New applications of palladium in organic syntheses
Abstract The palladium catalyzed vinylic substitution reaction with organic halides is a new and useful method for producing carbon-carbon bonds. The reaction is useful with aryl, heterocyclic, benzylic, and vinylic bromides and iodides. The most useful reactions are with bromides in which cases a triarylphosphine is required with the palladium catalyst. The best phosphine to employ for a specific synthesis can now be predicted based upon knowledge of how the phosphines influence vinylic substitution rates and side reactions forming phosphonium salts and palladium metal. Essentially all common functional groups can be tolerated by the reaction under the same conditions with the exception of α,β-unsaturated ketones and aldehydes. These react normally in the form of their acetals or ketals, however. Reactions proceed well with conjugated or isolated double bonds with mono-, di- and often tri-substituted olefins, generally in a stereospecific manner. A syn addition of the organopalladium compound to the olefin followed by a syn elimination of the palladium hydride is observed. The direction of addition of the organopalladium complex to the olefin depends upon the substituents present. Both steric and electronic influences have been observed. The direction of addition is dominated by steric effects; the organic group always adds, at least mainly, to the least substituted carbon of the double bond. Electron withdrawing substituents on the double bond generally cause exclusive addition of the organic group to the olefinic carbon not having the substituent. With electron donating groups, some addition to the carbon carrying the substituent is usually observed.