Palladium-catalyzed arylation of allylic benzoates using hypervalent siloxane derivatives

Arylsiloxane derivatives underwent palladium-catalyzed coupling with allylic esters to give adducts in high yields. The coupling reaction required inclusion of a fluoride activator and presumably occurred via a hypercoordinate intermediate. In addition, the coupling reaction proven to be both stereospecific (with inversion of configuration) and highly regioselective.; Highly functionalized arylsiloxanes were prepared from readily available aryl halides either by metallation or silylation protocols. In the metallation protocol, formation of the Grignard or lithium reagent, followed by condensation with tetraethyl orthosilicate gave the desired arylsiloxane in good yield. The major by-products of this reaction, di- and triaryl siloxanes, were shown to also transfer one of their aryl groups.; Arylsiloxanes can also be synthesized by silylation of aryl iodides and bromides employing Pd(0) or Rh(I) catalysts. The silylation reaction works best with para- and meta-substituted aryl derivatives that have electron-donating substituents. Electron deficient arene derivatives typically provided decreased yields of arylsiloxane.; Asymmetric variants of the siloxane coupling reaction have been unsuccessful. Both bidentate and monodentate chiral ligands were evaluated in this coupling reaction, but the enantioselectivities of adducts obtained in all cases were <10%. The results obtained from these experiments using chiral ligands and unstabilized nucleophiles are consistent with those previously reported by other researchers. A mechanism explaining the possible reasons for the low enantioselectivities obtained has been proposed that includes an η 3–η1 conversion. These results also clearly demonstrate the fact that achieving good enantioselectivites in allylic substitutions with unstabilized nucleophiles is far more difficult than in the analogous couplings with stabilized nucleophiles such as malonates where enantioselectivies of >90% are obtained in most cases.