MECHANISMS OF REACTION OF SULFATE ESTERS - A MOLECULAR-ORBITAL STUDY OF ASSOCIATIVE SULFURYL GROUP-TRANSFER, INTRAMOLECULAR MIGRATION, AND PSEUDOROTATION

Molecular orbital calculations for the reactions of 2-hydroxyethyl sul fate in neutral and ionized forms were used to examine the free energy profiles for intramolecular sulfuryl group transfer in the gas phase. Detailed analysis of reaction dynamics yielded structures for trigona l bipyramidal pentacoordinate reaction intermediates and for transitio n states at the MP2/6-31+G//HF/3-21+G(*) and MP2/6-31+G*//HF/3-21G(*) levels of calculation for anionic and neutral species, respectively. Application of a continuum dielectric model to these structures provid ed estimated free energy profiles for reaction in aqueous solution. Co mparison with the reactivity of phosphate esters suggests that the emp irical guidelines for reactions of phosphates may, in large part, be e xtended to reactions of sulfate esters. It is predicted that the activ ation barriers to intramolecular sulfuryl group transfer will be large , in accord with experiment and in contrast with phosphoryl transfer, The explanation is based upon (1) the requirement for the energeticall y unfavorable protonation of the sulfate moiety for nucleophilic attac k on sulfur; and (2) the relatively higher energy of the pentacoordina te relative to the tetracoordinate state for S over P species. However , ferry pseudorotation is facile and does not present a barrier to rea ction. The calculations suggest that nucleophilic substitution of alko xide on the protonated sulfate moiety has a low activation barrier and may occur with inversion or retention of stereochemistry at sulfur.