Asymmetric Diels–Alder reactions of N-allenoyloxazolidinones catalyzed by Cu(II)–bis(oxazoline) complexes

Catalytic asymmetric Diels–Alder reactions of N-allenoyloxazolidinones were investigated. Various chiral metal–bis(oxazoline) and metal–pyridinebis(oxazoline) complexes were screened. Cu(SbF₆)₂(H₂O)₂(t-BuBox) was found to be the most effective catalyst, giving the product in high yield, enantioselectivity, and endo:exo selectivity. The relative reactivity between N-allenoyloxazolidinones and N-alkenoyloxazolidinones was also investigated. A model for stereoinduction was proposed to account for the enantioselectivity and endo:exo selectivity.     

Reversal of enantioselection in an aldol reaction catalyzed by sterically congested bis(oxazoline)–Cu(II) complexes

The use of sterically congested C₂-symmetrical bis(oxazoline) ligands with methylene and ethylene spacers between the oxazoline rings results in the reversal of the enantioselection for aldol reactions catalyzed by bis(oxazoline)–Cu(II) complexes.     

Asymmetric hydrocyanation of olefins catalyzed by chiral diphosphite–nickel complexes

Chiral aryl diphosphite ligands derived from binaphthol were found to be effective in the nickel-catalyzed hydrocyanation of a variety of olefins. Enantioselective hydrocyanations of styrene, 4-substituted styrenes and norbornene were achieved with excellent regioselectivity and moderate enantioselectivity. The hydrocyanation of vinyl acetate gave 72.9% ee. The catalytic activity and the enantioselectivity of the Ni(0)–diphosphite complexes were found to be highly dependent on the structures of the ligands.     

The enantioselective diethylzinc addition to imines catalyzed by chiral Cu(II)–oxazoline complexes

A series of copper complexes of chiral bisoxazolines has been applied in the catalytic diethylzinc addition to N-sulfonyl imines. It has been found that the tridentate ligands 3–5 provided higher enantioselectivity than bidentate ones. Addition of 4 Å molecular sieves to the reaction system benefits the enantioselectivity. The optimal procedure for diethylzinc addition to different imines resulted in moderate yields and enantioselectivities of up to 82% ee.     

Asymmetric synthesis. Part 29: Asymmetric hydroformylation of styrene catalyzed by chiral spiro diphosphite–rhodium(I) complexes

Chiral diphosphite ligands L₁–L₃ were prepared by the reaction of (1S,5S,6R)-(cis,trans)-spiro[4.4]nonane-1,6-diol with chlorophosphites. These ligands were tested in the rhodium catalyzed hydroformylation of styrene and enantioselectivities up to 69% were achieved. High regioselectivities (97%) to 2-phenylpropanal and high yields (98%) were obtained under mild reaction conditions. The influence of reaction conditions is also discussed.     

Asymmetric domino Michael–Henry reaction of 1,2-diones with nitroolefins catalyzed by a chiral bisoxazolidine–Ni(acac)2 complex

The asymmetric domino Michael–Henry reaction of 1,2-cyclohexadione with nitroolefins catalyzed by chiral ligand bisoxazolidine 1 and Ni(acac)₂ has been developed. This process provided highly functionalized chiral bicycle[3,2,1] octane derivatives with the generation of four new stereogenic centers in high yields (76–99%), and with excellent enantioselectivities (up to 99%) and good diastereoselectivities (up to 9:1) under mild reaction conditions. The procedure presented is simple and makes this method suitable for practical use.     

Enhanced reactivity and enantioselectivity in catalytic glyoxylate-ene reactions using chiral bis(oxazoline)–copper complex in an ionic liquid

An optimal hydrophobic ionic liquid was discovered as a solvent for highly enantioselective glyoxylate-ene reactions catalyzed by a chiral bis(oxazoline)–copper complex. The reactivity and stereoselectivity were highly dependent upon the property of the ionic liquids; reactions between olefins and ethyl glyoxylate in [Bmim]SbF₆ at ambient temperature provided remarkably enhanced reactivity and stereoselectivity, which greatly exceed those of the corresponding reactions in dichloromethane. Furthermore, the metal–ligand complex was readily recycled up to eight times while exhibiting no significant decrease in reaction efficiency.     

