Complexes, optically active schiff bases series

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. 

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]. 

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—... 

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]. 

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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