Conformational equilibria and torsional barriers of the isopropyl groups in N,N‐diiso‐propylbenzamide and its thio and seleno analogues

Abstract

The conformations of the isopropyl groups and the barriers to conformational interconversion in N,N-diisopropylbenzamide (l), and its thio (2) and seleno (3) analogues have been studied by dynamic '€I NMR spectroscopy. In 1 only one conformation is observed, whereas 2 and 3 exist as mixtorc?s of three conformations. The use of the strongly deshielding C=X (X = 0, S, Se) group and comparison with earlier results on similar systems allows total assignment, in the case of 2, redying an earlier proposal. The temperature dependence of the NMR spectra of 2 and 3 clearly shows that the E-and 2-isopropyl groups rotate with very diiferent rates, thns providing experimental evidence for the stepwise nature of the conformational interchange.