Experimental and computational evidence for alpha-lactone intermediates in the addition of aqueous bromine to disodium dimethyl-maleate and -fumarate.
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 alpha-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.
Pirinççioğlu
Necmettin
N
Department of Chemistry, University of Dicle, TR-21280, Diyarbakir, Turkey. pirincn@dicle.edu.tr
Robinson
James J
JJ
Mahon
Mary F
MF
Buchanan
J Grant
JG
Williams
Ian H
IH
D001966:Bromine / Q000737:chemistry*; D039301:Computers, Molecular; D018360:Crystallography, X-Ray; D000069462:Dimethyl Fumarate; D005650:Fumarates / Q000737:chemistry*; D006868:Hydrolysis; D007783:Lactones / Q000138:chemical synthesis / Q000737:chemistry*; D008298:Maleates / Q000737:chemistry*; D008956:Models, Chemical; D015394:Molecular Structure; D012964:Sodium / Q000737:chemistry*; D013237:Stereoisomerism
D005650:Fumarates; D007783:Lactones; D008298:Maleates; D012964:Sodium; D000069462:Dimethyl Fumarate; C004780:dimethyl maleate; D001966:Bromine