Parallel Recognition by Kinetic Control with Imino Aldehyde Substrates that are Prone to Redistribution

One‐shot treatment of a mixture of Danishefsky diene 4 and tetraallyltin 8 with 3‐formylbenzylidene imines 1 in the presence of Sc(OTf)₃ catalyst provides a single product 13 as a result of exclusive Diels–Alder and allylation reactions on the imine and aldehyde functions, respectively. The chemoselectivities of the respective elementary reactions are improved in the parallel reaction, and the redistribution of substrate 1 that is induced readily by Sc(OTf)₃ is completely suppressed, thus offering a novel protocol for simplification of the multi‐step chemical process.     

Recycling Chiral Imidazolidin‐4‐one Catalyst for Asymmetric Diels–Alder Reactions: Screening of Various Ionic Liquids

Various imidazolium ionic liquids such as [Bmim]PF₆, [Bmim]SbF₆, [Bmim]OTf and [Bmim]BF₄ were screened for recycling an organic catalyst [(5S)‐5‐benzyl‐2,2,3‐trimethylimidazolidin‐4‐one ( 1 )] for asymmetric Diels–Alder reactions. Good yields and enantioselectivies (up to 85% yield and 93% ee) were obtained from reactions in [Bmim]PF₆ or [Bmim]SbF₆. However, reactions in [Bmim]OTf or [Bmim]BF₄ gave racemic products in low yields. Isolation of the products by simple extraction using diethyl ether allowed recycling of the ionic liquid containing the immobilized catalyst in subsequent reactions without significant decrease of yields and enantioselectivities.     

Ring‐opening polymerization of cyclohexene oxide by recyclable scandium triflate in room temperature ionic liquid

In this article, we provide a concept of a two‐phase polymerization system consisting of immiscible monomer and room temperature ionic liquid (IL). The catalyst is immobilized in the IL phase where polymerization takes place. The produced polymer is extracted by the monomer, and the remaining IL phase is catalytically active for more polymerizations. Thus, common volatile organic solvents are no longer needed. Ring‐opening polymerization of cyclohexene oxide (CHO) in 1‐n‐butyl‐3‐methylimidazolium tetrafluoroborate IL ([bmim][BF₄]) using scandium triflate [Sc(OTf)₃] catalyst serves as a realistic example of such concept. The yield of polyCHO in [bmim][BF₄] is higher than that in bulk. IL containing Sc(OTf)₃ can be used for at least three times. A circulatory polymerization process is carried out with added catalyst to keep a relatively high yield in following circulation processes. The assignments of proton signals of polyCHO in ¹H NMR are discussed in detail. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

A novel initiating system for ring‐opening polymerization of ε‐caprolactone: Synthesis of triarm star‐shaped poly(ε‐caprolactone)

The rare earth compound, scandium trifluoromethanesulfonate [Sc(OTf)₃], has been used as a water‐tolerant catalyst for the synthesis of star‐shaped poly(ε‐caprolactone)s (SPCLs) with trimethylol propane as trifunctional initiator in solvent at 40°C. Triarm SPCLs have been successfully prepared. The molar mass of SPCLs were determined by end‐group ¹H NMR analyses, which could be well controlled by the molar ratio of the monomer to the initiator, and were independent of the amount of Sc(OTf)₃ used. Differential scanning calorimetry analyses suggested that the maximal melting point, the cold crystallization temperature, and the degree of crystallinities of SPCLs increased with the increasing of the molar mass and were lower than the linear poly(ε‐caprolactone) (LPCL) with similar molar mass. Furthermore, polarized optical microscopy indicated that LPCL showed fast crystallization rate and good spherulitic morphology with apparent Maltese cross pattern, whereas SPCLs exhibit much lower crystallization rate and poor spherulitic morphology. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Bismuth Triflate‐Catalyzed Addition of Allylsilanes to N‐Alkoxycarbonylamino Sulfones: Convenient Access to 3‐Cbz‐Protected Cyclohexenylamines

Bismuth triflate was found to be an efficient catalyst in the Sakurai reaction of allyltrimethylsilanes with N‐alkoxycarbonylamino sulfones. The reaction proceeded smoothly with a low catalyst loading of Bi(OTf)₃⋅4 H₂O (2–5 mol%) to afford the corresponding protected homoallylic amines in very good yields (up to 96%). A sequential allylation reaction followed by ring‐closing metathesis delivers 6–8 membered 3‐Cbz‐protected cycloalkenylamines.     

