A variable-temperature N.m.r. study of the ring inversion in 7,12-dihydropleiadene and its 1-methoxy-derivative using a quantitative treatment of the collapse of an AB quartet

Abstract

The ¹H n.m.r. spectra of 7,12-dihydropleiadene (I; R = H) and the 7,7-(II) and 12,12-dideuterio-derivatives (III) of the 1-methoxy-compound have been measured over the range +60 to –35°. Using a full treatment of the collapse of an AB spin multiplet in the presence of exchange, theoretical spectra have been fitted to those observed. From the derived rate constants for the inversion of the seven-membered ring, Arrhenius plots were constructed from which ΔE and other activation parameters were obtained. For compounds (I; R = H) and (II), ΔF*= 14·0 and 13·7 kcal. mole^–1, respectively.

In the 1-methoxy-compound (I; R = OMe), the internal coupling constants for the 7- and 12-methylene groups were identical but the internal chemical shifts were not. That Δν_AB for the 7-(78·2 c./sec.)(at 60 Mc./sec.) and 12-methylene protons (20·8 c./sec.) is different accounts for the different linewidths of the signals from these methylene groups observed at normal probe temperature (35°).