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 intramolecular 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 excess of LiCl. p-Benzoquinone was employed as the oxidant and Li2PdCl4, as the catalyst. The reaction proceeds through an intramolecular tr ans addition of the allylsilane to a (pi-diene)palladium complex to produce a bicyclic (pi-allyl)palladium intermediate. Subsequent trans attack by chloride at the pi-allyl intermediate gives the product. The intermediate (pi-allyl)palladium complex was isolated and fully characterized. It was unambiguously demonstrated that the allylsilane had attacked the coordinated double bond trans to palladium (trans carbopalladation). The use of CuCl2, as the oxidant, instead of p-benzoquinone, gave a less stereoselective addition, but interestingly, with the opposite stereochemistry.