Cyanation of N‐Acylhydrazones with Trimethylsilyl Cyanide Promoted by a Brønsted Base and a Lewis Acid

Cyanation of N‐acylhydrazones using trimethylsilyl cyanide (TMSCN) proceeded well in the presence of an amine to afford the corresponding α‐hydrazinonitriles in high yields. For less reactive substrates, the combined use of an amine and a catalytic amount of scandium triflate [Sc(OTf)₃] was effective to promote the reactions. The mechanistic study suggested that the amine worked as a Brønsted base.     

High Regioselective Diels–‐Alder Reaction of Myrcene with Acrolein Catalyzed by Zinc‐Containing Ionic Liquids

The ambient zinc‐containing ionic liquids, MX‐ZnCl₂, functioning as both Lewis acid catalyst and green solvent, are employed for a high regioselective Diels–Alder reaction of myrcene with acrolein for the first time, where MX is either 1‐butyl‐3‐methylimidazolium chloride (BmimCl), 1‐ethyl‐3‐methylimidazolium bromide (EmimBr), N‐butylpyridinium bromide (BPyBr), or N‐ethylpyridinium bromide (EtPyBr). Compared with the analogous reaction performed over a ZnCl₂ catalyst in the conventional solvent dichloromethane, higher regioselectivity of the ‘para’ cycloadduct and excellent yield were achieved at shorter reaction time in these ionic liquids with optimized molar compositions of MX and ZnCl₂. These moisture‐insensitive ionic liquids can be easily separated from reaction products after simple washing with hexane, allowing their reuse with no obvious loss in activity.     

Hafnium Chloride Tetrahydrofuran Complex‐Catalyzed Diels–Alder Cycloadditions of 3‐Ethoxycarbonylcoumarins with 1,3‐Dienes under Solvent‐Free Conditions

Hafnium chloride tetrahydrofuran complex (HfCl₄⋅2 THF) is an efficient catalyst for the Diels–Alder cycloadditions of 3‐ethoxycarbonylcoumarins 1a – c and 1,3‐butadienes 2x – z under solvent‐free conditions furnishing the corresponding cycloadducts in excellent yields. This salt is an air stable Lewis acid and therefore the reactions can be performed in air atmosphere making the procedure simple to be carried out.     

Gold Catalysis: Oxepines from γ‐Alkynylfurans

A ketal group in a furyl position affords arene oxides from γ‐alkynylfurans even with the simple gold(III) chloride (AuCl₃) catalyst. These can either undergo Diels–Alder reactions, isomerise to stable oxepines by an oxygen‐walk reaction or by the addition of water selectively be converted to phenols which differ in the position of the hydroxy group from the normal phenols formed in the gold‐catalysed phenol synthesis. With a phenyl substituent on the furan, the 2‐hydroxymethylpyridinato‐gold(III) complex, not the usual arene oxide but an oxepine is obtained, still the arene oxide can be trapped from the valence‐tautomeric equilibrium by a Diels–Alder reaction.     

Metal triflates combined with caffeine based imidazolium salts: A new family of highly efficient and reusable catalysts

The direct alkylation of caffeine (1), by ethyl triflate gave 1,3,7-trimethyl-9-ethylxanthinium triflate (2) which led to 1,3,7-trimethyl-9-ethylxanthinium bis(trifluoromethanesulfonyl)amide (3) after methathesis with LiNTf₂; 3 proved to be an ionic solid which can be used for the recovery of metal triflates (M(OTf)_n with M = Sc, La, Yb, Cu, Hf, Bi). These reusable catalysts proved to be efficient Lewis acids for Diels–Alder reactions leading to very little or no polymerisation of the diene. In the case of bismuth (III) triflate, the catalyst can be recovered and reused at least 10 times without loss of activity.     

