Large chiral complexes

Immobilization of chiral complexes in PDMS membranes offers a method for the generation of new chiral catalytic membranes. The heterogenization of the Jacobsen catalyst is difficult because the catalyst loses its enantioselectivity during immobilization on silica or carbon surfaces whereas the encapsulation in zeolites needs large cages. However, the occlusion of this complex in a PDMS matrix was successful.212 The complex is held sterically within the PDMS chains. The Jacobsen catalyst occluded in the membrane has activity and selectivity for the epoxidation of alkenes similar to that of the homogeneous one, but the immobilized catalyst is recyclable and stable. 

O. Picazo et al., Large chiral recognition in hydrogen-bonded complexes and proton transfer in pyrrolo[2, 3-b]pyrrole dimers as model compounds. J. Org. Chem. 68, 7485-7489 (2003)... 

Heterogeneous photochemical processes are concerned with the effect of light on interacting molecules and solid surfaces. The concept of photoinduced surface chemistry is commonly used to integrate these processes. As cited earlier, they involve surface phenomena such as adsorption, diffusion, chemical reaction and desorption [3]. Experiments and theoretical calculations make clear that the photochemical behavior of an adsorbed molecule can be very different from that of a molecule in the gas or liquid phase [4]. Photochemical reactions of this type involve molecules and systems of quite different complexity, from species composed of a few atoms in the stratosphere to large chiral organic molecules that presumably were formed in prebiotic systems. 

In 1993, Shibuya and co-workers reported the stereoselective addition of DEHP to aldehydes to be catalysed by chiral complexes of titanium, specifically, dialkyl tartrate complexes of titanium that had been used previously with great success by Sharpless in the asymmetric oxidation of allylic alcohols (Scheme 9) [24]. Shibuya described moderate success in controlling enantioselectivities (<50% e.e.) with this extremely oxygen- and water-sensitive catalyst system. Rather large (10-20 mol %) catalyst loadings were required over a period of 15 h at 0 °C but the potential was definitely there. 

Helical coordination polymers containing large chiral cavities or charmels are interesting for stereospecific syntheses, separation of enantiomers and stereoselective catalysis. Crystals of 39 are obtained from a solution of nickel(II) acetate and benzoic acid in methanol which is covered with a solution of 4,4 -bipyridine in the presence of benzene or nitrobenzene [107]. Each helix turn contains three complex units and the chain consists of Ni(II) (with binding of benzoate) and the bipyridine. Because each helix is shifted against the next, large cavities are formed containing, for example, benzene or nitrobenzene. 

The technique of phase transfer catalysis is now twenty-five years old and has come of age. It has been applied to many areas of organic synthesis, from small and simple molecules to large and complex polymers. Successful chiral catalysts are beginning to be reported [61], and industry is starting to embrace phase transfer methods [62]. Phase transfer catalysis clearly has an important role to play in a cleaner future. 

A continuing interest in this area is to obtain more insight into the factors that govern the stereoselectivity of olefin coordination. NMR studies, including NOE and low-temperature experiments, concerning the effect of the olefin structure on the stability and stereoselectivities of a large number of prochiral olefins (GH2=GHR) in the alkene-amino acid chiral complex cis- (N,olefin)-(i ,Y)[PtX(77 -2-mb)(sarcosinato)] (2-mb = 2-methyl-3-buten-2-ol X = G1 )... 

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