Palladium catalysts bond formation

The corresponding reaction path for the hydrogenation of an alkene on an adatom, 14, is shown in Scheme 4.3. Here the 3mH2 site, 15, can adsorb the alkene to give the adsorbed species, 16 or 17, which are then saturated using the two hydrogen atoms on the active site. The adsorbed 7t-allyl, 20, may also take part with a palladium catalyst. The formation of this species provides the pathway for the facile double bond isomerization observed over palladium catalysts. ... 

Allylic malonate 100 completely isomerizes to the thermodynamically favored linear isomer 101 on treatment with a palladium catalyst [119]. Formation of a stabilized carbanion and Jt-(allyl)palladium species facilitates the C-C bond cleavage. Analogous isomerization is also catalyzed by a nickel complex [120]. These results demonstrate that the transition metal-catalyzed nucleophilic substitution of an allylic substrate with a carbon nucleophile is reversible, if the cleaved nucleophile is sufficiently stabilized. 

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. 

Muci AR, Buchwald SL (2002) Practical Palladium Catalysts for C-N and C-0 Bond Formation. 

There are relatively few examples of C-C bond formation on solid surfaces under UHV conditions. There are virtually no examples of catalytic C-C bond formation under such conditions. Perhaps the closest precedent for the present studies on reduced Ti02 can be found in the studies of Lambert et al. on single crystal Pd surfaces. Early UHV studies demonstrated that acetylene could be trimerized to benzene on the Pd(lll) surface in both TPD and modulated molecular beam experiments [9,10]. Subsequent studies by the same group and others [11,12] demonstrated that this reaction could be catalyzed at atmospheric pressure both by palladium single crystals and supported palladium catalysts. While it is not clear that catalysis was achieved in UHV, these and subsequent studies have provided valuable insights into the mechanism of this reaction as catalyzed by metals, including spectroscopic evidence for the hypothesized metallacyclopentadiene intermediates [10,13,14]. 

The Suzuki-Miyaura synthesis is one of the most commonly used methods for the formation of carbon-to-carbon bonds [7]. As a palladium catalyst typically tetrakis(triphenylphosphine)palladium(0) has been used, giving yields of44—78%. Recently, Suzuki coupling between aryl halides and phenylboronic acid with efficient catalysis by palladacycles was reported to give yields of 83%. 

Hydroboration of alkenes or alkynes followed by cross-coupling with organic electrophiles provides a straightforward method for the carbon-carbon bond formation (Scheme 1-19). The hydroboration of thioalkynes with catecholborane in the presence of a nickel or palladium catalyst yields P-(aLkylthio)-l-alkenylboronates (72a)... 

Palladium-catalyzed aromatic C—O bond formation is less developed than palladium-catalyzed aryl amination. Except when the aryl halide is strongly electron deficient,107-110 catalysts ligated by the conventional aryl phosphines such as DPPF and BINAP are ineffective for coupling of... 

Scheme 150).225 227 The pyran products predominate when the ratio of triphenylphosphine to palladium catalyst exceeds two whereas the linear oligomers are the major products when this ratio is close to unity. The suggested227 mechanism (Scheme 151) includes a step of insertion of C=0 into a C—Pd palladium-catalyzed reactions leading to the formation of pyranones (see Scheme 152)228 and piperidones (see Scheme 139 in Section V,A,2).211 It is useful to note that the 2,5-divinyltetrahydropyran derivative can be transformed catalytically by ruthenium trichloride into synthetically useful 3,4-dihydro-2//-pyran derivatives (Scheme 153).229... 

The Harmata group s initial report concerned a one-pot, one-operation procedure <99AG(E)2419> for the synthesis of enantiomerically pure 2,1-benzothiazines via the Buchwald-Hartwig reaction reported by Bolm <98TL5731 OOJOC169> for sulfoximine N-arylation. For example, treatment of ortho-bromobenzaldehyde 78 with enantiomerically pure N-H sulfoximine 77a in the presence of a palladium catalyst and base afforded the benzothiazine 79 in 78% yield (Scheme 22). Both C-N bond formation and condensation occurred during the reaction, a phenomenon that appears general for aldehydes like 78. 

A new domino lithium acetylide addition/rearrangement procedure on trans-1,2-dibenzoyl-3,5-cyclohexadiene furnished 3-alkylidene-2,3-dihydrofurans via an intriguing mechanism involving three bond formations and two bond cleavages in one single operation <06SL1230>. The reaction of dimedone with meso-diacetoxycyclohexene in the presence of a palladium catalyst led to the formation of the tricyclic product as depicted below <06S865>. 

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). 

Palladium catalyzed reaction of aryl halides and olefins provide a useful synthetic method for C-C bond formation reaction [171, 172], The commonly used catalyst is palladium acetate, although other palladium complexes have also been used. A sol-vent-free Heck reaction has been conducted in excellent yields using a household MW oven and palladium acetate as catalyst and triethylamine as base (Scheme 6.51) [173], A comparative study revealed that the longer reaction times and deployment of high pressures, typical of classical heating method, are avoided using this MW procedure. 

Metal-catalyzed cross-couplings are key transformations for carbon-carbon bond formation. The applicability of continuous-flow systems to this important reaction type has been shown by a Heck reaction carried out in a stainless steel microreactor system (Snyder et al. 2005). A solution of phenyliodide 5 and ethyl acrylate 6 was passed through a solid-phase cartridge reactor loaded with 10% palladium on charcoal (Scheme 2). The process was conducted with a residence time of 30 min at 130°C, giving the desired ethyl cinnamate 7 in 95% isolated yield. The batch process resulted in 100% conversion after 30 min at 140°C using a preconditioned catalyst. 

The ruthenium-, rhodium-, and palladium-catalyzed C-C bond formations involving C-H activation have been reviewed from the reaction types and mechanistic point of view.135-138 The activation of aromatic carbonyl compounds by transition metal catalyst undergoes ortho-alkylation through the carbometallation of unsaturated partner. This method offers an elegant way to activate C-H bond as a nucleophilic partner. The rhodium catalyst 112 has been used for the alkylation of benzophenone by vinyltrimethylsilane, affording the monoalkylated product 110 in 88% yield (Scheme 34). The formation of the dialkylated product is also observed in some cases. The ruthenium catalyst 113 has shown efficiency for such alkylation reactions, and n-methylacetophenone is transformed to the ortho-disubstituted acetophenone 111 in 97% yield without over-alkylation at the methyl substituent. 

Pyrimidine thioethers may also be synthesized via direct Pd-catalyzed C—S bond formation between halopyrimidines and thiolate anions. For very unreactive thiol nucleophiles such as 2-thiopyrimidine, both a strong base and a palladium catalyst are essential. Without a palladium catalyst or replacing f-BuONa with K2C03, the reaction failed to furnish the desired pyrimidine thioether [49]. 

Sato et al. carried out detailed studies on the possibilities of transformation of tetrazolo[l,5-tf]pyrazines 54 to 2-aminopyrazines 56 < 1994S931 >. These authors found that the generally used methods for this conversion fail because the starting compound exists in the stable bicyclic form 54, whereas partial formation of the azide valence bond isomer 55 would be necessary for the success of the transformation. Application of special reaction conditions succeeded, however hydrogenation over palladium catalyst in the presence of ammonium hydroxide or treatment with stannous chloride in a mixture of methanol and hydrochloric acid solved this problem. Thus, a great number of derivatives of 54 was reduced to the corresponding 2-aminopyrazine 56 in medium to high yields (45-100%). 

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