Meso Epoxides Research Articles

Mixing and matching chiral cobalt- and manganese-based calix-salen catalysts for the asymmetric hydrolytic ring opening of epoxides

Homochiral oligomeric salen macrocycles possessing aromatic spacers have been prepared as new calix-salen derivatives. The corresponding cobalt and manganese complexes were synthesized and characterized, and their catalytic activities have been studied in the challenging hydrolysis of meso epoxides. While manganese calix-salen complexes were not active in the studied reactions, the dual heterobimetallic system, using an equimolar combination of cobalt and manganese calix-salen derivatives proved to be more enantioselective than the sole cobalt system. Furthermore, as heterogeneous complexes, the catalytic mixture could be easily recovered by simple filtration and successfully reengaged in subsequent catalytic runs. Interestingly, no need for cobalt reactivation was noticed to maintain maximum efficiency of this dual system. The matched Co/Mn dual catalyst was also used to promote the dynamic hydrolytic kinetic resolution of epibromohydrin.

ChemInform Abstract: Enantioselective Bio‐Hydrolysis of Various Racemic and meso Aromatic Epoxides Using the Recombinant Epoxide Hydrolase Kau2.

Epoxide hydrolase Kau2 overexpressed in Escherichia coli RE3 has been tested with ten different racemic and meso,-disubstituted aromatic epoxides. Some of the tested substrates were bi-functional, and most of them are very useful building blocks in synthetic chemistry applications. As a general trend Kau2 proved to be an extremely enantioselective biocatalyst, the diol products and remaining epoxides of the bioconversions being obtained – with two exceptions – in nearly enantiomerically pure form. Furthermore, the reaction times were usually very short (around 1 h, except when stilbene oxides were used), and the use of organic co-solvent was well tolerated, enabling very high substrate concentrations (up to 75 g/L) to be reached. Even extremely sterically demanding epoxides such as cis-and trans-stilbene oxides were transformed on a reasonable time scale. All reactions were successfully conducted on a 1-g preparative scale, generating diol-and epoxide-based chiral synthons with very high enantiomeric excesses and isolated yields close to the theoretical maximum. Thus we have here demonstrated the usefulness and versatility of lyophilizedEscherichia coli cells expressing Kau2 epoxide hydrolase as a highly enantioselective biocatalyst for accessing very valuable optically pure aromatic epoxides and diols through kinetic resolution of racemates or desymmetrization of meso epoxides.

ChemInform Abstract: Unravelling a New Class of Chiral Organocatalyst for Asymmetric Ring‐Opening Reaction of meso Epoxides with Anilines.

AbstractThe chiral sulfinamide based organocatalyst (I) is synthesized and applied to the asymmetric ring‐opening reaction of meso epoxides with anilines.

Solvent-free aminolysis of aliphatic and aryloxy epoxides with sulfated zirconia as solid acid catalyst

Single-step and two-steps synthetic procedure for the synthesis of sulfated zirconia (SZ) was developed, which were calcined at 500, 600 and 700°C and characterized by various physico-chemical methods such as PXRD, FTIR, surface area, microanalysis, NH3-TPD and DRIFT analysis. These SZ materials were then employed as solid acid catalysts for the aminolysis of different aliphatic/aromatic terminal, aryloxy and meso epoxides with aromatic and aliphatic amines under ambient conditions. Amongst the catalyst prepared, SZ-2-600 prepared in two-steps and 600°C calcined was found to be the most efficient catalyst to give β-amino alcohols in up to 98% yield and >99% regioselectivity. The SZ catalyst was successfully recycled and reused up to six catalytic runs with intact efficiency.

Copolymerization of CO2and meso epoxides using enantioselective β-diiminate catalysts: a route to highly isotactic polycarbonates

We report a new class of catalysts for the enantioselective copolymerization of CO2 and alicyclic meso epoxides. C1-Symmetric β-diiminate zinc catalysts were designed on the basis of mechanistic information and subsequently optimized using structure–activity relationships and iterative ligand design. The optimized catalysts yield highly isotactic poly(cyclohexene carbonate) and poly(cyclopentene carbonate) with units of up to 94% ee under mild conditions. A diblock copolymer of CO2/cyclohexene oxide and CO2/cyclopentene oxide was synthesized, demonstrating the controlled behaviour of the catalyst. Solid-state structures and solution-state dynamics of the catalyst were studied to explain trends in enantioselectivities and turnover frequencies. A strong correlation between the isotacticity of the polymer and its melting temperature was observed. X-ray powder diffraction was used to measure crystallinity and study the changes in morphology observed upon annealing.

Unravelling a new class of chiral organocatalyst for asymmetric ring-opening reaction of meso epoxides with anilines.

