Pyridines dehydrogenative coupling

Among further specific methods, Newkome s group [99] has found that ethyl pyridin-3-carboxylate (534) underwent the dehydrogenative coupling at unhindered a-position to give the diethyl 2,2 -bipyridine-5,5 -dicarboxylate (535) in 35% yield, upon refluxing with 10% Pd-C under redueed pressure, Scheme 54. 

Pyridines and quinolines are prepared via dehydrogenative coupling of y-aminoalcohols and secondary alcohols in the presence of a bipyridyl-based ruthenium pincer complex (Scheme 6) (13CC6632). A number of cyclic and acyclic secondary alcohols were tolerated under these conditions. 

The bipyridyl herbicide Paraquat is made by reduction of pyridine to radical ions, which couple at the para positions. Oxidation and reaction with methyl bromide gives paraquat. Diquat is formed by dehydrogenation of pyridine and quaternization with ethylene dibromide. 

Pyridine HBr3 oxidizes 5a and 5b to hydrocarbon 70 via transanular bond-formation"1 70 can be dehydrogenated to [2.2]pyrenocyclophane 719S 96 On treatment of 69a-69c with the same reagent in each case the inner rings are coupled first9S). 

The cross-coupling reaction of the ketoxime ester 179 with the alkenylboronic acid leads to the 3-azahexa-l,3,5-triene 181 67t-electrocyclization of the triene 181 affords the 3,4-dihydropyridine 182, whose dehydrogenation by O2 aromatizes to the pyridine 180. Notably, this approach allows flexible control over the substitution pattern of the pyridine system to be synthesized. 

Metal-mediated Reaction. The o -position of this inner salt can undergo dehydrogenative cross-coupling with terminal aUcynes, such as phenylacetylene, under Cu-mediated reaction conditions. The a-alkynyl derivative readily cyclizes in situ with the loss of the Ts group to afford 2-phenylpyrazolo[ l,5-a]pyridine in a 22% yield. This inner salt does not work as well as the analogous l-(benzoylamino)pyridinium inner salt in this reaction. ... 

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