Pyridinium adducts

Pyridine may react as a nucleophile with radical cations to pyridinium salts, as in the electrolysis of anthracene in acetonitrile-pyridine which yields a bis-pyridinium-adduct 283 With tris (p-methoxyphenyl)ethylene a tripositive tripyridinium cation is formed (Eq. (124) ) 284 ... 

In the 1970s, O Leary and Samberg described the reaction of 2,4,6-trimethyl-pyrylium perchlorate 57 with a-chymotrypsin and observed the formation of a protein-pyridinium conjugate [94], Later on, the reaction of the tri-substituted pyrylium salt 58 with the proteins gelatin and a-chymotrypsin [95], glycophorin [96,97] and the Na /glucose cotransporter protein [98] was reported. In all cases, the reaction happened to exclusively involve the proteins lysine residues and formation of the corresponding pyridinium adducts were observed. 

Pyrylium salts bearing various metallo-carbonyl moieties at the para position of the heterocycle have been synthesized [99-103]. The manganese and rhenium pyrylium salts 59-60 [104] together with the chromium pyrylium ions 42, 43 and 61 [105] successfully labeled the protein BSA to afford the corresponding pyridinium adducts as evidenced by the comparison with the spectroscopic data of model pyridinium derivatives. Interestingly, the reaction can readily be monitored by UV-visible spectroscopy. 

Kinetic Acidities Of Methylbenzenes. One electron oxidation of methylbenzenes affords very acidic cation radicals that undergo fast deprotonations by a series of pyridine bases (Py).25 26 One electron oxidation of the benzylic radical thus formed yields the benzylic cation that adds to the nucleophilic pyridine to give the pyridinium adduct. 25 26... 

Ketoesters.—A Japanese group has developed a route to a-ketoesters based on an additive-type Pummerer rearrangement (Scheme 40) the method awaits application to substrates other than the one shown in the scheme. An apparently simple synthesis of benzoyl formates involves addition of pyridine-A-oxide to a-bromo-phenylacetates, and treatment of the resulting pyridinium adducts (which have been... 

Methyl propiolate and pyridine give a rather unstable 2 1 molar adduct which is the 1,2-dihydropyridine (112). The reaction sequence proposed to account for its formation is identical in principle to a similar scheme proposed earlier in the acridine series (Section II,A,2) and is also supported by the observation that the 1-benzoyl-pyridinium cation with the phenylacetylide anion yields (113). ... 

To the best of our knowledge, the hrst paper which mentioned an A-(l-haloalkyl)pyridinium compound appeared 66 years ago in the Chemische Berichte (Krohnke 33CB1386). Tlie author described the reaction of phenacyl pyridinium derivatives 1 with bromine in acetic acid to give the halides 2 (36CB2006 37CB864). Tire addition of bromine to the double bonds of A-vinylpyridinium salts 3 and 4 giving the adducts 5 and 6 has also been reported (51CB399) (Scheme 1). 

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. 

Further organic storing materials Phenyl bromide [14], pyridine, 1 -picoline, 2,6-lutidine [15-17] Arsonium salts [18, 19] Phosphonium salts [20] Pyridinium bromides [21] Aromatic amines [22] Urotropin-bromine adduct [23] Pyridinium and sulfonium salts [24] Propionitril [25]... 

This entry to supramolecular chemistry allows one, at the same time, to develop a host-guest chemistry because the space inside the cucurbituril barrel is sufficient to confine small "guesf molecules. This is illustrated with the crystallization of the supramolecular adduct [W3S4(H20)8Cl](pyH ccuc) Cl4a 5.5 H2O with a pyridinium cation inside the cucurbituril cavity [48]. The introduction of guest molecules may vary the net charge on the assembly and consequently the whole packing in the solid state. 

This event is clearly demonstrated by the behavior of 4-cyanopyridine which forms an adduct even with an excess of tribromoborane. However, 4-cyano-pyridinium chloride yields a saltlike dimeric iminoborane derivative 26> as depicted in Eq. (17). 

Alkenyloxazolidines.1 In the presence of pyridinium tosylate or BF, eth-erate, the N-protected norephedrine (1) cyclizes with the a,(3-unsaturated acetal 2 to give the 2-alkenyloxazolidine 3 as the major product. Cuprates add to 3 from the si face with high selectivity to give adducts (4), which are readily converted to (S)-3-alkylsuccinaldehydes in high enantiomeric purity. 

Work by Harbour, Chow and Bolton (1974) on the spin adducts of superoxide (or HOO )13 with nitrones paved the way for a number of investigations of superoxide and hydroperoxyl radical chemistry. Harbour and Bolton (1975) used DMPO to trap superoxide formed by spinach chloroplasts in the presence of 02. The signal strength was greatly enhanced when methylviologen was present, consistent with the hypothesis that this bis-pyridinium dication accepts an electron from the primary acceptor of photoprotein I, and then transfers it to molecular oxygen. 

Kinetics. The reaction of N-dodecyl 3-carbamoyl pyridinium bromide (I) with cyanide ion in the microemulsions was observed by following the 340 nm absorption maximum of the 4-cyano adduct (II). See equation (1). Following the work of Bunton, Romsted and Thamavit in micelles ( ), a 5/1 mole ratio of KCN to NaOH was employed to prevent cyanide hydrolysis. The pH of each reaction mixture was measured on a Coleman 38A Extended Range pH meter to insure that the system was sufficiently basic to allow essentially complete ionization of the cyanide. The appropriate amounts of cyanide and hydroxide were added to the mlcroemulslon sample within 10 minutes of running a reaction. Cyanide concentration varied between 0.02 and 0.08 M with respect to the water content. Substrate was Injected via a Unimetrics model 1050 syringe directly into a known volume of the yE-nucleophlle mixture in a 1.0 cm UV quartz cell. Absorbance at 340 nm was followed as a function of time on a Perkln-Elmer model 320 spectrophotometer at 25.0 + 0.3 C. Since the Initial bulk concentration of substrate was 10 M, cvanide was always present in considerable excess. 

---