Quantum chemical study on enantioselective reduction of keto oxime ether with borane catalyzed by oxazaborolidine. Part 2. Structures of catalyst‐alkoxyborane adduct with a four‐membered ring and succeeding reaction intermediates

Quantum chemical ab initio computations of the structures and properties of oxazaborolidine‐alkoxyborane adduct with a B? N? B? O four‐membered ring and succeeding reaction intermediates are carried out in the current work by means of the Hartree–Fock (HF) and the density functional methods. All the structures are optimized completely at the HF/6‐31G(d) and Becke's three‐parameter exchange functional and the gradient‐corrected functional of Lee, Yang, and Paar (B3LYP)/6‐31G(d) levels. As shown in the obtained results, the oxazaborolidine‐alkoxyborane adduct with a B? N? B? O four‐membered ring may be formed during the reduction of the carbonyl bond of the catalyst‐borane‐keto oxime ether adduct. The breakdown of the B? N? B? O four‐membered ring results in the formation of the adduct with a B? N? B? O? C? C? N seven‐membered ring and an oxime bond. The reduction of the oxime bond leads to the adduct with a chiral oxime carbon. The B(2)? N_{C? N} bond in the B? N? B? O? C? C? N seven‐membered ring of the adduct with a reduced oxime bond is weaker comparatively and thus may be more easily broken down. All the adducts have four stable structures. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 93: 294–306, 2003     

Quantum chemical study on enantioselective reduction of keto oxime ether with borane catalyzed by oxazaborolidine. Part 3. Properties of intermediates during hydride transfer

In the current article, the structures and properties of intermediates during the hydride transfer for the prior coordination of the carbonyl oxygen of keto oxime ether at B(2) of oxazaborolidine are discussed. All the structures are optimized completely by means of the Hartree–Fock (HF) and the density functional methods at the HF/6‐31G(d) and Becke's three‐parameter exchange functional and the gradient‐corrected functional of Lee, Yang, and Paar (B3LYP)/6‐31G(d) levels. The hydride transfer from BH₃ to the carbonyl carbon in oxazaborolidine‐borane‐keto oxime ether adduct results in the formation of the adduct 4a* with a seven‐membered ring. This adduct has four stable structures. Another hydride of BH₂ transfers to the oxime carbon in 4a* , leading to the adduct 5a* , which has also four stable structures. Among all the structures of 5a* , the most stable structure can generate (1S, 2R)‐cis amino alcohol, which is in agreement with that obtained in the experiment. This enantioselective reduction may go through the process in which oxazaborolidine‐borane‐keto oxime ether adduct is directly transformed into the adduct 4a* with a seven‐membered ring. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 93: 307–316, 2003     

Density functional study on enantioselective reduction of keto oxime ether with borane catalyzed by oxazaborolidine

Chiral amino alcohols have interesting biological activities and are used widely as chiral ligands in metal-mediated organic reactions[1―3]. Although many amino alcohols can be derived from the available amino acids, the asymmetric synthesis is an important method to get novel amino alcohols. Tillyer et al.[4] reported a new, highly stereoselective synthesis of cyclic (1S,2R)-cis amino alcohols A from keto oxime ethers B, via the enantioselective reduction catalyzed by oxazaborolidine C in …     

Quantum chemical study on enantioselective reduction of aromatic ketones catalyzed by chiral cyclic sulfur‐containing oxazaborolidines. Part 2. Structures of catalyst–borane–ketone adducts

In the present paper, the ab initio molecular orbital method is employed to study the structures of the adducts of borane and aromatic ketone to chiral cyclic sulfur‐containing oxazaborolidine used as a catalyst in the enantioselective reduction of aromatic ketone. The catalyst–borane–ketone adducts have four different structures. All the structures are optimized completely by means of the Hartree–Fock method at 6‐31g* basis sets. The structure which is of the greatest advantage to a hydride transfer from the borane moiety to the carbonyl carbon of aromatic ketone is the one with the next lowest formation energy, and the plausible transition state for the hydride transfer is predicted to be of a twisted boat structure. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 78: 252–260, 2000     

Quantum chemical study on enantioselective reduction of aromatic ketones catalyzed by chiral cyclic sulfur‐containing oxazaborolidines. Part 3. Structures of catalyst–alkoxyborane adducts

