Suzuki couplings with palladium carbenes

The first examples of microwave-assisted cross-couplings with organozinc compounds were recently reported [47]. In addition, the first high-speed synthesis of aryl boronates (Suzuki coupling reactants) has been performed under the action of single-mode irradiation with an in-situ-generated palladium carbene catalyst [48],... 

Both 6-alkanesulfanyl- and 6-alkanesulfonylpurines proved competent coupling partners in the palladium-catalyzed Suzuki reaction with boronic acids when an imidazolium carbene ligand was used <20050L1149>. The sulfones were found to be significantly more reactive (60 °C, THF) than the thioethers (90 °C, toluene) (Scheme 44). 

Imidazolium-linked ortbo-cyclophane reacts with nickel(II) and palladium(II) salts in the presence of acetate base to afford complexes where a metal centre is bound by a pair of heterocyclic carbenes, which themselves are part of a cyclophane skeleton. These cyclophane-metal complexes have been characterized by NMR spectroscopy and X-ray diffraction studies. They are highly active as promoters of Suzuki couplings [69]. 

The performance of complex 10 bearing a quinolinylidene as a remote car-bene ligand (Figure 5.5) was compared to that of simple imidazol-2-ylidene and phosphine-containing palladium catalysts in the Heck coupling of activated and non-activated aryl bromides with butyl acrylate. " The remote carbene complex showed much higher activity in these reactions as well as in the Suzuki coupling of a deactivated aryl bromide with phenylboronic acid. Since the complexes tested were structurally very different, purely electronic comparisons... 

Initial research in this area focused on the use of zerovalent Pd2(dba)3 as the palladium source, the carbene IMes, and CS2CO3 as base [43]. This reagent combination afforded a 59% yield in the coupling of 4-chlorotoluene with phenylboronic acid. The catalytic protocol could be simplified by the use of air-stable IMes-HCl that is deprotonated in situ with CS2CO3 isolated yields for the Suzuki-Miyaura coupling then reached >95% (Scheme 8). Subsequently a further simplification of the catalytic system was achieved, by extension of the reaction to air-stable Pd(II) precursors [44]. 

On the nature of the active species in palladium catalyzed Mizoroki-Heck and Suzuki-Miyaura couplings (particularly, Pd complexes with imi-nopyridine and N-heterocyclic carbenes as ligands) 06ASC609. 

Apart from palladacycles, a number of catalyst systems are currently known that show productivities up to 100,000 for Heck and Suzuki reactions of all kinds of aryl bromides. It is important to note that coupling reactions of electron-deficient aryl bromides (e.g., 4-bromoacetophenone), which are often used in academic laboratories, are not suitable as test reactions to judge the productivity of a new catalyst, because simple palladium salts without any Ugand give turnover numbers up to 100,000 with these substrates. Recently, palladium complexes in combination with sterically congested basic phosphines (e.g., tri-tcrt-butylphosphine), carbenes, and also phosphites led to productive palladium catalysts for the activation of various aryl chlorides. 

Trudell and Nolan have reported that palladium adducts of Arduengo s carbene hgand, l,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes) [59], are active catalysts for Suzuki cross-couplings of a wide variety of aryl chlorides with arylboronic acids (Equation 2.33) [60]. 

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