Cyanohydrins as substrates in benzoin condensation. : Regiocontrolled mixed benzoin condensation

O-Benzoylated cyanohydrins of aromatic aldehydes have been used as one of the substrates in the benzoin condensation. They were treated with aromatio aldehydes under phase-transfer conditions to result in benzoin benzoates. By this method aldehydes which fail to undergo condensation using traditional conditions could be converted into benzoins. By the Umpolung of reactivity of the pertinent aldehyde it was possible to prepare both isomeric unsymmetrical benzoins, including the thermodynamically less stable ones.     

Kinetics of the thiazolium ion-catalyzed benzoin condensation.

The formation of benzoin (Ph-CHOH-CO-Ph) from two molecules of benzaldehyde, catalyzed by 3-benzyl-5-(2-hydroxyethyl)-4-methylthiazolium bromide in methanol buffered with Et(3)N/Et(3)NH(+)Cl(-) has been studied. Initial-rate studies at various concentrations of PhCHO (0.1-1.7 M) showed that the reaction is close to being first order in PhCHO. Following the reaction in deuteriomethanol, (1)H NMR spectroscopy allowed rate constants for all three kinetically significant steps to be determined. These show that all three steps are partially rate-determining. A normal deuterium kinetic isotope effect for the overall reaction (k(H)/k(D) approximately 3.4) is observed using PhCDO, and a large inverse solvent isotope effect (k(D)/k(H) approximately 5.9) is observed using deuteriomethanol, consistent with the kinetic scheme presented here.     

Imidazole based solid-supported catalysts for the benzoin condensation

New polymer-supported imidazolium salts have been synthesised, and their utility as pre-catalysts in the benzoin reaction has been demonstrated.     

胶团噻唑衍生物对安息香不对称合成反应的催化作用

通过α-(1-萘基)-N-硫代甲酰乙胺与卤代酮反应制得六个光学活性的4-烷基-3-α-(1-萘基)乙基噻唑溴化物烷基碳链长=1,2,7.11,15,21). 将其用于催化水溶液中的安息香缩合反应, 所得产物收率约20-30%具有较高的光学纯度(47-57%). 在各种缓冲溶液中测定了S(+)-4-甲基-3-α-(1-萘基)乙基噻唑氯化物(Ta)的胶团性质和由它催化的不对称安息香缩合反应. 临界胶团浓度(cmc)证明(Ta)在反应中确以胶团形式催化. 在硼砂溶液中, 安息香的收率高达61%, 光学纯度23.6%.     

Neat benzoin condensation in recyclable room-temperature ionic liquids under ultrasonic activation

A new method for the benzoin condensation is proposed in imidazolium-based ionic liquids as solvents/catalysts. The use of a 30 kHz ultrasonic irradiation as activation method provides a quantitative yield of benzoin in 1-octyl-3-methyl-imidazolium bromide without degrading the ionic medium which can be-reused at least three times with no loss of activity.     

Asymmetric benzoin condensation promoted by chiral triazolium precatalyst bearing a pyridine moiety

Chiral triazolium salts bearing a pyridine ring were developed as N-heterocyclic carbene precursors. In the presence of the chiral triazolium salt and a base, the catalytic asymmetric benzoin condensation proceeded to afford the product in high level of chemical yield and enantioselectivity. A wide range of aromatic aldehydes were applicable to this reaction.     

Copper(II) complexes with condensation products of aminonaphthalene and benzoin derivatives

New copper(II) complexes with the products of condensation of benzoin with monopotassium 1-amino-8-hydroxy-2,4-naphthalenedisulfonate and 1,8-diaminonaphthalene (complexes I and II, respectively) have been synthesized. The compounds are identified and characterized by elemental analysis; X-ray powder diffraction; thermogravimetry, and electric conductivity data; magnetic susceptibility measurements; and IR, EPR, diffuse reflectance; EXAFS spectroscopy. The geometry and stability of the stereo isomers of complex I are theoretically studied with the use of the molecular mechanics method and semiempirical quantum-chemical calculations. The compositions of the inner spheres of complexes I and II and their coordination polyhedra—a distorted planar square (I) and a distorted octahedron (II)—are determined using different physicochemical methods.     

