Catalytic bond formation reactions

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. 

The simplest C-C bond formation reaction is the nucleophilic displacement of a halide ion from a haloalkane by the cyanide ion. This was one of the first reactions for which the kinetics under phase-transfer catalysed conditions was investigated and patented [l-3] and is widely used [e.g. 4-12], The reaction has been the subject of a large number of patents and it is frequently used as a standard reaction for the assessment of the effectiveness of the catalyst. Although the majority of reactions are conducted under liquiddiquid two-phase conditions, it has also been conducted under solidrliquid two-phase conditions [13] but, as with many other reactions carried out under such conditions, a trace of water is necessary for optimum success. Triphase catalysis [14] and use of the preformed quaternary ammonium cyanide [e.g. 15] have also been applied to the conversion of haloalkanes into the corresponding nitriles. Polymer-bound chloroalkanes react with sodium cyanide and cyanoalkanes under phase-transfer catalytic conditions [16],... 

As for heterogeneous olefin polymerization catalysis, the activity of rare-earth metal catalysts may be also enhanced in organic transformations by the use of silica supports or other carriers [7]. Indeed, several catalytic C-C and C-X (with X = H/D, Si, O) bond formation reactions as weU as functional group transformations witness to the potential of SOLn/AnC-based heterogeneous catalysts for fine chemical synthesis. 

Enantiomerically pure d.v-1 -amino-2-indanol and its derivatives have been used as ligands in numerous catalytic asymmetric carbon-hydrogen, carbon-carbon, and carbon-heteroatom bond formation reactions. The conformationally constrained indanyl platform has emerged as a particularly valuable backbone in a variety of catalytic processes leading to high levels of asymmetric induction. The aminoindanol 1 has also been used as a resolution agent (Chapter 8) as well as a chiral auxiliary (Chapter 24). For the synthesis of 1 see Chapter 24. 

Ca2+ is an essential ion in the photoassembly reaction in all PSII OECs examined to date. The experimental data indicated that Ca2+ is involved in accelerating the final step in the S-state catalytic cycle, involving the 0-0 bond formation reaction [29],... 

The examples discussed so far are all transition metal complexes. As we will see later (Chapters 4-9), most homogeneous catalytic processes are indeed based on transition metal compounds. However, catalytic applications of rare earth complexes have also been reported, although so far there has not been any industrial application. Of special importance are the laboratory-scale uses of lanthanide complexes in alkene polymerization and stereospecific C-C bond formation reactions (see Sections 6.4.3 and 9.5.4). 

Low valent metal complexes can generate aryl- or vinyl-metal bonds by oxidative addition of aryl or vinyl halides (Appendix 1), which can undergo various catalytic C-C bond formation reactions. The industrially most significant ones are described in this Section. 

MgO nanoparticles have proven to be very effective chemical reagents in the Claisen-Schmidt condensation, which is a very valuable C—C bond-formation reaction commonly employed in the pharmaceutical and fine chemical industries [295]. When MgO(lll) nanosheets were employed for the Claisen-Schmidt condensation of benzaldehyde and acetophenone, they were found to exhibit activity superior to other systems, such as AICI3, BE3, POCI3, alumina and other reported nano-crystaUine MgO samples [296]. This development is particularly noteworthy as it represents a potential heterogenization of the Claisen-Schmidt catalytic process, which offers numerous advantages including easier product recovery and catalyst recycling. 

These three techniques are employed along with others not mentioned here to investigate the catalytic nature of a reaction. It is difficult to obtain positive confirmation for one catalytic nature over another because of the ability of small amounts of homogeneous catalyst (concentrations below current detection methods) to catalyze reactions [11]. Leached atoms can readsorb rapidly to heterogeneous structures, either to a substrate or to the surface of the nanoparticles [17,18], In the following sections, we review some of the major results involving colloidal nanoparticles in solution-phase catalysis. The two reaction types that will be discussed in this chapter are redox reactions and carbon-carbon bond formation reactions. 

