Asymmetric borane reduction of ketones catalyzed by N‐hydroxyalkyl‐l‐menthopyrazoles

The equimolar mixture of N‐(hydroxyalkyl)pyrazoles and borane formed boric ester complex, in which the remaining borane was stabilized by the adjacent nitrogen of thr pyrazole ring. The borane complex derived from the chiral pyrazoles such as 3‐phenyl‐l‐menthopyrazole reduced p‐methylacetophenone ( 21 ) enantioselectively. When (2′S)‐2‐(2′‐phenyl‐2′‐hydroxyethyl)‐3‐phenyl‐l‐menthopyrazole ((2′S)‐ 10b ) was used, 21 was reduced into (S)‐p‐methylphenyl‐1‐ethanol ( 22 ) in moderate chemical and optical yields. Due to the inconvenience of the preparation and the lower optical yield, the use of N‐(α‐hydroxyalkyl)pyrazoles was unpromising for the enantioselective reduction of ketones by borane.     

Asymmetric borane reduction of prochiral ketones catalyzed by phosphinamides prepared from L‐serine

Synthesis of several new chiral phosphinamide catalysts with a proximal hydroxyl group from L ‐serine was described. These compounds have been successfully used in the asymmetric catalytic borane reduction of prochiral ketones. The optically active secondary alcohols were obtained with an enantiomeric excess (ee) up to 81% and excellent yields. © 2003 Wiley Periodicals, Inc. Heteroatom Chem 14:288–291, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.10145     

4-取代-脯氨醇催化的前手性酮不对称硼烷还原

(2S)-(二苯基羟甲基)-(4S)-苯氧基-四氢吡咯(10)由羟脯氨酸(6)经4步反应制得。用10催化6种前手性酮的不对称硼烷还原反应,其结果与(S)-二苯基脯氨醇催化的同一反应的结果进行了对比。     

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.     

Asymmetric borane reduction of achiral ketones mediated by a chiral oxazaborolidine derived from (−)-ephedrine

Asymmetric borane reduction of achiral ketones in the presence of a chiral oxazaborolidine derived from (−)-ephedrine yielded the corresponding alcohols in optical yields of 41 – 83 % ee.     

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     

Asymmetric hydrogenation of ketones catalyzed by silica‐supported chitosan‐iron‐nickel complex

A new silica‐supported biopolymer‐metal complex, silica‐supported chitosan‐iron‐nickel complex was prepared by a very simple method. This complex catalyst can be used as a catalyst in the asymmetric hydrogenation of propiophenone to (R)‐(+)‐1‐phenyl‐1‐propanol and acetophenone to (R)‐(+)‐1‐phenyl ethanol in 91.7 and 77.7% optical yields, respectively, at 110°C and under 70 kg/cm² pressure. The catalyst could be reused several times without any remarkable change in the catalytic activity. Copyright © 2004 John Wiley & Sons, Ltd.     

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%.     

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.     

Mildly and highly selective reductive deoxygenation of para‐di‐ and monoalkylaminophenyl ketones by borane

Para‐di‐ and monoalkylaminophenyl ketones were reduced selectively to para‐alkyl N,N‐di‐ and N‐monoalkylanilines in good to excellent yields by borane under mild, convenient, and neutral conditions. © 2005 Wiley Periodicals, Inc. Heteroatom Chem 16:2–5, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.20062     

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.     

Asymmetric hydrosilylation of ketones catalysed by a chiral rhodium complex

Synthesis of optically active alkoxysilanes by asymmetric hydrosilylation of ketones, in the presence of a chiral phosphine—rhodium complex, is described; optical yields up to 46% are observed.     

Asymmetric borane reduction using mixtures of homochiral amino alcohol ligands

The asymmetric borane reduction of acetophenone is investigated using mixtures of homochiral β-amino alcohol ligands. With stoichiometric amounts of a mixture of two- or three-amino alcohols, the e.e. remains at the level of the best amino alcohol for a wide composition range. A small but statistically significant enhancement in e.e. is observed when 10 mol% of an amino alcohol mixture of (1S,2R)-1-amino-2-indanol and (S)-phenylglycinol is used as chiral ligand.     

Asymmetric reduction of aminoketones with borane and chiral oxazaborolidine catalyst

A group of α‐ and β‐N, N‐dialkyl amino ketones were reduced enantioselectively by 2 moles of borane‐tetrahydrofuran in the presence of 10 mol% of the in‐situ formed chiral oxazaborolidine, followed by diluted hydrochloric acid. The resulting amino alcohol‐borane complexes were treated with hydrogen chloride‐glycol‐methanol to give the optically active amino alcohol with the ee up to 99%.     

A novel C3-symmetric prolinol-squaramide catalyst for the asymmetric reduction of ketones by borane

A novel C₃-symmetric prolinol-squaramide has been developed for the asymmetric reduction of ketones by borane. By using only 5 mol % catalyst 1a for the reaction, high yields and excellent enantioselectivities (up to 95% yield, 93% ee) were obtained. Moreover, 1a can be easily recovered by simple precipitation and re-used for four cycles without losing the selectivity.