Pyridine electron addition

For pyridones, pyrones, azinones (cf. 565) and also for A-oxides (cf 566) and [3-oxidocationic rings (cf. 567) the situation is more complex. The combined effect of the heteroatoms in such compounds is to act either as an electron source or as an electron sink depending on the requirements of the reaction (see Section 3.2.1.1.4). In practice, in reactions involving neutral species, substituents a or 7 to the heteroatom in 2- and 4-pyridones, 2- and 4-pyrones, and 2- and 4-thiinones (e.g. 565) and pyridine A-oxides (566) are usually activated by electron withdrawal almost as much as they are in pyridine itself. Additional nitrogen atoms increase the reactivity as expected. 

PDI)MnCH2SiMe3] , and a bimetallic aUcyl complex resulting from C-C coupling following deprotonation of the imine methyl groups in the backbone. Such reactivity demonstrates the chemical non-innocence of bis(imino)pyridines in addition to their interesting electronic properties. 

A trifunctional initiator was used to synthesize a (PIB- -PS)3 star-block copolymer using ring-substituted tricumyl chloride as initiator and TrCU as coinitiator via the sequential monomer addition methodology. The polymerization was conducted at -80 °C, in the presence of pyridine (electron donor) and 2,6-di-tert-butylpytidine (proton trap) in a 60/40 (v/v) methylcy-clohexane-methyl chloride solvent mixture. The copolymer synthesized exhibited a rather low PDI (1.17) and their mechanical, thermal, and morphological properties were examined extensively. 

Four types of nitrogen functionalities have been identified by XPS, which are illustrated in Fig. 24.13 pyridinic (Npy), pyrrolic (NH), quaternary (NQ), and an unknown (N-X). N-X is assigned mainly to pyridine N-oxides, but can also be nitro, nitroso groups, or shake-up satellite peaks [107]. Npy ( 399.0 eV) donates one p-electron, while NH ( 400.3 eV) donates two p-electrons. NQ, also called graphitic lutrogen, has a formal positive charge [107]. NQ exists in-plane, while Npy and NH are edge structures. However, NQ can also be protonated Npy and NH could also be pyridone, a hydroxylated pyridine. In addition, substitutional, interstitial, and sp -bonded N could also be present. This already intricate picture is... 

Because of the electronegativity of nitrogen, the dipole vector in both compounds points toward the heteroatom. The dipole moment of pyridine is larger than that of azacyclohexane (piperidine), because it is enhanced by the dipolar resonance forms in pyridine. In addition, the nitrogen is sp hybridized. (See Sections 11-3 and 13-2 for the effects of hybridization on electron-withdrawing power.)... 

Substituents are expected to alter the electron density at the multiply-bonded nitrogen atom, and therefore the basicity, in a manner similar to that found in the pyridine series. The rather limited data available appear to bear out these assumptions. The additional ring nitrogen atoms in triazoles, oxadiazoles, etc. are quite strongly base-weakening this is as... 

Heavily fluonnated aminobenzenes, pyridines, and pyrimidines are diazotized in strong-acid media Solid sodium nitrite added directly to the fluonnated amine dissolved in 80% hydrofluonc acid, anhydrous hydrogen fluoride, or (1 1 wt/wt) 98% sulfuric acid in (86 14 wt/wt) acetic and propionic acids affords the electrophilic fluoroarenediazonium ion Addition of an electron rich aromatic to the resultant diazonium solution gives the fluoroareneazo compound [10 II] (equa tions 9 and 10)... 

An interesting intermediate 30 was proposed to result from the sequential addition of pyridine to tetrachlorocyclopropene (31). Compound 30 represents an alkyl nitrogen ylide with two 1-chloroalkyl pyridinium moieties in the same molecule. Pyridines with electron-withdrawing groups and heterocycles with an electron-deficient nitrogen, for example, pyridine-3-carbaldehyde or quinoline, react with 31 to yield the corresponding mono-substituted products 32a and 32b (83JOC2629) (Scheme 8). 

The low yields of 6,6 -disubstituted-2,2 -bipyridincs 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. 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 71-electron would be smaller in a quinoline than in a pyridine derivative. 

Extension of this work by reacting 5-nitropyrimidine with 0,0-ketene acetals and with other cyclic and non-cyclic enamines showed that also with these electron-rich dienophiles the addition is regioselective and gives rise to the formation of 2-mono- or 2,3-disubstituted 5-nitropyridines (Scheme 30). Thus, reaction of 5-nitropyrimidine with the cyclic N,S-ketene acetals 4,5-dihydro-1 -methyl-2-methylthiopyrrole and 4,5,6,7-tetrahydro-1 -methyl-2-methylthioazepine gives in low yields 2,3-dihydro-1-methyl-5-nitropyr-olo[2,3-h]pyridine and the 5,6,7,8-tetrahydro-9-methyl-3-nitropyrido [2,3-Z)]azepine, respectively (89T2693) (Scheme 30). 

Treatment of 1,2,4-triazines 91a-91e with the electron-deficient die-nophile dimethyl acetylenedicarboxylate gave products, depending on the substituents [77LA( 10) 1718]. Pyrrolo-[2, -/][ ,2,4]triazines 92 were obtained via [4 + 2]-cycloaddition [77LA(9)1413, 77LA( 10)1718] with 91, but interaction with 91b in the absence of solvent gave, in addition to 92, the pyrido[2,l-/][l,2,4]triazine 93 and [l,3]oxazino[2,3-/][l,2,4]-triazine 94. In case of 91a pyridine and benzene derivatives were also formed in addition to 92 (Scheme 23). 

Coupling of the diazotetrazole with ethyl cyanoacetate gave 1034. Its cyclization in boiling acetic acid or pyridine afforded 1035 as the major product in addition to 1036. Mass spectral fragmentation of 1035 confirmed that the azole ring is more stable than the 1,2,4-triazine ring on electron impact [76JCS(P1) 1496] (Scheme 194). 

The observations that addition of pyridine increases the rate of decomposition, shifts the order of reaction from unity to zero, and considerably diminishes formation of 4-nitrophenol also warrants attention. This is compatible with the superior electron-donor properties of pyridine as compared to DMSO (Gutmann, 1976, 1977) generation of the corresponding diazopyridinium cation in one or several of the forms corresponding to 8.59 and 8.60 competes with formation of 8.58. 

Note As in related series, the addition of pyridine or (better) A,A-dimethylani-line (free of A-methylaniline, a common contaminant in some grades of this reagent) to phosphoryl chloride, appears to improve the yield of chloroqui-noxaline, especially if electron-withdrawing passenger groups are present. 

The reactivity of a remarkable electronically unsaturated tantalum methyli-dene complex, [p-MeCgH4C(NSiMe3)2]2Ta( = CH2)CH3, has been investigated. Electrophilic addition and olefination reactions of the Ta = CH2 functionality were reported. The alkylidene complex participates in group-transfer reactions not observed in sterically similar but electronically saturated analogs. Reactions with substrates containing unsaturated C-X (X = C, N, O) bonds yield [Ta] = X compounds and vinylated organic products. Scheme 117 shows the reaction with pyridine N-oxide, which leads to formation of a tantalum 0x0 complex. ... 

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