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Quartz, chiral support

The first successful experiments were reported by Schwab [16] Cu, Ni and Pt on quartz HI were used to dehydrogenate racemic 2-butanol 23. At low conversions, a measurable optical rotation of the reaction solution indicated that one enantiomer of 23 had reacted preferentially (eeright-handed quartz gave the opposite optical rotation it was deduced that the chiral arrangement of the crystal was indeed responsible for this kinetic resolution (for a review see [8]). Later, natural fibres like silk fibroin H5 (Akabori [21]), polysaccharides H8 (Balandin [23]) and cellulose H12 (Harada [29]) were employed as chiral carriers or as protective polymer for several metals. With the exception of Pd/silk fibroin HS, where ee s up to 66% were reported, the optical yields observed for catalysts from natural or synthetic (H8, Hll. H13) chiral supports were very low and it was later found that the results observed with HS were not reproducible [4],... [Pg.75]

Strategies to induce chirality in a prochiral substrate included modification of existing heterogeneous catalysts by addition of a naturally occurring chiral molecules, such as tartaric acid, natural amino acids, or alkaloids, and the implementation of chiral supports, which include quartz or natural fibers, for metallic catalysts. Both strategies have been successful on a limited basis. [Pg.229]

The desire to produce enantiomerically pure pharmaceuticals and other fine chemicals has advanced the field of asymmetric catalytic technologies. Since the independent discoveries of Knowles and Homer [1,2] the number of innovative asymmetric catalysis for hydrogenation and other reactions has mushroomed. Initially, nature was the sole provider of enantiomeric and diastereoisomeric compounds these form what is known as the chiral pool. This pool is comprised of relatively inexpensive, readily available, optically active natural products, such as carbohydrates, hydroxy acids, and amino acids, that can be used as starting materials for asymmetric synthesis [3,4]. Before 1968, early attempts to mimic nature s biocatalysis through noble metal asymmetric catalysis primarily focused on a heterogeneous catalyst that used chiral supports [5] such as quartz, natural fibers, and polypeptides. An alternative strategy was hydrogenation of substrates modified by a chiral auxiliary [6]. [Pg.143]

This is the earliest strategy for preparing a chiral solid hydrogenation-dehydrogenation catalyst (for reviews see [l-3,5,7,8]). Quartz, silk fibroin, cyclodextrin, and cellulose were applied as chiral supports of natural origin. With a Pd/silk catalyst up to 66 % optical yield was obtained in C=C bond hydrogenation, but subsequently the results proved irreproducible. [Pg.449]

The asymmetric adsorption of some organic and complex compounds on quartz crystals was described in Chapter 1.2. In 1932 Schwab and cowoikers were first to show that chiral quartz crystals can be used as chiral supports for metal catalysts Seven years later Stankiewicz, in his dissertation... [Pg.32]

An early approach toward chiral heterogeneous catalysts was the deposition of the catalyticaUy active metal or metal oxide particles onto intrinsically chiral supports such as quartz [24], cellulose [25], or synthetic chiral polymers [26-28]. Hydrogenation and dehydration reactions were tested, but enantioselective performance was found to be poor. In a recent review, Mallat et al. [29] attributed this poor enantioselectivity to the fact that only a small fraction of the metal atoms would... [Pg.109]

The first reported attempts of what was then called "absolute or total asymmetric synthesis" with chiral solid catalysts used nature (naturally ) both as a model and as a challenge. Hypotheses of the origin of chirality on earth and early ideas on the nature of enzymes strongly influenced this period [15]. Two directions were tried First, chiral solids such as quartz and natural fibres were used as supports for metallic catalysts and second, existing heterogeneous catalysts were modified by the addition of naturally occuring chiral molecules. Both approaches were successful and even if the optical yields were, with few exceptions, very low or not even determined quantitatively the basic feasibility of heterogeneous enantioselective catalysis was established. [Pg.75]

Thus, the Soai reaction is a template-directed self-replicating system that successfully maintains exponential growth kinetics and high autocatalytic efficiency over many turnovers. The results support the view that multiple and diverse ways exist to obtain chiral biomolecules via CPL or chiral inorganic crystals such as quartz combined with asymmetric autoctalysis. It is, however, important to remember that the Soai reaction must be carried out in nonaqueous solvents under prebiotically unrealistic conditions. [Pg.28]

This chapter summarizes data about the application of chiral metal catalysts supported on optically active quartz crystals in hydrogenation and other reactions. Despite the low enantioselective efficiency of these catalysts, recent result show that almost 100% enantioselectivity results when they are involved in autocatalytic processes. [Pg.31]

Schwab and Rudolph " prepared chiral catalysts by supporting the metals on the surfaces of ferreted fine powdered optically active quartz crystals, which proved to be active during as3nnmetric dehydrogenation and dehydration of racemic butan-2-ol. The dehydration-dehydrogenation reactions of butan-2-ol (Scheme 2.1.) were carried out in the vapor phase at... [Pg.32]

Chiral solid catalysts usually have two functions, activation and control. The activating function ensures that the solid actually catalyzes a reaction (chemical catalysis), and the control function provides the stereochemical direction that yields the required enantiomer. Early studies were carried out with metallic catalysts supported on inherently chiral solids such as quartz, cellulose (Harada and Yoshida, 1970), and polypeptides (Akabori et al., 1956 Beamer et al., 1967), in which the metal provided the activating function and the support provided the control function. More recent emphasis has been on binding chiral molecules to nonchiral supports. [Pg.276]

Before synthetic chiral stationary phases were developed, attempts were made to use naturally occurring chiral materials for the stationary phase. Quartz, wool, lactose and starch were inadequate but triacetylated cellulose has met with some success. The synthetic stationary phases introduced by Pirkle are able to interact with solute enantiomers in three ways, one of which is stereochemically dependent. Typically these interactions are based on hydrogen bonding, charge transfer (rc-donoi -acceptor based) and steric repulsive types. An independent chiral stationary phase therefore consists of chiral molecules each with three sites of interaction bound to a silica (or other) support. Early work in this area demonstrated that 5-arginine bound to Sephadex would resolve 3,4-dihydroxy-phenylalanine, and that direct resolution of chiral helicenes could be accomplished with columns packed with 2-(2,4,5,7-tetranitro-9-fluorenylideneaminoxy)-propionamide or tri-P-naphthol-diphosphate amide. Amino acid esters have also been resolved with a silica bound chiral binaphthyl crown ether, but better separations are achieved with A-acylated amino acid derivatives with amino-acid derived chiral stationary phases. [Pg.41]


See other pages where Quartz, chiral support is mentioned: [Pg.186]    [Pg.95]    [Pg.337]    [Pg.104]    [Pg.416]    [Pg.187]    [Pg.187]    [Pg.104]    [Pg.148]    [Pg.100]    [Pg.1280]    [Pg.356]    [Pg.356]    [Pg.161]    [Pg.312]    [Pg.930]    [Pg.110]    [Pg.484]   
See also in sourсe #XX -- [ Pg.101 ]

See also in sourсe #XX -- [ Pg.101 ]




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