Karabatsos

In contrast to the open-chain and dipolar models, which are based on conformations of the carbonyl compound not representing energy minima, Karabatsos proposed a different model assuming an early, reactant-like transition state in which the most stable conformation of the free carbonyl compound is preserved1314. Thus, the C-M bond eclipses the carbonyl double bond and, in order to minimize the energy of the transition state, the nucleophile approaches close to the small substituent on the stereogenic center (Figure 5). 

A further improvement of the theory of 1,2-asymmetric induction was introduced by Felkin15. Neither Cram s open-chain model nor the Karabatsos model is able to explain why the stereoselectivity increases when either the incoming nucleophile R2e or the substituent at the carbonyl group (R1) increases in bulk. To explain these experimental observations the following assumptions are made for the Felkin model ... 

Solvolytic experiments specifically designed to test Bartell s theory were carried out by Karabatsos et al. (1967), who were primarily interested in an assessment of the relative contributions of hyperconjugation and non-bonded interactions to secondary kinetic isotope effects. Model calculations of the (steric) isotope effect in the reaction 2- 3 were performed, as well as that in the solvolyses of acetyl chloride... 

The calculated isotope effects for the solvolyses of t-butyl chloride-dg (previously estimated by Bartell, 1961a) and acetyl chloride-ds were much weaker than the experimentally observed ones. The authors conclude (Karabatsos et al., 1967) that in ordinary systems where hyperconjugation (from the j8-position) is possible, the effect of non-bonded interactions accounts for only a small part (less than 10%) of the observed isotope effect. 

Karabatsos, G. J., Rotational Isomerism about sp2-sp3Carbon-Carbon Single Bonds, 5, 167. Kawabata, T., Memory of Chirality Asymmetric Induction Based on the Dynamic Chirality of Enolates, 23, 175. 

The populations of the rotamers are affected by steric effects. Karabatsos and Taller (77) examined the populations of unsymmetrical nitrosamines and found that in methyl-terr-butylnitrosamine only one isomer was observable. Nelsen and associates (78) similarly found that the populations of conformers of 2-nitroso-2-azabicyclo[2.2.2]octanes (40) are affected by the substituent in the 3-position. 

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