Application of MerCO as a chiral catalyst in asymmetric synthesis: asymmetric borane reduction of ketones

The borane reduction of aryl methyl ketones catalyzed by MerCO [(1R,2S,3R)-3-mercaptocamphan-2-ol] produced 1-aryl ethyl alcohols in 92% e.e. at 50°C in toluene. Significant temperature and solvent effects were observed in this reaction.     

Structurally well-defined, recoverable C3-symmetric tris(beta-hydroxy phosphoramide)-catalyzed enantioselective borane reduction of ketones

[reaction: see text] A series of new chiral C(3)-symmetric tris(beta-hydroxy phosphoramide) ligands have been synthesized via the reaction of trisphosphoramide ester and Grignard reagents. The catalytic asymmetric borane reduction of ketones with these new C(3)-symmetric chiral tris(beta-hydroxy phosphoramide)s was investigated. Structurally well-defined, recoverable ligand 1d is an efficient catalyst for the enantioselective borane reduction of both electron-deficient and electron-rich ketones, and high enantioselectivities were achieved (up to 98% ee).     

Diphenylamine-derived bis-hydroxyamide catalyzed asymmetric borane reduction of prochiral ketones

A series of bis-hydroxyamides were synthesized from diphenylamine-2,2′-dicarboxylic acid and chiral aminoalcohols. Their catalytic activity in asymmetric borane reduction was investigated. After the fine optimization of solvents, temperature, amount of borane complex, and the length of catalyst generating period, good to excellent yields (55–99%) and enantioselectivities (79–97% ee) can be achieved in the reduction of aromatic and alkyl prochiral ketones. A transition state structure was proposed on the basis of absolute configuration and controlled experiment.     

Asymmetric borane reduction of prochiral ketones using imidazolium-tagged sulfonamide catalyst

A novel sulfonamide catalyst based on a room temperature ionic liquid (RTIL) has been developed for the enantioselective reduction of ketones in refluxing toluene. The optically active secondary alcohol products were obtained in good enantiomeric excess and excellent yields. The imidazolium-tagged sulfonamide catalyst can be readily recovered and reused four times without any significant loss of catalytic activity.     

Catalytic asymmetric borane reduction of prochiral ketones using chiral camphor-derived mercapto-alcohols

Easily available D-(+)-camphor-derived chiral mercapto-alcohols 2 and 3 were employed for catalytic asymmetric borane reduction of aromatic ketones. Moderate enantioselectivities with e.e. 20.2-72.1% were obtained with 10 mol% catalyst. Opposite asymmetric induction was achieved when mercapto-alcohols 2 and 3 were used.     

C2-symmetric proline-derived tetraamine as highly effective catalyst for direct asymmetric Michael addition of ketones to chalcones

A C(2)-symmetric tetraamine catalyst was developed for the asymmetric Michael addition of ketones to chalcones. The corresponding adducts 1,5-dicarbonyl compounds were obtained in good chemical yields with high levels of diastereo- and enantioselectivities (up to >99 : 1 dr and 93% ee) under mild conditions. By studying the ESI-MS of the intermediates, a proposed mechanism was disclosed.     

Novel fluorous prolinol as a pre-catalyst for catalytic asymmetric borane reduction of various ketones

Novel prolinol carrying two perfluorohexylethyl groups at the α-position was prepared from l-proline as a starting chiral substrate. Catalytic asymmetric reduction of various ketones, including mono-, di-, and trifluoromethylated acetophenones, using fluorous oxazaborolidines derived from fluorous prolinol afforded the corresponding alcohols in good to excellent yields and with high enantioselectivities (up to 93.2% ee). The fluorous prolinol was recovered without any fluorous solvents or silica gel by simply cooling the organic phase and filtration.     

Chiral squaric prolinols: a new type of ligand for the asymmetric reduction of prochiral ketones by borane

A series of chiral bifunctional squaric prolinol ligands, having N, S substituents at C(3) of the squaric ring were synthesized and applied to the asymmetric borane reduction of prochiral ketones via an in situ formed chiral boron heterocycle, affording secondary alcohols with high yields and excellent enantiomeric excesses (up to 99%). The crystal structure of 5a was obtained and the mechanism of the catalytic asymmetric reduction is also discussed.     

