Schiff bases bidentate

The major developments of catalytic enantioselective cycloaddition reactions of carbonyl compounds with conjugated dienes have been presented. A variety of chiral catalysts is available for the different types of carbonyl compound. For unactivated aldehydes chiral catalysts such as BINOL-aluminum(III), BINOL-tita-nium(IV), acyloxylborane(III), and tridentate Schiff base chromium(III) complexes can catalyze highly diastereo- and enantioselective cycloaddition reactions. The mechanism of these reactions can be a stepwise pathway via a Mukaiyama aldol intermediate or a concerted mechanism. For a-dicarbonyl compounds, which can coordinate to the chiral catalyst in a bidentate fashion, the chiral BOX-copper(II)... 

The bidentate ligands were prepared by the Schiff-base condensation of two equivalents of the desired 2,6-dialkyl substituted anilines with acenaphthenequinone as in the scheme 1, The pre-catalysts, formed by addition of the ligand to (DME)NiBr2 are isolated and purified. The products were characterized by h, C NMR, GPC, DSC and Elemental Analysis. 

Infrared and thermal studies also add to our knowledge of these polymers. For example, the aromatic C-0 stretch of the Schiff-base ligands is shifted above 1300 cm-1 when coordinated, and coordinated carboxylate shows no CO stretch above 1600 cm-1 when bidentate. The... 

In contrast with the Schiff base salen, salicylaldehyde oxime (79) (salox) complexes of Co have received comparatively little attention, but a series of bis-bidentate divalent complexes of the form iraiis-Co(sa 1 ox)2( D M SO)2 have been reported.343 The heterocyclic bidentate oxime violurate (lH,3H-pyrimidine-2,4,5,6-tetrone 5-oximate, Hvi) (80) and its /V-methyl (mvi) and /V,/V -dimethyl (dmvi) derivatives form high-spin divalent [Co(vi)]+ and Co(vi)2 complexes, whereas [Co(vi)3] is low spin.344 The mixed-ligand Co(dmvi)2(phen) complex is also low spin. The crystal structure of m-Co(pxo)2Br2 (pxo = 2-acetylpyridine-l-oxide oxime) is isostructural with the Ni11 relative.345 The dichloro complex also adopts a cis configuration. The tridentate dioximes 2,6-diformyl-4-methylphenol dioxime and 2,6-diacetyl-4-methylphenol dioxime (Hdampo) form binuclear complexes of the type (81a) and (81b) respectively.346 Cobalt oxide nanoparticles were prepared by... 

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 ancillary N-donor and O-donor ligands are also important in the formation and stabilization of oxomanganese clusters. N-donor ligands are especially common ranging and range from bidentate to predesigned polydentate and include aliphatic, cyclic, Schiff base, and polypyridyl systems (Figures 2 and 3). 

The electrochemistry of Tc complexes containing either two variously substituted bidentate 8-ox type ligands [TcOCl(Lb)2], one 8-ox ligand, and one bidentate Schiff-base ligand [TcOCl(Lb)(La)], or one 8-ox ligand and one tridentate dianionic Schiff-base ligand such as... 

A class of ligands which is very flexible in terms of its denticity, donor atom geometry, and steric demands are Schiff bases derived from salicyladehyde or related carbonyl compounds carrying additional hydroxo groups. The use of bi- and tridentate ligands of this type allows the synthesis of mixed-chelate complexes with a number of O N, N N or N S bidentate ligands. The products are of considerable interest for nuclear medical applications as well as for homo-... 

Rhenium(IV) complexes with only one bidentate ligand have been described for chelating phosphines, Schiff bases, and A-heterocycles. The synthetic approaches are different and involve simple ligand exchange procedures as well as reduction of rhenium(V) or oxidation of rhe-nium(III) compounds. 

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