Palladium complexes Overman rearrangement

Cobalt Oxazoline Palladacycles (COPs) are organocobalt-palladium complexes which catalyse the asymmetric rearrangements of non-chiral allylic trichloroacetamidates with very high enantiomeric selectivity (>90%) to provide chiral allylic amines [it is an aza-Claisen rearrangement, The Overman Rearrangement Overman Carpenter Org React 66 2005, Kirsch, Overman and Watson J Org Chem 69 8101 2004] and in the presence of phenols stereospecific cross-couphng also occurs to provide chiral phenoxyallyl ethers with veiy high (>90%) enantiomeric selectivity [Kirsch, Overman and White Org Lett 9 911 2007, Overman Carpenter Org React 66 2005]. 

As a preliminary mechanistic proposal, we hypothesize that the palladimn(ll)-phosphoramidite catalyst acts as a chiral n-acid to activate the amine iV-oxide substrate (Scheme 1S.291. similar to the mechanism proposed for the Overman rearrangement of allylic trichloroacetimidates. While it is not clear whether the reactive species is oxide-bound complex 119a or olefin-bound complex 119b, we propose heterocycle 120 as an intermediate in this cyclization-induced mechanism Grob-type fragmentation eventually reveals O-allylhydroxylamine 117 and the palladium(n)-phosphoramidite catalyst, which can reenter the catalytic cycle. 

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). 

The first examples of the use of cyclopalladated complexes in enantioselective catalysis were reported in 1997 by Overman et al. - ° They applied a series of chiral cyclopalladated complexes to the rearrangement reactions shown in Scheme 5 and found that the palladium catalysts with ferrocene planar chirality exhibited good catalytic activity and enantioselectivity. 

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