Au(I)/Ag(I)-catalyzed annulation of sugar aldehyde tethered with 3-phenylprop-2-yn-1-yl ether with aryl amines for the pyrano[4,3-b]quinoline derivatives

Aryl amines undergo smooth annulation with O-phenylpropynyl sugar aldehyde in the presence of the Ph₃PAuCl (10 mol %)/AgSbF₆ (10 mol %) catalytic system to afford the corresponding tetrahydro-3aH-spiro[[1,3]dioxolo[4″,5″:4′,5′]furo[3′,2′:5,6]pyrano[4,3-b]quinoline-2,1′-cyclohexane] derivatives in good yields and selectivity.     

An unprecedented highly efficient solvent-free oxidation of alkynes to α,β-acetylenic ketones with tert-butyl hydroperoxide catalyzed by water-soluble copper complex

The catalytic system composed of CuCl₂ and 2,2′-biquinoline-4,4′-dicarboxylic acid dipotassium salt (BQC) was found to be highly efficient for the selective α-oxidation of internal alkynes to the corresponding α,β-acetylenic ketones, with aqueous tert-butyl hydroperoxide under mild conditions. For the first time, full conversions of alkynes were reached with excellent selectivities, and propargylic tert-butylperoxy ethers were observed and suggested as the reaction intermediates. In the case of terminal alkynes, the oxidations are sluggish and low yields ranging from 32% to 40% were obtained.     

Enantioselective addition of diethylzinc to aromatic aldehydes catalyzed by Ti(IV) complexes of C2-symmetrical chiral BINOL derivatives

Enantioselective addition of diethylzinc to a series of aromatic aldehydes is developed using new chiral C₂-symmetric ligand (S)-2,2′-(1,1′-binaphthyl-2,2′-diylbis(oxy))bis(methylene)bis(4-nitrophenol) (S)-2b. The catalytic system employing 10 mol % of (S)-2b and 120 mol % of Ti(OiPr)₄ was found to promote the addition of diethylzinc to a wide range of aromatic aldehydes with electron-donating and electron-withdrawing substituents, giving up to 89% ee and up to 95% yield of the corresponding secondary alcohol under mild conditions.     

A new and highly efficient water-soluble copper complex for the oxidation of secondary 1-heteroaryl alcohols by tert-butyl hydroperoxide

The water-soluble copper complex generated in situ from CuCl₂ and 2,2′-biquinoline-4,4′-dicarboxylic acid dipotassium salt (BQC), has been revealed as a highly efficient and selective catalyst for the oxidation of secondary 1-heteroaryl alcohols to the corresponding heteroaromatic ketones with aqueous tert-butyl hydroperoxide, under mild conditions. The catalytic system is compatible with different heterocycles such as pyridines, pyrroles, indoles, thiophens, furans, thiazoles, and imidazoles.     

Enantioselective synthesis of (2S,3′R,7′Z)-N-(3′-hydroxy-7′-tetradecenoyl)-homoserine lactone

A concise enantioselective total synthesis of (2S,3′R,7′Z)-N-(3′-hydroxy-7′-tetradecenoyl)-homoserine lactone is described. Key feature of this protocol is a catalytic asymmetric hydrogenation and a prophenol-zinc-catalyzed diazo addition to imine reaction as genesis of chirality. Moreover, flexibility is built in the synthesis to generate enantioenriched analogs using catalytic amount of enantioenriched C₂-symmetric ligands.     

Asymmetric addition of phenylacetylene to aldehydes catalyzed by soluble optically active polybinaphthols ligand

A chiral polymer ligand was synthesized by the polymerization of (S)-5,5′-dibromo-6,6′-dibutyl-2,2′-binaphthol (S-M-1) with (S)-2,2′-bishexyloxy-1,1′-binaphthyl-6,6′-boronic acid (S-M-2) via Pd-catalyzed Suzuki reaction. The application of the chiral polymer ligand to the asymmetric addition of phenylethynyl zinc to various aldehydes has been studied. The results show that the soluble chiral polybinaphthols ligand in combination with Et₂Zn and Ti(OⁱPr)₄ can exhibit excellent enantioselectivity for phenylacetylene addition to both aromatic and aliphatic aldehydes. The catalytically active center of the repeating unit S-1 used as a catalyst produced the opposite configuration of the propargylic alcohols to that of S-1, on the contrary, the chiral polymer gave the same configuration as the optically active binaphthol moiety of the polybinaphthols ligand. Moreover, the chiral polymer ligand can be easily recovered and reused without loss of catalytic activity as well as enantioselectivity.     

