Observations on the geometry of hydrogen transfer in [1,5] sigmatropic rearrangements

The present investigation was undertaken with the objective of identifying a [1,5] sigmatropic rearrangement occurring in a cyclic system which is not capable of such distention of the π framework. The case for study was the rearrangement of the 9aH-quinolizine to the 4H-quinolizine. The temperature dependence of the isotope effect was again applied as the criterion of TS++ geometry in H transfer. It has frequently been demonstrated, though not directly derivable (at present) from conventional transition-state theory of the isotope effect, that a bent TS++, i.e., one involving H transfer at an acute angle, can be correlated with a temperature-independent k/sub H//k/sub D/. That is to say, the finding of isotope effect parameters of [ΔE/sub a/]/sup H//sub D/ approx. = 0 and A/sub H//A/sub D/>>1.2 has been empirically shown to be most congruent with a bent TS++