Enantioselective Ring Research Articles

Enantioselective ring opening of epoxides with TMSN3 by macrocyclic oligomer-supported Cr(III)–salen catalysts

Three Cr(III)–salen catalysts supported onto macrocyclic oligomeric cyclooctene through linkers of different lengths were synthesized. The influences of the linker length and the oligomeric support on the activity and the enantioselectivity of catalysts were investigated in the asymmetric ring opening of a meso-epoxide and the kinetic resolution of terminal epoxides. For both reactions all three catalysts exhibited significantly higher catalytic efficiency than the unsupported monomeric Cr(III)–salen catalyst. The catalyst with the shortest linker 1 showed the highest reaction rate while the catalyst with the longest linker 3 afforded the highest enantioselectivity in both reactions. The recoverability of catalyst 1 was also studied.

Regio- and enantioselective friedel-crafts reactions of indoles to epoxides catalyzed by graphene oxide: a green approach.

Graphene oxide efficiently promotes high regio- and enantioselective ring opening reactions of aromatic epoxides by indoles addition, in solvent- and metal-free conditions. The Friedel-Crafts products were obtained with enantioselectivity up to 99 % ee. The complete inversion of stereochemistry indicates the occurrence of SN 2-type reaction, which assures high level of enantioselectivity.

ChemInform Abstract: Bimetallic Catalysis in the Highly Enantioselective Ring—Opening Reactions of Aziridines.

AbstractThe applied catalyst is readily prepared from commercially available yttrium isopropoxides.

Conformationally Rigid Chiral Bicyclic Skeleton‐Tethered Bipyridine N,N′‐Dioxide as Organocatalyst: Asymmetric Ring Opening of meso‐Epoxides

AbstractThe conformationally rigid chiral bicyclic skeleton tethered bipyridine N,N′‐dioxide (−)‐9 has been designed, synthesized and examined as an organocatalyst in the enantioselective ring opening of meso‐epoxides using tetrachlorosilane (SiCl4). The catalyst (−)‐9 is found to exhibit good enantioselectivity for substituted cis‐stilbene epoxides; whereas, the saturated cyclic meso‐epoxides display a moderate enantioselectivity. At −30 °C in chloroform, the catalyst (−)‐9 with 0.5 mol % loading generated the chlorohydrins in up to 97 % yield with up to 93 % ee. The possible creation of transient axial chiral environment around hypervalent silicon species due to the presence of conformationally rigid chiral bicyclic skeleton tethered bipyridine N,N′‐dioxide may be responsible for such enantioselectivity observed in the desymmetrization of meso‐epoxides.magnified image

Bimetallic catalysis in the highly enantioselective ring–opening reactions of aziridines

Structural and kinetic evidence for bimetallic mechanism.

ChemInform Abstract: Enantioselective Ring Opening of meso‐Epoxides with Aromatic Amines Catalyzed by Dinuclear Magnesium Complexes.

The dinuclear magnesium complexes generated in situ from the reaction of chiral multidentate semi-azacrown ether ligands with n-Bu2Mg were found to be efficient catalysts for enantioselective ring-opening of meso-epox- ides with aniline derivatives, affording the corresponding chiral β-amino alcohols in good yields with up to 90% ee.

Enantioselective fluoride ring opening of aziridines enabled by cooperative Lewis acid catalysis

The enantioselective ring opening of aziridines using a latent source of HF is described. A combination of two Lewis acids, (salen)Co and an achiral Ti(IV) cocatalyst, provided optimal reactivity and enantioselectivity for the trans β-fluoroamine product. The use of a chelating aziridine protecting group was crucial. Acyclic and cyclic meso N-picolinamide aziridines underwent fluoride ring opening in up to 84% ee, and the kinetic resolution of a piperidine-derived aziridine was performed with krel=6.6. The picolinamide group may be readily removed without epimerization of the fluoroamine. Preliminary studies revealed a bimetallic mechanism wherein the chiral (salen)Co catalyst delivers the nucleophile and the Ti(IV) cocatalyst activates the aziridine.

Enantioselective Ring Opening of meso‐Epoxides with Aromatic Amines Catalyzed by Dinuclear Magnesium Complexes

AbstractThe dinuclear magnesium complexes generated in situ from the reaction of chiral multidentate semi‐azacrown ether ligands with n‐Bu2Mg were found to be efficient catalysts for enantioselective ring‐opening of meso‐epoxides with aniline derivatives, affording the corresponding chiral β‐amino alcohols in good yields with up to 90% ee.

ChemInform Abstract: Access to Optically Active 3‐Azido‐ and 3‐Aminopiperidine Derivatives by Enantioselective Ring Expansion of Prolinols.

AbstractThe degree of the ring expansion process is dependent on the nature of the substituents and their relative stereoselectivity.

