Organic molecules asymmetric adsorption

An ideal approach to achieving chiral induction in a constrained medium such as zeolite would be to make use of a chiral medium. No zeolite that can accommodate organic molecules, currently exists in a stable chiral form. Though zeolite beta and titanosilicate ETS-10 have unstable chiral polymorphs, no pure enantiomorphous forms have been isolated. Although many other zeolites can, theoretically, exist in chiral forms (e.g., ZSM-5 and ZSM-11) none has been isolated in such a state. In the absence of readily available chiral zeolites, one is left with the choice of creating an asymmetric environment within zeolites by the adsorption of chiral organic molecules. 

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. 

We have shown how the band structure of photoexcited semiconductor particles makes them effective oxidation catalysts. Because of the heterogeneous nature of the photoactivation, selective chemistry can ensue from preferential adsorption, from directed reactivity between adsorbed reactive intermediates, and from the restriction of ECE processes to one electron routes. The extension of these experiments to catalyze chemical reductions and to address heterogeneous redox reactions of biologically important molecules should be straightforward. In fact, the use of surface-modified powders coated with chiral polymers has recently been reputed to cause asymmetric induction at prochiral redox centers. As more semiconductor powders become routinely available, the importance of these photocatalysts to organic chemistry is bound to increase. 

Nevertheless, photoactive MOFs also show unique photocatalytic properties that other materials cannot compete with, especially in organic synthesis applications. MOFs create the opportunity to combine photocatalyst with organocatalyst. One example is the chiral MOF, namely, Zn-PYIl, which exhibits high selectivity for photocatalytic asymmetric a-alkylation of aldehydes, as demonstrated in Fig. 4.13c. The Zn-PYll has also been synthesised via a PSM process of the parent MOF Zn-BClPl (top of Fig. 4.13c), which has been synthesised via solvothermal reaction from L-N-tert-butoxycarbonyl-2-(imidazole)-l-pyrrolidine (l-BCIP) [58]. The key point of the PSM process is the removal of the protective tert-butoxycarbonyl (Boc) moiety to expose active sites, which are likely to be the N — H of pyrrolidine of the L-BCIP molecules that is located within the channels according to dye adsorption test. This has been realised by microwave irradiation in dry lV,lV-dimethyl-formamide solution. The activated Zn-PYIl shows a high reaction efficiency (74 % in yield) and excellent enantioselectivity (92 % ee) in photocatalytic a-alkylation of aliphatic aldehydes compared to that of other MOFs. 

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