Nitrogen in pyridines

For another dramatic illu.slration of chemical shifts, have. students calculate the magnetic shielding of nitrogen in pyridine and compare it to its saturated cyclohexane analogue. 

Pyridinium ylide is considered to be the adduct car-bene to the lone pair of nitrogen in pyridine. The validity of this assumption was confirmed by Tozume et al. [12J. They obtained pyridinium bis-(methoxycarbonyl) meth-ylide by the photolysis of dimethyl diazomalonate in pyridine. Matsuyama et al. [13] reported that the pyridinium ylide was produced quantitatively by the transylidalion of sulfonium ylide with pyridine in the presence of some sulfides. However, in their method it was not easy to separate the end products. Kondo and his coworkers [14] noticed that this disadvantage was overcome by the use of carbon disulfide as a catalyst. Therefore, they used this reaction to prepare poly[4-vinylpyridinium bis-(methoxycarbonyl) methylide (Scheme 12) by stirring a solution of poly(4-vinylpyridine), methylphenylsulfo-nium bis-(methoxycarbonyl)methylide, and carbon disulfide in chloroform for 2 days at room temperature. 

It may be mentioned that this interpretation is in agreement with the deactivating and /3 directing influence of nitrogen in pyridine and its analogs14 with respect to substitution reactions. 

Since the nitrogen in pyridine is electron attracting it seemed reasonable to predict that the trihalopyridynes would also show the increased electrophilic character necessary to form adducts with aromatic hydrocarbons under similar conditions to those employed with the tetra-halogeno-benzynes. The availability of pentachloropyridine suggested to us and others that the reaction with w-butyl-lithium should lead to the formation of tetrachloro-4-pyridyl-lithium 82 84>. This has been achieved and adducts obtained, although this system is complicated by the ease with which pentachloropyridine undergoes nucleophilic substitution by tetrachloro-4-pyridyl lithium. Adducts of the type (45) have been isolated in modest yield both in the trichloro- and tribromo- 58) series. 

A number of other molecules in addition to those shown in Figures 6-7 and 6-8 are aromatic. The first five possible values of n are 0, 1,2, 3, and 4. These numbers correspond to 4n + 2 values of 2, 6, 10, 14, and 18, respectively. Pyridine (refer to Figure 6-7) illustrates the fact that aromatic compounds are not necessarily hydrocarbons. However, the replacement of the nitrogen in pyridine with oxygen places an sp hybridized atom in the ring, so the system is no longer aromatic. 

Figure 6.8. Comparison between the net charges and resonance shifts of nitrogen in pyridine, 1,3-diazine, 1,4-diazine, and 1,3,5-triazine (me). (Reproduced with permission from Ref. 43.)...

A surface for which the electrostatic potential is negative (a negative potential surface) delineates regions in a molecule which are subject to electrophilic attack, for example, above and below the plane of the ring in benzene, and in the ring plane above the nitrogen in pyridine. 

According to the acid-base concept of Pearson, A -phosphorins can be viewed as soft bases the lone electron pair at phosphoms is much more delocalized than the lone pair at nitrogen in pyridine. Thus, such soft Lewis acids as Hg ions are more likely to react with A -phosphorins (see p. 84). 

Atoms in aromatic rings are specified by lowercase letters. For example, the nitrogen in an amino acid is represented as N, the nitrogen in pyridin by n, and carbon in benzene by c. 

Pyrylium ions are six-membered heterocycles in which a positively charged sp2-hybridized oxygen replaces the nitrogen in pyridine. The pyrylium ring appears in many naturally occurring flower pigments. 

But if using a lone pair from the heteroatom would give an antiaromatic system, then the lone pairs will not overlap with the pi system.The nitrogen in pyridine and the oxygen in Problem 16-19(h) are examples. 

There exists a striking analogy in the behavior of carbocyclic nitro compounds and aromatic heterocycles and it has therefore been concluded that the hetero atom—e.g., nitrogen in pyridine—eflFects a similar electron displacement as does the nitro group in nitrobenzene. In the heterocyclic series, the stmcture parallel to o-chloronitrobenzene (III) would be that of 2-chloropyridine (VII). 

Note that the procedure described here can be extended without difficulty to atoms P which contribute severed orbitals and several p electrons of different symmetries carbon atoms in the sp hybridization state of acetylene and allene-type compounds 28>, heteroatoms with one n electron and a lone pair, like oxygen in the carbonyl group or nitrogen in pyridine 20,21). 

Figs. 9, 10 and 11 demonstrate effect of temperature on calculated In Va for hydrocarbon groups Fig. 9, for some polar groups Fig. 10 and for nitrogen in pyridine and triethylamine Fig. 11. 

There are four possible structures. The most significant feature of the aromatic ring is the proton at very large chemical shift (8.8) with only very small couphng. Protons next to nitrogen in pyridine rings have very large chemical shifts so this rules out all the structures except the second. 

Step 4. Now for the double bonds. Remember, two of the pyrrole units are pyrrolic, with hydrogen atoms on their nitrogens, while the other two are pyrrolenic, which means their nitrogens have no hydrogen attached and are akin to the nitrogens in pyridine. 

Protonation of the nitrogen of pyrrole destroys the aromatic system, thus forfeiting its resonance energy. Hence pyrrole is a very weak base. In fact, pyrrole can be protonated by very strong acids, but it is protonated on the carbon rather than on the nitrogen In pyridine, the electron pair on nitrogen is not part of the aromatic tt system and is available for protonation. 

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