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Heck-Mizoroki reaction NHCs

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. [Pg.14]

Scheme 6.6 Different NHC architectures used as hgands for the Mizoroki-Heck reaction... Scheme 6.6 Different NHC architectures used as hgands for the Mizoroki-Heck reaction...
Regarding bis-NHC chelating ligands, several structures that differ in the motifs used for the enlargement of the tether have been proposed as catalysts for the Mizoroki-Heck reaction. They range from non-functionalised aliphatic chains [23-25] to phenyl [26], biphenyl [27], binaphthyls [28] and to chains containing additional coordination positions like ethers [29], amines [30], and pyridines in an evolution towards pincer complexes [31-35], In most cases, the activity of aryl bromides in Mizoroki-Heck transformations was demonstrated to be from moderate to high, while the activation of chlorides was non-existent or poor (Scheme 6.7). [Pg.162]

Other classes of complexes that have been studied in depth in the Mizoroki-Heck reaction are those having a bidentate ligand containing both a NHC and a phosphine. The development of these structures was encouraged by early theoretical work from Rosch, who calculated that such ligands should be promising catalysts for this... [Pg.162]

As mentioned in the discussion of the reaction mechanism for this transformation, the active species is a dicoordinate Pd(0) complex, and it is unclear whether an associative or a dissociative process is operative for oxidative addition. In this context, different NHC complexes containing only one carbene ligand have been tested in the Mizoroki-Heck reaction. The most successful are those prepared by Beller, which were able to perform the Mizoroki-Heck reaction of non-activated aryl chlorides with moderate to good yields in ionic liquids (Scheme 6.13). The same compounds have also been applied to the Mizoroki-Heck reaction of aryldiazonium... [Pg.165]

Other successful examples of catalysts containing NHC ligands are found in palladium- and nickel-catalyzed carbon-carbon bond formations. The catalyst development with these metals has focused in particular on Heck-type reactions, especially the Mizoroki-Heck reaction itself [Eq. (42)] and various cross coupling reactions [Eq. (43)], e.g., the Suzuki-Miyaura reaction ([M] = and the Kumada-Corriu reaction ([M] = MgBr). " Related reactions like the Sonogashira coupling [Eq. (44)]326-329 Buchwald-... [Pg.42]

The Mizoroki-Heck reaction in liquid imidazolium salts as the solvent is a special case of an in situ system Under the reaction conditions NHC complexes of palladium are formed as the active catalyst from the solvent and the ligand-free palladium precursor. In general, ionic liquids are novel reaction media for homogeneous catalysis. They allow easy separation of product and catalyst after the reaction. ... [Pg.46]

PdX2(Cb)2 (X = halide, acetate) precursors may be formed from a Pd(II) salt (e.g. Pd(OAc)2) and A-heterocyclic azolium salts which are deprotonated into the NHC ligand [Ip, 64, 66a—c]. They are also generated in situ when iV-heterocyclic azolium salts are used as ionic liquid solvents [66d,e]. Isolated stable NHC-ligated Pd(0) complexes [67] are also used as catalysts in Mizoroki-Heck reactions [68]. [Pg.32]

DFT calculations by Rosch and cowoikers [74] showed that for PhPdCl(P Cb) complexes, where the bidentate hgand P Cb is a monophosphine Unked to an NHC, the reaction with an alkene proceeds via dissociation of the more labile phosphine that is, via a neutral Cb-linked ArPdX complex (Scheme 1.48). Later on, bidentate P Cb ligands generated in situ from phosphine-imidazohum salts proved to be efficient in Mizoroki-Heck reactions employing aryl bromides - even deactivated ones, as pioneered by Nolan and coworkers [Iq, 75]. Interestingly, the DFT calculations have paved the way to fhiitful experiments. [Pg.35]

The use of ionic liquids in Mizoroki-Heck reactions has been described in several reviews [9, 12, 24-26]. Modern trends of Mizoroki-Heck reactions in ionic liquids are the application of new ligand types, like A-heterocycfic carbene (NHC) ligands, supported catalysts, combination with ultrasound irradiation, microwave irradiation and multiphase catalysis. The combination of ionic liquids and activation methods is summarized in the representative chapters. [Pg.497]

In the quest for additional active catalysts for the Mizoroki-Heck reaction, the advent of N-heterocychc carbene (NHC) ligands was warmly welcomed [32], Transition metal-transition metal-phosphine complexes, and have therefore attracted considerable interest as competitive alternatives in Mizoroki-Heck chemistry, which requires high reaction temperatures. Since the seminal application of NHC ligands in Mizoroki-Heck arylations by Herrmann et al. [33], several research groups have introduced novel palladium catalyst-NHC ligand combinations. These were tested and assessed in standard couplings of simple iodo- or bro-moarenes 60 and activated acceptors such as acrylates 61 or styrene (63) [32], and a selection of impressive examples is summarized in Scheme 7.14. [Pg.230]

