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Salen Ligand (salen + ligand)
Selected AbstractsSalen Ligands Revisited: Synthesis and Application of a Planar Chiral "Ferro-Salen" LigandEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 5 2010Jochen Niemeyer Abstract Condensation of the O-protected planar chiral hydroxyferrocene carbaldehyde (Sp)- 1 with ethylenediamine, followed by deprotection gave rise to the "ferro-salen" ligand (Sp,Sp)- 3 in good yield. This scaffold was used for the preparation of a series of metal complexes [(Sp,Sp)- 4/5/6], which were subsequently applied for the Lewis acid catalyzed asymmetric trimethylsilylcyanation of benzaldehyde. [source] Synthesis and Electrochemical Study of an Original Copper(II)-Capped Salen,Cyclodextrin ComplexEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 29 2010Elise Deunf Abstract A new metallocapped cyclodextrin (CD) was synthesized by the regioselective debenzylation, induced by diisobutylaluminium hydride (DIBAL-H), of perbenzylated cyclodextrins. This reaction allowed for the efficient preparation of an unprecedented CD,salen type copper(II) complex. The electrochemical behavior of both the bound and unbound CD,salen compounds was investigated by cyclic voltammetry. Notably, it was shown that the presence of tert -butyl groups at the ortho - and para -positions of the salen aromatic rings stabilized the copper(II) phenoxyl radical species that was generated upon the one-electron oxidation of the starting compound. Importantly, this stabilization remained effective when the salen-type ligand was covalently attached to the CD. This allowed for investigations of the reactivity of the copper(II) phenoxyl radical complex towards a primary alcohol to be performed by cyclic voltammetry. This reaction can be considered as mimicking the behavior of galactose oxidase. However, under these conditions, no reactivity was observed in the presence of benzyl alcohol. This may be due to distortion, either of the initially square planar salen ligand after its grafting to the CD primary face, and/or of the CD itself. On the other hand, the electrochemical reduction of the un-grafted copper(II) salen-type ligand led to a transient anionic species that exhibited significant stability on the time-scale of the slow cyclic voltammetry measurement in the absence of the CD, but was unstable in the presence of the CD. In the latter case, it was demonstrated that the anionic species was protonated by the CD. Importantly, this protonation was not fast enough to prevent catalytic activation of iodomethane by the electro-generated copper(I)-capped salen CD complex. [source] Cyclic Carbonate Synthesis Catalysed by Bimetallic Aluminium,Salen ComplexesCHEMISTRY - A EUROPEAN JOURNAL, Issue 23 2010William Clegg Prof. Abstract The development of bimetallic aluminium,salen complexes [{Al(salen)}2O] as catalysts for the synthesis of cyclic carbonates (including the commercially important ethylene and propylene carbonates) from a wide range of terminal epoxides in the presence of tetrabutylammonium bromide as a cocatalyst is reported. The bimetallic structure of one complex was confirmed by X-ray crystallography. The bimetallic complexes displayed exceptionally high catalytic activity and in the presence of tetrabutylammonium bromide could catalyse cyclic carbonate synthesis at atmospheric pressure and room temperature. Catalyst-reuse experiments demonstrated that one bimetallic complex was stable for over 60 reactions, though the tetrabutylammonium bromide decomposed in situ by a retro-Menschutkin reaction to form tributylamine and had to be regularly replaced. The mild reaction conditions allowed a full analysis of the reaction kinetics to be carried out and this showed that the reaction was first order in aluminium complex concentration, first order in epoxide concentration, first order in carbon dioxide concentration (except when used in excess) and unexpectedly second order in tetrabutylammonium bromide concentration. Further kinetic experiments demonstrated that the tributylamine formed in situ was involved in the catalysis and that addition of butyl bromide to reconvert the tributylamine into tetrabutylammonium bromide resulted in inhibition of the reaction. The reaction kinetics also indicated