Sn(OTf)2 and Sc(OTf)3: Efficient and versatile catalysts for the controlled polymerization of lactones

A general and novel method for the controlled synthesis of aliphatic polyesters is presented. The evaluation of stannous (II) trifluoromethane sulfonate [Sn(OTf)₂] and scandium (III) trifluoromethane sulfonate [Sc(OTf)₃] as catalysts for the ring‐opening polymerization (ROP) of various lactones is described as a route to polyesters under mild and highly selective polymerization conditions. Size exclusion chromatograms of poly(ϵ‐caprolactone) initiated from ethanol in the presence of either Sn(OTf)₂ or Sc(OTf)₃ demonstrate the facile synthesis of narrowly dispersed products. Predictable molecular weights, typical of a living or controlled polymerization, were obtained with high yields. These catalysts are versatile and applicable toward the ROP of other cyclic (di)esters, including β‐butyrolactone, which produces the synthetic analogue of the biopolymer poly(β‐hydroxybutyrate). © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 2067–2074, 2000     

Ytterbium triflate catalyzed synthesis of β-enaminones

β-Enaminones have been synthesized in very good yield under solvent-free conditions from differently substituted amines and β-diketones in the presence of Yb(OTf)₃ as catalyst. The method is applicable to cyclic and acyclic ketones, aromatic and aliphatic amines without differences.     

TiCl2(OTf)-SiO2: A solid stable lewis acid catalyst for Michael addition of α-Aminophosphonates,Amines, Indoles and Pyrrole

TiCl₂(OTf)-SiO₂ is simply prepared by immobilization of TiCl₃(OTf) on silica gel surface and introduced as a non-hygroscopic Lewis acid catalyst for C-N and C-C bond formation via Michael addition reaction. A variety of structurally diverse nitrogen nucleophiles including α-aminophosphonates, aliphatic and aromatic amines and imidazole were evaluated as Michael donors. Friedel–Crafts alkylation of indoles and pyrrole was also investigated through Michael addition reaction in the presence of TiCl₂(OTf)-SiO₂ as a catalyst. The reactions were conducted at room temperature or 60 °C under solvent-free conditions and the desired Michael adducts were obtained in high to excellent yields.     

Microwave-enhanced bismuth triflate-catalyzed epoxide opening with aliphatic amines

In the presence of a catalytic amount of Bi(OTf)₃·4H₂O and under microwave irradiation, neat mixtures of epoxides and amines afforded smoothly the corresponding 2-amino alcohols. A wide variety of aliphatic amines were reacted with cycloalkene oxide, styrene oxide, and stilbene oxide. The reaction proceeded rapidly and afforded the 2-amino alcohols in high up to quantitative yields. All products could be obtained without aqueous work-up by simple filtration.     

Imidazolium-based polymer supported gadolinium triflate as a heterogeneous recyclable Lewis acid catalyst for Michael additions

A heterogeneous Lewis acid catalytic system has been developed by incorporating gadolinium triflate on to poly[styrene-co-(1-((4-vinylphenyl)methyl)-3-methylimidazolium) tetrafluoroborate] (1-Gd(OTf)₃), and the catalytic activity of this system has been examined for Michael additions of amines and thiols to α,β-unsaturated esters and acrylonitrile. The reactions proceed in moderate to excellent yields in the presence of catalytic system 1-Gd(OTf)₃. The catalytic system could be efficiently recycled and reused.     

Conjugate addition of aromatic amines to ethenetricarboxylates

The conjugate addition of amines is considered to be a useful reaction in synthetic organic chemistry. The reaction of reactive electrophilic olefins, ethenetricarboxylates, and aromatic amines with and without catalytic Lewis acids such as ZnCl₂ and ZnBr₂ at room temperature gave amine adducts in high yields. The products were converted to α-amino acid, dl-aspartic acid derivatives. Using Lewis acids such as Sc(OTf)₃ and Zn(OTf)₂ at higher temperature (40-80 °C), the reaction of ethenetricarboxylates and N-methylaniline gave an aromatic substitution product. A catalytic enantioselective conjugate addition using a chiral Lewis acid was also investigated. For example, the reaction of 1,1-diethyl 2-tert-butyl ethenetricarboxylate with N-methylaniline in the presence of chiral bisoxazoline-Cu(II) complex in THF at −20 °C for 17 h gave an amine adduct in 91% yield and 78% ee. On the other hand, the reaction with aniline and primary aniline derivatives gave adducts with almost no ee%.     

