Pschorr reactions

Another example of the analogy between pyrazole and chlorine is provided by the alkaline cleavage of l-(2,4-dinitrophenyl)pyrazoles. As occurs with l-chloro-2,4-dinitrobenzene, the phenyl substituent bond is broken with concomitant formation of 2,4-dinitrophenol and chlorine or pyrazole anions, respectively (66AHC(6)347). Heterocyclization of iV-arylpyrazoles involving a nitrene has already been discussed (Section 4.04.2.1.8(i)). Another example, related to the Pschorr reaction, is the photochemical cyclization of (515) to (516) (80CJC1880). An unusual transfer of chlorine to the side-chain of a pyrazole derivative was observed when the amine (517 X = H, Y = NH2) was diazotized in hydrochloric acid and subsequently treated with copper powder (72TL3637). The product (517 X = Cl, Y = H) was isolated. 

The Pschorr reaction has been employed successfully in the preparation of a pyrazolo[3,4-cjisoquinoline derivative (565) (78JHC1287). 

Protoporphyrin-IX, N-methyl-, 4, 396 Protoporphyrins, 4, 382 photooxygenation, 4, 402 Prototropic tautomerism polyheteroatom six-membered rings, 3, 1055 Prozapine properties, 7, 545 Pschorr reaction carbolines from, 4, 523 dibenzazepines from, 7, 533 dibenzothiophenes from, 4, 107 phenanthridines from, 2, 433 Pseudilin, pentabromo-synthesis, 1, 449 Pseudoazulene synthesis, 4, 526 Pseudobases in synthesis reviews, 1, 62 Pseudocyanines, 2, 331 Pseudothiohydantoin synthesis, 6, 296 Pseudouracil structure, 3, 68 Pseudoyangonin IR spectra, 3, 596 Pseudoyohimbine synthesis, 2, 271 Psicofuranine biological activity, 5, 603 as pharmaceutical, 1, 153, 160... 

Bischler-Napieralski reaction of 139 to a 3,4-dihydroisoquinoline, oxidation, dehydrogenation and reduction of the nitro to the amino function gave 140 which was subjected to a Pschorr reaction (Scheme 49). Quaternization was accomplished by methyl iodide to furnish the isoquinolininium salt 141 which underwent an ether cleavage on heating a solid sample or benzene or DMF solution to Corunnine (127) (73TL3617). 

The Pschorr reaction, originally applied to the synthesis of phen-anthrene and its derivatives,has been adapted to the formation of new heterocyclic systems.In its original form, it consisted of treating a diazonium salt with copper powder in acid solution in this way, rans-o-amino-a-phenylcinnamic acid was converted into phen-anthrene-9-carboxylic acid, Eq. (20). Variants of the reaction include cyclizations such as that in Eq. (21). The reaction may be horaolytic... 

Methods for intemuclear cyclization other than the Pschorr reaction have been used very little. Hey and Osbond prepared 7,8-di-... 

Arenediazonium species 1 can be reacted with another aromatic substrate 2, by the Gomberg-Bachmann procedure, to yield biaryl compounds 3. The intramolecular variant is called the Pschorr reaction ... 

In contrast to the Gomberg-Bachmann reaction, the intramolecular variant, the Pschorr reaction, is carried out in strongly acidic solution, and in the presence of copper powder. Diazonium biphenyl ethers are converted to dibenzofurans, e.g. 8 9 ... 

The Pschorr reaction also works with substrates containing a bridge other than oxygen. Thus various tricyclic products containing a biaryl subunit are accessible, e.g. carbazoles and fluorenes. 

The Gomberg-Bachmann reaction is usually conducted in a two-phase system, an aqueous alkaline solution, that also contains the arenediazonium salt, and an organic layer containing the other aromatic reactant. Yields can be improved by use of a phase transfer catalyst. Otherwise yields often are below 40%, due to various side reactions taking place. The Pschorr reaction generally gives better yields. 

The decomposition of 2-(2-phenylphenylsulfanyl)benzenesulfonyl chloride (5) at elevated temperatures leads to the formation of tribenzo[6,c/,/]thiepin(6) in 24% yield by an intramolecular cyclization pathway. Mechanistically, this thermolysis is comparable to the Pschorr reaction (cf. Houben-Weyl, Vol. 5/2b, p 420 Vol. 10/3, p 189) however, copper(I) chloride is now preferred as a catalyst, without any solvent.4 In the thermolysis of 5, 4-phenyldibenzothiophene (7) is formed as a byproduct in 14% yield. Octachloronaphthalene can also be employed as a catalyst however, the yield of 6 is somewhat lower (19%).4... 

Complexation with crown ethers increases the notoriously low solubilities of diazonium salts in most solvents (with the obvious exception of water). Therefore, it is possible to carry out phase-transfer reactions with complexed diazonium ions (review Gokel et al., 1985). Useful examples can be found in a paper from Gokel s group (Beadle et al., 1984a) on the Gomberg-Bachmann and Pschorr reactions (see Sec. 10.10). 

The Pschorr reaction can take place by two different mechanisms, depending on conditions (1) attack by an aryl radical (as in the Gomberg-Bachmann reaction) or (2) attack by an aryl cation (similar to the SnI mechanism discussed on p. 853). Under certain conditions the ordinary Gomberg-Bachmann reaction can also involve attack by aryl cations. 

Another free-radical arylation method consists of the photolysis of aryl iodides in an aromatic solvent. Yields are generally higher than in 14-17 or 14-21. The aryl iodide may contain OH or COOH groups. The mechanism is similar to that of 14-17. The aryl radicals are generated by the photolytic cleavage ArI AR + T. The reaction has been applied to intramolecular arylation (analogous to the Pschorr reaction). A similar reaction is photolysis of an arylthallium bis(trifluoroacetate) (12-21) in an aromatic solvent. Here too, an unsymmetrical biaryl is produced in good yields. ... 

The Pschorr reaction is a method of synthesis of phenanthrenes from diazotized Z-2-aminostilbenes. A traditional procedure involves heating with a copper catalyst. Improved yields are often observed, however, if the diazonium ion is treated with iodide ion. Suggest a mechanism for the iodide-catalyzed reaction. 

Intramolecular radical arylations are found to work quite well, however, e.g. the Pschorr reaction this involves the thermal decomposition of diazonium salts, e.g. (112), in the presence of copper powder as catalyst, and is used in the synthesis of phenathrenes such... 

Reduction of phenyldiazonium chloride in acetonitrile containing a high concentration of an aromatic substrate, which can act as a free-radical trap, leads to phenylation of the substrate in 14 - 33% yields together with 50 - 50% of benzene formed by phenyl radical attack on the acetonitrile [132], Intramolecular capture of the phenyl radical, in an electrochemical equivalent of the Pschorr reaction, is much more successful and phenanthrene derivatives can be prepared in 90 - 96% yield [133],... 

This section did not appear in CHEC-I(1984) and CHEC-II(1996) <1984CHEC(2)1, 1996CHEC-II(6)1>. Examples are the intramolecular Heck-type reaction of 2-benzyl-5-(2-bromophenyl)-4-phenylpyridazin-3(2//)-one and 5-(2-bromophenyl)-2-methyl-6-phenylpyridazin-3(2//)-one which yields 2-benzyldibenzo[/, ]phthalazin-l(2//)-one and 2-methyldibenzo[/,. ]cinnolin-3(270-one, respectively <2003T5919>. The same compounds were also obtained from the corresponding 2-aminophenyl (instead of 2-bromophenyl) derivatives via diazotization and subsequent Pschorr reaction. 

---