Semicorrin-copper

Figure 3. X-ray crystal structure of Pfaltz semicorrin-copper catalyst (40). [Adapted from (31).]...

Direct, controlled preparation of copper(I) complexes was achieved by Evans [7] from bisoxazolines and copper(I)triflate, which avoids the use of other methods for reduction or accidental reduction by the substrate, which may not always be efficient. When isobutene is the substrate one obtains only two enantiomers and no other products for styrene we obtain a cis and a trans product each occurring as a pair of enantiomers. We will illustrate this with styrene and the results of Pfaltz s semicorrin-copper. 

Pfaltz s semicorrin copper complexes - a real breakthrough more then ten years ago -can also be employed for intramolecular [2-1-1]-cycloadditions (Scheme 2) and achieve between 14 and 95 % ee depending on the substrate. [4] The related, otherwise very succes-ful, bis(oxazoline) complexes [5] (see above) do not appear to be as effective here though only one example has been investigated. 

Pfaltz and his coworkers (Fritschi et al., 1986, 1988a, 1988b Pfaltz, 1989, 1993) used the semicorrin-copper complex 8.160 and the corresponding 2 1 complex 8.162 as a reagent for cyclopropanation of styrene, buta-l,3-diene, 1-methyl-penta-2,4-diene, and hept-l-ene (8-71). The yield and the enantiomeric excess found for the two products are shown in Table 8-2. 

Table 8-2. Products of cyclopropanations (8-71) with the semicorrin-copper complex 8.160, after Fritschi et al. (1988 b).

The cyclopropanation with diazo compounds via decomposition is amenable to asymmetric induction using chiral metal chelates. Rhodium complexes of 3(S)-phthalimido-2-piperidinone and Al-(arenesulfonyl)proline 44 are typical. The latter catalyst is suitable for generating alkenylcarbenoids. For intramolecular reactions, the cognate complex 45 and the semicorrin-copper 46 are effective. 

Chiral C2-symmetric semicorrins (structure 4), developed by Pfaltz [11], were proven to be highly efficient ligands for the copper-catalyzed enantio-selective cyclopropanation of olefins. Variations of the substituents at the stereogenic centers led to optimized structures and very high enantioselectiv-ities [12]. 

In 1986, Pfaltz et al. introduced a new type of pseudo C2-symmetrical copper-semicorrin complex (68) as the catalyst (Scheme 60).227 228 The complexes (68) are reduced in situ by the diazo compound or by pretreatment with phenylhydrazine to give monomeric Cu1 species (69), which catalyze cyclopropanation. Of the semicorrin complexes, complex (68a) (R = CMe2OH) showed the best enantioselectivity in the cyclopropanation of terminal and 1,2-disubstituted olefins.227,228,17 It is noteworthy that complex (68a) catalyzes cyclopropanation, using diazomethane as a carbene source, with good enantioselectivity (70-75% ee).17... 

Scheme 61).22 149 150 152 229 230 The methylene-bridged bis(oxazoline)s (70) can afford either neutral copper complexes of the semicorrin type (72) or cationic copper complexes (73). Cyclopro-panation with copper complexes of type (70) shows similar stereochemistry to that with the corresponding copper semicorrin complexes.229-231 Alternatively, bis(oxazoline) ligand (71), bearing... 

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... 

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... 

During these studies [4] it was again proven that copper(I) containing one semicorrin was the species needed and thus preparing the catalyst from Cu(I)OTf saves one molecule of ligand and avoids other problems. 

Asymmetric cyclopropanations of alkenes and alkynes with a-diazocarbonyl compounds have been extensively explored in recent years and a number of very effective chiral catalysts have been developed2. Copper complexes modified with such chiral ligands as salicy-laldimines 38202,203, semicorrins 39204 208, bis(oxazolines) 40209-2" and bipyridines 41212 have... 

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. 

Pyrrolidinyl Enolate-, and Semicorrinate Anions as Chelate Ligands for fron(II) and Copper(II) Ions From Molecular to Collective Structures. 152... 

Metal Carbene TVansformations. The effectiveness of Rh2(55 -MEPY)4 and its 5R-form, Rh2 5R-MEPY)4, is exceptional for highly enantioselective intramolecular cyclopropanation and carbon-hydrogen insertion reactions. Intermolecular cyclopropanation occurs with lower enantiomeric excesses than with alternative chiral copper salicylaldimine or C2-symmetric semicorrin or bis-oxazoline copper catalysts, but intermolecular cyclopropenation exhibits higher enantio-control with Rh2(MEPY)4 catalysts. The methyl carboxylate attachment of Rh2(55-MEPY)4 is far more effective than steri-cally similar benzyl or isopropyl attachments for enantioselective metal carbene transformations. The significant enhancement in enantiocontrol is believed to be due to carboxylate carbonyl stabilization of the intermediate metal carbene and/or to dipolar influences on substrate approach to the carbene center. 

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 alcoholic medium, NaBH, converts a,P-unsaturated nitriles into cyanoethyl compounds under more severe conditions (refluxing alcohol for several hours). - i jpig enantioselective reduction of the double bond of each ( )- and (Z)-isomer of a,P-unsaturated nitriles derived from ketones has been evaluated using NaBH in EtOH/diglyme in the presence of semicorrin (/oCT catalyst the yields are good (about 75%), but the enantioselectivity remains modest (53-69%). - In contrast, it has been found that copper(l) hydride, a reagent that presumably operates by a mechanism akin to the Michael reaction, 754 cleanly and stereoselectively effects the reduction of a,P-iinsaturatcd to saturated nitriles in the presence of 2-butanol, as proton donnor, in THE. The use of LAH in THF at room temperature has also been reported in the stereoselective reduction of a,P-iinsaturatcd nitriles. 

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. 

Copper-Catalyzed Asymmetric Cyclopropanation with Chiral Semicorrin and Related Ligands... 

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. 

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. 

A very similar group of ligands 24-27 for copper or cobalt catalysts, used in asymmetric [1 +2] cycloadditions (Section D.l.6.1.5.) or hydrogenations of double bonds with sodium borohydride (Section D.2.5.1.2.1.3), are the semicorrins. Both enantiomers of the ligands can be obtained from comparatively cheap (2 )- or (.V)-pyroglutarnic acid19 (see Section 2.5.2. for these compounds). 

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