Mechanism and stereochemistry of diphosphate formation from dioxaphosphorinanes: A critical reassessment

The mechanism of diphosphate formation from (R)-2-chloro-2-oxo-5,5-dimethyl -4-(R)-phenyl-1,3,2-dioxaphosphorinane (5a) and 3-hydroxy-2-oxo-5, 5-dimeth yl-4-(R)-phenyl-1,3,2-dioxaphosyphorinane (6) has been investigated. The pr oducts formed are the ax-au diphosphate 7a and the ax-eq diphosphate 7b, wi th no evidence in the P-31 NMR spectrum for pentacoordinate chlorooxyanioni c phosphoranes 9. The structure of 7b has been established unambiguously by NMR spectroscopy, mass spectrometry, and elemental analysis, and the struc tures of 5a and 7a have been confirmed by X-ray crystallography. The mechan ism of the crucial diphosphate-forming reaction has been probed using O-18- labeling studies. The O-18-labeling patterns are consistent with the unsymm etric ax-eq diphosphate 7b arising from selective nucleophilic attack of th e axial oxygen of 6 on the chloride 5a with inversion of configuration at p hosphorus. The symmetric ax-as diphosphate 7a can be formed directly, as a result of selective nucleophilic attack of the axial oxygen of 6 on the chl oride 5a with retention of configuration, but the majority arises indirectl y by isomerization of the ax-eq diphosphate 7b. The isomerization apparentl y involves intermolecular exchange, with nucleophilic attack of the phospha te anion 6 on the equatorially substituted phosphorus atom of 7b with inver sion of configuration at phosphorus.