Solvent-free catalysis

The selectivities and activities of this reaction were greatly improved by running the catalysis solvent-free and by adding cocatalytic amounts of Schwesinger-type fluorides ( naked fluoride , e.g. phosphacenium-fluoride-P2 13) [14]. The fluoride effect is pronounced both in terms of activity and enantioselectivity (even inverting the sense of enantioselection in one example see Table 6.1). 

Walsh, P.J., Li, H. and de Parrodi, C.A. (2007) A Green chemistry approach to asymmetric catalysis Solvent-free and highly concentrated reactions. Chem. Rev., 107 (7), 2503-2545. 

Bigi F, Carloni S, Erullanti B, Maggi R, Sartori S (1999) A revision of the Biginelli reaction under solid acid catalysis. Solvent-free synthesis of dihydropyrimidines over montmorilion-ite KSF. Tetrahedron Lett 40 3465-3468... 

Keywords 1,4-Dicarbonyls, aldehydes, niobium pentachloride (NbCls), homogeneous catalysis, solvent-free, room temperature, condensation, Knoevenagel reaction, trisubstituted alkenes... 

Keywords Substituted amines, amines, a,p-unsaturated alkenes, ZrOCla-SHaO/montmorillon-ite KIO, heterogeneous catalysis, solvent-free, room temperature, Mannich reaction, Michael addition... 

The combination of ionic liquids with supercritical carbon dioxide is an attractive approach, as these solvents present complementary properties (volatility, polarity scale.). Compressed CO2 dissolves quite well in ionic liquid, but ionic liquids do not dissolve in CO2. It decreases the viscosity of ionic liquids, thus facilitating mass transfer during catalysis. The separation of the products in solvent-free form can be effective and the CO2 can be recycled by recompressing it back into the reactor. Continuous flow catalytic systems based on the combination of these two solvents have been reported [19]. This concept is developed in more detail in Section 5.4. 

New chemical synthesis routes leading to a better productivity and increased selectivity could be defined with regard to the new opportunities offered by HEX reactors. For example, they can lead to solvent-free operation or operations with at least dramatically reduced amount of solvent, to increase the reaction temperature or to engage in more efficient catalysis. 

There are different ways in which the nanoparticles prepared by ME-technique can be used in catalysis. The use of ME per se [16,17] implies the addition of extra components to the catalytic reaction mixture (hydrocarbon, water, surfactant, excess of a metal reducing agent). This leads to a considerable increase of the reaction volume, and a catal5fiic reaction may be affected by the presence of ME via the medium and solubilization effects. The complex composition of ME does not allow performing solvent-free reactions. 

Von Langermann, J., Mell, A., Paetzold, E. et al. (2007) Hydroxynitrile lyase in organic solvent-free systems to over come thermodynamic limitations. Advanced Synthesis and Catalysis, 349, 1418-1424. 

A new synthetic route for functionalized polyhydroxyalkyl-pyrimidines starting from unprotected aldoses and based on montmorillonite K-10 catalysis and solvent-free microwave irradiation conditions, has been reported by Yadav et al,m Thus, reaction of D-glucose and D-xylose with semicarbazide or thiosemicarbazide (186) in the presence of montmorillonite K-10, under microwave irradiation, proceeded via domino cycloisomerization, dehydrazination, and dehydration of the intermediate semi- or thiosemicarbazones (187) to afford l,3-oxazin-2-ones or l,3-oxazine-2-thiones (188) in one single step and in yields between 79% and 85% (Scheme 34). Other mineral catalysts tested, such as silica gel and basic alumina, were far less effective for this transformation and only silica gel was active at all, giving low yields (15-28%) of compounds 188a-d. The l,3-oxazin-2-ones(thiones) thus synthesized were subsequently converted into the target pyrimidines by reaction with aromatic... 

In addition to solvent-free processing, phase-transfer catalytic conditions (PTC) have also been widely employed as a processing technique in MAOS [15]. In phase-transfer catalysis, the reactants are situated in two separate phases, for example liquid-... 

Solvent-free Solid-Liquid Phase-transfer Catalysis (PTC)... 

The isomerization of an O-silyl ketene acetal to a C-silyl ester is catalyzed by a cationic zirconocene—alkoxide complex [92], This catalysis was observed as a side reaction in the zirconocene-catalyzed Mukaiyama aldol reactions and has not yet found synthetic use. The solvent-free bis(triflate) [Cp2Zr(OTf)2] also catalyzes the reaction in nitromethane (no reaction in dichloromethane), but in this case there may be competitive catalysis by TMSOTf (cf. the above discussion of the catalysis of the Mukaiyama aldol reaction) [91] (Scheme 8.51). 

Although beyond the scope of this book, a vast amount of work has been directed to supporting homogeneous catalysts on solid supports including silica, alumina and zeolites, and functionalized dendrimers and polymers [19]. These give rise to so-called solid-liquid biphasic catalysis and in cases where the substrate and product are both liquids or gases then co-solvents are not always required. In many ways solvent-free synthesis represents the ideal method but currently solvent-free methods can only be applied to a limited number of reactions [20],... 

Catalysis in reaction systems with undissolved substrates and products is not restricted to biocatalysis. High yields in sobd-state synthesis, sohd-to-sohd reactions, and solvent-free systems have also been reported for aldol condensation, Baeyer-Villiger oxidation, oxidative coupling of naphthols, and condensation of amines and aldehydes [1, 2]. 

Solid-liquid solvent-free phase-transfer catalysis (PTC) is specific for anionic reactions including base-catalysed isomerisation7. Usually, a catalyst (typically a tetraalky-lammonium salt or a cationic complexing agent) is added to an equimolar mixture of an electrophile and a nucleophile, one of which serves as both a reactant and the... 

SOLVENT-FREE PHASE TRANSFER CATALYSIS (PTC) REACTIONS 105... 

Solvent-free phase transfer catalysis (PTC) reactions 7.3.1. trans-1,2-Diphenylethylene (trans-stilbene)... 

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