Picolyl anions

The procedure reported here is based on a reaction discovered by Bunnett and Creary, and was first employed for preparative purposes by Bunnett and Traber.3 It is attractive because of the high yield obtained, the ease of work-up, and the cleanliness of the reaction. The reaction is believed to occur by the SRN1 mechanism, which involves radical and radical anion intermediates.2,4 The SRN1 arylation of other nucleophiles, especially ketone enolate ions,5 ester enolate ions,6 picolyl anions,7 and arenethiolate ions,8 has potential application in synthesis. 

The 2- and 4-picolyl anions are phenylated or mesitylated on reaction with chlorobenzene, phenyltri-methylammonium ion and 2-bromomesitylene under stimulation by light or potassium metal. The mechanism of reaction with bromobenzene and iodobenzene is not certain, with die aryne mechanism almost certainly intruding, and with iodobenzene some diarylation of the picolinyl anion results. The reaction of the 2-picolyl anion with 2-bromomesitylene, where an aryne process is impossible, is shown in equation (44). Similar reactions take place between the 4-picolyl anion and 2- or 4-bromopyridine or 2-chloro-quinoline.134... 

Phenylation of carbanions derived from olefins such as 1,3-pentadiene and l-(p-anisyl) propene and aromatics such as indene and fluorene are possible by the SrnI route in ammonia solvent. From the 1,3-pentadiene a mixture of mono-olefins and dienes is produced along with a small amount of di- and triphenylated material. Hydrogenation of the mixture gives 1-phenylpentane in 74% yield.106) Phenylation of 2- and 4-picolyl anions is conveniently effected in ammonia. When bromo- or iodobenzene is used for this purpose, reaction probably occurs by both SrnI and benzyne routes.111)... 

The pK values of the three methylpyridines determined by Fraser et al. [1] show marked differences 2- and 4-methylpyridine (pK 34 and 32, respectively) are much more acidic than 3-picoline (pK 37) because the negative charge in the anions can reside on nitrogen, which is not possible in the 3-picolyl anion [2], These differences are reflected in the behaviour of the compounds towards strongly basic... 

The conversion of 2-picoline into the picolyl anion mentioned above is a type of reaction which can occur with any pyridine derivative capable of generating an anion in the presence of a strong base. As a consequence, lateral reaction of a suitable alkyl, amino, hydroxyl or similar substituent is always a possible competitor with nuclear alkylation, amination or reaction at the ring nitrogen atom (see pp. 182-3, 379). 

None of the three picolines is affected by potassium t-butoxide in t-butanol in presence of oxygen, but change of solvent to dimethylformamide causes oxidation to the corresponding acids in good yields. Picolyl anions are probably involved2 8. 

The side-chain ethylation and propylation of picolines with olefins under pressure, in the presence of sodium, has been carried out a, 6O8, These reactions are evidently nucleophilic attacks by the picolyl anions upon the olefins. With increasing substitution of the side chain only the more acidic... 

The next route involved the addition of the picolyl anion derived from 10 to 3-trifluoromethylbenzonitrile, and subsequent cyclization to the tetrasubstituted imidazole (Scheme 2). 

Scheme 2. Picolyl Anion Adition to Benzonitrile Approach to 1.

Picolyl anions can be used as nucleophiles in Sgjjl reactions ... 

The anionic polymerization of 2-vinyl pyridine initiated by benzyl-2 pyridy lmethylmagnesiurn (benzyl picolyl magnesium) in hydrocarbon solvents, leads to highly isotactic polymers. 

Kim aud coworkers explored a known receptor 2,6-bis(bis(2-pyridylmethyl)aminomelhyl)-4-meihyl-phenol (H-bpmp) for naked-eye detection of phosphate anion in aqueous solution." Zn + coordination with bis(2-picolyl)amine (Dpa) together with the formation of a phenoxo-bridged dinuclear metal complex (32) affords the anion binding pocket The pH-sensitive dye pyrocatechol violet (8) was chosen as the indicator for the competition assay. When [Zn2(H-bpmp)] + was titrated into a solution of 8 at pH 7.0, the solution color changed from yellow (444 nm) to blue... 

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