Diazonium salts, aryl methylation

All these results are consistent with the hypothesis that aryl cations react in aqueous media at diffusion-controlled rates with all nucleophiles that are available in the immediate neighbourhood of the diazonium ion. On this basis Romsted and coworkers (Chaudhuri et al., 1993a, 1993b) used dediazoniation reactions as probes of the interfacial composition of association colloids. These authors determined product yields from dediazoniation of two arenediazonium tetrafluoroborates containing ft-hexadecyl residues (8.15 and 8.16) and the corresponding diazonium salts with methyl groups instead of Ci6H33 chains. ... 

Other limitations of the reaction are related to the regioselectivity of the aryl radical addition to double bond, which is mainly determined by steric and radical delocalization effects. Thus, methyl vinyl ketone gives the best results, and lower yields are observed when bulky substituents are present in the e-position of the alkene. However, the method represents complete positional selectivity because only the g-adduct radicals give reductive arylation products whereas the a-adduct radicals add to diazonium salts, because of the different nucleophilic character of the alkyl radical adduct. ... 

Aryl radicals are produced in the decomposition of alkylazobenzenes and diazonium salts, and by f)-scission of aroyloxy radicals (Scheme 3.73). Aryl radicals have been reported to react by aromatic subsitution (e.g. of Sh) or abstract hydrogen (e.g. from MMA10) in competition with adding to a monomer double bond. However, these processes typically account for <1% of the total. The degree of specificity for tail vs head addition is also very high. Significant head addition has been observed only where tail addition is retarded by sleric factors e.g. methyl crotonate10 and -substituted methyl vinyl ketones 79, 84). 

Several comparative studies are available in the literature, which clearly shows that, according to the latest results, tetrafluoro-pheny/ azides (AZ), trifluoro-methyl-phenyl diazirines (DZR), and particularly benzophenone (BP) are the best choice (Scheme 3.). Diazocarbonyl compounds, which played a historically role in the evolution of PAL, can also be considered together with aryl-diazonium salts [6] (not shown). For aryl azides the more common unsubstituted photophore is presented. It should be noted that tetrafluoro-phenyl azides have an increased reactivity towards CH-groups and they do not rearrange. Other substituted phenyl azides can readily alter the excitation wavelength allowing the application of milder or differential photoactivation. 

Arylation of activated double bonds with diazonium salts in the presence of copper catalysts is known as the Meerwin reaction. The reaction is postulated to either proceed through an organocopper intermediate or through a chlorine atom transfer from chiral CuCl complex to the a-acyl radical intermediate. Brunner and Doyle carried out the addition of mesityldiazonium tetrafluoroborate with methyl acrylate using catalytic amounts of a Cu(I)-bisoxazoline ligand complex and were able to obtain 19.5% ee for the product (data not shown) [79]. Since the mechanism of the Meerwin reaction is unclear, it is difficult to rationalize the low ee s obtained and to plan for further modifications. 

Photochemical arylation 4-24 Reduction of diazonium salts 4-29 Dimerization of diazonium salts 4-30 Methylation of diazonium salts 4-33 Coupling of Grignard reagents 4-34 Coupling of arylboranes 4-35 Coupling of other organometallic compounds... 

Fischer disconnection of the indole 30a (chapter 39) gives methyl pyruvate and an aryl hydrazine that would have to be made from the corresponding diazonium salt that would have to be protected on the other amino group 36. They decided to take a short cut by using the Japp-Klingemann reaction on the same diazonium salt with the easily made acetoacetate 35. 

Alkyl iodides have been widely used for selective alkylation of heteroaromatic bases. The method is based on rapid iodine abstraction by aryl radicals (obtained from benzoyl peroxide or diazonium salts) or by a methyl radical (obtained from MeCOOH, t-BuOH, t-BuOOH, (t-BuO)2, (MeCOO)2, MeS0Me/H202, or MeC0Me/H202) [2]. An example is depicted in Eq. (14) of Table 3. 

