Complexes, optically active schiff bases reaction

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]. 

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-... 

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... 

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... 

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... 

The first reports of a reaction of an amine with an aldehyde by Schiff [584] led to the establishment of a large class of ligands called Schiff bases. Among the most important of the Schiff bases are the tetradentate salen ligands (N,N -bis(salicy-laldehydo)ethylenediamine), which were studied extensively by Kochi and coworkers, who observed their high potential in chemoselective catalytic epoxidation reactions [585]. The best known method to epoxidize unfunctionalized olefins enantioselectively is the Jacobsen-Katsuki epoxidation reported independently by these researchers in 1990 [220,221]. In this method [515,586-589], optically active Mn salen) compounds are used as catalysts, with usually PhlO or NaOCl as the terminal oxygen sources, and with a O=Mn (salen) species as the active [590,591] oxidant [586-594]. Despite the undisputed synthetic value of this method, the mechanism by which the reaction occurs is still the subject of considerable research [514,586,591]. The subject has been covered in a recent extensive review [595], which also discusses the less-studied Cr (salen) complexes, which can display different, and thus useful selectivity [596]. Computational and H NMR studies have related observed epoxide enantioselectivities... 

Previously, Pasini [27] and Colonna [28] had described the use chiral titani-um-Schiff base complexes in asymmetric sulfide oxidations, but only low selec-tivities were observed. Fujita then employed a related chiral salen-titanium complex and was more successful. Starting from titanium tetrachloride, reaction with the optically active C2-symmetrical salen 15 led to a (salen)titani-um(IV) dichloride complex which underwent partial hydrolysis to generate the t]-0x0-bridged bis[(salen)titanium(IV)] catalyst 16 whose structure was confirmed by X-ray analysis. Oxidation of phenyl methyl sulfide with trityl hydroperoxide in the presence of 4 mol % of 16 gave the corresponding sulfoxide with 53% ee [29]. 

The lability of the Pd-N bond in cyclopalladated Schiff base complexes was evaluated. " The substituent on the nitrogen plays an important role determining the ease with which the Pd-N bond is being cleaved. Lopez et aL reported the synthesis of optically active cyclopalladated complexes of (Ligand exchange and insertion reactions were carried out with the bridged dimers. 

Cyclopentadlenyl Complexes.— The stereochemistiry and absolute configuration of the optically active chelate complex [CpCol NCgH C(Me)=NCH(Me)Ph ]I has been elucidated. It is the product of the reaction of [CpCo(CO)2 2 with the Schiff base. 

The base-catalyzed hydrophosphonyladon of aldehydes as a convenient method is widely used for the synthesis of chiral 1-hydroxyalkylphosphonates. We have developed an eflftcient and steroselective method for the synthesis of optically active 1-hydroxyalkylphosphonates and cyclic 1-hydroxyalkylphosphonates by asymmetric hydrophosphonyladon using Al-Schiff base complexes as catalysts. The addition of silver carbonate was found to enhance this catalytic reaction rate significantly. Under optimized reaction conditions, several optically active cyclic 1-hydroxyalkylphosphonates IVB including (S) and (R) configuration could be prepared by the asymmetric hydrophosphonyladon of aldehydes with corresponding cyclic phosphonates in 77-82 % yields with 99 % enantioselectivity. A series... 

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