由(1R, 2S)-麻黄碱制备的硼杂恶唑烷催化甲硼烷不对称还原苯乙酮

由(1R, 2S)-麻黄碱制得了五个新的手性硼杂恶唑烷1~5, 它们催化甲硼烷不对称还原苯乙酮, 获得了高产率的具有38.5~72.4%e. e.的R-1-苯基乙醇。讨论了催化剂的结构-活性关系及反应参数(催化剂用量、反应温度)对还原对应对映选择性的影响。     

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.     

Asymmetric reduction of representative ketones with a bis-chiral oxazaborolidine system

In recent years, many optically active β-amino alcohols, mostly derived from naturally occurring α-amino acids, have been incorporated into the asymmetric synthesis as chiral auxiliaries or ligands.1 An effective asymmetric catalysts, the oxazaborolidine-borane reagents, which were originally pioneered by Itsuno and Corey,2 were generally prepared from chiral β-amino alcohols by the reaction with boric acid or formed in situ in the presence of borane. These reagents provide excellent enanti…     

Reactivity of boranes with a titanium(IV) amine tris(phenolate) alkoxide complex; formation of a Ti(IV) tetrahydroborate complex, a Ti(III) dimer and a Ti(IV) hydroxide Lewis acid adduct

Treatment of the titanium(IV) alkoxide complex [Ti(Oi Pr)(OC6Me2H(2)CH2)3N] (2) with BH3.THF, as part of a study into the utility and reactivity of (2) in the metal mediated borane reduction of acetophenone, results in alkoxide-hydride exchange and formation of the structurally characterised titanium(iv) tetrahydroborate complex [Ti{BH4}(OC6Me2H2CH2)3N] (3). Complex (3) readily undergoes reduction to form the isolable titanium(III) species [Ti(OC6Me2H2CH2)3N]2 (4). Reaction of (2) with B(C6F5)3 results in formation of the Lewis acid adduct [Ti(OC6Me2H2CH2)3N][HO.B(C6F5)3] (5). In comparison, treatment of the less sterically encumbered alkoxide Ti(Oi Pr)4 with B(C6F5)3 results in alkoxide-aryl exchange and formation of the organometallic titanium complex [Ti(Oi Pr)3(C6F5)]2 (6). The molecular structures of 3, 4, 5 and 6 have been determined by X-ray diffraction.     

Molecular mechanism for H2 release from BH3NH3, including the catalytic role of the Lewis acid BH3

Electronic structure calculations using various methods, up to the coupled-cluster CCSD(T) level, in conjunction with the aug-cc-pVnZ basis sets with n = D, T, and Q, extrapolated to the complete basis set limit, show that the borane molecule (BH3) can act as an efficient bifunctional acid-base catalyst in the H2 elimination reactions of XHnYHn systems (X, Y = C, B, N). Such a catalyst is needed as the generation of H2 from isoelectronic ethane and borane amine compounds proceeds with an energy barrier much higher than that of the X-Y bond energy. The asymptotic energy barrier for H2 release is reduced from 36.4 kcal/mol in BH3NH3 to 6.0 kcal/mol with the presence of BH3 relative to the molecular asymptote. The NH3 molecule can also participate in a similar catalytic process but induces a smaller reduction of the energy barrier. The kinetics of these processes was analyzed by both transition-state and RRKM theory. The catalytic effect of BH3 has also been probed by an analysis of the electronic densities of the transition structures using the atom-in-molecule (AIM) and electron localization function (ELF) approaches.     

Stereoselective Borane Reduction of Acetophenone using 1,3,2-Oxazaborolidine Derived from (R)-4-(Diphenylhydroxymethyl)-1,3-Thiazolidine

A new chiral 1, 3, 2-oxazaborolidine derived from (R)-4-(diphenylhydroxymethyl)-l, 3-thiazolidine has been synthesized and its efficiency in the asymmetric reduction of acetophenone by borane has been investigated. It gives (R)-l-phenylethanol with high enantiomeric excess     

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     

Stoichiometric reactivity of dialkylamine boranes with alkaline earth silylamides

