Carbon Heck coupling reactions

Palladium-catalyzed carbon-carbon cross-coupling reactions are among the best studied reactions in recent decades since their discovery [102, 127-130], These processes involve molecular Pd complexes, and also palladium salts and ligand-free approaches, where palladium(O) species act as catalytically active species [131-135]. For example, the Heck reaction with aryl iodides or bromides is promoted by a plethora of Pd(II) and Pd(0) sources [128, 130], At least in the case of ligand-free palladium sources, the involvement of soluble Pd NPs as a reservoir for catalytically active species seems very plausible [136-138], Noteworthy, it is generally accepted that the true catalyst in the reactions catalyzed by Pd(0) NPs is probably molecular zerovalent species detached from the NP surface that enter the main catalytic cycle and subsequently agglomerate as N Ps or even as bulk metal. 

Reaction of organic halides with alkenes catalyzed by palladium compounds (Heck-type reaction) is known to be a useful method for carbon-carbon bond formation at unsubstituted vinyl positions. The first report on the application of microwave methodology to this type of reaction was published by Hallberg et al. in 1996 [86], Recently, the palladium catalyzed Heck coupling reaction induced by microwave irradiation was reported under solventless liquid-liquid phase-transfer catalytic conditions in the presence of potassium carbonate and a small amount of [Pd(PPh3)2Cl2]-TBAB as a catalyst [87]. The arylation of alkenes with aryl iodides proceeded smoothly to afford exclusively trans product in high yields (86-93%) (Eq. 61). 

There are a host of palladium-catalyzed C-C coupling reactions that have been developed since the mid-1960s, but only a couple of specific examples will be covered here (cf. Ref 14, 14a). Tsuji discovered Pd-catalyzed tr-allyl and carbon nucleophile coupling reactions in 1965.Mizoroki and Heck " separately developed vinylic coupling reactions (Equation (16)) in the early 1970s, and this is commonly referred to as a Heck reaction or coupling. 

Fujita, S., Yuzawa, K., Bhanage, B.M. et al. (2002) Palladium-catalyzed Heck coupling reactions using different fluorinated phosphine ligands in compressed carbon dioxide and conventional organic solvents. J. Mol. Catal. A Chem., 180, 35-42. 

Martinez, S., Vallribera, A., Cotet, C.L. et al. (2005) Nanosized metallic particles embedded in silica and carbon aerogels as catalysts in the Mizoroki Heck coupling reaction. New J. Chem., 29, 1342-5. 

Intramolecular oxo- or aminopaUadation provides an eflFicient tool for the preparation of various heterocyclic derivatives in recent years. In 2010, Alvarez et al. reported an effcient palladium-catalyzed tandem intramolecular oxopalladation/oxidative Heck coupling reaction toward benzofuians 196 [77] (Scheme 6.54). Nucleophilic attack of the oxygen to the activated carbon-carbon triple bond furnishes vinylpal-ladium intermediate 197, followed by alkene insertion into the C-Pd bond to afford alkylpalladium intermediate 198. The final product was generated via P-hydride elimination of intermediate 198. 

Carbon-carbon cross-coupling reactions dominate catalytic applications of NHC-Pd complexes, demonstrating the importance of this class of reaction and the unique suceess of Pd eompounds in these transformations. In the past 10 years, there has been a substantial body of work published on cross-coupling reactions catalysed by NHC-Pd complexes therefore, to provide some degree of structure to this ehapter, reactions have been broadly grouped under Heck, Suzuki and other types of reaction categories. Not surprisingly there is overlap between sections, and many publications report more than one class of reaction, however the layout does provide a coherent structure. 

Heck reaction, palladium-catalyzed cross-coupling reactions between organohalides or triflates with olefins (72JOC2320), can take place inter- or intra-molecularly. It is a powerful carbon-carbon bond forming reaction for the preparation of alkenyl- and aryl-substituted alkenes in which only a catalytic amount of a palladium(O) complex is required. 

Together with reactions named after Heck and Suzuki, the Stille reac-tion belongs to a class of modern, palladium-catalyzed carbon-carbon bond forming reactions. The palladium-catalyzed reaction of an organotin compound 2 with a carbon electrophile 1 is called Stille coupling. 

Carbon-carbon bond formation reactions and the CH activation of methane are another example where NHC complexes have been used successfully in catalytic applications. Palladium-catalysed reactions include Heck-type reactions, especially the Mizoroki-Heck reaction itself [171-175], and various cross-coupling reactions [176-182]. They have also been found useful for related reactions like the Sonogashira coupling [183-185] or the Buchwald-Hartwig amination [186-189]. The reactions are similar concerning the first step of the catalytic cycle, the oxidative addition of aryl halides to palladium(O) species. This is facilitated by electron-donating substituents and therefore the development of highly active catalysts has focussed on NHC complexes. 

Transition metal-catalysed reactions have emerged as powerful tools for carbon-carbon (C-C) bond formation [1], Cross-coupling reactions (Suzuki-Miyaura, Mizoroki-Heck, Stille, etc.) are recognised to be extremely reliable, robust and versatile. However, some other catalysed arylation reactions have been studied and have been reported to be very efficient [2]. In recent years, A -heterocyclic carbenes (NHC) have been extensively studied and their use as ligands for transition-metal catalysis has allowed for the significant improvement of many reactions [3]. This chapter highlights the use of NHC-bearing complexes in those arylation reactions. 

Another methodology that is widely used for C-C bond formation is the Heck coupling (Heck, 1985 T.suji, 1995). The Heck reaction involves the palladium-catalysed arylation of olefinic double bonds (Eqn. (12)) and provides an alternative to Friedel-Crafts reactions for attaching carbon fragments to aromatic rings. 

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