Palladium-Catalyzed Allylic C-H Alkylation

This reaction is an example of transition metal-catalyzed C-H activation, a field that has received an increasing amount of attention in recent years due to 

The experimental studies performed on the reaction revealed a positive Hammett p-value of 0.37, which indicates a buildup of negative charge in the ratedetermining transition state and suggests a pathway involving the abstraction of an allyhc proton. 

Using allylbenzene that had been fully deuterated in the benzylic position, the KIE for the reaction with nitro methylacetate was determined to be 5.5. Executing the KIE experiment in two separate flasks instead of as a direct competition afforded a significantly lower KIE value of 2.6. This is the KIE value for the whole catalytic cycle, while the previously determined value of 5.5 corresponds to the difference in reactivity in the single step in which the C-H bond is broken [7]. 

The lower KIE for the overall cycle indicates the existence of another step that has influence on the rate of the reaction. Isotope scrambling was not observed, which points to the irreversibility of the hydrogen/deuterium transfer reaction. Overall, the experimental results point to a C-H activation in which an irreversible proton abstraction is rate determining. 

The experimental preference of 4 1 for the linear product over the branched product is matched reasonably well by the calculated ratio of 8 1, considering the uncertainty of the calculation. 

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Also in 2012, an investigation of the palladium-catalyzed allylic C-H alkylation using Cat. A as catalyst to form compound 22 was performed by Fristrup and coworkers [53] on a reaction that was discovered by White et al. in 2008 (Scheme 8.9) [54]. 

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