Chiral sulfinamide based organocatalyst 11 was synthesized from readily available starting materials and used for the asymmetric ring-opening (ARO) reaction of meso epoxides with anilines. A high yield (up to 95%) of chiral β-amino alcohols with excellent enantioselectivity (ee up to 99%) was achieved in 24-30 h at rt under optimized reaction conditions. A probable mechanism for the catalytic ARO reaction is envisaged by (1)H and (13)C NMR experiments.

A broadly applicable and practical oligomeric (salen)Co catalyst for enantioselective epoxide ring-opening reactions

The (salen)Co catalyst (4a) can be prepared as a mixture of cyclic oligomers in a short, chromatography-free synthesis from inexpensive, commercially available precursors. This catalyst displays remarkable enhancements in reactivity and enantioselectivity relative to monomeric and other multimeric (salen)Co catalysts in a wide variety of enantioselective epoxide ring-opening reactions. The application of catalyst 4a is illustrated in the kinetic resolution of terminal epoxides by nucleophilic ring-opening with water, phenols, and primary alcohols; the desymmetrization of meso epoxides by addition of water and carbamates; and the desymmetrization of oxetanes by intramolecular ring opening with alcohols and phenols. The favorable solubility properties of complex 4a under the catalytic conditions facilitated mechanistic studies, allowing elucidation of the basis for the beneficial effect of oligomerization. Finally, a catalyst selection guide is provided to delineate the specific advantages of oligomeric catalyst 4a relative to (salen)Co monomer 1 for each reaction class.

Fe(OH)3 nano solid material: An efficient catalyst for regioselective ring opening of aryloxy epoxide with amines under solvent free condition

Iron hydroxide-Fe(OH)3 and iron oxides (Fe3O4 and Fe2O3) were successfully prepared and characterized. These materials were employed as efficient and environmentally benign heterogeneous catalysts for the epoxide ring opening reaction of various aryloxy, terminal and meso epoxides with aromatic and aliphatic amines under solvent free condition at room temperature. Out of these, nano-sized Fe(OH)3 (IH-1) showed better catalytic activity to give the product β-amino alcohols in excellent yield (up to ∼96%) and high regioselectivity in 10–360min. The catalyst was successfully recycled and reused eight times with no loss in catalytic activity.

ChemInform Abstract: Transition Metal Catalysis and Nucleophilic Fluorination

AbstractReview: 82 refs.

Transition metal catalysis and nucleophilic fluorination

Transition metal catalyzed transformations using fluorinating reagents have been developed extensively for the preparation of synthetically valuable fluorinated targets. This is a topic of critical importance to facilitate laboratory and industrial chemical synthesis of fluorine containing pharmaceuticals and agrochemicals. Translation to (18)F-radiochemistry is also emerging as a vibrant research field because functional imaging based on Positron Emission Tomography (PET) is increasingly used for both diagnosis and pharmaceutical development. This review summarizes how fluoride sources have been used for the catalytic nucleophilic fluorination of various substrates inclusive of aryl triflates, alkynes, allylic halides, allylic esters, allylic trichloroacetimidates, benzylic halides, tertiary alkyl halides and epoxides. Until recently, progress in this field of research has been slow in part because of the challenges associated with the dual reactivity profile of fluoride (nucleophile or base). Despite these difficulties, some remarkable breakthroughs have emerged. This includes the demonstration that Pd(0)/Pd(II)-catalyzed nucleophilic fluorination to access fluoroarenes from aryl triflates is feasible, and the first examples of Tsuji-Trost allylic alkylation with fluoride using either allyl chlorides or allyl precursors bearing O-leaving groups. More recently, allylic fluorides were also made accessible under iridium catalysis. Another reaction, which has been greatly improved based on careful mechanistic work, is the catalytic asymmetric hydrofluorination of meso epoxides. Notably, each individual transition metal catalyzed nucleophilic fluorination reported to date employs a different F-reagent, an observation indicating that this area of research will benefit from a larger pool of nucleophilic fluoride sources. In this context, a striking recent development is the successful design, synthesis and applications of a fluoride-derived electrophilic late stage fluorination reagent. This new class of reagents could greatly benefit preclinical and clinical PET imaging.

ChemInform Abstract: Enantioselective Nucleophilic Opening of meso Epoxides by Organolithium Reagents.

Aryl lithium reagents, complexed with (-)-sparteine, react enantioselectively with cyclic meso epoxides, to afford chiral aryl cyclanols. The enantiomeric excess, though moderate (27-87%), is the best in the literature for such a reaction. Activation by BF3 · OEt2 is needed, and is compatible with a diamine such as (-)-sparteine.

ChemInform Abstract: Asymmetric Ring Opening of Meso Epoxides with TMSCN Catalyzed by (Pybox)lanthanoid Complexes.

