Dimerization pyridines

While benzene and substituted benzenes usually do not dimerize, pyridine derivatives undergo [4+4]photodimerizations432) (4.29). 

Fig. 30 a Triple strand porphyrin array by Anderson [94]. The bottom picture shows the electronic absorption spectra of (a) the free base dimer, (b) the Zn-dimer-(pyridine)2 complex and (c) 37 in DCM. Spectrum (b) was recorded in the presence of excess pyridine and is scaled x2 to facilitate comparison with (c). Arrows highlight regions of increased or decreased absorption in the complex 37. Reproduced by permission of The Royal Society of Chemistry, b Triporphyrin bound to a hexameric wheel 38 [96]... 

Industrially, the most important derivatives are dimeric pyridines in the form of 4,4 -bipyridyl and 2,2 -bipyridyl. 2,2 -Bipyridyl can be synthesized by oxidative dimerization or by bromination to 2-bromopyridine and reaction with nonferrous metal catalysts such as copper. 

The monomers are electron pair acceptors, and donor molecules are often able to split the dimeric halide molecules to form adducts thus, whilst the dimeric halides persist in solvents such as benzene, donor solvents such as pyridine and ether appear to contain monomers since adduct formation occurs. Aluminium halides, with the one exception of the fluoride, resemble the corresponding boron halides in that they are readily hydrolysed by water. 

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. 

Pyridyl)hydrazine (Aldrich), 4-acetylpyridine (Acros), N,N,N -trimethylethylenediamine (Aldrich), methylrhenium trioxide (Aldrich), InQj (Aldrich), Cu(N0j)2-3H20 (Merck), Ni(N03)2-6Il20 (Merck), Yb(OTf)3(Fluka), Sc(OTf)3 (Fluka), 2-(aminomethyl)pyridine (Acros), benzylideneacetone (Aldrich), and chalcone (Aldrich) were of the highest purity available. Borane dimethyl sulfide (2M solution in THE) was obtained from Aldrich. Methyl vinyl ketone was distilled prior to use. Cyclopentadiene was prepared from its dimer immediately before use. (R)-l-acetyl-5-isopropoxy-3-pyrrolin-2-one (4.15) has been kindly provided by Prof H. Hiemstra (University of Amsterdam). 

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). 

Dimerization is reportedly catalyzed by pyridine [110-86-1] and phosphines. Trialkylphosphines have been shown to catalyze the conversion of dimer iato trimer upon prolonged standing (2,57). Pyridines and other basic catalysts are less selective because the required iacrease ia temperature causes trimerization to compete with dimerization. The gradual conversion of dimer to trimer ia the catalyzed dimerization reaction can be explained by the assumption of equiUbria between dimer and polar catalyst—dimer iatermediates. The polar iatermediates react with excess isocyanate to yield trimer. Factors, such as charge stabilization ia the polar iatermediate and its lifetime or steric requirement, are reported to be important. For these reasons, it is not currently feasible to predict the efficiency of dimer formation given a particular catalyst. 

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, 6-cyano-l,2-dihydro-thermal dimerization, 2, 370 Pyridine, 2-cyanomethyl-tautomerism, 2, 159 Pyridine, 4-cyanomethyl-tautomerism, 2, 159 Pyridine, 2-cyano-2,3,4,5-tetrahydro-metallation, 2, 387 Pyridine, 2,5-diacetyl-ipso substitution, 2, 301 Pyridine, 3,5-diacetyl-l,4-dihydro-Hantzsch synthesis, 2, 482 Pyridine, 4-dialkylamino-as acylation catalysts, 2, 34 Pyridine, 2,2-dialkyl-l,2-dihydro-... 

Note 2. Under acidic conditions and in the presence of water, the 18,20-hemiacetal formed from the 18-iodo-18,20-ether is transformed into a dimeric anhydro product, resistant to further oxidation. Pyridine is added to prevent this transformation. 

Monomer-oligomer equilibria. [Ni(Me-sal)2], mentioned above as a typical planar complex, is a much studied compound. In pyridine it is converted to the octahedral bispyridine adduct (/zsoo = 3.1 BM), while in chloroform or benzene the value of is intermediate but increases with concentration. This is ascribed to an equilibrium between the diamagnetic monomer and a paramagnetic dimer, which must involve a coordination number of the nickel of at least 5 a similar explanation is acceptable also for the paramagnetism of the solid when heated above 180°C. The trimerization of Ni(acac)2 to attain octahedral coordination has already been referred to but it may also be noted that it is reported to be monomeric and planar in dilute chloroform solutions. 

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. 

The Michael dimerization (activated double bond-amino group interaction) affords the intermediate 211 whose tautomeric form 212 closes the tetrahydropy-ridine cycle 213 which undergoes aromatization with elimination of water and ammonia to isomeric pyridine 214. 

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. 

Trifluoroacetohydroximoyl bromide etherate is a synthon for thiadiazo-lines and oxadiazolines (87JHC1391). A fused isoxazolopyridine was obtained with excess malononitrile but, unexpectedly, dimeric malononitrile gave a highly substituted 2-(CF3)-pyridine, albeit in 9% yield (87BCJ4480). 

Unsaturated nitriles Dicyanobutenes Methyl pyridines Ethyl disulfides (HSSH) Acrylonitrile dimers... 

Ru04 reacts with pyridine to form Ru03(py), probably a dimer Py2(0)2Ru(jU-0)2Ru(0)2Py2, an aerobically assisted oxidant [48c]. 

The activation of silylene complexes is induced both photochemically or by addition of a base, e.g. pyridine. A similar base-induced cleavage is known from the chemistry of carbene complexes however, in this case the carbenes so formed dimerize to give alkenes. Finally, a silylene cleavage can also be achieved thermally. Melting of the compounds 4-7 in high vacuum yields the dimeric complexes 48-51 with loss of HMPA. The dimers, on the other hand, can be transformed into polysilanes and iron carbonyl clusters above 120 °C. In all cases, the resulting polymers have been identified by spectroscopic methods. 

Whether these results will also have an impact on the theory of metallaallenes is difficult to predict at least for the compounds Cp (CO)2Mn = M = Mn(CO)2Cp, (M = Ge, Sn, Pb) a linear structure is established and also linear p-carbido complexes are known [198], Recently, a germanium compound has been synthesized which is directly comparable with 22. In this case, the starting material for the synthesis is not a monomeric base adduct, but a dimeric germylene complex which is cleaved by Na2Fe(CO)4 in pyridine to form 72 [199],... 

Quinone diazides can also be obtained by the diazo group transfer reaction of 4-tosyl azide. For example, 9-diazo-10-anthrone (2.55) is formed from anthrone (2.54) if the reaction is carried out in an ethanol-piperidine mixture. On the other hand, if ethanol is replaced by pyridine, dimerization with loss of molecular nitrogen takes place and the azine 2.56 is isolated (Scheme 2-32 Regitz, 1964 Cauquis et al., 1965). In the preceding discussion tosyl azide was shown to be an electrophilic reagent. It therefore seems likely that it is not the anthrone 2.54 but its conjugate base which reacts with tosyl azide. 

Aroyl-pyridine werden mit Titan(III)-chlorid/Lithiumalanat mit bis zu 93% d.Th. zu den entsprechenden Alkoholen reduziert. Als Nebenprodukt fallen dimere Verbindungen an z.B.5 ... 

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