An in-depth look at the effect of Lewis acid catalysts on Diels–Alder cycloadditions in ionic liquids

Pyridinium salts: from synthesis to reactivity and applications

This review highlights the pyridinium salts in terms of their natural occurrence, synthesis, reactivity, biological properties, and diverse applications.

Origin of reversal of stereoselectivity for [4+2] cycloaddition reaction between cyclopentadiene and methyl methacrylate in the presence of the chloroloaluminate ionic liquid (1-ethyl-3-methyl-imidazolium chloride): in silico studies

The origin of stereoselectivity for [4+2] cycloaddition reaction of methyl methacrylate with cyclopentadiene was investigated with the B3LYP-D3(BJ)/6-31+G(d)//B3LYP/6-31+G(d) level of theory in the presence of the ionic liquid 1-ethyl-3-methyl-imidazolium chloride (EMI+Cl–) and its acidic chloroaluminate melt, 1-ethyl-3-methyl-imidazolium heptachlorodialuminate (EMI+Al2Cl7–). The reaction of methyl methacrylate with cyclopentadiene was examined in the gas phase to rationalize the effect of the ionic liquid ion pairs EMI+Cl– and EMI+Al2Cl7–. The DFT calculated results were found to be in good agreement with the experimentally observed results. The much-discussed hydrogen bonding effect of the imidazolium cation with the dienophile is less important to govern the stereoselectivity for the cycloaddition reaction. The atoms in molecules theory was used to examine the role of hydrogen bonding between the EMI+ cation and methyl methacrylate in the transition state geometries. The calculated activation barriers with the M062X/6-31+G(d)//B3LYP/6-31+G(d) and MP2/6-311+G(d,p)//B3LYP/6-31+G(d) levels of theory also predict the similar exo/endo-stereoselectivity trend for the cycloaddition reactions.

N-hexylpyridinium bis(trifluoromethylsulfonyl)imide and Lewis acids — catalytic systems for Diels-Alder reaction

A comparative study of the Diels-Alder reactions between cyclopentadiene (1) and dienophiles (2 a–c) in N-hexylpyridinium bis(trifluoromethylsulfonyl)imide in a temperature range of 20–45°C is reported. The reaction rate constants and activation energies were calculated. Moreover, the catalytic systems based on N-hexylpyridinium bis(trifluoromethylsulfonyl)imide and Lewis acids were tested as a reaction media to perform the Diels-Alder reaction. Yb, Y, Mg, Zn triflates and chlorides (0.005 to 0.1 mmol) were used as catalysts. The recycling of catalytic system consisted of YCl3 and ionic liquid was performed.

Ionic liquids - deanol derivatives as the Diels-Alder reaction solvents

The Diels-Alder reaction between cyclopentadiene and dienophiles in deanol derivatives containing bis(trifluoromethylsulfonyl)imide anion as media have been studied. The effect of the substituents attached to the cation on the endo:exo selectivity as well as the reaction yield have been evaluated in the absence and presence of Lewis acid catalyst - Y(OTf)3. Catalytic activity of metal triflates and the recycling of chosen catalytic systems have also been investigated

Life cycle assessment of an ionic liquid versus molecular solvents and their applications

Ionic liquids (ILs) have been claimed as "greener" replacements to molecular solvents. However, the environmental impacts of the life cycle phases and comparison with alternative methods have not been studied. Such a life cycle assessment (LCA) is essential before any legitimate claims of "greenness" can be made and is the subject of this paper. The model IL selected is 1-butyl-3-methyl-imidazolium tetrafluoroborate ([Bmim][BF4]) and its use as a solvent for the manufacture of cyclohexane and in a Diels-Alder reaction was assessed. These uses are compared with more conventional synthesis methods. The results indicate that processes that use IL are highly likely to have a larger life cycle environmental impact than more conventional methods. Sensitivity analysis shows that the result is robust to errors and variation in the data. For cyclohexane synthesis, the industrial gas phase process is the greenest, but the three solvents compared for the Diels-Alder reaction showed comparable life cycle impact. Although ILs are not the most attractive alternatives, the result may change if their separation efficiency, stability and recyclability are improved. Because there are many kinds of ILs, with many applications, two examples are not enough to reach any general conclusions about the greenness of all ILs. However, the life cycle data and approach of this study can be used for evaluating the greenness of more kinds of solvents, processes, and emerging technologies.

Ionic Liquids as Powerful Solvent Media for Improving Catalytic Performance of Silyl Borate Catalyst to Promote Diels−Alder Reactions

Use of the acidic chloroaluminate ionic liquid, including the recycled ones, can improve the catalytic activity of the toluene-coordinated silyl borate in enhancing rates, stereoselectivities, and yields of Diels-Alder reactions.

Reaction Rates as a Function of Scale within Ionic Liquids: Microscale in Droplet Microreactors versus Macroscale Reactions in the Case of the Grieco Three-Component Condensation Reaction

Task-specific ionic liquids (TSILs) and more specifically binary task-specific ionic liquids (BTSILs), a unique subclass, have been shown to be excellent supports for solution-phase chemistry. The negligible volatility of ionic liquids enables their use as stable droplet microreactors in atmospheric environments without oil protection or confinement. These droplets can be moved, merged and mixed by electrowetting on a chip. Solution-phase synthesis can be performed on these open digital microfluidic labs-on-a-chip as illustrated by a study of the Grieco three-component reaction in [tmba][NTf(2)]-droplet (tmba=N-trimethyl-N-butylammonium NTf(2)=bis(trifluoromethylsulfonyl)imide) microreactors. A detailed study of matrices and scale effects on conversion and kinetic rates of this three-component condensation is presented in this paper. Reactions have been shown to be slower in droplets than in batches in the absence of additional mixing. Also, a significant influence of the ionic-liquid matrix has been observed. Finally, an increase of droplet's temperature resulted in a kinetics enhancement so as to reach macroscale reaction rates, probably because of a much better mixing of reaction's components involving a Marangoni's effect.

An Asymmetric, bifunctional catalytic approach to non-natural alpha-amino acid derivatives

A catalytic, asymmetric process for the synthesis of 1,4-benzoxazinones from o-benzoquinone imides and ketene enolates is reported. Addition of Lewis acids (Zn(OTf)2, In(OTf)3, and in particular Sc(OTf)3) creates a bifunctional catalytic system that dramatically increases the reaction rate and the yield of these non-natural amino acid precursors while preserving the remarkable enantioselectivity inherent to the reaction. Cocatalyst Sc(OTf)3 increases the yield by up to 42% while producing products in >99% ee.

Biodegradable Ionic Liquids: Selected Synthetic Applications

3-Methyl-1-(propyloxycarbonylmethyl)imidazolium octylsulfate 1a and 3-methyl-1-(pentyloxycarbonylmethyl)imidazolium octylsulfate 2a are ionic liquids that have previously been shown to be readily biodegradable in the CO2 headspace test (ISO 14593). In the present study, these ionic liquids were evaluated as reaction media for Diels–Alder and hydrogenation reactions. The comparison of the performance of these two designer solvents in these reactions with those conducted in other, non-biodegradable ionic liquids has demonstrated that they are comparable and viable solvents.

Lewis base ionic liquids

Ionic liquids which are (weak) Lewis bases have a number of interesting and useful properties different to those of traditional ionic liquids, including volatility and the possibility of being distillable in some cases, a base catalysis effect in others and enhancement of the acidity of dissolved acids.