The chiral cyclic sulfur‐containing oxazaborolidine catalyst reacts with aromatic ketone in the presence of borane to form the catalyst–alkoxyborane adduct with a B‐O‐B‐N four‐membered ring. The ab initio molecular orbital method is employed to study the structures of the catalyst–alkoxyborane adduct. All the calculated systems are optimized completely by means of the Hartree–Fock method at 6‐31g* basis sets. The B‐O‐B‐N four‐membered ring is stable, although there is strong tensile stress in the four‐membered ring. The catalyst–alkoxyborane adduct exists in four stable structures. Among these structures, the largest energy difference is only about 4 kJ/mol. In the catalyst–alkoxyborane adduct, the B(2) N(3) bond in the catalyst is weakened greatly. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 78: 261–268, 2000     

Quantum chemical study on enantioselective reduction of aromatic ketones catalyzed by chiral cyclic sulfur‐containing oxazaborolidines. Part 1. Structures and properties of catalysts

The ab initio molecular orbital method is employed to study the structures and properties of chiral cyclic sulfur‐containing oxazaborolidine, as a catalyst, and its borane adducts. All the structures are optimized completely by means of the Hartree–Fock method at 6‐31g* basis sets. The catalyst is a twisted chair structure and reacts with borane to form four plausible catalyst–borane adducts. Borane–sulfur adducts may be formed, but they barely react with aromatic ketone to form catalyst–borane–ketone adducts, because they are repulsed greatly by the atoms arising from the chair rear of the catalyst with a twisted chair structure. Borane–N adduct has the largest formation energy and is predicted to react easily with aromatic ketone to form catalyst–borane–ketone adducts. The formation of the catalyst–borane adducts causes the B_BH3 H_BH3 bond lengths of the BH₃ moiety to be increased and thus enhances the activity of the enantioselective catalytic reduction. The borane–N adduct is of great advantage to hydride transfer. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 78: 245–251, 2000     

手性含硫恶唑硼烷催化芳酮不对称还原反应的量子化学研究

用HF方法在6-31G^*基组下,对手性含硫恶唑硼烷催化苯乙酮不对称还原反应进行了量子化学从头算研究。还原反应经历了催化剂-硼烷加合物、催化剂-硼烷-酮加合物、催化剂-烷氧基硼烷加合物的生成以及催化剂-烷氧基硼烷加合物的离解过程。催化剂-硼烷加合物、催化剂-硼烷-酮加合物和催化剂-烷氧基硼烷加合物的生成分别为放热、吸热、放热过程;催化剂-烷氧基硼烷加合物离解成催化剂烷氧基硼烷为吸热过程。催化剂-硼烷-酮加合物和催化剂-烷氧基硼烷加合物都存在四种稳定的结构。最有利于氢转移的催化剂-硼烷-酮加合物结构是次低能量结构,并且具有扭曲的船形结构。催化剂-烷氧基硼烷加合物含有一个B-O-B-N四元环,尽管四元环有较大的张力,但加合物仍有较高的稳定性。     

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     

Chiral terpene auxiliaries. Part 1: Highly enantioselective reduction of ketones with borane catalyzed by an oxazaborolidine derived from (−)-β-pinene

(1R,2S,3R,5R)-3-Amino-6,6-dimethyl-2-hydroxybicyclo[3.1.1]heptane was synthesized in three steps from (−)-β-pinene. It was used for the in situ generation of a B-methoxy-oxazaborolidine catalyst for the asymmetric reduction of alkyl-aryl ketones with borane-dimethyl sulfide complex. In the presence of 3 mol % of the catalyst, the product alcohols were obtained in high yields and with enantiomeric excesses in the range of 93-98%.     

Ab inito Study on Enantioselective Reduction of Ketones Catalyzed by Thiazolidino[3,4—c]oxazaborolidines

The ab initio molecular orbital method is employed to study the enantioselective reduction of acetophenone with borane catalyzed by thiszolidino[3,4-c]oxazaborolidine.Computation result shows that the controlling step for the reduction is the decomposition of the catalyst-alkoxyborane adduct and the reduction leads to S-alcohols.The transition atate of the hydride transfer from the borane moiety to the carbonyl carbon of acetophenone is a twisted chair structure with a B(2)-N(3)-BBH3-HBH3-CCo-OCO6-membered ring.     

