Alkene oxidative arylations

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... 

Scheme 2. Oxidative arylation of alkenes by electrophilic palladium (I I).

The arylation of activated alkenes with aryl halides in the presence of base was discovered by R. F. Heck in 1971 and is now one of the standard methods for C—C bond formation. The catalysts are mostly palladium or nickel phosphine complexes, which react via a succession of oxidative addition and insertion reactions, as shown in the following simplified cycle ... 

This transformation matured to being a highly efficient tool for the synthesis of substituted alkenes [116-118]. Thus, for instance, effective palladium-catalyzed regioselective oxidative arylation of alkenes [119] were developed, which enabled oxidative coupling to proceed at ambient temperature (Scheme 9.46). 

The rate of epoxidation of alkenes is increased by alkyl groups and other ERG substituents, and the reactivity of the peroxy acids is increased by EWG substituents." These structure-reactivity relationships demonstrate that the peroxy acid acts as an electrophile in the reaction. Low reactivity is exhibited by double bonds that are conjugated with strongly EWG substituents, and very reactive peroxy acids, such as trifluoroperoxyacetic acid, are required for oxidation of such compounds. " Strain increases the reactivity of alkenes toward epoxidation. Norbornene is about twice as reactive as cyclopentene toward peroxyacetic acid." trani-Cyclooctene is 90 times more reactive than cyclohexene." Shea and Kim found a good correlation between relief of strain, as determined by MM calculations, and the epoxidation rate. ° There is also a correlation with ionization potentials of the alkenes. Alkenes with aryl substituents are less reactive than unconjugated alkenes because of ground state stabilization and this is consistent with a lack of carbocation character in the TS. 

Also oxidative arylation of alkenes can be carried out by the reaction of phenyl-phosphonic acid 182 with alkenes [83]. 

Regioselectivity is one of the major problems of Mizoroki-Heck reactions. It is supposed to be affected by the type of mechanism ionic versus neutral, when the palladium is ligated by bidentate P P ligands. The ligand dppp has been taken as a model for the investigation of the regioselectivity. Cabri and Candiani [Ig] have reported that a mixture of branched and linear products is formed in Pd°(P P)-catalysed Mizoroki-Heck reactions performed from electron-rich alkenes and aryl halides (Scheme 1.26a) or aryl ttiflates in the presence of halide ions (Scheme 1.26b). This was rationalized by the so-called neutral mechanism (Scheme 1.27). The neutral complex ArPdX(P P) is formed in the oxidative addition of Pd°(pAp) yj Qj. Q aj.yj triflates in the presence of halides. The carbopalladation... 

While all these approaches point to various possible and often elegant solutions to the waste problem of the Mizoroki-Heck reaction, each still has its drawbacks. Thus, in the short term, customized solutions to minimize the waste effluent will be required for specific synthetic applications, until, in the long run, the ideal, generally applicable method is found. This could on the one hand ensue from the development of shape-selective C—H activation catalysts that by themselves are able to define the position of functionalization, to be used in the oxidative arylation of alkenes with either molecular oxygen or hydrogen peroxide... 

The hydroarylation of aHc5mes, which has emerged as a powerful alternative to the oxidative arylation of alkene, is not discussed in this chapter as it is not a true Heck alkenylation. Jia, C., Piao, D., Oyamada, J. et al. (2(X)0) Efficient activation of aromatic C—H bonds for addition to C—C multiple bonds. Science, 287,1992-5. 

Scheme 1.12 Palladium

Scheme 1.13 Palladium-catalyzed oxidative arylation of alkenes with arenes (Fujiwara,...

Also of interest is that Fe(acac)j catalyzes the oxidative arylation of vinylsilanes such as 298 using Cl(CH2)2Br as an oxidant [221]. The presence of a coordinating pyridyl group correctly placed, is needed to obtain the arylated alkene with complete regio- and stereocontrol (Scheme 4.66). 

The highly selective sy/j-double arylation or alkenylation reaction of alkynes or alkenes with aryl or alkenylstannane reagents was realized via a possible oxidative addition, insertion, transmetalation, and reductive elimination mechanism (eqs 31 and 32). ... 

Oxoesters are oxidized with Mn(OAc)3 to the corresponding radicals that can add intermolecularly or intramolecularly (eq Ib)" to generate alkyl radicals. In the presence of Cu(OAc)2 the latter are rapidly quenched and oxidized to give alkenes. Radical arylation with alkyl iodides can be induced with dibenzoyl peroxide the yield of the reaction can be improved using a catalytic amount of Cu(0Ac)2-H20, which minimizes hydrogen abstraction by the intermediate radical but introduces a competitive electron-transfer oxidation of the intermediate radical. The oxidative addition of disulfides to alkenes (Trost hydroxysulfenylation ) can be promoted by catalytic amounts ofCu(OAc)2. ... 

The reactions of the second class are carried out by the reaction of oxidized forms[l] of alkenes and aromatic compounds (typically their halides) with Pd(0) complexes, and the reactions proceed catalytically. The oxidative addition of alkenyl and aryl halides to Pd(0) generates Pd(II)—C a-hondi (27 and 28), which undergo several further transformations. 

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