Oxazoline based copper catalysts

During the course of their pioneering work on the copper-catalyzed cyclopropanation reaction of alkenes [25], the copper-catalyzed aziridination of alkenes using PhI=NTs was also discovered by Evans et al. in 1991 [26], A wide range of alkenes can be smoothly converted to the corresponding aziridines in the presence of Cu(I) and Cu(II) salts, such as Cu(MeCN)4C104 and Cu (acac)2, respectively (Scheme 2.16). Notably, aliphatic alkenes afforded the desired aziridines without allylic C-H amidation products. In the hterature, only one example of the enantioselective aziridination of styrene using chiral bis (oxazoline)-based copper catalyst was demonstrated. At the... 

The success of bis(oxazolines) (43) in the copper-catalyzed cyclopropanation reaction has prompted numerous researchers to modify these structures in an attempt to improve the catalysts. To date, none have approached the success and generality exhibited by /erf-butyl bis(oxazoline) (55c) although some afford improved selectivities in specific cases. As a corollary to its success in this reaction, the copper-catalyzed cyclopropanation has taken on the aspects of a testing ground for new bis(oxazoline)-based ligands. The plethora of publications in this area will be summarily condensed in the rest of this section, and emphasis will be placed only on those ligands that provide improvements over 55c and those publications that deal with structural or mechanistic advances. 

Michael reaction. Different ligands for copper catalysts are available. For example, an oxazoline prepared from valine," a phosphine from proline,"" and a carbohydrate-based thiolhave been tested. 

After the introduction of chiral Schiff base and semicorrin ligands as enan-tioselective copper catalysts, the next major advance in copper systems is based on bis(oxazoline) ligand 7 (65) and 8 (66) (Scheme 11). With 7, up to 99% ee of cyclopropane was obtained for the cyclopropanation of styrene with EDA. Same enantioselections but higher diastereoselectivity (transrcis = 94 6) was obtained when 2,6-di-Zert-butyl-4-methylphenyl diazoacetate (BDA) was used instead of EDA. It was noted that the catalytic system with 7 was the only copper sys-... 

The first asymmetric copper-catalyzed cyclopropanation using a homogeneous catalyst was reported by Noyori in 1966 [31], This reaction, which allowed the cycloaddition on styrene, was carried out with a chiral Schiff base copper complex and produced poor enantiomeric excess (Fig. 11). Further refinement of the chiral ligands produced later much better catalysts, such as the bis-oxazoline (Box) derivatives, able to provide enantioselectivities up to 99%. 

Zeolites are very useful catalysts for a large variety of reactions, from acid to base and redox catalysis [27]. Hutchings et al. reported that bis(oxazoline)-modified Cu (II)-HY catalysts are effective for asymmetric carbonyl- and imino-ene reactions and aziridination of styrene [28, 29]. Recently Djakovitch and Kohler [30-34] found that Pd(II)-NaY zeolite activates aryl halides towards Heck olefination, a-arylation of malonate, and amination reactions. It is well known that alkali-exchanged faujasite zeolites are solid base catalysts [35]. Owing to the usefulness of zeolites in organic chemistry, and our interest, we recently reported the use of modified alkali-exchanged zeolite Y, NaY zeolite [36] with electron rich copper catalyst in the Y-arylation of nitrogen heterocycles with aryl halides to afford A -arylheterocycles in excellent yields under mild conditions without the use of any additive. 

Asymmetric Diels-Alder reactions have also been achieved in the presence of poly(ethylene glycol)-supported chiral imidazohdin-4-one [113] and copper-loaded silica-grafted bis(oxazolines) [114]. Polymer-bound, camphor-based polysiloxane-fixed metal 1,3-diketonates (chirasil-metals) (37) have proven to catalyze the hetero Diels-Alder reaction of benzaldehyde and Danishefsky s diene. Best catalysts were obtained when oxovanadium(lV) and europium(III) where employed as coordinating metals. Despite excellent chemical yields the resulting pyran-4-ones were reported to be formed with only moderate stereoselectivity (Scheme 4.22). The polymeric catalysts are soluble in hexane and could be precipitated by addition of methanol. Interestingly, the polymeric oxovanadium(III)-catalysts invoke opposite enantioselectivities compared with their monomeric counterparts [115]. 

Sulikowski has devised an asymmetric approach to 1,2-disubstituted mitosenes based on the intramolecular cyclization of diazoesters in the presence of chiral copper(I) catalysts <97T16521>. For instance, cyclization of the methyl ester 94 with a bis(oxazoline) copper(I) catalyst afforded a mixture of diastereomeric indolines which were then oxidized to the mitosene 95 using DDQ. Similar cyclizations of chiral esters (e.g., menthyl, etc.) afforded optically pure... 

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