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If one considers the intermediate above, there are immediately two possibilities for subsequent reaction:

1. A Wittig-type olefination which would lead to a vinyl silane: This reaction happens to be unproductive as the vinyl silane can't do any further chemistry under the reaction conditions

2. A Peterson-type olefination: Which gives a vinyl phosphine (sp.), which may further be deprotonated to give another ylid (with concomitant formation of TMSOH) which will carry on to eventually give an allene.

My question is why is the Peterson faster than the Wittig. The immediate thing that springs to mind is the difference between acid and base catalysed Peterson reactions in which the base mediated version snaps shut before the C-C bond has a chance to rotate (i.e. kinetically very fast), but this isn't an explanation.

Potentially more promising is comparing the electronegativity/positivity of the elements, and noting that silicon is significantly more electropositive and hence a better electrophile however in the Peterson, the leaving group isn't such a driving force as the phosphine oxide from the Wittig.

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  • $\begingroup$ The Si-O bond is stronger, a stronger interaction is established faster and it is harder reverted. $\endgroup$ – RBW Oct 15 '16 at 17:36
  • $\begingroup$ Is this from a particular publication? $\endgroup$ – jerepierre Oct 17 '16 at 21:40
  • $\begingroup$ @Marko Are you invoking the Hammond Postulate? This is a kinetic issue, but your argument is a thermodynamic one (hard to imagine either of the two pathways being reversible). $\endgroup$ – Zhe Oct 19 '16 at 19:40
  • $\begingroup$ @jerepierre, not to my understanding- just a thought experiment but I'm sure there are examples $\endgroup$ – NotEvans. Oct 19 '16 at 22:12
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Seems like the silicon atom is more oxophilic as compared to the phosphorus atom. This could in principal be explained by the higher strength of the $\ce{Si-O}$ bond, as indicated by the dissociation energies $D$ (see http://www.wiredchemist.com/chemistry/data/bond_energies_lengths.html): $$\begin{array}{ccc} \hline \textbf{Bond} & \textbf{Dissociation energy / kJ mol}\mathbf{^{-1}} & \textbf{Bond length / pm} \\ \hline \ce{Si-O} & 452 & 163 \\ \ce{P-O} & 335 & 163 \\ \hline \end{array}$$ The $\ce{Si-O}$ bond is stronger thus given an explanation why the Peterson olefination is favoured. It seems that the bond lengths $r_b$ are the same, though.

By the way, a nice discussion of oxophilicity was given recently in Inorg. Chem. 2016, 55 (18), 9461.

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You're looking for the faster rate, so given that a triphenylphosphonium (ever see a model of one of these things?) is a lot bulkier than a trimethylsilyl group, I would argue that the sterics is the main driving force behind the selectivity of this reaction.

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  • $\begingroup$ Good point : ) I guess only way to really find out if its sterics or oxophilicity would be to repeat the experiment with the $\mathrm{PMe_3}$ system. $\endgroup$ – logical x 2 Oct 17 '16 at 21:50

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