Zeolites asymmetric epoxidation

In the present work, the Jacobsen s catalyst was immobilized inside highly dealuminated zeolites X and Y, containing mesopores completely surrounded by micropores, and in Al-MCM-41 via ion exchange. Moreover, the complex was immobilized on modified silica MCM-41 via the metal center and through the salen ligand, respectively. cis-Ethyl cinnamate, (-)-a-pinene, styrene, and 1,2-dihydronaphtalene were used as test molecules for asymmetric epoxidation with NaOCl, m-CPBA (m-chloroperoxybenzoic acid), and dimethyldioxirane (DMD) generated in situ as the oxygen sources. 

The addition of activated molecular sieves (zeolites) to the asymmetric epoxidation milieu has the beneficial effect of permitting virtually all reactions to be earned out with only 5-10 mol % of the Ti-tartrate catalyst [3,4]. Without molecular sieves, only a few of the more reactive allylic alcohols are epoxidized efficiently with less than an equivalent of the catalyst. The role of the molecular sieves is thought to be protection of the catalyst from (a) adventitious water and (b) water that may be generated in small amounts by side reactions during the epoxidation process. 

The ability of zeolites to adsorb and retain small molecules such as water forms the basis of their use in the noncatalytic synthesis of fine chemicals (Van Bekkum and Kouwenhoven, 1988, 1989). One of the best recent examples is the use of NaA zeolite in the Sharpless asymmetrical epoxidation of ally lie alcohols (see Chapter 10) (Gao et al., 1987 Antonioletti et al 1992). In this Ti(IV)-catalyzed epoxidation by t-butyl hydroperoxide in the presence of diethyl tartrate (reaction 6.4), it has been demonstrated that the inclusion of zeolites (3 A or 4 A) leads to high conversion (>95%) and high enantioselectivity (90-95% ee). The effect of the zeolite is quite dramatic. It is believed that the role of the zeolite is to protect the titanium isopropoxide catalyst from water, perhaps generated during the reaction. 

This chapter focuses on several recent topics of novel catalyst design with metal complexes on oxide surfaces for selective catalysis, such as stQbene epoxidation, asymmetric BINOL synthesis, shape-selective aUcene hydrogenation and selective benzene-to-phenol synthesis, which have been achieved by novel strategies for the creation of active structures at oxide surfaces such as surface isolation and creation of unsaturated Ru complexes, chiral self-dimerization of supported V complexes, molecular imprinting of supported Rh complexes, and in situ synthesis of Re clusters in zeolite pores (Figure 10.1). 

Chiral Mo complexes bearing ligands derived from a (2S,4R)- or (2S,4S)-4-hydroxyproline compound (13a and 13b) have been tethered to the internal surface of a mesoporous zeolite USY (251). The supported asymmetric Mo catalyst was tested for the enantioselective epoxidation of allylic alcohols. 

In the presence of the immobilized (2S,4S)-4-hydroxyproline Mo catalyst (13b), nerol and geraniol react selectively with r-BuOOH to form the 2,3-epoxide with ee values of 64 and 47%, respectively. Surprisingly, when Mo is complexed by the diastereomeric (2S,4R) form (13a), racemic epoxidation is observed. The enantioselective catalysis appears to be promoted by immobilization in the zeolite USY pores. Indeed, in the epoxidation of nerol, an ee of 10% was found for the homogeneous asymmetric Mo complex, whereas the supported catalyst favors the selective production of the (2S,3R)-epoxide (64% ee). 

Long-range electron transfer is postulated to occur from ferrocene to tris(bipyridine)iron(III) constructed within the pores of a NaY zeolite. The iron bipyridine complex is too large to move throughout the faujasite pores to the surface, thus requiring the long-range transfer. The asymmetric catalyst, titanium tartrate, has been prepared inside NaY and used as an immobilized catalyst for the epoxidation of cinnamyl alcohol. ... 

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