Long Range Splittings in the NMR Spectra of Isomeric 2-Bromo-2-butenes

Abstract

The recent publication of Fraser on long-range coupling between the protons in the methyl groups of tiglic and angelic acids and their methyl esters prompts us to report a similar observation in the NMR spectra of cis- and trans-2-bromo-2-butene. In this case, also, we observe coupling between the methyl group protons that are separated from one another by five bonds (four single bonds and one double bond). Furthermore, the magnitude of this coupling is larger when the two methyl groups are trans disposed about the olefinic linkage than when they are cis oriented. The possibilities of such splittings have been alluded to previously to account for the barely perceptible fine structure in one of the peaks in the NMR spectrum of methyl photosantonate. Such splittings across five bonds have also been observed in the NMR spectra of plumericin and in the spectra of other butene derivatives.