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Helical Chiral Pyridine N‐Oxides: A New Family of Asymmetric Catalysts

Helical Chiral Catalysts: The gram-scale synthesis of 1-azahelicenes, structural characterization of the corresponding N-oxides, and the application of this new family of catalysts to the catalytic, enantioselective ring-opening of meso epoxides is described (see scheme). Included is the first demonstration that the appropriate structural modifications beneath the plane of the pyridine N-oxide can serve as a powerful means for tuning the catalyst enantioselectivity. Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Enantioselective Desymmetrization of meso Epoxides with Anilines Catalyzed by a Niobium Complex of a Chiral Multidentate Binol Derivative.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

Enantioselective Desymmetrization of meso Epoxides with Anilines Catalyzed by a Niobium Complex of a Chiral Multidentate Binol Derivative

Molecular recognition in the desymmetrization of meso epoxides with anilines is displayed by a Lewis acid catalyst formed from niobium(V) methoxide and a novel tetradentate binol derivative. The catalyst has a remarkable ability to distinguish between different meso epoxides (see scheme) and also efficiently mediates the ring opening of unsymmetrically disubstituted epoxides with high chemo- and stereoselectivity.

(Salen)Ti(IV) complex catalyzed asymmetric ring-opening of epoxides using dithiophosphorus acid as the nucleophile

A series of chiral bis-Schiff bases were synthesized starting from (1 R,2 R)-(+)-diaminocyclohexane, (+)- cis-1,2,2-trimethyl-1,3-diaminocyclopentane, ( R)-2,2 ′-diamino-1,1 ′-binaphthalene, and (1 S,2 S)-diphenyl-1,2-ethanediamine. The enantioselective ring-opening of meso epoxides with dithiophosphorus acids catalyzed by a (salen)Ti(IV) complex formed in situ upon the treatment of Ti(OPr- i) 4 and the aforementioned chiral Schiff base was realized. The resulting products were obtained with low to good enantioselectivities (up to 73% ee). The (salen)Ti(IV) complex containing the backbone of 1,2-diaminocyclohexane exhibited the best enantioselectivity. The substituents in dithiophosphorus acids and those on the salen aromatic ring have a significant influence on the reaction. Moderate enantioselectivity were obtained for the (salen)Ti(IV) complex catalyzed ring-opening of racemic monosubstituted epoxides. High regioselectivity was observed for the alkyl substituted epoxides, whereas poor regioselectivity was obtained for the aryl substituted ones.

Synthetic and Mechanistic Studies of Strained Heterocycle Opening Reactions Mediated by Zirconium(IV) Imido Complexes

The reactions of the bis(cyclopentadienyl)(tert-butylimido)zirconium complex (Cp(2)Zr=N-t-Bu)(THF) (1) with epoxides, aziridines, and episulfides were investigated. Heterocycles without accessible beta-hydrogens undergo insertion/protonation of the C-X bond to produce 1,2-amino alcohols (X = O) and 1,2-diamines (X = NR), whereas heterocycles with accessible beta-hydrogens undergo elimination/protonation to produce allylic alcohols (X = O) and allylic sulfides (X = S). Mechanistic investigations support a stepwise pathway with zwitterionic intermediates for the first reaction class and a concerted pathway for the second reaction class. Additionally, the feasibility of chirality transfer from the planar-chiral ebthi (ebthi = ethylenebis(tetrahydroindenyl)) ligand was demonstrated with a chiral analogue, (ebthi)-Zr=NAr(THF) (Ar = 2,6-dimethylphenyl), 2, through the diastereoselective ring opening of meso epoxides.

The First Example of Enantioselective Isocyanosilylation of meso Epoxides with TMSCN Catalyzed by Novel Chiral Organogallium and Indium Complexes.

AbstractFor Abstract see ChemInform Abstract in Full Text.

The first example of enantioselective isocyanosilylation of meso epoxides with TMSCN catalyzed by novel chiral organogallium and indium complexesElectronic supplementary information (ESI) available: Characterization data, chiral analysis and determination of absolute configuration. See http://www.rsc.org/suppdata/cc/b2/b212511k/

The desymmetrization ring opening of meso epoxides using trimethylsilyl cyanide catalyzed by organogallium and indium complexes with binaphthol monoether derivatives as chiral ligands gave beta-isocyanohydrins with moderate to excellent enantioselectivities of up to 95% ee.

Reagent-controlled stereoselectivity in titanocene-catalyzed epoxide openings: reductions and intermolecular additions to alpha,beta-unsaturated carbonyl compounds.

The generation and addition reactions of metal bound radicals derived from normal and meso epoxides by electron transfer from titanocene(III) reagents is described. The control of enantioselectivity and diastereoselectivity of these transformations is investigated by variation of the ligands of the metal complex. The reaction can lead to unprecedented and highly selective reactions, in which synthetically useful alcohols may be prepared. The synthesis presented also circumvents the use of toxic metals. Another advantage is that there is no loss of two functional groups as usually observed in reductive radical chain reactions.