In Pschorr reaction

In Pschorr reactions involving anilides, amido groups must be protected to avoid the formation of triazinones122 iV-alkylation has commonly been employed, but the resulting iV-alkylphenanthridones are difficult to dealkylate. Similarly, protection by AT-alkylation has been employed in the case of the benzylamine (104 (R = Me).138 In a procedure designed to circumvent this difficulty, the N-sulfonylated diazonium chlorides, e.g., 104a-c, are decomposed catalytically acid hydrolysis, followed by oxidation, of the 5,6-dihydro derivative... 

Diazonium compounds, as intermediates in hydfazone oxidation 710 as intermediates in Pschorr reactions 611... 

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... 

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... 

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 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... 

There are three areas of activity in the field of arenediazonium salts in interaction with metals and transition elements which have some similarities to metals. First is the use of copper in the reactions of Sandmeyer (1884), Pschorr (1896), Gomberg-Bachmann (1924), and Meerwein (1939). Other transition metal catalysts (Ti and Pd) have been used for such reactions since the 1970s (see Secs. 10.8 and 10.9). Up to now only one intermediate has been directly identified, the aryldiazenido palladium complex (ArN2Pd(PPh3)3]+BF4 (Yamashita et al., 1980 see Sec. 10.9, Scheme 10-64). 

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. ... 

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. 

The Pschorr reaction is valuable in the synthesis of carbolines. The 3-aminopyridine (165) gives the a-carboline (166 78%) under free-radical conditions, but the aniline... 

The Pschorr Reaction allows the preparation of biaryl tricyclics by intramolecular substitution of one arene by an aryl radical. This radical is generated in situ from an aryl diazonium salt by copper catalysis. Although excess copper salts are used, the yield is normally moderate. 

The decomposition of 3,4-dihydro-4-oxo-3-phenyl-l,2,3-benzo-triazines (110) in phosphoric acid is mechanistically similar to the Pschorr reaction in the sense that it involves intramolecular arylation... 

Studies on the mechanism of the Pschorr reaction have led to a number of interesting cyclizations onto a pyridine nucleus. Hey and Osbond441 were the first to carry out such a reaction 7,8-dihydro-benzo[/]quinoline (211) was obtained in 41 % yield from the N-nitroso-acetanilide (210). The diazonium salt of the amine 212 also underwent cyclization in spite of the presence of a second deactivating... 

Among the initiation methods available, photochemistry has already been used successfully for Pschorr reactions [122]. A new study, from which one example is shown in Scheme 24, revealed a dramatic influence of the substituents on the 2-bromobenzophenone 66. Methoxy groups in the radical ring led to a highly electrophilic cr-aryl radical which proved unreactive for hydrogen abstraction and cyclization. Substituents in the non-radical ring that stabilize the cyclohexadienyl... 

Biaryl formation is important when cationic substrates are used, such as diazonium salts (usually in the intramolecular version, the photo-Pschorr reaction [113]) or quaternary ammonium salts [114], In this case PET generates a neutral radical, and this expels nitrogen or an amine molecule yielding an aryl radical. Although the present discussion generally does not include ma-... 

The Pschorr reaction(33) has been useful in securing morphinandienones, many of which are difficult to obtain by phenolic oxidation. The f -(-)-2 -aminobenzylisoquinoline (20) was diazotized and the diazonium salt non-catalytically decomposed with heat to salutaridine (19) (Scheme 2.3). 

Only moderate yields of biphenyls result from the Gomberg reaction, in which an acidic solution of a diazonium salt is made alkaline with sodium hydroxide solution in a two-phase mixture with an aromatic hydrocarbon such as benzene (Scheme 8.20). A variation of this reaction uses a copper catalyst. The reaction is more successful in the intramolecular version and the Pschorr reaction (discussed in Chapter 12) offers a useful route to phenanthrene. 

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