- An Addition-Isomerization Mechanism for the Anionic Polymerization of MesP=CPh2 and m -XylP=CPh2
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We report that the anionic polymerization of P-mesityl and m-xylyl-substituted phosphaalkenes follows an unusual addition-isomerization mechanism. Specifically, the polymerization of ArP=CPh2 [Ar = Mes (1a), m-Xyl (1b)] involves the hindered nucleophilic anion intermediate, a -P(Ar)-CPh2-, which undergoes a proton migration from the o-CH3 of the Mes/m-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 (2a) or MeP(CHPh2)-6-MeC6H3-2-CH2-CPh3 (2b), which were both crystallographically characterized. Polymerization of 1a or 1b in THF solution using n-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) 3a or 3b. A large kinetic isotope effect (≤23) was determined for the n-BuLi-initiated polymerization of 1a-d9 compared to 1a 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 o-CH3 of the Mes moiety to the -CPh2 moiety has an activation energy (Ea = +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 [Ea = 14.0 ± 0.9 kcal mol-1 (1a), 15.6 ± 2.8 kcal mol-1 (1b)].
- Rawe, Benjamin W.,Priegert, Andrew M.,Wang, Shuai,Schiller, Carl,Gerke, Sonja,Gates, Derek P.
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- SYNTHESIS AND STRUCTURE OF ARYL-SUBSTITUTED PHOSPHA-ALKENES
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An improved synthesis of the two triarylsubstituted phospha-alkenes mesityl (diphenylmethylene)phosphine (1a) and 2,6-dimethylphenyl(diphenylmethylene)phosphine (1b) described.Of several variants, the preferred route starts with the aryl bromides 4 which
- Knaap, Th.A. van der,Klebach, Th.C.,Visser, F.,Bickelhaupt, F.,Ros, P.,et al.
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p. 765 - 776
(2007/10/02)
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