Benzaldehyde-supported ionic liquids

The same authors have also reported 1,3-dipolar cycloadditions using 2-hydroxy and 3-hydroxybenzaldehydes grafted on a soluble ionic liquid support [62]. New benzaldehyde-supported ionic liquids were prepared via two different routes. In the first approach the synthesis started from an N-alkylimidazole and 2-chloroethanol, thermolysis of which, followed by anion exchange to form the BF4 or PF ionic liquid, gave the desired supports. After esterification with an acid-functionalized 2-hydroxybenzaldehyde, excellent yields of the benzaldehyde-supported ionic liquids were obtained. The synthetic approach is shown in Scheme 7.13. 

Table 7.9 Asymmetric cyanosilylation of benzaldehyde in ionic liquids using supported VO(salen) complexes.

The second synthetic approach involved the reaction of 1-imidazole, chloroetha-nol, and sodium ethoxide to form (2-hydroxyethyl)imidazole. Treatment of this with dicyclohexylcarbodiimide (DCC) to form the N,N -dicyclohexyl isourea followed by reaction with hydroxybenzaldehyde and quaternization with an alkyl iodide gave the iodide-based supported benzaldehyde. Anion exchange then was performed to generate the desired supported ionic liquids (Scheme 7.14). 

For application in 1,3-dipolar cycloaddition reactions, diversity could be introduced by first reacting the benzaldehyde-grafted ionic liquid with different alkyl amines using microwave heating. This was followed by conventional heating with an imidate to give the desired supported product. Treatment of the bound cydoad-duct with NaOMe in methanol resulted in cleavage from the ionic liquid phase (Scheme 7.15). 

A series of supported chiral VO(salen) complexes anchored on silica, single-wall carbon nanotube, achvated carbon or ionic liquids have been prepared through the simple methods based on the addition of mercapto groups to terminal C=C double bonds (Scheme 7.17) [58]. The four recoverable catalysts and the standard VO(salen) complex 37 were tested for the enantioselechve cyanosilylation of benzaldehyde using trimethylsilyl cyanide (Table 7.9). It should be noted that the ionic liquid-supported IL-VO(salen) showed the highest catalyhc achvity, though the ee-value was considerably reduced compared to the soluble 37 in [bmim][PF6] (entries 4 and 5). 

Of the inorganic supports, best results were reported for a mesoporous MCM-41 [337]. Support on ionic-liquid phases has been studied by different groups with variable results [338, 339], Of the non-conventional organic polymers, non-covalent immobilization on poly(diallyldimethylammonium) is notable [340], Catalysts 133 (15 mol.%) promoted the aldol reaction of acetone and benzaldehydes to afford the corresponding (i-hydroxyketones in 50-98% yields and 62-72% ee, which are clearly lower than those reported for other polymer-supported systems. Recycling of the catalysts was possible at least six times without loss of efficiency. More recently, proline has been attached to one DNA strand while an aldehyde was tethered to a complementary DNA sequence and made to react with a non-tethered ketone [341], To date, the work has focused more on conceptual development than on the analysis of its practical applications in organic synthesis. 

Scheme 7.15. 1,3-Dipolar cycloaddition reactions using ionic liquid-supported benzaldehydes.

Synthesis of thiazolidinones [PEG -RMIM]X ionic liquids have been used for rapid synthesis of a small library of amido 4-thiazolidinones from amine, aldehyde, and mercaptoacid components (Scheme 7.22) [74]. In an initial feasibility study, acid-functionalized benzaldehydes were first coupled to the [PEG -RMIM]X ionic liquids. Imines were formed by reaction of the supported aldehydes with primary amines. The reactions were run in open vessels. Optimum results were obtained by irradiating the reaction mixture with low power at 100 °C for 20 min. The imines were then condensed with mercaptoacids to give the desired thiazolidinones which were then cleaved from the ionic liquid support by amide formation. Microwave irradiation was again used in this cleavage step. The procedure entailed addition of a small amount of solid potassium tert-butoxide to a premixed mixture of the amine and supported thiazolidinone and microwave exposure for 10-20 min at 100 or 150 °G depending on the amine used. In another study, a series of one-... 

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