Studies in stereochemistry by nuclear magnetic resonance

Abstract

Nuclear Magnetic Resonance Spectroscopy is used to help solve two independent problems:- (i) The kinetic isomerization of β-aminocrotononitrile in both solid and solution phases is studied by using the relative intensity of the cis and trans methyl peaks from the n.m.r. At temperatures below 46°C, a solid state equilibrium of >95% cis isomer is formed, and above 46°C a 1:1 mixture of the two isomers forms as the compound slowly fuses. In solution, the trans isomer appears more stable at any temperature. The studies indicate β-aminocrotononitrile exists as either a pure cis monomer, or as a dimer or polymer of the cis and trans isomers. Dilution studies, acid catalysed kinetics, deuterium isotope effect studies, etc., support this observation. The structure of this dimer is proposed, and a reaction scheme supporting the experimental results suggested. (ii) Several ethyl ethers with an asymmetric centre next to the oxygen atom are studied by n.m.r. In each case, the methylene protons of the ethyl groups are non-equivalent, the chemical shift between them being about 20 cycles (at 60mc). This non-equivalence is attributed not only to the asymmetry of the carbon atom, but also to the induced asymmetry of the lone-pair electrons on the ether oxygen.