ALLYLSILANES AS CARBON NUCLEOPHILES IN THE PALLADIUM-CATALYZED 1,4-OXIDATION OF CONJUGATED DIENES

Palladium-catalyzed oxidation of cyclic 1,3-dienes 3, 5, 9, 11, and 13 , with an allylsilane group in the side chain, led to an intramolecula r 1,4-syn-addition to the conjugated diene through a carbocyclization. Acyclic trienesilanes 7 also underwent analogous 1,4-oxidations. The reaction was carried out in acetone-acetic acid (2:1) with a slight ex cess of LiCl. p-Benzoquinone was employed as the oxidant and Li2PdCl4, as the catalyst. The reaction proceeds through an intramolecular tr a ns addition of the allylsilane to a (pi-diene)palladium complex to pro duce a bicyclic (pi-allyl)palladium intermediate. Subsequent trans att ack by chloride at the pi-allyl intermediate gives the product. The in termediate (pi-allyl)palladium complex was isolated and fully characte rized. It was unambiguously demonstrated that the allylsilane had atta cked the coordinated double bond trans to palladium (trans carbopallad ation). The use of CuCl2, as the oxidant, instead of p-benzoquinone, g ave a less stereoselective addition, but interestingly, with the oppos ite stereochemistry.