Enone isomerization probe

In this study, benzaldehyde and benzaldehyde-methyllithium adduct were fully optimized at HF/6-31G and their vibrational frequencies were calculated. The authors used MeLi instead of lithium pinacolone enolate, since it was assumed that the equilibrium IBs are not much different for the MeLi addition and lithium enolate addition. Dehalogena-tion and enone-isomerization probe experiments detected no evidence of a single electron transfer to occur during the course of the reaction. The primary carbonyl carbon kinetic isotope effects and chemical probe experiments led them to conclude that the reaction of lithium pinacolone enolate with benzaldehyde proceeds via a polar mechanism. 

The mechanism of the isomerization was probed by using the deuterated propargyl alcohol 25. The labeling pattern in the produced a,y3-enone 26 showed that the isomerization proceeded via a 1,2-shift of a hydrogen atom attached at the pro-pargylic carbon. On the basis of these results, a mechanism was proposed as depicted in Scheme 12.5. 

---