Water-tolerant silica-based trifloaluminate ionic liquids dedicated for Diels-Alder reaction
Anna WOLNY1, Piotr LATOS1, Anna CHROBOK1
1Faculty of Chemistry, Department of Chemical Organic Technology and Petrochemistry, Silesian University of Technology, Gliwice, Poland
Reactions with typical Lewis acids, such as AlCl3 or BF3 are performed under strict anhydrous conditions to avoid corrosion, what brings additional costs. Water-tolerant trifloaluminate ionic liquids (ILs), synthesised from 1-alkyl-3-methylimidazolium triflates and aluminium triflate are new and unique Lewis acidic catalysts with medium-strength Lewis acidity (AN = ca. 65). The existence of oligonuclear, multiply-charged trifloaluminate anions, with multiple bridging triflate modes was confirmed using spectroscopic analysis [1-3].
In this work, encouraged by the past results [2,3], we decided to support trifloaluminate ILs on multimodal porous silica surface, in hope to develop a high-performance and recyclable SILP catalyst. Ionic liquid was anchored to the silica surface via three methods: (a) direct bonding of 1-methyl-3-(triethoxysilylpropyl)imidazolium trifloaluminate IL [tespmim][OTf]-[AlOTf3] to the silica surface, (b) bonding of [tespmim][OTf] to the silica surface following by complexation reaction with Al(OTf)3, or (c) bonding of [tespmim]Cl at first, then ion exchanging of chloride to triflate anion and next complexation with Al(OTf)3, what resulted in creation of three supported ionic liquid-like phase (SILLP) catalysts. Ionic liquid loading on the silica surface was determined with TGA (25.27 wt% (a), 19.25 wt% (b), 19.39 wt% (c)) and the chemical grafting of IL was confirmed by 29Si MAS NMR.
Catalytic activity of obtained Lewis acidic SILP catalysts were tested in the Diels-Alder reaction, which is one of the most powerful transformation for the production of pharmaceuticals, agrochemicals, flavours and fragrances. In the preliminary studies reaction conditions of the model Diels-Alder reaction between maleic anhydride and isoprene were optimised. The following parameters were chosen: SILP catalyst amount (0.1% mol for dienophile), dienophile:diene molar ratio (2:3) and the dichloromethane amount (0.5 mL). Cycloadduct was obtained with high yields in the presence of all tested catalysts (97.02% for (a), 96.97% (b), 96.51% (c)) yields after 180 minutes at room temperature. The trials of recycle of catalysts were successful specially for catalyst obtained via (a) path. The catalyst retained high catalytic activity and selectivity over ten cycles, while the other catalysts (b) and (c) started to loss their catalytic activity in fifth cycle. It can be the result of aggregation of the triflate salts on the silica surface during their synthesis.
New SILP catalyst (a) was found to be very robust in terms of substrate scope during the tests in other Diels-Alder reactions between maleic anhydride and various dienes as cyclopentadiene (99.9% yield, 1 min), isoprene (97.02% yield, 180 min), α-terpinene (97.9% yield, 60 min) and myrcene (98.7%, 60 min). Search for highly active, selective and stable catalyst led us finally to the implementation of SILLP catalyst in the continuous flow reactor, what made the whole process even more environmentally friendly and green chemistry suitable.
[1] Kobayashi, S., Sugiura, M., Kitagawa, H., Lam, W. W.-L. Chem. Rev., 2002, 102, 2227–2302.
[2] Latos, P., Culkin, A., Barteczko, N., Boncel, S., Jurczyk, S., Brown, L. C., Swad?ba-Kwa?ny, M. Front. Chem., 2018, 6, 535.
[3] Latos, P., Szelwicka, A., Boncel, S., Jurczyk, S., Swadzba-Kwasny, M., Chrobok, A. ACS Sustain. Chem. Eng., 2019, 7, 5184 – 5191.