Covalent chiral ligand/catalyst

Glos and Reiser [23] introduced aza-bis(oxazolines) as new chiral ligands for copper and palladium catalysts. Because of the structural flexibility of these compounds they also prepared an immobilized ligand by covalent grafting to methoxypoly(ethyleneglycol) (structures 14 and 15 in Scheme 9). 

Two strategies have been adopted for asymmetric cyclopropanation. First, there are auxiliary-based methods, involving a covalently attached adjacent chiral moiety on either the olefin or the cyclopropylating agent. The second process, on the other hand, employs a chiral ligand on a metal catalyst. This method is more applicable to route b or c, and this is an issue that warrants further discussion. 

Obviously, the way to heterogeneous enantioselective catalysis is open when chiral ligands are introduced into the polymers. This idea was first proposed by Okamoto and Still (231). However, the first catalytic results were published by Cazaux and Caze (235). These authors use the asymmetric Mo(VI) complex (9) covalently bound to a cross-linked polystyrene resin for the enantioselective epoxidation of geraniol. Unfortunately, the enantiomeric excess obtained was low—far below that obtained with the Ti-tartrate catalyst (Section II,B). Moreover, the polymeric structure appeared to be unstable. 

Asymmetric synthesis via enolate intermediates has been extensively studied. Asymmetric induction can be divided into five main categories (1) a chiral auxiliary covalently linked to an enolate moiety,2,3 (2) a chiral ligand of a countercation of an enolate,4-6 (3) a chiral electrophile,7,8 (4) a chiral Lewis acid,9,10 and (5) a chiral phase-transfer catalyst.11,12 Rather than reviewing these examples, we introduce here the principle of asymmetric induction for... 

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. 

Covalent attachment of ligands to polymer supports retains their complexing properties156 and widens their applications. For instance, immobilized crown poly-thers and cryptands used as phase-transfer catalysts can be recycled55. Chiral ligands have been used for a chromatographical separation of D- and L-amino acids75. ... 

Since the development of solid-phase peptide synthesis by Merrifield in the 1960s [12], the approach of covalently bonded (chiral) ligands and catalysts has gained... 

A chiral bis(oxazoline) ligand was covalently functionahzed onto a hierarchically ordered mesocellular mesoporous silica and applied to the asymmetric Henry reaction between various aldehydes and nitromethane at ambient temperature. As a chiral heterogeneous catalyst, this functionalized mesoporous material showed excellent enantioselectivity in the asymmetric Henry reaction when the free silanol groups of the mesoporous silica were capped by trimethylsilyl groups. And also this catalyst could be magnetically separated from the reaction mixture without significant loss of reactivity or enantioselectivity [94]. 

Non-covalently Immobilized Catalysts Based on Chiral Salen Ligands. . 152... 

In the last 20 years a great deal of effort has been focused towards the immobilization of chiral catalysts [2] and disparate results have been obtained. In order to ensure the retention of the valuable chiral hgand, the most commonly used immobihzation method has been the creation of a covalent bond between the ligand and the support, which is usually a solid, hi many cases this strategy requires additional functionalization of the chiral hgand, and this change - together with the presence of the very bulky support - may produce unpredictable effects on the conformational preferences of the catalytic complex. This in turn affects the transition-state structures and thus the enantioselectivity of the process. 

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