3,3 -Disubstituted 2,2 -bipyridine

In the absence of an added solvent, 3-alkyIpyridines, 4-alkyl-pyridines, and 3,4-dialkylpyridines all gave yields of substituted 2,2 -bipyridines that were up to three times greater than that of 2,2 -bipyridine from pyridine under similar conditions. With 3-ethyl-4-methylpyridine a marked improvement in yield was ob.served when the reaction was carried out at about 150°C in a vacuum, rather than at the atmospheric boiling point (195°C) of this base. This effect has also been observed with some other bases but the amount of 3,3, 5,5 -tetramethy 1-2,2 -bipyridine from 3,5-lutidine could not be increased in this way, and this pyridine was as unreactive as the 2-substituted pyridines. This finding is undoubtedly related to the reluctance of 3-substituted pyridines to form 3,3 -disubstituted 2,2 -bipyridines. 

Varela, J.A., Castedo, L., Maestro, M., Mahia, J. and Saa, C. (2001) Regiocontrolled one-step synthesis of 3,3 -disubstituted 2,2 -bipyridine ligands by cobalt(I)-catalyzed cyclotrimerization. Chemistry—A European Journal, 7(23), 5203-5213. 

The facility for 4-metalation shown by 3-substituted pyridines is continued with bipyridyl derivatives, and thus 3,3 -bis[2-(trimethyIsilyl)ethoxy]-methyl-2,2 -bipyridine gave the 4,4 -dilithio species on treatment with n-BuLi, and this then afforded 4,4 -disubstituted bipyridines, often in very good yield (Scheme 111)(88HCA957). 

Fig. 6. Chemical structure of the investigated metallosurfactants with two (left) and one (middle) disubstituted bipyridines. Right room temperature time-resolved emission decays of complex 12 in n-hexane (sohd line, —) and upon addition of one drop of ethanol (dotted line, ---). Emission was monitored at 644 and 623 nm, respectively.

Figure 2.31 Molecular structure of Eu(F )3 (dmphen)(EtOH)dmphen. (Displacement ellipsoids for non-FI atoms are shown at the 50% probability level and H atoms are represented by circles of arbitrary size) [26b]. (Reprinted from Inorganica Chimica Acta, 360, C.R. De Silva, J.R. Maeyer, R. Wang, GS. Nichol, Z. Zheng, Adducts of europium -diketonates with nitrogen p,p -disubstituted bipyridine and phenanthroline ligands Synthesis, structural characterization, and luminescence studies, 3543-3552, 2007, with permission from Elsevier.)...

Because bipyridines substituted at the 3 and 3 positions exhibit a large steric repulsion between substituents while in the cis configuration,27 they bind metals more weakly and form strained, nonplanar structures.28 However, a series of 3,3 -disubstituted bipyridines were coordinated to ruthenium, and it was demonstrated that molecular distortions could be used to advantage in modulating physical properties of the resulting complexes.29... 

Figure 26 Threading of the disubstituted bipyridine (26) leading to the complex Ru(25.26) and subsequent...

To solve these problems, we developed the synthesis of symmetrically disubstituted bipyridine ligands 150, which possess two electropolymerizable bithiophenic groups fixed at an internal (3-position of thiophene by an alkylsuUknyl or an alkoxy spacer [161]. The analysis of the electropolymerization of these compounds shows that the association of low oxidation potential polymerizable groups and two-site precursors allows us to synthesize stable functional polymers. 

Four 2-substituted pyridines were found to give the expected 6,6 -disubstituted 2,2 -bipyridines in yields corresponding to only about 3% of the amount of 2,2 -bipyridine formed from pyridine itself under comparable conditions. It is also of interest that with three 2-methyl-pyridines the expected 6,6 -dimethyl-2,2 -bipyridines were accompanied by smaller amounts of 2,2 -bipyridines having no methyl groups in the 6,6 -positions. Moreover, a very small amount of 5,5 -dimethyl-2,2 -bipyridine (8) was isolated following reaction with 2,5-lutidine (6) but no 3,3 -dimethyl-2,2 -bipyridine could be detected. The absence of this compound suggests that 3,3, 6,6 -tetramethyl-2,2 -bipyri-dine (9) is not an intermediate, but that the 2-methyl group is lost before the formation of the 2,2 -bipyridine (6—>8). 

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 -... 

With films based on a 2,2 -bipyridine ligand which is 4,4 -disubstituted with electron-with-drawing carboxyester groups the selectivity is drastically changed, since HCOO" is obtained with a high yield (p >90%) at a very low overvoltage (—0.75 V vs. AglAgCl).98 This difference was... 

A number of syntheses of substituted 2,3 -bipyridines are worthy of note. Tetracyclone heated at 215°C with nicotinonitrile affords 3,4,5,6-tetraphenyl-2,3 -bipyridine, whereas 3,4-di(2-pyridyl)pyridine is obtained by an oxidative degradation of the corresponding 6,7-disubstituted thiazolo[3,2-a]-pyridinium salt. Nicotinic acid on UV irradiation in aqueous solution at pH 4-6 gives 2,3 -bipyridine-5-carboxylic acid, whereas irradiation of picolinic acid in the same pH range in the absence of metal ions gives some 2,3 -bipyridine. 6,6 -Diphenyl-2,3 -bipyridine is thought to be formed from... 

Schubert, U. S. Kersten, J. L. Pemp, A. E. Eisenbach, C. D. Newkome, G. R. A new generation of 6,6 -disubstituted 2,2 -bipyridines towards novel oligo(bipyridine) building blocks for potential applications in materials science and supramolecular chemistry. Eur. J. Org. Chem. 1998, 2573-2581. 

Both the emission quantum yields and lifetimes are significantly increased on cooling the solution to 77 K, implying that the radiative decay pathways are favored in the more rigid environment the emission lifetimes are typically governed by the energy gap law [17, 20, 74, 75], Table 3 summarizes the excited-state decay parameters for the MLCT excited states of /ac-tClRe1, 4 -(X)2-bpy)(CO)3] complexes (where 4,4 -(X)2-bpy is 4,4 -disubstituted-2,2 -bipyridine). 

The ring opening of bitriazolopyridines, (15), with the addition of various reagents leads to the formation of 6,6 -disubstituted bpys.54,55 Bipyridines substituted with a secondary alcohol, an ester linkage, and a ketone were synthesized using sulfuric acid, acetic acid, and selenium dioxide, respectively (Scheme 8).54... 

The low yields of 6,6 -disubstituted-2,2 -bipyridines recorded in Table I are probably the result of steric retardation of the adsorption of 2-substituted pyridines. This view is supported by the observation that 2-methylpyridine is a much weaker poison for catalytic hydrogenations than pyridine.15 On the other hand, the quinolines so far examined (Table II) are more reactive but with these compounds the steric effect of the fused benzene ring could be partly compensated by the additional stabilization of the adsorbed species, since the loss of resonance energy accompanying the localization of one n-electron would be smaller in a quinoline than in a pyridine derivative. 

As an example, it was reported that the TfO salt of the tetracoordinated silver(l) complex with 2,2 -bipyridine-4,4 -disubstituted ligands 33 yielded a novel ionic liquid crystal displaying columnar mesomorphism [69]. The coordination and the ionic bonds drive the supramolecular assembly because the ligand is nonmesogenic or smectic, but when coordinated to the metal, it gives columnar mesomorphism... 

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