Guanidinium salts ionic liquid

The addition of hexa- and pentasubstituted guanidinium salts as ionic liquids has improved the chemical yields obtained in the reaction between acetone and various aliphatic and aromatic aldehydes catalyzed by proline (1, 20 mol%) at -25°C. The corresponding aldol products 4 were obtained with yields ranging from 33% to 82% and enantioselectivities from 48% to 99% [26]. 

Most of these studies were carried out with rhodium catalysts. A remarkable exception was reported by Rieger and coworkers [105]. They tested four cobalt-containing ionic liquids for the hydroformylation of 1-hexene (CO/Hj = 30 bar, 120 "C, 12 h) (Scheme 7.16). The methyl guanidinium salt of 1 showed higher activitythan CojfCOlg. With 1-methyl-l//-l,2,4-triazol-2-ium (2) as counter-ion, only low conversion was noted. Not surprisingly, 3 and 4 showed no conversion... 

It must be noted that a remarkable improvement of both the chemical yield (from 6% to 82%) and the enantioselectivity (up to >99% ee) of L-proline-catalysed aldol reactions of a wide range of aldehydes with acetone was found by Liebscher et al. when hexasubstituted or pentasubstituted guanidinium salts were added as ionic liquids. Furthermore, this study showed that guanidinium salts could be advantageous over imidazolium salt-based ionic liquids. 

The aldol reactions were also p>erformed in ionic liquid solvents namely N,N-diethyl-N -hexyl-N -methyl-N",N"dipropylguanidinimn iodide and the commercially available 1-butyl-3-methylimidazolium tetrafluoroborate [bmimJpFJ (Shah, Khan et al. 2009). Equimolar ratios of cyclohexanone and 4-bromobenzaldehyde were used and the reaction mixture was extracted with diethyl ether. The guanidinium salt gave improved results than the neat reaction. Application of the commercially available [bmimjpFJ resulted in a rapidly diminishing enantioselectivity for the recycling. 

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