T. Kurihara, H. Okutomi, Y. Miseki, H. Kato, A. Kudo, "Highly Efficient Water Splitting over Photocatalyst without Loading Cocatalyst" Chem. Lett., 35, 274 (2006).
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In addition to the azepane cocatalyst, this reaction requires the use of the more strongly reducing photoredox catalyst Ir(ppy)3 and the addition of lithium hexafluoroarsenide (LiAsF6) to promote single-electron reduction of the aryl ketone.
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The Mortreux system consists of the molybdenum precatalyst molybdenum hexacarbonyl Mo(CO)6 and resorcinol cocatalyst. In 1975 T.J. Katz proposed a metal carbyne (i.e. alkylidyne) and a metallacyclobutadiene as intermediates. In 1981 R.R. Schrock characterized several metallacyclobutadiene complexes that were catalytically active.
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Late transition metal hydrophosphination catalysts, i.e. those reliant on the nickel-triad and neighboring elements, generally require alkenes and alkynes with electron withdrawing substituents. A strong base is required as a cocatalyst. Mechanism proposed for hydrophosphination catalyzed by a Pt(II) phosphido complex.
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These catalysts are metallocenes together with a cocatalyst, typically MAO, −[O−Al(CH3)]n−. The idealized metallocene catalysts have the composition Cp2MCl2 (M = Ti, Zr, Hf) such as titanocene dichloride. Typically, the organic ligands are derivatives of cyclopentadienyl. In some complexes, the two cyclopentadiene (Cp) rings are linked with bridges, like −CH2−CH2− or >SiPh2.
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Oxidative carbonylation is an alternative to phosgenation. The advantage is the avoidance of phosgene. Using copper catalysts, dimethylcarbonate is prepared in this way: : 2 MeOH + CO + 1/2 O2 -> MeO(CO)OMe + H2O Diphenyl carbonate is also prepared similarly, but using palladium catalysts. The Pd-catalyzed process requires a cocatalyst to reconvert the Pd(0) to Pd(II).
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Tetrahedron 2009, 65, 8185 300px MBH reaction can involve proline derivative as a cocatalyst. It was proposed that imidazole nucleophilic catalyst and proline effect the reaction via iminium intermediate.Chem. Eur, J. 2009, 15, 1734 With (S)-proline and DABCO, α-amido sulfones and α,β-unsaturated aldehydes undergo a highly enantioselective aza-MBH reaction (46-87% yield, E/Z 10:1-19:1, 82-99% ee).
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Isotactic PB-1 is synthesized commercially using two types of heterogeneous Ziegler–Natta catalysts. The first type of catalyst contains two components, a solid pre-catalyst, the δ-crystalline form of TiCl3, and solution of an organoaluminum cocatalyst, such as Al(C2H5)3. The second type of pre-catalyst is supported. The active ingredient in the catalyst is TiCl4 and the support is microcrystalline MgCl2.
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The precatalyst are prepared by alkylation and arylation of the phosphinimide complexes is possible through alkyllithium or Grignard reagents, giving products such as CpTi(NPR3)Me2. The zirconium complexes (R3PN)2ZrCl2 can be alkylated or arylated through simple substitution. These organoTi and organoZr complexes are activated by treatment with MAO and B(C6F5)3 as a cocatalyst to activate polymerization through methyl abstraction. The phosphinimide catalyst is thought to be homogeneous and single sited.
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Commercial synthesis of syndiotactic polypropylene is carried out with the use of a special class of metallocene catalysts. They employ bridged bis-metallocene complexes of the type bridge-(Cp1)(Cp2)ZrCl2 where the first Cp ligand is the cyclopentadienyl group, the second Cp ligand is the fluorenyl group, and the bridge between the two Cp ligands is -CH2-CH2-, >SiMe2, or >SiPh2. These complexes are converted to polymerization catalysts by activating them with a special organoaluminum cocatalyst, methylaluminoxane (MAO).
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The White catalyst was originally developed for use in a branched allylic acetoxylation reaction. An enantioselective version of this reaction was subsequently reported, using chromium(III) salen fluoride as a chiral cocatalyst. A macrolactonization reaction based on the branched allylic esterification was developed for the preparation of 14- to 19-membered macrolides. This method was applied to the total synthesis of 6-deoxyerythronolide B. In addition to acetate, a wide variety of carboxylic acids may be employed as nucleophiles in the branch allylic esterification reaction.
