Phosphorane-phosphonium salt equilibria

Up to this point, the criterion used to ascertain the presence of an equilibrium, i.e. the simultaneous presence of two well defined entities, was mainly based on 1H, 13C or 31P NMR determinations at different temperatures or in various solvents. However, it turns out that the chemical shift in 31P NMR spectra (usually negative with respect to the H3P04 reference) is often inadequate to permit a choice among various possibilities, namely a true phosphorane, a phosphonium salt or an equilibrium between these two forms. 

Mechanistic Studies.- A number of substitution reactions of alcohols, phenols, or amines with 1,3,2-dioxaphospholans, e.g. (73), oxazaphospholans, and diazaphos-pholans have been followed by n.m.r. and shown to involve H-phosphoranes, e.g. (74). The reactions are run in toluene or without solvent, and without addition of an acidic catalyst in some systems, with 4-chlorophenol as the nucleophile, an equilibrium was established between an H-phosphorane (75) and a phosphonium salt (76), but the authors still favour (75) as the true intermediate. The equilibrium constants for a series of exchange reactions between (thio)phosphites (77) and (thio)phosphoro-dichloridites (78), and some analogous bromidites and a fluoridite, have been measured.The constants increase with increased electron donor ability of R. 

Heating a mixture of pentachlorophosphole (26) and phenylacetylene (27) gave the phosphorane (28) which, on the basis of variable temperature NMR, appeared to be in equilibrium with the phosphonium salt (29) The most... 

The chlorination of (72) at —100 °C gives the trichlorophosphorane (73), which rearranges above — 40 °C to the dichlorophosphate (74) with no evidence for the formation of the phosphonium salt (75). Likewise, the halogenation of (76a,b) at low temperature gives the phosphoranes (77a,b) which, with the exception of (77b, X = Br), are stable at room temperature. The latter compound, which was shown to be in equilibrium with the phosphonium salt (78), slowly gave a spirophosphorane (79) by cleavage of the ether linkage. The... 

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