Cis- trans isomerization of dimethyl 2,3-dibromofumarate

Abstract

The isomerization of dimethyl 2,3-dibromofumarate in chloroform solutions was investigated by the combination of nuclear magnetic resonance (NMR) and density functional theory (DFT) calculations. The bromination of dimethyl acetylenedicarboxylate leading to dimethyl 2,3-dibromofumarate produces the trans isomer initially, which however converts into the more stable cis isomer. The conversion from trans to cis is spontaneous and greatly accelerated by light.

Fig. 1. Bromination reaction leading to dimethyl 2,3-dibromofumarate.

Fig. 2. (a) Selected NMR spectra of the reaction mixture taken at different stages of the process. (b) Relative concentrations of the reagents as obtained from the NMR spectra. SM denotes the starting material. Lines connecting points are intended as a guide for the eye.

Fig. 3. UV-vis spectra acquired during the synthesis. Black line – the absorption spectrum of the starting material; red line – typical transmission curve of glass, here a standard glass slide.

Fig. 4. Relative concentrations of reagents monitored by NMR. The experiment was performed in situ, inside the NMR chamber. The arrows indicate points when (1) a small light bulb was turned on inside the NMR chamber and (2) the NMR tube with the reaction mixture was taken out of the NMR chamber and exposed to light in the lab. Solid lines are exponential decay fits to the data points before light was introduced, as described in the text.