Boron hydrides. On the importance of multiple isotopic substitution for the observation of primary inverse hydrogen kinetic isotope effects

Davis, Robert Earl; Kenson, Robert E.; Kibby, Charles L.; Lloyd, H. H.

doi:10.1039/c19650000593

Cited by 2 publications

(1 citation statement)

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“…It is probable that the inverse effect arises from the three secondary effects outweighing the primary effect (27,31,32) and this view is substantiated by the observation that reduction of methyl alkyl ketones with morpholine-borane (33) shows a small normal isotope effect.…”

Section: The Terms Steric Approach Control Andmentioning

confidence: 66%

Deuterium Isotope Effects in the Reduction of Cyclohexanones. The Concepts of Steric Approach Control and Product Development Control

Wigfield¹,

Phelps²

1972

Can. J. Chem.

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Definitions for the terms steric approach control and product deuelopment control in relation to the degree of bonding in the transition state are put forward. Kinetic deuterium isotope effects in the reduction of a graded series of ketones by NaBH, and NaBD, have been measured. These effects are inverse, and show no systematic dependence on the degree of steric hindrance of the ketone. This evidence appears to invalidate the steric approach control-product development control explanation of the axial to equatorial product ratios (1). The primary isotope effect is very small, which is consistent with an early transition state in these reductions, as demanded by other explanations of product ratios, but since several other factors causing small isotope effects may be applicable, the evidence does not constitute proof of an early transition state.The main conclusions of this work have already been published in preliminary form (2).Les notions de "steric approach control" et 'kroduct deuelopment control" en relation avec le degri. de liaison dans I'ttat de transition, sont precisees. Les effets isotopiques du deuterium sur la cinbtique de reduction d'une serie de cetones par NaBH, et NaBD,, ont Ct C mesurts. Ces effets sont inverses et ne montrent aucune dependance systematique avec le degrt d'empschement sttrique de la cCtone. Cette mise en evidence montre que I'explication des rapports du produit axial et equatorial par le "steric approach control" et "product development control", n'est pas valable (1). L'effet isotopique primaire, trts faible, est compatible avec un debut d'6tat de transition dans ces rbductions. Ce rbsultat, deji mentionnt dans d'autres etudes, ne constitue pas une preuve puisque de nombreux autres facteurs peuvent Etre la cause de ces faibles effets isotopiques. Les conclusions principales de ce travail ont deja Ct C publikes dans une note preliminaire (2).

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Section: The Terms Steric Approach Control Andmentioning

confidence: 66%