The reaction of diazonium salts with aqueous solutions of 1,2-diaminocyclohexane mixed with formaldehyde affords the l-[2-aryl-l-diazenyl]-3-( 3-[2-aryl-diazenyl]perhydrobenzo[ imidazol-l-yl methyl)perhy-drobenzoMimidazoles 1252 in 23-66% yields (Scheme 314) <2006JHC217>. 

Methylation, Vinylation, and Arylation of Diazonium Salts Methyl-de-diazoniation, and so on... 

The reagent reacts with a diazonium salt to form an oxime which on acid hydrolysis affords an aryl methyl ketone. ... 

Only half-a-dozen of these compounds are known (Table XLI) and at least this number of methods of preparing them have been reported. Borsche and Manteuffel125 have found that 2-pyrazolin-4,5-diones are formed as the by-products when oc-ketoesters are treated with aryl-diazonium salts. The principal products are 4-arylazo-2-pyrazolin-5-ones. Nitric acid oxidation of 3-methyl-l-phenyl-2-pyrazolin-5-one forms the corresponding 4,5-dione.809 Wislicenus and Gtiz1842 heated 4 -bromo - 4 - nitro -1 - (4 - bromophenyl) - 3 - methyl - 2 -pyrazolin - 5 - one in water and obtained the analogous 4-oxo compound. Acid hydrolysis of rubazonic acids, which are 4-imino-2-pyrazolin-5-ones, leads to 2-pyrazolin-4,5-diones.424 809 Oxidation of 4,4 -bis(2-pyrazolin-5-ones) or of rubazonic acids with nitric acid809 forms the 4,5-diones. 

To test whether any free aryl radicals can be formed in the course of reactions of the Sandmeyer type, decompositions of aqueous solutions of stabilized diazonium salts, such as the borofluorides, have been carried out in the absence of oxygen or of traces of oxidizing agents, in the presence of the water-soluble vinylic monomers, acrylonitrile and methyl methacrylate. Polymerization has regularly been observed when one-electron transfers of type (3) above are possible and the isolated polymers, after washing free from simple aromatic materials, have been shown, by inspection of infrared spectra and often by detection of halogens derived from the diazonium salts, to contain aromatic nuclei as end groups. 

Another common transformation of diazonium salts is their conversion to aryl halides by reaction with cuprous salts (CuX), in what is known as the Sandmeyer reaction, named after Traugott Sandmeyer (Switzerland 1854-1922). This means that the Ar-NH2 ArX conversion is possible, where Ar = an aryl group. When 132 is treated with cuprous bromide (CuBr), the product is bromobenzene (18). The reaction works with many other cuprous salts as well, including cuprous chloride (CuCl). A variation of this reaction treats the diazonirun salt with cuprous cyanide (CuCN) to give a nitrile. In this manner, 4-methylanihne (24) is treated with HCI and NaN02 and then with CuCN to give 4-methyl-l-cyanobenzene (134). 

Kochi (1956a, 1956b) and Dickerman et al. (1958, 1959) studied the kinetics of the Meerwein reaction of arenediazonium salts with acrylonitrile, styrene, and other alkenes, based on initial studies on the Sandmeyer reaction. The reactions were found to be first-order in diazonium ion and in cuprous ion. The relative rates of the addition to four alkenes (acrylonitrile, styrene, methyl acrylate, and methyl methacrylate) vary by a factor of only 1.55 (Dickerman et al., 1959). This result indicates that the aryl radical has a low selectivity. The kinetic data are consistent with the mechanism of Schemes 10-52 to 10-56, 10-58 and 10-59. This mechanism was strongly corroborated by Galli s work on the Sandmeyer reaction more than twenty years later (1981-89). 

Methyl-, benzyl- and toluene-sulphonamide react with aromatic diazonium compounds to give isolable triazenes which decompose in the presence of alkali with the formation of the corresponding aromatic azide and the alkyl- or aryl-sulphinic acid salt . This sequence (equation 123), known as the Dutt-Wormall reaction, is mechanisti-... 

Several 1-aryl-6,6a-diselena-l,2-diazapentalenes (169) were made by allowing 3-methyl(ene)-l,2-diselenolium salts (168) to react with arene-diazonium fluoroborates [Eq. (42)]. The structure of the sulfur analog was established by X-ray analysis. ... 

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