Effects of solvent and base on the palladium-catalyzed amination: PdCl2(Ph3P)2/Ph3P-catalyzed selective arylation of primary anilines with aryl bromides

A readily accessible catalytic system, PdCl₂(Ph₃P)₂/Ph₃P, was developed for the selective arylation of primary anilines with aryl bromides. The strong influence of solvents and bases on the catalytic activity was observed. In refluxing o-xylene, triphenylphosphine shows high efficiency for Pd-catalyzed intermolecular amination reactions. By changing the bases, mono- and diarylation of primary amines could be selectively achieved in high yields. Moreover, the catalytic system showed good toleration for the steric hindrance of anilines. A series of N,N,N′,N′-tetraaryl-1,1′-biphenyl-4,4′-diamines, important intermediates of OLED hole transport materials, were synthesized facilely via coupling reactions between 4,4′-diaminobiphenyls and aryl bromides.     

A quantum chemical study on the mechanism of chiral N-oxides-catalyzed Strecker reaction

The mechanism for the Strecker reaction of silyl cyanide (H₃SiCN) and benzaldehyde N-methylimine (PhCHNCH₃) catalyzed by chiral 3,3′-dimethyl-2,2′-bipyridine N,N′-dioxide was investigated using the density functional theory (DFT) at the B3LYP/6-31G* level. The calculations revealed that the non-catalyzed reaction proceeded in a concerted way via a five-membered ring transition state, while the catalytic one occurred stepwisely via a hexacoordinate hypervalent silicate intermediate. It was predicted that both non-catalyzed and catalytic Strecker reactions involved two competitive reaction pathways, that is, addition followed by isomerization or isomerization followed by addition. The calculations indicated that two reaction pathways were comparable for both non-catalyzed and catalytic Strecker reactions. In the catalytic reaction, the strong electron donor (N-O) of chiral N-oxide played an important role in enhancing the reactivity and nucleophilicity of H₃SiCN by coordinating O atom to the Si atom of H₃SiCN. Chiral N-oxide could be used as a good catalyst for the reaction, which was in agreement with the experimental observations.     

DAB-Cy as an inexpensive and effective ligand for palladium-catalyzed homocoupling reaction of aryl halides

A novel catalytic system of PdCl₂(CH₃CN)₂ with N,N′-dicyclohexyl-1,4-diazabutadiene (DAB-Cy) ligand was successfully used in reductive coupling of aryl halides.     

Catalytic photocarboxylation of 1,1-diphenylethylene with N,N,N′,N′-tetramethylbenzidine and carbon dioxide

Photocarboxylation of 1,1-diphenylethylene with N,N,N′,N′-tetramethylbenzidine (TMB) in MeCN under bubbling of CO₂ proceeded with high catalytic efficiency, giving 3,3-diphenylacrylic acid (DPA) and 3-hydroxy-3,3-diphenylpropionic acid (20). The turnover number (TON=(DPA+20)/TMB) reached 17. Similarly, 1-phenyl-1-cyclohexene yielded cis-2-acetamido-2-phenylcyclohexanecarboxylic acid with TON 5.9. As compared with related N,N-dimethylaniline derivatives, TMB is more resistant to photodecomposition, has the much larger absorbance in the S₀→S₁ transition, and has the lower quenching efficiency by CO₂. Probably these factors are partly responsible for the high TON observed for TMB.     

γ-Selective allylic substitution reaction with Grignard reagents catalyzed by copper N-heterocyclic carbene complexes and its application to enantioselective synthesis

The reaction of allylic compounds with alkyl Grignard reagents in the presence of a catalytic amount of copper N-heterocyclic carbene (NHC) complexes proceeded predominantly in an S_N2′ reaction pathway to give γ-substituted product in excellent yield. The method was applied to asymmetric reaction by using optically active NHC ligands.     

Copper-based reverse ATRP process of styrene in mixed solvents

Xylene/N,N-dimethylformamide (DMF) and xylene/ethanol were employed as mixed solvents, respectively, for the reverse atom transfer radical polymerization (R-ATRP) of styrene with the azobisisobutyronitrile (AIBN)/CuBr₂/N,N,N′,N′′,N′′-pentamethyldiethylenetriamine (PMDETA) initiating system. With a limited amount of DMF added in, CuBr₂/PMDETA complex could dissolve well in the reaction system, so the control of polymerization was enhanced compared with the one in which simplex xylene was used as solvent. But the polarity of DMF leaded kinetics to deviation from first order. Ethanol could also improve the solubility of catalyst and be scavenged quickly by argon at 110°, therefore the impact of polarity of solvent on kinetics was negligible. Induction periods were not observed here indicating rapidly establishment of equilibrium between Cu(I) and Cu(II). This method that adding a little amount of polar solvent with low boiling point into non-polar solvent gives a new way to solve the problem of poor solubility of the catalyst in R-ATRP.     