Effect of borane source on the enantioselectivity in the enantiopure oxazaborolidine‐catalyzed asymmetric borane reduction of ketones

The effect of borane source on enantioselectivity in the enantiopure oxazaborolidine‐catalyzed asymmetric borane reduction of ketones has been investigated by using (S)‐3,1,2‐oxazaborobicyclo[3.3.0]octane and (S)‐7,3,1,2‐thiaxazaborobicyclo[3.3.0]octane as catalysts. The results indicate that the enantioselective order of different borane sources is borane–dimethyl sulfide < borane–N,N‐diethylaniline < borane–THF for the asymmetric reduction of a ketone under the same conditions. © 2007 Wiley Periodicals, Inc. Heteroatom Chem 18:740–746, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20370     

New heterogeneous catalyst for enantioselective borane reduction of ketones: phosphinamide anchored to nickel boride

A new example of heterogenization of a homogeneous catalyst by anchoring the chiral molecule to nickel boride is reported. The enantioselectivity in the reduction of acetophenone reaches that reported under homogeneous conditions.     

A new series of chiral catalysts for the enantioselective borane reduction of ketones

A new series of 1,3,2-oxazaborolidine catalysts substituted in position 4 by the (CH3)3C(CH2)n group (n=2, 3, 4, 5) were synthesized and applied to the borane reduction of prochi-ral ketones. The relationship between catalyst structure and enantioselectivity was discussed.     

Influences of electronic effects and anions on the enantioselectivity in the oxazaborolidine-catalyzed asymmetric borane reduction of ketones

The influence of electronic effects on the enantioselectivity of the oxazaborolidine-catalyzed asymmetric borane reduction of ketones has been observed and investigated with use of para-substituted acetophenones and propiophenones with a variety of functional groups and B-unsubstituted and B-methoxyoxazaborolidines derived from (S)-2-(diphenylhydroxymethyl)pyrrolidine with borane and trimethyl borate as catalysts in toluene and THF. The results indicate that Hammett linear free energy electronic effects on the enantioselectivity in the asymmetric reduction were observed and rationalized. Tuning electronic effects of the catalyst can improve the enantioselectivity in the reduction. Another interesting finding to be noted is that anions heavily affect the enantioselectivity, especially for the B-methoxy catalyst, because of their coordination with the boron atom in the catalysts.     

A C2-symmetric chiral bipyridyldiol–titanium complex as a catalyst for the asymmetric trimethylsilylcyanation of substituted benzaldehydes

This article describes a novel C₂-symmetric ligand that comprises a central bipyridine-pinene-derived core and two functionalized two diphenylmethanol subunits. The [8′-(hydroxy-diphenyl-methyl)-10,10,10′,10′-tetramethyl-[5,5′]bi[6-aza-tricyclo[7.1.1.0^2,7]undecyl]-2(7),3,5,2′(7′),3′,5′-hexane-8-yl]-diphenyl-methanol 1 is an effective catalyst in the enantioselective addition of trimethylsilylcyanide to various aromatic aldehydes with asymmetric inductions of up to 98% ee. Importantly, the correlation between Hammett substituent constants and the enantiomeric excess, and the electron-releasing group at the meta- and para-positions of substituted benzaldehydes were demonstrated to be associated with the high enantioselectivity of the trimethylsilylcyanation reaction that involves trimethylsilylcyanide.     

Enantioselective borane reduction of aromatic ketones catalyzed by chiral aluminum alkoxides

The asymmetric borane reduction of prochiral ketones with an alkoxide catalyst prepared in situ from aluminum tri-iso-propoxide and (R)-binaphthol was examined. Using these conditions, alcohols were obtained in high yield and e.e.'s of up to 83%.     

芳基取代的手性恶唑啉铝氢化物的制备及其对酮羰 基的不对称还原反应

通过两种不同方法合成了三个羟苯基恶唑啉手性配体,并用其制成新手性还原剂双恶唑啉铝氢化物(DIOAL-H),研究了它对酮羰基的不对称还原反应。     

A C2-symmetric nickel diamine complex as an asymmetric catalyst for enecarbamate additions to butane-2,3-dione

Butane-2,3-dione was activated towards nucleophilic addition of enecarbamates by a series of metal triflate complexes of a C2-symmetric diamine to give stereogenic, aldol-like, t-alcohols, a novel nickel(II) triflate complex was identified as a good catalyst for this asymmetric transformation, and an aquo nickel(II) complex was identified by XRD techniques.     

Enantioselective borane reduction of aromatic ketones using chiral BINOL derivatives as ligands in an aluminum catalyst

Chiral aluminum complex-catalyzed asymmetric borane reduction of aromatic ketones has been successfully carried out in the presence of (R)-BINOL derivatives as ligands. Secondary alcohols were obtained in high yields with good enantioselectivities (up to 90% e.e.).