Oxidation Schiff base-type ligands

The major breakthrough in this field was achieved in 2003 by Legros and Bolm [164], who reported a highly enantioselective iron-catalyzed asymmetric sulfide oxidation. Optically active sulfoxides were obtained with up to 96% ee in good yields under very simple reaction conditions using Fe(acac)3 as precatalyst in combination with a Schiff base-type ligand (Table 3.9). Furthermore, inexpensive and safe 35% aqueous hydrogen peroxide served as terminal oxidant. 

A solid-phase sulfur oxidation catalyst has been described in which the chiral ligand is structurally related to Schiff-base type compounds (see also below). A 72% ee was found using Ti(OPr-i)4, aqueous H2O2 and solid-supported hgand 91 . More recently, a heterogeneous catalytic system based on WO3, 30% H2O2 and cinchona alkaloids has been reported for the asymmetric oxidation of sulfides to sulfoxides and kinetic resolution of racemic sulfoxides. In this latter case 90% ee was obtained in the presence of 92 as chiral mediator. ... 

A phosgene-free route to aromatic isocyanates, such as M DI and TDI, was reported by Fernandez et al. [42] (Scheme 5.7) According to the patent, the one-pot synthesis involves the use of an immobilized Schiff base type of ligand catalyst that facilitates the oxidative carbonylation of aromatic amines to the corresponding isocyanates. However, 2,2,2-trifluoroethanol (TFE), 1,2-dichlorobenzene, and carbon monoxide were used in this process, so this would not be a totally environmentally friendly process even if these reagents could be recycled and reused. 

The oxidation of substituted o-diphenols to the corresponding o-benzoquinones in the presence of 4 different binuclear copper complexes has been investigated by Bolus and Vigee [14]. The initial rates of oxidation were within a factor of 4 for the substrates catechol, 4-methylcatechol, dopamine, 3,4-dihydroxybenzoic acid, and 3,4-dihydroxybenzonitrile. The ligands of the catalyst complexes were Schiff-base type derivatives of 2,6-diformyI-4-methylphenol with diamines and amino acids their synthesis and characterization have been reported [15]. 

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... 

Niobium and tantalum halides form adducts with various nitrogen donor ligands including aliphatic and aromatic amines nitriles, Schiffs bases and imidazoles (Table 5). The reactions of MXS with pyridine and related ligands such as bipy or phen depend critically on the reaction conditions. With py at low temperature MX5 (X = Cl, Br) yielded 1 1 adducts that are rapidly reduced to [MX4(py)2] on increasing the temperature, with formation of l-(4-pyridyl)pyridinium halide. Similarly, bipy and phen reduced the metal in MeCN to oxidation state +IV and formed monoadducts of type [MX bipy)] at room temperature, while at 0°C the same reactions yielded [NbCls(bipy)(MeCN)] and [TaX5(bipy)(MeCN)J (X = C1 or Br). NbBrs and Tals formed [MX5(bipy)2], which were formulated as the eight-coordinate [MX4(bipy)2]X.1 Reduction of the metal can however be prevented, even at room temperature,... 

A one-sided binding of the complex to the support resulted in further improvements (92). This requires synthesis of nonsymmetric salen-type ligands, which is complicated by the tendency of such ligands to equilibrate to give mixtures containing symmetric Schiff bases. Excellent results were obtained with monomer 7d, diluted in a methacrylate polymer, by using a combination of meta-chloroperbenzoic acid (mCPBA) and A-methyl-morpholine-A-oxide (NMO) as the oxidants ... 

In the search for model systems, hundreds of new compounds have been made, those with carboxylate ligands being the most common. However, other ligands employed have been bipyridine, pyrazolylborates, triazacyclononanes, and Schiff bases of various sorts. These have been obtained in oxidation states II-V with both unitary and mixed valencies. The structures vary from quite simple to extremely complicated. Some of the core types are shown in Fig. 17-D-2. 

Manganese compounds of the type (22-XXXVIII) catalyze the epoxidation of alkenes with Phl=0, OCT, and similar oxidants. There is a debate about the mechanism, though a metallaoxirane (22-XXXIX) intermediate is likely with chiral Schiff base ligands the reaction is enantioselective.164... 

Next to the rich oxygenation chemistry of Mn in Mn-Pc and Mn-POR complexes, there exists catalytic chemistry of Mn with non-heme-type ligands, mostly bioinspired. In Photosystem II, a non-heme multinuclear Mn redox center allows to oxidize water, while in catalase the active center is a dinuclear Mn species [34], Biomimctic models for these biological redox centers use ligands such as 2,2 -bipyridine (BPY), triaza- and tetraazacycloalkanes and Schiff bases such as Me(Salen) and Mc(saloph) (structure sec below) [23J. Usually, the complexes activate heterolytically peroxides, with Mn valency changes such as ... 

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