Catalytic asymmetric cyclopropanation at a chiral platform

Novel chiral Robson-type macrocyclic complexes M(2)-L [where M = Mn(II), Mn(III), Co(II) and Co(III) and L denotes tetra-Schiff base chiral ligands, L1 or L2] have been synthesized by metal template condensation of 2,6-diformyl-4-methyl-phenol, with 1R,2R-diaminocyclohexane (L1) or 1R,2R-diphenylethylenediamine (L2). The dinuclear Co(II) and Co(III) complexes catalyze asymmetric cyclopropanation of styrene with diazoacetate cooperatively and with high enantioselectivity.     

Catalytic asymmetric synthesis of chiral phosphanes

Chiral phosphanes, important ligands for metal-catalyzed asymmetric syntheses, are often prepared with compounds from the chiral pool, by using stoichiometric chiral auxiliaries, or by resolution. In some cases, this class of valuable compounds can be prepared more efficiently by catalytic asymmetric synthesis. This Concepts article presents an overview of these synthetic methods, including recent advances in catalysis by metal complexes, biocatalysis, organocatalysis, and ligand-accelerated catalysis.     

Catalytic asymmetric protonation of lithium enolates using amino acid derivatives as chiral proton sources

[reaction: see text] Asymmetric protonation of lithium enolates was examined using commercially available amino acid derivatives as chiral proton sources. Among the amino acid derivatives tested, Nbeta-l-aspartyl-l-phenylalanine methyl ester was found to cause significant asymmetric induction in the protonation of lithium enolates. The enantiomeric excess (up to 88% ee) of the products obtained in the presence of a catalytic amount of the chiral proton source was higher than those obtained in the stoichiometric reaction.     

Catalytic asymmetric Michael reactions using a chiral rhodium complex

Catalytic asymmetric Michael reaction of β-keto esters and methyl vinyl ketone was achieved using a chiral diamine-based Rh complex to give the Michael adducts in up to 75% e.e.     

NMR studies of a ternary complex reagent of lithium ester enolate, chiral diether, and lithium diisopropylamide in an asymmetric Michael reaction

A ternary complex reagent of lithium ester enolate-chiral diether-lithium diisopropylamide was formed in an equimolar mixture of these reagents in toluene based on low-temperature NMR spectroscopy. The use of [⁶Li,¹⁵N]-lithium diisopropylamide as a lithiodeprotonation and complexing reagent produced two sets of doublet peaks in ⁶Li NMR of a 1:1:1 mixture of lithium enolate-chiral diether-lithium diisopropylamide, indicating the formation of a ternary complex reagent.     

Catalytic asymmetric synthesis of chiral allylic esters

Trichloroacetimidate derivatives of prochiral (Z)-2-alken-1-ols react at room temperature with carboxylic acids to give chiral 3-acyloxy-1-alkenes in high enantiopurity in the presence of di-mu-acetatobis[(eta5-(S)-(pR)-2-(2'-(4'-methylethyl)oxazolinyl)cyclopentadienyl,1-C,3'-N)(eta4-tetraphenylcyclobutadiene)cobalt]dipalladium (COP-OAc) or its enantiomer. This reaction has broad scope, proceeds with predictable high stereoinduction, is accomplished at room temperature using high substrate concentrations and low catalyst loadings, and likely proceeds by a novel mechanism.     

Catalytic asymmetric synthesis of chiral allylic esters

A broadly useful catalytic enantioselective synthesis of branched allylic esters from prochiral (Z)-2-alkene-1-ols has been developed. The starting allylic alcohol is converted to its trichloroacetimidate intermediate by reaction with trichloroacetonitrile, either in situ or in a separate step, and this intermediate undergoes clean enantioselective S(N)2' substitution with a variety of carboxylic acids in the presence of the palladium(II) catalyst (R(p),S)-di-μ-acetatobis[(η(5)-2-(2'-(4'-methylethyl)oxazolinyl)cyclopentadienyl-1-C,3'-N)(η(4)-tetraphenylcyclobutadiene)cobalt]dipalladium, (R(p),S)-[COP-OAc](2), or its enantiomer. The scope and limitations of this useful catalytic asymmetric allylic esterification are defined.     

