DFT study on the preactivation reaction of a palladium catalyst precursor in phosphine-free Heck reactions

Abstract  The mechanism of the preactivation process of trans-dichlorobis(diethanolamine-N)palladium(II) complex is investigated using density functional theory. The role of diethanolamine (a solvent for the reaction in the absence of a strong base) and acetonitrile (solvent for the reaction in the presence of a strong base) is analyzed by using a discrete model. The Onsager model is applied to assess the effect of the bulk medium. Both models show that diethanolamine activates the complex and thus is a better suited solvent for the Heck reactions of the investigated complex. Graphical abstract        

Olefin epoxidation with tert-butyl hydroperoxide catalyzed by functionalized polymer-supported copper(II) Schiff base complex

Abstract  

A new polymer-supported Cu(II) Schiff base complex has been synthesized and characterized by elemental (including metal) analysis, FT-IR spectroscopy, UV–Vis diffuse reflectance spectroscopy, thermogravimetric analysis, and scanning electron microscopy. The catalytic performance of this complex was evaluated in the epoxidation of styrene in acetonitrile/N,N-dimethylformamide (9:1) mixture with 70% tert-butyl hydroperoxide as an oxidizing agent under liquid phase reaction conditions for selective synthesis of styrene oxide. Suitable reaction conditions have been optimized by considering the effects of various reaction parameters such as temperature, reaction time, solvent, oxidant, catalyst amount, and styrene to hydroperoxide molar ratio for the maximum conversion of styrene as well as selectivity of styrene oxide. We have also investigated the epoxidation reaction of various olefins under the optimized reaction conditions. Comparison between catalytic activities of the polymer-supported Cu(II) Schiff base complex and its homogeneous analogue showed that the polymer-supported catalyst was more active. This heterogeneous complex was reused for five times. The selectivity of the heterogeneous catalyst does not change even after five times of reusing.     

A new aspect of Heck catalyst formation

Abstract  

The mechanism of the formation of the active Pd(0) complex from trans-dichlorobis(diethanolamine-N)palladium(II) complex in the presence of strong base was investigated by using density functional theory (M06 method). Our investigation shows that in the basic environment trans-dichlorobis(diethanolamine-N)palladium(II) complex undergoes abstraction of the alcoholic proton, and coordination of alkoxide oxygen to palladium. The intermediate complex, in which hydrogen is coordinated to Pd, undergoes reductive elimination of HCl, yielding the catalytically active low ligated Pd(0) complex.     

A density functional theory investigation of the bromide oxidation mechanism by a vanadium bromoperoxidase model complex

Abstract  

Density functional theory has been used to study the mechanism of bromide oxidation by the oxo-peroxo complex K[VO(O₂)Hheida] (heida = N-(2-hydroxyethyl)iminodiacetic acid), which has the highest reported rate constant for bromide oxidation of any vanadium complex. Two possible mechanisms were explored, involving bromide attack on a protonated or unprotonated peroxo atom. The direct nucleophilic attack of bromide on a protonated peroxo begins the reaction, i.e. the protonated peroxo ligand is the active site of reaction. We examined five different transition states in the mechanism. Two transition states were found to have lower activation barriers. A reduction in the potential energy barriers, when calculated using the polarisable continuum model, indicates that with the involvement of acetonitrile as a solvent the transition states become more stable.     

Manganese(II), cobalts(II) and nickel(II) complexes of tris(2-pyridyl)phosphine and their catalytic activity toward oxidation of tetralin

Abstract  

Monomeric Mn²⁺, Co²⁺ and Ni²⁺ complexes of tris(2-pyridyl)phosphine (P(2-py)₃ were synthesized through the reaction of the hydrated metal(II) chlorides with P(2-py)₃ in near-quantitative yields. The solid-state structure of the Mn complex was determined by single-crystal X-ray diffraction. All three complexes were tested as homogeneous catalysts for the oxidation of tetralin to α-tetralone with tert-butyl hydroperoxide (TBHP) as oxidant. The influences of temperature, solvent, catalyst molar ratio and time of the reaction on the catalyzed reactions were investigated.     

Heck-type reaction of aryldiazonium silica sulfates

Abstract  

Aryldiazonium silica sulfates were employed as new and efficient reagents for Heck-type arylation reactions with styrene and methyl acrylate. The reactions were carried out in an organic solvent, in the presence of Pd(OAc)₂ as precatalyst without use of base and free ligand. The trans isomers of desired products, methyl cinnamates and stilbenes, were obtained with full conversion. Use of a catalytic amount of Pd(OAc)₂ produced the corresponding products in short reaction times and excellent yields.     

