Chemoselective Electrochemical Hydrogenation of Ketones and Aldehydes with a Well‐Defined Base‐Metal Catalyst

Abstract Hydrogenation reactions are fundamental functional group transformations in chemical synthesis. Here, we introduce an electrochemical method for the hydrogenation of ketones and aldehydes by in situ formation of a Mn‐H species. We utilise protons and electric current as surrogate for H2 and a base‐metal complex to form selectively the alcohols. The method is chemoselective for the hydrogenation of C=O bonds over C=C bonds. Mechanistic studies revealed initial 3 e− reduction of the catalyst forming the steady state species [Mn2(H−1L)(CO)6]−. Subsequently, we assume protonation, reduction and internal proton shift forming the hydride species. Finally, the transfer of the hydride and a proton to the ketone yields the alcohol and the steady state species is regenerated via reduction. The interplay of two manganese centres and the internal proton relay represent the key features for ketone and aldehyde reduction as the respective mononuclear complex and the complex without the proton relay are barely active.

The hydrogenation of acetone by Mn−H is thermodynamically favoured over proton reduction, which is evident from the solution hydricity of H2 and isopropanol in MeCN (Scheme S 2).

Scheme S 2. Thermodynamic cycle for the formation of isopropanol (left) and H2 (right) via hydride transfer.
The hydricity of isopropanol in MeCN was calculated previously, the one of H2 was determined experimentally. 6 The equilibrium of phenol ∆ R p a,1 0 has to be considered in both reactions and the same is to say for the hydricity ∆ R H−,MH 0 of the putative M−H species. The difference in the hydricity of isopropanol and H2 is about −49 kJ/mol. However, protonation of acetone has to be considered in the latter case, ∆ R p a,2 0 , which subtracts about 3 kJ/mol, as the pKa of protonated acetone in water is −7. 2,7 which was converted to a pKa of about 0.6 in acetonitrile by the empirical formula from Ref 8, second group. Thus, the driving force for isopropanol formation is larger than for H2 formation.

Experimental Section
General Manipulations of air-sensitive reagents were carried out by means of common Schlenk-type techniques involving the use of a dry argon or nitrogen atmosphere or performed in an MBraun glovebox. The complexes are light sensitive and thus, are prepared, handled and stored in the dark. All reagents were purchased in chemical grades of 99% or higher and used without further purification.

Electrochemical Measurements
General information on the electrochemistry: All CV measurements have been conducted in brown glassware with a glassy carbon (GC) working electrode (diameter 3 mm, ALS or CHI), a platinum wire counter electrode (Chempur), and an Ag/AgNO3 reference electrode. All data were referenced versus the Fc +|0 redox couple by adding ferrocene at the end of the measurements. The measurements have been conducted with Gamry Reference 600, or 600+ potentiostats. iR compensation was performed by the positive feedback method, which is implemented in the PHE200 software of Gamry.
Scheme S 3. Reduction chemistry of 1 in thf in the absence of substrates. 11 The species depicted in grey have not been observed spectroscopically. The CO stretching frequencies of the transient reduced species are shown in blue.            In the presence of large excess of phenol, a prominent catalytic wave appears at around −2.4 V, which belongs to the hydrogen evolution reaction. The wave decreases with increasing amounts of acetone indicating that the reaction is successfully supressed. However, since the wave belonging to the hydrogenation reaction of acetone and the one belonging to the hydrogen evolution reaction overlap, the current cannot be reliable determined under pseudo first order conditions for acetone, i.e. in the presence of large excess of phenol.     The experiment was stopped, when the current dropped down to about 10 % of the initial current. The charge passed counts for 2.7 electrons.

IR-SEC Experiments
IR-SEC experiments were conducted in an OTTLE cell. 12 The cell is equipped with a platinum mesh working electrode in the optical path, a pseudo-Ag-reference, and a platinum counter electrode. The IR spectra were recorded with a Bruker Invenio-R spectrometer equipped with a MCT-Detector (15.500 -350 cm −1 ). The scan rate for the linear sweep voltammogram was 0.0025 Vs −1 and IR-spectra were recorded every twelve seconds, that is an IR-spectrum was recorded every 30 mV.
We investigated 1 by IR-SEC in the absence of acetone and phenol in thf. The species distribution was were very similar to the species distribution previously observed in dmf (Scheme S 3). 11 For a detailed discussion and assignment of the individual species we refer to this paper.