催化动力学光度法测定痕量锂

建立了测定痕量Li+的催化动力学光度法。在硼酸缓冲溶液(pH10.0)中,Li+能催化H2O2氧化苋菜红的褪色反应。在540nm波长处,所测得反应体系的吸光度的下降程度与Li+的浓度在0.1~100μg/10 mL间呈线性关系。Li+的检出限为0.3×10-10g/mL,相对标准偏差为1.5%~2.5%,回收率为97.2%~104.5%。方法可用于工业废水Li+检测。     

Catalytic enantioselective Meerwein-Eschenmoser Claisen rearrangement: asymmetric synthesis of allyl oxindoles

The first catalytic, enantioselective Meerwein-Eschenmoser Claisen rearrangement has been achieved. Palladium(II) BINAP or phosphinooxazoline catalysts were employed to generate oxindole products with 100% conversion and up to 92% ee.     

Catalytic asymmetric silane alcoholysis: practical access to chiral silanes

A new asymmetric synthesis of chiral silanes has been developed. Chiral phosphine-modified copper complexes catalyze the alcoholysis of prochiral dihydrosilanes with good to excellent stereoselectivity at silicon. The application of this methodology to the tandem silylformylation-allylsilylation of alkynes has been demonstrated.     

Practical asymmetric synthesis of the herbicide (S)-indanofan via lipase-catalyzed kinetic resolution of a diol and stereoselective acid-catalyzed hydrolysis of a chiral epoxide

Racemic indanofan [(+/-)-1] was efficiently converted to enantiopure (S)-indanofan [(S)-1] by a combination of enzymatic resolution and chemical inversion techniques. An additional important technique is the use of an o-xylene complex of a hemiketal (S)-3c as a precursor, which can be quantitatively converted to (S)-indanofan and easily purified by recrystallization from o-xylene.     

Catalytic asymmetric cyanosilylation of ketones by a chiral amino acid salt

In this Communication, we describe the first highly enantioselective cyanosilylation of ketones catalyzed by simple chiral amino acid salt. A broad range of aromatic, alpha,beta-unsaturated, heterocyclic ketones catalyzed by l-phenylglycine sodium salt gave the corresponding trimethylsilyl ethers of cyanohydrins in 75-96% yields with 86-97% enantioselectivities. A catalytic cycle based on experimental phenomena and studies has been proposed to explain the origin of this activation. The reaction employed commercially available and fully recyclable catalysts and involves a simple experimental procedure.     

A NMR and theoretical study of the aggregates between alkyllithium and chiral lithium amides: control of the topology through a single asymmetric center

The complexes between methyllithium and chiral 3-aminopyrrolidine (3-AP) lithium amides bearing a second asymmetric center on their lateral amino group were studied using multinuclear ((1)H, (6)Li, (13)C, (15)N) low-temperature NMR spectroscopies in tetrahydrofuran-d(8). The results indicate that lithium chelation forces the pyrrolidine ring of the 3-AP to adopt a norbornyl-like conformation and that robust 1:1 noncovalent complexes between methyllithium and 3-AP lithium amides form in the medium. A set of (1)H-(1)H and (1)H-(6)Li NMR cross-coupling correlations shows that the binding of methyllithium can take place along the "exo" or the "endo" face of this puckered structure, depending on the relative configuration of the lateral chiral group. This aggregation step renders the nitrogen of the 3-amino group chiral, the "exo" and "endo" topologies corresponding to the (S) and (R) configurations, respectively, of this atom. Density functional theory calculations show that the "exo" and "endo" arrangements are, for both diastereomers, almost isoenergetic even when solvent is taken into account. This result suggests that the formation of the mixed aggregates is under strict kinetic control. A relationship between the topology of these complexes and the sense of induction in the enantioselective alkylation of aromatic aldehydes by alkyllithiums is proposed.     

