Pyridine-based catalysts bipyridine

In 1960 Rapoport and his co-workers found that some 2,2 -biquinoline is formed when quinoline w as used as a solvent for dehydrogenations in the presence of palladiuin-on-carbon catalyst, and they showed that several related bases (including pyridine) gave 2,2 -biai yls when refluxed at atmospheric pressure with a 5% pal-ladium-on-carbon catalyst. With a pyridine-to-catalyst ratio of 10 1, 11% conversion of pyridine to 2,2 -bipyridine was observed after heating for 24 hr. 

Rhodium-on-carbon has also been found to bring about the formation of 2,2 -biquinoline from quinoline, the yield and the percentage conversion being similar to that obtained with palladium-on-carbon. On the other hand, rhodium-on-carbon failed to produce 2,2 -bipyridine from pyridine, and it has not yet been tried with other bases. Experiments with carbon-supported catalysts prepared from ruthenium, osmium, iridium, and platinum have shown that none of these metals is capable of bringing about the formation of 2,2 -biquinoline from quinoline under the conditions used with palladium and rhodium. ... 

It would be expected that the stabilization of the adsorbed species by an extended conjugated system should increase with the number of aromatic rings in the adsorbed azahydrocarbon. However, data suitable for comparison are available only for phenanthridine, benzo-[/]quinoline, and benzo[h] quinoline. The large difference in the yields of biaryl obtained from the last two bases could be caused by steric interaction of the 7,8-benz-ring with the catalyst, which would lower the concentration of the adsorbed species relative to that with benzo[/]quinoline. The failure of phenanthridine to yield any biaryl is also noteworthy since some 5,6-dihydrophenanthridine was formed. This suggests that adsorption on the catalyst via the nitrogen atom is possible, but that steric inhibition to the combination of the activated species is involved. The same effect could be responsible for the exclusive formation of 5,5 -disubstituted 2,2 -dipyridines from 3-substi-tuted pyridines, as well as for the low yields of 3,3, 5,5 -tetramethyl-2,2 -bipyridines obtained from 3,5-lutidine and of 3,3 -dimethyl-2,2 -... 

If it is assumed that 2,2 -bipyridine is bonded to the catalyst by both nitrogen atoms, then the position of the chemisorbed molecule on the metal is rigidly fixed. Unless two molecules of this base can be adsorbed at the required distance from each other and in an arrangement which is close to linear, overlap of the uncoupled electrons at the a-position cannot occur. The failure to detect any quaterpyridine would then indicate that nickel atoms of the required orientation are rarely, if ever, available. Clearly the probability of carbon-carbon bond formation is greater between one chemisorbed molecule of 2,2 -bipyridine and one of pyridine, as the latter can correct its orientation relative to the fixed 2,2 -bipyridine by rotation around the nitrogen-nickel bond, at least within certain limits. 

The inherent difference in the reactivity of the 2-, and 3-positions of the pyridine ring was also exploited in an industrial application of the carbonylative coupling of pyridines. 2,5-Dibromo-3-methylpyridine was converted into the 2-monoamides using different amines with a 98 2 selectivity. Keys to the success of the coupling, which was run on the 100 kg scale, were the use of DBU as base and the replacement of the phosphine in the catalyst with 2,2 -bipyridine.84 The carbonylation of 3,5-... 

Boron-substituted pyridine reagents can be used to construct the bipyridine ring system by coupling them with halopyridines in the presence of a Pd° catalyst and a base (Suzuki method). Various ligands have been made in this manner in moderate to high yields, including 2,3-bipyridine (85%)41 and 3,5-dimethyl bipyridine (60%) 42 One valuable feature of the Suzuki method is that it is compatible with stannanes. A pyridyl diethylborane has been coupled to a tributyl tin-functionalized pyridyl bromide.43 This compatibility is useful for polypyridine syntheses because subsequent Stille coupling of the bipyridyl stannane is possible. 

On oxidation with O2 catalyzed by a Co(II) complex of 6,6 -bis(benzoylamino)-2,2 -bipyridine (233) in toluene containing an appropriate base such as pyridine (20 °C, 24 h), a quantitative conversion of 23 to 74 was observed. In addition, the durability of this complex as an oxygenation catalyst is much higher than that of 229 [Co(salen)]. Furthermore, the catalytic activity of 233 can be restored by heating it to 200 °C under reduced pressure because of its high thermal stability . ... 

The best known catalysts of the water gas shift reaction are the binary metal carbonyls ([Fe(CO)5], [M(CO)6] M = Cr, Mo, W [342,345-347]) and carbonyl clusters ([Ru3(CO)i2], [Rh fCO) ], [Ir4(CO)i2]) [347], In the basic solutions under 35-120 bar CO, elevated temperature (100-125 °C) is needed for reasonable catalytic activity. Often these complexes are used in solutions of nitrogen-containing bases (amines, pyridine or picolines) or the precursors are themselves complexes of N-donor ligands (substituted bipyridines and phenantrolines [348], pyridine, 2-, 3- or 4-picoline [349],... 

Elucidation of the kinetics of the phenylselenoetherification of CH2= CHCH2CH2CH2OH with PhSeCl in the presence of a base (pyridine, triethylamine, quinoline, 2,2 -bipyridine) as catalyst, using UV-VIS spectrophotometry, revealed that the cyclization is facilitated by a hydrogen bonding between the base and the OH group. Calculations at the (MP2(fc)/6-311+G //B3LYP/6-311+G ) level suggest an 5N2-like mechanism for the transition state of the cyclization step. ... 

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