Stereospecific adsorption

The Kinetics of the Stereospecific Polymerization of a-Olefins G. Natta and I. Pasquon Surface Potentials and Adsorption Process on Metals... 

The enantiomorphs of a given sugar were found to exhibit no appreciable difference in Ka for example, the values for D- and L-mannose were 0.626 and 0.627, respectively. It appears, therefore, that stereospecific adsorption to the gel, which could possibly result from the presence of the dissymmetric residues of D-glucopyranose in the dextran matrix, is not a significant factor. 

The results summarized in the previous sections show that, for the considered sample of a-TiCU, there are two types of adsorption of the triethyl-aluminum on a-titanium trichloride, one of them is connected with active centers which are directly active in the stereospecific polymerization of propylene. 

The Kinetics of the Stereospecific Polymerization of a-Olefins G. Natta and I. Pasquon Surface Potentials and Adsorption Process on Metals R. V. Culver and F. C. Tompkins Gas Reactions of Carbon P. L. Walker, Jr., Frank Rusinko, Jr., and L. G. Austin The Catalytic Exchange of Hydrocarbons with Deuterium C. Kemball... 

Quartz can be chiral. A study by Bernal et al. (44) showed that quartz may have played a role as an adsorptive substrate which served as a template in the abiotic evolution of biopolymers. Additionally, it has been reported that the stereospecific adsorption of various organic molecules such as vitamin B12 and d,l camphor can occur on quartz. Moreover, it is also possible to separate the two enantiomers by adsorption onto powdered d or 1 quartz (45). 

The increase in isotacticity seems to be essentially connected to the decrease of the initial rate, as practically no change in the isotacticity index with polymerization time was detected. Moreover, while the atactic productivity decreases monotonically with the EB/TEA ratio in both systems, the isotactic productivity has a more complex behavior with the binary catalyst it remains almost unchanged up to EB/TEA s 0.25 and then falls, whereas with the ternary catalyst it increases up to EB/TEA 0.2 and then rapidly drops. On the grounds of these results, Spitz suggested that the reversible adsorption on the catalytic surface of the TEA EB complex (which is supposed to be very fast) changes the non specific centers into stereospedfic, though less active, centers, while the slower adsorption of free EB reversibly poisons both types of sites. The differences between the binary and the ternary catalysts would arise mainly from the presence, in the latter, of a larger number of potential stereospecific sites. 

The understanding of direct-initation mechanisms. In this context, tire use of metal salts of strong acids has opened a new area of fundamental studies. Indeed, we believe that the chemical insertion of the olefin between the metal cation and the anion following its adsorption on the salt surface closely resembles recent mechanistic proposals put forward by Zambelli and collaborators to rationalise initiation in stereospecific polymerisation (propene with vanadium-based catalysts). We are inclined to conclude that the cationic nature of stereospecific polymerisations of the Ziegler-Natta type is supported by our findings , viz. electrophilic attack by... 

The zeolite is rigid and ordered, and lacks conformational adaptability, in contrast to an enzyme, which can coil, uncoil, and twist around. Yet the zeolite can incorporate transition metal functions—these are of prime importance in enzyme catalysis—and it can effect redox reactions reactions over zeolites can be inhibited by competitive adsorption of reactants, products, solvents, or poisons—a phenomenon observed in biological and some other inorganic heterogeneous catalytic systems Rideal kinetics have been identified in some zeolite-catalyzed alkylations, a pattern which has its parallels in the enzyme field a few cases of stereospecificity (such as orfho-alkylation effects, unusual olefin isomer ratios), where a transition state not otherwise attainable intervenes, may exist. What better group of catalysts than zeolites might there have been to activate the evolutionary process in the dark, fermenting Pre-Cambrian seas some 1,000,000,000 years ago ... 

Some fundamental studies were made earlier. It was found that the hydrodimerization of an O, j6-unsaturated ketone (enone) was at least partially stereospecific, with the d, I dimer formed from the /ranj-enone and mainly the meso dimer formed from the cw-enone. Kanetsuna and Nonaka [50] also examined both the stereoselectivity and stereospecificity of the hydrodimerization in detail. It was indicated that either the reaction mechanism or the stereochemical course is changed by the nature (protic or aprotic) of the reaction media. Particularly, the latter is greatly influenced by the manner of adsorption of an anion radical intermediate on the cathode. It was also postulated to explain the experimental results that the radical anion is adsorbed at the )6-position, where the radical p electron is delocalized and the adsorption is affected by the nature of the media. 

The stereospecificity, stereoselectivity, and regioselectivity of the fluoroacetamidation of a variety of olefins have been studied in detail from synthetic and mechanistic aspects by Laurent and coworkers, as in Eq. (51) [331-334]. The significant stereochemical results observed may be rationalized by considering coplanar adsorption on the anode of carbo-nium cation intermediates derived from the olefins. 

It has recently been proposed that kink sites of high Miller index metal surfaces should be considered as chiral when the step lengths on either side of the kink are unequal [22], Two such surfaces, which are not superimposable, can be defined - by analogy with the Cahn-Ingold-Prelog rules - as, e. g., Ag(643) and Ag(643). Theoretical calculations predicted that adsorption of chiral molecules should be stereospecific on such surfaces, but the only experimental evidence yet available is the electro-oxidation of d- and L-glucose on Pt(643) and Pt(531) surfaces [23]. It was speculated that with the polycrystalline metal catalyst, which contains equal numbers of (/ )- and (S)-type kink sites, preferential adsorption of a chiral modifier on one type of kink site would leave the other type of site free for catalysis. 

Thus we have a continuous number of phenomena in which structural correspondence plays an important part, these phenomena belonging to different fields of science to crystal chemistry, to the science of alloys, to epitaxy, to stereospecific adsorption, to heterogenous catalysis, to enzyme reactions, to antimetabolites, to the action of physiologically active substances and to a number of other biochemical agents, as well as to immunology. Similar views were recently expressed by Seifert concerning epitaxy [16). 

Natta (194) discovered the phenomenon of stereospecific polymerization responsible for the formation of valuable isotactic polymers of a-olefins in the presence of heterogeneous catalysts and of activators and explained it by oriented adsorption. However, the latter is an obligatory condition of the multiplet theory, according to which the reacting atoms come into contact with the sui face, and the substituents must be oriented in one direction, namely, off the surface (195). According to Natta, the electronic properties also play an important part in stereo-specific polymerization. 

Acetylcholine and its antagonists are clearly quite stereospecific at the postjunctional membrane that is, the cholinergic receptor is stereoselective, especially muscarinic receptor (Ariens, 1971 Ariens and Simonis, 1967). From the results of electrochemical adsorption some knowledge of this stereospecificity can be obtained. However, it should be noted that the experimental evidence for the adsorption of acetylcholine and drugs onto a charged surface provides information of the competitive inhibitory effects on the attachment of acetylcholine to the cholinergic receptor site. 

Stereospecific Adsorption of Nitrous Oxide, Cyclopropane, Water, and Ammonia on the Co(II)A Synthetic Zeolite... 

Communicated in writing) Dr. Eley s review of enzyme catalysis made only brief reference to the very marked stereospecificity of enzyme reactions. Adsorption of substrates on enzymic proteins cannot account satisfactorily for this unless there is at least a two-point chemisorption throughout the whole of the reaction process. 

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