Mechanistic Investigations of Cooperative Catalysis in the Enantioselective Fluorination of Epoxides

This report describes mechanistic studies of the (salen)Co- and amine-cocatalyzed enantioselective ring opening of epoxides by fluoride. The kinetics of the reaction, as determined by in situ (19)F NMR analysis, are characterized by apparent first-order dependence on (salen)Co. Substituent effects, nonlinear effects, and reactivity with a linked (salen)Co catalyst provide evidence for a rate-limiting, bimetallic ring-opening step. To account for these divergent data, we propose a mechanism wherein the active nucleophilic fluorine species is a cobalt fluoride that forms a resting-state dimer. Axial ligation of the amine cocatalyst to (salen)Co facilitates dimer dissociation and is the origin of the observed cooperativity. On the basis of these studies, we show that significant improvements in the rates, turnover numbers, and substrate scope of the fluoride ring-opening reactions can be realized through the use of a linked salen framework. Application of this catalyst system to a rapid (5 min) fluorination to generate the unlabeled analog of a known PET tracer, F-MISO, is reported.

Access to Optically Active 3-Azido- and 3-Aminopiperidine Derivatives by Enantioselective Ring Expansion of Prolinols

The activation of N-alkyl prolinols by XtalFluor E allowed the formation of an aziridinium intermediate that can react with tetrabutylammonium azide (nBu(4)NN(3)) to produce 3-azidopiperidines and/or 2-(azidomethyl)pyrrolidines, in a ratio up to 100/0. These 3-azidopiperidines can be reduced to the corresponding 3-aminopiperidines.

ChemInform Abstract: Enantioselective Ring Opening Reaction of meso‐Epoxides with Aromatic and Aliphatic Amines Catalyzed by Magnesium Complexes of BINOL Derivatives.

AbstractThe reaction with anilines is catalyzed by the (R)‐BINOL/Bu2Mg/MeLi system and tolerates a wide range of functional groups [cf.

ChemInform Abstract: Catalytic Enantioselective Ring‐Opening Reaction of meso‐Epoxides with ArSeH Using a (Salen)Ti(IV) Complex.

AbstractOptimum reaction conditions are examined for the enantioselective ring opening of epoxides with arylselenols using a (salen)Ti(IV) complex.

Iridium-Catalyzed Enantioselective Ring Opening of Azanorbornene Derivatives

Iridium-Catalyzed Enantioselective Ring Opening of Azanorbornene Derivatives

Enantioselective epoxide ring opening catalyzed by bis(tetrahydroisoquinoline) N,N′-dioxides

Four bis(tetrahydroisoquinoline) N,N′-dioxides were used as catalysts for the epoxide ring opening with tetrachlorosilane under various conditions. A strong solvent effect on asymmetric induction was observed for each of the used catalysts. The highest achieved asymmetric induction for the opening of meso-stilbene oxide was 69% ee. Regarding the cycloalkene oxides, 56% ee was obtained in the reaction with cyclooctene oxide.

Enantioselective desymmetrization of meso-epoxides with anilines catalyzed by polymeric and monomeric Ti(IV) salen complexes

The active catalysts for the enantioselective ring opening (ARO) of meso-stilbene oxide, cis-butene oxide, cyclohexene oxide, cyclopentene oxide, and cyclooctene oxide with various substituted anilines were generated in situ by the reaction of Ti(O(i)Pr)(4) with poly-[(R,R)-N,N'-bis-{3-(1,1-dimethylethyl)-5-methylene salicylidene} cyclohexane-1,2-diamine]-1 and (1R,2R)-N,N'-bis[3,5-di(tert-butyl)salicylidene] cyclohexane-1,2-diamine-2. These catalysts in the presence of nonracemic imine as an additive provided β-amino alcohol in excellent yield (99%) and chiral purity (enantiomeric excess (ee) up to 99%) for the ARO of meso-stilbene oxide with aniline. The same protocol was less effective for the ARO of cyclic epoxides; however, when triphenylphosphine was used as an additive, there was a significant improvement in catalyst performance for the ARO of cyclohexene oxide (yield, 85-90%; ee, 63-67%). Both in situ generated polymeric and monomeric catalysts performed in a similar manner except that the polymeric catalyst Ti(IV)-1 was more active and recycled several times with retention of enantioselectivity when compared with the monomeric catalyst Ti(IV)-2, which was nonrecyclable.

Enantioselective Ring Opening Reaction of meso‐Epoxides with Aromatic and Aliphatic Amines Catalyzed by Magnesium Complexes of BINOL Derivatives

AbstractCatalyzed by the Mg complexes of BINOL derivatives, the enantioselective ring opening reaction of various meso‐epoxides proceeded smoothly with either aromatic or aliphatic amines as the nucleophiles to afford the corresponding chiral β‐amino alcohols in moderate‐to‐high yields with good to excellent enantioselectivities.

ChemInform Abstract: Novel Concepts in Directed Biaryl Synthesis. Part 12. First Atropo- Enantioselective Ring Opening of Achiral Biaryls Containing Lactone Bridges with Chiral Hydride-Transfer Reagent Derived from Borane.

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.

ChemInform Abstract: Novel Concepts in Directed Biaryl Synthesis. Part 10. The Atropo- Enantioselective Ring Opening of Achiral Lactone-Bridged Biaryls Using Chirally Modified Aluminum Hydrides.

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.

ChemInform Abstract: Enantioselective Ring Opening of Epoxides by Fluoride Anion Promoted by a Cooperative Dual‐Catalyst System.

AbstractChemInform 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.