Introduction

Alcohols find in pharmaceuticals, natural products, agrochemicals, dyes, and polymers1,2,3. The existing methods for carbonyl reduction frequently require metals (e.g. palladium, platinum, ruthenium, osmium, nickel and iron) or stoichiometric reductant reagents (e.g. NaBH4, LiAlH4 and H2) that commonly show limited substrate scope (Fig. 1a)4,5. These systems have demonstrated some drawbacks, such as: using precious, low abundance and toxic metals. On the other hand, hydrogen gas is very flammable and lacks compatibility with a diverse range of functional groups6. Also, the reductants are costly, and metal salts form as by-products during of reaction7.

Figure 1
figure 1

Hydrogenation strategies of carbonyl compounds. Strategy (a): classic chemical approach, Strategy (b): previous electrochemical methods, and Strategy (c): This work.

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In recent years, synthetic organic electrochemical protocols have received significant attention since electrochemistry proposes a mild and environmentally-friendly alternative to conventional chemical approaches for organic reactions8,9,10,11. To the best of our knowledge, the electrochemical conversion of carbonyl to corresponding alcohols has a short history of merely several decades, and most of the methodology leads to pinacol products12,13,14,15. Calvo et al. reported the electrochemical reduction of ketones to corresponding alcohols using the palladium wire as a working electrode16. Gutierrez et al. developed an electrochemical system using Pd, Rh and Pt catalysts. The resulting catalyst system was successfully applied to reduce butyraldehyde, furfural, acetophenone, and benzaldehyde17. Mechanistic studies of electrochemical hydrogenation of carbonyl compounds were conducted by Lilga’s group18. Lercher and Gutiérrez's group examined the carbon-supported Pd, Re, Pt and Ni catalysts for benzaldehyde reductio19. Cathodic reduction of 5-hydroxymethylfurfural was performed on the solid electrode in the presence and the absence of glucose by Koper's group20. A similar study in 2016 showed that the reduction of 5-hydroxymethylfurfural on solid electrodes in an acidic solution produced 2,5- Bis(hydroxymethyl) furan, 2,5- Bis(hydroxymethyl) tetrahydrofuran, and 2,5-dimethylfuran21. Li et al. pointed to mechanisms of electrochemical reduction of furfural on metallic Cu electrodes in acidic media. This mechanism may happen through either electrocatalytic hydrogenation or electro-reduction paths22. In a subsequent study, Attard’s group examined the reduction of ketopantolactone (KPL) and ethyl pyruvate (EP) on platinum electrodes23,24. Koper and co-workers reported the hydrogenation of acetophenone, 4-acetylpyridine, and acetone at platinum single-crystal electrodes (Fig. 1b)25,26.

Artificial photosynthesis is a chemical process that biomimetic natural photosynthesis. In the mentioned process, water and carbon dioxide are transformed into carbohydrates and oxygen using sunlight27. Indeed, in natural photosynthesis, electrons from the water-oxidation reaction are used for carbon dioxide reduction. Oxygen-evolution reaction (OER) by water-oxidation reaction is a fundamental electrode reaction owing to its essential applications in electrochemical energy apparatus such as regenerative fuel cells and rechargeable metal-air batteries28,29,30,31. Ru and Ir oxides have been employed in the OER reactions as catalysts, though low abundance and high cost have limited their widespread use32. Recently, increasing attempts have been made to design Ni, Co, Mn and Fe-based transition metal oxides or hydroxides that replaced Ru- and Ir-based OER catalysts33,34,35,36,37,38,39. Ni and Cu foams have been widely noticed as electrodes in OER reactions due to their 3D conductive frameworks and porous structures40,41,42,43. The high porosity and surface area of 3D nanoporous foams show increased conductivity compared to 2D planar electrodes44. For example, Li et al. thermally synthesized a 3D NiO/Ni foam as OER catalysis. The oxidation of nickel compounds under thermal conditions led to the NiO film45. Also, a simple one-step process for synthesizing NiO on the surface of Ni foam during electrochemical alcohol oxidation, without the need for an external thermal or nickel source, is reported in our previous article46.

While carbonyl compounds are reduced on solid-metal electrodes using expensive homogeneous and heterogeneous metals, we propose an alternative electrochemical methodology without requiring specialized reductants via low-cost OER electrons (Fig. 1c).

Results and discussion

Reduction of carbonyl and other functional groups

Changing several electrodes as anode and cathode seriously affected the reaction efficiency (Table 1). Running the reaction with aluminum, magnesium, iron, and titanium cathode produced 4%, 37%, 32% and 5% yields (entries 1–4, respectively). Testing of other anodes, such as titanium, did not improve the flow of the reaction, while nickel–iron foam showed an improvement process (entries 5 and 6). The iron foam anode improved by 95% product with high selectivity (entry 7). The highest yield was obtained using nickel foam as both the anode and cathode, while the formation of by-products was not observed (entry 8).

