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

Article abstract of DOI:10.1002/chem.202002075

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.

Full text of DOI:10.1002/chem.202002075

Accepted Article
Accepted Article
Title: Chemoselective Electrochemical Hydrogenation of Ketones and
Aldehydes with a Well-Defined Base-Metal Catalyst
Authors: Igor Fokin and Inke Siewert
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
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to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
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content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.202002075
Link to VoR: https://doi.org/10.1002/chem.202002075
01/2020

10.1002/chem.202002075
Chemistry - A European Journal
1 Chemoselective Electrochemical Hydrogenation of Ketones and
2 Aldehydes with a Well-Defined Base-Metal Catalyst
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Igor Fokin, Inke Siewert*
Universität Göttingen, Institut für Anorganische Chemie, Tammannstr. 4, 37077 Göttingen, Germany
E-Mail: inke.siewert@chemie.uni-goettingen.de
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Hydrogenation reactions are fundamental functional group
transformations in chemical synthesis. Herein, we introduce an
electrochemical method for the hydrogenation of ketones and aldehydes
via 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 3e−
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.
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20 Introduction
21 Hydrogenation reactions are of utmost academic
51 In a putative electrochemical hydrogenation protocol
52 using a molecular catalyst, the key M−H species for
53 the catalytic hydrogenation reaction is formed
54 electrochemically upon reduction and protonation of
55 the metal ion (Figure 1, top).
22 and industrial importance as they represent one of
23 the key reactions for the synthesis of bulk chemicals
24 as well of elaborated organic molecules.1 Initially,
25 research on hydrogenation reactions focused on the
26 use of noble metal complexes based on Ru, Rh, Ir,
27 Pd, Pt and remarkable achievements have been made
28 using such complexes. Recently the focus shifted to
29 the use of non-precious metal catalysts, as these
30 metals exhibit a higher availability in earth’s crust.2
31 In the first instance, the direct reduction of C=O
32 bonds with H2 yielding the alcohols seems to be a
33 very atom and redox economic approach. However,
34 the reaction protocols for such hydrogenation
35 reactions require often harsh conditions with high H2
36 pressure at elevated temperature. Furthermore, each
37 ton of H2 produced via steam reforming produces
38 11.88 tons of CO2 and the process operates with an
39 external energy efficiency of only 60.4%, and thus,
40 more sustainable protocols are highly desirable.3
41 Another approach, yet rarely investigated in
42 molecular catalysis, is the use of protons and electric
43 current as hydride source.4 Electrochemistry has
44 been used recently more frequently to construct
45 (organic) molecules, as it allows to use energy
46 directly from renewable sources. It represents a
47 versatile tool for more redox, atom and energy
48 economic synthesis.5 The reactive species can be
49 formed in-situ under mild conditions in high local
50 concentration near the electrode surface.6
56
57 Figure 1. Top: Electrochemical hydrogenation of
58 acetone to isopropanol. Bottom: Selected Mn
59 (pre)catalysts for the hydrogenation of ketones.
60 However, such M−H species also represent key
61 intermediates in the electrochemical hydrogen
62 production catalysed by metal complexes, and thus,
63 the in-situ formed metal hydride species may react
64 with excess of protons forming H2 instead of reacting
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with the ketone.7 From a thermodynamic point of
view, the hydrogenation, for example, of acetone is
thermodynamically favoured over proton reduction
due to the hydrogenation free energy (ΔG0 ~ −23 kJ
mol−1, see SI for detailed analysis).8 This is also
evident from the solution hydricity of isopropanol,
which is larger than the hydricity of H2. In other
words, hydride transfer from a putative LnM−H
species has a higher driving force for the former
57 promotes the initial, heterolytic cleavage of H2
58 forming the Mn−H species and the proton is
59 accepted by the basic ligand site.
60 Results and discussion
61 1 was synthesised and characterised as described
62 previously.14 It exhibits several reduction processes,
63 between −1.9 V and −2.9 V, which are coupled by
64 chemical reactions as described previously, and the
65 potentials depend on the solvent (Figure 2, Table 1,
66 Figure S14, Figure S15).14 All potentials in this
67 paper are referenced vs. the Fc+|0 redox couple and a
68 GC working electrode has been used for all CV
69 measurements.
