Schiff Base Catalyst Research Articles

Enantioselective Intramolecular Cyclopropanation of cis‐Alkenes by Chiral Ruthenium(II) Schiff Base Catalysts and Crystal Structures of (Schiff Base)Ruthenium Complexes Containing Carbene, PPh3, and CO Ligands.

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Aluminum Schiff base catalysts derived from β‐diketone for the stereoselective polymerization of racemic lactides

Aluminum Schiff base catalysts derived from β‐diketone for the stereoselective polymerization of racemic lactides

Stereochemical Influence of the Ligand on the Structure of Manganese Complexes: Implications for Catalytic Epoxidations

Manganese complexes of the ligand HphoxCOOR (R=H or Me) have been synthesized and characterized by X-ray analysis, ESI-MS, ligand-field spectroscopy, electrochemistry, and paramagnetic 1H NMR. The ligands, chirally pure or racemic, influence the structures of the complexes formed. Manganese(III) complexes of the ligand HphoxCOOMe are square-pyramidal or octahedral with two ligands bound in a trans fashion in the solid state. The racemic ligand (RS-HphoxCOOMe) as well as the enantiopure ligand (R-HphoxCOOMe) forms manganese complexes with similar solid-state structures. Ligand-exchange reactions occur in solution giving rise to meso complexes as confirmed by ESI-MS and deuteration studies. The manganese(III) complex of R-HphoxCOOH is octahedral, with two dianionic ligands bound in a fac-cct fashion in a tridentate manner. The manganese(III) complex of RS-HphoxCOOH is also octahedral with two dianionic ligands now bound in a trans fashion in a didentate manner and with two water molecules occupying axial sites. The paramagnetic 1H NMR spectra of the complexes have been interpreted on the basis of the relaxation times with the help of the inversion-recovery pulse technique. The binding of imidazole with the metal center depends on the chirality of the ligands in the metal complexes of HphoxCOOMe. Imidazole coordination was found to occur with the metal complex that contains two ligands with the same chirality (R and R) (R-1), while no imidazole coordination was found upon reaction with the metal complex that contains two ligands with opposite chirality (R and S) (RS-1). Epoxidation reactions of various alkenes with H2O2 as the oxidant reveal that the complexes give turnover numbers in the range of 10-35, the epoxide being the major product. The catalytic activity depends on the additives used, and a clear base effect is observed. The turnover numbers have been found to be higher in the complexes where no binding of N-Meim is observed. The latter fact unambiguously shows that imidazole binding is not a prerequisite for higher turnover numbers, in contrast to the Mn-Schiff base catalysts.

A Catalytic Enantioselective Hetero Diels−Alder Approach to the C20−C32 Segment of the Phorboxazoles

[reaction: see text] An efficient synthesis of the C20-C32 segment of the phorboxazoles has been achieved using an enantioselective hetero Diels-Alder reaction catalyzed by Jacobsen's Cr(III) amino indanol Schiff base catalyst.

Autoxidation of 2,6-di-tert-butylphenol with cobalt Schiff base catalysts by oxygen in CO2-expanded liquids

CO2-expanded acetonitrile and methylene chloride have been used in this first detailed study of catalytic O2 oxidations in these remarkably effective reaction media. The autoxidation of 2,6-di-tert-butylphenol (DTBP) with the cobalt Schiff-base (Co(salen*) in these so-called CO2-expanded liquids (CXLs) has been extensively studied using precisely controlled and monitored batch reactions. The dependence of conversion, selectivity and turn-over-frequency on various reaction parameters, including temperature, [O2], [catalyst], and solvent composition has been evaluated. The rates of O2-oxidation in CXLs are typically 1–2 orders of magnitude greater than those obtained with either the neat organic solvent or supercritical CO2 as reaction media. In keeping with the proposed mechanism, the dependence of both the selectivity and conversion on O2 concentration and catalyst concentration indicates that the O2 adduct, and not free O2, serves as oxidant in two critical steps in these systems. The increase in conversion with increasing temperature supports formation of the phenoxy radical as the rate determining step. In contrast, the temperature independence of selectivity is as expected for two competing radical coupling reactions. The balance between O2 solubility and mixed-solvent dielectric constant determines some of the benefits of the CXLs. Because of the greatly increased solubility of O2 in CXLs, the conversion in those media is substantially greater than that in either scCO2 or the neat organic solvent. However, conversion eventually decreases with increasing CO2 content of the solvent because of the decreasing dielectric constant of the medium. The solubilities of O2 and Co(salen*) have been determined in CXLs based on methylene chloride.

