Addition Of Lewis Acids Research Articles

Catalytic asymmetric alkylation of acylsilanes.

The highly enantioselective addition of Grignard reagents to acylsilanes is catalyzed by copper diphosphine complexes. This transformation affords α-silylated tertiary alcohols in up to 97% yield and 98:2 enantiomeric ratio. The competing Meerwein-Ponndorf-Verley reduction is suppressed by the use of a mixture of Lewis acid additives. The chiral catalyst can be recovered as a copper complex and used repeatedly without any loss of catalytic activity.

Nucleophilic Addition of Lewis Acid Complexed α-Amino Carbanions to Arynes: Synthesis of 1-Aryl-N-methyl-1,2,3,4-tetrahydroisoquinolines

The direct C-1 arylation of N-methyl-1,2,3,4-tetrahydroisoquinolines via coupling of α-amino carbanions derived from Lewis acid complexed tetrahydroisoquinolines and in situ generated arynes is described. This process provides an easy access to the title compounds and a new synthetic route to (±)-cryptostyline alkaloids.

The polymeric quaternary ammonium salt supported on silica gel as catalyst for the efficient synthesis of cyclic carbonate

A polymeric quaternary ammonium salt, polydiallyldimethylammonium bromide (PDDA-Br), was supported on silica gel and employed as a recyclable heterogeneous catalyst for the catalytic synthesis of cyclic carbonate. The supported catalysts had been characterized by FT-IR, XRD, TGA and SEM methods. It was observed that PDDA-Br, silica and Lewis acid exhibited strong synergistic effect for the cycloaddition reaction. The addition of Lewis acid like ZnBr2, ZnCl2 and FeCl3 AlCl3 would improve the catalytic activity significantly. Especially, the combination of PDDA-Br with ZnBr2 afforded the most efficient catalyst for the cycloaddition reaction. Silica gel has high BET surface area and allowed good dispersion of PDDA-Br and ZnBr2. Moreover, the as-prepared catalyst was highly stable and leaching-resistance. It could be reused at least 10 times without any loss of activity.

Effect of Sodium Cation on Metallacycle β‐Hydride Elimination in CO2–Ethylene Coupling to Acrylates

The catalytic conversion of carbon dioxide and olefins into acrylates has been a long standing target, because society attempts to synthesize commodity chemicals in a more economical and sustainable fashion. Although nickel complexes have been known to successfully couple CO2 and ethylene for decades, a key β-hydride elimination step has proven a major obstacle to the development of a catalytic process. Recent studies have shown that Lewis acid additives can be used to create a lower-energy pathway for β-hydride elimination and facilitate a low number of catalytic turnovers. However, the exact manner, in which the Lewis acid promotes β-hydride elimination remains to be elucidated. Herein, we describe the kinetic and thermodynamic role that commercially relevant and weakly Lewis acidic sodium salts play in promoting β-hydride elimination from nickelalactones synthesized from CO2 and ethylene. This process is compared to a non-Lewis acid promoted pathway, and DFT calculations were used to identify differences between the two systems. The sodium-free isomerization reaction gave a rare CO2 -derived β-nickelalactone complex, which was structurally characterized.

Preparation of a novel solid acid catalyst with Lewis and Br{\o}nsted acid sites and its application in acetalization

A novel melamine--formaldehyde resin (MFR) supported solid acid with Lewis and Bronsted acid sites was synthesized through the immobilization of acidic ionic liquid and cuprous ion on MFR. The scanning electron microscopy (SEM) characterization showed that addition of PEG-2000 in the synthesis of MFR could promote the formation of regular particles with diameters around 3.7 m m. The XRD pattern demonstrated that some cuprous ions were aggregated. The catalytic performance of this acid catalyst was evaluated by acetalization. The results showed that the catalytic activity of MFR with Bronsted acid could be improved by addition of Lewis acid. The solid acid was very efficient for the acetalization of carbonyl compounds and diols with moderate to excellent yields and there was no loss of catalytic activity even after being recycled for 6 runs.

