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[摘要]：Hammett anal. of the palladium-catalyzed allyl-aryl coupling reaction has demonstrated that the rate of the coupling reaction is enhanced by electron-withdrawing groups on the aryl siloxane. The pos. slope of the Hammett plot indicated involvement of a charged transition state in which neg. charge on the aryl ring is stabilized inductively. This result is consistent with either transmetalation or reductive elimination being the rate-detg. step in the coupling process. Furthermore, the influence of ligand on the metal site has been assessed from competition studies as a function of ligand type, cone angle, and electronic effects. From the relative ratios of coupling products produced in the Hammett study, it is possible to gather insight into the role of the electronic as well as the steric effects of ligands on the mechanism of the coupling reaction. |
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