Polyaniline salts as polymer‐based solid acid catalyst for low molecular weight poly(lactic acid)

Polymer‐based solid acid catalyst, polyaniline (PANI) salt, is used for the first time to synthesize polymer. In continuation of our work to synthesize organic chemicals using PANI‐based solid acid catalyst, in this work, polylactic acid is synthesized by the condensation polymerization of lactic acid using PANI salts. PANI salts are characterized by FTIR, FE‐SEM, and TGA analyses. Polymerization of lactic acid in xylene solvent at 140°C for 24 h with the use of very low amount of PANI catalysts gave polylactic acid (PLLA) in the order: PANI‐MSA (46%) > PANI‐TFA (33%) > PANI‐Bi(OTf)₃ (27%) > PANI‐Cu(OTf)₂ (20%) > PANI‐Yb(OTf)₃ (15%). Molecular weights of PLLA synthesized using PANI‐MSA and PANI‐TFA are found to be 4385 and 4830, respectively. This methodology gives highly crystalline polymer with mushroom cap‐like morphology. Advantage of this methodology is the use of easily synthesizable, recyclable, easily handlable, cheaper, and eco‐friendly nature of the catalyst. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 41147.     

Diastereo‐ and Enantioselective Synthesis of trans‐2,3‐Disubstituted 2,3‐Dihydropyran‐4‐one Derivatives

trans‐Diastereoselective hetero‐Diels–Alder reactions took place in the presence of SiCl₄/activator systems. The reactions of aldehydes with a derivative of Danishefsky’s diene afforded the corresponding pyrones with high yields and diastereoselectivity upon activating SiCl₄ with suitable neutral Lewis bases. Aldol intermediates deriving from a Mukaiyama‐type pathway were isolated in many cases. The employment of a chiral activator allowed us to convert Danishefsky’s diene (or its disubstituted derivative) into both aldols and pyrones in good to high enantiomeric excesses.     

Brønsted acid-free controlled polymerization of tetrahydrofuran catalyzed by recyclable rare earth triflates in the presence of epoxides

We report a series of novel, highly efficient, hydrolytically stable and recyclable Lewis acid rare earth triflate catalysts for the living/controlled bulk ring-opening polymerization (ROP) of tetrahydrofuran (THF) in the presence of epoxides. The rare earth triflates [RE(OTf)₃] include that of Sc, Y, La, Nd, Dy and Lu. Epoxides include propylene oxide (PO), cyclohexene oxide (CHO) or styrene oxide (SO). Especially RE(OTf)₃/PO shows high activities, producing PTHF with up to 62 percent conversion and relatively narrow molecular weight distributions (MWDs) (as low as 1.14). Molecular weights (MWs) can be readily controlled from 1.7 to 56.1 kDa by changing monomer to catalyst molar ratios. The Sc(OTf)₃/PO catalyst gives an α,ω-dihydroxyl telechelic PTHF with MWs close to calculated values and MWDs below 1.2. The rare earth catalysts are easily recovered by simple water extractions. For instance, Sc(OTf)₃ can be used at least six times without an obvious decrease of catalytic activity. A polymerization mechanism involving the first two propagation steps of the alkyltetrahydrofuranium cation is proposed to account for the induction periods observed.     

High Atom Efficiency in Sc(OTf)3‐Catalyzed Allylation of Aldehydes with Tetraallyltin

The high atom efficiency was achieved through addition of Ac₂O in the Sc(OTf)₃‐catalyzed allylation of aldehyde with tetraallytin in a 4 : 1 molar ratio.     

Controlled Reversible Anchoring of η6‐Arene/TsDPEN‐ Ruthenium(II) Complex onto Magnetic Nanoparticles: A New Strategy for Catalyst Separation and Recycling

A new catalyst separation and recycling protocol combining magnetic nanoparticles and host‐guest assembly was developed. The catalyst, (η⁶‐arene)[N‐(para‐toluenesulfonyl)‐1,2‐diphenylethylenediamine]ruthenium trifluoromethanesulfonate [Ru(OTf)(TsDPEN)(η⁶‐arene)] bearing a dialkylammonium salt tag, was easily separated from the reaction mixtures by magnet‐assisted decantation, on basis of the formation of a pseudorotaxane complex by using dibenzo[24]crown‐8‐modified Fe₃O₄ nanoparticles. The ruthenium catalyst has been successfully reused at least 5 times with the retention of enantioselectivity but at the expense of relatively low catalytic activities in the asymmetric hydrogenation of 2‐methylquinoline.     

