On intramolecular dyotropy: Structural effects on reaction rates, crystal structure-molecular mechanics correlations and primary deuterium kinetic isotope effects. (Parameters for intramolecular recognition)

Abstract

Previous attempts to prepare the pentacyclic triene 17 for comparison of the rate of intramolecular dyotropy with the kinetics of similar irreversible rearrangements of norbornene ring-substituted analogues had given only dyotropomer 18 with an estimated minimum ratio k1(17)/k 1(5) ∼2 × 105 at 36 °C. In the following it is shown that the steric proximity, dCH, of transferring H atoms to receptor sp2 carbons in the reaction zone cavity together with MM-calculated π-energy differences between dyotropomers can rationalise the large rate enhancement observed for the triene 17 compared with 5 and its analogues. For a series of compounds modelled on 5, in which dCH variations are quite small, observed differences in dyotropic rate are identified as arising from the interplay of molecular geometry changes and small changes in π-energy at the receptor alkene site occasioned by proximate polar groups, the electronic changes associated with aromatisation of the appended donor-site ring remaining essentially constant across the series. When the electronic energy changes associated with dyotropy for a pair of analogous structures are very closely similar, a rate-spread of ca. 104 can be identified with a change in dCH of 0.1-0.17 Å. Similar kinetic effects concomitant on small parallel structural variations, virtually identical in relative-rate terms to those in the triene series, are seen in the irreversible dyotropy of a series of analogous pyrazolines modelled on compound 36 and may be likewise rationalised. Kinetic comparisons for a group of aryl-ring substituted analogues of pyrazoline 36 reveal quite modest substituent effects, consistent with reactant-like transition-states for these quantitative, exothermic rearrangements. Inter-series comparison of alicyclic trienes with pyrazolines indicate that when dCH values are essentially identical in representative examples, a rate-differential of 10 2-103 between the two series can be identified principally with the differing electronic requirements for triene and (slower) pyrazoline rearrangements. Primary deuterium kinetic isotope effects (kr2H/k12D, d In[k12H/k 12D]/dt and especially A2H/A2D) reveal strong evidence for non-classical behaviour especially for pyrazoline 38.