ON INTRAMOLECULAR DYOTROPY - STRUCTURAL EFFECTS ON REACTION-RATES, CRYSTAL-STRUCTURE MOLECULAR MECHANICS CORRELATIONS AND PRIMARY DEUTERIUMKINETIC ISOTOPE EFFECTS - (PARAMETERS FOR INTRAMOLECULAR RECOGNITION)

Previous attempts to prepare the pentacyclic triene 17 for comparison of the rate of intramolecular dyotropy with the kinetics of similar ir reversible rearrangements of norbornene ring-substituted analogues had given only dyotropomer 1 8 with an estimated minimum ratio k1 (17) /k 1 (5) approximately 2 x 10(5) at 36-degrees-C. In the following it is shown that the steric proximity, d(CH), of transferring H atoms to rec eptor sp2 carbons in the reaction zone cavity together with MM-calcula ted pi-energy differences between dyotropomers can rationalise the lar ge rate enhancement observed for the triene 17 compared with 5 and its analogues. For a series of compounds modelled on 5, in which d(CH) va riations are quite small, observed differences in dyotropic rate are i dentified as arising from the interplay of molecular geometry changes and small changes in pi-energy at the receptor alkene site occasioned by proximate polar groups, the electronic changes associated with arom atisation of the appended donor-site ring remaining essentially consta nt across the series. When the electronic energy changes associated wi th dyotropy for a pair of analogous structures are very closely simila r, a rate-spread of ca. 10(4) can be identified with a change in d(CH) of 0.1-0.17 angstrom. Similar kinetic effects concomitant on small pa rallel structural variations, virtually identical in relative-rate ter ms to those in the triene series, are seen in the irreversible dyotrop y of a series of analogous pyrazolines modelled on compound 36 and may be likewise rationalised. Kinetic comparisons for a group of aryl-rin g substituted analogues of pyrazoline 36 reveal quite modest substitue nt effects, consistent with reactant-like transition-states for these quantitative, exothermic rearrangements. Inter-series comparison of al icyclic trienes with pyrazolines indicate that when d(CH) values are e ssentially identical in representative examples, a rate-differential o f 10(2)-10(3) between the two series can be identified principally wit h the differing electronic requirements for triene and (slower) pyrazo line rearrangements. Primary deuterium kinetic isotope effects (k(1)2H /k(1)2D, d/n[k(1)2H/k(1)2D]/dt and especially A2H/A2D) reveal strong e vidence for non-classical behaviour especially for pyrazoline 38.