Semicorrin ligand

Subsequently, bis(oxazoline) ligands ((70) and (71)), structurally related to the semicorrin ligand, were introduced and further expanded the scope of asymmetric cyclopropanation... 

The next major contribution in asymmetric cyclopropanation was the introduction of chiral semicorrin ligands 184 by Fritschi et al.95 This ligand has been used for coordinating with copper and has been found to provide improved enantiocontrol in the cyclopropanation of monosubstituted olefins. Copper(I), coordinated by only one semicorrin ligand, is believed96 to be the catalytically active oxidation state. The copper(I) oxidation state can be reached directly... 

ASYMMETRIC COPPER AND COPPER(II) CATALYZED REACTIONS 5. Semicorrin Ligands... 

For a review on chiral semicorrin ligands in asymmetric catalysis, see Pfaltz, A. Synlett 1999,... 

A remarkable complex (33) with a C2-symmetric semicorrin ligand has been recently developed by Pfaltz and coworkers.64 A copper(II) complex was used as a procatalyst, but (33) was shown to be the active cyclopropanation catalyst. As shown in Table 3, this complex resulted in spectacular enantioselecti-vities in the range of 92-97% ee. Once again, the (15,35,4/ )-menthyl group attenuated the selectivity. Unfortunately, even though respectable yields were obtained with dienes and styrenes, the reaction with 1-heptene was rather inefficient. 

Cu complexes with bis-oxazoline ligands 6 that were first reported by Masamune and co-workers [39] have incited considerable interest because of the exceptional enantiocontrol that can be achieved with their use as catalysts for cyclopropanation reactions. Concurrent investigations by Evans [40], who added 7 Masamune, who provided 8 and 9 [41] and Pfaltz [42], who investigated a similar series, established that the C2-symmetric bis-oxazoline ligands are suitable alternatives to semicorrin ligands for Cu in creating a highly enantioselective environment for intermolecular cyclopropanation. For the first time, diazoacetates with ester substituents as small as ethyl could be used to achieve enantioselectivity >90% ee in reactions with styrene (Table 5.4). 

Oxazolines are excellently suited for the com-plexation of metals. Based on this knowledge, a variety of metal-oxazoline complexes have been synthesized in recent years and used with outstanding results. In 1986 Pfaltz developed the semicorrin ligands 27 and thereby laid the foundation for future developments [13]. However, the more recently developed bis(oxa-zoline) ligands are more easily accessible and have therefore found wider application (Fig. 5). 

In close structural analogy to the semicorrinate ligands of 9, the bidendate, chiral C2-symmetric 5-azasemicorrins 10 and bis(4,5-dihydrooxazol-2-yl)methane systems n is6,2i7 12 218,219 12 a, and 14 perform exceptionally well in copper-catalyzed enantioselective cyclo-... 

In close structural analogy to the semicorrinate ligands of 9, the bidendate, chiral C2-symmetric 5-azasemicorrins 10216 and bis(4,5-dihydrooxazol-2-yl)methane systems li,196 217 12,218 219 12a,197 13,220 and I4197a perform exceptionally well in copper-catalyzed enantioselective cyclo-propanation reactions with diazo esters. With 10, 11, and 12 a, the active catalyst is prepared in situ by adding a catalytic amount of a copper(I) salt with a weakly coordinating anion [copper(I) triflate,196,217,219 copper(I) perchlorate197] to the free ligand with the enolizable system 12 (as well as with 12a and 14) it has been prepared by reaction with copper(I) terh butoxide or from the copper(I) bischelate complex by reduction with phenylhydrazine. 

Intramolecular cyclopropanations with unsaturated diazo ketones have also been reported. Furthermore, enantioselective cyclopropanation with diazomethane can be achieved in up to 75% ee. In detailed mechanistic discussions, a copper(I) species, complexed with only one semicorrin ligand, and formed by reduction and decomplcxation, is suggested as the catalytical-ly active species, cisjtrans Stereoselection and discrimination of enantiotopic alkene faces should take place within a copper-carbene-alkene complex25-54"56. According to these interpretations, cisjtrans selectivity is determined solely by the substituents of the alkene and of the diazo compound (especially the ester group in diazoacetates) and is independent of the chiral ligand structure (salicylaldimine or semicorrin)25. 

Semicorrin ligands have also been used in asymmetric cyclopropanation of silyl enol ethers to give optically active silyloxycyclopropanes which, after acid-catalyzed ring opening, serve as convenient precursors in the synthesis of chiral y-oxo carboxylic esters50. 

Similar to semicorrin ligands, chiral bis(4,5-dihydro-2-oxazolyl) systems 3 are used in copper-catalyzed asymmetric cyclopropanation of styrene with diazoacetates57 61, 9 ". In a similar way. bis(oxazolinyl)pyridine type ligands have also been used100. 

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-... 

In the realm of catalytic enantioselective conjugate reductions, Pfaltz reported a seminal result in 1989 with the introduction of chiral semicorrin ligands [173, 174], The cobalt complex ofligand 236 mediates the enantioselective reduction of esters 235 and 238 with NaBH4 (Equations 48 and 49) [173]. The fact that the configuration of the product is correlated to the olefin isomer of the employed acceptor (cis versus trans) provides convenient access either to 237 or to ent-137 (94 % ee). 

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