Ionic liquids condensation

Miscellaneous reactions in neutral ionic liquids Kitazume et al. have also investigated the use of [EDBU][OTf as a medium in the formation of heterocyclic compounds [58]. Compounds such as 2-hydroxymethylaniline readily condense with... 

Davis and co-workers have carried out the first examples of the Knoevenagel condensation and Robinson annulation reactions [61] in the ionic liquid [HMIM][PFg] (HMIM = l-hexyl-3-methylimidazolium) (Scheme 5.1-33). The Knoevenagel condensation involved the treatment of propane-1,3-dinitrile with a base (glycine) to generate an anion. This anion added to benzaldehyde and, after loss of a water molecule, gave l,l-dicyano-2-phenylethene. The product was separated from the ionic liquid by extraction with toluene. 

Even within the small numbers of studies conducted to date, we are already seeing potentially dramatic effects. Free radical polymerization proceeds at a much faster rate and there is already evidence that both the rate of propagation and the rate of termination are effected. Whole polymerization types - such as ring-opening polymerization to esters and amides, and condensation polymerization of any type (polyamides, polyesters, for example) - have yet to be attempted in ionic liquids. This field is in its infancy and we look forward to the coming years with great anticipation. 

S., Kula, M.R., and Kragl, U. (2003) Enzymatic Condensation Reactions in ionic liquids, in Ionic Liquids as Green Solvents, ACS Symposium Series, Vol. 856 (eds R.D. Rogers and... 

There is rapid diffusion of CO2through condensed phases, e.g., polymers and ionic liquids. 

Scheme 10.30. Domino Knoevenagel condensation/catalytic hydrogenation reaction in ionic liquid.

In addition to the ionic liquid-mediated procedure in solution (see Scheme 6.112), Leadbeater and coworkers also presented a solid-phase protocol for a one-pot Mannich reaction employing the above mentioned chlorotrityl linker [67]. In this approach, p-chlorobenzaldehyde and phenylacetylene were condensed with readily prepared immobilized piperazines (Scheme 7.56). 

The very first report on the use of ionic liquids as soluble supports was presented by Fraga-Dubreuil and Bazureau in 2001 [102]. The efficacy of a microwave-induced solvent-free Knoevenagel condensation of a formyl group on the ionic liquid (IL) phase with malonate derivatives (E1CH2E2) catalyzed by 2 mol% of piperidine was studied (Scheme 7.89). The progress of the reaction could be easily monitored by 1H and 13C NMR spectroscopy, and the final products could be cleaved from the IL... 

The first use of room temperature ionic liquids as potential novel soluble phases for combinatorial synthesis has recently been described. As model reaction the Knoevenagel condensation of salicyl aldehyde grafted on to an imidazolium-derived ionic liquid was studied under the action of microwave irradiation (Scheme 12.19) [66]. Reactions were performed without additional solvent in the presence of a basic catalyst, utilizing microwave irradiation in a designated monomode microwave reac-... 

In 2002 Mehnert and co-workers were the first to apply SILP-catalysis to Rh-catalysed hydroformylation [74], They described in detail the preparation of a surface modified silica gel with a covalently anchored ionic liquid fragment (Scheme 7.7). The complex N-3-(3-triethoxysilylpropyl)-4,5-dihydroimidazole was reacted with 1-chlorobutane to give the complex l-butyl-3-(3-triethoxysilylpropyl)- 4,5-dihydroimidazolium chloride. The latter was further treated with either sodium tetrafluoroborate or sodium hexafluorophosphate in acetonitrile to introduce the desired anion. In the immobilisation step, pre-treated silica gel was refluxed with a chloroform solution of the functionalised ionic liquid to undergo a condensation reaction giving the modified support material. Treatment of the obtained monolayer of ionic liquid with additional ionic liquid resulted in a multiple layer of free ionic liquid on the support. 

In classical Hantzsch procedure, an enaminocarbonyl is formed in sim by condensation of ammonia source onto the 1,3-dicarbonyl substrate. But many groups have used a three-component modified-Hantzsch protocol in which the preformed enamine is introduced as a partner. Thus, utilization of cyclic or acyclic 1,3-dicarbonyl compounds, aldehydes, and acyclic or cyclic enamines has been reported, leading regioselectively to diversely substituted 1,4-DHP derivatives (Scheme 7). The sequence involving such starting materials was performed in numerous efficient systems, and more particularly in the following (1) microwave-assisted reaction in acetic acid [50], DMF [51], or an acetic acid/DMF system [52] (2) sonification in ethylene glycol [53] and (3) use of ionic liquids such as [bmim]BF4 [54]. 

Several NaOH-treated ionic liquids for self- and cross-aldol condensation reactions of propanal provide an interesting example illustrating improved product selectivity in a system in which competing reactions take place (109). In the self-aldol condensation reaction of propanal, 2-methylpent-2-enal is formed. The reaction progresses through an aldol intermediate and produces the unsaturated aldehyde. The NaOH-treated ionic liquid [BDMIM]PF gave the highest product... 

The effect of hydrophobicity of some ionic liquids was explored (119) in a recent example of a condensation synthesis involving a quaternary ammonium amide. The water produced was continuously removed from the reaction mixture, producing a much higher yield than had previously been possible for this reaction in other media. 

II reaction under similar conditions at temperatures between 80 and 100°C and with a four-fold excess of 2-methylpentanal (to compensate for the low solubility), the selectivity for the Aldol II product (80%) was 20% higher in [BMIMJEF NaOH than in the water/NaOH system, both at 100% propanal conversion. The increased selectivity was attributed to the higher solubility of the reactant 2-methylpentanal in the ionic liquid phase than in the water phase. The higher solubility of 2-methylpentanal effectively suppressed the self-aldol condensation in the ionic liquid. 

OTf)3 (5 mol%). The use of bismuth triflate/[bmim]BF4 is the ideal catalytic system for these condensations because the recovery and reuse of bismuth triflate is especially easy in ionic liquids compared to toluene. 

Ionic liquids are also alternative reaction media considered as green, mostly because of their lack of vapor pressure and the possibility of reuse [68, 69]. Base-catalyzed Knoevenagel reactions have been described in these media [70-73]. The development of task specific ionic liquids in which the solvent and promoter are coupled is a real breakthrough in the field and some of them have been used in the Knoevenagel condensation [74—81]. 

Thiazolium ion based ionic liquids (OIL) have been used to promote the benzoin condensation of benzaldehyde. 4- And 5-methylthiazoles are readily alkylated with n-butyl bromide to give the corresponding bromide salt. Anion exchange with sodium tetrafluoroborate gave the tetrafluoroborate salt 53 as a stable yellow orange oil. When activated with a small quantity of triethylamine (5 mol%) the oil promotes the coupling of benzaldehyde to benzoin <99TL1621>. 

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