Hyperconjugation, enol formation

In constrast, kinetic regioselectivity does not usually correspond to the thermodynamic stability ratio between the two enolates. Indeed, when the ketone is ionised in protic solvents which make equilibration possible, the more substituted enolate is formed (e.g. [45] and [46] are in the ratios 10 90 and 40 60 for the lithium and sodium ion pairs, respectively, in dimethyl ether) (House, 1972). This means that the hyperconjugative effect, which is predominant in the enolate, is less important than inductive and steric effects in the transition state, a result which is in agreement with the carbanion character. The regioselectivity of preparative enolate formation in organic solvents has been reviewed by D Angelo (1976). 

With respect to the intermolecular nitrene addition to alkenes, the formation of aziridines is generally observed. Nevertheless, the use of sulfonimidamides has allowed the discovery of a highly chemoselective intermolecular aUylic C(sp )-H amination that can be applied to several classes of alkenes. Various terpenes and aUyl enol carbonates, particularly, undergo allylic amination in excellent yields of up to 98% (Scheme 28). The chemoselectivity was supposed to be controlled by the substrate. Hyperconjugation of the aUylic C—H bonds with the adjacent Jt-system would increase their reactivity, a result corroborated by the exclusive formation of the aziridine from P-caryophyUene whose structure does not display such a hyperconjugative effect for the aUylic C—H bonds. 

---