ON THE ORIGINS OF ENHANCED REACTIVITY OF 5-MEMBERED CYCLIC PHOSPHATE-ESTERS - THE RELATIVE CONTRIBUTIONS OF ENTHALPIC AND ENTROPIC FACTORS

The hydrolysis of five-membered cyclic phosphate and phosphonate esters is about 106-fold more rapid than the hydrolysis of related acyclic esters. The origin of the enhanced reactivity of the cyclic esters has been ascribed to enthalpic factors, associated with ground-state strain, and to entropic factors, associated with optimal orbital orientations. The temperature dependence of the rates of alkaline hydrolysis of ethyl and methyl esters of ethylene phosphate and of ethyl and methyl esters of propylphostonate have been determined. The enthalpies of activation for the cyclic esters are much less than those for the corresponding acyclic esters while differences in entropies of activation are dependent on the nature of the alkyl substituent, varying from less than 1 cal mol-1 deg-1 (eu) for the methyl esters to about 8 eu for the ethyl esters. The data in the present study indicate that the report of an unusually low entropy of activation (17 eu less than the acyclic analogue) for the hydrolysis of the cyclic phosphonate ester, ethyl propylphostonate (Aksnes, G.; Bergesen, K. Acta Chem. Scand. 1966 30, 2508) is in error and thus cannot support assumptions that the high reactivity of cyclic esters is due primarily to entropic effects. The acceleration of the rate of hydrolysis of cyclic phosphate esters is therefore due to enthalpic factors, consistent with interpretations based on ring strain. © 1990, American Chemical Society. All rights reserved.