10 reaction than for H+ (cf. ΔΔG0 ~ −46 kJ mol−1, see SI
11 for detailed calculation).9 However, the barrier is
12 expected to be smaller in the latter case and thus,
13 catalysts are required with large overpotentials for
14 H2 evolution or the reaction should be run near the
15 thermodynamic potential. Furthermore, the direct
16 reduction of the ketone forming the ketyl radical
17 must be prevented.5b,10,11
70 Table 1. Peak potentials vs. Fc+|0 /V of the reduction
71 processes of 1 in various solvents, ν = 0.1 Vs−1, 0.1
n
n
72 M Bu4NPF6 (dmf and CH2Cl2), 0.2 M Bu4NPF6
73 (thf), GC electrode.
18 The electrochemical hydrogenation of ketones and
19 aldehydes with polypyridine rhodium complexes has
20 been investigated previously, though, C=O and C=C
21 bonds were hydrogenated with such systems.4a-e
22 Selectivity is also a major issue for protocols using
23 metal or metal modified electrodes, electric current
24 and protons. In such systems, adsorbed H atoms,
25 which are generated electrochemically under acidic
26 conditions, have been proposed as active species.12
solvent Ep,c,1
Ep,c,2
Ep,c,3
Ep,c,4
Ep,c,5
dmf14
thf
−2.06 −2.20 −2.40 −2.54 −2.76
−1.94 −2.20 −2.47 −2.63 −2.87
CH2Cl2 −1.87 −2.14
74
27 However,
chemoselective
electrochemical
28 hydrogenation protocols for unsaturated C=O
29 compounds forming the corresponding alcohols are
30 still challenging, as the desired hydrogenation
31 reaction competes with the hydrogenation of the
32 C=C bond, which is thermodynamically favoured
33 (see SI for calculation), the direct reduction of the
34 ketone forming unselectively the follow up products
35 of the ketyl radical, and the hydrogen evolution
36 reaction (HER).12f,i,13
75
76 Figure 2. CV data of 1 in thf, c1 = 1 mM,
77 cphenol = 10 mM, cacetone = 100 mM, 0.2 M nBu4NPF6,
78 ν = 0.1 Vs−1.
37 We had reasonable evidence that the binuclear
38 manganese complex 1 may serve as a catalyst in the
39 electrochemically-driven hydrogenation reaction of
40 C=O bonds (Figure 1). We knew from a previous
41 study on 1 that (i) the proton relay in 1 facilitates the
42 formation of Mn−H species upon electrochemical
43 reduction and protonation, (ii) the Mn−H species
44 reacts with CO2 forming formate, and (iii) it shows
45 only very low reaction rate for the H2 evolution.14
46 Furthermore, 1 exhibits several structural features,
47 which proved to be beneficial in manganese
79 In order to decrease the driving force for HER as an
80 undesirable side reaction, phenol as weak acid was
81 chosen, since this leads to an estimated standard
82 potential for hydrogen evolution of −2.16 V in thf.18
83 Adding 10 equiv of phenol to a solution of 1 (1 mM)
84 led to an increased current at Ep,cat = −2.38 V in thf
85 and at Ep,cat = −2.35 V in CH2Cl2 (ν = 0.1 Vs−1, Figure
86 2, Figure S14, Figure S15). Upon adding 100 equiv
87 of acetone to these mixtures, the catalytic waves shift
88 positively, that are to peak potentials Ep,cat = −2.21 V
89 in CH2Cl2 and Ep,cat = −2.19 V in thf, which is near
90 the peak potential of the 2. reduction wave in 1. The
91 peak potential shifts upon adding acetone indicated
92 different reactivities, and thus, controlled potential
93 electrolysis (CPE) experiments were run to evaluate
94 product formation.
48 hydrogenation
catalysts.
