Asymmetric Baeyer—Villiger oxidation: control of stereoelectronic demand

The recent development of asymmetric Baeyer—Villiger oxidation of prochiral and racemic ketones is briefly summarized, focusing on the regio- and stereocontrol of the oxidation attained by regulating the stereoelectronic demand in the step of rearrangement of the Criegee intermediate.Based on the report presented at the International Conference Modern Trends in Organoelement and Polymer Chemistry dedicated to the 50th anniversary of the A. N. Nesmeyanov Institute of Organoelement Compounds of the Russian Academy of Sciences (Moscow, May 30–June 4, 2004).Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1784–1794, September, 2004.     

Vanadyl salen complexes covalently anchored to an imidazolium ion as catalysts for the cyanosilylation of aldehydes in ionic liquids

Two vanadyl salen complexes having peripheral styryl substituents have been reacted with 1-methyl-3-(3-mercaptopropyl)-imidazolium chloride using azoisobutyronitrile as radical initiator. The resulting compounds contain at the same time a vanadyl salen complex and one imidazolium cation. In agreement with the expectations in view of their structure, these compounds were insoluble in conventional organic solvents, but completely miscible in imidazolium ionic liquids. These vanadyl salen complexes bonded to an imidazolium cation are highly active and reusable catalysts for the cyanosilylation of aldehydes. Moderate enantiomeric excesses were obtained using the chiral version of this complex.     

Oxovanadium(V)-catalyzed enantioselective Meerwein-Ponndorf-Verley cyanation of aldehydes using acetone cyanohydrin

Oxovanadium(V)(salen) complex 4 was found to catalyze Meerwein-Ponndorf-Verley cyanation of aliphatic aldehydes with good to high enantioselectivity. This cyanation showed a positive nonlinear effect.     

Syntheses,structures and electrochemical properties of ruthenium(II/III) complexes with tetradentate Schiff base ligands

Three unsymmetrical tetradentate Schiff base ligands, H₂salipn, H₂salipn-Br₄ and H₂salipn-Cl₂, have been synthesized from the typical condensation reactions of treating 1,2-diaminopropane with salicylaldehyde, 3,5-dibromosalicylaldehyde and 5-chlorosalicylaldehyde, respectively. Treatment of [RuCl₂(PPh₃)₃] with one equivalent of H₂salipn or H₂salipn-Br₄ in the presence of triethylamine in tetrahydrofuran (THF) afforded the corresponding ruthenium(III) complexes [Ru^IIICl(PPh₃)(salipn)] (1) and [Ru^IIICl(PPh₃)(salipn-Br₄)] (2). Interaction of [RuHCl(CO)(PPh₃)₃] with one equivalent of H₂salipn-Cl₂ or H₂salipn-Br₄ under the same conditions led to isolation of ruthenium(II) complexes [Ru^II(CO)(PPh₃)(salalipn-Cl₂)] (3) and [Ru^II(CO)(PPh₃)(salalipn-Br₄)] (4), respectively, in which one of the imine bonds was nucleophilically attacked by hydride to result in the formation of a mixed imine-amine ligand. The molecular structures of 1?1.5CH₂Cl₂, 2, 3?0.5CH₂Cl₂ and 4 have been determined by single-crystal X-ray crystallography. The electrochemical properties of 1–4 were also investigated. Their cyclic voltammograms displayed quasi-reversible Ru(IV)/Ru(III) and Ru(III)/Ru(II) couples with E^o ranging from 0.67 to 1.05 V and 0.74 to 0.80 V vs. Ag/AgCl (0.1 M), respectively.     

Salen Supported Molecular Borosilicates

Various Salen ligands (Salen( ^t Bu)H₂=N, N-ethylenebis(3,5-di-tertbutyl(2-hydroxy)benzylidenimine) were used to prepare borosilyl and -O bridged borosilyl compounds having the formula, L{B(OSiMe₃)₂}₂ (L=Salen( ^t Bu) (1), Salpen( ^t Bu) (2), Salben( ^t Bu) (3), Salhen( ^t Bu) (4) and L(BOSiMe₃)₂(-O) (L=Salen( ^t Bu) (5) and Salben( ^t Bu) (6)). In the case of 5 and 6 the formation of the B–O–B linkage takes precedence over the formation of a B–O–Si linkage. All of the compounds were characterized by Mp, elemental analysis, ¹H and ¹¹B NMR, IR, MS and in the case of 1, 2, and6 by X-ray crystallography.     

Catalytic, asymmetric synthesis of α,α-disubstituted amino acids

Copper(salen) complex 1 has been found to catalyse the asymmetric alkylation of enolates derived from a variety of amino acids. There is a clear relationship between the size of the side chain in the substrate and the enantioselectivity of the process, so that the enantioselectivity decreases in the order alanine>aminobutyric acid>allylglycine>leucine>phenylalanine>valine. A transition state model which accounts for the influence of the size of the side chain on the enantioselectivity of the reactions is presented.     

