Chiral Borated Esters in Asymmetric Synthesis： 1. The First Asymmetric Reaction Catalyzed by Chiral Spiroborated Esters with an O3BN Framework

The first asymmetric reaction catalyzed by chiral spiroborated esters with an O3BN framework was reported. In the presence of 0.1 equivalent of (R,S)-1 or (S,S)-1, acetophenone was reduced by 0.6 equivalent of borane in THF at 0-5℃ for 2 h to give (R)-1-phenylethanol of up to 76% ee and 73% isolated yield. Influence of reaction conditions on the stereoselectivity of the reduction was investigated and a possible catalytic mechanism of the chiral spiroborated esters toward the reduction was also suggested.     

Quantum chemical study on the mechanism of enantioselective reduction of prochiral ketones catalyzed by oxazaborolidines

The ab initio molecular orbital study on the mechanism of enantioselective reduction of 3,3-dimethyl butanone-2 with borane catalyzed by chiral oxazaborolidine is performed. As illustrated, this enantioselective reduction is exothermic and goes mainly through the formations of the catalyst-borane adduct, the catalyst-borane-3,3-dimethyl butanone-2 adduct, and the cata-lyst-alkoxyborane adduct with a B-O-B-N 4-member ring and through the decomposition of the catalyst-alkoxyborane adduct with the regeneration of the catalyst. During the hydride transfer in the catalyst-borane-3,3-dimethyl butanone-2 adduct to form the catalyst-alkoxyborane adduct, the hydride transfer and the formation of the B-O-B-N 4-member ring in the catalyst-alkoxyborane adduct happen simultaneously. The controlling step for the reduction is the transfer of hydride from the borane moiety to the carbonyl carbon of 3,3-dimethyl butanone-2. The transition state for the hydride transfer is a twisted chair structure and the reduction leads to     

Asymmetric Reduction of Acetophenone O-Methyloxime with Reagents Prepared from L-Proline and Borane

Asymmetric reduction of acetophenone O-methyloxime with reagents in situ prepared from L-pronne and borane has been investigated. A series of conditions opfimiTation were made, including an examination of the effect of the temperature of ratalyst pretreatment, the temperature of reduction, the amount of borane, the additive, the solvent, the reducing agent and various chiral anxiHaries on the enanfioselecfivlty. Under the opti-mal condition, (S)-a-phenylethylamlne was obtained in 53% yield with 83.1% ee in the presence of 25 mol% L-proline.     

Quantum chemical study on asymmetric catalysis reduction of imine

The asymmetric catalysis reaction is considered to be an important way by which chiral compounds are generated. Chiral 1,3,2-oxazaborolidine, as an effective asymmetric catalyst, is used widely in the enantioselective reduction of prochiral ketones, imines, and carbon-carbon double bonds[1—3]. Up to now, a number of quantum chemical modeling investigations of the en-antioselective reduction of prochiral ketones with borane catalyzed by chiral oxazaborolidines have been carried out[4—7]. Howe…     

Enantioselective reduction of acetophenone analogues using carrot and celeriac enzymes system

<正>The enantioselective reduction of acetophenone analogues catalyzed by carrot and celeriac was performed in moderate conversions and excellent enantiomeric excesses.The steric factors and electronic effects of the substituents at the aromatic ring were found to significantly affect the efficiency of the enantioselective reduction of acetophenone analogues,while they had a little effect on the enantioselectivity of acetophenone analogues reduction.It was also found that the conversions of acetophenone analogues reduction at 33℃by means of both biocatalysts were three times as great as those at room temperature.     

