Oxazoline-based catalysts

Scheme 8 Hydrogenation of terminal olefins with oxazoline-based catalysts bearing an axial chiral biarylphosphite moiety. Full conversion was observed for all entries...

Catalyst 58, in which the oxazoline ring has been replaced with an imidazoline, gave ee-values in the low 90% region for substrates 36 and 38-40 [42]. However, for certain substrates (see Section 30.5), replacement of the oxazoline by an imidazoline has resulted in significantly higher enantioselectivity. Recently, a number of pyridine- and quinoline-derived iridium complexes 59-62 have been developed, which gave promising enantioselectivities with substrates 36-39 [43, 44]. However, these catalysts cannot yet compete with the most efficient oxazoline-based complexes and complex 14. 

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. 

Lim and Sulikowski (84) explored the intramolecular C-H insertion in 119 alpha to the nitrogen atom as a rapid entry to the mitomycin skeleton and the antitumor agent FR-900482. Rhodium(II) based catalysts provide nearly racemic products. Bis(oxazoline) (55b) affords highest selectivities in this system and chloroform was found to be the optimal solvent, Eq. 71. The authors note that the reaction is somewhat capricious. 

Sigman and co-workers developed a novel oxazoline-based hydrogen-bonding catalyst (122) for HDA reactions. The modular nature of the catalyst design al-... 

Next, Evans et al. [15] reported that Cu-based catalysts were superior in the Diels-Alder reaction of the oxazolidinone 9 with cyclopentadiene 8. ITie (5,5)-bis(oxazo-line)-Cu(II) and -Zn(II) complexes were very effective catalysts of the reaction. The optimum tert-butyl ligand 13-Cu(II) complex afforded (2S)-endo-ll with > 98 % ee. In contrast, the optimum catalyst system for the phenyl-substituted ligand 12-Zn complex afforded the enantiomeric (R) product, (2R)-endo-ll, with 92 % ee. The different direction of asymmetric induction was explained in terms of the geometry of cata-lyst-dienophile complexes at the corresponding metal centers. The bis(oxazoline)-Zn(II) complex-catalyzed reaction proceeded via the tetrahedral chiral Zn-dienophile complex VIII, in a manner similar to the bis(oxazoline)-Mg catalyst reported by Corey [13], whereas the reaction catalyzed by the cationic bis(oxazoline)-Cu complex proceeded via the square-planar Cu(II)-dienophile intermediate VII, so the diene preferred to approach from the opposite si face of the bound dienophile with s-cis configuration, avoiding steric repulsion by one of the tert-butyl substituents on the oxazoline rings. 

A magnesium-based catalyst system which employs either a bis(oxazoline) or amido-mono(oxazoline) ligand derived from phenylglycine has been reported to effect an interesting Diels-Alder between unsymmetrical alkylidene 26 and cyclopentadiene (Scheme 23) [79]. The reaction generates a quaternary center and is noteworthy in its ability to preferentially deliver cycloadduct 29, despite a superficial similarity between the two carbonyl substituents (OEt vs. Ph). This selectivity has been rationalized on the basis of an inferred steric preference for the phenyl group to reside perpendicular to the alkylidene, thus creating a marked bias in the diastereomeric transition states [80]. 

Cu-Based Dendritic Catalysts. Chow et al. designed a Cu(II)-bis(oxazoline) dendritic catalyst (Figure 10.4) to accelerate the Diels-Alder reaction between cyclopentadiene and... 

The epimine oxazoline ratio depends on the nature of the substrate as well as on the reaction conditions, especially on the type of base catalyst, the temperature, and the solvent. In this respect, the formation of 4,6-O-benzylidene-... 

Entry Catalyst Ar Oxazoline Base Solvent Temp. (°C) Conf. Yield (%) ee (%)... 

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). 

There are two other examples from Buchmeiser [107] and Mingotaud [108] that describe catalyst modification at the X-type Hgand for use in aqueous media, although both of these are based on a micellar catalysis approach (see section MiceUar Catalysis Approaches ). Buchmeiser s system involved the use of an amphiphilic, poly(2-oxazoline)-based block copolymer that was functionalized and subsequently attached to the catalyst through a fluorinated silver carboxylate. 

As test reactions, the a-hydrazination of ethyl 2 methylacetoacetate and the Henry reaction of nitromethane and 2-nitrobenzaldehyde were chosen. In both cases, a significant difference between the bis-oxazoline (BOX) and trisox-based catalysts was notable. The performance of all the trisox-based catalysts proved to be inferior for both reactions, both in terms of the activities and enantio-selectivities. While the attachment of a linker at the ligand backbone and the immobilization only moderately affected the BOX-based catalysts for both reactions, the effect on the trisox-based catalysts led to a dramatic decrease in activity. 

It is well known that a zwitterionic enolate intermediate can be generated via the addition of a Lewis base to ketenes, which can be oxidized by an appropriate oxaziridine to form the corresponding imine and zwitterionic epoxide. The obtained zwitterionic epoxide intermediate is expected to add to the in situ generated imine to furnish the final products. Based on this finding. Ye reported a novel enantioselective formal [3 + 2] cycloaddition of ketene 40 to racemic oxaziridine 41 for the synthesis of oxazolin-4-one. By using NHC 43 or 44 as the Lewis base catalyst [23], the product 42 could be obtained in good yield with high diastereo and enantioselectivity (Scheme 2.12). 

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... 

In contrast with the impressive evolution of asymmetric reactions with paUadium(0) catalyst, palladium(II)-catalyzed asymmetric Wacker-type oxidations have received only scant attention. For instance, Alper and Hamel and Cao and Zhang described asymmetric Pd(II)-catalyzed car-bonylation of aUyUc alcohols. Chiral bis(oxazolines) based on binaphtyl (Boxaxs ) or biphenyl backbone were also successfiiUy applied in the asymmetric Wacker-type cycUzation of aUylphenoIs. Sasai and co-workers reported on the oxidative cychzation of alkenyl alcohols with spiro bis(isoxazolines) (SPRIXs). ... 

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. 

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