Scheme 7.14 Different protocols for Mizoroki-Heck reactions using NHC-ligands. Scheme 7.14 Different protocols for Mizoroki-Heck reactions using NHC-ligands.
The reaction of Michael acceptors (21) with aryl iodides, catalysed by NHC complexes of Pd(0), can switch between the Mizoroki-Heck reaction (22) and the conjugate addition (23) by choice of the base. " ... [Pg.379]

The Mizoroki Heck reaction (Equation (9.1)) is a valuable synthetic tool for the catalytic formation of a range of aryl-olefins, and other related building blocks, widely used in the pharmaceutical and fine chemicals industries. Many Pd-based compounds are known to catalyse the reaction to varying extents. In the past 10 12 years, NHC-based systems were added to the list, and seemed to provide some advantages with respect to other catalysts. [Pg.253]

In recent years, ionic liquids have become extremely popular as a green alternative to conventional reaction solvents. Given that a number of the most popular ionie liquids used are 1,3-dialkylated imidazolium salts, their true role in transition metal catalysed reactions, particularly in the presence of base, must be scrutinised. Following the initial report of a successful Mizoroki-Heck reaction performed in an ionic liquid by Earle and co-workers, Xiao and co-workers were able to isolate a number of isomeric [(NHC)2PdBr2] complexes from the reaction mixture [eqn (2.6)]. Under stoichiometric conditions, heating [Pd(OAc)2] and sodium acetate in the ionic liquid, BMIM HBr (l-butyl-3-methylimidazolium bromide), the dimeric palladium carbene complex, [(BMIM)PdBr(p-Br)]2 could be readily obtained, which upon further heating generated bis(NHC)-Pd complexes. [Pg.81]

Some of the most stable palladium-NHC complexes reported to date feature pincer-type CNC or CCC ligands, where the two terminal ligands are NHCs (Figure 2.4). For example, complex 92 decomposed in refluxing N,N-dimethylacetamide (DMA) (bp = 165 °C), depositing palladium black after eight hours, while 93 was unchanged after 24 hours at this temperature. In Mizoroki-Heck reactions, catalyst 93 had no induction period and no loss of catalytic activity was found in the presence of metallic mercury. [Pg.107]

Related reactions, that have been catalysed by NHC/Pd systems, are the intramolecular Mizoroki-Heck using catalysts formed in situ from imidazolium salts and a Pd(0) source [69], and the arylation of allylic alcohols by a benzothiazole-Pd complex [70,71] (Scheme 6.14). [Pg.166]

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. [Pg.191]

A similar situation prevails in other C-C and C-N coupling reactions since they also contain a PdL key intermediate. It is therefore no surprise that mixed NHC/phosphane ligand systems have been employed for the Mizoroki-Heck, Suzuki-Miyaura and StiUe reactions [238,255-258]. In all these cases, the incorporation of a phosphane ligand instead of the second NHC ligand improves the activity of the catalytic reaction. Similar results are reported for the allylic alkylation of dimethylmalonate using mixed NHC/phosphane palladium catalysts [252]. [Pg.116]

The benchmark was set by Herrmarm s carbene complex 65 which, under optimized reaction conditions, showed a respectable TON of 13000. In a similar study by Baker et al., the Mizoroki-Heck arylation of iodobenzene (60b) with 63 in the presence of catalyst 66 bearing a chelating NHC ligand with a cyclophane backbone showed an outstanding TON of 7100000 [34]. In another study, Buchmeiser et al. introduced a new class of NHC based on 1,3-disubstituted tetrahydropyrimi-... [Pg.230]

Scheme 1.19 The first NHC-catalyzed Heck-Mizoroki reaction reported by Herrmann s group [51]. Scheme 1.19 The first NHC-catalyzed Heck-Mizoroki reaction reported by Herrmann s group [51].

See other pages where Heck-Mizoroki reaction NHCs is mentioned: [Pg.161]    [Pg.161]    [Pg.162]    [Pg.165]    [Pg.44]    [Pg.52]    [Pg.118]    [Pg.240]    [Pg.32]    [Pg.37]    [Pg.128]    [Pg.503]    [Pg.232]    [Pg.205]    [Pg.118]    [Pg.90]    [Pg.100]    [Pg.504]    [Pg.163]    [Pg.203]    [Pg.322]    [Pg.47]    [Pg.118]    [Pg.119]    [Pg.258]   
See also in sourсe #XX -- [ Pg.230 , Pg.231 ]




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