Palladium-catalyzed oxidation of 1,3-dienes in acetic acid using an oxidation system of Mn02 and catalytic amounts of p-benzoquinone selectively gives l,4-diacetoxy-2-alkenes. The reaction proceeds with high stereoand regioselectivity, and by ligand control the reaction can be made to take place with either cis or trans 1,4-diacetoxylation across the diene in cyclic systems. Also in an acyclic system the 1,4-relative stereochemistry can be controlled as shown by the stereoselective oxidation of (EE)-and (£rZ)-2,4-hexadiene to their corresponding dl (>88% di) and meso (>95% meso) diacetates 15 and 18, respectively. Evidence is provided that supports a mechanism involving a trans acetoxypalladation of the conjugated diene to give an intermediate (ir-allyl)palladium complex, followed by either a cis or trans attack by acetate on the allyl group. The cis attack is best explained
The regiochemistry of copper-catalyzed reactions between Grignard reagents and allylic substrates has been studied. A dual regiocontrol was obtained in the Li2CuC14-catalyzed Grignard reaction with primary allylic acetates. Reaction conditions that favor formation of an intermediate dialkylcuprate (fast addition of Grignard reagent, low temperature, low concentration of catalyst) gave a-substitution, whereas reaction conditions favoring formation of a monoalkylcopper intermediate (slow addition of Grignard reagent, increased temperature, increased concentration of catalyst) led to a y-substitution. A remarkable solvent effect was observed for CuCN-catalyzed Grignard coupling with primary allylic acetates. In ether a highly y-selective reaction took place, but in T H F a-substitution predominated. Other allylic substrates such as allylic sulfones and allylic chlorides were also studied. The latter substrates showed a preference for y-substitution, which is explained by their high reactivity.Allylic compounds are important substrates in organic synthesis, and they have attracted a lot of mechanistic interest over the years, in particular with respect to nucleophilic displacement, i.e. SN2 and SN2'.' A number of studies dealing with the regio-and stereochemistry of nucleophilic substitution of allylic substrates have appeared.Recently, transition metals have become popular tools for the activation of allylic substrates.* By coordination of the double ( I ) Magid, R. M. Tetrahedron 1980Tetrahedron , 36, 1901 (2) Consiglio, G.; Waymouth, R. M. Chem. Reo. 1989, 89, I . Scheme I P R-NU
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