Catalytic activity of Fe(II) and Cu(II) PNP pincer complexes for Suzuki coupling reaction

Iron and copper PNP pincer complexes of the type [Fe(L)SO₄] and [Cu(L)OCOCH₃] are reported and represented as C‐1 and C‐2 catalyst. Both the complexes were synthesized using bis(diphenylphosphino)pyridine‐2,6‐diamine [L], and salts of ‘Fe’ and ‘Cu’ by direct coordination method. The as synthesized complexes were characterized using FTIR, UV–Vis, mass analysis and TGA. The effect of reaction time, catalyst load, solvent and base on the reaction between phenylboronic acid and para substituted bromobenzenes in the presence of the catalysts were investigated for evaluating the catalytic efficiency of the complexes. The results obtained highlight the enhanced C‐C coupling reactions with the use of 0.4 mol% of the catalyst C‐1 in 14 h and 0.6 mol% of C‐2 in 16 h respectively with Cs₂CO₃ base and ACN as solvent media. Of the two complexes reported, C‐1 with iron as catalytically active metal is more stable and active towards coupling which is reflected in its better coupling yields in lesser reaction time compared to copper bearing C‐2 complex.     

Microwave-enhanced cyanation of aryl halides with a dimeric <Emphasis Type="Italic">ortho</Emphasis>-palladated complex catalyst

Abstract  

A new dimeric ortho-palladated complex of 2-methoxyphenethylamine was synthesized and characterized and its application as a cyanation catalyst was investigated. The main advantages of this catalyst are its easy preparation, handling, stability, and moisture insensitivity. Thus, [Pd{C₆H₃(CH₂CH₂NH₂)-4-OMe-5-κ ²-C,N}(μ-Br)]₂ showed excellent catalytic activity for the cyanation of aryl iodides and bromides with K₄[Fe(CN)₆], in DMF in the presence of K₂CO₃ under microwave irradiation and conventional heating at 130 °C to give the desired cyanoarene products in good to high yields. The less reactive aryl chlorides also react with K₄[Fe(CN)₆] to give moderate yields of the aromatic nitriles. The effects of various parameters such as solvent, base, and amount of catalyst were studied. The reaction is suitable for a wide variety of substituted aryl halides with different electronic properties. Application of microwave irradiation improved the yields of the reactions and reduced the reaction times from hours to minutes.     

An efficient, high yielding, and eco-friendly method for the synthesis of 14-aryl- or 14-alkyl-14H-dibenzo[a,j]xanthenes using polyvinylsulfonic acid as a recyclable Br?nsted acid catalyst

Abstract  

Aryl or alkyl-14H-dibenzo[a,j]xanthene derivatives were synthesized efficiently by the reaction of 2-naphthol and aromatic or aliphatic aldehydes in the presence of polyvinylsulfonic acid (PVSA) as a reusable Br?nsted acid catalyst under aqueous conditions at 90 °C. This reaction was studied under different conditions, and several solvents were examined. In terms of reaction yield and time, water was found to be the optimum solvent. The catalytic performance of PVSA was then compared with various acidic catalysts under optimized reaction conditions. The catalyst is well characterized using IR and DSC techniques.     

N‐heterocyclic carbene–palladium(II) complex supported on magnetic mesoporous silica for Heck cross‐coupling reaction

Magnetic mesoporous silica was prepared via embedding magnetite nanoparticles between channels of mesoporous silica (SBA‐15). The prepared composite (Fe₃O₄@SiO₂‐SBA) was then reacted with 3‐chloropropyltriethoxysilane, sodium imidazolide and 2‐bromopyridine to give 3‐(pyridin‐2‐yl)‐1H‐imidazol‐3‐iumpropyl‐functionalized Fe₃O₄@SiO₂‐SBA as a supported pincer ligand for Pd(II). The functionalized magnetic mesoporous silica was further reacted with [PdCl₂(SMe₂)₂] to produce a supported N‐heterocyclic carbene–Pd(II) complex. The obtained catalyst was characterized using Fourier transform infrared spectroscopy, scanning electron microscopy, energy‐dispersive X‐ray analysis, vibrating sample magnetometry, Brunauer–Emmett–Teller surface area measurement and X‐ray diffraction. The amount of the loaded complex was 80.3 mg g^?1, as calculated through thermogravimetric analysis. The formation of the ordered mesoporous structure of SBA‐15 was confirmed using low‐angle X‐ray diffraction and transmission electron microscopy. Also, X‐ray photoelectron spectroscopy confirmed the presence of the Pd(II) complex on the magnetic support. The prepared magnetic catalyst was then effectively used in the coupling reaction of olefins with aryl halides, i.e. the Heck reaction, in the presence of a base. The reaction parameters, such as solvent, base, temperature, amount of catalyst and reactant ratio, were optimized by choosing the coupling reaction of 1‐bromonaphthalene and styrene as a model Heck reaction. N‐Methylpyrrolidone as solvent, 0.25 mol% catalyst, K₂CO₃ as base, reaction temperature of 120°C and ultrasonication of the catalyst for 10 min before use provided the best conditions for the Heck cross‐coupling reaction. The best results were observed for aryl bromides and iodides while aryl chlorides were found to be less reactive. The catalyst exhibited noticeable stability and reusability.     

