Arenediazonium

Aldehydes can also be prepared by the carbonylation of aryl and alkenyl halides and triflate, and benzyl and allyl chlorides using tin hydride as a hydride source and Pd(PhjP)4 as a catalyst[377]. Hydrosilancs arc used as another hydride source[378]. The arenediazonium tetralluoroborate 515 is converted into a benzaldehyde derivative rapidly in a good yield by using Et ,SiH or PH MS as the hydride source[379]. 

Arenediazonium salts are also used for the couplina[563], (Z)-Stilbene was obtained unexpectedly by the reaction of the ti-stannylstyrene 694 by addition-elimination. This is a good preparative method for cu-stilbene[564]. The rather inactive aryl chloride 695 can be used for coupling with organostannanes by the coordination of Cr(CO)3 on aromatic rings[3.565]. 

Reaction with arenediazonium salts Adding a phe nol to a solution of a diazonium salt formed from a primary aromatic amine leads to formation of an azo compound The reaction is carried out at a pH such that a significant portion of the phenol is pres ent as its phenoxide ion The diazonium ion acts as an electrophile toward the strongly activated ring of the phenoxide ion... 

Triarylbismuthines have been synthesized by means of the Nesmeyanov reaction that employs an arenediazonium salt such as the tetrafluoroborate, a bismuth trihahde, and a reduciag agent (51). The decomposition of iodonium salts ia the preseace of bismuth trichloride and metallic bismuth also leads to the formation of triarylbismuthines, Ar Bi (52) ... 

REOUCTIVE ARYLATIQN OF ELECTRON-DEFICIENT OLEFINS BY ARENEDIAZONIUM SALTS INDUCED BY TITANIUM(III) SALTS... 

It was noted early by Smid and his coworkers that open-chained polyethylene glycol type compounds bind alkali metals much as the crowns do, but with considerably lower binding constants. This suggested that such materials could be substituted for crown ethers in phase transfer catalytic reactions where a larger amount of the more economical material could effect the transformation just as effectively as more expensive cyclic ethers. Knbchel and coworkers demonstrated the application of open-chained crown ether equivalents in 1975 . Recently, a number of applications have been published in which simple polyethylene glycols are substituted for crowns . These include nucleophilic substitution reactions, as well as solubilization of arenediazonium cations . Glymes have also been bound into polymer backbones for use as catalysts " " . 

New diazotization techniques for the Balz-Schiemann reaction feature alter native mtrosating agents in place of aqueous sodium nitrite or substitution of other salts such as arenediazonium hexafluorophosphates for arenediazoniuin fluoroborates... 

Arenediazonium hexafluorophosphates represent a promising alternative to diazonium fluoroborates because of decreased water solubility and, in some instances, higher yields of fluoroaromatics [27, 28] (equation 6)... 

The relative efficacy of this procedure compared with the standard Balz-Schie mann arenediazonium fluoroborate conditions for difunctional aromatics is shown in Table 66 in reference 14 and in equation 7 The arenedidzonium liexafluorophosphate... 

Other less widely used alternative diazonium salts for aryl fluoride synthesis include arenediazonium hexafluoroantimonates, ArN2SbFg, arenediazonium hexafluoroarsonates, ArN AsFg, and arenediazonium hexafluorosilicates, (ArNjljSiFg 114]... 

The Bergmann variation of the Balz Schiemann reaction is a two-step process featuring copper- or copper halide-catalyzed decomposition of aqueous or acetone solutions of arenediazonium fluoroborates containing alkyl or halogen substituents [30] A recent modification is a one-step technique featuring simultaneous diazoti-zation and decomposition by addition of aqueous sodium nitrite at 25 °C to a mixture of fluorobonc acid, copper powder, and 2 isopropyl 6 methylaniline to give 2-isopropyl-6 methylfluorobenzene in 73% yield [37]... 

Depending on structure, photolysis of films of arenediazonium fluoroborates and hexafluorophosphates at room temperature gives aryl fluorides m 10-75% yield [32] In situ photochemical decomposition of arenediazonium fluoroborates... 

This technique has been applied to the synthesis of fluormated dopamine and other compounds of biological significance Fluoroheterocycles such as fluoro-imidazoles [JJ, 36] and fluoropyrazoles [37] can also be prepared by the photo Balz-Schiemann technique (equation 9) Photochemically induced in situ fluoro dediazoniation can also be applied to arenediazonium fluorides in hydrogen fluo nde-pyndine media Thus, o-fluoroanisole is obtained in 73% yield at 20 °C after I8h [dS]... 

