Palladium oxidative rearrangment

The palladium-catalyzed reductive N-heteroannulation using carbon monoxide has been reported (Scheme 102). As part of this study, substrate 551 was examined in an attempt to synthesize 552. However, 552 was found to be unstable and underwent further oxidative rearrangement in air to form spiro compound 555 via intermediates 553 and 554 <2006T10829>. 

The process was later improved by the use of a p-toluenesulfonyl substituent at the allylic carbon atom (equation 42). The authors claim that this modification has a powerful influence on both the selectivity and mechanism of the oxidation, exclusive oxidative rearrangement then being observed. Several other methods of achieving allylic oxidation using palladium catalysts have also been reported,although these are generally of less importance. 

A related oxidative rearrangement of cephem dioxides has been reported in which an alkene is oxidized stereospecifically with rearrangement to the allylic alcohol in good yield by simple exposure to a palladium/caibon catalyst, as depicted in equation (12). Adventitious oxygen preadsotbed on the catalyst seems the likely oxida The reaction fails on the parent ccphem or its monoxide, or on the free acid of the dioxide. This reaction would seem to hold some promise for furdier utility in the cephem field and odier related systems. 

The oxidative rearrangement of cyclic alkenes and ketones often leads to ring expansion or ring contraction reactions. The reagents generally used for this purpose are hypervalent main group oxidants such as thallium(III), lead(lV), iodine(III) and selenium(IV), alAough palladium(II) has been used as well. 

Hydroxyketones are versatile intermediates in the synthesis of pharmaceutical intermediates and heterocyclic molecules. a-Aryl hydroxyketones have been prepared by reaction of aryl aldehydes with 1,4-dioxane followed by reduction with lithium aluminum hydride (LAH) and by the selective LAH reduction of a-silyloxy a,P-unsaturated esters." WissneC has shown that treatment of acid chlorides with tris(trimethylsilyloxy)ethylene affords alkyl and aryl hydroxymethyl ketones. 1-Hydroxy-3-phenyl-2-propanone (3) has been generated by the osmium-catalyzed oxidation of phenylpropene and by the palladium-catalyzed rearrangement of phenyl epoxy alcohoP both in 62% yield. 

Oxidative palladium-catalysed rearrangement of diaryl alkenyl carbinols to /1,/1-diaryl Q ,j8-unsaturated ketones occurs with the geometry of the alkene product determined by the substitution pattern on the aryl rings (Scheme 139). ... 

A similar process is also possible using diphenylsulfoxide as the oxygen atom source, which is transferred to a palladium(II) carbenoid intermediate however, the oxidative rearrangement... 

The 5-oxohexanal 27 is prepared by the following three-step procedure (1) 1,2-addition of allylmagnesium bromide to an a, / -unsaturated aldehyde to give the 3-hydroxy-1,5-diene 25, (2) oxy-Cope rearrangement of 25 to give 26, and (3) palladium catalyzed oxidation to afford 27. The method was applied to the synthesis of A -2-octalone (28), which is difficult to prepare by the Robinson annulation[25]. 

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