Oxazoline complex

Rhodium complexes with chelating bis(oxazoline) ligands have been described to a lesser extent for the cyclopropanation of olefins. For example, Bergman, Tilley et al. [32] have prepared a family of bis(oxazoline) complexes of coordinatively unsaturated monomeric rhodium(II) (see 20 in Scheme 13). Interestingly, the use of complex 20 in the cyclopropanation reaction of styrene afforded mainly the cis cyclopropane cis/trans = 63137), with 74% ee and not the thermodynamically favored trans isomer. No mechanistic suggestions are proposed by the authors to explain this unusual selectivity. 

Clarke and Shannon also supported copper bis(oxazoline) complexes onto the surfaces of inorganic mesoporous materials, such as MCM-41 and MCM-48, through the covalent binding of the ligand, modified by alkoxysilane functionalities [59]. The immobilized catalysts allowed the cyclopropanation of styrene with ethyldiazoacetate to be performed as for the corresponding homogeneous case, and were reused once with almost no loss of activity or selectivity. 

The same group [97] studied the corresponding rhodium oxazoline complexes that led to very similar results in terms of activity (81% yield) and enantioselectivity (68%), however. 

The only example of immobilization of a bis(oxazoline) complex by adsorption onto silica was published very recently [70]. The complexes 6a-Cu(OTf)2 and 6b-Cu(OTf)2 were adsorbed onto a chromatographic grade of silica gel and the resulting solids used as catalysts in two Diels-Alder reactions. 

Numerous examples of nucleophilic attack on coordinated nitrile ligands are found in the literature, particularly when the transition metal is platinum(II).224 The nucleophilic attack of two equivalents of CIO I2CII20 on the electrophilic nitrile carbon atoms of both nitrile ligands in cis-or /r<7H.v-[PtC12(RCN)2] (R = Et, Prn, Pr1, Bu p-CF3C6H4, p- and o-MeC6H4) affords the corresponding A2-l,3-oxazoline complexes ((76) and (77), respectively), in which the heterocycle acts as a... 

Recently, several mechanistic studies have been performed by means of calculations based on density functional theory. - Pfaltz s model proposed for asymmetric cyclopropanation using copper-semicorrin or -bis(oxazolines) complex has been supported by calculation.295 Another calculation also supports the parallel approach.296... 

Pro-chiral pyridine A-oxides have also been used as substrates in asymmetric processes. Jprgensen and co-workers explored the catalytic asymmetric Mukaiyama aldol reaction between ketene silyl acetals 61 and pyridine A-oxide carboxaldehydes 62 <06CEJ3472>. The process is catalyzed by a copper(II)-bis(oxazoline) complex 63 which gave good yields and diastereoselectivities with up to 99% enantiomeric excess. 

A single report appears in the literature regarding the use of chirally modified palladium catalysts in reductive enyne cyclization.60 Upon exposure of 1,6-enyne 36a to the indicated palladium pyridine-oxazoline complex in the presence of EtjSiH, cyclization product 36b is formed in good yield, but with only modest levels of asymmetric induction (Scheme 26). 

Figure 5-6. Rationale for the different topicities of bis(oxazoline) complexes.

Copper(II)-bis(oxazoline) complexes (48) are robust, valuable catalysts for a wide variety of stereoselective Diels-Alder reactions. In a key step en route to... 

Tris(oxazoline) complexes have also been investigated as ligands in the allylic oxidation reaction. Katsuki and co-workers (116) observed that Cu(OTf)2 com-plexed to the tris(oxazoline) 160 is a more selective catalyst than one derived from CuOTf, Eq. 99, in direct contrast to results observed with bis(oxazohnes) or pyridyl bis(oxazohnes) as ligands (cf. Section III.A.3). When the reaction is conducted at -20°C, the cyclopentenyl benzoate is delivered in 88% ee albeit in only 11% yield after 111 h. Larger cycloalkenes are less selective (cyclohexene 56% ee, cyclohep-tene 14% ee, cyclooctene 54% ee). 

The control of enantioselectivity in the reduction of carbonyl compounds provides an opportunity for obtaining the product alcohols in an enantiomerically enriched form. For transfer hydrogenation, such reactions have been dominated by the use of enantiomerically pure ruthenium complexes [33, 34], although Pfaltz and coworkers had shown by 1991 that high levels of enantioselectivity could be obtained using iridium(I) bis-oxazoline complexes [35]. 

Structurally very close to the N,N,O-tridentate ketiminate systems, magnesium complexes bearing bis-amido-oxazolinate complexes 44a-e were used in the PLA preparation from L-lactide in the presence of benzyl alcohol. The low reactivity of 44d, e is due to the presence of a pendant functionality engaging the metal center, and the steric bulk of the phenyl group hindering the coordination of benzyl alcohol or of a monomer to the metal center, contributing to a diminution of the propagation [69]. 

With chiral catalysts, the reaction becomes enantioselective. Among the successful catalysts are diisopropoxyTi(IV) BINOL and copper bis-oxazoline complexes. 

Bolm and co-workers expanded the diastereoselective lithiation to include the ri -cyclopentadienylrhenium(l) tricarbonyl oxazoline complex 451 (Scheme 8.148). The selectivity was determined to be 9 1 favoring diastereomer 452. The strucmre of 452 was determined by crystallography. Interestingly, lithiation of 451 with jec-butyllithium resulted in the formation of nucleophilic addition products. 

Ghosh and co-workers have also demonstrated that the Cu(II)-bis(oxazoline) complexes of conformationally constrained inda-box ligands 9a and ent-9a are excellent catalysts for the enantioselective Diels-Alder reaction. Using copper(II) trrflate as the metal source, the reaction resulted in selectivities up to >99 1 endo/ exo ratio with endo ee up to 99% (2R isomer), as shown in Table 9.10 (Fig. 9.24). Of particular interest, Cu(II)-phe-box ligand 6-derived catalyst complex exhibited considerably lower enantioselectivity (30%)." Furthermore, they have shown that the use of Mg(II) as the chelating metal resulted in a reversal of stereochemistry [up to 98 2 endo/exo and 61% endo ee for the (25) isomer]. Davies also showed that the use of copper(II) triflate with his stmcturally related inda-box ligands 9b and 34a led to similar selectivities. 

Lewis acid catalyzed 1,3-dipolar cycloadditions of olehns and nitrones are useful synthetic transformations that have the potential of dehning up to three contiguous chiral centers. Presumably, such reactions can also be catalyzed by chiral Lewis acids derived from metal-bis(oxazoline) complexes. 

Evans has reported that the cationic C2-symmetric chiral Lewis acid Cu(ll)bis(oxazoline) complex promotes the hetero-Diels-Alder reaction of 0 ,/3-unsaturated acyl phosphonates with enol ethers to give the cycloadducts with excellent ee (Scheme 52). As well as simple dihydropyrans, various fused bis-dihydropyrans are also reported <1998JA4895, 2000JA1635>. 

Table 7 Asymmetric cyclization/hydrosilylation of dimethyl diallyl malonate catalyzed by palladium pyridine-oxazoline complexes...

OXORHENIUM(V) OXAZOLINE COMPLEXES FOR OXYGEN ATOM TRANSFER... 

Oxorhenium(V) Oxazoline Complexes for Oxygen Atom Transfer 55... 

Naud, F., Malan, C., Spindler, F., Ruggeberg, C., Schmidt, A.T. and Blaser, H.-U. (2006) Ru-(phosphine-oxazoline) complexes as effective, industrially viable catalysts for the enantioselective hydrogenation of aryl ketones. Adv. Synth. Catal., 348, 47. 

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