Pyridil

Pyridazinotriazole, iV-amino-oxidation, 5, 109-110 Pyridazinyl ketones, 3, 32 Pyridazinyl sulfides quaternization, 3, 18 2,2 -Pyridil... 

A benzilic acid rearrangement can be achieved with 2,2 -pyridil and with the quinoline analogues (75) under mild conditions by using methanolic nickel(II) or cobalt(II) acetates. The product acid is isolated as a metal complex (Scheme 71) (69AJC1439). 

Figure 8-43. In the case of complexes of 2,2 -pyridil, the benzilic acid rearrangement may be followed and complexes of the intermediate may be isolated.

The oxidation of the metal complexes of l,10-phenanthroline-5,6-quinone is thought to proceed in a similar manner, with the first step being a benzilic acid rearrangement. Rearrangements of this type may also be followed directly in nickel(u) and cobalt(m) complexes of 2,2 -pyridil. The first step of the reaction involves nucleophilic attack on an O-bonded carbonyl group to form a hydrate, followed by a benzilic acid rearrangement. In this case, the benzilic acid rearrangement products may be isolated as metal complexes (Fig. 8-43). 

The generality of the rearrangement is further illustrated by the reaction of 2,2 -furil with hydroxide ion in dry ether (Table 1). Likewise, 2,2 -pyridil is rearranged in hot methanol solution (40 min) to give the sodium salt of 2,2 -pyridilic acid (86%). Acidification, however, affords bis(2-pyridyl)methanol by decarboxylation since 2,2 -pyridilic acid (16) is structurally similar to a -keto acid. Benzilic rearrangement of 2,2 -pyridil with methanolic nickel(ll) and cobalt(II) acetates results in the formation of metal complexes of 2,2 -pyridilic acid (17 92%). A plausible mechanism is summarized in Scheme 4. Rearrangement is also observed with 2,2 -quinaldil, but benzil, 2,2 -furil or 1-phenyl-2-(2 -pyridyl)ethane-1,2-dione are not susceptible to these metal template reactions. 

Solid 2,2 -pyridil (1.08 g 5.10 mmol) is combined with 0.82 g (5.2 mmol) of 2,3-diaminonaphthalene in a 100-mL round-bottomed flask containing a magnetic stir bar. To this is added 30 mL of absolute ethanol. The flask is equipped with a reflux condenser. This solution is then stirred and heated at reflux for 5 h. The round bottom is removed from the heat and the solution is allowed to cool to room temperature, at which time solid yellow crystals of the dpb product form. The product is removed by vacuum filtration. Purification is achieved by chromatography on adsorption alumina using methylene chloride as the eluent as described above. The desired yellow band containing the dpb product elutes prior to the unreacted starting materials. The purified product is then washed with 50 mL of diethylether and dried under vacuum for 4 h. Yield 1.14 g, 67%. 

The pyridils are not especially remarkable. Some examples have been reported to undergo the benzilic acid rearrangement , but others with ethanolic potash give an aldehyde (which subsequently undergoes the Cannizzaro reaction) and a carboxylic acid . Heated with lead oxide, 2,2 -pyridil gives di-(2-pyridyl) ketone . An x-ray crystallographic study of 2,2 -pyridil has been made" . 

Pyridil allowed to react with hydrated Ni-acetate in methanol 2,2 -pyridilic acid Ni-complex. Y almost 100%. - This extremely mild rearrangement is dependent on chelation. Also with cobaltous acetate and limitations s. D. S. C. Black, Chem. Gommun. 1967, 311. 

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