Palladium-catalyzed 1,4-additions to conjugated

Palladium-catalyzed 1,4-additions to conjugated dienes can be divided into two classes (1) non-oxidation reactions that (2) oxidation reactions. In the former class, a palladium(O) catalyst is employed and the first step in the catalytic cycle is often an activation of one of the reactants by its oxidative addition to Pd(0). In the second class, a palladium(II) complex is the active catalyst which oxidizes the substrate diene with formation of Pd(0). Reoxidation of Pd(0) to Pd(II) by an oxidant regenerates the active catalyst. 

J.-E. Backvall, Palladium-Catalyzed 1,4-Additions to Conjugated Dienes, review in Metal-catalyzed Cross Coupling Reactions, Eds P. Stang and F. Diederich, WILEY-VCH, Weinham, 1998, pp. 339-385. 

Palladium(ll)-catalyzed 1,4-additions to conjugated dienes also involve the foimation of a (jr-allyl)palladium intermediate. These, in all known cases, are oxidation reactions. 

Palladium(II)-catalyzed 1,4-additions to conjugated dienes, in which at least one alkoxide function is added, require the presence of an alcohol function. In all cases known so far, this involves an alkoxypalladation of tlie conjugated diene to give an intermediate 4-alkoxy-l,2,3-(jr-allyl)palladium complex. Subsequent nucleophilic attack on the jr-allyl... 

A palladium-catalyzed 1,4-dialkoxylation of conjugated dienes was obtained when the 1,4-oxidation was performed in an alcohol as the solvent [105]. In this case it is necessary to run the reaction in the presence of a catalytic amount of a strong acid such as methanesulfonic acid or perchloric acid. Cyclic dienes gave a highly stereoselective 1,4-ds addition of the two alkoxy groups [Eq.(48)j. Also, the reaction with acyclic conjugated dienes proceeded in a, 4-syn addition. Thus, ( , )-hexa-2,4-diene gave E)- 2R, 5R ) dimethoxyhex-3-ene. The mechanism is depicted in Scheme 8-27. 

Palladium-Catalyzed Intramolecular 1,4-Additions to Conjugated Dienes. 

A number of intramolecular Pd-catalyzed 1,4-oxidations of conjugated dienes were developed.f In these reactions, two nucleophiles are added across the diene, one of which adds intramolecularly. So far, only heteroatom nucleophiles have been employed. In order to extend these intramolecular 1,4-oxidations to carbon nucleophiles, it was found that a vinylpalladium species can be obtained in situ from an alkyne via a chloropalladation. The approach is particularly attractive since it involves a Pd(II) chloride salt and could be compatible with the rest of the catalytic cycle. Reaction of dienyne with LiCl, and benzoquinone in the presence of palladium acetate as the catalyst, afforded the carbocyclization products. The reaction resulted in an overall stereoselective fltiri-addition of carbon and chlorine across the diene t B (Scheme 23). 

The related metal-catalyzed 1,4-addition reaction is shown in Scbone 9 (15). The conjugate addition of arylmercury halides or telraarylstananes toenones is catalyzed by palladium(Il) diloride in an acidic solution. The mechanism is reported to proceed through the transmetalation giving Ar-Pd-Q, the insotion of enone to the C-Pd bond, and finally, the protonolysis of the C-Pd or the O-Pd bond with acid. Thus, both reactions catalyzed by ibodium(I) and palladium(II) complexes are concluded to proceed through a quite similar inxKess. 

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