The
selective
49 hydrogenation of ketones by a manganese complex
50 was reported for the first time in 2016.15 A few other
51 reports appeared subsequently, and most of the
52 catalysts have in common that they bear a pincer
53 ligand, a basic site in the ligand backbone, and
54 carbonyl ligands at the Mn centre (Figure 1,
55 bottom).2g,16 The bifunctionality of the catalysts is
56 supposed to be critical for catalyis.2g,15,16,17 It
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A CPE experiment at −2.2 V in CH2Cl2 showed
indeed the formation of isopropanol in the crude
reaction mixture. However, CH2Cl2 turned out to be
not suitable as a solvent, since the current dropped
quickly and Faraday efficiency (FE) for isopropanol
formation was rather low (~ 10%, Figure S16). The
current decreased with the first 20 minutes of the
electrolysis and was re-established after polishing
the electrode, which indicates a fouling of the
56 with yields of 65% (entry 7). The unsaturated
57 aldehyde pent-4-enal, was selectively converted to
58 the alcohol, too (entry 8).
59 We had no evidence that any of the carbonyl
60 compounds is reduced directly at the electrode
61 surface as none of them is reduced at the potential of
62 the catalysis in the absence of 1 as evident from the
63 CV data (Figure S18, Figure S20, Figure S22, Figure
64 S24, Figure S26, Figure S28, Figure S30, Figure
65 S32).
10 electrode surface under catalytic conditions. A CPE
11 experiment in thf at −2.2 V revealed isopropanol
12 formation with higher FE and the current held more
13 constant (Figure S17). Thus, we focused on thf. We
14 optimised the reaction conditions at first. The
15 reaction must be run in a divided cell and an excess
16 of phenol is necessary to achieve reasonable
17 conversion. Catalyst concentration of 1 mM proved
18 to be ideal in terms of current flow and FE for
19 acetone formation. Reasonable conversion of
20 acetone to isopropanol was accomplished with a
21 carbon foam electrode, 5-mol % catalyst and 7.5
22 equiv of phenol. In these optimised reaction
23 conditions, H2 formation was successfully supressed
24 and counted for only 5% of the injected charge,
25 whereas 88% of the charge counted for isopropanol
26 formation. When the ratio of acetone to catalyst was
27 increased, a TON up to 32 was observed, however,
28 the conversion of acetone was lower.
66 Table 2. Overview on the electrosynthesis
67 experiments.
29 With these optimised conditions in hand, we
30 investigated the substrate scope of the protocol
31 (Figure S17-Figure S32, Table 2). Acetone was
32 hydrogenated with yields of 60% and FE of 88%.
33 Next, we explored the influence of the carbon chain
34 lengths. Symmetrical (entry 2) and unsymmetrical
35 ketones (entry 3) with longer chain lengths were
36 successfully hydrogenated, with yields of 65% and
37 75%, respectively. The catalyst and phenol
38 concentration were maintained the same and the
39 currents were similar to the one in experiment in
40 entry 1.
68
69 $The yields were determined by 1H-NMR spectroscopy using the
70 internal standard benzene.
41 The method is not limited to the hydrogenation of
42 ketones, but also feasible for aldehydes. 1-pentanal
43 was hydrogenated with a yield of 99% under
44 otherwise identical conditions (entry 4). The
45 aldehyde with a longer chain length, i.e. 1-hexanal,
46 was also successfully employed (entry 5).
71 The selectivity for the hydrogenation of C=O bonds
72 over C=C bonds is similar to what has been
73 displayed for manganese compounds in thermal
74 hydrogenation protocols using H2,2g,15,16,17 however,
75 selective electrochemical protocols using base metal
76 complex are unknown to the best of our knowledge.
77 The electrochemical hydrogenation of similar
78 unsaturated and saturated substrates at metal
79 electrodes often suffers from electrochemical
80 efficiency due to simultaneous H2 evolution as well
81 as selectivity. Various products are often formed,19
82 e.g. hydrogenolysis, hydrogenation of the double
83 bond, coupling products of the ketyl radical,10
84 cyclisation products for unsaturated carbonyl
85 compounds20 due to direct reduction of the ketone.