Tuning Double Layer Structure of WO3 Nanobelt for Promoting the Electrochemical Nitrogen Reduction Reaction in Water

Electrochemical fixation of nitrogen to ammonia with highly active, highly selective and low cost electrocatalysts is a sustainable alternative to the extremely energy-and capital-intensive Haber-Bosch process. Herein, we demonstrate a near electroneutral WO_3 nanobelt catalyst to be a promising electrocatalyst for selective and efficient nitrogen reduction. The concept of near electroneutral interface is demonstrated by fabricating WO_3 nanobelts with small zeta potential value on carbon fiber paper, which ensures a loose double layer structure of the electrode/electrolyte interface and allows nitrogen molecules access the active sites more easily and regulates proton transfer to increase the catalytic selectivity. The WO_3/CFP electrode with optimal surface charge achieves a NH_3 yield rate of 4.3 μg·h~(-1)·mg~(-1) and a faradaic efficiency of 37.3% at-0.3 V vs. RHE, rivalling the performance of the state-of-the-art nitrogen reduction reaction electrocatalysts. The result reveals that an unobstructed gas-diffusion pathway for continually supplying enough nitrogen to the active catalytic sites is of great importance to the overall catalytic performance.     

Synthesis of a New Chiral Cyclicβ-Amino Alcohol

The enantioselective chiral l,3,2-oxazaborolidine-catalyzed reduction of prochiralketones by borane has been intensively investigated'-'. Most oxazaborolidine catalystshave been prepared from chiral D-amino alcohols. In our previous study on the catalyststructure-enantioselectivity relationships, we prepared (R)-2-amino-1,l-diphenyl-3~(2naphthyl)-l-propanol I which proved to be a good precursor of chiral catalyst for theenantioselective borane reduction of prochiral ketones4-9. This let'ter d…     

Determination of Sulpiride by Linear Sweep Stripping Voltammetry at a Mercury Electrode

This paper reports the electrochemical properties of sulpiride at a mercury electrode,especially its adsorptive characteristic,Sulpiride dissolved in a supporting electrolyte of a McIlvaine buffer at pH 6.8 represents a sensitive and well defined reduction wave by linear sweep stripping voltammetry.This method is based on the pre-concentration and the reduction of sulpiride at a hung mercury drop electrode.The reduction peak potential is -1.72V(vs.Ag-AgCl) and the peak current is proportional to the concentration of sulpiride in the range of 0.1-0.6 μg/mL.The detection limit is 0.0250μg/mL obtained under the experimental conditions selected in this work.The electrochemical properties of this system were investigated.and the proposed method was applied to the determination of sulpiride in pharmaceutical tablets with satisfactory results,It was compared well with the UV spectrophotometric method,showing a superior sensitivity.     

Stereoselective synthesis of 3-butylphtalide via CBS catalytic reduction

Optically active 3-butylphtalide of high enantiomeric excesses (up to 93% e.e.) was synthesized from the reduction of o-pentanoylbenzoic ester with borane using B-methyloxyoxazaborolidine (1b) as the chiral catalyst.     

Studies on the Oxazaborolidine-catalyzed Enantioselective Reduction of 3-Morpholin-4-yl-1-phenyl-1-propanone with Density Functional Theory

Density functional theory (DFT) has been applied to study the enantioselective reduction of 3-morpholin-4-yl-l-phenyl-l-propanone with borane catalyzed by (S)-4-benzyl-5,5-diphenyl-l,3,2-oxazaborolidine at the B3LYP/6-31G* level. All molecular species involved in the four reaction steps have been fully optimized and the structural parameters are provided, and the micro process of reaction was also investigated. The catalyst-alkoxyborane adduct formed in step Ⅲ exhibits a B-O-B-N tetra-atomic ring. Reaction coordination calculations show that BH3 can react with 3-morpholin-4-yl-l-phenyl-l-propanone spontaneously, resulting in the need of 2 tool BH3 in the reaction.     

(1R,3R,4S)‐1‐Benzyl‐3‐(tert‐butyldimethylsilyloxy)‐4‐(hydroxymethyl)pyrrolidine–borane: novel B—H...H—O hydrogen bonding

The absolute configuration of the title cis‐(1R,3R,4S)‐pyrrolidine–borane complex, C₁₈H₃₄BNO₂Si, was confirmed. Together with the related trans isomers (3S,4S) and (3R,4R), it was obtained unexpectedly from the BH₃·SMe₂ reduction of the corresponding chiral (3R,4R)‐lactam precursor. The phenyl ring is disordered over two conformations in the ratio 0.65:0.35. The crystallographic packing is dominated by the rarely found donor–acceptor hydroxy–borane O—H...H—B hydrogen bonds.     

