1-Oxo-2-oxa-1-phosphabicyclo[2.2.2]octane:  A New Mechanistic Probe for the Basic Hydrolysis of Phosphate Esters

Synthesis of the title compound 2 was accomplished in a multistep sequence starting from hypophosphorous acid. In strong base, the bicyclic phosphinate 2 hydrolyzes 2 orders of magnitude faster than the bicyclic phosphate OP(OCH2)3CCH3 (1a) and the acceleration is entirely enthalpic. This rate enhancement is attributed to the greater ease with which 2 achieves the five-coordinate transition state. The molecular structure of 2, determined by X-ray means, compared with that of 1a reveals no evidence of strain within either bicyclic framework. The observed acceleration does not support the contribution of a stereoelectronic effect in the hydrolysis of six-membered ring phosphates.