Electrocatalytic Semihydrogenation of Alkynes with [Ni(bpy)3]2+

Electrifying the production of base and fine chemicals calls for the development of electrocatalytic methodologies for these transformations. We show here that the semihydrogenation of alkynes, an important transformation in organic synthesis, is electrocatalyzed at room temperature by a simple complex of earth-abundant nickel, [Ni(bpy)3]2+. The approach operates under mild conditions and is selective toward the semihydrogenated olefins with good to very good Z isomer stereoselectivity. (Spectro)electrochemistry supports that the electrocatalytic cycle is initiated in an atypical manner with a nickelacyclopropene complex, which upon further protonation is converted into a putative cationic Ni(II)–vinyl intermediate that produces the olefin after electron–proton uptake. This work establishes a proof of concept for homogeneous electrocatalysis applied to alkyne semihydrogenation, with opportunities to improve the yields and stereoselectivity.


Electrochemical experiments
All electrochemical experiments were performed outside of the glovebox, in DMF 0. For experiments assessing post-activity electrodes, at the end of a standard electrolysis, the working electrode (carbon foam) was quickly disconnected, taken out of the solution and was not rinsed. The final electrolyte solution was removed from both chambers of the electrolysis cell. The cell, which was not rinsed, was then filled with the same electrolyte containing the alkyne and acid but exempt of Ni. The nonrinsed, previously used working electrode was plunged in the fresh electrolyte and a new electrolysis performed.
Isolation procedure. The electrolysis was performed as described above but with following alteration: the concentrations At the end of the electrolysis, the cathodic electrolyte (5 mL) was mixed with 5 ml deionized water. The mixture warms following water addition and was allowed to cool down to room temperature. The DMF/water phase was extracted 3 times with 10 mL pentane. The combined pentane phases were washed with 10 mL distilled water and directly filtered over a short silica pad (ca 9 g). Full elution of the product with additional volumes of pentane was followed by thin layer chromatography. The desired fractions were combined and the solvent removed on rotary evaporator. The obtained oil was taken in ca 5 mL of diethyl ether and the solvent removed on rotary evaporator to afford the final product as an oily colorless solid (48.2 mg; 53% yield).
Spectroscopic measurements were recorded during chronoamperometry at -1.9 V (vs reference electrode).
UV-SEC experiments were performed in a thin-layer quartz glass cell (1 mm optical path length, 013511 spectroelectrochemical cell kit, ALS Co., Ltd, Japan) using a gold mesh in the optical path, a platinum wire, and the AgNO3/Ag electrode described above as working, counter, and reference electrodes, respectively. The cell was filled with 1 mL of the electrolytic solution under investigation. The spectra were recorded every 10 s for 10 min.
IR-SEC experiments were conducted in an optically transparent thin-layer electrochemical (OTTLE; Department of Chemistry, University of Reading) cell fitted with NaCl windows, equipped with a gold mesh working electrode in the optical path, a Ag wire pseudo-reference electrode, and a platinum mesh counter electrode. 2 The OTTLE cell was filled with 0.3 mL of the solution under investigation. The spectra were recorded every 30 s for 5 min.

Analytical methods
Samples were analyzed by gas chromatography using gas chromatographs equipped with a flame ionization detector (GC- Integrals of the GC-FID and NMR peaks of the substrates and products were normalized over the one of the internal standard (mesitylene) for quantification. The quantification of carbon balance, alkyne conversion, alkene yield, faradaic efficiency (F.E) toward alkenes and turnover numbers (TONs) were calculated using the following equations: Where C i (S), C t (S), C i (SH ! ) and C t (SH ! ) are concentrations in alkyne S or alkene SH2 at the beginning of reaction (Ci) and at the given time (Ct), # (SH ! ) is the amount of alkene at a given time, $ (Ni) is the amount of Ni at the beginning of the reaction, Qt is the charge passed through the system at a given time and F is the Faraday constant (96485 C·mol -1 ).
Carbon balance, alkyne conversion, yields in alkenes were quantified from GC-FID measurements, unless otherwise noted.
The reported Z/E ratios of products are evaluated based on the integration of the signals of the olefinic protons in 1 H NMR, unless otherwise noted. In the case of 4-octenes, the Z/E ratio was calculated from the GC-FID chromatograms using two standard isomers. The presence of detectable amounts of alkane products was assessed using GC-FID when reference alkane compounds are available. When reference alkane compounds are not available, GC-MS was used to provide a qualitative evaluation of the presence of alkane.
Analysis of the post-activity electrodes was performed following methods established in literature. 3,4 The working electrode was namely disconnected right after electrolysis and taken out of the solution.
For electron microscopy, the working electrode was dried under a N2 stream overnight and the dried sample was mounted on an aluminum stub with gold holder and introduced in the electron microscope. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) were recorded on S-5500 (Hitachi, Japan) microscope at an acceleration voltage of 30 kV.
For metal trace titration, the working electrode was digested in a 69% HNO3 solution for 15 min at 200 o C using a microwave oven (MARS6, CEM, USA) and the resulting solution submitted to analysis by inductively coupled plasma mass spectrometry (ICP-MS; ICPMS-2030, Shimadzu, Japan).

Thermodynamic calculation of standard potential
The interconversion of an alkyne S with the corresponding alkene SH2 in a solvent (s) with the presence of an acid (HA) is described by: with the corresponding standard potential E S/SH 2 ,HA,s 0 .
In the case of benzoic acid (BA) as a proton source in DMF, the parameters are as follow: Thus: In first approximation here, we do not account for the homoconjugation of the acid with the corresponding base, although the phenomenon is known and quantified for BA in DMF.
In the case of the hydrogenation of 4-octyne 1 to 4-octene 1H2 in DMF, the parameters are as follow: (1)

Electrochemical behavior of Ni
Fig. S1 CVs of Ni.

