Proton NMR spectra and conformations of N-alkyl-N-benzyl- and N-alkyl-N-(.ALPHA.-chlorobenzyl)carbomoyl chlorides.

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Rotational isomerization about the amide C–N bond of twelve N-alkyl-N-benzylcarbamoyl chlorides (C6H5CHX–NR–COCl, 1: X=H, 2: X=Cl; R=CH3, C2H5, n- and i-C3H7, n- and t-C4H9) was studied by proton NMR spectroscopy. The values of ΔG\neweq for the isomerization are 15.8–17.3 kcal/mol for compounds 1 and 13.3–15.9 kcal/mol for compounds 2. Another kind of slow rotation was observed about the benzyl–nitrogen bond of 1(R=t-C4H9) and the α-chlorobenzyl–nitrogen bond of 2(R=CH3) at low temperature. The chiral centers of compounds 2 produce large nonequivalences for the geminal protons of the 2-methylene group of 2(R=n-C3H7) as well as 2(R=n-C4H9) and for the geminal methyl protons of 2(R=i-C3H7), reflecting some conformational preference of these compounds.
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