Allylic substitution solvent effects

The palladium(n) catalyst, because of its Lewis acidity, may play a role in the addition of allylic tin to the ketone however, acylation of crotyltin was not reported to form a tertiary alcohol using palla-dium(II). It appears that solvent effects dominate in these cases. As part of the same study, substitute vi-nylstannanes were shown to undergo acylation with retention of configuration however, the resulting a, -unsaturated ketones were not configurationally stable to the reaction conditions. Isomerically pure (Z)-l-propenylstannane was acylated to afford a 50 50 mixture of alkenes (equation 87). The (Z)-a, -un-saturated ketone was shown to isomerize to a mixture of (Z)- and ( )-isomers under the reaction conditions. Mixtures of (Z)- and ( )-2-substituted vinylstannanes were acylated to afford mainly the ( )-a, -unsatuTated ketone (equation 88). ... 

In addition to enantiocontrol, the problem of regiocontrol arises in these reactions. There are various factors that influence the regioselectivity of allylic substitutions [3,4,13, 36, 37, 38, 39]. Electronic effects exerted by the catalyst and the allylic substituents, steric interactions between the nucleophile, the allyl system and the catalyst, and the relative stabilities of the Ti-olefin complexes formed after nucleophilic addition, can all play a role. The relative importance of these factors varies with the catalyst, the substrate, the nucleophile, the solvent and other reaction parameters and is difficult to predict. 

A solvent effect on the rate of the rearrangement was also determined (Scheme 11.21). The significance of the solvent effect depended on the position of the methoxy substituent. For the 6-methoxy-substituted allyl vinyl ether 9c, the rearrangement proceeded 68 times faster in methanol-d4 than in benzene-dg. 

Baechler and coworkers204, have also studied the kinetics of the thermal isomerization of allylic sulfoxides and suggested a dissociative free radical mechanism. This process, depicted in equation 58, would account for the positive activation entropy, dramatic rate acceleration upon substitution at the a-allylic position, and relative insensitivity to changes in solvent polarity. Such a homolytic dissociative recombination process is also compatible with a similar study by Kwart and Benko204b employing heavy-atom kinetic isotope effects. 

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