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Early transition metals can easily form oxides; therefore, protection groups, like the use of methylaluminoxane (MAO) due to its Lewis acidity, can be used to prevent side reactions from happening. As a cocatalyst, MAO is well known for their use in metallocene chemistry as they activate metallocene complexes for olefin polymerization. To remove the MAO protecting group, the reaction can be treated with acid. Instead of MAO, trimethylsilyl (TMS) have also been used to protection functional groups such as amine, since the amine functionality can easily react with other olefins to form branched polymer chains.
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1999, 121, 10219-10220 The phenolic oxygen of β-ICD was shown to be important in the reaction, implying the function of Bronsted acid moiety. β-ICD and its related versions are effective catalysts for various other substrates. 300px Cyclopentenone and various aromatic and aliphatic aldehydes undergo an asymmetric reaction using Fu's planar chiral DMAP catalyst in isopropanol (54-96% yield, 53-98% ee). In this case, magnesium iodide as a Lewis acid cocatalyst was required to accelerate the reaction.Chem. Commun. 2010, 46, 2644-2646 P-Chiral phosphines have been investigated. 250px Simple diamines can also be employed as MBH catalysts.
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The group proved [Ru(bpy)3]Cl2 to be an efficient photocatalyst for the formal [2+2] cycloaddition of enones and yields potential for development of new reaction protocols with reduced environmental impact. Yoon's group has also researched into crossed intermolecular [2+2] heterodimerizations, proving the possibility of using two dissimilar enone substrates to successfully produce these dimers. This method bypasses some synthetic limitations of cycloadditions conducted under standard UV photolysis conditions. Yoon reviews the ways how cocatalyst strategies can be applied to synthesis, ranging from developments in organic photochemistry and the precedents that brought interest in photocatalytic synthesis.
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Acta Crystallogr., 19: 967-971. β-In2S3 has two subtypes. In the T-In2S3 subtype, the tetragonally-coordinated vacancies are in an ordered arrangement, whereas the vacancies in C-In2S3 are disordered. The disordered subtype of β-In2S3 shows activity for photocatalytic H2 production with a noble metal cocatalyst, but the ordered subtype does not.Chai, B.; Peng, T.; Zeng, P.; Mao, J. (2011.) “Synthesis of Floriated In2S3 Decorated With TiO2 Nanoparticles for Photocatalytic Hydrogen Production Under Visible Light.” J. Mater. Chem., 21: 14587. . β-In2S3 is an N-type semiconductor with an optical band gap of 2.1 eV.
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As the vast majority of CW polymerization catalysts is based on late- transition metal complexes, having generally lower oxophilicity, these complexes were demonstrated also to provide copolymerisation of olefins with polar monomers like acrylates, alkylvinylketones, ω-alken-1-ols, ω-alken-1-carboxylic acids etc., which was the main initial intention of development of this class of catalysts. These random copolymers could further be utilized in the construction of sophisticated amphiphilic grafted copolymers with hydrophobic polyolefin core and shell based on hydrophilic arms, in some cases made of stimuli-responsive polymers. Example of a nickel precatalyst that, upon activation with suitable cocatalyst (MAO, organoaluminiums), promotes chain walking.
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Although this reaction relies on the use of a secondary amine organocatalyst to generate the enamine species which is oxidized in the proposed mechanism, no enantioselective variant of this reaction exists. Diagram of Photoredox beta-arylation of aldehydes The development of this direct β-arylation of aldehydes led to related reactions for the β-functionalization of cyclic ketones. In particular, β-arylation of cyclic ketones has been achieved under similar reaction conditions, but using azepane as the secondary amine cocatalyst. A photocatalytic "homo-aldol" reaction works for cyclic ketones, allowing the coupling of the beta-position of the ketone to the ipso carbon of aryl ketones, such as benzophenone and acetophenone.
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In one application (C6F5)3B forms noncoordinating anions by removing anionic ligands from metal centers.Fuhrmann, H.; Brenner, S.; Arndt, P.; Kempe, R. “Octahedral Group 4 Metal Complexes That Contain Amine, Amido, and Aminopyridinato Ligands: Synthesis, Structure, and Application in α-Olefin Oligo- and Polymerization”, Inorganic Chemistry, 1996, 35, 6742-6745. Illustrative is a reaction that give rise to alkene polymerization catalysts where tris(pentafluorophenyl)boron is used as an activator or cocatalyst: :(C6F5)3B + (C5H5)2Zr(CH3)2 → [(C5H5)2ZrCH3]+[(C6F5)3BCH3]− In this process, the strongly coordinating methyl group transfers to the boron to expose a reactive site on zirconium. The resulting cationic zirconocene species is stabilised by the non coordinating borane anion.
