Carbon-hydrogen bonds pyridine

Oxidation of methylpyridines in 60-80 % sulphuric acid at a lead dioxide anode leads to the pyridinecarboxylic acid [213]. The sulphuric acid concentration is critical and little of the product is formed in dilute sulphuric acid [214]. In these reactions, electron loss from the n-system is driven by concerted cleavage of a carbon-hydrogen bond in the methyl substituent. This leaves a pyridylmethyl radical, which is then further oxidised to the acid, fhe procedure is run on a technical scale in a divided cell to give the pyridinecarboxylic acid in 80 % yields [215]. Oxida-tionof quinoline under the same conditions leads to pyridine-2,3-dicarboxylic acid [214, 216]. 3-HaIoquino ines afford the 5-halopyridine-2,3-dicarboxylic acid [217]. Quinoxaline is converted to pyrazine-2,3-dicarboxylic acid by oxidation at a copper anode in aqueous sodium hydroxide containing potassium permanganate [218]. 

This gas-phase reaction provides the only method currently available for determining hydrogen-bonded pyridine free base. The method requires neither assumptions, extrapolations, nor approximations the p factor of the reaction is sufficiently small that the reactivities of the pyridine and benzene derivatives can be compared directly. The method was first introducted for determining the electrophilic reactivity of pyridine [62JCS4881 71JCS(B)2382] using 1-arylethyl acetates (9.109). Subsequent determinations used I-aryl-l-methylethyl acetates. (9.110) and i-arylethyl methyl carbonates (9.111) [79JCS(P2)228],... 

NAD oxidizes a compound by accepting a hydride ion from it. In this way, the number of carbon-hydrogen bonds in the compound decreases (the compound is oxidized) and the number of carbon-hydrogen bonds in NAD increases (NAD is reduced). NAD can accept a hydride ion at the 4-position of the pyridine ring because the electrons can be delocalized onto the positively charged nitrogen atom. Although NAD could also accept a hydride ion at the 2-position, the hydride ion is always delivered to the 4-position in enzyme-catalyzed reactions. 

Regioselective functionalization of unreactive carbon-hydrogen bonds, in particular, arylation of pyridines by using aryl iodide, silver acetate, and catalytic palladium acetate 06SL3382. 

The activation of unreactive C-H bonds remains a challenge for synthetic organic chemists. Activation of such bonds provides the opportunity to functionalize relatively cheap and abundant hydrocarbons. The clear advantages of directly forming carbon-carbon bonds from carbon-hydrogen bonds have driven the development of a variety of reactions in this area. Chatani et al. have reported carbonylation of sp C-H bonds of secondary amines. In this reaction, various secondary amines were employed as substrates, and it was found that the presence of a pyridine ring adjacent to the amine group was essential for the carbonylation to proceed. 

Full details (see Vol. 1, p. 185) of the Japanese work on the preparation of trifluoromethylarenes from trifluoroiodomethane and iodoarenes in the presence of copper powder and a dipolar aprotic solvent have become available, and it appears that the best solvent in some cases is pyridine. This method (but with DMF as solvent) has also been used to prepare the compounds PhR [R = Me03C (CF3)3, CF3 0 (CF2)2, or perfluoro-2-tetra-hydrofurfuryl] in good yields from iodobenzene and the corresponding polyfluoroiodo-compounds. Perfluoroalkyl-copper compounds are very probably involved in such reactions, and the reactions of preformed n-perfluoroheptylcopper in dimethyl sulphoxide with the aromatic carbon-hydrogen bonds of benzene, toluene, p-xylene, nitrobenzene, and chlorobenzene also lead to (perfluoroalkyl)arenes (some replacement of chlorine occurs in the case of chlorobenzene). Homolytic substitution by perfluoro-heptyl radicals, perhaps within the co-ordination sphere of the copper atom,... 

Ockam s razor, we concluded that an intemiediate was fonned at both sec, and tert, positions. We postulated that this was formed by insertion of an Fe oxenoid species into the carbon-hydrogen bond. At the secondary position, the iron-carbon bond was stable, but at the tertiary position, the weaker bond fragmented into radicals which coupled with pyridine in the ordinary way. All the appropriate blank experiments were carried out to show that secondary radicals, had they been formed at the lower oxygen pressures, would have been captured by the solvent pyridine. [6]... 

Yan Y, Gu J, Bocarsly AB (2014) Hydrogen bonded pyridine dimer a possible intermediate in the electrocatalytic reduction of carbon dioxide to methanol. Aerosol Air Qual Res 14 515-521... 

While complexes 1-5 did not promote saturated alkane or fluorocarbon activation, the lessons learned from die synthetic and reactivity studies have led to the synthesis of the tris(triflate) complex 13, which is considerably more reactive due to the labile triflate ligand sphere. A promising example of the enhanced reactivity of 13 is its electrophilic substitution reactions with benzene and pyridine to form titanium-carbon bonds. These "titanations" are formal carbon-hydrogen bond activation processes and should allow access to the many useful organic reactions that titanium-carbon bonds are known to undergo (37,38). 

Choice of catalyst and solvent allowed considerable flexibility in hydrogenation of 8. With calcium carbonate in ethanol-pyridine, the sole product was the trans isomer 9, but with barium sulfate in pure pyridine the reaction came to a virtual halt after absorption of 2 equiv of hydrogen and traws-2-[6-cyanohex-2(Z)-enyl]-3-(methoxycarbonyl)cyclopentanone (7) was obtained in 90% yield together with 10% of the dihydro compound. When palladium-on-carbon was used in ethyl acetate, a 1 1 mixture of cis and trans 9 was obtained on exhaustive hydrogenation (S6). It is noteworthy that in preparation of 7 debenzylation took precedence over double-bond saturation. 

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