Diazonium salts, aryl with alkenes

The diazonium salts 145 are another source of arylpalladium com-plexes[114]. They are the most reactive source of arylpalladium species and the reaction can be carried out at room temperature. In addition, they can be used for alkene insertion in the absence of a phosphine ligand using Pd2(dba)3 as a catalyst. This reaction consists of the indirect substitution reaction of an aromatic nitro group with an alkene. The use of diazonium salts is more convenient and synthetically useful than the use of aryl halides, because many aryl halides are prepared from diazonium salts. Diazotization of the aniline derivative 146 in aqueous solution and subsequent insertion of acrylate catalyzed by Pd(OAc)2 by the addition of MeOH are carried out as a one-pot reaction, affording the cinnamate 147 in good yield[115]. The A-nitroso-jV-arylacetamide 148 is prepared from acetanilides and used as another precursor of arylpalladium intermediate. It is more reactive than aryl iodides and bromides and reacts with alkenes at 40 °C without addition of a phosphine ligandfl 16]. 

Some observations are important for improvement of the yield and for the elucidation of the mechanism of the Meerwein reaction. Catalysts are necessary for the process. Cupric chloride is used in almost all cases. The best arylation yields are obtained with low CuCl2 concentrations (Dickerman et al., 1969). One effect of CuCl2 was detected by Meerwein et al. (1939) in their work in water-acetone systems. They found that in solutions of arenediazonium chloride and sodium acetate in aqueous acetone, but in the absence of an alkene, the amount of chloroacetone formed was only one-third of that obtained in the presence of CuCl2. They concluded that chloroacetone is formed according to Scheme 10-50. The formation of chloroacetone with CuCl2 in the absence of a diazonium salt (Scheme 10-51) was investigated by Kochi (1955 a, 1955 b). Some Cu11 ion is reduced by acetone to Cu1 ion, which provides the electron for the transfer to the diazonium ion (see below). 

Alkenes activated by an electron-withdrawing group (Z may be C=C, halogen, C=0, Ar, CN, etc.) can be arylated by treatment with a diazonium salt and a cupric chloride catalyst. This is called the Meerwein arylation reaction Addition of... 

Aiyl diazonium salts can be used coupled with alkenes in a Heck-like reaction (14-20). Other reactive aryl species also couple with aryldiazonium salts in the... 

Free-radical addition of an aryl group and a hydrogen has been achieved by treatment of activated alkenes with a diazonium salt and TiCls. ... 

The compound ArX can be added across double bonds, in a free-radical process, by treatment of alkenes with diazonium salts, although Meerwein arylation (substi-... 

Normally, the most practical vinyl substitutions are achieved by use of the oxidative additions of organic bromides, iodides, diazonium salts or triflates to palladium(0)-phosphine complexes in situ. The organic halide, diazonium salt or triflate, an alkene, a base to neutralize the acid formed and a catalytic amount of a palladium(II) salt, usually in conjunction with a triarylphosphine, are the usual reactants at about 25-100 C. This method is useful for reactions of aryl, heterocyclic and vinyl derviatives. Acid chlorides also react, usually yielding decarbonylated products, although there are a few exceptions. Likewise, arylsulfonyl chlorides lose sulfur dioxide and form arylated alkenes. Aryl chlorides have been reacted successfully in a few instances but only with the most reactive alkenes and usually under more vigorous conditions. Benzyl iodide, bromide and chloride will benzylate alkenes but other alkyl halides generally do not alkylate alkenes by this procedure. 

Aryldiazonium salts react with bis(dibenzylideneacetone)palladium to form arylpalladium salts and nitrogen. Therefore, diazonium salts may be employed to catalytically arylate alkenes under mild conditions. Since many aryl halides are made from diazonium salts this variation could even be more convenient than using aryl halides. The reaction proceeds in good to excellent yields in nonaqueous solvents, using sodium acetate as the base at room temperature with terminal alkenes and cyclopentene." Internal alkenes usually give poor yields, however. 

Palladium(II) salts apparently oxidize arylamines to arylpalladium salts since alkenes are arylated by reaction with only an aromatic amine and a palladium salt. However, yields are generally low.100 Much better yields are obtained if /-butyl nitrite is added and, of course, this forms the diazonium salt in situ. This not only saves a step but some diazonium salts which are too unstable to be isolated may be used as well. The reactions are carried out in the presence of acetic or chloroacetic acid with 5-10% bis(di-benzylideneacetone)palladium as catalyst (equation 41).101... 

In contrast to classical Meerwein arylations, non-activated alkenes are well suited for this reaction type for two reasons. First, due to the relatively slow formation of azo compounds by addition of aryl radical 49 to 48, this undesired pathway cannot compete successfully with the attack of 49 on the alkene to give radical adduct 50. Second, a nucleophilic alkyl radical 50 arises from the addition step, which is effectively trapped by electrophilic salt 48 to give azo compound 51. As a result of several improvements, the methodology is now applicable for a wide range of polar to non-polar alkenes with almost no restrictions on the substitution pattern of the diazonium salt [101, 102]. Moderate diastereoselectivities have been obtained in first attempts with chiral auxiliaries [103]. The azo compounds accessible, such as 51, can be converted to carboamination products 52 by hydrogenation and to various other heterocycles. 

The Pd )-catalyzed reaction of aryl diazonium salts with mono-substituted alkenes [1] was found to be an interesting alternative to the well-known Pd - catalyzed arylhalide alkene coupling (Heck type reaction) or the copper mediated reaction of aryl diazonium salts with alkenes (Meerwein arylation) [2], The reaction can be run without isolation of the diazonium salt in presence of only 0.5 to 1 mol% of the Palladium catalyst in a one pot procedure, in high yield and under nuld conditions. The resulting styrene is reduced in a subsequent hydrogenation step with an in situ generated heterogeneous Pd-catalyst. The combination of three reaction steps without isolation of intermediates and the virtually complete recovery of the Pd-metal at the end of the reaction sequence makes this process [4] extremely efficient. 

Diazonium salts are the most reactive source of arylpalladium species and the reaction can be carried out at room temperature using Pd2(dba)3 as a catalyst in the absence of both phosphine ligand and base [44]. The reaction of diazonium salts means indirect substitution reaction of an amino group of anilines or aromatic nitro group with an alkene. The use of diazonium salts is synthetically more convenient than the use of aryl halides, because many aryl halides, particularly iodides, are prepared from diazonium salts. 

Besides the traditional coupling between alkenes and aryl (vinyl) halides, other functionalized aryl derivatives can also couple with alkenes in the Heck reaction, including aryl silanes,stannanes, bismuth, antimony,triflates, boric acid, phosphonic acid, carboxylic acid, and diazonium salt. ... 

Aryl diazonium ions can also be used to form certain types of carbon-carbon bonds. The reaction of diazonium salts with certain conjugated alkenes results in... 

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). 

The conversion of ArN2 salts into ArH takes place in hexamethyl-phosphoramide (by a long radical chain process), and also by the use of the naphthalene anion radical CioHs" in THF. Alternatively, deamination of ArNH2 can be carried out with Bu ONO in DMF. Unsymmetrical biaryls can be formed in a phase-transfer reaction between the diazonium fluoroborate (in an arene solvent) and KOAc complexed with 18-crown-6 polyether. ° Meerwein aryl-ation of fluorinated olefins with ArN2 and copper(ii) halides has been described, but an alternative procedure for the overall addition of ArHal to an alkene involves the use of ArNH2, Bu ONO, and copper(ii) halides in acetone or... 

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