Comment on "The Pentacovalent Phosphorus Intermediate of a Phosphoryl Transfer Reaction"

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Abstract
However, the apical P-O bond lengths for this trigonal bipyramidal intermediate (2.0 and 2.1 Å) are significantly longer than those in small-molecule phosphorane structures [e.g., 1.65 Å and 1.67 Å for pentacyclohexyloxy-phosphorane (3)]. These distances are not unreasonable for a transition state structure with partial apical bonds (4), but by definition this is not a stable species. Even at 93 K, the half-life for decomposition of an activated complex is -11 s, so this cannot be a kinetically trapped transition state. To be thermodynamically viable, such a TS complex would need to be comparable in stability with the starting and product complexes, which implies that the kinetic barrier to reaction has disappeared (and so the transfer reaction should become ultrafast at low temperature) and that the thermodynamic properties of the transition state are very unusual (because the change in temperature from 291 to 93 K would have to lead to a decrease in ΔG‡ of some 12 kcal mol-1 for the reaction within the ES complex). From a structural point of view, we were also concerned that some of the attributes of various Fourier maps described by Lahiri et al. (1) did not seem consistent with a PO3n- moiety.

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