Pyridine-based catalysts 2-hydroxypyridine

Figure 1.13 Generation of rhodium-based supramolecularcatalysts by assembly of pyridine/hydroxypyridine pairs (a) Self-assembly modes of pyridine-based phosphines, (b) Alkene hydroformylation with supramolecular rhodium-diphosphine catalysts (c) CAChe minimized 3D structure ofthe rhodium-diphosphine complex (other ligands from the metal omitted for clarity).

Swain (114) has recently repeated the experiments of Lowry, using pyridine as the nucleophilic group (N) and phenol as the electrophilic group (E) and Swain and Brown (115) have gone one step further, using 2-hydroxypyridine, a weaker base than pyridine, which has both (N) and (E) groups on the same molecule. They find 2-hydroxypyridine a still more powerful catalyst than the mixture of phenol and pyridine. The... 

Park and coworkers developed a highly efficient procedure for the synthesis of 3-hydroxypyridines in 2012 [15]. With dirhodium complex Rh2(tfacam)4 (tfacam = trifluoroacetamide) as the catalyst and 6-diazo oxime ethers as substrates, based on carbenoid-mediated N—O bond insertion, a variety of pyridines were produced in 5—10 min in good to excellent yields. Recently, they reported their new achievements on transformation of a-diazo oxime ethers to pyridines with rhodium as the catalyst (Scheme 2.10). 

The findings of Swain and Brown 55) support the ternary mechanism proposed by Lowry for the mutarotation of tetra-O-methylglucose. It was found that the mutarotation of tetra-O-methylglucose in benzene in the presence of both an acid (phenol) and a base (pyridine) followed third-order kinetics but was first-order with respect to each component tetra-O-methyl-glucose, pyridine, and phenol 55). 2-Hydroxypyridine was found to be a very effective bifunctional catalyst, and since both acid and base functions were in the same molecule, the mutarotation followed second-order kinetics. Its catalytic action was essentially independent of the other acid and base species present. Although it is a much weaker acid or base than either phenol or pyridine, its catalysis of the mutarotation of tetra-O-methylglucose in benzene was much greater than that of either pyridine or phenol, or a mixture of both 55). 

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