Reactions of β-diketiminato group 2 silylamides, [HC{(Me)CN(2,6-(i)Pr(2)C(6)H(3))}(2)M(THF)(n){N(SiMe(3))(2)}] (M = Mg, n = 0; M = Ca, Sr, n = 1), and an equimolar quantity of pyrrolidine borane, (CH(2))(4)NH·BH(3), were found to produce amidoborane derivatives of the form [HC{(Me)CN(2,6-(i)Pr(2)C(6)H(3))}(2)MN(CH(2))(4)·BH(3)]. In reactivity reminiscent of analogous reactions performed with dimethylamine borane, addition of a second equivalent of (CH(2))(4)NH·BH(3) to the Mg derivative induced the formation of a species, [HC{(Me)CN(2,6-(i)Pr(2)C(6)H(3))}(2)Mg{N(CH(2))(4) BH(2)NMe(2)BH(3)}], containing an anion in which two molecules of the amine borane substrate have been coupled together through the elimination of one molecule of H(2). Both this species and a calcium amidoborane derivative have been characterised by X-ray diffraction techniques and the coupled species is proposed as a key intermediate in catalytic amine borane dehydrocoupling, in reactivity dictated by the charge density of the group 2 centre involved. On the basis of further stoichiometric reactions of the homoleptic group 2 silylamides, [M{N(SiMe(3))(2)}(2)] (M = Mg, Ca, Sr, Ba), with (CH(3))(2)NH·BH(3) and (i)Pr(2)NH·BH(3) reactivity consistent with successive amidoborane β-hydride elimination and [R(2)N[double bond, length as m-dash]BH(2)] insertion is described as a means to induce the B-N dehydrocoupling between amine borane substrates.     

Asymmetric Reduction of Acetophenone with Borane Catalyzed by Chiral Oxazaborolidinones Derived from L-α-Amino Acids

Some chiral oxazaborolidinones were prepared from L-α-amino acids reacting with borane and used as a new type of catalysts for asymmetric reduction of acetophenone with borane to afford R-(+)-1-phenyl ethanol with 23–76%ee in high yield.     

由(1S,2S)—伪麻黄碱制备噁唑硼烷催化甲硼烷不对称还原含氟酮

前手性酮的对映选择性还原在不对称合成中占有极其重要的地位。人们对这类反应进行了广泛的研究,其中Ｉｔｓｕｎｏ和Ｃｏｒｙ等发现的用手性硼杂口恶唑烷催化不对称还原的方法,已成为高对映选择性还原羰基化合物最成功的方法之一［１,２］。近年来,国内外许多文献报道...     

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.     

Reductive openings of acetals: explanation of regioselectivity in borane reductions by mechanistic studies

The mechanisms of regioselective reductive openings of acetals were investigated in several model systems by a combination of Hammett plots, kinetic experiments, density functional calculations, and (11)B NMR. The regioselectivity of borane reductions of cyclic acetals can be controlled by the choice of borane. Lewis acid activation of BH3 x NMe3 increases the reaction rate and renders the borane the most electrophilic species, which associates to the more electron-rich oxygen of the acetal. In contrary, without activation, the regioselectivity is instead directed by the Lewis acid, as exemplified by the reaction with BH3 x THF.     

Dehydrogenation mechanisms and thermodynamics of MNH2BH3 (M=Li, Na) metal amidoboranes as predicted from first principles

Electronic structure calculations have been used to determine and compare the thermodynamics of H(2) release from ammonia borane (NH(3)BH(3)), lithium amidoborane (LiNH(2)BH(3)), and sodium amidoborane (NaNH(2)BH(3)). Using two types of exchange correlation functional we show that in the gas-phase the metal amidoboranes have much higher energies of complexation than ammonia borane, meaning that for the former compounds the B-N bond does not break upon dehydrogenation. Thermodynamically however, both the binding energy for H(2) release and the activation energy for dehydrogenation are much lower for NH(3)BH(3) than for the metal amidoboranes, in contrast to experimental results. We reconcile this by also investigating the effects of dimer complexation (2×NH(3)BH(3), 2×LiNH(2)BH(3)) on the dehydrogenation properties. As previously described in the literature the minimum energy pathway for H(2) release from the 2×NH(3)BH(3) complex involves the formation of a diammoniate of diborane complex ([BH(4)](-)[NH(3)BH(2)NH(3)](+)). A new mechanism is found for dehydrogenation from the 2×LiNH(2)BH(3) dimer that involves the formation of an analogous dibroane complex ([BH(4)](-)[LiNH(2)BH(2)LiNH(2)](+)), intriguingly it is lower in energy than the original dimer (by 0.13 eV at ambient temperatures). Additionally, this pathway allows almost thermoneutral release of H(2) from the lithium amidoboranes at room temperature, and has an activation barrier that is lower in energy than for ammonia borane, in contrast to other theoretical research. The transition state for single and dimer lithium amidoborane demonstrates that the light metal atom plays a significant role in acting as a carrier for hydrogen transport during the dehydrogenation process via the formation of a Li-H complex. We posit that it is this mechanism which is responsible, in condensed molecular systems, for the improved dehydrogenation thermodynamics of metal amidoboranes.     

In situ Raman spectroscopic study of the pressure induced structural changes in ammonia borane

The effect of static compression up to 65 GPa at ambient temperature on ammonia borane, BH(3)NH(3), has been investigated using in situ Raman spectroscopy in a diamond anvil cells. Two phase transitions were observed at approximately 12 GPa and previously not reported transition at 27 GPa. It was demonstrated that ammonia borane behaves differently under compression at quasi-hydrostatic and non-hydrostatic conditions. The ability of BH(3)NH(3) to generate second harmonic of the laser light observed up to 130 GPa suggests that the non-centrosymmetric point group symmetry is preserved in the material up to very high pressures.     

