Dimerization of pyridine

Figure 13 Dimers of pyridine showing configurational isomerism of the cis conformers.

The influences at work on these interactions have been discussed in two important papers (80JOC1347, 80JOC1354). It is found that influences of media can be expressed in terms of two effects, differential stabilization of the different dipoles of the two tautomeric species by the dielectric constant of the medium, the dipole reaction field and by differential hydrogen bonding. For self-association pyridin-4-one is shown to form oligomers, in contrast to the well-known dimerization of pyridin-2-ones. This self-association can shift the position of apparent tautomeric equilibrium (hence the warnings previously noted about effect of concentration on Kr). 

In summary, although radical-anion formation in the dimerization of pyridine has been observed by ESR, the nature of the overall pathway (anionic or radical) remains to be elucidated. 

Bipyridyl (20) (63AHC(2)179) is synthesized by oxidative dimerization of pyridine over hot Raney nickel, while 4,4 -bipyridyl (21) (B-79MI10705) is made by free radical coupling of the pyridine radical anion generated by sodium in liquid ammonia, followed by air oxidation (Scheme 6). Quaternization of (21) with methyl chloride gives paraquat while reaction of (20) with 1,2-dibromoethane gives diquat. 

Measurements on copper) I) chloride show the vapour to be the dimer of formula CU2CI2, but molecular weight determinations in certain solvents such as pyridine show it to be present in solution as single molecules, probably because coordination compounds such as py -> CuCl (py = pyridine) are formed. 

These results show that in the phenylation of thiazole with benzoyl peroxide two secondary reactions enter in competition the attack of thiazole by benzoyloxy radicals, leading to a mixture of thiazolyl benzoates, and the formation of dithiazolyle through attack of thiazole by the thiazolyl radicals resulting from hydrogen abstraction on the substrate and from the dimerization of these radicals. This last reaction is less important than in the case of thiophene but more important than in the case of pyridine (398). 

Unusual heterocyclic systems can be obtained by photodimerizations and for five-membered heterocycles with two or more heteroatoms such dimerizations need be effected on their ring-fused derivatives. Cyclobutanes are usually obtained as in the photodimerization of the s-triazolo[4,3-a]pyridine (540) to the head-to-head dimer (541). These thermally labile photodimers were formed by dimerization of the 5,6-double bond in one molecule with the 7,8-double bond in another (77T1247). Irradiation of the bis( 1,2,4-triazolo[4,3-a]pyridyl)ethane (542) at 300 nm gave the CK0ifused cyclobutane dimer (543). At 254 nm the cage-like structure (544) was formed (77T1253). 

Pyridine, and its monomethyl and 3,5-dimethyl derivatives " combine exothermically with dimethyl acetylenedicarboxylate in ether yielding some ether soluble materials, including trimethyl pyrrocoline-1,2,3-tricarboxylate (Section III,F,3) and deep red ether-insoluble gums. A number of crystalline compounds have been isolated from these gums by fractional crystallizations and will now be considered in detail. In the case of pyridine, Diels et al. ° isolated a red labile 1 2 molar adduct, which they formulated as (75), which isomerized rapidly on standing to a yellow stable adduct (76). These formulations are no longer accepted. Diels and Alder also suggested that the acetylenic ester first dimerized to the diradical (74) which then combined with the pyridine. 

The formation of pyridine 210 appears to start with dimerization of aminobutenone 207 due to carbonyl-amino group interaction. Then the intermediate 208 undergoes [3,3]-sigmatropic rearrangement, whereupon dihydropyridine 209 eliminates ammonia. 

Chloro-l//-l-benzazepines 2 are obtained as unstable red oils in excellent yields by heating 1 //-l-benzazepin-2(3//)-ones 1 with phosphoryl chloride in pyridine.208 Reaction conditions are important since in the absence of pyridine, or in dichloromethane solution, only poor yields of dimers, e.g. 3, are produced. The chlorobcnzazepines are stable for only short periods (24 hours in anhydrous pyridine) and rapidly polymerize. Isolation of the pure chloro compounds is difficult since they undergo very rapid hydrolysis to the benzazepinones. 

Treatment of UCI4 with the lithium complex obtained from dicyclohexylcar-bodiimide followed by crystallization from pyridine afforded a dinuclear uranium(rV) oxalamidinate complex in the form of dark green crystals in 94% yield (Scheme 191). The same compound could also be obtained by first reducing UCI4 to LiUCli (or UQs+LiCl) followed by reductive dimerization of di(cyclo-hexyl)carbodiimide as shown in Scheme 191. The molecular structure of this first oxalamidinato complex of an actinide element is depicted in Figure 31. ° ... 

A head-to-head dimerization of a-olefin catalyzed by a bis(imino)pyridine iron complex has been reported by Small and Marcucci [126]. This reaction delivers linear internal olefins (up to 80% linearity) from a-oleftns. The linearity of products, however, depends on the catalyst structure and the reaction conditions. 

The limitation of availability of coordination sites in the allyl compounds is further illustrated by the fact that Zr(allyl3)Br dimer reacts with only one molecule of pyridine (without a valency change) to form the monomeric... 

Cycloadditions are in general an effective way of constructing cyclobutane rings. A wide variety of heterocyclic systems dimerize in this way. 1,3-Diacetylindole, for example, affords the head-to-tail dimer 242 on irradiation in ethanol.185 Ethyl 2-ethoxy-l,2-dihydroquinoline-l-carboxy-late is similarly converted in diethyl ether into the trans head-to-head dimer.186 Notable among many analogous photodimerizations are those reported in 1,4-dihydropyridines,187 in furo[3,2-b]pyridin-2(4//)-ones,188 in 8-methyl-s-triazolo[4,3-a]pyridine,189 and in 2H-2-benzazepine-1,3-diones.190 The [ 2 + 2] dimerization of amidopyrine is the first reported example of a photocycloaddition in a 4-pyrazolin-3-one.191... 

Ifcobs is directly proportional to pyridine concentration. Therefore a plot of kobs vs. [pyridine] is linear, with a slope (k ) equal to the second order rate constant for ylide formation, and an intercept (k0) equal to the sum of all processes that destroy the carbene in the absence of pyridine (e.g.) intramolecular reactions, carbene dimerization, reactions with solvent, and, in the case of diazirine or diazo carbene precursors, azine formation. 

These routes are dimerization to furoxans 2 proceeding at ambient and lower temperatures for all nitrile oxides excluding those, in which the fulmido group is sterically shielded, isomerization to isocyanates 3, which proceeds at elevated temperature, is practically the only reaction of sterically stabilized nitrile oxides. Dimerizations to 1,2,4-oxadiazole 4-oxides 4 in the presence of trimethylamine (4) or BF3 (1 BF3 = 2 1) (24) and to 1,4,2,5-dioxadiazines 5 in excess BF3 (1, 24) or in the presence of pyridine (4) are of lesser importance. Strong reactivity of nitrile oxides is based mainly on their ability to add nucleophiles and particularly enter 1,3-dipolar cycloaddition reactions with various dipolarophiles (see Sections 1.3 and 1.4). 

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