Sodium hydroxide-catalyzed transfer hydrogenation of carbonyl compounds and nitroarenes using ethanol or isopropanol as both solvent and hydrogen donor
Graphical abstract
Introduction
Economic, regulatory and environmental concerns have prompted a remarkably increasing demand for sustainable, practical and “green” catalytic processes [1]. This demand pushes industry and academia to shift their focus toward improvement of catalyst recovery, restriction of catalyst leaching, development of abundant, cheap and less toxic catalysts, as well as the use of green solvents [1], [2].
Reduction of carbonyl compounds yielding alcohols is a very important transformation in organic synthesis, its industrial application span, fine chemical conversions to pharmaceuticals synthesis. Direct hydrogenation with pressures of H2 gas [3] and transfer hydrogenation from a hydrogen donor molecule [4] are two often employed strategies (Chart 1). As a key example of green catalysis, transfer hydrogenation methodology has become in recent years a center of attraction because it does not require pressurized hydrogen gas and elaborate experimental setups, the hydrogen donors are readily available, inexpensive, easy to handle, and the major side product (such as acetone) can be recycled.
Transition metal-catalyzed transfer hydrogenation of carbonyls, involving first-, second- and third-row transition metals of groups 8–10, has attracted growing interest owing to their high efficiency and selectivity [4], [5]. However, the noble metals (such as Ru, Rh, Pd, Ir, Os and Pt) among them are very expensive, in addition, regulatory organizations limit the metal residual levels in pharmaceutical products to ppm or less levels because of their inherent toxicity. Although more abundant and biocompatible iron seems an excellent candidate for an economic and “greener” alternative, most of the reactions do not proceed in the absence of uneasy-to-get and environmentally-unfriendly ligands [6]. Furthermore, organocatalytic [7] and base-catalyzed transfer hydrogenation [8] have also emerged recently, and received considerable attention. The importance of the hydroxide bases [8] has been first illustrated in 2009 by the reports of the groups of Polshettiwar and Varma with KOH [8a] and Ouali et al., with NaOH [8b] without any transition metal complex for the hydrogenation of carbonyls by 2-propanol. In this context, we have further investigated base-catalyzed transfer hydrogenation.
In all cases of transfer hydrogenation, 2-propanol and formate are the mainly used “sacrificial” reducing sources and solvents. The use of a green solvent is one of the 12 principles of green chemistry [1a]. A green solvent must therefore, possess specific features including low toxicity, non-mutagenicity, widespread availability, and reproducibility. These common green solvents used in organic synthesis include in particular H2O, glycerine, EtOH, some ionic liquids, and supercritical CO2. As one of renewable and cheapest reagents, ethanol, usually produced by fermenting starch, has the potential to be ideal an alternative to 2-propanol and formate in transfer hydrogenation [9]. However, the successful application of ethanol as hydrogen source was rarely reported [10], mainly due to its ability to produce stable transition metal complexes containing carbonyl with the catalysts that are used for the transfer hydrogenation process.
Nitro derivatives are a major family of polluants. Their reduction products of nitroarenes, functionalized anilines are important precursors and intermediates for the manufacture of pharmaceuticals, agrochemicals, pigments, dyes, rubbers, polymers, rubbers, corrosion inhibitors and photographic developers [11], [12]. Reduction of poisonous nitroarenes [13] based on catalytic hydrogenation, metal mediated reductions and electrolytic reduction is the traditional synthesis methods for anilines [14]. Recently, catalytic transfer hydrogenation has emerged as a green and efficient route for the formation of anilines, however, the uses of expensive transition metals and/or ligands are necessary in the transformation [15], in addition, the product contamination by these noble metals restricts the application of such systems in several fields, and especially in biomedicine. Thus, it is highly desirable to develop more economic and pharmaceutically safe methodologies for synthesis of anilines, as well as degradation of nitroarenes.
Herein, we report that abundant and cheap NaOH promotes transfer hydrogenation of carbonyls including ketones and aldehydes compounds forming primary and secondary alcohols, respectively, using EtOH as both hydrogen source and solvent under relatively mild conditions. Additionally, nitroarenes are hydrogenated to form anilines and azobenzenes based on the NaOH-catalyzed transfer hydrogenation protocol with 2-PrOH as both hydrogen donor and solvent.
Section snippets
Investigation of the optimal reaction conditions for transfer hydrogenation of carbonyl compounds
In a preliminary experiment, 4-chloroacetophenone 1a was chosen as a test substrate to identify the optimal reaction conditions. The transfer hydrogenation was initially carried out using EtOH (2 mL) as hydrogen source and solvent, in the presence of 2 equiv of NaOH at 80 °C. The conversion increased with increased reaction time in the range of 1–15 h, providing 95% conversion (Fig. 1a). It was further found that the amount of NaOH is one of the most crucial factors for the formation. Within 15 h,
Concluding remarks
In summary, we have shown that abundant, cheap sodium hydroxide was able to efficiently catalyze transfer hydrogenation of carbonyl compounds, without any transition metals and ligands, using the renewable and green solvent ethanol as both the hydrogen source and reaction solvent. In the process, ethanol is a highly interesting alternative to formic acid or 2-propanol as the hydrogen atom source. This catalytic system held several advantages, such as high efficiency, prevention of metal
General
All reactions and manipulations were performed under nitrogen using standard Schlenk techniques, unless otherwise noted. All commercially available reagents were used as received, unless indicated otherwise.
Flash column chromatography was performed using silica gel (300–400 mesh). 1H NMR spectra were recorded with 300 MHz spectrometer, and 13C NMR spectra were recorded at 75 with 300 MHz spectrometer.
Typical experimental procedure: transfer hydrogenation of carbonyl 1 using EtOH as solvent and hydrogen source in the presence of NaOH
A dried Schlenk tube (100 mL) equipped with a magnetic stirring bar was charged under a nitrogen
Acknowledgements
Financial support from the China Scholarship Council (CSC) (PhD grant to DW, the Ministère de l’Enseignement Supérieur et de la Recherche (PhD grant to C.D.), the University of Bordeaux, the Center National de la Recherche Scientifique (CNRS) and L’Oréal are gratefully acknowledged.
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