2,2 -Pyridil rearrangement

Oxidation of the pyridoins, e.g. with iodine, gives the corresponding a-diketones. The significant electron withdrawal exerted by the 7r-deficient heterocyclic system on carbonyl groups at the a- and y-positions can be demonstrated in the behaviour of the diketones under benzilic acid rearrangement conditions. Whereas 3,3 -pyridil (73) gives a normal... 

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. 

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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