Rearrangements, Claisen transition metal catalysts

A number of Claisen rearrangements of hetero-aromatic compounds have been reported, which are also an important means for the preparation of substituted heterocycles [18]. Under thermal conditions, the reaction of 2-allyloxypyridine 13 gave a mixture of N-allylated and C-allylated products 14,15 [19]. A transition metal catalyst is effective for the selective formation of N-allylated product 14 under mild conditions [20]. 

Although most catalysts for the various rearrangements are complexes of the late transition metals, TiCLi catalyzes the amino-Claisen rearrangement shown in equation (39). In this case, the thermal rearrangement gives identical prodnct yields. 

The Claisen rearrangement of 2-allylthiopyridine to l-allyl-2-pyri-dinethione is also catalyzed by transition metals. The role of the catalyst, PdCl2(PhCN)2, is particularly important in this case since the reaction is thermodynamically unfavorable, and presumably the product is stabilized by complex formation. When the product is freed from the metal by addition of excess pyridine, it readily reverts to starting material on heating (80JOC5221). 

The catalytic influence of ammonium chloride on the rate of the reaction was discussed by Claisen in his first report19. Since then, numerous catalysts have been introduced to affect rate enhancements of Claisen rearrangements, e.g., Bronsted and Lewis acids, bases or transition metal complexes. The literature concerning catalytic effects in the Claisen rearrangement has been thoroughly covered until 1984 0,122. 

In contrast, the late transition metal palladium is more coordinative to soft carbon-carbon multiple bonds rather than hard oxygen. The bidentate coordination is further advantageous over the weak monodentate coordination of y,6-unsaturated carbonyl product to set the catalytic cycle. Recently we reported the (R)-DABNTf-Pd(II) complex as an effective catalyst for asymmetric Claisen rearrangement. (R)-DABNTf-Pd(II) catalyst gave the (RyR)-anti-50 in 83% ee (Scheme 35) [171]. 

In sharp contrast, the late transition metals are more coordinative to soft carbon-carbon multiple bonds rather than hard oxygen. These binding modes are further classified into mono- and bi-dentate coordinations, depending on the ligands on the metal catalysts, or the substituent pattern in the Claisen diene systems and solvents employed. Bi-dentate coordination of the Claisen substrate is advantageous over the weak mono-dentate coordination of the Claisen rearrangement product, y,d-unsaturated carbonyl compounds, to release the metal complex allowing the catalytic cycle. Furthermore, enantiodiscrimination by chiral late transition metal complexes is based on the discrimination of two enantiotopic diene faces in the enantiomeric six-membered transition states. 

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