Triazine-based polyfluorinated triquaternary liquid salts: Synthesis, characterization, and application as solvents in rhodium(I)-catalyzed hydroformylation of 1-octene

Abstract

Silylation of N-(2-hydroxyethyl)imidazole, HOCH2CH2Im (1), with hexamethyldisilazane gave N-(2-trimethylsilyloxyethyl)imidazole, Me3SiOCH2CH2Im (2), which underwent quaternization reactions with the alkyl halides and gave three new N-(trimethylsilyloxyethyl) imidazolium halides, Me3SiOCH2CH2Im+RX-, where Im+ = imidazolium and R/X = Me/I (3), CH2CH2F/Br (4), and CH2CH2CF3/I (5). The ethyl ether, 6, formed from 1 and ethyl bromide was quaternized with CF3CH2CH2I to obtain 7, followed by anion exchange with LiN(SO2CF3)2 to obtain 8. The metathesis reactions of 3-5 with cyanuric fluoride in acetonitrile at 25°C gave tris[2-(N′-alkylimidazolium)ethoxy] triazine trihalides, N3C3(OCH2CH2Im+ RX-)3, where R/X = Me/I (9), CH2 CH2F/Br (10), and CH2CH2CF3/I (11). Two neutral trimetric compounds, N3C3(OCH2 CH2Im)3 (12) and N3C3 (OCH2CH2NMe2)3 (14), were prepared from reactions of cyanuric fluoride and Me3SiOCH2 CH2NMe2 (13), or 2, respectively. The quaternization of 12 with MeI gave tris[oxoethyl(trimethyl)ammonium]triazine, N3 C3(OCH2CH2N+Me3 I-)3 (14). Subsequent exchange of the halides in 9-11 and 15 with the weakly coordinating anions of AgOSO2CF3, LiN(SO2CF3)2, AgNO3, or AgClO4 resulted in new triquaternary salts, 16-23, that were characterized by NMR, elemental analyses, and, for some of the compounds, mass spectroscopy. Physical (melting point and density) and thermal properties of 9-23 determined with a differential scanning calorimeter (DSC) and a thermogravimetric analyzer (TGA) are reported. In Rh(I)-catalyzed hydroformylation of 1-octene, with Ph2P(NMPBTA) (26) [NMPBTA = N-methylpyridinium bis(trifluoromethane-sulfonyl)amide] as ligand, the turnover frequency (TOF), conversion, isomer selectivity (n/i), and recyclability were compared when triquaternary salts 17 and 20 or monoquaternary 8 were used as solvents in the biphasic hydroformylation process. A change of 26/Rh(I) ratio from 4 to 20 resulted in a significant increase of n/i selectivity with 17 or 20, but was marginal with 8 as solvent.