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.     

Hydroboration with pyridine borane at room temperature

Treatment of pyridine borane (Py.BH3) with iodine, bromine, or strong acids affords activated Py.BH2X complexes that are capable of hydroborating alkenes at room temperature. Evidence is presented for an unusual hydroboration mechanism involving leaving group displacement. In contrast to THF.BH3, hydroboration with Py.BH2I selectively affords the monoadducts. The crude hydroboration products are converted into synthetically useful potassium alkyltrifluoroborate salts upon treatment with methanolic KHF2.     

Electronic structures of methylated azaferrocenes and their borane adducts: photoelectron spectroscopy and electronic structure calculations

The preparation and crystal structure of a new 2,5-dimethylazaferrocene (DMAF) adduct of borane (DMAF-BH(3)) are reported. The electronic structures of 1,2,3,4,5-pentamethylazaferrocene (PMAF), its borane adduct 1,2,3,4,5-pentamethylazaferrocene-BH(3) (PMAF-BH(3)) and of DMAF-BH(3) have been studied by HeI-HeII UV photoelectron spectroscopy (UPS) and high-level DFT methods. UPS data reveal the influence of borane and methyl substitution on the electronic structures of azaferrocenes, which have been rationalized by inductive and hyperconjugative effects. UPS data also allow for the estimation of azaferrocene lone-pair stabilization upon coordination to borane, and suggest that pyridine is a slightly stronger sigma donor toward BH(3) than either azaferrocene studied. UV-Vis spectroscopic data for the radical cation of PMAF were also obtained and compared with the UPS data.     

Sodium magnesium amidoborane: the first mixed-metal amidoborane

The first example of a mixed-metal amidoborane Na(2)Mg(NH(2)BH(3))(4) has been successfully synthesized. It forms an ordered arrangement in cation coordinations, i.e., Mg(2+) bonds solely to N(-) and Na(+) coordinates only with BH(3). Compared to ammonia borane and monometallic amidoboranes, Na(2)Mg(NH(2)BH(3))(4) can release 8.4 wt% pure hydrogen with significantly less toxic gases.     

Molecular addition compounds. 17. Borane and chloroborane adducts with organic sulfides for hydroboration

The following sulfides have been examined as borane carriers in comparison with dimethyl sulfide and 1,4-oxathiane: tert-butyl methyl sulfide, isoamyl methyl sulfide, ethyl isoamyl sulfide, tert-butyl isoamyl sulfide, diisoamyl sulfide, tetrahydrothiophene, tetrahydro-thiopyran, thioanisole, 3-ethylthiotetrahydrofuran, bis(3-tetrahydrofuryl) sulfide, and bis(2-methoxyethyl) sulfide. Their complexing ability toward borane increases in the following order: thioanisole < ether-sulfides < dialkyl sulfides < dimethyl sulfide. Borane adducts of the sulfides are liquids above 0 degrees C. The thioanisole adduct loses diborane at room temperature. The reactivity of the adducts toward 1-octene increases in the reversed order of the complexing ability of the sulfides. Diisoamyl sulfide has a mild, ethereal, agreeable aroma, its synthesis is economical and the borane adduct, 4.2 M in BH3, is stable over prolonged periods at room temperature. The sulfide can be recovered from hydroboration-oxidation products by distillation. Consequently, diisoamyl sulfide is a new promising borane carrier. Bis(2-methoxyethyl) sulfide, easily synthesized from the low cost thiodiethanol, is three times more soluble in water than 1,4-oxathiane. Its borane adduct is 6.0 M in BH3 and can substitute for more expensive borane-1,4-oxathiane in hydroboration reactions. Applications of these new borane adducts in the synthesis of mono- and dichloroborane adducts was also studied. The equilibrium ratios observed for the new chloroborane adducts were similar to that observed for dimethyl sulfide adducts. However, the hydroboration of 1-octene with these new chloroborane adducts are much faster than the corresponding adducts of dimethyl sulfide, which are currently used extensively.     

Effect of conformational control of chiral oxazaborolidine by π-π stacking interaction of a pentafluorophenyl group toward asymmetric borane reduction

A pentafluorophenyl group can act as a stereo-controlling group in oxazaborolidine-catalyzed asymmetric borane reduction through intramolecular π-π stacking interaction with a phenyl group. The intramolecular π-π interaction in oxazaborolidine bearing pentafluorophenyl group is confirmed by calculations and ¹H NMR study. The interaction affects the enantioselectivity of the asymmetric reduction of acetophenone while the extent is small.