Enantioselective copper-catalyzed cyclopropanation of styrene by means of chiral bispidine ligands

Enantiopure C₁- and C₂-symmetric bispidine ligands have been synthesized and screened in the asymmetric copper-catalyzed cyclopropanation of styrene. In order to improve the enantiomeric excesses (ee) of the cyclopropane derivatives, the best performing C₁-symmetric diamine ligand was selected for studies on the reaction conditions. The optimized procedure allowed us to obtain up to 91% ee for the cis-cyclopropane derivative and up to 79% ee for the trans one.     

A P-chiral bisdiamidophosphite ligand with a 1,4:3,6-dianhydro-d-mannite backbone and its application in asymmetric catalysis

A novel readily available P,P-bidentate diamidophosphite ligand with P^∗-stereocentres is prepared from an inexpensive C₂-symmetric 1,4:3,6-dianhydro-d-mannite. By using this efficient ligand, up to 98% ee is achieved in Pd-catalysed asymmetric allylic alkylation, up to 92% ee in Pd-catalysed asymmetric allylic amination and up to 87% ee in Rh-catalysed asymmetric hydrogenation. The influence of the precatalyst, substrate and solvent on the enantioselectivity is discussed.     

BINAM-prolinamides as recoverable catalysts in the direct aldol condensation

New BINAM-prolinamides were developed and tested as organocatalysts in the direct aldol condensation between aldehydes and several aliphatic ketones. C₂-symmetrical (Sa)-BINAM-l-prolinamide gives the best enantioselectivities for this transformation, being recovered and reused after the reaction by simple extractive techniques. The reaction was performed in DMF/H₂O at 0 °C to give the aldol products in up to 95% ee for acetone. For 2-butanone, the corresponding iso-regioisomers were regioselectively obtained in up to 96% ee working in DMF at rt. In the case of cyclohexanone, dr up to 10:1 in favour of the anti products, which were obtained in 90–93% ee, was achieved.     

Metal/linked-BINOL complexes: Applications in direct catalytic asymmetric Mannich-type reactions

Development of metal/linked-BINOL complexes and their applications in direct catalytic asymmetric Mannich-type reactions of hydroxyketones are reviewed. A Et₂Zn/linked-BINOL complex was effective for diastereo- and enantioselective synthesis of β-amino alcohols. By choosing the proper protective groups on the imine nitrogen, either anti- or syn-β-amino alcohol was obtained in excellent enantioselectivity (up to >99.5% ee) using the same zinc catalysis. Y{N(SiMe₃)₂}₃/linked-BINOL complex was effective for various hydroxyketones, affording syn-β-amino alcohols with high enantioselectivity (up to 98% ee). To broaden the nucleophile scope to carboxylic acid derivatives, N-acylpyrrole was utilized as an ester equivalent donor. In(O-iPr)₃/linked-BINOL complex was effective for generating an In-enolate from N-acylpyrrole in situ, giving Mannich adducts with high enantioselectivity (up to 98% ee).     

Copper-catalyzed enantioselective 1,4-conjugate addition of dialkylzinc reagents to α,β- and α,β,γ,δ-unsaturated ketones

An enantioselective Cu(II)-catalyzed conjugate addition of dialkylzinc reagents to α,β- or α,β,γ,δ-unsaturated ketones with chiral cyclohexane-based amidophosphine ligands was developed. With 2 mol% of Cu(OAc)₂·H₂O/L5, the conjugate addition of diethylzinc to α,β-unsaturated ketones was achieved in good-to-excellent yields (up to 98%) and high enantioselectivities (up to 92% ee). This catalytic system was shown to be efficient for the 1,4-conjugate addition of Et₂Zn to (2E,4E)-1,5-diphenylpenta-2,4-dien-1-one with 85% yield and 90% ee. Moreover, with 1 mol% of Cu(OTf)₂/L11, the conjugate addition of α,β,γ,δ-unsaturated ketones was accomplished with 1,4-regioselectivity, good yields (79–86%) and excellent enantioselectivities (up to 97% ee).     

Palladium-catalyzed asymmetric allylic nucleophilic substitution reactions using chiral tert-butanesulfinylphosphine ligands

The asymmetric allylic alkylation of racemic 1,3-diphenyl-2-propenyl acetate 3 with dimethyl malonate proceeded smoothly in the presence of lithium acetate, BSA (N,O-bis(trimethylsilyl)acetamide), [Pd(η³-C₃H₅)Cl]₂, and chiral tert-butanesulfinylphosphine ligand 2c to give the allylic alkylation product in good yield and high enantiomeric excess (up to 93% ee), while the enantioselectivities of allylic amination of 3 with various amines were moderate (up to 76% ee).     

Carbohydrates as chiral auxiliaries in enantioselective synthesis of four stereoisomers of optically active α,γ-substituted γ-butyrolactones

Using several easily accessed and inexpensive chiral auxiliaries derived from carbohydrates, all four stereoisomers of the optically active α,γ-substituted γ-butyrolactones were obtained respectively in high enantiomeric purities (up to 96% ee for trans and up to >99% ee for cis) by the SmI₂-induced reductive coupling reaction in the presence of a proton source.     

