Asymmetric epoxidation solid support

Different immobilization methods were employed, and the new heterogenized catalysts were applied in asymmetric hydrogenation and epoxidation of olefins, as well as in ring-opening reactions of epoxides. In some cases, the typical catalytic properties of homogeneous catalysts could be transferred to the heterogeneous systems. The immobilization of complexes on solid supports facilitated recovery and recycling of the catalysts. 

The preparation of the silica supported a-fluorotropinone is easy to reproduce as a commercially available solid support was employed the asymmetric epoxidation has been applied to trans- and trisubstituted alkenes affording the corresponding... 

An example of catalysts which are themselves heterogeneous are the poly-amino acids used for the asymmetric Julia-Colonna-type epoxidation of chalcones using alkaline hydrogen peroxide (Section 10.2) [8]. Because of the highly efficient synthesis of epoxides, this process also has attracted industrial interest (Section 14.3). Since recent work by the Berkessel group revealed that as few as five L-Leu residues are sufficient for epoxidation of chalcone, several solid-phase-bound short-chain peptides have been used, leading to enantioselectivity up to 98% ee [14], For example, (L-Leu)5 immobilized on TentaGel S NH2 , 8, was found to be a suitable solid-supported short-chain peptide catalyst for epoxidations. 

Whereas these solid catalysts tolerate water to some extent, or even use aqueous H2O2 as the oxidant, the use of homogeneous Ti catalysts in epoxi-dation reactions often demands strictly anhydrous conditions. The homogeneous catalysts are often titanium alkoxides, possibly in combination with chiral modifiers, as in the Sharpless asymmetric epoxidation of allylic alcohols (15). There has recently been an increase in interest in supporting this enantioselective Ti catalyst. 

They found that the erythrolthreo-% i,cX N Xtj is X-substituent dependent, acting through H-bonding, which was demonstrated by the TFDO Ic epoxidation of 73. An example of cyclohexene epoxidation by dioxiranes derived from various ketones grafted on solid supports has also appeared <1996MI273>. Shi and co-workers reported <1996JA9806> excellent ee s of asymmetric epoxidation of different /ra t-olefms by fructose-derived ketones 74 before then, only low enantioselectivities (9-20%) have been reported on this type of reaction. 

The asymmetric epoxidation of functionalized alkenes still attracts considerable attention. Synthetic chemists continue to be in search of new and improved routes to epoxides, since they provide versatile intermediates for natural product synthesis. The topic of preparative techniques for chiral epoxides is seldom broached without the mention of the Sharpless epoxidation. Indeed, the impact of this protocol cannot be overestimated, as new applications continue to be reported. For example, linear poly(tartrate ester) ligands have been used this past year to generate a solid-supported Sharpless-type catalyst <97CC123>. 

Since the introduction of the first peptide organocatalyst in the 1980s, a considerable number of new peptide frameworks have been developed that are able to effectively catalyse several important transformations including alcohol esterifications, 1,4-conjugate additions, aldol reactions, Strecker synthesis, asymmetric cyanohydrin synthesis and alkene epoxidation are discussed. A few successful examples of solid-supported peptides and reactions in ball milling under solvent-free conditions have been demonstrated. These methods combine the advantages of being economically and environmentally friendly processes. 

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