Several investigators have attempted to identify the nature of the metal species that catalyze these coupling reactions in solution. Due to the harsh conditions of these reactions and the thermodynamic instability of colloidal particles, it is highly possible and probably likely that multiple catalytic mechanisms occur simultaneously. A few review articles on carbon-carbon bond formation reactions catalyzed with colloidal transition metal nanoparticles are available [7, 11]. 

Significant progress in the application of this concept has also been made in catalytic C—C bond formation reactions. Chirik and co-workers recently reported an interesting case involving [2jc +2jc] cycloaddition of dienes and enynes using the bis-dinitrogen... 

In the course of evolution, nature has devised a multitude of enzymes which are capable of stereoselectively forming C-C bonds in vivo. Depending on the kind of substrate employed, nucleophilic acylation reactions are accomplished in nature by means of different lyases such as transketolases and pyruvate decarboxylases, which all require thiamine (1) as coenzyme [1,2,3,4,5,6]. For the synthetic organic chemist, asymmetric catalytic C-C bond formation reactions, in... 

We reported on the synthesis of monomeric (NHC)Pd(allyl)Cl, (NHC)Pd(acac)Cl, ° and (NHC)Pd(carboxylate) complexes," among many architectures," and we studied their activation mechanisms and catalytic activities. The synthesis of most of these complexes is directly related to a successful in situ generation and use of NHC and the appropriate palladium source. All these complexes display very high activity as precatalysts for C-C and C-N bond formation reactions. As an added advantage, all of these Pd(ll) complexes are air and moisture stable, and some are already commercially available. Here, we focus on the synthesis and applications of two of those well-defined complexes (NHC)Pd(allyl)Cl and (NHC)Pd(acac)Cl. The multigram syntheses described herein are straightforward and afford the desired complexes in excellent yields. 

Stereochemistry of the C-0 bond cleavage has been initially elucidated indirectly by the following experiments. Catalytic C-C bond formation reaction of (Z)-3-acetoxy-5-carbomethoxycyclohexene with sodium dimethyl malonate... 

Allylic alkylations are among the most widely applied catalytic C-C bond formation reactions in organic chemistry. In the 1980s already mononitrosyl ferrate complexes of type 31 were reported to be active in regioselective allylic alkylations [94, 95]. However, this pioneering work suffered from low turnover numbers and the reaction had to be carried out under CO atmosphere. 

A basic site can be described in terms of its ability to accept a proton (Bronsted base) or to donate electrons (Lewis base). In order for base-catalysed reactions to occur, the basicity must be sufficient to stabilise an anionic or polarised species with a significant negative charge that forms part of the catalytic cycle. For a typical base-catalysed C-C bond formation reaction (such as a Michael addition or the Knoevenagel condensation (Scheme 9.18)), the basic site stabilises the intermediate carbanion, which can then act as a nucleophile in the C-C bond formation. Surface sites on MgO are typical strong basic sites. 

In 2007, synthesis and complexation of a PSiP-pincer hgand, in which a sihcon atom and two phosphorus atoms are tethered by a phenylene group, was first reported by Turculet and coworkers [11-19]. They reported that the PSiP-ruthenium complex exhibited catalytic activity for transfer hydrogenation of ketones [11], and the PSiP-platinum and -palladium complexes efficiently catalyzed reduction of COj to methane by silanes [18[. Shortly after the Turculefs first report in 2007, we reported the first example of utilization of the phenylene-bridged PSiP-pincer complex in carbon-carbon bond formation reactions of unsaturated hydrocarbons [20[. 

However, till now catalytic reactions of ethene and carbon dioxide are not known. But the great number of successful stoichiometric reactions between these two molecules makes it probable to achieve the catalytical bond formation in the near future. A short survey about the various reactions of ethene and other alkenes with carbon dioxide is given in the following sections in the order of transition metals applied. 

Catalytic processes from the viewpoint of green chemistry include catalytic reductions and oxidations methods, solid-acid and solid-base catalysis, as well as carbon-carbon bond formation reactions (31). 

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