Understanding the unusual regioselectivity in the nucleophilic ring-opening reactions of gem-disubstituted cyclic sulfates.: Experimental and theoretical studies

The regioselectivity of the nucleophilic ring-opening reactions of three gem-disubstituted cyclic sulfates with sodium azide has been studied from both experimental and theoretical viewpoints. It is found that, depending on the substituent present in the cyclic sulfate, the reaction displays reversed regioselectivity, which allows one or another regioisomer to be obtained with selectivities greater than 4:1. The theoretical calculations show that, contrary to previous understanding, the intrinsic preference in all cases is azide attack at the less-substituted C-beta position, a consequence of similar stereoelectronic effects in the three sulfates considered. The observed preference for C-alpha attack in the case of the ester sulfate is explained in terms of differential solvent effects, which are in turn due to subtle differences in the charge transfer in the different transition structures.