A convenient synthesis of functionalized isoxazolines and related 5-hydroxyisoxazolidine-4-carboxylates

The effectiveness of Sc(OTf)₃ as a Lewis acid catalyst for the 1,4-addition reaction of carbamates to appropriate 2-acetyl-2-pentenoic esters is herein reported. In particular, N-benzyl-(tert-butyldimethylsilyloxy)carbamate added efficiently to the title substrates to afford a new class of polyfunctionalized isoxazolidines and isoxazolines.     

An approach to the Paal-Knorr pyrroles synthesis catalyzed by Sc(OTf)3 under solvent-free conditions

A facile synthesis of N-substituted pyrroles by the Paal-Knorr condensation has been accomplished using a simple procedure. Among different metal triflates screened, 1 mol % Sc(OTf)₃ efficiently promoted the reaction to give excellent yield (89-98%) under mild reaction conditions. Additionally, Sc(OTf)₃ could be recovered easily after the reactions and reused without evident loss in activity.     

Highly Efficient Aminolysis of Epoxides Catalyzed by Reusable Zirconyl Triflate,ZrO(OTf)2

Efficient ring opening of epoxides with aromatic amines catalyzed by ZrO(OTf)₂ is reported, and the corresponding β amino (beta-amino acid) alcohols were obtained in high yields in CH₃CN as solvent. The reactions were carried out at room temperature and in the presence of only 1.25 mol% of ZrO(OTf)₂. This catalyst can be reused several times without loss of its activity.     

Highly Regioselective 5-endo-tet Cyclization of 3,4-Epoxy Amines into 3-Hydroxypyrrolidines Catalyzed by La(OTf)3

Highly regioselective intramolecular aminolysis of 3,4-epoxy amines has been achieved. Key features of this reaction are (1) chemoselective activation of epoxides in the presence of unprotected aliphatic amines in the same molecules by a La(OTf)₃ catalyst and (2) excellent regioselectivity for anti-Baldwin 5-endo-tet cyclization. This reaction affords 3-hydroxy-2-alkylpyrrolidines stereospecifically in high yields. DFT calculations revealed that the regioselectivity might be attributed to distortion energies of epoxy amine substrates. The use of this reaction was demonstrated by the first enantioselective synthesis of an antispasmodic agent prifinium bromide.     

Highly Selective Hydrothiolation of Unsaturated Organosilicon Compounds Catalyzed by Scandium(III) Triflate

The first use of a Lewis acid catalyst in the addition reaction of both aromatic and aliphatic thiols to unsaturated organosilicon compounds is reported. In catalytic tests, scandium(III) triflate demonstrates high catalytic activity in this process. Under mild conditions (25 °C, room temperature, 1–10 h) a number of thioether‐functionalized organosilicon species are obtained with appreciable selectivity. This study constitutes the first example of allylsilane hydrothiolation that gives the Markovnikov regioisomer as the main product. Ethynylsilanes are also successfully used in the hydrothiolation reaction in the presence of Sc(OTf)₃.     

Lanthanum-pyBOX complexes as catalysts for the enantioselective conjugate addition of malonate esters to β,γ-unsaturated α-ketimino esters

Abstract

In this paper, we report the application of chiral complexes of La(III) with pyBOX ligands as Lewis acid catalysts in the conjugate addition of malonic esters to N-tosyl imines derived from β,γ-unsaturated α-keto esters to give the corresponding chiral α,β-dehydroamino esters. pyBOX complexes with La(III), Yb(III), Sc(III), and In(III) triflates were assessed in this reaction but only La(III) showed good activity and enantioselectivity, while Yb(III) provided the expected product with low yield and stereoselectivity, and the Sc(III) and In(III) complexes were completely inactive. The complex of La(OTf)₃ with the diphenyl-pyBOX ligand prepared in situ provided the best results and allowed obtaining chiral α,β-dehydroamino esters 3 with excellent yields, E:Z diastereomeric ratios (29:71–99:1) and high enantiomeric excesses (20–95%). The reaction could be applied to imines having a substituted aromatic ring or a heterocycle attached to the double bond, although the presence of electron-withdrawing groups on the aromatic ring was detrimental for stereoselectivity. The reaction products were obtained with the S configuration at the stereogenic center and the Z configuration at the enamine double bond as determined by NOESY experiments and X-ray analysis. Based on the experimental results a stereochemical model involving a nine-coordinate La(III) species has been proposed.     

Highly efficient and versatile chemoselective addition of amines to epoxides in water catalyzed by erbium(III) triflate

Er(OTf)₃ is proposed as a highly efficient and reusable catalyst for the opening of epoxides in water with aliphatic as well as aromatic amines leading to the synthesis of β-amino alcohols. The aqueous conditions employed in the present method will make it ‘environmentally friendly’ and potentially useful for industrial applications.     