Ultrasound promoted (17 kHz) decomposition of arenediazonium fluoro borates can also be effected m the presence of tnethylamine tnhydrofluoride in Freon 113 media For example, fluorobenzene can be obtained in 92-95% yield by this method [39] (equation 10)... 

The discovery of the Balz-Schiemann reaction in 1927 replaced the earlier Wallach procedure (1886) based on fluorodediazoniation of arenediazonium piper-idides (aryltriazenes) in aqueous hydrogen fluoride [40 41]... 

Diazotization of aromatic amines in aqueous 70% hydrogen fluoride at 0 °C followed by in situ decomposition of the arenediazonium fluorides has been replaced by diazotization in anhydrous hydrogen fluoride [52, 53] (equation 13)... 

When aqueous solutions are mixed with arenediazonium chlorides, the corresponding salts are formed. These salts decompose between 80 and 120 C with a release of mtrogen and formation of aryloxyfluorooxosulfomobis(fluorosuLfonyl)meth-... 

Heating of ori/w-arenediazonium carboxylates or 1,2,3-areno-thiadiazole-1,1 -dioxides,... 

Arenediazonium ions 1 can undergo a coupling reaction with electron-rich aromatic compounds 2 like aryl amines and phenols to yield azo compounds 3. The substitution reaction at the aromatic system 2 usually takes place para to the activating group probably for steric reasons. If the para position is already occupied by a substituent, the new substitution takes place ortho to the activating group. 

Arenediazonium ions are stable in acidic or slightly alkaline solution in moderate to strong alkaline medium they are converted into diazohydroxides 4 ... 

The optimal pH-value for the coupling reaction depends on the reactant. Phenols are predominantly coupled in slightly alkaline solution, in order to first convert an otherwise unreactive phenol into the reactive phenoxide anion. The reaction mechanism can be formulated as electrophilic aromatic substitution taking place at the electron-rich aromatic substrate, with the arenediazonium ion being the electrophile ... 

For aryl amines the reaction mixture should be slightly acidic or neutral, in order to have a high concentration of free amine as well as arenediazonium ions. Aryl ammonium species—ArNH3+—are unreactive. The coupling of the diazonium species with aromatic amines proceeds by an analogous mechanism ... 

Arenediazonium ions are relatively weak electrophiles, and therefore react only with electron-rich aromatic substrates like aryl amines and phenols. Aromatic compounds like anisole, mesitylene, acylated anilines or phenolic esters are ordinarily not reactive enough to be suitable substrates however they may be coupled... 

The opposite case—reaction of an arenediazonium species with an aliphatic substrate —is possible if a sufficiently acidic C—H bond is present e.g. with /3-keto esters and malonic esters. The reaction mechanism is likely to be of the Sel-type an electrophilic substitution at aliphatic carbon ... 

Aliphatic primary amines also undergo the diazotization reaction in weakly acidic solution however the resulting aliphatic diazonium ions are generally unstable, and easily decompose into nitrogen and highly reactive carbenium ions. The arenediazonium ions are stabilized by resonance with the aromatic ring ... 

However even arenediazonium ions generally are stable only at temperatures below 5°C usually they are prepared prior to the desired transformation, and used as reactants without intermediate isolation. They can be stabilized more effectively through complexation by crown ethers." ... 

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 ... 

An arenediazonium ion 1 in aqueous alkaline solution is in equilibrium with the corresponding diazohydroxide 4 The latter can upon deprotonation react with diazonium ion 1, to give the so-called anhydride 5. An intermediate product 5 can decompose to a phenyl radical 6 and the phenyldiazoxy radical 7, and molecular nitrogen. Evidence for an intermediate diazoanhydride 5 came from crossover experiments " ... 

When the arenediazonium compound 1 is treated with sodium acetate, instead of alkali hydroxide, the reaction proceeds via an intermediate nitroso compound, and is called the Hey reaction. 

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. 

Arylhydrazones from reaction of /3-dicarbonyl compounds with arenediazonium salts... 

The coupling of arenediazonium compounds 1 to 1,3-dicarbonyl substrates 2 (Z = COR) is known as the Japp-Klingemann reaction As suitable substrates, /3-ketoacids (Z = COOH) and /3-ketoesters (Z = COOR) can be employed. As reaction product an arylhydrazone 4 is obtained. 

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