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 k,(17)/k1(5) -2 x 1 O5 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 M M-calculated n-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 o n 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 n-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. lo4 can be identified with a change in dcH of 0.1-0.17 A. Similar kinetic effects concomitant o n 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 1 02-1 O3 between the two series can be identified principally with the differing electronic requirements for triene and (slower) pyrazoline rearrangements. Primary deuterium kinetic isotope effects (k12H/k12D, d In [k12H/k12D] /dt and especially 4,/&,) reveal strong evidence for non-classical behaviour especially for pyrazoline 38.The remarkable increase in rate for intramolecular chemical change when compared with the intermolecular mode has considerable implications for synthetic methodology and also the detailed course of enzymic ~atalysis.~ Compounds which display intramolecular hydrogen dyotropy free from side reactions at conveniently measurable rates, and which furnish crystals suitable for X-ray crystallographic and/or neutron diffraction molecular structure analysis provide suitable models for investigating the interplay of reaction-zone proximity effects and intrinsic steric and electronic energy changes which may accompany the dyotropic process. Information so obtained is also of considerable potential value for understanding the detailed stereochemical requirements for bond-forming and bond-breaking event^.^ Since th...

Noyori Hydrogenation: Aromaticity, Synchronicity, and Activation Strain Analysis

“…In other words, the transformation seems to be easier for late transition states when the new C–H bond is already developed. This result agrees with kinetic and structural studies in related intramolecular (dyotropic) double hydrogen atom migrations , which indicate that changes of only 0.1–0.17 Å in the initial C···H bond length (also called precompression factor) translate into a rate spread of 10 4 s –1 . , …”

Dyotropic Reactions: Mechanisms and Synthetic Applications

“…Small changes in π-energy at the receptor π-bond, occasioned by the proximity of polarizing substituents on the adjacent methylene bridge, contribute to the modest rate-spread observed for the series of trienes 145 (about an order of magnitude). 105,106 It is also worthy of note that while primary deuterium kinetic isotope effects in the rearrangements of compounds 147 reveal unambiguous evidence for a tunneling contribution to the kinetics, the analogous dyotropy of compounds 145 shows comparatively smaller tunneling components. 105,107,108 Vogel and co-workers reported the thermally induced [ σ 2 s + σ 2 s + π 2 s ] process in the endo,endo-11-oxatetracyclododecene derivative 149 (Scheme 60), a process similar to the rearrangement observed in isodrins 145.…”

“…105,106 It is also worthy of note that while primary deuterium kinetic isotope effects in the rearrangements of compounds 147 reveal unambiguous evidence for a tunneling contribution to the kinetics, the analogous dyotropy of compounds 145 shows comparatively smaller tunneling components. 105,107,108 Vogel and co-workers reported the thermally induced [ σ 2 s + σ 2 s + π 2 s ] process in the endo,endo-11-oxatetracyclododecene derivative 149 (Scheme 60), a process similar to the rearrangement observed in isodrins 145. 109 The thermochemical data [∆H q ) 35-39 kcal/mol and ∆S q ) 2-11 cal/(K mol)] and the deuterium labeling experiments are consistent with a concerted intramolecular mechanism involving the stereospecific 4,5-endo a 9,10-endo double hydrogen atom migration.…”

1,2,3,4-Tetrachloro-11,11-dimethoxy-1,4,5,6,9,10-hexahydro-endo-1,4-exo-6,9-dimethanonaphthalene

“…A number of related structures have been determined, for example, aldrin (1,2,3,4,10,10-hexachloro-1,4,4a,5,8,8a-hexahydro-1,4:5,8-dimethanonaphthalene;DeLacy & Kennard, 1972), isodrin (1,2,3,4,10,10-hexachloro-1,4,4a,5,8,8a-hexahydro-endo-1,4-endo-5,8-dimethanonaphthalene;Kennard et al, 1979) and dechloroethoxyisodrin (endo,endo-3,5,6,11,-11-pentachloro-4-ethoxytetracyclo[6.2.1.1 3,6 .0 2,7 ]dodeca-4,9diene; Mackenzie et al, 1993).…”