Steric occlusion

Figure 4 Excluded volume for the Di agonist pharmacophore. The mesh volume shown by the black lines is a cross section of the excluded volume representing the receptor binding pocket. Dihydrexidine (see text) is shown in the receptor pocket. The gray mesh represents the receptor essential volume of inactive analogs. The hydroxyl binding, amine binding, and accessory regions are labeled, as is the steric occlusion region.

Various strategies have been pursued in order to immobilise chiral epoxidation catalysts and these encompass covalent attachment to solid supports,[41] steric occlusion in nanosized cages of zeolites,[42 44] entrapment in a polydimethylsiloxane membrane145,461 and fluorous biphasic systems.1471 However, these approaches frequently require tedious ligand modifications and often lead to a marked decrease in both selectivity and activity of the transition metal catalyst. 

Figure 4. The faujasite trap steric occlusion visualized in an uncomplicated way in an olefin reaction at relatively low temperatures hexene data from Ref. 60...

Fig. 9-4. Steric occlusion of the ligand, (a) If the matrix is heavily substituted with ligand, binding of the target protein to the ligand may be blocked, (b) At a lower concentration of ligand the target protein can freely interact with the ligand.

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. 

In contrast to earlier polymer-supported complex catalysts in which complexes were immobilized through electrostatic interaction, covalent bonds, or coordinative bonds, in this case the complex is captured in Ihe elastomer network by occlusion in a dense polymer in Ihe absence of any supplementary chemical bonding and only as result of steric restrictions. In the hydrogenation of methyl acetoacetate by this catalyst an ee of 70% was obtained in polyethyleneglycol solution at 60 °C. Afer regeneration of the catalyst and reuse, its activity and enantioselectivity were almost unchanged. 

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