Catalytic system salen

Song et al. [62] reported poly-salen Co(III) complexes 18, 19 as catalyst for HKR (Figure 5) of terminal alkene epoxides. The polymeric catalysts provided product epoxides with excellent conversion (>49%) and high chiral purity (ee s, 98%) and the catalytic system could be recycled once with retention of activity and enantioselectivity. 

An even more active related catalytic system has recently been reported by Lee s group. This (salen)cobalt(III) catalyst containing the salen ligand depicted in Fig. 12 exhibits a highly unusual coordination mode for the normally tetradentate salen ligand [35]. That is, whereas the f-butyl-substituted phenolate analog displays conventional imine coordination, the salen ligand in Fig. 12 is proposed to be bound... 

Various other chromene derivatives 176a-d could be epoxidized with Katsuki s Mn-salen catalyst 173d using either H2O2 or TMS2O2 as oxidant. With this catalytic system several axial ligands (none, 7V-methylimidazole, pyridine TV-oxide) and additives (none. 

Warnmark et al. [12] have reported the formation of a dynamic supramolecular catalytic system involving a hydrogen bonding complex between a Mn(ll I) salen and a Zn(II) porphyrin (Figure 1.4). The salen sub-unit acts as the catalytic center for the catalytic epoxidation of olefins while the Zn-porphyrin component performs as the binding site. The system exhibits low selectivity for pyridine-appended styrene derivatives over phenyl-appended derivatives in a catalytic epoxidation reaction. The... 

Chromium(III) salen will form chromium(V) derivatives in much the same way as porphyrin complexes do. By reaction of iodosylbenzene with chromium(III)salen oxochromium(V) complexes are obtained (equation 74) which will effect oxygen atom transfer to phosphines and alkenes such as norbornene (equation 75) in stoichiometric and catalytic systems.1270 The... 

In conclusion, the chiral salen Co(III) complexes immobilized on Si-MCM-41 colud be synthesized by multi-grafting method. The asymmetric synthesis of diols from terminal olefins was applied with success using a hybrid catalyst of Ti-MCM-41/chiral Co(III) salen complexes. The olefins are readily oxidized to racemic epoxides over Ti-MCM-41 in the presence of oxidants such as TBHP, and then these synthesized diols are generated sequentially by epoxide hydrolysis on the salen Co(lll) complexes. This catalytic system may provide a direct approach to the synthesis of enantioselective diols from olefins. 

The fixation of C02 into a three-membered ring has also been promoted, under relatively severe conditions, by other catalytic systems such as tetrapheny-lantimony halides (333 K, 5 MPa) [68e] or the p-methoxyphenol/DMAP system (DMAP = 4,6-(dimethylamino)pyridine 393 K, 3.6MPa, 48h) [68f] or, under much milder conditions, by alkali [68g-k] or tetralkylammonium halides [68j], or by (Salen)Cr(III)(DMAP) [681]. It is worth noting that, with 2-alkyl or 2-aryl substituted aziridines, alkali or tetralkylammonium halides catalyze the formation of the 4-substituted regioisomer as the main or unique product, whereas the chromium(III) catalyst promoted the preferential conversion to the 5-subshtuted regioisomer with high selectivity and yield. 

Chromium(salen) catalysts are excellent reagents for the desymmetrization of OT to-epoxides. Thus, tfr-stilbene oxide is converted to the (3, 3 )-aminoalcohol in the presence of catalytic quantities of chromium-salen complex in methylene chloride solution open to the atmosphere. The addition of small quantities of triethylamine was found to dramatically increase enantioselectivities (by almost 25%). This catalytic system also promotes an efficient aminolytic kinetic resolution (AKR) of racemic epoxides with 2-type symmetry (Equation 18) <20040L2173, 1999TL7303>. W fo-Epoxides can be opened with aromatic amines in water in the presence of 1 mol% of an Sc(ni) catalyst ligated to 1.2mol% of a chiral bipyridine ligand <2005OL4593>. 

It was found that iron phthalocyanine and the Co(II)-complexes of salen-type molecules, especially the Co(II)-complex of N,N -bis(salicylidene)-l,2-phenylenediamine (salophen) activated efficiently the molecular oxygen in the catalytic system. 

Very recently, White and coworkers introduced the chiral Lewis acid Crm(salen) as cocatalyst into Ll/Pd11 catalytic system. The oxidative allylic acetoxyaltion of terminal olefins 1 afforded the corresponding branched allylic acetates 3 in high regioselectivity and moderate enantio-selectivities (up to 63% ee) (Scheme 6) [22], The asymmetric induction possibly results from the coordination between Cr salen) and BQ, and the adduct of Cr,n(salen) BQ promotes the acetoxylation of rc-allyl-palladium complex to form enantioenriched branched allylic acetates. 

Ally lie amination. 1-Alkenes can be functionalized with double bond migration that results in derivatives of l-amino-2-alkenes, when they are treated with TsNHCOOR and catalytic amounts of Pd(OAc)2 under O2. Maleic anhydride, 4A-molecular sieves and NaOAc are the proper additives for this reaction. Alternatively, the same transformation is accomplished with benzoquinone as the oxidant, together with l,2-bis(benzenesulfinyl)-ethane and (salen)Cr tl. A heterobimetallic catalytic system is involved. However, only the Pd(II) catalyst is needed for oxidative cyclization of homoallylic A-tosylcarbamates. ... 

Addition to C=0. Several protocols have been developed for the preparation of chiral cyanohydrins or their trimethylsilyl ethers. Catalytic systems including vanadyl-salen... 

Analogously, in the presence of silica-supported palladium catalysts, benzene is oxidized under ambient conditions to give phenol, benzoquinone, hydroquinone and catechol [37b]. Palladium chloride, used for the catalyst preparation, is believed to be converted into metallic palladium. The synthesis of phenol from benzene and molecular oxygen via direct activation of a C-H bond by the catalytic system Pd(OAc)2-phenanthroline in the presence of carbon monoxide has been described [38]. The proposed mechanism includes the electrophilic attack of benzene by an active palladium-containing species to to produce a a-phenyl complex of palladium(ll). Subsequent activation of dioxygen by the Pd-phen-CO complex to form a Pd-OPh complex and its reaction with acetic acid yields phenol. The oxidation of propenoidic phenols by molecular oxygen is catalyzed by [A,A"-bis(salicylidene)ethane-l,2-diaminato]cobalt(ll)[Co(salen)] [39]. 

The third class of systems to be discussed here appeared in the last years, reflecting the growing need in simple and non-toxic systems for sulflde oxidation. Chiral [Fe "(salen)Cl] complexes were synthesized and used as catalysts for asymmetric oxidation of several alkylaryl sulfides with PhlO. Surprisingly, the reactive intermediate in this system was found different from those detected in Mn-salen and Cr-salcn based catalytic systems. Namely, the intermediate was shown to be a iodosylbcnzcnc-irondata obtained are analyzed to understand some catalytic properties of these systems. 

Scheme 4. Kochi s catalytic cycle for the Cr" (salen)/PhlO catalytic system and Giiheany correction [15].

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