Aromatic compounds from aryl iodides

The palladium-catalyzed carbonylation of aryl halides in the presence of various nucleophiles is a convenient method for synthesizing various aromatic carbonyl compounds (e.g., acids, esters, amides, thioesters, aldehydes, and ketones). Aromatic acids bearing different aromatic fragments and having various substituents on the benzene ring have been prepared from aryl iodides at room temperature under 1 atm... 

Cycloaddition of 2-nitrosopyridine 48 with nitrile oxides can give either di-A -oxides such as 49 or 3-mono-A -oxides such as 50 (93JHC287). In general, greater electron withdrawing character in the aromatic substituent appears to favor formation of the di-A -oxides. Sulfur ylides such as compound 51 are obtained from aryl isothiocyanates and l-amino-2-methylthiopyridinium iodides (84JCS(P1)1891) nitrogen ylides can be obtained from a similar reaction (86H(24)3363). 

Arylamines are converted by diazotization with nitrous acid into arenediazonium salts, ArN2+ X-. The diazonio group can then be replaced by many other substituents in the Sandmeyer reaction to give a wide variety of substituted aromatic compounds. Aryl chlorides, bromides, iodides, and nitriles can be prepared from arenediazonium salts, as can arenes and phenols. In addition to their reactivity toward substitution reactions, diazonium salts undergo coupling with phenols and arylamines to give brightly colored azo dyes. 

Fused pyrazole compounds have been prepared from A-alkyl substituted pyrazoles. For example, a palladium-catalyzed/norbornene-mediated sequential coupling reaction involving an aromatic sp2 C-H functionalization as the key step has been described, in which an alkyl-aryl bond and an aryl-heteroaryl bond were formed in one pot <060L2043>. A variety of highly substituted six-membered annulated pyrazoles 59 were synthesized in a one-step process in moderate yields from IV-bromoalkyl pyrazoles 57 and aryl iodides 58. 

The electrochemical analysis allowed the determination of kinetic constants for this reaction46. Thus, in the presence of bromobenzene, the rate constant for the oxidative addition was found to be equal to about 70 M 1 s 1. The a-arylnickel complexes are unstable, except those obtained from o-tolyl or mesityl bromide as starting substrates. In these particular cases, the arylnickel complexes can be prepared by electrolysis from an ArBr/NiBr2(bpy) equimolar ratio. However, the exhaustive electrolysis of an aromatic iodide in the presence of ZnBr2, in DMF and at —1.4 V/SCE, leads to the corresponding arylzinc compound but the yield remains low (<20%). Indeed, the aryl iodide is mainly converted to ArH according to, very likely, a radical process (Scheme 11). 

For insoluble development in Heck-type reactions is P-C and N-C bond formation, which results from coupling of aryl halides with phosphorous compounds [38] and amines [39]. The first application in aqueous medium was achieved by coupling of a dialkyl phosphite with an aromatic iodide to give an arylphosphonate in 99% yield. In 1996, Stelzer and co-workers presented a P-C cross-coupling reaction between primary and secondary phosphines and functional aryl iodides to water-soluble phosphines [Eq. (9)], which are potentially applicable as ligands in aqueous-phase catalysis [40]. 

Conversion of diazonium compounds to aryl chlorides, bromides, or cyanides is usually accomplished using cuprous salts, and is known as the Sandmeyer reaction. Since a CN group is easily converted to a CO2H group (eq. 10.13), this provides another route to aromatic carboxylic acids. The reaction with KI gives aryl iodides, usually not easily accessible by direct electrophilic iodination. Similarly, direct aromatic fluorination is difficult, but aromatic fluorides can be prepared from diazonium compounds and tetrafluoroboric acid, HBF4. 

Another Pd-catalyzed reaction of aryl- and alkylzinc halides used carboxylic anhydrides as starting organic compounds (Scheme 127).328 One of the advantages of this method is that anhydrides can be synthesized in situ from the corresponding sodium salts of carboxylic acids and ethyl chloroformate. The scope of the method includes aliphatic and aromatic anhydrides, phenyl-, ethyl-, isopropyl-, and n-butylzinc iodides. 

Finally, polymer 594 has been used as an arene-catalyst to activate nickel from nickel(II) chloride and lithium, in order to perform hydrogenation of different organic substrates such as afkenes, afkynes, carbonyl compounds and their imines, alkyl and aryl halides (chlorides, bromides and iodides), aromatic and heteroaromatic compounds as well as nitrogen-containing systems such as hydrazines, azoxy compounds or Af-amino oxides, giving comparable results to those obtained in the corresponding reaction in solution . 

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