Table 1 Optimization of reaction conditions. For all of these reactions, < 10% maximum error is offered.
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Replacing LiClO4 with other electrolytes such as NaCl, LiBr, and KIO3 led to a notable drop in the yield (entries 9–11). Unsurprisingly, no reaction occurred without electrolytes (entry 12). Moreover, by changing the electrolyte to 0.2 mmol, the amount of product decreased to 59% (entry 13). While CH3CN and ethanol were almost equally efficient, the latter choice was for its superior safety profile (entry 14). However, the reaction did not proceed when ethanol was used instead of a mixture of ethanol and H2O (entry 15). Changing the voltage to 2.3 V or 1.3 V decreased the yield of the reaction (entries 16 and 17).

So, the final optimized conditions for reducing benzaldehyde to benzyl alcohol are as follows: LiClO4 as the supporting electrolyte (0.5 mmol) in H2O: EtOH (6 mL) under Constant- potential conditions in an undivided cell.

With the optimized reaction conditions in hand, we probed a wide range of aldehydes and ketones to explore the ability and compatibility of this electrochemical hydroxylation reaction. As shown in Table 2, various benzaldehydes with different substituents offered the corresponding alcohols in moderate to excellent yields.

Table 2 Substrate scope of carbonyl compounds.
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Simple benzaldehyde could furnish the desired benzyl alcohol in a good yield (2a). Fortunately, o-bromo, o-chloro, and m- chloro benzaldehyde worked well in this system and provided the products (2b, 2c, 2d) in 75–100% yields. Our results indicated that electron-withdrawing groups such as cyano, chloro, and bromo at the para position afforded 21%, 59%, and 90% isolated yields. Electron-donating groups like isopropyl, methyl, and methoxy at the para position were all converted in 30–100% yields (2 h–2j). The reduction of 2, 6-dichlorobenzaldehyde was more challenging than the 2, 4-dichlorobenzaldehyde due to its satirically hindered. 2, 4-Dimethylbenzaldehyde and 2, 4-dichlorobenzaldehyde provided the corresponding alcohols in 100% yield (2 l and 2 m). Notably, 2, 4-dimethoxybenzaldehyde produced the desired alcohol in a trace yield (2n). The 1-naphthaldehyde and 2-naphthaldehyde substrates provided the desired 2o and 2p in 45% and 57% yield, respectively. As examples of ketones, Acetophenone successfully transformed into 1-phenylethanol in 88% yield (2q), while the reduction of benzophenone failed (2r). No product was detected when cyclohexanone and hexanal were utilized as reaction substrates (2 s and 2t). In all cases, the yield obtained from this method has been very successful compared to the results of previous reports.

To find further applications for the procedure, we attempted to extend the reaction scope to challenging substrates such as C=C and C≡C bond reductions. The hydrogenation of alkene and alkyne has also been an essential tool in synthesizing medicines and pharmaceutical industries48,49. However, these reactions often require the use of noble transition-metal catalysts (e.g. Pd and Pt) or Renay-Ni, which is less expensive albeit hard to handle50,51,52. The summarized -electrochemical hydrogenation of unsaturated organic compounds reports were collected in a review article by Ye in 202153. Under the present system, styrene and phenylacetylene afforded ethylbenzene in a 99% yield. Further investigation indicates a low level of selectivity found under our method for the C=O bond reduction over C=C hydrogenation. To verify this claim, we investigated the reaction of cinnamaldehyde and 3-phenylpropiolaldehyde. The GC results showed that hydrogenation of C=C was faster than C=O reduction (Fig. 2a,b). To demonstrate the scalability of this new technology, a 1.061 g scale reduction of benzaldehyde was performed using inexpensive nickel foam as anode and cathode in a breaker, and the corresponding product 2a resulted in a near quantitative 70% isolated yield (Fig. 2c).

Figure 2
figure 2

Hydrogenation of alkyne and alkene (a). A comparison of the activity of carbonyl and alkene/ alkyne functional group (b). Scaling up the hydrogenation reaction(c).

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We observed a featureless cyclic voltammogram without any cathodic peak for a mixture of EtOH/ benzaldehyde and EtOH/ H2O/ benzaldehyde, whereas cyclic voltammetry recorded from benzaldehyde displayed a new cathodic peak near − 2.2 V (Fig. 3a). Likewise, Fig. 3b exhibits linear sweep voltammograms (LSV) prepared using electrolyte solutions containing benzaldehyde, EtOH, EtOH/ H2O, and a mixture of EtOH/ H2O/ benzaldehyde, suggesting that benzaldehyde reduction takes place at near − 2.0 V (see more details in supporting information)18.