Asymmetric reduction of acetophenone over heterogeneous oxazaborolidine catalysts

The reduction of acetophenone over the heterogeneous catalyst NiB_2–0.1(oxazaborolidine)_0.1 by borane in THF at 273 K is quicker than in homogeneous phase. The catalyst presented high enantioselectivity (ee ≥90%) at low molar ratio provided the ketone was added very slowly to the catalyst and borane complex.     

A rhodium(III) complex of the linear diborazine H3B·NMe2BH2·NMe2H: an intermediate in the dehydrocoupling of H3B·NMe2H

The solid‐state structure of the rhodium complex (dimethylamine–dimethylaminoborane–borane‐κ²H,H′)dihydridobis(triisopropylphosphane‐κP)rhodium(III) tetrakis[3,5‐bis(trifluoromethyl)phenyl]borate, [RhH₂(C₄H₁₈B₂N₂)(C₉H₂₁P)₂](C₃₂H₁₂BF₂₄), is reported. The complex contains the linear diborazine H₃B·NMe₂BH₂·NMe₂H, a kinetically important intermediate in the transition‐metal‐mediated dehydrocoupling of H₃B·NMe₂H, ultimately affording the dimeric amino‐borane [H₂BNMe₂]₂. The structure of the title complex contains a distorted octahedral Rh^III centre, with mutually trans phosphane ligands and cis hydride ligands. The diborazine is bound through two Rh—H—B σ‐bonds and exhibits a gauche conformation with respect to the B—N—B—N backbone.     

Asymmetric Catalysis with a Phosphoramidite Derived from (−)‐(aR)‐ [1,1′‐Binaphthalene]‐8,8′‐diol

Treatment of (aR)‐[1,1′binaphthalene]‐8,8′‐diol ((−)‐ 1 ) with hexamethylphosphorous triamide afforded the N,N‐dimethylphosphoramidite (−)‐ 3 (Scheme 1). The synthesis of the analogous N,N‐diisopropylphosphoramidite 4 failed, however, and afforded the acyclic phosphonamidate (−)‐ 5 . The application of the cyclic phosphoramidite (−)‐ 3 towards asymmetric catalysis was investigated. The borane reduction of acetophenone ( 6 ) to (R)‐1‐phenylethanol ( 7 ) in the presence of (−)‐ 3 proceeded with 96% ee (Scheme 2). The use of (−)‐ 3 as ligand in several Cu‐catalyzed addition and substitution reactions resulted in enantioselectivities ranging from 0 to 50% (Schemes 3 and 4).     

The first example of a mixed alkoxide hydride of boron: sodium boron isopropoxide trihydride

The title compound, poly[[μ‐trihydro(isopropoxy)borato]sodium(I)], [Na(C₃H₁₀BO)]_n, forms unique polymeric layers normal to the c axis via Na⁺...O [2.3405 (15) Å] and Na⁺...H(borane) [2.22 (3) and 2.28 (3) Å] interactions. This arrangement builds on distorted tetrahedral Na⁺, oxygen and boron environments, with one of the borane hydride units uncoordinated, and highlights potential H₃B—O‐based chemistry.     

A d10 Ni–(H2) Adduct as an Intermediate in H?H Oxidative Addition across a Ni?B Bond

Bifunctional E H activation offers a promising approach for the design of two‐electron‐reduction catalysts with late first‐row metals, such as Ni. To this end, we have been pursuing H₂ activation reactions at late‐metal boratranes and herein describe a diphosphine–borane‐supported Ni—(H₂) complex, [(^PhDPB^iPr)Ni(H₂)], which has been characterized in solution. ¹H NMR spectroscopy confirms the presence of an intact H₂ ligand. A range of data, including electronic‐structure calculations, suggests a d¹⁰ configuration for [(^PhDPB^iPr)Ni(H₂)] as most appropriate. Such a configuration is highly unusual among transition‐metal H₂ adducts. The nonclassical H₂ adduct is an intermediate in the complete activation of H₂ across the Ni B interaction. Reaction‐coordinate analysis suggests synergistic activation of the H₂ ligand by both the Ni and B centers of the nickel boratrane subunit, thus highlighting an important role of the borane ligand both in stabilizing the d¹⁰ Ni—(H₂) interaction and in the H—H cleavage step.     

