Palladium silk

Chapter 3 presents data about enantioselective hydrogenation reactions on metal catalysts supported on chiral carriers. Discussed are palladium supported on modified silica gels, effective chiral colloidal catalysts, template catalytic systems, palladium-silk, palladium-wool, and palladium polypeptide catalysts. 

Silk-palladium s. Palladium-silk Siloles s. Dibenzosiloles C-Siloxanecarboxylic acids... 

Asymmetric hydrogenations have been reported with palladium on silk 123>, palladium on modified cellulose 124> and on modified ion exchange resins 125 >. Also with Raney Nickel modified with amino adds 126> and peptides, 27>. Platinum-carbon catalysts exhibiting shape selectivity have been made by coating them with a thermosetting resin, which is carbonized. In such a way an organic molecular sieve skin is formed over the original catalyst 128>. 

Heterogeneous catalysts modified by the addition of chiral substances have been used to hydrogenate olefins asymmetrically, but only a few effective chiral heterogeneous catalyst systems have been found. Palladium deposited on silk fibroin was used to hydrogenate 4-benzylidene-2-methyl-5-oxazolone asymmetrically to give, after hydrolysis, optically active phenylalanine (Fig. lc). The optical purity1 of the product was found to be dependent on the origin of the fibroin and its chemical pretreatment (4-6). 

Raney nickel has been modified with amino acids and other chiral reagents to give catalysts that have been used to effect asymmetric reductions (7). However, these catalysts suffer from some of the same kinds of vagaries that have been observed for the palladium on silk fibroin catalysts. For example, the optical purities of the products were found to be very dependent on pH and the method of catalyst preparation. 

Enantioselective heterogeneous catalytic hydrogenation using a chiral catalyst was pioneered by Aka-bori and Izumi, who prepared a palladium catalyst supported on silk fibroin. The oxime acetates of diethyl a-ketoglutarate or of ethyl phenylpyruvate were hydrogenated to form glutamic acid (7-15% ee) and phenylalanine (30% Similarly, a palladium-poly-L-leucine catalyst was used for the asym-... 

The carbonates, sulphates, and borates are decomposed. The sulphides of the alkalies and alkaline earths are decomposed while the sulphides of arsenic, antimony, molybdenum, zinc, cadmium, tin, iron, lead, copper, mercury, and palladium are not attacked. Cobalt sulphate is not attacked, while the sulphates of the alkalies and alkaline earths are attacked and dissolved. Alkali tungstates, ammonium arsenite and arsenate, copper arsenite, ammonium magnesium arsenate, ammonium molybdate and vanadate, potassium cyanide and ferrocyanide are decomposed. Paraffin is not attacked shellac, gum arabic, gum tragacanth, copal, etc., are decomposed. Celluloid is slowly attacked. Silk paper, gun cotton, gelatin, parchment are dissolved. M. Meslans 22 has studied the esterification of alcohol by hydrofluoric acid. 

Since silk fibroin of cultured silkworms contains relatively large amount of achiral glycine residue, silk fibroin cS wild lkworms with a h er content of chiral alanine residue was examined as the carrier of palladium and optical yields of S<+) 14.4% and R-(+) 32.1% were obtained in reaaions (1) and (2), re ctively (3). l n amino groups of silk fibroin of cultured silk worm were acetylated and used as the carrier, the optical yield in reaction (2) was R-(+) 65,9%. 

The active site of the catalyst has been considered to be metallic palladium deposited in the -structure of silk fibroin as an a mmetric chelate. However, as a result of the examination of the reproducibility of these atymmetric hydrogenation reactions, the asymmetric modification Cpommii ) of palladium by S-a-amino acid was suspected, the latter being concomitantly formed by the l rdrolysis of silk protein with hydrochloric acid from palladium(II) chloride in the course of the catalyst preparation (4). 

Historically, it took many years to achieve the almost perfect stereocontrol of an enantioselechve hydrogenahon reaction using a heterogeneous catalyst, with a resultant 97-99% enanhomeric excess (ee) of the product One of the oldest such examples is that of tartaric acid-modified platinum black for the hydrogenahon of an oxime to give a chiral amine of <20% ee [2]. Nonetheless, many reviews in this field have provided a clear history of the enanhoselechve catalysis from silk-palladium-a palladium metal supported on a chiral silk fiber to hydrogenate a... 

Scheme 10.1 Enantioselective hydrogenation and optical yield over the silk-palladium catalyst.

Izumi Y. (1959) Studies with silk-palladium catalyst. I-III. Bull. Chem. Soc. Jpn. 32,932 - 945. 

Alternatively, the surface of a metal can be modified with an enantiomerically pure additive. For example, in 1956 palladium metal/silk fibroin was used for the hydrogenation of alkenes with moderate enantioselectivity. However, most success with modified metal siufaces has been achieved in the hydrogenation of ketones. The reduction of 3-keto esters with a Raney nickel/tartaric acid/sodium bromide catalyst provides good enantioselectivities. For example, P-keto ester (3.108) affords the P-hydroxy ester (3.109). Platinum metal modified with cinchona alkaloids has been successfully used with a-keto esters. The reaction yields product with up to 90% ee in the reduction of a-keto ester (3.110). ... 

Pd-asbestos, Pd-pumice Pd-C, Pd-silk Pd-Pb-CaCOg Bis( triphenylphos-phine)(maleic anhydride)palladium PdO... 

A number of asymmetric reductions have been reported using asymmetric hydrogenation catalysts. Akabori et al. (1956) reduced (XII) on a palladium on silk catalyst and obtained, after hydrolysis, optically active phenylalanine. Hartung (Beamer et al., 1960) has discussed such asymmetric reductions in terms of asymmetric active sites on the catalyst surface. An ordinary catalyst is envisaged as having equal numbers of enantiomorphic sites, some of which, however, may be linked so that only one at a time may function. To prepare a catalyst having a disparity... 

The first study, conducted by Sun et al., demonstrated the feasibility of the approach by producing hbers from a polymer poly(dodecylthiophene) (PDT) and a metal salt [palladium (II) diacetate (Pd(OAc)2)], either of which can form hbers by themselves. A subsequent study, by Yu et al. showed that ultra-hne hbers can be produced using a highly dilute solution of a polymer in the core, which would otherwise produce droplets due to jet break-up when spun alone. This concept was illustrated by the use of dilute solutions of PAN (3 and 5 wt%) in the core wrapped in PAN-co-PS. Removal of the sheath after spinning resulted in very hne PAN hbers (100 nm or less) with narrow and unimodal distribution. More recently, in a study based on silk and PEO (core/sheath) polymers, with water as the solvent for both, it was reported that the crystallized silk hbers (converted from the random coil I conformation to the stable y -sheet silk II structure) could be produced after anneaUng at high humidity (90% RH at 25°C) and subsequent water extraction of the PEO sheath. ... 

The idea of using a protein as a chiral source originates from the work by Akabori et al. (1956) who tried an asymmetric reduction of acetoximes from benzoylformate and a-ketoglutarate by means of hydrogenation catalyzed by palladium on silk fibroin. Reduced products were obtained in 30 and 7 % optical yields, respectively. 

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