Factors affecting the competitive reduction of 8,8-dimethylnaphthalene-1,4,5(8H)-trione in a Diels–Alder cycloaddition with hydroxysulfinyldienes

Competing reduction and cycloaddition products were formed in the reaction of 8,8-dimethylnaphthalene-1,4,5(8H)-trione with a hydroxysulfinyldiene. The ratio of reduction to cycloaddition products depended on the stereochemistry of the diene and on the solvent employed, being higher in ethanol than in benzene. The ratio was also affected by the addition of Lewis acids, decreasing in the order BF3 = Al2O3 > MgCl2 > ZnCl2. The results help to explain and predict the occurrence of these competing processes in Diels–Alder cycloadditions involving quinonedienophiles.

Reactivity of Nitrido Complexes of Ruthenium(VI), Osmium(VI), and Manganese(V) Bearing Schiff Base and Simple Anionic Ligands

Nitrido complexes (M≡N) may be key intermediates in chemical and biological nitrogen fixation and serve as useful reagents for nitrogenation of organic compounds. Osmium(VI) nitrido complexes bearing 2,2':6',2″-terpyridine (terpy), 2,2'-bipyridine (bpy), or hydrotris(1-pyrazolyl)borate anion (Tp) ligands are highly electrophilic: they can react with a variety of nucleophiles to generate novel osmium(IV)/(V) complexes. This Account describes our recent results studying the reactivity of nitridocomplexes of ruthenium(VI), osmium(VI), and manganese(V) that bear Schiff bases and other simple anionic ligands. We demonstrate that these nitrido complexes exhibit rich chemical reactivity. They react with various nucleophiles, activate C-H bonds, undergo N···N coupling, catalyze the oxidation of organic compounds, and show anticancer activities. Ruthenium(VI) nitrido complexes bearing Schiff base ligands, such as [Ru(VI)(N)(salchda)(CH3OH)](+) (salchda = N,N'-bis(salicylidene)o-cyclohexyldiamine dianion), are highly electrophilic. This complex reacts readily at ambient conditions with a variety of nucleophiles at rates that are much faster than similar reactions using Os(VI)≡N. This complex also carries out unique reactions, including the direct aziridination of alkenes, C-H bond activation of alkanes and C-N bond cleavage of anilines. The addition of ligands such as pyridine can enhance the reactivity of [Ru(VI)(N)(salchda)(CH3OH)](+). Therefore researchers can tune the reactivity of Ru≡N by adding a ligand L trans to nitride: L-Ru≡N. Moreover, the addition of various nucleophiles (Nu) to Ru(VI)≡N initially generate the ruthenium(IV) imido species Ru(IV)-N(Nu), a new class of hydrogen-atom transfer (HAT) reagents. Nucleophiles also readily add to coordinated Schiff base ligands in Os(VI)≡N and Ru(VI)≡N complexes. These additions are often stereospecific, suggesting that the nitrido ligand has a directing effect on the incoming nucleophile. M≡N is also a potential platform for the design of new oxidation catalysts. For example, [Os(VI)(N)Cl4](-) catalyzes the oxidation of alkanes by a variety of oxidants, and the addition of Lewis acids greatly accelerates these reactions. [Mn(V)(N)(CN)4]2(-) is another highly efficient oxidation catalyst, which facilitates the epoxidation of alkenes and the oxidation of alcohols to carbonyl compounds using H2O2. Finally, M≡N can potentially bind to and exert various effects on biomolecules. For example, a number of Os(VI)≡N complexes exhibit novel anticancer properties, which may be related to their ability to bind to DNA or other biomolecules.

Selective addition of amines to 5-trifluoromethyl-2,4-dichloropyrimidine induced by Lewis acids

A variety of 2,4-diamino-pyrimidine systems can be prepared from the corresponding 2,4-dichloro-pyrimidine by sequential addition of two amines, the first adding selectively to the 4-position. In contrast it was found that 2,4-dichloro-5-trifluoromethyl-pyrimidine yields a 1:1 mixture of the two possible isomers. Lewis acids were employed to increase the ratio of isomers to >10:1 in favor of the 2-addition product. Optimization of the effect of Lewis acid additives on this and other dichloropyrimidine systems will be discussed.

ChemInform Abstract: Synthesis of Isoquinuclidines from Highly Substituted Dihydropyridines via the Diels—Alder Reaction.