Mechanistic pathways for oxidative addition of aryl iodides to the low-ligated diethanolamine palladium(0) complex in phosphine-free Heck reactions

A set of reactions of different activated olefins and aryl iodides with the trans-dichlorobis(diethanolamine-N)palladium(II) complex (trans-[PdCl₂(DEA)₂]) as a precatalyst was performed, in the presence of diethanolamine (DEA) as a weak base, and NaOEt as a strong base. It was established that the presence of NaOEt slightly lowered the yields, but significantly accelerated the reactions. This experimental finding is in agreement with our computational investigation that shows that significantly higher activation barrier is required for the preactivation reaction in the presence of a weak base than in the presence of a strong base. The reaction between the catalytically active DEA-Pd(0)-Cl complex, formed in the preactivation reaction, and iodobenzene was investigated using density functional theory. Two mechanisms for the oxidative addition of the activated complex were found. The first mechanism is based on a nucleophilic attack of Pd on I of iodobenzene, and yields an intermediate tetracoordinated Pd complex (aI2). The second mechanism begins with a nucleophilic attack of Pd on the benzene ring, and yields a tricoordinated intermediate complex (bI4). It was concluded, on the basis of structural and energetical properties of aI2 and bI4, that the second mechanism is significantly more favorable. It was shown that the oxidative addition requires noticeable lower activation energy than that required for the preactivation process. Thus, our investigations indicate that oxidative addition is not the rate determining step for the Heck reactions investigated in this work, but preactivation step.     

Operationally convenient asymmetric synthesis of (S)-2-amino-3,3-bis-(4-fluorophenyl)propanoic acid

This study demonstrated that alkylation of chiral glycine Schiff base 3 with chloride 4 can be efficiently conducted in acetonitrile as a solvent using commercial-grade potassium tert-butoxide as a base. High reaction rate (1 h) chemical (>90%) and stereochemical (>95% de) outcomes of the alkylation step render this procedure reliable and operationally convenient for multi-gram synthesis of enantiomerically pure amino acid 1. Due to the simplicity of experimental procedures and commercial availability all reagents involved, this procedure can be easily reproduced in regular biochemistry laboratories allowing for systematic biological studies and medicinal applications of compound 1.     

Facile nucleophilic fluorination of primary alkyl halides using tetrabutylammonium fluoride in a tert-alcohol medium

Nonpolar protic reaction media such as t-amyl alcohol allow the aliphatic, nucleophilic fluorination reaction of primary haloalkane systems to fluoroalkanes, using tetrabutylammonium fluoride (TBAF), to proceed chemo-selectively at a reasonable reaction rate under mild conditions to afford the fluoro-product in high yield. As an example, the nucleophilic fluorination of 2-(3-iodopropoxy)naphthalene (1a) as the primary haloalkane model compound, with TBAF in acetonitrile as a polar aprotic solvent, CsF in t-amyl alcohol as a nonpolar protic solvent, and TBAF in t-amyl alcohol for 1 h provided 2-(3-fluoropropoxy)naphthalene (2a) in 38, 5, and 76% yields, respectively.     

Synthesis,characterization, and ethylene oligomerization of three novel dendritic nickel catalysts

Three dendritic nickel complexes C1–C3 were synthesized from three poly(amido amine) dendrimers, salicylic aldehyde and nickel chloride hexahydrate via Schiff base condensation reaction and coordination reaction. The structures of the dendritic ligands and nickel complexes were characterized by FT-IR, UV, ¹H NMR, ESI–MS, and elemental analysis. When activated with aluminum co-catalysts, three complexes C1–C3 were able to catalyze ethylene oligomerization. The catalytic activities and the product distribution of complexes C1–C3 were depended on the reaction parameter, co-catalyst, solvent, and the structure of the pre-catalyst. When using ethyl aluminum sesquichloride (EASC) as co-catalyst in toluene, the catalytic activity of complex C3 containing the longest bridging methylene groups reached the highest value of 1.63 × 10⁶ g·(mol Ni·h)^?1 with 69.15% C₁₁ in the product at 30 min, 25 °C, 0.5 MPa, and Al/Ni ratio of 900.     

l-Proline: an efficient catalyst for the synthesis of new spirooxindoles

Abstract  

l-Proline was found to be a versatile organocatalyst for the synthesis of new spirooxindole derivatives in good yields under mild reaction conditions using 1-propanol as a solvent.     

