Intramolecular thermal (4σ + 2π) dyotropy: primary 2H kinetic isotope effects. Experimental limiting barrierparameters for quantum tunnelling in 2H transfer processes and mechanistically significant substituent effects
✍ Scribed by Kenneth Mackenzie; K. Brian Astin; Edward C. Gravett; Robert J. Gregory; Judith A. K. Howard; Claire Wilson
- Book ID
- 101287248
- Publisher
- John Wiley and Sons
- Year
- 1998
- Tongue
- English
- Weight
- 106 KB
- Volume
- 11
- Category
- Article
- ISSN
- 0894-3230
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✦ Synopsis
Primary deuterium kinetic isotope effects (PDKIE) in parazoline-annelated syn-sesquinorbornenes exhibiting irreversible intramolecular (4s 2p) thermal dyotropy reveal unambiguous evidence for a tunnelling contribution to the kinetics in one instance but not for a close analogue. For analogous dyotropy of a cyclohexadieneannelated syn-sesquinorbornene, the tunnelling components of the kinetic behaviour is small by comparison. The H atom traverse between alternative loci for the pyrazolines, deduced from x-ray and neutron diffraction data, is in agreement with approximate barrier parameters obtained by fitting of the PDKIE data to the Bell equation; barrier penetration is 3.22 kcal below the computed barrier corrected for the tunnelling contribution. The relative kinetic effect of systematic variation of the p-donor/acceptor groups on aryl ring substituents at C and N in the pyrazoline ring is consistent with a pericyclic process for dyotropy of these compounds, but not with rearrangement mediated by biradicals resulting from single H atom transfer in the rate-limiting step. Computer modelling of the transition state for dyotropy of these compounds is also consistent with a thermal, orbital symmetry conserved pericyclic reaction.