A stereo- and regioselective Diels-Alder reaction for the synthesis of highly substituted isoquinuclidines from dihydropyridines and electron-deficient alkenes has been developed. While reactions with activated dienophiles proceed readily under thermal conditions, the use of Lewis acid additives is necessary to facilitate cycloadditions for less reactive alkenes. This procedure affords the target compounds in high yields and diastereoselectivities.

Synthesis of isoquinuclidines from highly substituted dihydropyridines via the Diels-Alder reaction.

A stereo- and regioselective Diels-Alder reaction for the synthesis of highly substituted isoquinuclidines from dihydropyridines and electron-deficient alkenes has been developed. While reactions with activated dienophiles proceed readily under thermal conditions, the use of Lewis acid additives is necessary to facilitate cycloadditions for less reactive alkenes. This procedure affords the target compounds in high yields and diastereoselectivities.

Synthesis of 3,4‐Dihydroisoquinolin‐1‐ones from N‐Boc‐(β‐Arylethyl)carbamates via Isocyanate Intermediates

AbstractMild reaction conditions for the regioselective synthesis of isoquinolin‐1‐ones and related fused‐ring heterocycles from N‐Boc‐protected (β‐arylethyl)carbamates are described. The reactions involved the use of Tf2O and 2‐chloropyridine and isocyanates are likely to be key intermediates. The method was extended to substrates bearing less nucleophilic aryl moieties by using Lewis acid additives, such as BF3·Et2O, to enhance the Friedel–Crafts‐type cyclization of the isocyanate intermediates. This method allowed the synthesis of various substituted isoquinolin‐1‐ones, β‐carbolines, thiophene‐fused ring systems and tetrahydrobenzoazepin‐1‐ones in good yields and with high regioselectivities.

Regiochemical Reversals in Nitrosobenzene Reactions with Carbonyl Compounds: α-Aminooxyketone versus α-Hydroxyaminoketone Products.

The Lewis acid-catalyzed reaction of nitrosobenzene with a ketone can produce an α-aminooxyketones or an α-hydroxyaminoketone, with reaction regiochemistry switching from the latter to the former, dependent upon the addition of Lewis acid or sterically-hindered solvent. While the latter (C-N bond formation) is easily explained by attack of the enolate α-carbon at N, the former (C-O bond formation) has been an enigma, with few proposed explanations, and none which explain simultaneously formation of both products and all the regiochemical reversals. Herein, the regiochemistry reversal is proposed to occur via (1) nucleophile formation governed by Hard and Soft Acids and Bases (HSAB) theory, (2) a nucleophilic attack by the enolate O at N, followed by (3) a [2,3]-sigmatropic rearrangement. This mechanistic pathway and HSAB considerations account for formation of both products and explain the three reported regiochemistry reversals, which are observed upon the introduction of (A) Lewis acid catalyst, (B) AcOH, or (C) solvent bulkiness.

Optimized Synthesis, Structural Investigations, Ligand Tuning and Synthetic Evaluation of Silyloxy‐Based Alkyne Metathesis Catalysts

Nitride- and alkylidyne complexes of molybdenum endowed with triarylsilanolate ligands are excellent (pre)catalysts for alkyne-metathesis reactions of all sorts, since they combine high activity with an outstanding tolerance toward polar and/or sensitive functional groups. Structural and reactivity data suggest that this promising application profile results from a favorable match between the characteristics of the high-valent molybdenum center and the electronic and steric features of the chosen Ar(3)SiO groups. This interplay ensures a well-balanced level of Lewis acidity at the central atom, which is critical for high activity. Moreover, the bulky silanolates, while disfavoring bimolecular decomposition of the operative alkylidyne unit, do not obstruct substrate binding. In addition, Ar(3)SiO groups have the advantage that they are more stable within the coordination sphere of a high-valent molybdenum center than tert-alkoxides, which commonly served as ancillary ligands in previous generations of alkyne metathesis catalysts. From a practical point of view it is important to note that complexes of the general type [(Ar(3)SiO)(3)Mo≡X] (X = N, CR; R = aryl, alkyl, Ar = aryl) can be rendered air-stable with the aid of 1,10-phenanthroline, 2,2'-bipyridine or derivatives thereof. Although the resulting adducts are themselves catalytically inert, treatment with Lewis acidic additives such as ZnCl(2) or MnCl(2) removes the stabilizing N-donor ligand and gently releases the catalytically active template into the solution. This procedure gives excellent results in alkyne metathesis starting from air-stable and hence user-friendly precursor complexes. The thermal and hydrolytic stability of representative molybdenum alkylidyne and -nitride complexes of this series was investigated and the structure of several decomposition products elucidated.