The importance of four-membered NHCs in stabilizing Breslow intermediates on benzoin condensation pathway

N-heterocyclic carbenes (NHCs) have been established to be effective organocatalysts for facilitating the benzoin condensation and many other reactions. These reactions involve the formation of a Breslow intermediate (BI), which exhibits umpolung chemistry. To facilitate organocatalysis, several new cyclic carbenes are being introduced, four-membered NHCs are of special interest. Whether these NHCs can exhibit catalytic influence or not, can be evaluated by exploring the potential energy surface (PES) of the benzoin condensation reaction. Quantum chemical analysis has been carried out to compare the PES of these four-membered NHCs with that of standard five-membered NHCs to explore their catalytic ability. The barrier for the first step of the reaction for the formation of BI is comparable in all the cases. But the barrier for the second step of the reaction leading to the benzoin formation from BI is estimated to be very high for the four membered NHCs. These results indicate that the probability of identifying and isolating the BI is very high in comparison to the completion of benzoin condensation reaction in the case of the four-membered NHCs.     

Facile route to benzils from aldehydes via NHC-catalyzed benzoin condensation under metal-free conditions

A simple and efficient one-pot procedure for the synthesis of α-diketones from aldehydes via benzoin condensation under the influence of a catalytic amount of azolium salt combined with DBU has been developed. Thus, aldehyde was allowed to react with the azolium salt/DBU catalytic system at room temperature, and then the reaction mixture was heated to 70 °C under air atmosphere to afford the corresponding 1,2-diketone in good yield. This would be an efficient alternative method of synthesizing α-diketones from aldehydes under metal-free conditions.     

An investigation of the catalytic potential of mono- and dicationic imidazolium N-heterocyclic carbenes in the benzoin condensation

The catalytic potential of imidazolium salts in the benzoin condensation was investigated. Various aromatic aldehydes were tested in the benzoin condensation under the optimised protocol to afford α-hydroxyketones using N-heterocyclic carbenes derived from mono- and dicationic imidazolium salts. The products were obtained in good yields within short reaction times. Dicationic imidazolium salts with a long aliphatic chain between the imidazole rings were found to be more effective pre-catalysts for the benzoin condensation in comparison to the corresponding monocationic salts having the same aliphatic chain length.     

Catalytic intramolecular crossed aldehyde--ketone benzoin reactions: a novel synthesis of functionalized preanthraquinones

Stereochemically and functionally rich polycyclic compounds are obtained by the first crossed aldehyde-ketone benzoin reaction in excellent yield under mild reaction conditions (5-20 mol % thiazolium salt, 10-70 mol % DBU, tBuOH, 40 degrees C, 30 min). This novel catalytic methodology offers a convenient approach to sophisticated molecular architectures useful for the stereocontrolled construction of polycyclic compounds as well as the fully regiocontrolled synthesis of anthra- and naphthoquinones.     

Chromogenic oxazines for cyanide detection

[reaction: see text] We have designed two heterocyclic compounds for the colorimetric detection of cyanide. The skeleton of both molecules fuses a benzooxazine ring to an indoline fragment and can be assembled efficiently in three synthetic steps starting from commercial precursors. The two compounds differ in the nature of the substituent on the carbon atom at the junction of the fused heterocycles, which can be either a methyl or a phenyl group. In the presence of cyanide, both molecules are converted quantitatively into cyanoamines with the concomitant appearance of an intense band in the visible region of the absorption spectrum. The developing absorption is a result of the opening of the benzooxazine ring with the formation of a 4-nitrophenylazophenolate chromophore. Nuclear magnetic resonance spectroscopy and X-ray crystallographic analyses demonstrate that the covalent attachment of a cyanide anion to the indoline fragment is responsible for these transformations. The chromogenic process is particularly fast for the methyl-substituted oxazine and can be exploited to detect micromolar concentrations of cyanide in water. Furthermore, the colorimetric response of this compound to cyanide does not suffer the interference of the halide anions, which instead are known to complicate the detection of cyanide in conventional sensing protocols. Thus, our mechanism and compounds for the colorimetric identification of cyanide can lead to the development of practical strategies for the convenient determination of this toxic anion in aqueous environments.     

Tandem conjugate cyanide addition-Dieckmann condensation in the synthesis of the ABCD-ring system of lactonamycin

An efficient synthesis of the ABCD-ring system of lactonamycin (1) is reported in this Letter. The key step is the tandem cyanide conjugate addition-Dieckmann condensation of alkyne 17 to afford a fully functionalized anthracene. Selective reduction of the cyano group with subsequent lactam formation affords the tetracyclic core of lactonamycin 19. [reaction: see text]     

Catalytic condensation of ethyl cyclopentanonecarboxylate with Schiff bases

Ethyl cyclopentanonecarboxylate reacts with Schiff bases to give amino keto esters, from which 5,6-benzoquinoline derivatives are synthesized in the presence of hydrochloric acid. Compounds of the enamine type are obtained in the presence of acetic acid.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, pp. 683–688, May, 1973.     