Figure 3
figure 3

(a) Cyclic voltammograms of benzaldehyde (black), benzaldehyde in EtOH (green), and benzaldehyde in EtOH: H2O (red) on nickel foam. (b) Linear sweep voltammetry scans (LSV) with LiClO4 and NH4PF6 as the supporting electrolyte when containing benzaldehyde (black), benzaldehyde in EtOH (green), EtOH: H2O (blue), and benzaldehyde in EtOH and H2O (red). Scan rate set at 20 mV/s with a nickel foam working electrode. An Ag/AgCl electrode was used as the reference and a nickel foam as the counter electrode.

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Although the mechanism of carbonyl reduction was not discussed herein, the experimental and theoretical study of benzaldehyde reduction by Rousseau and Lilga’s group strongly implied that in protic solvents (H2O or EtOH), alcohol is the favorite product.

Unlike protic solvents in aprotic conditions, dimerization was preferred, and the pinacol product was obtained18. In addition, the current electrochemical methodology failed to produce stable C–N products for the reduction of imines such as N-benzylideneaniline. However, aldehyde reduction afforded only trace quantities of benzyl alcohol (Fig. 4).

Figure 4
figure 4

Electrochemical reduction of N-benzyl-1-phenylmethanimine.

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A comparison of direct cathodic reduction of benzaldehyde with other chemical and electrochemical methods is summarized in Table 3.

Table 3 Comparison of the efficiency of the present method with other previous methods for reducing benzaldehyde.
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Conclusions

In summary, an innovative electrochemical protocol for reducing carbonyl groups to the corresponding alcohols was developed. The reaction was performed under external reductant–free conditions to provide a broad range of alcohol and alkane compounds in good yields. Furthermore, the scalability of our procedure was confirmed with the reduction of benzaldehyde on a 1.0 g scale. We believe this green and mild approach to preparing alcohols and alkanes will apply in academic and industrial modern fields.

Methods

Materials

All chemical materials were used without further purification. Lithium perchlorate (LiClO4), benzaldehyde, 4-methoxybenzaldehyde, 4-isopropyl benzaldehyde, 4-chlorobenzaldehyde, 3-chlorobenzaldehyde, 2,4-dichlorobenzaldehyde, 2,6-dichlorobenzaldehyde, 2, 4-dimethylbenzaldehyde, 2, 4-dimethoxybenzaldehyde, cyclohexanone, hexanal, 2-chlorobenzaldehyde, 3-chlorobenzaldehyde, 4-cyanobenzaldehyde, 4-methylbenzaldehyde, 1-naphthaldehyde, 2-naphthaldehyde, acetophenone, benzophenone, cinnamaldehyde, and solvents were purchased from Sigma-Aldrich or Merck Company. 3-phenylpropiolaldehyde was synthesized according to the procedure followed by Kim47. The products were characterized by gas chromatography (GC) and thin-layer chromatography (TLC). Preparative thin layer chromatography (PTLC) separations were carried out on 0.25 or 0.5 mm E. Merck silica gel plates (60F-254).

The nickel foam (Nanobazar), iron foam (Suzhou JSD foam metal Co., Ltd), iron-nickel foam (Suzhou JSD foam metal Co., Ltd), aluminum foil (Suzhou JSD foam metal Co., Ltd), and titanium foil (Suzhou JSD foam metal Co., Ltd) purchased from commercial sources. The electrochemical reactions were conducted through a Model JPS-302D tracking dual DC power supply. Cyclic voltammetry analysis was carried out on a Palmsens (Emstat3 +) electrochemical workstation, using a nickel foam as the working electrode, a Pt electrode as a counter electrode, and an Ag/AgCl as a reference electrode. The cyclic voltammogram was recorded at a scan rate of 20 mV/s. GC yield was performed on a Model TOF LC/MS gas chromatograph spectrometer. 1H NMR spectra were measured in CDCl3 using a 400 MHz spectrometer (Bruker, Switzerland). 13C NMR spectra were measured in CDCl3 at 101 MHz. The following abbreviations were used to explain the multiplicities: s = singlet, d = doublet, t = triplet, q = quartet, m = multiple.

General experimental procedure

In a 10 ml beaker with a stirring bar, the starting material (0.5 mmol), LiClO4 (0.5 mmol) in 6.0 mL H2O, and EtOH (5:1) were added successively. The electrolytic cell was equipped with nickel foam (1 cm × 1 cm) as an anode and a cathode. The solution was electrolyzed at ambient temperature under a constant voltage (3.4 V). After completing the reaction, the mixture was extracted with ethyl acetate. The obtained organic layer dried over Na2SO4 and was used for GC and NMR spectroscopy.

Procedure for gram-scale synthesis of 2a

In a 100 ml beaker with a stirring bar, the starting material (10.0 mmol), LiClO4 (5.0 mmol) in 60 mL H2O, and EtOH (5:1) were added successively. The electrolytic cell was equipped with nickel foam (5 cm × 5 cm) as an anode and a cathode. The solution was electrolyzed at ambient temperature under a constant voltage (6.8 V). After completing the reaction, the mixture was extracted with ethyl acetate. The obtained organic layer dried over Na2SO4 and was used for GC and NMR spectroscopy.