Ligands chiral Schiff base

The first example of asymmetric catalytic ring-opening of epoxides with sp2-hybridized carbon-centered nucleophiles was reported by Oguni, who demonstrated that phenyllithium and a chiral Schiff base ligand undergo reaction to form a stable system that can be used to catalyze the enantioselective addition of phenyllithium to meso-epoxides (Scheme 7.24) [48]. Oguni proposed that phenyllithium... 

In 2000, Gennaii et al. discovered a new family of chiral Schiff-base ligands, with the general structure, Af-alkyl-p-(A -salicylideneamino)alkanesulfonamide, depicted in Scheme 2.28. These ligands were successfully implicated in the copper-catalysed conjugate addition of ZnEt2 to cyclic enones (Scheme 2.28) and, less efficiently, to acyclic enones such as benzalacetone (50% ee) or chalcone... 

In 1966, Nozaki et al. reported that the decomposition of o-diazo-esters by a copper chiral Schiff base complex in the presence of olefins gave optically active cyclopropanes (Scheme 58).220 221 Following this seminal discovery, Aratani et al. commenced an extensive study of the chiral salicylaldimine ligand and developed highly enantioselective and industrially useful cyclopropanation.222-224 Since then, various complexes have been prepared and applied to asymmetric cyclo-propanation. In this section, however, only selected examples of cyclopropanations using diazo compounds are discussed. For a more detailed discussion of asymmetric cyclopropanation and related reactions, see reviews and books.17-21,225... 

Use of chiral ligands allows asymmetric synthesis of optically active branched aldehydes. In the early 1970s, two groups independently reported the first examples of asymmetric hydroformylation (109). Optical yields of less than 2 % were obtained by using styrene as substrate and a chiral Schiff base-Co or phosphine-Rh complex as catalyst. 

Manganese compounds of the type (22-XXXVIII) catalyze the epoxidation of alkenes with Phl=0, OCT, and similar oxidants. There is a debate about the mechanism, though a metallaoxirane (22-XXXIX) intermediate is likely with chiral Schiff base ligands the reaction is enantioselective.164... 

Many other reports of ligand libraries for specific catalytic applications have been reported. Among them, Gilbertson and co-workers reported a chiral phosphine library, tested in the rhodium-catalyzed asymmetric hydrogenation of an enamide (158,159), and a similar library for the palladium-catalyzed allylation of malonates (160, 161) Hoveyda and co-workers (162, 163) reported a chiral Schiff base library, screened in the titanium-catalyzed opening of epoxides with (TMSCN) (trimethyl silyl cyanide) ... 

Reaction of the chiral Schiff-base ligand H2L ((97) Scheme 18) with NbIV(NEt2)4 produced LNbv(NEt)(NEt2) (98) (43%) and L Nb(NEt2)2 (99) (16%) with modified ligand L. 174 Radical pathways were suggested. 

Possible control of stereochemistry was investigated with the use of chiral Schiff bases L-ll (R = Ph, Cy) as ligands or their recrystallized catalyst Cu-3 in the polymerizations of MMA with a bromide initiator in xylene at 90 °C.103 Unfortunately, the tacticity of the polymers is not different from that in conventional polymerizations, while control of molecular weights was achieved. 

Oguni discovered that phenyllithium in the presence of 5 mol % of chiral Schiff base hgands created a stable and efficient catalyst system. The addition to cyclohexene oxide occurred in quantitative yield to form the phenylcyclohexanol in 90% ee (Scheme 11) [24]. Oguni proposed that deprotonation of the phenol and/or 1,2-addition to the imine ligand 15 formed the catalytically-active species. 

There are several other reports of successful application of this type of chiral Schiff base ligand in copper-catalyzed asymmetric cyclopropanation44" 51- 93-94 including intramolecular versions52. 

The copper-catalyzed cyclopropanation of alkenes with diazoalkanes is a particularly important synthetic reaction (277). The reaction of styrene and ethyl diazoacetate catalyzed by bis[/V-(7 )- or (5)-a-phenyl-ethylsalicylaldiminato]Cu(II), reported in 1966, gives the cyclopropane adducts in less than 10% ee and was the first example of transition metal-catalyzed enantioselective reaction of prochiral compounds in homogeneous phase (Scheme 90) (272). Later systematic screening of the chiral Schiff base-Cu catalysts resulted in the innovative synthesis of a series of important cyclopropane derivatives such as chrysanthemic acid, which was produced in greater than 90% ee (Scheme 90) (273). The catalyst precursor has a dimeric Cu(II) structure, but the actual catalyst is in the Cu(I) oxidation state (274). (S)-2,2-Dimethylcyclopropanecar-boxylic acid thus formed is now used for commercial synthesis of ci-lastatin, an excellent inhibitor of dehydropeptidase-I that increases the in vivo stability of the caibapenem antibiotic imipenem (Sumitomo Chemical Co. and Merck Sharp Dohme Co.). Attempted enantioselective cyclopropanation using 1,1-diphenylethylene and ethyl diazoacetate has met with limited success (211b). A related Schiff base ligand achieved the best result, 66% optical yield, in the reaction of 1,1-diphenylethylene and ethyl diazoacetate (275). 

The need in new inexpensive, safe and effective processes for asymmetric sulfide oxidations is determined by pharmaceutical industry requirements [38], Recently, inexpensive and active. systems based on hydrogen peroxide as oxidant and non-toxic chiral iron(III) complexes as catalysts have been reported [39-41 ]. Different mctal-salen complexes have also been previously employed as catalysts for oxidation of sulfides with PhIO Mn "(salen) [42-44], salen) 45], salen) [46], The mechanism proposed in [46] involves intermediate formation of 0x0 iron(lV)-salcn cation radical, that seems doubtful based on the experimental results obtained. In this Chapter we present asymmcinc version of the latter system [(salen )Fc ClJ/PhlO (where salcn stands for the corresponding chiral Schiff base ligands. Scheme 5) and an NMR investigation of the active intermediates. 

Starting from his initial work in 1989 on organometaUic compounds with a ferroelectric phase, Espinet developed the chemistry of metallomesogens based on dimeric palladium derivatives of chiral Schiff bases (Pd2(/r-Cl)2L 2 where L represents a chiral Schiff base ligand.Several examples of these complexes (6.21-6.23) are shown in Figure 6.14... 

Metallomesogens based on dimeric palladium (II) derivatives of enantiopure Schiff bases Pd2(lt-C1)2L 2 (6.21-6.23) where L represents a chiral Schiff base ligand. 

A new family of hybrid organic-inorganic catalysts with dispersed active sites inside ordered mesoporous materials has been prepared by anchorage of transition-metal ligands of Schiff base-type and chiral amino alcohols like (li ,2S)-ephedrine on micelle-templated silicas [180]. Metalation of the grafted ligands with manganese was followed by UV-VIS spectroscopy. 

Chiral Catalysts Containing Group 10 Metals (Ni, Pd, and Pt). The catalyst formed in situ from Ni(acac)2 and bomane aminoalcohols (DAB or DAIB) catalyze the enantioselective addition of diethylzinc to chalcones (254) (Fig. 21). Nickel(II)-chiral Schiff-base (the ligand derived from 1,2-diaminocyclohexane or 1,2-diaminopropane with pyrone derivative) complexes were efficient in epoxida-tion of nonfunctionalized olefins (255). Bis-ferrocenyl-triphosphane (PIGIPHOS) formed catalytically active complex with Ni(II) (256). Nickel-catalyzed asymmetric hydrocyanation of vinylarenes using glucose-derived phosphinite ligands was observed (257). 

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-... 

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