Reduction of aldehydes and ketones to their corresponding alcohols in 1-butyl-3-methylimidazolium tetrafluoroborate.

Abstract

The use and release of volatile organic solvents (VOS) to the atmosphere have become detrimental to both human health and his environment. In addition, the non-recyclability and the excessive consumption of these solvents in chemical industry influence their high costs. This has incited the growing need towards the design of new processes and benign solvents, helping in minimizing these raised issues. Ionic liquids (ILs) which are acknowledged as green and environmentally friendly solvents, are assigned as promising alternatives for replacing these VOS. The review of some ILs aspects such as structure, synthesis methods, physicochemical properties, solvent applications in reduction reactions, and their recyclability gave an impressive trend and a motivation towards our work. Synthesis of 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]), as typical IL and investigation of its solvent efficiency, in the reduction of aldehydes and ketones to their corresponding alcohols, were the core objectives of this research. [BMIM][BF4] was synthesized via microwave (MW) and conventional methods. The characterization of the synthesized [BMIM][BF4] was done by means of Fourrier transform infrared (FTIR), nuclear magnetic resonance (1H- and 13C-NMR,) and liquid chromatograph-mass spectroscopy (LC-MS) techniques. Physicochemical properties of [BMIM][BF4] related to its solvent application, such as water content, density, viscosity and thermal stability, were explored. It was observed that synthesis method impacted on the purity and yield of the products, as well as the reaction times. 1H-NMR spectrum of [BMIM][BF4] showed the high resonance peak of hydrogen atom on carbon at position-two, (C2)-H, around 8.66 ppm, implying the high polarity of this hydrogen atom towards reduction reactions. [BMIM][BF4] solvent property, in comparison with normal organic solvent viz. ethanol, was investigated through the reduction reactions. The reduced aldehydes and ketones were benzaldehyde (Ph-CHO), acetophenone (Ph-COCH3), ferrocenecarboxyaldehyde (Fc-CHO), and acetylferrocene (Fc-COCH3). Two different methods viz. conventional or ultrasound (US) were also employed in aldehydes and ketones reduction by NaBH4, as reducing agent, in either synthesized [BMIM][BF4] or dry ethanol as solvent. Further to this, similar reduction reaction was carried out under solvent-free conditions for a better investigation of solvent effect. Additionally, reduction reaction of similar substrates under H2 gas with 10% palladium supported on activated charcoal (10% Pd/C), as catalyst, was also investigated. iii | The obtained alcohols were characterized by FTIR and NMR. The recyclability of [BMIM][BF4] was studied by use of both reducing agents, in the reduction of Ph-CHO, as typical example. Under similar reaction conditions, [BMIM][BF4] provided higher product yields than dry ethanol. This showed that during reduction process, [BMIM][BF4] was fast proton donor than dry ethanol, as correlating with 1H-NMR spectrum of [BMIM][BF4]. This observation together with its efficient recyclability make [BMIM][BF4] more efficient and cost effective. Furthermore, green approaches such as MW and US examined in this study showed high potential in terms of better product yields and short reaction times.