2,4,6-Triphenylpyridine

Krohnke observed that phenacylpyridinium betaines could be compared to 3-diketones based on their structure and reactivity, in particular, their ability to undergo Michael additions. Since 3-dicarbonyls are important components in the Hantzsch pyridine synthesis, application of these 3-dicarbonyl surrogates in a synthetic route to pyridine was investigated. Krohnke found that glacial acetic acid and ammonium acetate were the ideal conditions to promote the desired Michael addition. For example, N-phenacylpyridinium bromide 50 cleanly participates in a Michael addition with benzalacetophenone 51 to afford 2,4,6-triphenylpyridine 52 in 90% yield. 

Irradiation of an acetone solution of 2,4,6-triphenylpyridine 1-oxide results in an inseparable mixture of 2,4,6-triphcnylpyridinc (2) and 2,4,6-triphenyl-l,3-oxazepine (3).10... 

Triphenyl-47/-l,2-diazepine is transformed into a mixture of 2,4,6-triphenylpyridine and the rearranged diazepines 3,5,6-triphenyl-4//-l,2-diazepine (28) and 3,5,6-triphenyl-l H-, 2-di-azepine (29) on prolonged heating.111... 

Studies of gas-phase S"n2 reactions at sp carbon have been made by Fourier transform ion cyclotron resonance mass spectrometry (FTlCRMS) and complemented by both semiempirical and ab initio MO calculations. The particular processes of interest involved intramolecular reactions in which neutral nucleophiles displace neutral leaving groups within cationic substrates, e.g. A-(2-piperidinoethyl)-2,4,6-triphenylpyridinium cation (59), in which the piperidino moiety is the nucleophile and 2,4,6-triphenylpyridine (60) is the leaving group. No evidence has been obtained for any intermolecular gas-phase 5) 2 reaction involving a pyridine moiety as a leaving group. The quantum mechanical treatments account for the intramolecular preference. 

At the same time the pyridine is oxidized to a pyridine radical cation which by dimerization and proton loss forms 4,4 -dipyridyl. However, if 2.4.6-triphenylpyridine 31 is used instead of pyridine,it forms the stable radical cation 33, which can be observed in the ESR-spectrum in addition to the pyryl radical 22... 

Pyrylium cations form pyridines with ammonia and pyridinium salts with primary amines (B-82MI 505-02). For example, 2,4,6-triphenylpyrylium cation (261 Z=0) yields 2,4,6-triphenylpyridine with ammonia, the corresponding 1-methylpyridinium salt with methylamine, and pyridine 1-arylimines with phenylhydrazine. Xanthylium ions (210), where ring opening cannot readily occur, form adducts (262) with ammonia, amines, amides, ureas, sulfonamides and imides. Similar adducts (e.g. 263) are formed by benzo[( ]pyrylium ions. 

A series of complexes with the ir-bonded aromatic and heteroaromatic frameworks 6 and 9 was prepared by ligand exchange. An example is the preparation of the r 6-tricarbonylchromium complex of indole (30) (68JOM359). Ligand exchange may lead to complexes 9. Thus, interaction of 2,4,6-triphenylpyridine with L3Cr(CO)3 yields the trinuclear jr-complex... 

The synthetic route (3.137), leading to trinuclear complex 708 at the expense of phenyl fragments of 2,4,6-triphenylpyridine, is very interesting [5,298] ... 

In agreement with this hypothesis, the steric hindrance to solvation, estimated from the difference in measured and additive pKa values (A) is greater for 2-t-butyl-4,6-diphenyl than for 2,4,6-triphenylpyridine, indicating an expected larger steric effect in the former (83JCS(P2)45). The best result is obtained with benzothiazol-2-yl, which tends to have a big steric effect in the plane of the pyridinium (entry 8) (Scheme 119). 

Only two azine aryloxyls have been described, the 2,4,6-triphenylpyridin-3-oxyl (111) and its pyrimidin-5-oxyl analog. ESR spectra have been published but not analyzed for these persistent species.364-367... 

Triphenylpyridine or 2,6-dimethylpyridine, instead of triethylamine, is used as a base and in addition benzyltriethylammonium chloride is necessary to achieve lactonization. Prostaglandin Fzal, 15-lactone (37) was synthesized by this method [25]. Thus, as shown in Scheme 12, a 1,2-dichloroethane solution of the hydroxy acid 35 was added to a 1,2-dichloroethane solution of 34, 2,6-dimethylpyridine and benzyltriethylammonium chloride under reflux over 4.25 h. After reflux for another 1 h, the lactone 36 was obtained in 91% yield. 

PCC can be modified to show selectivity for the oxidation of allylic alcohols in steroidal systems. A solution of PCC in dichloromethane with 2% pyridine at ca. 2 C was found to be an effective and selective oxidant (Table 6). In contrast to chromate oxidations of saturated alcohols in rigid systems. Parish and coworicers found that quasiequatorial allylic alcohols were oxidized faster than axial ones. Similar prcperties were also found for solutions of PCC and 3,5-dimethylpyrazole (2%) in dichloromethane. In adtUtion, Parish and coworicers also examined several other aromatic amines for the ability to promote allylic selectivity. 2,2 -Bipyridine, pyrazine, pyridazine, j-triazine and 2,4,6-triphenylpyridine all had some effect, but their efficacy appear to be substrate dependent. Most recently the combination of PCC... 

The reaction of 3 and 4 with piperidine gave the ring-opened products isoquinolylbutadiene 313 and quinolylbutadiene in 76% and quantitative yields, respectively (71LA65). With 2,4-diphenylbenzo[u]quinolizinium salt (314), piperidine attacks position 6 to give 2,4,6-triphenylpyridine derivative 315 (76%) [81H(15)1175], In the reaction of 2 with piperidine at the 6-position, 67/-benzoquinolizine 316 is formed (71LA65). 

Examples of stable salts of heteroaromatic radical anions isolated as crystalline compounds are the hthium, sodiiun, and potassium salts of 2,4,6-triphenylpyridine (6, Scheme 5) [34]. 

The photolysis of aminimides produces acylnitrenes, which have usually been trapped in intermolecular reactions.151,159 Again, isocyanates often accompany the reactions. The thermolysis of 2,4,6-triphenylpyridine N-acylimines at 170-250°C affords isocyanates in good yields.160... 

Methyl-2,4,6-triphenylpyridine was obtained from 2-dimethylamino-4//-pyran 121 (Ar = Ar = Ph,R = H,R = Me) on heating with aqueous ammonia (84JPR657). 

Thermolysis of 2,4,6-triphenylpyridine (V-imidoylimines gives N,N -diarylcarbodiimides in high yields (Eq. 37)89,90 it provides a useful synthetic entry to symmetrical and unsymmetrical diarylcarbodiimides. 

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