Selected Thoughts on Chiral Crystals, Chiral Surfaces, and Asymmetric Heterogeneous Catalysis

Abstract

Although the uses of zeolitic materials in heterogeneous catalysis are extensive, the opportunity to develop applications of zeolites in enantioselective catalysis remains underexploited. In part, the relative paucity of chiral zeolite structures is one underlying reason, although some significant progress has been made in recent years in the preparation and structural characterization of chiral zeolitic materials, including the recent report of ITQ‐37, the first example of a chiral mesoporous zeolite. However, as elaborated in this Minireview article, the preparation of a chiral zeolite structure is itself not sufficient to ensure that the material will be useful in enantioselective catalysis, and a number of other key issues must also be addressed. In addition to highlighting these issues, this Minireview surveys more widely a number of concepts that are pivotal to the design of chiral solids for the catalysis of enantioselective transformations. In addition to chiral zeolites, a range of other chiral materials, including organic molecular solids, and chiral catalysts, including metal–organic frameworks, chiral metal surfaces, and single‐site immobilized organometallic species, are discussed, together with an appraisal of future directions for asymmetric heterogeneous catalysis.

Comparison of the structure-directing effect of ephedrine and pseudoephedrine during crystallization of nanoporous aluminophosphates

“…Molecular structures of the organic molecules were described with the cvff forcefield [23]. Due to the strong basicity of ephedrine (pK a = 9.6) [24] the low pH of the synthesis medium (3)(4)(5), and the necessity for charge-balancing the negative charge imposed by the incorporation of dopants, protonated EPH and PsEPH molecules were studied. The atomic chargedistribution of the protonated cations was obtained from DFT+D calculations, using the B3LYP hybrid functional and the ESP charge calculation method, setting the total net charge to +1.…”

“…As a consequence, the metabolism of living beings differentiates between enantiomers of chiral compounds, very frequently having only one enantiomer the desired therapeutic effect [2]. In this context, the design of chiral solids able to discriminate between enantiomers of a chiral compound, either during separation or catalytic processes, represents one of the greatest challenges in contemporary chemical research [3,4]. In the quest for chiral functional solids, zeolites and crystalline nanoporous materials in general have been proposed as ideal candidates since they could potentially combine their high surface area and characteristic shapeselectivity with a potential enantioselectivity that might be enhanced by the confinement effect [5][6][7][8].…”

Reflections on the value of electron microscopy in the study of heterogeneous catalysts

“…It is chemically rather straightforward to introduce well-defined single catalytically active sites into these nanoporous hosts, and the various ways of doing so (either during formation or by post synthesis modification) have been described elsewhere [42][43][44]66,69]. One of the key features of nanoporous, well-ordered solids is that they possess very large surface areas.…”

Summarizing comments on the discussion and a prospectus for urgent future action

“…One needs either homochiral crystals or constrained chiral crystals for such conversions. It has become clear [5][6][7] that the appropriate metal-organic framework (MOF), of the kind described and used by Wu & Lin [8], and others [9][10][11][12], are superior chiral solids because they can readily be prepared as homochiral crystals. Wu & Lin [8] have beautifully demonstrated the efficacy of such designed catalysts for reactions such as the addition of diethylzinc to 1-naphthaldehyde to produce (R)-1-(2-napathyl)-propanol with complete conversion and 90% enantioselective excess (ee).…”