Bonding in pyridine

Some workinvolvingLiAlH4 was also reported. Reduction of carvenone with LiAlH4 in ether does not affect the double bond in pyridine 8 % of the... 

The force constants obtained for sulphoxides and sulphones are about 7 x 10 5 and 10 x 10 10 dyne cm - respectively174. The values of the force constants for sulphoxides are very similar to those of the N—O bond in pyridine N-oxides in which the k values are approximately 6-7 x 105 dyne cm-119. 

Similarly, the large a-CH couplings of pyridine, quinoline, and isoquinoline can be used to locate substituents or fused rings in these heterocycles [73 d, i], N-Oxidation and protonation further enhance Jcu of the a-CH bond in pyridine (Table 4.68) due to the positive charge at nitrogen. To conclude, the sites of oxidation and protonation can be derived from the magnitudes of JC I in pyridines, azines, and their fused derivatives [467]. 

Figure 1.24 Examples of closed shell interactions (a) Aurophilic interactions in [Au2( J.-C1) (PPh3)2] (C104), (b) halogen bonding in pyridine—I-CCR and I5 and (c) secondary bonding in [ HgCl(C6H4N2Ph) 2].

NMR spectroscopy has been useful in determining the extent of charge transfer in species such as the pyrido-neimines and pyridonemethides. This demonstrates that the predominant canonical form of pyridoneimine is the aromatic zwitterion 19, while the major canonical form for pyridonemethide is the quinoid 20 <2001JOC8883>. In a similar vein, 13C NMR spectra have been used to study polarization of the ethylene bond in pyridine chalcone derivatives such as 21 as a result of conjugation with the pyridine moiety <1999JST(482)371>. 

Flash thermolysis of 3-aryl[l,2,3]triazolo[l,5-a]pyridines (277) under mild conditions (380-500°C, 10-3 torr) affords carbazoles (279, 280) in nearly quantitative yields.232 The triazoles exist as the valence tautomeric diazo compounds 278 in the gas phase.189 The substitution patterns in the products (279 and 280) demonstrate that the reactions take place exclusively by a carbene-nitrene rearrangement in which the pyridylcarbenes insert into the 2,3-bond in pyridine (Scheme 52). 

By pyrolysis of 14C-labeled phenyl(4-pyridyl)diazomethane it was shown that the carbene 283 rearranges predominantly via expansion into the 3,4-bond in pyridine (Eq. 81). The chemical yield of 2-azafluorene (284) was up to 70%.232... 

Easy thermal cleavage of the N-N bond in pyridine N-methylimines66 has precluded isolation of such compounds. The other cleavage product, presumably methylnitrene, has not been detected or trapped, but phenyl-nitrene was detected in the thermal cleavage of the N-N bond of 1,2,3-triazole N-phenylimines.106... 

The details of bonding in pyridine will be explained in chapter 29, on page 724 of the textbook. 

The difference in bonding in pyridine and pyrrole is reflected in their properties. Although both are weak bases, pyridine is 10 -10 times more basic than pyrrole. When pyridine acts as a Br0nsted base, protonation of nitrogen converts an unshared pair (N ) to a bonded pair (N—H) while leaving the aromatic system intact. 

Nickel-catalyzed activation of the C(2)-H bond in pyridine N-oxides allowed the latter to react with aUcynes under mild conditions to afford ( )-2 alkenylpyridine N-oxides in modest to good yields (Scheme 9) [27]. The compounds obtained can be deoxygenated readily to give various substituted pyridines, which are regarded as useful intermediates for pharmaceuticals and materials. 

Heterocyclic aromatic compounds contain C and H atoms other than carbon and hydrogen (Fig. 6). For the monocyclic molecules Hiickel s rule is applicable. For example both pyridine and pyrrole contain six ti electrons. Unlike the former the lone pair of the latter is delocalized. Armit and Robinson have shown a connection between the electronic sextet and the heteroaromaticity. Due to the electronegativity difference between carbon and nitrogen the bonds in pyridine are not of equal length and the delocalization is not perfect. Five membered heteroaromatics with oxygen and sulfur are furan and thiophene respectively. Pyrazole/imidazole, triazoles and tetrazoles are five membered heteroaromatics with two, three and four nitrogen atoms respectively. Three important aromatic six membered heterocyclic molecules are pyrimidine, pyrazine and pyridazine. Benzofused... 

We discussed fhe sfrucfure and bonding in pyridine and imidazole in Section 21.2D. In accounting for the relative basicities of fhese and other heterocyclic aromatic amines, it is important to determine first whether the unshared pair of electrons on nitrogen is a part of fhe An + 2) tt elecfrons giving rise to aromaticity. In the case of pyridine, fhe unshared pair of elecfrons is nof a part of the aromatic sextet. 

Since the latter are useful sources of pyridine derivatives, methods for their deoxygenation have been much studied. These compounds lack the oxidizing properties found with some amine oxides. Polarographic studies show that in acid media pyridine 1-oxide has a much lower reduction potential than oxides of tertiary amines with exocyclic nitrogen atoms -. The half-wave potentials are not useful guides to the abilities of the N-O bonds in pyridine oxides to take part in oxygen transfer reactions, and in 4-nitropyridine 1-oxide the nitro group is reduced first . 

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