Phosphorus Ins and Outs

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Science  24 Jun 2011:
Vol. 332, Issue 6037, pp. 1485
DOI: 10.1126/science.332.6037.1485-a
CREDIT: STOLLENZ ET AL., ANGEW. CHEM. INT. ED. 50, 10.1002/ANIE.201100893 (2011)

Both amines and phosphines are shaped like pyramids. The difference is that amines rapidly undergo so-called umbrella inversions, whereby the three groups pushed downward by the electron lone pair at the nitrogen vertex suddenly point upward (or vice versa). In phosphines, the inversion is much slower: At room temperature, the three groups bound to P tend to stay put. Stollenz et al. were therefore initially puzzled by the behavior of a diphosphine in which three flexible (CH2)14 chains bridge two P atoms: Facile conversion was taking place between the isomer with both electron pairs facing in and the one with both pairs facing out. Some study of models convinced them of a rather distinct mechanism, in which one of the methylene chains was passing through the ring bounded by the other two, in effect turning the compound inside out. To bolster this hypothesis, they heated the compound to high enough temperature to induce the more common umbrella inversions and isolate in/out isomers, and then verified that these new isomers could not convert to the in/in and out/out variants at room temperature—a result in accord with the topological restrictions of the inside-out pathway.

Angew. Chem. Int. Ed. 50, 10.1002/anie.201100893 (2011).

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