Schiff base ligands hydrogenation

From a practical viewpoint the recently discovered vanadium-based and iron-based asymmetric sulfoxidation with hydrogen peroxide is worth mentioning [305, 306]. For vanadium, in principle as little as 0.01 mol% of catalyst can be employed (Fig. 4.111). With tridentate Schiff-bases as ligands, formed from readily available salicylaldehydes and (S)-tert-leucinol, ees of 59-70% were obtained for thioanisole [305], 85% ee for 2-phenyl-l,3-dithiane [305] and 82-91% ee for tert-butyl disulfide [307]. For iron, similar results were obtained using 4 mol% of an iron catalyst, synthesized in situ from Fe(acac)3 and the same type of Schiff base ligands as in Fig. 4.111 (see Ref. [306] for details). 

The one-stage condensation of tren with 5,5 -diformyl-2,2 -bipyridine made it possible to also obtain the macrobicyclic Schiff base, whose hydrogenation on the palladium catalyst yielded the clathrochelate tris-bipyridine tranbpy ligand 1 (Scheme 91) [195]. 

Iridium(I) complexes with Schiff base ligands effectively transfer hydrogenate ketones. Use of optically active ligands allows asymmetric reduction of prochinal ketones to optically active alcohols ... 

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. 

N-H - 0, C=0- H-Osoivent, 0-H---0, N-H - 0=C hydrogen bonds and C=0- Jt, C-H- 7t, and it - it stacking interactions, derived from both Schiff base and rednced Schiff base ligands are discussed to provide the readers with some nseful insights into the factors behind the observed strnctnral diversity that direct the formation of snpramolecnlar strnctures in the solution and/or solid state. 

Scheme 8.34 Hydrogenation of a Schiff base with tetrasulfonated diphosphine ligand.

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... 

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