Complexes, optically active schiff bases

The modified Sharpless reagent was also successfully applied288 for the asymmetric oxidation of a series of 1,3-dithiolanes 248 to their S-monooxides 249 (equation 134). It was observed that the optical induction on sulphur (e.e. from 68 to 83%) is not significantly affected by the substituents R1 and R2. Asymmetric oxidation of a few aryl methyl sulphides by organic hydroperoxides in the presence of a catalytic amount of the optically active Schiff base-oxovanadium(IV) complexes gave the corresponding sulphoxides with e.e. lower than 40%289. 

The Schiff bases being derivatives of aldehydes or ketones and various amines have received considerable attention because of their interesting physical and chemical properties, involvement in biologically important reactions and widespread application of their metal complexes. Increasing interest in optically active Schiff bases is connected with the discovery at the beginning of the 1990s of the so-called Jacobsen catalysts used in several asymmetric reactions showing excellent enantioselectivity. 

The optically active Schiff bases containing intramolecular hydrogen bonds are of major interest because of their use as ligands for complexes employed as catalysts in enantioselective reactions or model compounds in studies of enzymatic reactions. In the studies of intramolecularly hydrogen bonded Schiff bases, the NMR spectroscopy is widely used and allows detection of the presence of proton transfer equilibrium and determination of the mole fraction of tautomers [21]. Literature gives a few names of tautomers in equilibrium. The OH-tautomer has been also known as OH-, enol- or imine-form, while NH tautomer as NH-, keto-, enamine-, or proton-transferred form. More detail information concerning the application of NMR spectroscopy for investigation of proton transfer equilibrium in Schiff bases is presented in reviews.42-44... 

Optically active Schiff-base oxovana-dium(IV) complexes catalyze the asymmetric oxidation of sulfides to sulfoxides by peroxides [86]. The catalytically active species is VO(V) rather than VO(IV) and is formed in situ under the reaction conditions. A series of related complexes based on the optically active ligand shown in Eig. 15 shows linear dependence of their oxidation Ef values on the Hammett parameters of functional group X. These values ranged from 0.18 V versus Cp2Ee/DMSO for X = NO2 to —0.18 V for X = OCH3 [87]. A few complexes of planar tetradentate non-Schiff base ligands have also been investigated [88]. 

Free ligands have been studied in order to obtain an insight into their structure, both in solution and in the solid state, and for comparison with their metal complexes. H NMR spectroscopy has been used to investigate the keto-enol equilibrium and the nature of the hydrogen bonds. In the case of optically active Schiff bases UV and CD spectra provided information about structure in solution. The Schiff bases that have been most widely examined are derivatives of acetylacetone, salicyl-aldehyde and hydroxymethylenecamphor, whose prototypes with en are shown in Figure 13. 

Schiff Bases. An extension of work on VO and UOl" complexes of optically active Schiff bases has resulted in the isolation of two series of titanium complexes of stoicheio-metry. TiOSB and TiOSB,HA [SB = quadridentate Schiff base dianion from sali-cylaldehyde and ( —)-propylenediamine, ( —)-butanediamine, meso-butanediamine, (-)-cyclohexylamine, or (-l-) Stilbenediamine A = CIO4, C2O4, or Cl]. The low solubilities of the complexes and an i.r. band at ca. 800 cm attributable to v(Ti—... 

Key Words Schiff bases, Imines, Complexes, Optically active, Equilibria, NMR, CP MAS NMR. 

Rh2(55-mepy)4] was first used for cyclo-propanation. Enantioselective cyclopropana-tion is industrially important since synthetic pyrethroids, which are used as insecticides, contain substituted three-membered rings, whose configuration is crucial for their biological effect. [9] Enantioselective cyclopropa-nation has tradition. It was this reaction type which in 1966 opened up the field of enantioselective homogeneous catalysis with transition metal complexes. The copper(II) complex of the Schiff base from salicylaldehyde and optically active 1-phenylethylamine at that time reached 6% ee. [10] With optimized opti-... 

Schiff base-oxovanadium(IV) complexes, as optically active oxidizing agents 291... 

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... 

Another class of ligands for ATH is represented by multidentate Schiff bases and their derivatives. Zassinovich and Mestroni reported on the effective reduction of alkyl aryl ketones catalyzed by a series of lr(l) complexes with chiral bidentate pyridylaldimines, of the form [lr(cod)(NNR )]C104 (76a-f see Scheme 4.31). It was observed that both the activity and selectivity depended heavily on the nature of the subshtuents at the chiral center of the ligand, and also at the prochiral center of the substrate. Optical yields of up to 50% (R-isomer) at 100% conversion were obtained in the ATH of BuC(0)Ph and PhCH2C(0)Ph using [lr(cod)(PPEl)]C104 as the precatalyst (0.1% mol, 83 °C, PrOH, KOH) [66]. 

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. 

Although complexes with these ligands are common in palladium(II) chemistry, their occurrence is more scarce in platinum(II) compounds. Nevertheless these complexes can be prepared, examples being platinum(II) complexes of the optically active quadridentate Schiff base of salicylaldehyde and (R)-l, 2-diamines.1212 An alternative synthesis involves formation of the Schiff base by reaction of a complexed amino ligand on platinum(II) with amide acetates (equation 372).1213... 

The first enantiomer-selective polymerization was performed with propylene oxide (172) as a monomer [245], The polymerization was carried out with a ZnEt2/(+)-bor-neol or ZnEt2/(-)-menthol initiator system. The obtained polymer was optically active and the unreacted monomer was rich in (S)-isomer. Various examples are known concerning the polymerization and copolymerization of 172 [246-251 ]. A Schiff base complex 173 has been shown to be an effective catalyst In the polymerization at 60°C, the enantiopurity of the remaining monomer was 9% ee at 50% monomer conversion [250],... 

In addition to its use in the preparation of the square pyramidal Mo and W complexes 38 and 39, the Schiff base derived from pyridine carbaldehyde-(2) and S-(—)-a-phenyl ethyl amine54 was also used for the synthesis of optically active Co complexes of the tetrahedral type60. Unlike the Mo and W compounds, the separated Co diastereoisomers 40a, 40b one of which is shown in Scheme 20, are optically stable. The rigidity of the tetrahedral Co complexes and the nonrigidity of the square pyramidal Mo and W complexes give a further indication of the intramolecular character of the epimerization of 38 and 39. 

Brunner has continued his studies on optically active manganese carbonyl complexes and has reported that treatment of Mn(CO)5Br with ort/to-Me2NC6H4PPh2 (PN) yields two enantiomers of/ac-[Mn(CO)3(PN)Br], Treatment of this complex with carbon monoxide in the presence of A1C13 produces the cation [Mn(CO)4(PN)] +, which was isolated as its hexafluorophosphated salt. Addition of menthoxide anions to the manganese carbonyl cation yields the diastereoisomers of Mn(CO)3(PN) (CO2C30H 9) however, these could not be separated due to their instability. Reaction of Mn(CO)5Br with the Schiff base NN (1) leads to formation of two isomers of... 

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