Bi(OTf)3-catalyzed allylation of epoxides: a facile synthesis of homoallylic alcohols

Epoxides react smoothly with tetraallyltin in the presence of 2 mol% of Bi(OTf)₃ under mild reaction conditions to afford the corresponding homoallylic alcohols in excellent yields with high regioselectivity while aryl aziridines produce exclusively allyl amines in good yields under similar conditions.     

Secondary benzylation using benzyl alcohols catalyzed by lanthanoid, scandium, and hafnium triflate

The combination of a secondary benzyl alcohol and a metal triflate (e.g., La, Yb, Sc, and Hf triflate) in nitromethane was a highly effective secondary-benzylation system. Secondary benzylation of carbon (aromatic compounds, olefins, an enol acetate), nitrogen (amide derivatives), and oxygen (alcohols) nucleophiles was carried out with a secondary benzyl alcohol and 0.01-1 mol % of a metal triflate in the presence of water. Secondary benzyl alcohols and nucleophiles bearing acid-sensitive functional groups (e.g., tert-butyldimethylsilyloxy and acetoxy groups and methyl and benzyl esters) could be used for alkylation. Hf(OTf)4 was the most active catalyst for this alkylation, and trifluoromethanesulfonic acid (triflic acid, TfOH) was also a good catalyst. The catalytic activity of metal triflates and TfOH increased in the order La(OTf)3 < Yb(OTf)3 < TfOH < Sc(OTf)3 < Hf(OTf)4. A mechanistic study was also performed. The reaction of 1-phenylethanol (4a) in the presence of Sc(OTf)3 in nitromethane gave an equilibrium mixture of 4a and bis(1-phenylethyl) ether (54). Addition of a carbon nucleophile to the equilibrium mixture gave alkylated product in high yield.     

Carbonylation of C−N Bonds in Tertiary Amines Catalyzed by Low‐Valent Iron Catalysts

The first iron catalysts able to promote the formal insertion of CO into the C?N bond of amines are reported. Using low‐valent iron complexes, including K₂[Fe(CO)₄], amides are formed from aromatic and aliphatic amines, in the presence of an iodoalkane promoter. Inorganic Lewis acids, such as AlCl₃ and Nd(OTf)₃, have a positive influence on the catalytic activity of the iron salts, enabling the carbonylation at a low pressure of CO (6 to 8 bars).     

Lanthanide Triflate Catalyzed One-Pot Synthesis of Dihydropyrimidin-2(1H)-thiones by a Three-Component of 1,3-Dicarbonyl Compounds,Aldehydes, and Thiourea Using a Solvent-Free Biginelli Condensation

Abstract

Novel one-pot Biginelli-type reaction has been developed. Aromatic and aliphatic aldehydes with β-dicarbonyl compounds and thiourea in the presence of catalytic amount 5 mol% of Yb(OTf)₃ at 100° C for 60–90 min under solvent-free conditions proceeded smoothly to afford corresponding dihydropyrimidin-thiones. The yields of the classical Biginelli reaction can be increased from 20–50% to 81–91% while the reaction time was shortened from 18–48 h to 60–90 min. In addition the catalyst can be easily recovered and reused.     

Sc(OTf)3-catalyzed cyclocondensation of 2-propylresorcinol with diethoxymethane. Formation and fragmentation of resorcin[n]arenes

The cyclocondensation of an equimolar amount of 2-propylresorcinol with diethoxymethane in the presence of Sc(OTf)₃ in acetonitrile produced a mixture of resorcin[n]arenes (n=4-7) with a kinetically controlled product distribution. However, the reaction with an excess amount of diethoxymethane produced the thermodynamically favored resorcin[4]arene as the major product. The fragmentation/recombination mechanism of the resorcin[n]arenes was discussed.     

A general and mild copper-catalyzed three-component synthesis of protected homoallyl amines

A mild and highly efficient copper-catalyzed, one-pot, three-component reaction is developed for the synthesis of homoallyl amines. Reaction of an aldehyde, a carbamate, and allyltrimethylsilane in the presence of 5 mol % Cu(OTf)₂ furnishes the corresponding protected homoallylic amines in good to excellent yields.     

Sc(OTf)3 catalysed multicomponent synthesis of chromeno[2,3-d]pyrimidinetriones under solvent-free condition

An efficient, simple, and environmentally friendly synthesis of a series of chromeno[2,3-d]pyrimidine-trione derivatives has been accomplished via the three-component reaction of a barbituric acid, dimedone/cyclohexane-1,3-dione, and aromatic aldehydes using Sc(OTf)₃ as a recyclable catalyst under solvent-free condition. This method exploits the use of Sc(OTf)₃ as a Lewis acid catalyst in organic synthesis and offers many rewards such as excellent product yield and easy work-up procedure. Harmless reaction conditions, as well as the absence of side-products, are another green aspects of this protocol.