Manganese alkane complexes: an IR and NMR spectroscopic investigation

Manganese propane and manganese butane complexes derived from CpMn(CO)(3) were generated photochemically at 130-136 K with the alkane as solvent and characterized by FTIR spectroscopy and by (1)H NMR spectroscopy with in situ laser photolysis. Time-resolved IR spectroscopic measurements were performed at room temperature with the same laser wavelength. The ν(CO) bands in the IR spectra of the photoproducts in propane are shifted to low frequency with respect to CpMn(CO)(3), consistent with formation of CpMn(CO)(2)(propane). The (1)H NMR spectra conform to the criteria for alkane complexes: a high-field resonance for the η(2)-CH protons that shifts substantially on partial deuteration of the alkane and exhibits a coupling constant J(C-H) on (13)C-labeling of ca. 120 Hz. The NMR spectrum of each system exhibits two diagnostic product resonances in the high-field region for the η(2)-CH protons, corresponding to CpMn(CO)(2)(η(2)-C1-H-alkane) and CpMn(CO)(2)(η(2)-C2-H-alkane) isomers. Partial deuteration of the alkane at C1 results in characteristic strong isotopic perturbation of equilibrium of the η(2)-CH resonance of CpMn(CO)(2)(η(2)-C1-H-alkane). With propane-(13)C(1), the η(2)-CH resonance of CpMn(CO)(2)(η(2)-C1-H-alkane) isomer exhibits (13)C satellites with J(C-H) = 119 Hz. The corresponding resonance of CpMn(CO)(2)(η(2)-C2-H-alkane) is identified by use of propane-2,2-d(2). The lifetimes of the (η(2)-C1-H-alkane) isomers of the manganese complexes were determined by NMR spectroscopy as 22 ± 2 min at 134 K (propane) and 5.5 min at 136 K (butane). The corresponding spectra and lifetimes of the CpRe(CO)(2)(alkane) complexes were measured for reference (CpRe(CO)(2)(propane) lifetime ca. 60 min at 161 K; CpRe(CO)(2)(butane) 13 min at 171 K). The lifetimes determined by IR spectroscopy were similar to those determined by NMR spectroscopy, thereby supporting the assignments. These measurements extend the range of alkane complexes characterized by NMR spectroscopy from rhenium and rhodium derivatives to include less stable manganese derivatives.     

Experimental and theoretical investigation of epoxide quebrachitol derivatives through spectroscopic analysis

Two synthetic epoxide derivatives, important intermediates in organic synthesis, were obtained from L-quebrachitol, and their conformations were proposed based on spectroscopic analysis. Density functional theory (DFT) calculations of infrared and NMR spectra were shown to be reliable enough for organic chemistry applications. The observed structures were determined with the aid of the DFT spectroscopic data, stressing the relevance and utility of combined experimental/theoretical studies and also the usefulness of the (13)C NMR B3LYP/6-31G(d,p) calculations.     

Kinetic and NMR spectroscopic studies of chiral mixed sodium/lithium amides used for the deprotonation of cyclohexene oxide

The mixed-metal complex formed from n-butylsodium, n-butyllithium, and a chiral amino ether has been studied by NMR spectroscopy. Three different mixed-metal amides were used as chiral bases for the deprotonation of cyclohexene oxide. The selectivity and initial rate of reaction were compared for sodium-amido ethers, lithium-amido ethers, and mixtures of sodium and lithiumamido ethers in diethyl ether and tetrahydrofuran, respectively. The mixed sodium/lithium amides are more reactive than the single sodium and lithium amides, whereas the stereoselectivities are higher when lithium amides are used. The alkali-metal/gamma-amido ethers exhibit both higher initial reaction rates and stereoselectivities than their beta-amido ether analogues. NMR spectroscopic studies of mixtures of n-butylsodium (nBuNa), n-butyllithium (nBuLi), and the gamma-amino ethers in diethyl ether show the exclusive formation of dimeric mixed-metal amides. In diethyl ether, the lithium atom of the mixed-metal amide is internally coordinated and the sodium atom is exposed to solvent; however, in tetrahydrofuran, both metals are internally coordinated.     

Arsenate adsorption on ruthenium oxides: A spectroscopic and kinetic investigation

Arsenate adsorption on amorphous (RuO(2)1.1H(2)O) and crystalline (RuO(2)) ruthenium oxides was evaluated using spectroscopic and kinetic methods to elucidate the adsorption mechanism. Extended X-ray absorption fine structure spectroscopy (EXAFS) was used to determine the local coordination environment of adsorbed arsenate. Additionally, pressure-jump (p-jump) relaxation spectroscopy was used to investigate the kinetics of arsenate adsorption/desorption on ruthenium oxides. Chemical relaxations resulting from the induced pressure change were monitored via electrical conductivity detection. EXAFS data were collected for two initial arsenate solution concentrations, 3 and 33 mM at pH 5. The collected spectra indicated a similar coordination environment for arsenate adsorbed to RuO(2)1.1H(2)O for both arsenate concentrations. In contrast the EXAFS spectra of RuO(2) indicated differences in the local coordination environments for the crystalline material with increasing arsenate concentration. Data analysis indicated that both mono- and bidentate surfaces complexes were present on both RuO(2)1.1H(2)O and RuO(2). Relaxation spectra from the pressure-jump experiments of both ruthenium oxides resulted in a double relaxation event. Based on the relaxation spectra, a two step reaction mechanism for arsenate adsorption is proposed resulting in the formation of a bidentate surface complex. Analysis of the kinetic and spectroscopic data suggested that while there were two relaxation events, arsenate adsorbed to ruthenium oxide surfaces through both mono- and bidentate surface complexes.