that no kinetic resolution of racemic epoxides was possible with this class of catalysts, even when the catalyst was derived from a chiral salen ligand. However, it was shown that if enantiomerically pure styrene oxide was used as substrate, then enantiomerically pure styrene carbonate was formed. On the basis of the kinetic and other experimental data, a catalytic cycle that explains why the bimetallic complexes display such high catalytic activity has been developed. [source] Photoisomerization of a Maleonitrile-Type Salen Schiff Base and Its Application in Fine-Tuning Infinite Coordination PolymersCHEMISTRY - A EUROPEAN JOURNAL, Issue 12 2010Chun-Wei Lin Abstract Strategically designed salen ligand 2,3-bis[4-(di- p -tolylamino)-2-hydroxybenzylideneamino]maleonitrile (1), which has pronounced excited-state charge-transfer properties, shows a previously unrecognized form of photoisomerization. On electronic excitation (denoted by an asterisk), 1Z*,1E isomerization takes place by rotation about the C2C3 bond, which takes on single-bond character due to the charge-transfer reaction. The isomerization takes place nonadiabatically from the excited-state (1Z) to the ground-state (1E) potential-energy surface in the singlet manifold; 1Z and 1E are neither thermally inconvertible at ambient temperature (25,30,°C), nor does photoinduced reverse 1E*,1Z (or 1Z*) isomerization occur. Isomers 1Z and 1E show very different coordination chemistry towards a ZnII precursor. More prominent coordination chemistry is evidenced by a derivative of 1 bearing a carboxyl group, namely, N,N,-dicyanoethenebis(salicylideneimine)dicarboxylic acid (2). Applying 2Z and its photoinduced isomer 2E as building blocks, we then demonstrate remarkable differences in morphology (sphere- and needlelike nanostructure, respectively) of their infinite coordination polymers with ZnII. [source] Cooperative Catalysis in the Hydrolytic Kinetic Resolution of Epoxides by Chiral [(salen)Co(III)] Complexes Immobilized on Gold ColloidsADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 7-8 2008Thomas Belser Abstract Chiral salen ligands were incorporated into self-assembled thiolate monolayers (SAMs) on gold colloids. Treatment of the immobilized ligand with Co(OAc)2,4,H2O yielded the corresponding [(salen)Co(II)] complex, and aerobic oxidation in the presence of triflic acid afforded the catalytically active [(salen)Co(III)] complex. Functionalized gold colloids with a diameter of 3.4,nm, coated with a mixed monolayer of n -octanethiolates and thiolates with chiral [(salen)Co(III)] end groups were studied as catalysts in the hydrolytic kinetic resolution (HKR) of hexene-1-oxide. Extremely high selectivitiy and significant rate acceleration relative to homogeneous monomeric catalysts were observed. Recovery of the immobilized catalyst was accomplished by simple filtration, and catalyst reoxidation and repeated recycling (seven times) was possible with no loss of reactivity or enantioselectivity. [source] Synthesis of Metal,(Pentadentate-Salen) Complexes: Asymmetric Epoxidation with Aqueous Hydrogen Peroxide and Asymmetric Cyclopropanation (salenH2: N,N, -bis(salicylidene)ethylene-1,2-diamine)CHEMISTRY - A EUROPEAN JOURNAL, Issue 17 2007Hiroaki Shitama Abstract It is known that the rates and stereochemical outcomes of epoxidations and cyclopropanations using a metallosalen (salenH2: N,N, -bis(salicylidene)ethylene-1,2-diamine) complex as catalyst are affected by a trans effect of the apical ligand of the complex. By taking into consideration this trans effect, we have synthesized optically active pentadentate salen ligands bearing an imidazole or pyridine derivative as the fifth coordinating group, and have prepared the corresponding manganese(III) and cobalt(II) complexes, in which the fifth ligand is expected to intramolecularly coordinate to the metal center and exert a trans effect. Indeed, high enantioselectivity has been achieved in epoxidations using aqueous hydrogen peroxide as the terminal oxidant and in cyclopropanations with these complexes as catalysts. In general, metallosalen-catalyzed reactions have been carried out in the presence of an excess of a donor ligand; however, the present reactions do not need the addition of any extra donor ligand. [source] | ||||||||||