Base-catalyst mediation

SCHEME 8.56. The base-catalyst-mediated asymmetric vinylogous Mukaiyama aldol reaction by Denmark and Heemstra. 

The sensitivity of cellular constituents to environmental extremes places another constraint on the reactions of metabolism. The rate at which cellular reactions proceed is a very important factor in maintenance of the living state. However, the common ways chemists accelerate reactions are not available to cells the temperature cannot be raised, acid or base cannot be added, the pressure cannot be elevated, and concentrations cannot be dramatically increased. Instead, biomolecular catalysts mediate cellular reactions. These catalysts, called enzymes, accelerate the reaction rates many orders of magnitude and, by selecting the substances undergoing reaction, determine the specific reaction taking place. Virtually every metabolic reaction is served by an enzyme whose sole biological purpose is to catalyze its specific reaction (Figure 1.19). 

In contrast to the situation with copper-based catalysis, most studies on ruthenium-based catalysts have made use of preformed metal complexes. The first reports of ruthenium-mediated polymerization by Sawamoto and coworkers appeared in I995.26 In the early work, the square pyramidal ruthenium (II) halide 146 was used in combination with a cocatalyst (usually aluminum isopropoxide). 

The same group reported a palladium-mediated oxidation of methane to a methanol derivative employing a CuCl2 and Pd/C-based catalyst system and dioxygen in a trifluoroacetic acid/water mixture.18 A system was also described, which mediated the oxidation of ethane (Equation (10)). 

Uozumi has explored a series of (25, 4/ )-4-hydroxyproline-derived 2-aryl-6-hydroxy-hexahydro-lFf-pyrrolo[l,2-c]imidazolones as potential alternatives to cinchona alkaloid-based catalysts for the alcoholative ASD of meio-anhydrides (Fig. 16) [226]. Uozumi screened a small library of catalysts prepared by a four-step, two-pot reaction sequence from 4-hydroxyproline in combination with an aldehyde and an aniline. The most selective member, compound 67, mediated the methanolytic ASD of cw-hexahydrophthalic anhydride in 89% ee when employed at the 10 mol% level for 20 h at -25 °C in toluene [226]. 

It is well established that metallic copper or copper salts efficiently catalyze N- and O-arylation reaction using pentavalent and trivalent organobismuth compounds [5-9, 24]. The C-arylation reaction of phenols and active methylene compounds using pentavalent organobismuth compounds are usually mediated by a base. However, in some cases, copper catalysts mediate C-arylation using pentavalent organobismuth compounds. 

In a pair of papers last year, Scott Nelson of the University of Pittsburgh expanded the range of the ketene aldol . In the first paper (J. Am. Chem. Soc. 2004,126, 14), he employed a chiral Al-based catalyst 3. This catalyst mediated additions such as propionyl bromide 1 to 2 to give 4 in 98 2 syn/anti ratio and 95% . 

J. H. Koh, and J. Park, Dynamic kinetic resolution of allylic alcohols mediated by ruthenium- and lipase-based catalysts,... 

Pyrrolidine-amine-based catalysts such as 18 and 19 can also mediate the addition of ketones to alkylidene malonates (Scheme 2.40) [14]. Aldehydes (e.g., isova-leraldehyde) do not react with vinylidenemalonate 16 under these conditions. 

Surprisingly few studies have been directed towards the development of noncinchona alkaloid-based catalysts for the alcoholative ASD of meso-anhydrides, or indeed any of the enantioselective alcoholysis processes. Uozumi has reported a series of (2S, 4R)-4-hydroxyproline-derived 2-aryl-6-hydroxyhexahydro-lfi-pyr-rolo[l,2-c] imidazolones which mediate the methanolytic ASD of ds-hexa-hydrophthalic anhydride in up to 89% ee when employed at the 10 mol% level for 20 h at —25 °C in toluene [186]. Additionally, Nagao has described the use of a bifunctional chiral sulfonamide for the thiolytic ASD of meso-cyclic anhydrides in up to 98% ee when employed at the 5 mol% level for 20 h at rt in ether [187]. 

The activation energies available for binary and ternary catalyst systems are in a range from 20 to 70 kj mol x, which according to Odian is characteristic for polymerizations mediated by Ziegler/Natta-catalysts [456]. It is interesting to note that the polymerization of dienes catalyzed by NdCl3-based catalyst systems show lower activation energies than Nd-carboxylate-based catalyst systems. The influence of halide donors on the temperature depen-... 

Louie and Grubbs prepared an iron-based catalyst for atom transfer radical polymerization (ATRP) [49]. By heating a solution of Iz Prim and FeX2 (X = Br, Cl), crystals of Fe(Iz Prim)2X2 were obtained. These complexes mediated the homogeneous ATRP of styrene and methyl methacrylate with... 

Commercially available aminophosphine 4 provided even better yields in the coupHng of acycHc secondary amines [42]. The resulting catalyst was found to be so active that the reaction could often be conducted at room temperature. For example, Di- -butylamine was efficiently reacted with 4-bromotoluene in 96% isolated yield at room temperature, Eq. (27). In addition, electron-rich, electronically neutral, and electron-deficient aryl bromides were effectively utihzed with this new system. The 4/Pd-based catalysts also mediate the coupling of J -alkylanihnes that bear electron-donating substituents on the amine partner. A Xantphos/Pd-catalyst is effective in the coupling of electron-poor alkylaryl-amines with electron-poor aryl bromides. 

Silver can mediate oxidation reactions and has shown unique reactivity. In a few cases, namely, nitrene-, carbene-, and silylene-transfer reactions, novel reactivity was found with homogeneous silver catalysts. Some of these reactions are uniquely facilitated by silver, never having been reported with other metals. While ligand-supported silver catalysts were extensively utilized in enantioselective syntheses as Lewis acids, disappointingly few cases were reported with oxidation reactions. Silver-catalyzed oxidation reactions are still underrepresented. Silver-based catalysts are cheaper and less toxic versus other precious metal catalysts. With the input of additional effort, this field will undoubtedly give more promising results. 

Transition metals have already established a prominent role in synthetic silicon chemistry [1 - 5]. This is well illustrated by the Direct Process, which is a copper-mediated combination of elemental silicon and methyl chloride to produce methylchlorosilanes, and primarily dimethyldichlorosilane. This process is practiced on a large, worldwide scale, and is the basis for the silicones industry [6]. Other transition metal-catalyzed reactions that have proven to be synthetically usefiil include hydrosilation [7], silane alcdiolysis [8], and additions of Si-Si bonds to alkenes [9]. However, transition metal catalysis still holds considerable promise for enabling the production of new silicon-based compounds and materials. For example, transition metal-based catalysts may promote the direct conversion of elemental silicon to organosilanes via reactions with organic compounds such as ethers. In addition, they may play a strong role in the future... 

Due to increasing environmental problems, the use of metal catalysts will gradually be reduced in the future the reduction in the early use of lead- and chromium-based catalysts is evidence of this. With stricter regulations governing the release of metals, the cost of catalyst recovery and environmental remediation is quickly making noncatalytic processes for the production of carboxylic acids preferable. Coupled with recent advances in the field of biocatalysis, metal-mediated oxidations may give way to alternative processes as we enter a new millennium of chemistry. 

---