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

Reference:

Pirinccioglu, N., Robinson, J. J., Mahon, M. F., Buchanan, J. G. and Williams, I. H., 2007. Experimental and computational evidence for alpha-lactone intermediates in the addition of aqueous bromine to disodium dimethyl-maleate and -fumarate. Organic and Biomolecular Chemistry, 5 (24), pp. 4001-4009.

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Abstract

Structural analysis of the bromo-beta-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: cis alkene yields erythro lactone, and trans alkene yields threo lactone. 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 a-lactone. The cyclic bromonium species is not an intermediate. An alternative pathway leading directly from cis alkene to cis lactone, via an unusual frontside displacement mechanism, is over 20 kJ mol(-1) higher in free energy. Hydrolysis of the bromo-beta-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.

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