Rhodium‐Catalyzed Enantioselective Intramolecular [4+2] Cycloaddition using a Chiral Phosphine‐Phosphite Ligand: Importance of Microwave‐Assisted Catalyst Conditioning

The use of modular α,α,α′,α′‐tetraaryl‐1,3‐dioxolane‐4,5‐dimethanol (TADDOL)‐ and 1,1′‐bi‐2‐naphthol (BINOL)‐derived phosphine‐phosphite ligands (L₂*) in the asymmetric rhodium‐catalyzed intramolecular [4+2] cycloaddition (“neutral” Diels–Alder reaction) of (E,E)‐1,6,8‐decatriene derivatives (including a 4‐oxa and a 4‐aza analogue) was investigated. Initial screening of a small ligand library led to the identification of a most promising, TADDOL‐derived ligand bearing a phenyl group adjacent to the phosphite moiety at the arene backbone. In the course of further optimization studies, the formation of a new, more selective catalyst species during the reaction time was observed. By irradiating the pre‐catalyst with microwaves prior to substrate addition high enantioselectivities (up to 93% ee) were achieved. The new cyclization protocol was successfully applied to all three substrates investigated to give the bicyclic products in good yield and selectivity. ³¹P NMR and ESI‐MS measurements indicated the formation of a [Rh(L₂*)₂]⁺ species as the more selective (pre‐) catalyst.     

Oligomerization of n‐Butenes in a Two‐Phase Reaction System with Homogeneous Ni/Al‐Catalysts

The oligomerization of n‐butenes in a two‐phase catalyst system consisting of a Lewis acidic ionic liquid (IL) and a nonpolar phase is described. A highly active catalyst system with NiCl₂(PMe₃)₂ was investigated, resulting in 91 % dimers with N‐methylpyrrole as a buffer. The reaction was found to be dependent on the molar ratio of the IL to the catalyst. Additionally, the influence of the Lewis acid was investigated and AlCl₃ was determined as the most suitable for activating the nickel complex. The concentration of AlCl₃ played an important role in achieving high catalyst activity and dimer selectivity. Several reactions with different pyrrole derivatives demonstrated that the use of a buffer with low steric hindrance was also important with regard to increased dimer selectivity.     

Chiral N,N′‐Dioxide–Scandium(III)‐Catalyzed Asymmetric Epoxidation of 2‐Arylidene‐1,3‐diketones with Hydrogen Peroxide

The catalytic asymmetric epoxidation of 2‐arylidene‐1,3‐diketones using aqueous 30% H₂O₂ as oxidant has been successfully realized by a N,N′‐dioxide–scandium(III) triflate [Sc(OTf)₃] complex, giving the corresponding products in moderate to good yields (up to 85%) with excellent enantioselectivities (up to 99% ee) under mild reaction conditions without extra additives.

     

Sulfonic Acid‐Functionalized Ionic Liquids as Metal‐Free,Efficient and Reusable Catalysts for Direct Amination of Alcohols

A series of sulfonic acid‐functionalized (SO₃H‐functionalized) ionic liquids was synthesized and used as metal‐free, highly selective and efficient catalysts for the direct amination of alcohols. Notably, the activities of the series of SO₃H‐functionalized ionic liquids were compared and a 92% isolated yield was obtained using 3‐tetradecyl‐1‐(butyl‐4‐ sulfonyl)imidazolium trifluoromethanesulfonate ([BsTdIM][OTf]) as the catalyst. Importantly, the catalytic system has wide substrate scope including benzylic, allyl, propargylic, aliphatic alcohols with sulfonamide, amide, carbamate, aromatic amine and N‐heterocyclic compounds. Interestingly, the system was also suitable for a multi‐gram scale direct amination of alcohols. Additionally, the reusable nature of [BsTdIM][OTf] makes this protocol more attractive and avoids the disposal and neutralization of acidic catalysts. Moreover, preliminary experiments indicated that this reaction should proceed via an S_N1 pathway.