Theoretical study with vibration–rotation and dipole moment calculations of quartet states of the CrCl molecule

The potential energy curves have been investigated for the 10 lowest quartet electronic states in the ^2s+1Λ^± representation below 30,000 cm^?1 of the molecule CrCl via CASSCF and MRCI (singly and doubly excitation with Davidson correction) calculations. The harmonic frequency ω_e, the internuclear distance r_e, the rotational constant B_e, the electronic energy with respect to the ground state T_e, and the permanent dipole moment μ have been calculated. By using the canonical functions approach, the eigenvalues E_v, the rotational constant B_v, and the abscissas of the turning points r_min and r_max have been calculated for the considered electronic states up to the vibrational level v = 19. Seven electronic states have been studied here theoretically for the first time. The comparison of these values to the theoretical results available in the literature shows a good agreement. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Effects of electrolytes used in roughening gold substrates by oxidation–reduction cycles on surface-enhanced Raman scattering

In this work, the effects of electrolytes used in roughening gold substrates by electrochemical methods on surface-enhanced Raman scattering (SERS) were first investigated. First, gold substrates were roughened by triangular-wave oxidation–reduction cycles (ORC) in aqueous solutions containing different kinds of 0.1 M electrolytes. Then Rhodamine 6G (R6G) was used as Raman probe to examine this effect of electrolytes used on the SERS observed. The result indicates that the highest intensity of SERS of R6G was obtained on the roughened Au substrate prepared in 0.1 M NaCl, which was less used in the literature. Meanwhile, it was also found that the rougher surface morphology observed, which is contributive to the higher SERS obtained, is corresponding to the smaller cathodic peak area shown in the cyclic voltammograms for roughening the Au substrate.     

Theoretical study on the Diels–Alder reaction of cyclopentadiene with methacrolein catalyzed by diethylimidazolium cation

The theoretical studies of how room temperature ionic liquids control desired reactions are very scarce in contrast with their increasing applications in many fields as recyclable solvents, catalysts, and reaction mediums. The present work considers the Diels–Alder (D‐A) reaction of cyclopentadiene with methacrolein in the presence of diethylimidazolium salts as the first prototype of our systemic studies about important organic synthesis reactions catalyzed by room temperature ionic liquids. We show the mechanism details of the D‐A reactions with and without the dialkylimidazolium cation and rationalize the experimental findings based on the results from the quantum chemistry calculations at the AM1, HF/6‐31G(d), and B3PW91/6‐31G(d,p) levels of theory, respectively. It is found that the diethylimidazolium cation acts as a Lewis acid center to catalyze the D‐A reaction, which decreases the barrier and increases the asynchronicity of the D‐A reaction, but does not change the potential energy surface profile of the reaction compared to the noncatalyzed process. The present results rationalize the early experimental findings well and provide the first prototype for theoretically understanding the D‐A reaction in the presence of dialkylimidazolium salts. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Highly enantioselective reduction of achiral ketones with NaBH4/Me3SiCl catalyzed by (S)-α,α-diphenylpyrrolidinemethanol

The reducing agent prepared from sodium borohydride, trimethylsilyl chloride and a catalytic amount of (S)-α,α-diphenylpyrrolidinemethanol has been successfully applied to the enantioselective reduction of ketones. The optically active secondary alcohols were obtained in excellent enantiomeric excess and almost quantitative chemical yield.     

Quantum chemical study of IrFn (n = 1–7) clusters: An investigation of superhalogen properties

In present investigation, the interactions of iridium (Ir) atom with fluorine (F) atoms have been studied using the density functional theory. Up to seven F atoms were able to bind to a single Ir atom which resulted in increase of electron affinities successively, reaching a peak value of 7.85 eV for IrF₇. The stability and reactivity of these clusters were analyzed by calculating highest occupied molecular orbital (HOMO)–LUMO gaps, molecular orbitals and binding energies of these clusters. The unusual properties of these clusters are due to the involvement of inner shell 5d‐electrons, which not only allows IrF_n clusters to belong to the class of superhalogens but also shows that its valence can exceed the nominal value of 2. © 2012 Wiley Periodicals, Inc.     