Amperometric method for on-line cyanide detection

An amperometric method for the detection of cyanide has been developed based on the anodic dissolution of gold in solutions containing cyanide. The method is characterized by a cyclic regeneration of the electrode surface. During the regeneration, the surface layers on the electrode are dissolved. By selection of suitable measuring potentials for the cyanide, linearity can be shown to exist between current and cyanide concentration over the concentration ranges from 0 to 10 mg/l and 10 to 100 mg/l. The lowest detection level is about 0.05 mg CN^–/l. Determinations in the concentration range investigated are possible at a measuring potential of E=270 mV. The method is suitable for detecting the end point during the decontamination of cyanide-containing sewage. The influence on the results of pH, temperature, flow rate, heavy metal content, type of decontamination agent and the presence of brighteners is discussed.     

Efficient and mild benzoin condensation reaction catalyzed by simple 1-N-alkyl-3-methylimidazolium salts

The benzoin reaction, catalyzed by simple 1-N-alkyl-3-methylimidazolium salt-based ionic liquid via carbene intermediate, to give the α-hydroxyl ketone proceeds in CH₂Cl₂ under mild conditions.     

Ultramicro detection of cyanide and cyano compounds

Summary Ultramicro capillary methods for testing of cyanides, thiocyanates, hexacyanoferrates(II) and (III) are proposed. Traces of any of them can be detected in the presence of the others. A mixture containing 4-(2-pyridilazo)-resorcinol and mercuric nitrate has been found to be a very sensitive reagent for hydrogen cyanide.A capillary technique is described for the azide-iodine test.

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Zusammenfassung Spuren von Cyanid, Thiocyanat, Hexacyanoferrat(II) und (III) können mit Hilfe der vorgeschlagenen Kapillarmethode nebeneinander nachgewiesen werden. Eine Mischung von 4-(2-Pyridylazo)-resorcin (PAR) und Quecksilber-nitrat dient als sehr empfindliches Reagens für Blausäure. Eine Kapillar-methode für die Jod-Azid-Reaktion wurde beschrieben.

     

Some derivatives of benzoin

Extension of the reaction time of benzoin (I) with anthranilamide (II) gave 2,3,5,6-tetraphenyl-pyrazine (IV) in addition to the expected benzamide III. Condensation of I with II in the presence of ammonium acetate or ammonium formate yielded IV, and IV with 4-quinazolinone (VII), respectively. Reaction of I with ammonium acetate only led to IV and 2,3,5,6-tetraphenylpyrrole (VIII). When I was heated with 2-aminothiazole, benzil, desoxybenzoin, and VIII were formed. A mechanism has been extended to explain the formation of IV and VIII.     

New methods for the detection of cyanide based on displacement of the glutathione ligand of glutathionylcobalamin by cyanide

Glutathionylcobalamin (GSCbl) is a vitamin B₁₂ derivative that contains glutathione as the upper axial ligand to cobalt via a Co–S bond. In the present study, we discovered that cyanide reacted with GSCbl, generating cyanocobalamin (CNCbl) and reduced glutathione (GSH) via dicyanocobalamin (diCNCbl) intermediate. This reaction was induced specifically by the nucleophilic attack of cyanide anion displacing the glutathione ligand of GSCbl. Based on the reaction of GSCbl with cyanide, we developed new methods for the detection of cyanide. The reaction intermediate, violet-coloured diCNCbl, could be applied for naked eye detection of cyanide and the detection limit was estimated to be as low as 520 μg L^?1 (20 μM) at pH = 10.0. The reaction product, CNCbl, could be applied for a spectrophotometric quantitative determination of cyanide with a detection limit of 26 μg L^?1 (1.0 μM) at pH = 9.0 and a linear range of 26–520 μg L^?1 (1.0–50 μM). In addition, the other reaction product, GSH, could be applied for a fluorometric quantitative determination of cyanide with a detection limit of 31 μg L^?1 (1.2 μM) at pH = 9.0 and a linear range of 31–520 μg L^?1 (1.2–20 μM). These new GSCbl-based methods are simple, highly specific and sensitive with great applicability for the detection of cyanide in biological and non-biological samples.