Dioxaphospholanes equilibria

In general, it was found that low temperatures and the presence of a five-membered iminophosphane ring (e.g. an R2R3P=dioxaphospholane ring, or an R2R3P= oxazaphospholane ring, for A ) tend to favour the formation of an aminophosphorane, in accordance with the stability sequence acyclic < monocyclic < spirocyclic. Similarly, one can deduce, from Scheme 28, that if the XZ bond cannot be broken, Z cannot migrate to the N atom and the only possibility will be the equilibrium b. 

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. 

Phosphorus Compounds.—Treatment of the dioxaphospholen (635) with sulphur yields a mixture of the oxathiole (636) and fran -dibenzoylstilbene, PhCOCPh=CPhCOPh." 2-Phenyl-l,3,2-dioxaphospholan (637 R = Ph) dimerizes in solution to a ten-membered-ring compound, which was isolated as (638) by sulphurization." The cyclic thiophosphorous acid (639) exists in equilibrium with the betaine (640) in aqueous ethanol/ The spiro-compound (641), which contains a phosphorus-hydrogen bond, is formed by the action of (637 R = Cl) on diethyl tartrate. The phosphazene (642) spontaneously cyclizes to the spiro-compound (643). " Stable derivatives (645) and (646 R = Br) of five-co-ordinate phosphorus have been obtained by the action of bromine on the o-phenylene phosphite (644)/ The chlorine analogue (646 R Cl) reacts with 3,5-dimethylpyrazole to give the corresponding pyrazole derivative (646 R = 3,5-dimethylpyrazol-l-yl), which reversibly dimerizes to the zwitterion (647). ... 

In contrast, the polymerization of 2-oxo-l,3,2-dioxaphospholane (R = -(CH2>2 ) occurs much more rapidly, and no initiator is needed. This polymerization is approximately 85% complete at 24 C. Unlike the previous equilibrium, this is quite rapid, and the polymer cannot be purified by fractional precipitation. A variable temperature NMR... 

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