Butylene linked palladium N-heterocyclic carbene complexes: Synthesis and catalytic properties

New bis(NHC)-Pd complexes were synthesized and characterized by elemental analysis, ¹H NMR, ¹³C NMR, and IR spectroscopy. The reaction of Pd(OAc)₂ and bis(benzimidazolium) salts in DMSO gave the monomeric palladium complex in which the N-heterocyclic carbene was bound to the metal centre. The crystal and molecular structure of the cis-dibromo{1,1′-di[2,3,4,5,6-pentamethylbenzyl]-3,3′-butylenedibenzimidazol-2,2′-diylidene}-palladium(II) complex was determined by single-crystal X-ray diffraction. The activity of the Pd(II) complexes in the direct arylation of benzothiazole with arylbromides was investigated. A preliminary catalytic study showed that these bis(NHC)-Pd complexes were highly active in the direct arylation of benzothiazole with arylbromides.     

Flow-injection spectrophotometry of vanadium by catalysis of the bromate oxidation of N,N'-bis(2-hydroxyl-3-sulfopropyl)-tolidine

A highly sensitive flow-injection method is proposed for the catalytic determination of vanadium(V) at sub-nanogram per milliliter levels using a new indicator reaction. The method is based on the catalytic effect of vanadium(V) on the bromate oxidation of N,N′-bis(2-hydroxyl-3-sulfopropyl)-tolidine. 1,2-Dihydroxybenzene-3,5-disulfonate was used as an activator in the vanadium(V)-catalyzed reaction and significantly enhanced the sensitivity of the method. Vanadium(V) in the range 0.01-3.0 ng ml⁻¹ was easily determined with sampling rate of about 30 h⁻¹. Vanadium(IV) could be also determined. The limit of detection (S/N=3) was 0.008 ng ml⁻¹ and the relative standard deviations were 1.4 and 1.6% for ten determinations of 0.2 ng ml⁻¹ vanadium(IV) and vanadium(V), respectively. Interferences from metal ions could be suppressed by the addition of ethylenediamine-N,N,N′,N′-tetrakis(methylenephosphonic acid) as a masking agent. The proposed method was successfully applied to the determination of vanadium in water samples.     

Synthesis and optical properties of conjugated N,N-dimethyl and thienyl end-capped 2,5-(arylethynyl)thiophene oligomer structures

End-capped (N,N-dimethylaminophenyl) and 2′-thienylethynyl 2,5-thiophene oligomer structures were synthesized by heterocoupling between the terminal acetylenes such as: p-(N,N-dimethylaminophenyl)ethyne (3) [or 1-(p-(N,N-dimethylaminophenyl)-2-p-(ethynylphenyl)ethyne, 4]; p-(β-ethenyl-2′-thienyl)phenylethyne (E-9) [or p-(β-ethynyl-2′-thienyl)phenylethyne, 11], and 2,5-diiodothiophene, catalyzed by the Cl₂Pd(PPh₃)₂/CuI system, in good to excellent yields. The 2,5-di[(3′,5′-di(trimethylsilylethynyl)phenyl]_x-1-ethynyl]thiophene oligomers were prepared by heterocoupling between 3′,5′-di[(trimethylsilylethynyl)phenyl]_x-1-ethyne (n = 0-2) terminal acetylenes and 2,5-diiodothiophene, in excellent yields. The terminal acetylenes were efficiently prepared by a specific protection-deprotection methodology. All the ethynylphenyl compounds obtained show fluorescence radiation emission, with a bathochromic shift of the wavelength that increases with the chain conjugation.     

Theoretical studies of iridium-mediated tautomerization of substituted pyridines

Room temperature reaction of [Ir(COD)₂]BF₄ (COD = 1,5-cyclooctadiene) and amide-tethered or simple 2,3′-bipyridyls gave iridium(I) complexes bearing chelating protic pyridylidenes. This protic pyridylidene tautomer is stabilized by both chelation effect and by hydrogen bonding. The mechanistic details of this tautomerization of N-heterocycles to N-heterocyclic carbenes (NHCs) were investigated using the density functional theory (DFT). DFT studies suggested that cyclometalation of 2,3′-bipyridyls took place to give an iridium(III) hydride, which subsequently undergoes formal 1,3-hydrogen shift from the iridium to the pyridyl nitrogen atom. Two possible mechanisms of this formal 1,3-hydrogen shift process have been examined: the β-insertion of the hydride into an olefin followed by proton abstraction and the water-assisted proton transfer via a cyclic transition state. The latter mechanism is strongly favored in the presence of a catalytic amount of water, and this mechanism is applicable to the tautomerization of both amide-tethered and amide-free 2,3′-bipyridyls.     

