ИСТИНА

The mechanism of decarboxylative elimination was studied over 60 years ago with inorganic bases in protic solvents. It was proposed that Z-β-bromostyrenes result from concerted anti-fragmentation. An alternative mechanism with intermediate β-lactone is also considered in the literature. Nowadays this reaction remains a popular means to prepare β-bromostyrenes but is usually performed with Et3N in DMF, which strongly improves its Z-selectivity. We studied the effect of amine base, added water and tetraalkylammonium salts on the rate and Z/E-selectivity of decarboxylative elimination of 2,3-dibromo-3-arylpropanoic acids in DMF. The addition of common ion salt, [Bu4N]Br, did markedly improve the Z-selectivity, while even 1% of water resulted in increased formation of E-isomer. beta-Lactones were detected by IR and NMR but kinetic analysis showed that beta-lactones are not intermediates, but side products in decarboxylative elimination.