Pyridines transfer hydrogenation

The pyridine nucleotide dehydrogenase reactions fall into two large groups those which transfer hydrogen from or to the A (or pro R) side of the 4 position of the nicotinamide ring, and those which use the B (or pro S) side 1.64,65),... 

Furo[3,4-7]pyridines can be prepared in a stereoselective synthesis involving a ruthenium-catalyzed asymmetric transfer hydrogenation reaction <2001TL1899>. The reaction proceeds with exceptionally high yield and ee (Equation 50). 

Table 7 Scope of the enantioselective transfer hydrogenation of pyridines...

Cascade Transfer Hydrogenation of Quinoline and Pyridine Derivatives... 

Scheme 3.48 Transfer hydrogenation of pyridine and 1,10 phenanthroline derivatives.

In this transfer hydrogenation, aromatization of the dihydropyridine (Hantzsch ester) to form a pyridine derivative is essential for it to act as the hydride source. 

A method has been described for the preparation of N-formyl-a-amino-acid t-butyl esters by formylation of the free base with a mixture of formic acid, DCC, and pyridine, thus overcoming the incompatibility of the t-butyl ester group with the usual acidic formylation conditions. The transfer hydrogenation technique has been recommended for the removal of N-benzyloxycarbonyl and benzyl ester groups from peptides. N-Acetyl-a-amino-acids are converted into the corresponding A-ethyl derivatives by reaction with Meerwein s reagent followed by reduction with sodium borohydride. ... 

A new approach to stereoselective transfer hydrogenation of imines was the application of chiral phosphoric acid esters as organocatalysts [50-52]. The mechanism is based on the assumption that the imine is protonated by a chiral Bronsted acid, which acts as the catalyst. The resulting diastereomeric iminium ion pairs, which may be stabilized by hydrogen bonding, react with the Hantzsch dihydropyridine at different rates to give an enantiomerically enriched amine and a pyridine derivative [50-52]. The exact mechanism is still under discussion however computational density functional theory (DFT) studies ]53, 54] suggest a three-point contact model. ... 

The post-synthetic methodology has also been used to incorporate catalytic centres. The incorporation of chiral l,l -bi-2-naphthols (BINOLs) into CMPs has been used to perform catalytic reactions using the alcohol groups to bind to catalytically active centres such as phosphoric acids for asymmetric transfer hydrogenations or titanium for diethyl-zinc additions. Other metal-containing functionalities have also been incorporated into CMP networks. The coordination of metals to pyridine or phenylpyridine via Sonogashira reactions has also been reported for catal5Aic transformations such as reductive amination or aza-Henry reactions, a-atylations and oxyaminations. ... 

In addition to quinolines, pyridines were also found to be suitable substrates for this cascade reduction. Chiral tetrahydropyridines 36 and 37 were synthesized with up to 92% ee by employing (R)-7 as the catalyst [25]. Phenanthroline, which has the same functional group as quinoline and pyridine, was reported by Metallinos et al. to undergo transfer hydrogenation to the corresponding octahy-drophenanthroline 38 with up to 99% ee, whereas most other substrates provided moderate ee [26]. 

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