Asymmetric addition of trimethylsilylcyanide to N-benzylimines catalyzed by recyclable chiral dimeric V(V) salen complex

Chiral dimeric vanadium (V) salen complex (10 mol%) derived from 5,5-Methylene di-[(S,S)-{N-(3-tert-butyl salicylidine)-N′-(3′,5′-di-tert-butyl salicylidene)]-1,2-cyclohexanediamine] with vanadyl suphate followed by auto oxidation was used as efficient catalyst for enantioselective Strecker reaction of N-benzylimines with TMSCN at −30 °C. Excellent yield (92%) of α-aminonitrile and high chiral induction was achieved (ee up to 94%) in case of 2-methoxy substituted N-benzylimines in 10 h. The catalytic system worked well up to four cycles with retention of enantioselectivity.     

Highly efficient and selective biocatalytic acylation studies on triazolylsugars

Different acid anhydrides (of C₂ to C₇ aliphatic fatty acids and benzoic acid) have been used to study the selective acylation of primary/secondary hydroxyl groups in 2-phenyl-4-(d-threo-1′,2′,3′-trihydroxypropyl)-2H-1,2,3-triazole, 2-phenyl-4-(d-erythro-1′,2′,3′-trihydroxypropyl)-2H-1,2,3-triazole, 2-phenyl-4-(d-arabino-1′,2′,3′,4′-tetrahydroxybutyl)-2H-1,2,3-triazole and 2-phenyl-4-(d-lyxo-1′,2′,3′,4′-tetrahydroxybutyl)-2H-1,2,3-triazole in the presence of Candida antarctica lipase B in diisopropyl ether. Among the different acid anhydrides, butanoic anhydride was found to be the most efficient acylating agent (for butanoylation); for acetylation, vinyl acetate gave the best results. The reactions with both these acylating agents were highly selective and efficient yielding exclusively the monoacylated products in 95-99% yields in 1-5 h.     

New optically active organoantimony (BINASb) and bismuth (BINABi) compounds comprising a 1,1′-binaphthyl core: synthesis and their use in transition metal-catalyzed asymmetric hydrosilylation of ketones

Racemic 2,2′-bis[diarylstibano]-1,1′-binaphthyls [(±)-BINASbs] and 2,2′-bis[di(p-tolyl)bismuthano]-1,1′-binaphthyl [(±)-BINABi], which are the antimony and bismuth congeners of BINAP, have been prepared from 2,2′-dibromo-1,1′-binaphthyl (DBBN) via 2,2′-dilithio-1,1′-binaphthyl intermediate by treatment with the appropriate metal halides [(p-Tol)₂SbBr, Ph₂SbBr and (p-Tol)₂BiCl]. The optical resolution of the (±)-BINASbs could be achieved via the separation of a mixture of the diastereomeric Pd-complexes derived from the reaction of (±)-BINASbs with di-μ-chlorobis{(S)-2-[1-(dimethylamino)-ethyl]phenyl-C¹,N}dipalladium(II). Optically active (R)-BINASb and (R)-BINABi could be also obtained from optically active (R)-DBBN by the same procedure. The enantiopure BINASbs have been shown to be effective chiral ligands for the rhodium-catalyzed asymmetric hydrosilylation of ketones.     

Synthesis and structural characterization of a dichloro zinc complex of N,N′-bis-(2,6-dichloro-benzyl)-(R,R)-1,2-diaminocyclohexane: Application to ring opening polymerization of rac-lactide

A novel dichloro zinc complex (L¹)ZnCl₂, where L¹ is N,N′-bis-(2,6-dichloro-benzyl)-(R,R)-1,2-diaminocyclohexane, has been synthesized and characterized. The dimethyl derivatives, generated in situ from the well characterized dichloro zinc complexes (L¹)ZnCl₂ and (L²)ZnCl₂, where L² is N,N′-bis-(benzyl)-(R,R)-1,2-diaminocyclohexane, were employed as initiators for the ring opening polymerization (ROP) of rac-lactide (rac-LA). The complexes were found to be highly efficient initiators yielding the polylactide (PLA) with a narrow molecular weight distribution. The catalytic activity and heterotactic selectivity of the Zn(II) complexes were affected by the substituents on the phenyl groups of benzyl moieties in (R,R)-1,2-diaminocyclohexane. The dimethyl derivative of (L²)ZnCl₂ produced highly stereocontrolled PLA with P_r = 0.75 at −25 °C.