On intramolecular dyotropy: structural effects on reaction rates and crystal structure-molecular mechanics correlations for some new examples

Abstract

In an earlier paper we discussed the effect on reaction rate for thermal intramolecular dyotropy exhibited for a series of trienes, 1–4 and pyrazolines 6–9 having identical structural elements proximate to the 2 H receptor π-bond. Both sets of compounds displayed closely similar rate-ratios across the series correlating with structural modification. The set of homopyrazolines 14–17 analogous to 6–9, now reported, correlate similarly and all three sets of compounds obey a linear free energy relationship log k₁^A=m log k₁^B+C, suggestive of a common reaction mechanism for H dyotropy for these compounds. We also report on the kinetic behaviour of new, oxygen-bridged trienes 22 and 23, analogues of triene 1; over quite large temperature ranges, translating into a rate-spread of ∼ 10⁵, Arrhenius plots for rearrangement of 1 and 23 are essentially linear, indicating that these compounds behave classically with scant evidence for quantum tunnelling from this result and the previously reported indecisive PDKIE data. We also describe solvent effects on the rearrangement rates for representative analogues of triene 1, and for trienes 22 and 23, and briefly report on the solid-state thermochemical behaviour of the triene 3, analogous to 1. A new molecular framework exhibiting relatively slow thermal H dyotropy is also reported. The information we have obtained rather strongly suggests a concerted, but non-synchronous transfer of 2 H in the dyotropic process.