Enantioselective Intramolecular [2+2] Photocycloaddition Reactions of 4‐Substituted Coumarins Catalyzed by a Chiral Lewis Acid

Eight coumarins, which carry a terminal alkene tethered by a CH(2)XCH(2) group to their 4-position (X = CH(2), CMe(2), O, S, NBoc, NZ, NTs, NBn), were synthesized in overall yields of 51-80 %. Starting materials for the syntheses were either commercially available 4-hydroxycoumarin or 4-formylcoumarin. The intramolecular [2+2] photocycloaddition of these coumarins gave diastereoselectively products with a tetracyclic 3,3a,4,4a-tetrahydro-1H-cyclopenta[2,3]cyclobuta[1,2-c]chromen-5(2H)-one skeleton. Direct irradiation at λ = 300 nm in dichloromethane (c = 10 mM) led to product formation in good yields for most substrates, presumably via a singlet excited state intermediate. Due to the low coumarin absorption at λ >350 nm the photocycloaddition was slow upon irradiation at λ = 366 nm. Addition of a chiral oxazaborolidine-based Lewis acid (50 mol %) increased the reaction rate at λ = 366 nm and induced a significant enantioselectivity in the [2+2] photocycloaddition. Six out of eight coumarin substrates (X = CH(2), CMe(2), O, NBoc, NZ, NTs) gave the respective products in yields of 72-96 % and with 74-90 % enantiomeric excess (ee) upon irradiation in dichloromethane (c = 20 mM) at -75 °C. The Lewis acid presumably acts by coordination to the coumarin carbonyl oxygen atom, which leads to a bathochromic shift (redshift) of the UV absorption and which increases the singlet state lifetime. A second electrostatic interaction of the hydrogen atom at C3 with the oxygen atom of the oxazaborolidine is likely.

Synthesis of modified uracil and cytosine nucleobases using a microwave-assisted method

Modified nucleobases and nucleic acids have found many biological and pharmaceutical applications. Here we report a microwave-directed synthesis of a variety of modified uracil and cytosine nucleobases with high yields under solvent-free conditions. The reaction yields were further improved by addition of Lewis acid. The crystal structures of 5-isopropyl-6-methyluracil and 6-phenyluracil were also determined.

Cascade synthesis of bis-N-sulfonylcyclobutenes via Cu(i)/Lewis acid-catalyzed (3 + 2)/(2 + 2) cycloadditions: observation of aggregation-induced emission enhancement from restricted CN photoisomerization

A remarkable role of Lewis acid additives in syntheses of bis-N-sulfonylcyclobutenes via copper(I) catalyzed (3 + 2)/(2 + 2) cycloaddition cascade of sulfonyl azides and terminal alkynes is described. In addition, these cyclobutenes display a unique aggregation-induced emission enhancement (AIEE), reported for the first time, arising predominantly from restricted rotation in C=N photoisomerization in the solid state.

Catalytic alkyne metathesis and stoichiometric metal–alkylidyne formation from N[tbnd]Mo(OR)3 complexes promoted by Lewis acids

Nitride complex NMo(OCMe2CF3)3 is synthesized in 78% yield on a multigram scale. Although the irreversible but sluggish conversion of terminal molybdenum–nitride complexes NMo(OCMe(CF3)2)3 (1) and NMo(OC(CF3)3)3(NCMe) (2) to their propylidyne analogs EtCMo(OR)3(DME) (OCMe(CF3)2 (3), OC(CF3)3 (4)) via metathesis with 3-hexyne occurs, the analogous reaction with NMo(OR)3 (OROCMe2CF3 (5), OCMe3 (6)) complexes does not occur under similar reaction conditions. However, the kinetic barrier to alkylidyne formation from 5 and 6 with internal alkynes can be overcome through the addition of simple Lewis acids, including MgBr2, MgI2 and BPh3 in specific instances. Although this typically leads to accelerated decomposition of the alkylidyne complex so formed, the combination of metal–nitride complex plus exogenous Lewis acid frequently leads to alkyne metathesis of the test substrates 1-phenyl-1-propyne and 1-phenyl-1-butyne under milder conditions than possible in the absence of Lewis acid, in some cases at room temperature. The interaction of solvent, ancillary alkoxide ligands, and Lewis acid is complex and was not predicted a priori. New benzylidyne complexes 4-MeOC6H4CMo(OC(CF3)3)3(MeOC6H4CN) (22%), 4-Ph–C6H4CMo(OC(CF3)3)3(4-PhC6H4CN) (46%) were isolated in low yield via the nitride-to-alkylidyne route upon reaction with suitable diarylalkynes. Several related alkylidyne complexes were formed but could not be separated cleanly from the alkyne reagents used.