Calculation of bipyridine released from Ni
The peak current at a reversible CV wave of a freely diffusing species is given by equation 3-1: 10 where ip,c is peak current, F is Faraday constant (96485.3 C·mol -1 ), S is the surface area of working electrode, C 0 is concentration of the analyte in bulk, D is diffusion coefficient of the analyte, ν is scan rate, R is universal gas constant (8.314 J·K -1 ·mol -1 ), and T is absolute temperature.
From the cathodic peak current at the bpy 0/couple (E1/2 = -2.60 VFc) observed in CVs of bipyridine at different concentrations ( Fig. S3), we estimated a diffusion coefficient of bipyridine under our conditions at: D(bpy) = 6.08 10 -5 cm 2 ·s -1 . Using that value, the concentration of bipyridine released from Ni upon addition of 1 could be recovered from the cathodic peak current at -2.65 VFc (Fig. S2b). The values are reported in Table S1 and Fig. S4. S13 Table S1. Concentration of released bipyridine from Ni upon addition of 1. [

iR-corrected potentials applied in electrolysis
In our experimental electrolytic setup (two-compartment cell split by a P3 frit), the application of iR compensation during electrolysis led to an oscillating behavior. For that reason, electrolyses were performed without compensating for ohmic drop. Nevertheless, an estimation of the potentials corrected from ohmic drop can be obtained using the ohmic drop measured prior to electrolysis (Rcell), the potential applied during electrolysis (Eapp) and the current averaged during electrolysis (<i>).
An estimate of the iR-corrected potential is then given by: Eapp,corr = Eapp -<i>×Rcell. The corresponding values are reported in Table S3. We note that these Eapp,corr values represent conservative negative estimates of the potentials applied at initial time, since the magnitude in current decays as the electrolyses proceed and the alkyne is consumed.      The electrolysis in an electrolyte saturated with H2 but exempt of acid (BA) under conditions otherwise identical to our standard ones shows only minor conversion of the alkyne 1 (3.4%), with olefin evolution below traces (Fig. S16). This result further assesses that, in our system, the nature of the conversion process is an electrocatalytic hydrogenation and not an electrochemically-assisted hydrogenation. At the end of an electrolysis run under our standard conditions for 45 min (Fig. S18a, mauve area), the working electrode (carbon foam) was quickly disconnected, taken out of the solution and was not rinsed. This procedure is aimed to prevent the re-dissolution of any Ni deposits that can occur in the absence of (cathodic) applied potential and lead to false negative rinse-tests. 4 The final electrolyte solution was removed from both chambers of the electrolysis cell (also not rinsed) and replaced by fresh electrolyte containing 1 and BA but exempt of Ni. The non-rinsed, previously used working electrode was plunged in the fresh electrolyte and a new electrolysis performed. This second electrolysis produces only traces of conversion (3.9 % at 45 min; Fig. S18a, grey area). A similar experiment but stopping the first electrolysis at 15 min (this duration being our shortest estimate to reach full alkyne conversion) produces a similar result (Fig. S18b), although with a remainder of activity in the second electrolysis (at a rate at least 20 times slower) likely due to the presence of active Ni complex in the remainder of initial electrolyte carried over by the non-rinsed electrode and cell.

Without and with applied potential
We also note that no induction period is observed prior to activity during electrocatalytic runs with Ni (see Fig. S11,15), which would indicate the degradation of the molecular complex into another species responsible for catalysis.

S30
was not rinsed, such results can indicate a deposit of Ni strongly adsorbed on the electrode or Ni species dissolved in the remaining film of electrolyte covering the electrode. Although we cannot conclude on the exact speciation of these deposits (adsorbed molecular complexes, clusters or nanoparticles), SEM pictures of an electrode following an electrolysis under our standard conditions (including [Ni]) and disconnected as described above (Fig. S20) do not evidence Ni deposits, which suggest that these deposits would be of small size (< 10 nm).
Collectively, these results conclusively demonstrate that, if Ni deposits may form during electrolysis, these deposits are not the species responsible for electrocatalytic alkyne semihydrogenation in our conditions.            The IR-SEC experiment conducted with Ni and 1-phenyl-1-propyne 2 (used to avoid overlay with solvent signature) produces a band at 1924 cm -1 (Fig. S40b,d), which is not observed in the absence of 2. This value is intermediate between the ones encountered for unsaturated C-C bond stretching in disubstituted alkynes (2200-2250 cm -1 region) and in 1,2disubstituted alkenes (1600-1650 cm -1 region). 14 Fig. S10b). This increase suggests that the reduced Ni-hydride is also competent to transfer the hydride to the triple C-C bond. The electrocatalytic activity towards alkyne is further assessed by electrolysis of 1 with TFA (Ni/1/TFA 1:10:100), which produces the alkene 1H2 in 38% yield and 3.8 TONs (Table S2). This result confirms that TFA is a suitable acid for alkyne semihydrogenation  S56 electrocatalyzed by Ni. Altogether, a mechanism for alkyne semihydrogenation shuttling via a Ni hydride complex is thus more plausible with strong acids such as TFA (Fig. S41b).

Additional mechanistic discussion
An electrocatalytic wave developing from -1.80 VFc being also observed at high excess of BA vs 2 (Ni/2/BA 1:5:1-50; Fig.   S8a) suggest that a Ni hydride mechanism may also operate with that acid, although at potentials more cathodic than the mechanism involving a nickelacyclopropene intermediate.