</sub> = +18.5 kcal mol<sup>–1</sup>). For comparison, this computational value was quite close to the
experimentally measured activation energy of propagation ArPCPh<sub>2</sub> in THF [<i>E</i><sub>a</sub> = 14.0 ± 0.9
kcal mol<sup>–1</sup> (<b>1a</b>), 15.6 ± 2.8 kcal
mol<sup>–1</sup> (<b>1b</b>)]