Allylic Rearrangement and Isomerization

Rearrangement of allylic esters is catalysed by both Pd(II) and Pd(0) compounds, although their catalyses differ mechanistically. Allylic rearrangement of allylic acetates takes place by Pd(0) formed from Pd(OAc)2-Ph3P as a catalyst [191]. An equilibrium mixture of 453 and 454 in a ratio of 1.9 1.0 was obtained [192]. The Pd(0)-Ph3P-catalysed rearrangement is explained by the formation of a 7i-allylpalladium complex. The rearrangement of y -tolylsulfone from tertiary 455 to primary 456 occurs [193]. 

Rearrangement of the acetate of the optically active cyanhydrin 463 proceeds stereoselectively to yield the oc,/ -unsaturated nitrile 464 with 89% ee [197]. Allyl trichloroacetimidate 465 is rearranged from O to N to give 466 at room temperature with retention of the stereochemistry [198]. 

Cope and Claisen rearrangements proceed under milder conditions in the presence of Pd(II) catalyst [194,199]. Cope rearrangement of the linear 1,5-diene 467 to 468 

Mechanistic studies on the Pd(II)-catalysed Cope rearrangement of the deuterated 1,5-diene 471 has been carried out. After coordination of the alkene to PdCl2, the 4-palladacyclohexyl cation 472 is formed by carbopalladation as the rate determining step. The isomerization ends with the cleavage of 472 to afford 473 [202], 

PdCl2(PhCN)2-catalysed Claisen rearrangement of the allyl vinyl ether 474 derived from cyclic ketone at room temperature affords the syn product 475 with high diastereoselectivity [203]. In contrast to thermal Claisen rearrangement, the Pd(II)-catalysed Claisen rearrangement is always stereoselective, irrespective of the geometry of allylic alkenes. The anti product is obtained by the thermal rearrangement in the presence of 2,6-dimethylphenol at 100 °C for lOh. 

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