Catalytic asymmetric hydrogenation of indoles using a rhodium complex with a chiral bisphosphine ligand PhTRAP

Highly enantioselective hydrogenation of N-protected indoles was successfully developed by use of the rhodium catalyst generated in situ from [Rh(nbd)₂]SbF₆ and the chiral bisphosphine PhTRAP, which can form a trans-chelate complex with a transition metal atom. The PhTRAP–rhodium catalyst required a base (e.g., Cs₂CO₃) for the achievement of high enantioselectivity. Various 2-substituted N-acetylindoles were converted into the corresponding chiral indolines with up to 95% ee. The hydrogenations of 3-substituted N-tosylindoles yielded indolines possessing a stereogenic center at the 3-position with high enantiomeric excesses (up to 98% ee).     

Chiral 1,2-diaminocyclohexane as organocatalyst for enantioselective aldol reaction

A simple and commercially available chiral 1,2-diaminocyclohexane as catalyst, hexanedioic acid as co-catalyst could efficiently catalyze the asymmetric aldol reaction in MeOH-H₂O. Cyclic ketones as aldol substrates gave the anti-β-hydroxyketone products with moderate to good yields, diastereoselectivity and enantioselectivity (up to 78% yield, >20:1 anti/syn, 94% ee). Hydroxyacetone as aldol substrate afforded the syn-α, β-dihydroxyketones as major products in up to 85% yield with good enantioselectivity (up to >20:1 syn/anti, 93% ee).     

A new electronically deficient atropisomeric diphosphine ligand (S)-CF3O-BiPhep and its application in asymmetric hydrogenation

A new electronically deficient atropisomeric diphosphine ligand (S)-CF₃O-BiPhep was synthesized from 1-bromo-3-(trifluoromethoxy)benzene in high yield. The key steps included oxidative coupling with anhydrous ferric chloride and optical resolution by (+)-DMTA. The ligand afforded high performance for Ir-catalyzed asymmetric hydrogenation of quinolines with ee up to 92% and TON up to 25,000. It was also successfully applied to the Pd-catalyzed asymmetric hydrogenation of simple indoles with ee up to 87% and Rh-catalyzed asymmetric 1,4-addition of phenylboronic acid to 2-cyclohexenone with 97% ee.     

High acceleration of the direct aldol reaction cocatalyzed by BINAM-prolinamides and benzoic acid in aqueous media

The enantioselective direct aldol reaction, organocatalyzed by recoverable BINAM-prolinamide derivatives can be highly accelerated by a catalytic amount of a carboxylic acid without a detrimental of the obtained enantioselectivities. From the study of suitable acids and reaction conditions, benzoic acid in aqueous DMF or in water was shown to give the best results with high yields and enantioselectivities. Thus, the reaction between p-nitrobenzaldehyde and acetone catalyzed by (S_a)-BINAM-l-Pro and benzoic acid can be carried out at −20 °C in only 8.5 h to give the expected product with 86% ee. In the case of butan-2-one, the iso- and the anti-isomers are obtained in a 1:1 isomer ratio up to 99% ee. Cyclohexanone gives the anti-aldol in up to 99% dr and 97% ee in only 2 h. The opposite diastereoselectivity is obtained in the case of cyclopentanone with lower ee up to 65% for the syn and 85% for the anti-isomer.     

α-Chloroacetone as a donor in the BINAM-l-prolinamide organocatalyzed aldol reaction: application to the enantioselective synthesis of α,β-epoxy ketones

Recoverable (S_a)-BINAM-l-prolinamide in combination with benzoic acid catalyzed the direct aldol reaction between α-chloroacetone and several aldehydes in different solvents, including water. It is possible to obtain mainly one of the isomers with good regio-, diastero-, and enantioselectivity by choosing the appropriate solvent and reaction conditions. Thus, α-chloroacetone mainly gives the anti-aldol isomer in DMF/H₂O with up to 97% ee. The crude α-chloro-β-hydroxy ketones obtained are transformed stereospecifically into the corresponding enantioenriched trans-α,β-epoxy ketones derivatives with up to 97% ee through an S_N2 displacement reaction by treatment with Et₃N.     

Chiral pyridine N-oxides derived from monoterpenes as organocatalysts for stereoselective reactions with allyltrichlorosilane and tetrachlorosilane

The synthesis of enantiomerically pure C₂-symmetric dipyridylmethane ligands and related N,N′-dioxides is reported. A procedure for the synthesis of a few new enantiomerically pure C₂-symmetric pyridine N-oxides and the preparation of four pyridine N-oxides with oxygen and nitrogen atoms as further coordinating elements in the heterocycle framework is described. All compounds were prepared from naturally occurring monoterpenes. These new compounds were assessed as organocatalysts in two different reactions, namely the allylation of aldehydes with allyltrichlorosilane that afforded homoallylic alcohols in good yields and up to 85% ee and the stilbene oxide opening by the addition of tetrachlorosilane that gave chlorohydrin in quantitative yield and up to 70% ee.     

