Purely organic catalysts

Purely organic catalysts (Continued) electrochemistry, 342 hydrogen-bonded associates, 328 phase-transfer reactions, 333 photochemistry, 341 polymerization, 332... 

Considerable effort has been devoted to the development of enantiocatalytic MBH reactions, either with purely organic catalysts, or with metal complexes. Paradoxically, metal complex-mediated reactions were usually found to be more efficient in terms of enantioselectivity, reaction rates and scope of the substrates, than their organocatalytic counterparts [36, 56]. However, this picture is actually changing, and during the past few years the considerable advances made in organocatalytic MBH reactions have allowed the use of viable alternatives to the metal complex-mediated reactions. Today, most of the organocatalysts developed are bifunctional catalysts in which the chiral N- and P-based Lewis base is tethered with a Bronsted acid, such as (thio)urea and phenol derivatives. Alternatively, these acid co-catalysts can be used as additives with the nucleophile base. 

The triazolium salt 2 has also been used as a purely organic catalyst [17]. It is an active catalyst for asymmetric benzoin-type condensation reactions yielding the reaction products with enantiomeric excesses of 20-80%, which at the time marked a major advance with respect to the previously established catalysts (Scheme 5, Eq. 1) [18]. It was also found to catalyze the asymmetric intramolecular Stetter reaction with moderate to good enantioselectivities (41-74% ee) (Scheme 5, Eq. 2) [19]. 

III. Biomimetic oxidation catalysis based on purely organic catalysts 17... 

Fortunately, as the reaction is transferred from a purely organic solvent system to mixed organic-aqueous media, which are employed in most RP-HPLC separations, the apparent multiplicity of maxima in the time profile of the intensity dependence seems to be suppressed or to collapse to a reasonably simple biexponential-like dependence. As shown in Figure 11, simply changing the solvent from ethyl acetate to 95% aqueous acetonitrile and the catalyst from triethylamlne to imidazole produces a single maximum profile, one that is more easily modeled mathematically, as defined in Equation 4 ... 

The development of catalysts for the efficient oxidation of catechol and its derivatives in water is topic of ongoing work in this laboratory. Towards this end, polyethylene glycol side-chains were incorporated in a pentadentate salen ligand to enhance the water solubility of the complexes derived thereof. A dinuclear copper(II) complex is found to catalyze the oxidation of 3,5-di-tert.-butylcatechol into 3,5-di-tert-butyl-o-benzoquinone more than twice as fast in aqueous organic solution as in purely organic solvents (ly,at/knon= 140,000). Preliminary data are discussed. 

C02 as co-solvent Here, C02 is used to solubilize a catalyst, which is insoluble in the pure organic liquid, in an expanded organic phase. Once the C02 is removed at the end of the reaction, the catalyst precipitates and can be removed by filtration. 

Concept A variety of metal-based chiral catalysts effect enantioselective chemical transformations that allow for concise and efficient synthesis of optically pure organic molecules. These transformations,... 

The reverse reaction is catalysed by copper sulphate in an ethanol/water (50 50) mixture297 298. Indium(III) chloride catalysis of Diels-Alder reactions was also reported, but the effects were poor and comparison to uncatalysed reactions was made only in a few cases299,300. A very versatile Lewis acid catalyst for such reactions is methylrhenium trioxide (MTO)300. This catalyst can be used without a solvent, in pure organic solvents like chloroform and even in pure water. While the catalyst is active in the latter two solvents (Table 22), it gives the best results in water (Table 23). 

Thauei R. K., Klein, A. R. and Hartmann, G. C. (1996) Reactions with molecular hydrogen in microorganisms Evidence for a purely organic hydrogenation catalyst. Chem. Rev. (Washington, D. C.), 96, 3031 2. 

Pure alumina catalyst prepared either by hydrolysis of aluminum isopropoxide or by precipitation of aluminum nitrate with ammonia, and calcined at 600-800°, contains intrinsic acidic and basic sites, which participate in the dehydration of alcohols. The acidic sites are not of equal strength and the relatively strong sites can be neutralized by incorporating as little as 0.1 % by weight of sodium or potassium ions or by passing ammonia or organic bases, such as pyridine or piperidine, over the alumina. 

Despite the fact that aryl bromides are generally less reactive, o- and p-bromotoluenes could be efficiently vinylated with ethene in DMF/H2O with [Pd(OAc)2] + P(o-tolyl)3 as catalyst and Et3N as base [16]. With careful choice of reaction parameters (90 °C and 6 bar of ethene) all bromotoluene was converted to high purity ortho- or para-vinyltoluene. Under the conditions used, the reaction mixture forms two phases. In this case the main role of water is probably the dissolution of triethylamine hydrobromide which otherwise precipitates from a purely organic reaction medium and causes mechanical problems with stirring. 

Two patterns are possible in the activation mechanism by simple chiral Lewis base catalysts. One is through the activation of nucleophiles such as aUyltrichlorosilanes or ketene trichlorosilyl acetals via hypervalent silicate formation using organic Lewis bases such as chiral phosphoramides or A-oxides. " In this case, catalysts are pure organic compounds (see Chapter 11). The other is through the activation of nucleophiles by anionic Lewis base conjugated to metals. In this case, transmetal-lation is the key for the nucleophile activation. This type of asymmetric catalysis is the main focus of this section. 

The main emphasis has so far been placed on metal-containing catalysts, both in the field of dendritic catalysis and in organic catalysis in general. However, the current trend is increasingly towards catalysis with purely organic compounds. A number of example of such metal-free - and dendritic - catalysts,... 

Apart from enzymes and catalytic antibodies, chemical catalysis has been shown to be of utility for such a transformation by using either metal-containing complexes or purely organic molecules. In both cases, the catalyst plays a key role in activating both the pronucleophilic carbonyl compound and the electrophilic aldehyde it also imparts effective stereoinduction. The most representative metal-ligand complexes and organocatalysts yet reported in this context [7] are depicted in Figs. 1 and 2, respectively. 

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