Palladium acetate rearrangements

Palladium-catalyzed oxidation of 1,4-dienes has also been reported. Thus, Brown and Davidson28 obtained the 1,3-diacetate 25 from oxidation of 1,4-cyclohexadiene by ben-zoquinone in acetic acid with palladium acetate as the catalyst (Scheme 3). Presumably the reaction proceeds via acetoxypalladation-isomerization to give a rr-allyl intermediate, which subsequently undergoes nucleophilic attack by acetate. This principle, i.e. rearrangement of a (allyl)palladium complex, has been applied in nonoxidative palladium-catalyzed reactions of 1,4-dienes by Larock and coworkers29. Akermark and coworkers have demonstrated the stereochemistry of this process by the transformation of 1,4-cyclohexadiene to the ( r-allyl)palladium complex 26 by treatment... 

In the presence of tetrakis(triphenylphosphine)palladium a-cyano allylic acetates rearrange to y-acetoxy-a,/3-unsaturated nitriles. The products can he converted into furanc derivatives (equation II).2... 

Mixtures of regioisomers are frequendy obtained in these reactions.80 The problem is most serious with primary allylic alcohols without a- or p-substituents. Even the 2-arylated products generally rearrange to saturated aldehydes. Ally alcohol itself, when reacted with iodobenzene and triethylamine, with palladium acetate as catalyst, for example, produces a 71 % yield of an 84 16 mixture of 3-phenyl- and 2-phenyl-propanal (equation 28). 

The details of the cupric salt reaction with the palladium adduct are not clear. Exchange to form a cupric alkyl is one possibility or complex formation,"probably with chloride bridges between the palladium adduct and cupric chloride, may occur with subsequent anion shift from palladium to carbon or perhaps an Sn2 displacement of the complex metal group by an anion may occur. Rearrangements producing 1,3 and 1,4 substituted products from linear olefins have also been observed. For example, 1-butene produced several percent of 1,3- and 1,4-chloro acetates and diacetates under the reaction conditions used 16>. "Hydrido-palladium acetate or chloride" -complexes would seem to be likely intermediates in these arrangements. 

In accessing chiral allyl vinyl ethers for Claisen rearrangement reactions, Nelson et al. employed the iridium-mediated isomerization strategy. Thus, the requisite enantioenriched diallyl ether substrate 28 was synthesized via a highly enantioselective diethylzinc-aldehyde addition protocol10 (Scheme 1.1k). The enantioselective addition of Et2Zn to cinnamaldehyde catalyzed by (—)-3-exo-morpholinoisobomeol (MIB 26)11 provided an intermediate zinc alkoxide (27). Treatment of 27 with acetic acid followed by 0-allylation in the presence of palladium acetate delivered the 28 in 73% yield and 93% ee. Isomerization of 28 with a catalytic amount of the iridium complex afforded the allyl vinyl ether... 

In the synthesis of racemic 3-phosphoshikimic acid derivatives a palladium-catalyzed rearrangement of a cyclohexenyl phosphate was used for the stereochemical control of the C-3 oxygen function29. This rearrangement is considerably slower than the analogous reactions of allylic acetates and requires a stoichiometric amount of bis(acetonitrile)palladium(II) chloride. 

In a finding of greater practical significance. Overman and coworicers showed that the reactions could be carried out with catalytic amounts of the palladium(II) complex, and that the catalytic effect was broadly applicable to acyclic 1,5-dienes as well7 In a typical example (equation 32), 2-methyl-3-phe-nyl-l,5-hexadiene rearranges in 1 h at room temperature in 87% yield in the presence of 0.06 equiv. of bis(benzonitrile)palladium dichloride, in contrast to the thermal rearrangement which has t n = 13 h at 177 °C. The cat yst thus provides an estimated rate acceleration of about 10 °. The product is a 93 7 mixture of ( )- and (Z)-isomers, corresponding to the equilibrium ratio. Palladium acetate and tetra-kis(triphenylphosphine) were ineffective as catalysts. One serious limitation is that the catalyzed reaction occurs only with those 1,5-dienes which possess an alkyl or aryl substituent at C-2 or C-5 (but not both). 

A polycyclic system with a l,3-dioxa-2-silacyclohepta-4,6-diene fragment was obtained in low yield from bis(diazocarbonyl)silane 671. In the first stage, a biscarbene or a biscarbenoid, generated in the presence of copper triflate or palladium acetate, forms 1,2-diacylcyclopropene, then rearranging into cyclic cumulene triene 672 by migration of a silicon atom to an oxygen atom. The latter dimerizes to [4]-radialen 673 or reacts with furan to afford 674 (Scheme 32) (89AG175()). 

One of the traditional methods for applications of the aliphatic Claisen rearrangement has used Mercuric salts to prepare vinyl ether as illustrated in the use of the allylic alcohol (28) en route to aldehyde 29. The chemical development group at Boehringer-Ingelheim has developd a mild palladium acetate-phenanthroline catalyst 31 for the sequential allyl vinyl ether-Claisen process utilizing commercially available triethyleneglycol divinyl ether which avoids the use of mercuric acetate. ... 

Cooper(I) carboxylates give esters with primary (including neopentyl without rearrangement), secondary, and tertiary alkyl, allylic, and vinylic halides. A simple Sn mechanism is obviously precluded in this case. Vinylic halides can be converted to vinylic acetates by treatment with sodium acetate if palladium(II) chloride is present. ... 

Entry 6 is an example of application of the chiral diazaborolidine enolate method (see p. 572). Entry 7 involves generation of the silyl ketene acetal by silylation after conjugate addition of the enolate of 3-methylbutanoyloxazolidinone to allyl 3,3,3-trifluoroprop-2-enoate. A palladium catalyst improved the yield in the rearrangement... 

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