Pyridine, protonation

Q1. Measure the coupling between the two pyridine protons accurately. Now check the value against data quoted in Table 5.5 and all should become clear... 

Scheme 3 shows the details of the synthetic strategy adopted for the preparation of heteroleptic cis- and trans-complexes. Reaction of dichloro(p-cymene)ruthenium(II) dimer in ethanol solution at reflux temperature with 4,4,-dicarboxy-2.2 -bipyridine (L) resulted the pure mononuclear complex [Ru(cymene)ClL]Cl. In this step, the coordination of substituted bipyridine ligand to the ruthenium center takes place with cleavage of the doubly chloride-bridged structure of the dimeric starting material. The presence of three pyridine proton environments in the NMR spectrum is consistent with the symmetry seen in the solid-state crystal structure (Figure 24). 

Blocking either the 2- or 4-position allows lithiation at the other LDA or BuLi can be used to 4-lithiate the 2-blocked 3-fluoropyridines 319,284 320285 and 321,286 287 and BuLi 2-lithiates the 4-blocked 3-fluoropyridine 322.284 288 289 Note the selectivity for removal of the acidified pyridine protons in 321 and 322 over the coordination-activated ones adjacent to the pivalanilide group. A double orthodirecting effect ensures 4-lithiation of 323.290... 

The chlorine atom in 4-chloropyridine can be replaced photochemically by the dimethyl ketyl radical755. Irradiation of 4-chloropyridine in a 4 1 mixture of 2-propanol and water gives a low yield (2%) of 2-(4 -pyridyl)-2-propanol. Sensitization by benzophenone increases the yield to 25%, but the product is now accompanied by 6% of diphenyl-(4-pyridyl)methanol. The major product is believed to be formed via hydrogen abstraction from 2-propanol by photoexcited pyridine. Protonated pyridines do not undergo this abstraction process, and accordingly the product yield decreases under acidic conditions. The radical (259) formed from the pyridine will combine (at position 4) with the dimethyl ketyl radical (260) and elimination of HC1 from the adduct (261) completes the reaction (equation 194). 

Pyridine shows all the characteristics of aromatic compounds. It has a resonance energy of 113 kJ/mol (27 kcal/mol) and it usually undergoes substitution rather than addition. Because it has an available pair of nonbonding electrons, pyridine is basic (Figure 16-11). In an acidic solution, pyridine protonates to give the pyridinium ion. The pyridinium ion is still aromatic because the additional proton has no effect on the electrons of the aromatic sextet It simply bonds to pyridine s nonbonding pair of electrons. 

Pyridines in their transition metal complexes coordinate as )] (N), q (C,C), (N,C), or q ligands (Figured). By far the most common coordination mode is (N), in which the lone electron pair on the nitrogen donates to a Lewis acidic metal center (see Lewis Acids Bases). The M-N bond lies in the plane of the pyridine ring (Figure 6). Few reactivity studies of (N) pyridine ligands have been reported. One of potential interest for HDN is the base-catalyzed exchange of the pyridine protons for deuterium... 

Imidazole and its derivatives form an interesting and important class of heterocyclic aromatic amines. Imidazole is approximately 100 times more basic than pyridine. Protonation of imidazole yields an ion that is stabilized by the electron delocalization represented in the resonance structnres shown ... 

The special properties associated with pyridine a- and y-positions are evident again in the reactions of alkyl-pyridines protons on alkyl groups at those positions are particularly acidified because the enaminate anions formed by side-chain deprotonation are delocalised. The ability to form side-chain anions provides a useful means for the manipulation of a- and y-side-chains. 

Of more preparative value are the reactions of nucleophilic radicals, such as HOCHa and RaNCO, which can be easily generated under mild conditions, for example HOCH2 from ethylene glycol by persulfate oxidation with silver nitrate as catalyst. These substitutions are carried out on the pyridine protonic salt, which provides both increased reactivity and selectivity for an a-position the process is known as the Minisci reaction (cf. 3.4.1). It is accelerated by electron-withdrawing substituents on the ring. 

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