Contribution of ring strain and the stereoelectronic effect to the hydrolysis of cyclic five-membered ring phosphorus esters

Abstract

The rate of base catalyzed hydrolysis of cyclic five-membered ring 2-oxo-2-phenyl-1,2-oxaphospholan, 1, and 2-oxo-2-phenyl-1,3,2-dioxaphospholan, 2 , and their acyclic analogues ethyl ethylphenylphosphinate, 3 , and diethyl phenylphosphonate, 4 , were measured. The cyclic ester 1 hydrolyzes 6.2 × 10 3 times faster than 3, corresponding to an activation free energy difference of 5.2 kcal Mol . This provides a good estimate of the magnitude of ring strain in these cyclic esters. In contrast, cyclic ester 2 hydrolyzes 1.5 × 10 6 times faster than its acyclic analog 4 , corresponding to an activation free energy difference of 8.4 kcal mol . This later energy difference is thought to be derived from both a ring strain effect (~ 5.2 kcal mol ) and a stereoelectronic effect (~3.2 kcal mol ).