A d10 Ni–(H2) Adduct as an Intermediate in HH Oxidative Addition across a NiB Bond

Bifunctional E? H activation offers a promising approach for the design of two‐electron‐reduction catalysts with late first‐row metals, such as Ni. To this end, we have been pursuing H₂ activation reactions at late‐metal boratranes and herein describe a diphosphine–borane‐supported Ni—(H₂) complex, [(^PhDPB^iPr)Ni(H₂)], which has been characterized in solution. ¹H NMR spectroscopy confirms the presence of an intact H₂ ligand. A range of data, including electronic‐structure calculations, suggests a d¹⁰ configuration for [(^PhDPB^iPr)Ni(H₂)] as most appropriate. Such a configuration is highly unusual among transition‐metal H₂ adducts. The nonclassical H₂ adduct is an intermediate in the complete activation of H₂ across the Ni? B interaction. Reaction‐coordinate analysis suggests synergistic activation of the H₂ ligand by both the Ni and B centers of the nickel boratrane subunit, thus highlighting an important role of the borane ligand both in stabilizing the d¹⁰ Ni—(H₂) interaction and in the H—H cleavage step.     

Asymmetric Induction From Ethyl Lactate in the Reduction of Acetophenone to Phenylethanol

Ethyl lactate was used for the first time as a solvent for acetophenone reduction to phenylethanol by borane and sodium tetrahydridoborate. Moderate enantiomeric excess was obtained with the pure reducing agents that could be improved up to 46% by employing stoichiometric amounts of the Lewis acid ZnCl₂. These are the highest e.e. values published for solvent‐induced chiral synthesis.     

Borane chain transfer reaction in olefin polymerization using trialkylboranes as chain transfer agents

This article discusses a new borane chain transfer reaction in olefin polymerization that uses trialkylboranes as a chain transfer agent and thus can be realized in conventional single site polymerization processes under mild conditions. Commercially available triethylborane (TEB) and synthesized methyl‐B‐9‐borabicyclononane (Me‐B‐9‐BBN) were engaged in metallocene/MAO [depleted of trimethylaluminum (TMA)]‐catalyzed ethylene (Cp₂ZrCl₂ and rac‐Me₂Si(2‐Me‐4‐Ph)₂ZrCl₂ as a catalyst) and styrene (Cp*Ti(OMe)₃ as catalyst) polymerizations. The two trialkylboranes were found—in most cases—able to initiate an effective chain transfer reaction, which resulted in hydroxyl (OH)‐terminated PE and s‐PS polymers after an oxidative workup process, suggesting the formation of the B‐polymer bond at the polymer chain end. However, chain transfer efficiencies were influenced substantially by the steric hindrances of both the substituent on the trialkylborane and that on the catalyst ligand. TEB was more effective than TMA in ethylene polymerization with Cp₂ZrCl₂/MAO, whereas it became less effective when the catalyst changed to rac‐Me₂Si(2‐Me‐4‐Ph)₂ZrCl₂. Both TEB and Me‐B‐9‐BBN caused an efficient chain transfer in the Cp₂ZrCl₂/MAO‐catalyzed ethylene polymerization; nevertheless, Me‐B‐9‐BBN failed in vain with rac‐Me₂Si(2‐Me‐4‐Ph)₂ZrCl₂/MAO. In the case of styrene polymerization with Cp*Ti(OMe)₃/MAO, thanks to the large steric openness of the catalyst, TEB exhibited a high efficiency of chain transfer. Overall, trialkylboranes as chain transfer agents perform as well as B? H‐bearing borane derivatives, and are additionally advantaged by a much milder reaction condition, which further boosts their applicability in the preparation of borane‐terminated polyolefins. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 3534–3541, 2010