Highly selective and recyclable hydrogenation of α‐pinene catalyzed by ruthenium nanoparticles loaded on amphiphilic core–shell magnetic nanomaterials

A multifunctional nanomaterial (Fe₃O₄@SiO₂@C_X@NH₂) comprising a magnetic core, a silicon protective interlayer, and an amphiphilic silica shell is successfully prepared. Ru nanoparticles catalyst loaded on Fe₃O₄@SiO₂@C_X@NH₂ is used in hydrogenation of α‐pinene for the first time. The novel nanomaterial with amphipathy can be used as a solid foaming agent to increase gas–liquid–solid three‐phase contact and accelerate the reaction. Under the mild conditions (40 °C, 1 MPa H₂, 3 h), 99.9% α‐pinene conversion and 98.9% cis‐pinane selectivity are obtained, which is by far the best results reported. Furthermore, the magnetic nanocomposite catalyst can be easily separated by an external magnet and reused nine times with high selectivity maintaining.     

Hydrogen bonding. Part 77. Molecular orbital study of C–HF hydrogen bonds in tetramethylammonium fluoride; potential effects on solid state structure

The infrared (IR) spectrum of tetramethylammonium fluoride suggests that it contains the strongest C–HF⁻ hydrogen bonds yet observed. Ab initio 3-21G(^*) calculations were used to examine potential solid state arrangements of cation about anion. The favored state is one in which four cations surround each F⁻ in a D_2d arrangement and four F⁻ surround each cation. Each F⁻ acts as acceptor of four hydrogen bonds of −10.8 kcal mol⁻¹, one from each cation. This arrangement, similar to that of tetramethylammon chloride, is consonant with the IR spectrum of the cation in solid tetramethylammonium fluoride. In the preferred form of the monomeric gas phase ion-pair F⁻ lies against one triangular face of the T_d cation with three CHF hydrogen bonds of −11.5 kcal mol⁻¹ each. Constraint of F⁻ in the gas phase ion-pair to interaction with a single cation hydrogen results in a tightly bound molecular complex between HF and trimethylammonium methylide with an interaction energy of −27 kcal mol⁻¹; however, this structure is not seen elsewhere and apparently does not play a role in the solid salt.     

Organometallic dyes: Part 1. Synthesis of orange to cyan dyes based on donor–conjugated–acceptor chromogenes using ferrocene as the donor group

A novel series of organometallic donor–conjugated–acceptor dyes derived from ferrocene as the donor group have been synthesized via the Knoevenagel reaction of ferrocene carboxaldehyde and various active methylene compounds to give a range of dyes ranging from orange to blue–green in color. The most bathochromic dye is that derived from dialkyl thiobarbituric acid and the least is that derived from the tetralone. The dyes showed an unusual negative solvatochromism as the solvent polarity increased. All dyes synthesized are expected to have some non‐linear optical properties, as evidenced from the pronounced solvatochromism. Copyright © 2001 John Wiley & Sons, Ltd.     

Theoretical study on the reaction mechanisms and stereoselectivities of DABCO‐catalyzed Rauhut–Currier/cyclization reaction of methyl acrylate with 2‐benzoyl‐3‐phenyl‐acrylonitrile

With the aid of density functional theory (DFT) calculations, we have investigated the mechanisms and stereoselectivities of the tandem cross Rauhut–Currier/cyclization reaction of methyl acrylate R₁ with (E)‐2‐benzoyl‐3‐phenyl‐acrylonitrile R₂ catalyzed by a tertiary amine DABCO. The results of the DFT calculations indicate that the favorable mechanism (mechanism A) includes three steps: the first step is the nucleophilic attack of DABCO on R₁ to form intermediates Int1 and Int1‐1, the second step is the reaction of Int1 and Int1‐1 with R₂ to generate intermediate Int2(SS,RR,SR&RS), and the last step is an intramolecular S_N2 process to give the final product P(SS,RR,SR&RS) and release catalyst DABCO. The S_N2 substitution is computed to be the rate‐determining step, whereas the second step is the stereoselectivity‐determining step. The present study may be helpful for understanding the reaction mechanism of similar tandem reactions.