Asymmetric copper-catalyzed Diels–Alder reaction revisited: control of the structure of bis(oxazoline) ligands

Synthesis of 1,4-bis(oxazoline) ligands bearing a bicyclo[2,2,2]backbone derived from 9,10-dihydro-9,10-ethanoanthacene trans-dicarboxylic acid was revisited. Starting from l- or d-amino alcohols and either (S,S) or (R,R)-dihydroethano trans-dicarboxylic acid, a complete series of ligands was evaluated in the copper-catalyzed Diels–Alder reaction. The most efficient ligands with a phenyl substituent on the oxazoline ring afforded enantiomeric excess up to 98%. This is different from previous results indicating that the best enantioselectivity involved a diastereomeric ligand with the meso-backbone.     

Intramolecular C–H insertion catalyzed by dirhodium(II) complexes using CO2 as the reaction media

Abstract

In this work, the intramolecular C–H insertion of diazoacetamides catalyzed by dirhodium(II) complexes and using CO₂ as solvent is disclosed. The expected lactams were obtained in yields over 97%. The asymmetric intramolecular C–H insertion was also achieved and the β-lactam 14 was obtained in >97% yield and 65% ee using the chiral dirhodium(II) catalyst Rh₂(S-PTTL)₄. Finally, the dirhodium(II) complex Rh₂(OAc)₄ was used in two consecutive cycles in which complete conversion to the lactam was observed.     

Enantioselective one-pot three-component synthesis of propargylamines catalyzed by copper(I)–pyridine bis-(oxazoline) complexes

A one-pot three-component coupling of aldehydes and amines in presence of terminal alkynes has been efficiently catalyzed by copper (I) complex of i-Pr-pybox-diPh 2b or s-Bu-pybox-diPh 2c. The process is simple and allows the synthesis of various propargylamines in good to excellent enantioselectivities (up to 99% ee) and in higher yields. The nature of the substituents attached to imines plays a vital role on the enantioselectivities obtained. The presence of gem-diphenyl group at C-5 position and secondary alkyl substituents at the C-4 chiral center of the oxazoline rings of the chiral ligands was found to be crucial for higher enantioselectivities. A transition state model involving π–π stacking is also proposed for the stereochemical outcome.     

Asymmetric cyclopropanation catalyzed by copper–Schiff’s base complexes

The asymmetric cyclopropanation of styrene with alkyl diazoacetate were performed with copper complexes of Schiff bases, derived from substituted salicylaldehydes and a new chiral amino alcohol, as the catalysts. The electronic and steric properties, as well as the position of those substituents show obvious effects on the enantioselectivities, and ee higher than 98% were achieved under optimal conditions.     

Asymmetric intramolecular C–H insertion of sulfonyldiazoacetates catalyzed by Rh(II)

The asymmetric C–H insertion of alkyldiazosulfones has been studied. High selectivity was achieved using a combination of a chiral catalyst and a chiral auxiliary (Rh₂(S-pttl)₄ and menthyl ester).     

Enantioselective Morita–Baylis–Hillman reaction catalyzed by a chiral phosphine oxide

An application of a hypervalent silicon complex, generated from a chiral phosphine oxide catalyst and silicon tetrachloride, to the enantioselective organocatalytic Morita–Baylis–Hillman reaction is described. A chloride anion liberated from the hypervalent silicon complex smoothly generated a γ-chloro silyl enol ether that subsequently reacted with an aldehyde to afford the Baylis–Hillman adducts in good yields and with good enantioselectivities.     

Asymmetric conjugate addition of α-keto esters to nitroolefins catalyzed by chiral CuII hydroxo complexes

As a part of our research project on hard anion-late transition metal complexes as mild acid–base catalysts, we describe herein that Cu^II hydroxo complexes having chiral N-substituted-diaminocyclohexanes are mild and selective catalysts, which are applicable to the catalytic asymmetric conjugate addition of α-keto esters to nitroolefins. The reaction proceeded diastereoselectively without the detectable formation of self-aldol products, affording the corresponding coupling products with anti-stereochemistry in an enantioselective manner.     

Asymmetric Morita-Baylis-Hillman reactions catalyzed by chiral Brønsted acids

Chiral BINOL-derived Br?nsted acids catalyze the enantioselective asymmetric Morita-Baylis-Hillman (MBH) reaction of cyclohexenone with aldehydes. The asymmetric MBH reaction requires 2-20 mol % of the chiral Br?nsted acid 2e or 2f and triethylphosphine as the nucleophilic promoter. The reaction products are obtained in good yields (39-88%) and high enantioselectivities (67-96% ee). The Br?nsted-acid-catalyzed reaction is the first example of a highly enantioselective asymmetric MBH reaction of cyclohexenone with aldehydes.