Oxazoline, complexes with

Metallomesogens have been shown to form helical supramolecular organisations in their mesophases [95]. Chiral oxazoline complexes with various metal ions and six alkyl chains did not show LC behaviour, but when mixed with trinitrofluorenone form achiral smectic A phases [96]. Furthermore, when a branch was included in the structure of the ligands (Fig. 12) the corresponding complexes with copper(II) and palladium(II) form columnar mesophases which have a helical organisation [97]. The presence of the stereogenic centre near the central metal ion in these complexes (Fig. 12) is enough to cause the parallel molecules to stack in a tilted manner with... 

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. 

Helmchen and coworkers employed a,co-amino-1,3-dienes as substrates [51]. By using palladium complexes with chiral phosphino-oxazolines L as catalysts, an enantiomeric excess of up to 80 % was achieved. In a typical experiment, a suspension of Pd(OAc)2, the chiral ligand L, the aminodiene 6/1-90 and an aryltriflate in dimethylformamide (DMF) was heated at 100 °C for 10 days. Via the chiral palladium complex 6/1-91, the resulting cyclic amine derivative 6/1-92 was obtained in 47% yield and 80% ee (Scheme 6/1.23). Using aryliodides the reaction time is shorter, and the yield higher (61 %), but the enantiomeric excess is lower (67% ee). With BINAP as a chiral ligand for the Pd°-catalyzed transformation of 6/1-90 and aryliodide, an ee-value of only 12% was obtained. 

Many examples of asymmetric reactions catalyzed by copper complexes with chiral ligand systems have been reported. In particular, various copper-bis(oxazoline) catalysts (e.g., complexes (H) to (L), Scheme 48) are effective for carbon-carbon bond-forming reactions such as aldol,204 Mukaiyama-Michael, Diels-Alder,206 hetero Diels-Alder,207,208 dipolar cycloaddition,209,210... 

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). 

The 1-t-butylphospholane sulfide intermediate to TangPhos was also used to prepare the P,N ligands 48 by reacting the lithium complex with C02 and then oxazoline formation with a range of chiral amino alcohols [69b, 74]. The Ir complexes of these ligands have been successfully used in the reduction of / -methylcinnamic esters (80-99% ee) and methylstilbene derivatives (75-95% ee), a particularly challenging class of unfunctionalized olefins [4 c]. 

Another successful aza Diels-Alder reaction involves 2-azadienes of type 125 with dienophile 126 in the presence of bis(oxazoline) catalyst 106a.53 Product 127 is obtained in a high exo endo ratio, as well as high enantiomeric excess for the exo-isomer of 127. The high enantioselectivity and high yield rely on the chiral catalyst 106a, which activates the dienophile by complexation with an appropriate functional group and does not irreversibly coordinate with the... 

Heck reactions have also been used by Helmchen et al. for a two-component domino process of a,co-amino-1,3-dienes.1721 By using palladium complexes with chiral phosphino-oxazolines as catalysts an enantiomeric excess of up to 80 % is achieved. In a... 

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). 

Interestingly, the Fe2+ ion in the core can be easily removed by base, the complex dissociates and the individual polymer dimers can be analyzed. Block copolymers of 2-ethyl-2-oxazoline with other substituted oxazolines have also been made [121]. Ru2+(4,4 dichloromethyl-2,2 bipyridine)3 has also been used as the multifunctional initiator for the ATRP of styrene at 110°C [122], It is interesting to note that the Cu+ ions necessary for the polymerization reaction are solubilized via complexation with other bipyridine species. 

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

Meyers and Shimano discovered the unusual deprotonation behavior of ethoxy-vinyllithium-HMPA complex (EVL-HMPA) for the deprotonation of the trans-oxazoline 366 and the cw-oxazoline 367. The EVL-HMPA complex is prepared by deprotonation of ethyl vinyl ether with ferf-butyllithium in THE followed by addition of HMPA. Reaction of the frani-oxazoline 366 with both the EVL-HMPA complex and conventional alkyllithium reagents (RLi) resulted in deprotonation at the benzylic 5-position. In contrast, deprotonation of 367 occurred at the 4-position with an alkyllithium reagent RLi, whereas benzylic deprotonation predominated with the EVL-HMPA complex (Scheme 8.117). ° The authors proposed that EVL-HMPA complexes with the 5-phenyl substituent prior to deprotonation. 

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. 

---