Asymmetric adsorption

Extensive studies of stereoselective polymerization of epoxides were carried out by Tsuruta et al.21 s. Copolymerization of a racemic mixture of propylene oxide with a diethylzinc-methanol catalyst yielded a crystalline polymer, which was resolved into optically active polymers216 217. Asymmetric selective polymerization of d-propylene oxide from a racemic mixture occurs with asymmetric catalysts such as diethyzinc- (+) bomeol218. This reaction is explained by the asymmetric adsorption of monomers onto the enantiomorphic catalyst site219. Furukawa220 compared the selectivities of asymmetric catalysts composed of diethylzinc amino acid combinations and attributed the selectivity to the bulkiness of the substituents in the amino acid. With propylene sulfide, excellent asymmetric selective polymerization was observed with a catalyst consisting of diethylzinc and a tertiary-butyl substituted a-glycol221,222. ... 

Another hypothesis on homochirality involves interaction of biomolecules with minerals, either at rock surfaces or at the sea bottom thus, adsorption processes of biomolecules at chiral mineral surfaces have been studied. Klabunovskii and Thiemann (2000) used a large selection of analytical data, provided by other authors, to study whether natural, optically active quartz could have played a role in the emergence of optical activity on the primeval Earth. Some researchers consider it possible that enantioselective adsorption by one of the quartz species (L or D) could have led to the homochirality of biomolecules. Asymmetric adsorption at enantiomor-phic quartz crystals has been detected L-quartz preferentially adsorbs L-alanine. Asymmetrical hydrogenation using d- or L-quartz as active catalysts is also possible. However, if the information in a large number of publications is averaged out, as Klabunovskii and Thiemann could show, there is no clear preference in nature for one of the two enantiomorphic quartz structures. It is possible that rhomobohedral... 

It is not easy to obtain both enantiomers of non-charged chiral complexes in optically pure form. Chromatographic separation by the use of asymmetric adsorption on an optically active adsorbent(for example, quartz, starch and partial separation of enantiomers. This is probably because asymmetric adsorption is. very weak. To make matters worse, we do not know the stereochemistry of asymmetric adsorption, nor how to improve the efficiency of separation. 

According to Klabunovskii and Patrikeev (281), the appearance of optical activity in the products of asymmetric catalysis is attributable to asymmetric adsorption on quartz thus, it is adsorption that is asymmetric, not catalysis. 

Chapter 1 considers the possible relationships of earthly clays and other minerals to the origin of chirality in organic molecules. Attempts to establish experimental evidence of asymmetric adsorption on clays were unsuccessfiil, but die search for chirality did find naturally occurring enantiomorphic crystals like quartz. Asymmetric adsorption of organic molecules on quartz crystals such as separation of racemic mixtures, like Co or Cr complexes, alcohols and other compounds, allowed for the conclusion that quartz crystals can serve as possible sources of chirality but not of homochirality. This latter conclusion results fi om the finding that all studied locations of quartz crystals contain equal amounts of d- and /-forms. The preparations of synthetic adsorbents such as imprinting silica gels are also considered. More than 130 references are analyzed. 

Selection of optically pure crystals can be accomplished with the aid of a polariscope in which the thin plates of quartz are examined between crossed Nicol prisms in polarized light. These plates are sawed perpendicular to the optical axis of well formed natural crystals. Brasilian twins exhibiting bright interference patterns in crossed Nicol prisms must be broken out. Only optically pure areas are selected for further use, for example, for asymmetric adsorption and for asymmetric catalysis. Piezoquartz as plates of an oblique cut, called the "Cut of Curie", are used in radio electronics and can be identified by the sign of their optically rotation according to the method of Lemmlein According to this method, the etched plate of piezoquartz is... 

Therefore it was sufficient to resolve this complex into enantiomers to determine its spatial structure. This complex does not form diastereomers and the only method of its resolution was asymmetric adsorption on crystals of quartz - a very inert material. In this maimer, a solution of samples of the complex was vigorously shaken with quartz powder, the solution was filtered and its optical rotation measured. On /-quartz the optical rotation of the remaining solutions reached from + 0.05 to + 0.075 and on <7-quartz they reached from -0.075 to -0.13 . Thus the configuration of the complex proved to correspond to the czx-structure. The z/-quartz preferentially adsorbs the (+)-complex and the /-quartz the (-)-complex. Brazilian twin quartz gave negative results. 

It is well known that silica plays an important role in living matter. This was shown by the selective adsorption of some sugars on quartz crystals . Consequently, Schwab and Wahl tried to determine the asymmetric adsorption of silicic acid specimens recovered from the mushroom Euspongia and from the Japanese sponge. However, they did not find any asymmetric adsorption. It turned out that the stmcture of this silicic acid is similar to the structure of silica. It possesses no helical structure and cannot play any role in the formation of optically active compounds in organisms. 

Table 1.4. shows that the differential adsorption of Z-Ala on /-quartz and of Z)-Ala on t/-quartz was as much as 1.0-1.8%. Interestingly, results received by the same author on the asymmetric adsorption of racemic DL-Ma HCl in which only one of the enantiomers D- or Z-Ala ) was labeled with or showed differential adsorptions as high as 20% (Table 1.5). 

Table 1.6. Asymmetric adsorption-desorption of H-DZ,-Ala-rPr ester on /- and d-quartz, measured by GLC (modified data adapted from Kavasmaneck et...

Thus, these data also eonfirmed the earlier findings of Tsuchida and Karagunis The positive results in asymmetric adsorption of amino acids, as shown in the work of Boimer and coworkers ...required carefully contrived and rigorously maintained anhydrous conditions, with mere traces of moisture nullifying the effect... ... 

Kavasmaneck P R. (1978) Asymmetric adsorption by quartz a model for the origin of optical activity, Diss.Thesis, Stanford Univ., 246 pp., Diss. Abstr. Int. B. 36 (9) 4508-4509 Chem. Abstr. 1976,84,146534. 

Ikeda Y. (1954) Partial resolution of racemic hydroperoxydes by asymmetric adsorption on rock quartz. Bull. Liberal Arts, Coll. Wakayama Univ. (Nat. Sci.) 4,27-28 Chem. Abstr. 49,9583 (1955). 

Kamai G.Kh., Klabunovskii E.I., Gatilov Yu.F. and Khodakov G.S. (1961) Resolution of tertiary arsenic compounds into optical isomers using asymmetric adsorption on natural dissymmetric adsorbents, Dokl. Acad. Sci. USSR, 139, 1112-1113, Chem. Abstr. 1962, 56,41f Chernyaev I.I., Korablina L.S. and Muraveiskaya G.S. (1965) On optical resolution and photochemical isomerization of as5mimetric c/s -diammines, Zh. neorg. khim. (J. Inorg. Chem. (russ.), 10, 1045-1052. 

Furoyama S., Sawada M., Hashiya K. and Morimoto T. (1982) Asymmetric adsorption of alanine by quartz powder from ethanol solution. Bull. Chem. Soc. Jpn. 55, 3394-3397. 

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... 

Scheme 2.9. Asymmetric adsorption and decomposition of the racemic mixture of (/ )-(-)- and (5)-(-l-)-butan-2-ol over Cu-/-quartz catalyst (adapted from Klabunovskii et al.

Supported chiral Ni and Ni-Cu catalysts i 9-i60-24s,248,249 special interest because they allow the elucidation of the nature of the metal-support interaction and the asymmetric adsorption of modifier and substrate molecules by use of the IR spectra of adsorbed molecules. 

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