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The interaction between an excited photocatalyst and organic molecule can show a diverse sample of reactive intermediates that can be manipulated to form a synthetic bond construction. This impacts the photocatalyst and the photoactivation steps such as the interaction with the excited state of the photocatalyst or controlling the rate and selectivity of the photoactivation steps. Additionally, Yoon takes a dual approach to the asymmetric of enantioselective [2+2] photocycloadditions by using visible light that can absorb transition metal and a Lewis acid cocatalyst. Yoon was able to see that each catalyst can be enabled to be independent resulting in a broader scope and greater flexibility and efficiency in enantioselective photochemical cycloadditions.
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Hydrocyanation is important due to the versatility of alkyl nitriles (RCN), which are important intermediates for the syntheses of amides, amines, carboxylic acids and esters. The most important industrial application is the nickel-catalyzed synthesis of adiponitrile (NC-(CH2)4-CN) synthesis from buta-1,3-diene (CH2=CH-CH=CH2). Adiponitrile is a precursor to hexamethylenediamine (H2N-(CH2)6-NH2), which is used for the production of certain kinds of Nylon. The DuPont ADN process to give adiponitrile is shown below: :Butadiene hydrocyanation This process consists of three steps: hydrocyanation of butadiene to a mixture of 2-methyl-butene-3-nitrile (2M3BM) and pentene-3-nitrile (3PN), an isomerization step from 2M3BM (not desired) to 3PN and a second hydrocyanation (aided by a Lewis acid cocatalyst such as aluminium trichloride or triphenylboron) to adiponitrile.
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The severe limitations on the arene component scope in this reaction is due primarily to the need for an arene radical anion that is stable enough not to react directly with enamine or enamine radical cation. In the proposed mechanism, the activated photoredox catalyst is quenched oxidatively by an electron-deficient arene, such as 1,4-dicyanobenzene. The photocatalyst then oxidizes an enamine species, transiently generated by the condensation of an aldehyde with a secondary amine cocatalyst, such as the optimal isopropyl benzylamine. The resulting enamine radical cation usually reacts as a 3 π-electron system, but due to the stability of the radical coupling partners, deprotonation of the β-methylene position gives rise to a 5 π-electron system with strong radical character at the newly accessed β-carbon.
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The use of copper-cocatalyst in addition to palladium complexes in Sonogashira's procedure enabled the reactions to be carried under mild reaction conditions in excellent yields. A rapid development of the Pd/Cu systems followed and enabled myriad synthetic applications, while Cassar-Heck conditions were left, maybe unjustly, all but forgotten. The reaction's remarkable utility can be evidenced by the amount of research still being done on understanding and optimizing its synthetic capabilities as well as employing the procedures to prepare various compounds of synthetic, medicinal or material/industrial importance. Among the cross-coupling reactions it follows in the number of publications right after Suzuki and Heck reaction and a search for the term "Sonogashira" in Scifinder provides over 1500 references for journal publications between 2007 and 2010.
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Much more productive are processes based on the biotechnologically easily accessible itaconic acid, which produces exclusively itaconic anhydride in yields of up to 98% at temperatures of 165-180 °C and pressures of 10-30 mmHg in the presence of catalytic quantities of strong acids, such as concentrated sulphuric acid. Synthese von Itaconsäureanhydrid aus Itaconsäure In order to avoid overheating and thus higher proportions of citraconic anhydride, the dehydration reaction can also be carried out in higher boiling aromatic solvents such as toluene or xylene in the presence of acidic montmorillonite or in cumene in the presence of methanesulfonic acid. In both variants yields of 95-97 % of itaconic anhydride are achieved. A more recent process of cyclizing dicarboxylic acids with diethyl carbonate in the presence of a chromium-salene complex with µ-nitrido-bis(triphenylphosphane) chloride as cocatalyst quantitatively provides itaconic anhydride contaminated with citraconic anhydride (18%) already at 40 °C in 1 millimolar preparations.
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