An unexpected highly diastereoselective double Baylis-Hillman reaction of per- (or poly)fluorophenyl aromatic aldimines with methyl vinyl ketone

Aza-Baylis-Hillman reaction of per- (or poly)fluorophenyl aromatic aldimines 1 with methyl vinyl ketone (MVK) was studied. It was found that both the used Lewis base and solvent can significantly affect the reaction. Using triphenylphosphine as a Lewis base, the reaction of 1 with MVK proceeded smoothly to give the normal Baylis-Hillman adduct 2 along with the double Baylis-Hillman adduct 3 as by-product in THF. When 1,4-diazabicyclo[2.2.2]octane was used as a Lewis base in DMF, the aza-Baylis-Hillman reaction of 1 with MVK gave the double aza-Baylis-Hillman adduct 3 exclusively in moderate to good yields with excellent diastereoselectivities. The double Baylis-Hillman adduct 3 was conveniently converted to fluorine-containing 4-alkylidene-2-cyclohexen-1-ones under mild reaction conditions in good yields.     

Cyclopentadienyl molybdenum dicarbonyl η-allyl complexes as catalyst precursors for olefin epoxidation. Crystal structures of Cp′Mo(CO)2(η-C3H5) (Cp′ = η-C5H4Me, η-C5Me5)

The complexes Cp′Mo(CO)₂(η³-C₃H₅) [Cp′ = η⁵-C₅H₅ (1), η⁵-C₅H₄Me (2), η⁵-C₅Me₅ (3)] have been prepared, structurally characterised by X-ray diffraction (2, 3), and tested as catalyst precursors for the epoxidation of olefins at 55 °C. Complex 1 gave a turnover frequency (TOF) of 310 mol mol_Mo⁻¹ h⁻¹ in the epoxidation of cis-cyclooctene with tert-butylhydroperoxide (TBHP, in decane) as oxidant, and 1,2-epoxycyclooctane was obtained quantitatively within 6 h. A similar result was obtained for complex 2, while the TOF for 3 was about one order of magnitude lower, suggesting a possible activity dependence on the ring substituents. For 1 the use of 1,2-dichloroethane as solvent increased the initial reaction rate to 361 mol mol_Mo⁻¹ h⁻¹, with no decrease in epoxide selectivity. Under these conditions the reaction rates for other olefins increased in the order 1-octene < trans-2-octene < cyclododecene < (R)-(+)-limonene < cis-cyclooctene, and, with the exception of limonene, the corresponding epoxide was the only product. For 1 the selective epoxidation of cis-cyclooctene could also be achieved in aqueous solution, using TBHP or H₂O₂ as oxidants, which gave epoxide yields of 99% and 27% at 24 h, respectively. The possibility of facilitating catalyst recycling by using ionic liquids as solvents was investigated.     

Enantioselective transacetylation of (R,S)-β-citronellol by propanol rinsed immobilized Rhizomucor miehei lipase

Background  

Use of enzymes in low water media is now widely used for synthesis and kinetic resolution of organic compounds. The frequently used enzyme form is the freeze-dried powders. It has been shown earlier that removal of water molecules from enzyme by rinsing with n-propanol gives preparation (PREP) which show higher activity in low water media. The present work evaluates PREP of the lipase (from Rhizomucor miehei) for kinetic resolution of (R,S)-β-citronellol. The acylating agent was vinyl acetate and the reaction was carried out in solvent free media.     

The synthesis of functionalized pyranophenalenones

Abstract  

Protonation of the highly reactive 1:1 intermediate produced in the reaction between alkyl or aryl isocyanides and electron-deficient acetylenic esters with 3-hydroxy-1H-phenalene-1-one leads to a vinylisonitrilium cation, which undergoes an addition reaction with the conjugate base of the 3-hydroxy-1H-phenalene-1-one to produce biologically interesting dialkyl 10-(alkyl or arylamino)-7-oxo-7H,8H-naphtho[1,8-gh]chromene-8,9-dicarboxylates in moderate to fairly good yields at room temperature.     

Sulfamic-acid-catalyzed simple and efficient synthesis of 4-aryl-3-methyl-1-phenyl-1H-benzo[g]pyrazolo[3,4-b]quinoline-5,10-diones under solvent-free conditions

Abstract  

An efficient synthesis of 4-aryl-3-methyl-1-phenyl-1H-benzo[g]pyrazolo[3,4-b]quinoline-5,10-diones from the three-component condensation reaction of 3-methyl-1-phenyl-1H-pyrazol-5-amine, aromatic aldehydes, and 2-hydroxy-1,4-naphthoquinone under solvent-free conditions in good to excellent yields and short reaction times using sulfamic acid as heterogeneous acid catalyst has been investigated.