Palladium Acetate Addition with Chloride Elimination

VII. Palladium Acetate Addition with Chloride Elimination... 

Terminal alkynes react with propargylic carbonates at room temperature to afford the alka-l, 2-dien-4-yne 14 (allenylalkyne) in good yield with catalysis by Pd(0) and Cul[5], The reaction can be explained by the transmetallation of the (7-allenylpailadium methoxide 4 with copper acetylides to form the allenyKalk-ynyl)palladium 13, which undergoes reductive elimination to form the allenyl alkyne 14. In addition to propargylic carbonates, propargylic chlorides and acetates (in the presence of ZnCb) also react with terminal alkynes to afford allenylalkynes[6], Allenylalkynes are prepared by the reaction of the alkynyl-oxiranes 15 with zinc acetylides[7]. 

Benzyl chloride reacts easily with methyl acrylate in the presence of tri-n-butylamine and palladium acetate (1 mol %) as catalyst.51 The product is a mixture of (E)-methyl 4-phenyl-3-butenoate (67%) and (E)-methyl 4-phenyl-2-butenoate (9%), arising from elimination-addition reactions of the palladium hydride group which largely isomerize the initial elimination product. 

Abnormal olefin arylation reactions which are of interest mechanistically and preparatively occur with some allylically substituted compounds. The ailylic esters and ethers appear normal and produce cinnamyl derivatives exclusively while ailylic alcohols and chlorides are abnormal. Ailylic alcohols and "arylpalladium acetates form 3-arylaldehydes from primary ailylic alcohols and 3-arylketones from secondary alcohols 3°). The mechanism of reaction apparently involves anti-Markovnikov addition of the palladium compound to the double bond followed by elimination of the hydrogen atom on the hydroxyl-bearing carbon rather than the benzylic hydrogen. This again would be elimination of the more electronegative hydrogen atom. 

This elimination is reminiscent of the last step in the aqueous palladium chloride oxidation mentioned above and this reaction also may involve multiple hydride addition-elimination steps. Minor amounts of the normal products and Markovnikov products are also generally found in these reactions. Cupric chloride can be used as a reoxidant although the yields are generally lower than with an all acetate, non-catalytic reaction. 

Carbonate esters are obtained by selenium-mediated reaction of alcohols with carbon monoxide in the presence of the alcohol alkoxide. The synthesis of allylic acetates by acetate-ion displacement on allylic chlorides is markedly catalysed by palladium(ii) chloride an addition-elimination mechanism is invoked to explain the overall stereochemistry of an S- 1 process. 

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