Nickel(II) catalysed co-polymerisation of CO and ethene: Formation of polyketone vs. polyethylene – The role of co-catalysts

The square planar Ni complex (SP-4-3)-[Ni(2-tol)(PPh 3)(N,O)] (N,O = (Z)-4,4,5,5,6,6,6-heptafluoro-3-oxo-2-(pyrrolidine-2-ylidene)-hexanenitrile) is an active catalyst for the co-polymerisation of CO and ethene yielding aliphatic polyketone. Addition of Lewis acids like BPh 3 or B(C 6F 5) 3 as co-catalyst accelerates the polymerisation but not the lifetime of the catalyst and leads to a mixture of polyethylene and polyketone. Addition of the Lewis base triphenylphosphane (PPh 3) completely suppresses the formation of polyethylene. The polymers formed were characterised by IR spectroscopy, 13C cross polarisation (CP) magic angle spinning (MAS) solid state NMR spectroscopy, differential scanning calorimetry (DSC) and size exclusion chromatography (SEC).

Recent developments in metal-catalyzed living radical polymerization

This review presents a short overview of recent developments in metal-catalyzed living radical polymerization, mainly focusing on our recent research studies related to the subject. Metal-catalyzed living radical polymerization or atom transfer radical polymerization, which was originally developed via evolution of the metal-catalyzed Kharasch or atom transfer radical addition to chain-growth polymerization via reversible activation, has now been widely developed in many aspects. The effective metal catalysts include various transition metals, such as ruthenium, copper, iron and nickel, and highly active and versatile catalytic systems have been developed by designing ligands, applying lower oxidation metal species and using additives to widen the scope of controllable monomers and to minimize the amount of metal catalysts and the residual metals in the products. The development of the initiating systems has enabled the synthesis of a wide variety of novel, well-defined polymers, including end-functionalized, block, graft and star polymers, but also more complicated polymers possessing multiple controlled structures. Furthermore, metal-catalyzed living radical polymerization has been judiciously combined with stereospecific radical polymerization based on the use of polar solvents or Lewis acid additives, resulting in the dual control of the molecular weight and the tacticity of the resulting polymers and enabling the preparation of stereoblock and stereogradient polymers.

Oxidation of a Cyclometalated Pd(II) Dimer with “CF3+”: Formation and Reactivity of a Catalytically Competent Monomeric Pd(IV) Aquo Complex

The reaction of [(bzq)Pd(OAc)](2) (bzq = benzo[h]quinoline) with "CF(3)(+)" reagents to afford the monomeric Pd(IV) aquo complex (bzq)Pd(CF(3))(OAc)(2)(OH(2)) is demonstrated. Heating this Pd(IV) adduct at 60 °C for 12 h leads to highly chemoselective aryl-CF(3) bond-forming reductive elimination. The rate and yield of this transformation are both significantly enhanced by the addition of Brønsted and Lewis acidic additives. The mechanism of C-CF(3) bond formation from (bzq)Pd(CF(3))(OAc)(2)(OH(2)) has been studied, and the major pathway is proposed to involve pre-equilibrium acetate dissociation followed by C-CF(3) coupling. Finally, this monomeric Pd(IV) complex is shown to be a kinetically competent intermediate for C-H trifluoromethylation with "CF(3)(+)" reagents. Collectively, these studies provide valuable insights about the speciation, nuclearity, and reactivity of Pd intermediates in catalytic C-H trifluoromethylation reactions.