Spectroscopy, NMR and DFT studies on molecular recognition of crown ether bridged chiral heterotrinuclear salen Zn(II) complex

A barium-containing crown ether bridged chiral heterotrinuclear salen Zn(II) complex BaZn2L(ClO4)2, where L is a folded dinuclear chiral (R,R)-salen ligand, has been synthesized and characterized by elemental analysis, 1H NMR, UV-vis, IR, circular dichroism (CD) spectra, and mass spectra. As a folded dinuclear chiral host, its recognition with achiral guests (imidazole derivatives), rigid bidentate guest (1,4-diazobicyclo[2,2,2]octane, DABCO) and chiral guests (amino acid methyl esters) was investigated by means of UV-vis spectrophotometric titration, CD spectra. The association constants of D-amino acid methyl esters are found to be higher than those of their L-enantiomer. The sandwich-type binding of BaZn2L(ClO4)2-DABCO supramolecular assembly was specially studied via 1H NMR titration and 1H ROESY. To understand the recognition on molecular level, density functional theory (DFT) calculations on B3LYP/LanL2DZ were performed on the minimal energy conformations of host, guests, and host-guest complexes. The minimal energy conformations were obtained by molecular mechanics (MM) optimization and molecular dynamics (MD) simulation. The results of single point energy, HOMO energy, and charges transfer were analyzed. The results of theoretical calculations are in good agreement with the experimental data.     

Synthesis and characterization of biocompatible poly (L-lactide) using zinc (II) salen complex

Abstract

A biocompatible zinc (II) complex based on a tetradentate N,N,O,O-type salen ligand was synthesized, characterized and used for the solvent-free ring-opening polymerization (ROP) of L-lactide in bulk at 180?°C to prepare high molecular weight poly(L-lactide) (M_n : 82,600?Da; M_w : 140,000?Da; PDI: 1.70). Poly(L-lactide) (PLLA) was characterized using FTIR, ¹H NMR, ¹³C NMR, GPC, TGA, DSC, WAXD, and MALDI-ToF. Kinetic measurement was carried out and first-order behavior to monomer was observed. The k _app was found as 6?±?0.001?×?10^?4?s^?1. The biocompatibility of the PLLA was confirmed by in vitro cytotoxicity against NIH/3T3 fibroblast cell line and can be used in biomedical applications.     

Transition Metal Complexes of a Salen–Fullerene Diad: Redox and Catalytically Active Nanostructures for Delivery of Metals in Nanotubes

A covalently‐linked salen–C₆₀ (H₂L) assembly binds a range of transition metal cations in close proximity to the fullerene cage to give complexes [M(L)] (M=Mn, Co, Ni, Cu, Zn, Pd), [MCl(L)] (M=Cr, Fe) and [V(O)L]. Attaching salen covalently to the C₆₀ cage only marginally slows down metal binding at the salen functionality compared to metal binding to free salen. Coordination of metal cations to salen–C₆₀ introduces to these fullerene derivatives strong absorption bands across the visible spectrum from 400 to 630 nm, the optical features of which are controlled by the nature of the transition metal. The redox properties of the metal–salen–C₆₀ complexes are determined both by the fullerene and by the nature of the transition metal, enabling the generation of a wide range of fullerene‐containing charged species, some of which possess two or more unpaired electrons. The presence of the fullerene cage enhances the affinity of these complexes for carbon nanostructures, such as single‐, double‐ and multiwalled carbon nanotubes and graphitised carbon nanofibres, without detrimental effects on the catalytic activity of the metal centre, as demonstrated in styrene oxidation catalysed by [Cu(L)]. This approach shows promise for applications of salen–C₆₀ complexes in heterogeneous catalysis.     

Assessment of the various ionization methods in the analysis of metal salen complexes by mass spectrometry

Metal salen complexes are one of the most frequently used catalysts in enantioselective organic synthesis. In the present work, we compare a series of ionization methods that can be used for the mass spectral analysis of two types of metalosalens: ionic complexes (abbreviated as Com⁺X^?) and neutral complexes (NCom). These methods include electron ionization and field desorption (FD) which can be applied to pure samples and atmospheric pressure ionization techniques: electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI) and atmospheric pressure photoionization (APPI) which are suitable for solutions. We found that FD is a method of choice for recording molecular ions of the complexes containing even loosely bonded ligands. The results obtained using atmospheric pressure ionization methods show that the results depend mainly on the structure of metal salen complex and the ionization method. In ESI spectra, Com⁺ ions were observed, while in APCI and APPI spectra both Com⁺ and [Com + H]⁺ ions are observed in the ratio depending on the structure of the metal salen complex and the solvent used in the analysis. For complexes with tetrafluoroborate counterion, an elimination of BF₃ took place, and ions corresponding to complexes with fluoride counterion were observed. Experiments comparing the relative sensitivity of ESI, APCI and APPI (with and without a dopant) methods showed that for the majority of the studied complexes ESI is the most sensitive one; however, the sensitivity of APCI is usually less than two times lower and for some compounds is even higher than the sensitivity of ESI. Both methods show very high linearity of the calibration curve in a range of about 3 orders of magnitude of the sample concentration. Copyright © 2014 John Wiley & Sons, Ltd.