Structure‐Based Analysis and Optimization of a Highly Enantioselective Catalyst for the Strecker Reaction.

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Structure-based analysis and optimization of a highly enantioselective catalyst for the strecker reaction.

A mechanistic investigation of the asymmetric Strecker reaction catalyzed by a metal-free Schiff base catalyst was conducted. The active site of the catalyst, the relevant stereoisomer of the imine substrate, and the solution structure of the imine-catalyst complex were elucidated using a series of kinetics, structure-activity, and NMR experiments. An unusual bridging interaction between the imine and the urea hydrogens of the catalyst was identified and supported by computation. Rational optimization of catalyst structure based on the mechanistic insight led to an improved catalyst for the asymmetric Strecker reaction.

Enantioselective epoxidation of non-functionalised alkenes using a urea–hydrogen peroxide oxidant and a dimeric homochiral Mn(III)-Schiff base complex catalyst

The catalytic enantioselective epoxidation of chromenes, indene and styrene using a urea–hydrogen peroxide adduct as an oxidising agent and the novel dimeric homochiral Mn(III)-Schiff base catalyst 1 has been investigated in the presence of carboxylate salts and nitrogen and oxygen coordinating co-catalysts. Conversions of more than 99% were obtained with all alkenes except styrene. Absolute chiral induction, as determined by 1H NMR using the chiral shift reagent (+)-Eu(hfc) 3, was obtained in the case of nitro- and cyanochromene. The catalyst could be re-used for up to five cycles with some loss of activity due to degradation of the catalyst under epoxidation condition with retention of e.e.'s.

Enantioselective catalytic addition of HCN to ketoimines. Catalytic synthesis of quaternary amino acids.

[formula: see text] Highly enantioselective addition of HCN to ketoimines has been achieved for the first time using readily accessible and recyclable Schiff base catalysts. Essentially quantitative isolated yield and enantioselectivity of up to 95% ee was obtained. Furthermore, some of the Strecker adducts could be recrystallized in high recovery, yielding optically pure materials. Conversion of the alpha-aminonitrile adducts to the corresponding alpha-quaternary alpha-amino acids was effected in high yield by a formylation/hydrolysis sequence.

Schiff Base Catalysts for the Asymmetric Strecker Reaction Identified and Optimized from Parallel Synthetic Libraries

ADVERTISEMENT RETURN TO ISSUEPREVCommunicationSchiff Base Catalysts for the Asymmetric Strecker Reaction Identified and Optimized from Parallel Synthetic LibrariesMatthew S. Sigman and Eric N. JacobsenView Author Information Department of Chemistry and Chemical Biology Harvard University, Cambridge, Massachusetts 02138 Cite this: J. Am. Chem. Soc. 1998, 120, 19, 4901–4902Publication Date (Web):May 2, 1998Publication History Received13 January 1998Published online2 May 1998Published inissue 1 May 1998https://doi.org/10.1021/ja980139yCopyright © 1998 American Chemical SocietyRIGHTS & PERMISSIONSArticle Views16425Altmetric-Citations743LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit Read OnlinePDF (140 KB) Get e-AlertsSupporting Info (1)»Supporting Information Supporting Information SUBJECTS:Catalysts,Ligands,Peptides and proteins,Reaction products,Stereoselectivity Get e-Alerts

Alumina-anchored Cobalt(II) Schiff Base Catalyst for the Isomerization of Trimethyldicyanoquadricyclane to the Norbornadiene

Abstract N,N′-Disalicylidene-4-carboxy-1,2-phenylenediiminatocobalt(II) supported on polyaminesulfone modified alumina beads has been prepared. The immobilized complex revealed efficient catalysis for the conversion of trimethyldicyanoquadricyclane (TMDC-Q) to the corresponding norbornadiene (TMDC-NBD). Using a bench-scale fixed bed reactor, TMDC-Q in chlorobenzene (1 mol dm−3) isomerized to TMDC-NBD quantitatively during the flow through catalyst bed of only 20 cm with the temperature increase from 24.5 °C to 75 °C.