Experimental and computational evidence for α-lactone intermediates in the addition of aqueous bromine to disodium dimethyl-maleate and -fumarate

Abstract

Structural analysis of the bromo-β-lactones obtained by addition of bromine to aqueous solutions of disodium 2,3-dimethylmaleate and 2,3-dimethylfumarate reveals stereochemistries opposite to those originally assigned in 1937: cisalkene yields erythrolactone, and transalkene yields threolactone. B3LYP/6-31+G(d) calculations using a PCM description of aqueous solvation confirm the validity of our proposed mechanism, in which the first-formed intermediate in each case is an α-lactone. The cyclic bromonium species is not an intermediate. An alternative pathway leading directly from cisalkene to cislactone, via an unusual frontside displacement mechanism, is over 20 kJ mol^–1 higher in free energy. Hydrolysis of the bromo-β-lactones yields bromohydrins whose stereochemistries as determined by X-ray crystallography indicate stereospecific formation by acyl–oxygen cleavage of the lactone ring, again contrary to the original view.