A 1H NMR and molecular modelling investigation of diastereotopic methylene hydrogen atoms

The ¹H NMR spectra of methyl 3‐bromo‐2‐methylpropionate (1a) and the corresponding chloro compound (2a) show no long‐range coupling between the methyl and methylene protons. In contrast, in the analogous dihalocompounds, methyl 2,3‐dibromo‐2‐methylpropionate (1b) and methyl 2,3‐dichloro‐2‐methylpropionate (2b), one of the methylene protons exhibits a large ⁴J_HH coupling (0.8 Hz) to the methyl group, but the other proton shows no observable splitting. This can be explained quantitatively by calculations of the conformational preferences in these compounds combined with the known orientation dependence of the ⁴J_HHcouplings. One conformer predominates in the dihalo compounds 1b and 2b, and this is responsible for the ⁴J_HH coupling. In 1a and 2a all three conformers are populated and the ⁴J_HH couplings average to zero. The technique is a potentially general method of unambiguously assigning diastereotopic methylene protons. Copyright © 2002 John Wiley & Sons, Ltd.