Bisoxazoline-based chiral catalysts

Asymmetric epoxidation of olefins with ruthenium catalysts based either on chiral porphyrins or on pyridine-2,6-bisoxazoline (pybox) ligands has been reported (Scheme 6.21). Berkessel et al. reported that catalysts 27 and 28 were efficient catalysts for the enantioselective epoxidation of aryl-substituted olefins (Table 6.10) [139]. Enantioselectivities of up to 83% were obtained in the epoxidation of 1,2-dihydronaphthalene with catalyst 28 and 2,6-DCPNO. Simple olefins such as oct-l-ene reacted poorly and gave epoxides with low enantioselectivity. The use of pybox ligands in ruthenium-catalyzed asymmetric epoxidations was first reported by Nishiyama et al., who used catalyst 30 in combination with iodosyl benzene, bisacetoxyiodo benzene [PhI(OAc)2], or TBHP for the oxidation of trons-stilbene [140], In their best result, with PhI(OAc)2 as oxidant, they obtained trons-stilbene oxide in 80% yield and with 63% ee. More recently, Beller and coworkers have reexamined this catalytic system, finding that asymmetric epoxidations could be perfonned with ruthenium catalysts 29 and 30 and 30% aqueous hydrogen peroxide (Table 6.11) [141]. Development of the pybox ligand provided ruthenium complex 31, which turned out to be the most efficient catalyst for asymmetric... 

This process (hetero Diels-Alder reaction leading to a dihydropyran system) may be also conducted in an asymmetric version application of chiral transition-metal catalysts based on BINOL, BDMAP, bisoxazolines, etc. provides adducts in very high optical purity (ee up to 99%) [1,6], In a series of papers Jurczak reported recently a highly enantioselective cycloaddition of 1-methoxy-1,3-butadiene and butyl glyoxylate catalyzed with chiral salen complexes [21],... 

Siegel and Schmalz [79] reported a new approach to optically active ferrocenes with planar chiraHty, based on a Cu-catalyzed C-H insertion (Scheme 17). Both a five- and a six-membered ring could be formed in this way. Using a copper-bisoxazoline catalyst, good yields and ees in the range of 60-80% could be obtained, whereas chiral dirhodium catalysts proved to be ineffective in this case. 

Allylpalladium complexes with BOX-type ligands and glucopyrano-oxazoline-palladium catalysts were used as catalysts for enantioselective allylic substitution (277). A chiral bisoxazoline ligand (BOXBZ) developed by Pfaltz has been used for asymmetric carbo- and heteroannulation reactions (278). An axial binaphthyl-based ligand possessing oxazolyl substituents (BOXAX) was developed by Hayashi and co-workers and successfully applied for the asymmetric Wacker-type cyclization (279). 

A hybrid strategy that involves synergism between metal catalyst and BB organo-catalyst has been applied to the aza-Henry reaction [62], Thus, the aza-Henry reaction between tert-butyl 2-nitropropionate and iminoesters is catalyzed by combined use of quinine (16) and a Cu-bisoxazoline catalyst (20mol%). The approach is based on chiral molecular recognition of the organic and the metallic catalysts and produces the densely functionalized adduct 48 in good diastereo- and enanti-oselectivity (Scheme 29.29). 

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