Anomalous ¹H NMR spectra of 3,4-dihydroisoquinolines and related compounds

Abstract

Solutions of 3,4-dihydro-6,7-methylenedioxyisoquinoline (1) and analogues2-5 and9 as well as isoquinolines7 and8 in certain samples of CDCl₃, CCl₄, DMSO-d₆ or acetone-d₆ gave rise to anomalous¹HNMR spectra with extreme line broadening, signals due to protons at C-l and C-3 most often not being seen. The spectra of1 and3 were most striking in this respect.¹HNMR spectra of the quaternary salt 10 and the model schiff base 11 were normal. Several hypotheses for the observed line broadening have been considered and rejected, a slow equilibrium1⇆13 being one of them. NaBH₄ reduction of 1 and 2 followed by mass spectrometric analysis of the crude tetrahydroisoquinolines16 and17 ruled out a slow equilibrium 1 ⇆14 and 2 ⇆15 as contributory cause for line broadening. The crude reduction products unexpectedly contained N-ethyl species 22 and25. Their formation is rationalised.