Enantioselective synthesis of α-methylene-γ-butyrolactones using chiral Pd(II)-SPRIX catalyst

Combination of Pd(OCOCF₃)₂ and (M,S,S)-iPr-SPRIX ligand promoted the intramolecular cyclization of 2-alkynoates very effectively to form α-methylene-γ-butyrolactones in good yields with up to 92% ee.     

Iridium-catalyzed asymmetric hydrogenation of pyridine derivatives, 7,8-dihydro-quinolin-5(6H)-ones

[Ir(COD)Cl]₂/MeO-Biphep/I₂ catalyst system is highly effective for asymmetric hydrogenation of pyridine derivatives 7,8-dihydro-quinolin-5(6H)-ones with high enantioselectivities (up to 97% ee).     

General stereoretentive preparation of chiral secondary mixed alkylmagnesium reagents and their use for enantioselective electrophilic aminations

A general preparation of enantiomerically and diastereomerically enriched secondary alkylmagnesium reagents was reported as well as their use for performing highly stereoselective transition-metal free electrophilic aminations leading to α-chiral amines in up to 97% ee. Thus, the reaction of t-BuLi (2.2 equiv.) with a mixture of chiral secondary alkyl iodides and the commercially available magnesium reagent Me₃SiCH₂MgCl in a 2 : 1 mixture of pentane and diethyl ether at up to −50 °C provided optically enriched secondary mixed alkylmagnesium species of the type alkyl(Me)CHMgCH₂SiMe₃ with high retention of configuration (up to 99% ee). The resulting enantiomerically enriched dialkylmagnesium reagents were trapped with electrophiles such as non-enolizable ketones, aldehydes, acid chlorides, isocyanates, chlorophosphines and O-benzoyl hydroxylamines providing α-chiral tertiary alcohols, ketones, amides, phosphines and tertiary amines in up to 89% yield (over three reaction steps) and up to 99% ee.

We report a general method for the preparation of enantiomerically enriched secondary alkylmagnesium reagents, which undergo highly stereoselective transition-metal free electrophilic aminations, leading to α-chiral amines in up to 97% ee.     

Iridium-catalyzed asymmetric transfer hydrogenation of quinolines with Hantzsch esters

The iridium-catalyzed enantioselective transfer hydrogenation of quinolines with Hantzsch esters was developed with up to 88% ee using [Ir(COD)Cl]₂/(S)-SegPhos/I₂ as a catalyst.     

Facile one-pot synthesis of BINOL- and H8-BINOL-based aryl phosphites and their use in palladium catalysed asymmetric allylation

Novel P-monodentate aryl phosphite ligands have been synthesised in one step from (R)-BINOL, (R)-H₈-BINOL and (R)-H₈-3,3′-dibromo-BINOL. With the new aryl phosphites, up to 86% ee was observed in the asymmetric Pd-catalysed amination of 1,3-diphenyl-2-propenyl acetate with sodium diformylamide. In the enantioselective alkylation of 1,3-diphenyl-2-propenyl acetate with dimethyl malonate, up to 97% enantioselectivity was achieved.     

Enantioselective C-S bond formation by iron/Pybox catalyzed Michael addition of thiols to (E)-3-crotonoyloxazolidin-2-one

The enantioselective Michael addition of thiols to (E)-3-crotonoyloxazolidin-2-one was effectively catalyzed by the Fe(BF₄)₂·6H₂O/(S,S)-ip-Pybox catalyst, and the addition product was obtained with up to a 95% ee.     

Neutral p-cymene ruthenium complexes with P-stereogenic monophosphines. New catalytic precursors in enantioselective transfer hydrogenation and cyclopropanation

A family of eight neutral, pseudotetrahedral piano-stool ruthenium complexes C, of the type [RuCl₂(p-cymene)(PArPhR)] (Ar = 1-naphthyl, 9-phenanthryl and 2-biphenylyl; R = Me, i-Pr, OMe, –CH₂SiMe₃ and –CH₂SiPh₃) have been prepared and characterised, including the X-ray crystal structure for C6 (Ar = 2-biphenylyl; R = i-Pr). These complexes catalyse the asymmetric hydrogen transfer reaction of acetophenone in refluxing 2-propanol in the presence of potassium tert-butoxyde, reaching full conversions and up to 45% ee after 24 h towards the S enantiomer of 1-phenylethanol. Cationic complexes formed upon treatment of C with one equivalent of AgSbF₆ or (Et₃O)PF₆ are active in the cyclopropanation reaction of styrene and α-methylstyrene by ethyl diazoacetate. Low to moderate conversions (up to 58%), diastereoselectivities (up to 40% de), and moderate enantioselectivities (up to 69% ee) have been found. For both reactions, bulky complexes and C6 in particular lead to the best results.