Heck reaction with aryl diazonium salts

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. 

The Heck arylation of 2-substituted 3,4-dihydro-2//-pyrans with diazonium salts exhibits /ra 5-diastereoselectivity and leads to the 2-aryl-5,6-dihydro-27/-pyrans (Scheme 7) <03CC1656>. Tris(dihydropyranyl)indium undergoes Pd-mediated cross coupling reactions with aryl halides to give mainly 6-aryl-3,4-dihydro-2//-pyrans (Scheme 8) <030L2405>. 

The diazonium group can be replaced by a number of groups. " Some of these are nucleophilic substitutions, with S l mechanisms (p. 432), but others are free-radical reactions and are treated in Chapter 14. The solvent in all these reactions is usually water. With other solvents it has been shown that the Sj-jl mechanism is favored by solvents of low nucleophilicity, while those of high nucleophilicity favor free-radical mechanisms. The N2 group can be replaced by CP, Br, and CN, by a nucleophilic mechanism (see OS IV, 182), but the Sandmeyer reaction is much more useful (14-20). Transition metal catalyzed reactions are known involving aryl-diazonium salts, and diazonium variants of the Heck reaction (13-10) and Suzuki coupling (13-12) were discussed previously. As mentioned on p. 866 it must be... 

Aryl diazonium salts can be used coupled with alkenes in a Heck-Uke reaction (12-15)7 Other reactive aryl species also couple with aryldiazonium salts in the presence of a palladium catalyst7 A Suzuki type coupling (13-9) has also been reported using arylboronic acids, aryldiazonium salts and a palladium catalyst/ ... 

The Matsuda-Heck reaction of an aryl diazonium salt 82 with 1,1,1-trifluoropropene... 

In 1991 at the Central Research Laboratories of Hoechst AG we became interested in the palladium-catalyzed olefination (Heck reaction) of aryl halides and aryl diazonium compounds which is arguably one of the most powerfiil methods for the synthesis of substituted olefins. In collaboration with Herrmann and co-workers we have shown that active catalyst mixtures obtained by using in situ mixtures of Pd(II) salts and commercially available tri-o-tolylphosphine consist under the conditions of the Heck reaction primarily of cyclometaUated paUadacycles l... 

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. ... 

One of the first examples of a Pd-catalyzed coupling reaction realized on an industrial scale is the Matsuda-Heck reaction of an aryl diazonium salt with 1,1,1-trifluoropropene developed by Baumeister and co-workers at Ciba-Geigy and today performed on ton scale by Novartis for the synthesis of a sulfonylurea herbicide (Prosulfuron). By combining three synthetic steps (diazotization, aUcenylation, and hydrogenation) in a... 

Still, there are base-free Mizoroki-Heck reactions, which are known to occur with arene diazonium salts. In this particular case, the absence of base is advantageous, as spontaneous decomposition of diazonium salts is known to be accelerated by nucleophiles and bases. High precatalyst loadings are required, as, for example, in the arylation of disubstited acrylates 10 (9 11, Scheme 2.2) [10] a base-free reaction might only be sustained for a few turnovers. 

Diazonium salts are another useful source of free radicals, and the formation of the reactive species can be achieved by reductive electrolysis or direct treatment with diazonium tetrafluoroborate salts [39]. By this route, several aryl derivatives could be introduced onto the nanotube sidewalls [40]. Aryl groups bearing halogen or alkyne functionalities are particularly interesting as they can be further reacted in Pd-catalyzed coupling reactions (Suzuki, Heck) or in click chemistry reactions to create products with great potential in materials science [41]. 

Although aryl and vinyl halides have found vast applications in Heck coupling, aromatic diazonium salts, particularly the isolable tetrafluoroborates, proved successful in the work of Kikukawa et al. [23 a, b] and a one-pot reaction with the C-C-coupling step [23 c-e]. Extensions of this tandem diazotation Heck reaction technique were reported by Beller et al. (eq. (5)) [23 f], including heterogeneous Pd/carbon catalysts [23 g]. 

Heck reactions are compatible with water (see Chapter 3.2.4) [35], which increases the speed of reaction in the presence of quaternary ammonium salts [36 a]. It is not surprising, then, that aqueous solvents (e. g. CH3CN/H2O) and water-soluble catalysts such as Pd(TPPTS)3 where TPPTS = P(C6H4-m-S03Na)3 [35, 67] can be employed successfully (eq. (10)). However, only aryl and vinyl iodides and aromatic diazonium salts (generated in situ from arylamines in aqueous media) are, up to the present, accessible to this method [36 b-h]. 

Recently colloidal Pd nanoparticles protected with tetraalkylammonium salts have been attracting attention as active catalysts. They are used for Heck and Suzuki-Miyaura reactions without phosphine ligands [6,7]. Most simply, Pd(OAc)2 is used without a ligand, forming some kind of colloidal or soluble Pd(0) species in situ in reactions of active substrates such as aryl iodides and diazonium salts. Pd on carbon without phosphine is active for some Heck and other reactions, but not always. These Pd(0) catalysts without ligands are believed to behave as homogeneous catalysts [8]. 

Aryl halides are frequently prepared from the corresponding aryldiazonium salts by diazotation procedures. However, diazonium salts can be subjected directly to very mild Heck arylation conditions, which deliver coupled products (entry 19). Preferably, the reaction is executed in nonaqueous solvents such as acetonitrile, acetone, or methylene chloride with sodium acetate as base and with palladiumbis(dibenzylideneacetone) as catalyst. Alternatively, a combination of the amine and f-butyl nitrite, in a mixture of acetic acid and monochloroacetic acid, can provide the desired product directly, which makes the isolation of a diazonium salt unnecessary (entry 20). " It is also possible to use aromatic acid anhydrides as oxidative addition precursors (entry 21). Clearly, anhydrides are very interesting starting materials for a number of Heck reactions due to price and absence of halide salt formation. 

Diazonium salts are very useful starting materials for Heck reactions and other cross-coupling reactions (see 4.2.2). The first protocols have been developed by Matsuda and colleagues, and the reaction sometimes is being called Heck-Matsuda reaction. Since the 1970 s, a number of applications have emerged. Moro et al. used an arenediazonium salt in a Heck arylation reaction to obtain the resulting trans-stilbene derivative in excellent yield with complete stereocontrol (Scheme 5-18). ... 

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