47 Lastly, we show that the protocol is selective for
48 C=O bonds over C=C bonds as two unsaturated
49 ketones and one aldehyde were converted
50 chemoselectively. Electrolysis of 6-methylhept-5-
51 en-2-one yielded the corresponding alcohol.
52 However, the yield was lower likely due to the steric
53 bulk (entry 6). Terminal double bonds are also
54 tolerated as shown in the hydrogenation reaction of
55 hex-5-en-2-one forming selectively hex-5-en-2-ol
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However, at Raney-Nickel electrodes, the selectivity
for alcohol formation is reported to be good, though,
FE for aliphatic ketones were only moderate.21 The
surface orientation also plays a crucial role for
selectivity as recently shown on Pt electrodes.
Acetone is reduced nearly selectively to propanol at
stepped Pt(111) crystals and to propane at stepped
Pt(100) electrodes, which was rationalised by
differences in the coordination number of the surface
53 concentration, cacetone = acetone concentration, x, y =
54 reaction order, A = electrode area.
55 The reaction order in phenol was estimated under
56 pseudo first order conditions with varying amounts
57 of phenol. The current tentatively increases
58 proportional with the square root of the phenol
59 concentration (Figure S38). This means that the
60 catalysis is first order in phenol according to eq (1)
61 and (2), as the catalytic current icat is proportional to
10 active sites.22
62 the square root of the apparent rate constants kobs
.
11 Next, we investigated the mechanism of the reaction
12 of 1, acetone and phenol in detail. Acetone is the
13 source for isopropanol as confirmed by 13C isotope
14 labelling studies: When 13C=O-labelled acetone was
15 used, 13C−OH-labelled isopropanol was formed. No
16 other 13C-labelled products were observed by 13C-
17 NMR spectroscopy, which tentatively excludes
18 direct reduction of acetone forming the ketyl-radical
19 (Figure S3, Figure S4). This was corroborated by CV
20 study of acetone and 1, in which no catalytic wave
21 was observed (Figure S36) and in a CPE experiment
22 of 13C=O-labelled acetone and phenol without
23 catalyst, in which we do not observe any conversion
24 at all (Figure S3). Thus, we assume that the alcohol
25 is not formed via direct electrochemical reduction of
26 the ketones,5b,10,11 but instead via formation of a
27 transient Mn−H species.
63 This would be in line with one internal proton
64 transfer from the phenol unit to the metal centre
65 forming the Mn−H species, and proton transfer from
66 external phenol. The reaction order in acetone could
67 not be determined. Large excess of PhOH leads to a
68 prominent wave for hydrogen evolution, which
69 decreases with increasing amounts of acetone,
70 indicating that this reaction is supressed successfully
71 with increasing amounts of acetone (Figure S39).
72 However, the redox process assigned to the H2
73 evolution reaction is only ~180 mV more negative
74 than the catalytic wave for the hydrogenation
75 reaction, and thus reliable determination of the
76 current under large excess of phenol is not possible.
77 IR-spectroelectrochemical experiments (IR-SEC)
78 were conducted to get structural information on the
79 in-situ formed reduced Mn(CO)3 species during
80 catalysis (Figure 3.).26 The redox chemistry of 1 in
81 dmf has been investigated in detail previously by
82 various methods, and the species formed in thf upon
83 reduction are essentially the same (Figure S46).14
84 Initial reduction of 1 in the absence of any substrate
85 leads to the formation of A (blue trace). The CO
86 stretching frequencies of A at 2017, 1923, and
87 1904 cm−1 are characteristic for a dinculear Mn
88 species in which both MnI ions are coordinated by
89 one X-type ligand, three facial carbonyl ligands, and
90 the α-diimine units of L. Thus, A was previously
91 assigned to [Mn2(H−1L)(CO)3Br] (H−1L denotes the
92 ligand deprotonated at the phenol unit forming a
93 phenolate).14 That is, initial reduction of 1 induces
94 homolytic O−H bond cleavage, loss of one bromide
95 ligand and coordination of the resulting phenolate to
96 the metal ion. The all-over reaction for the formation
97 of A from 1 is redox-neutral for the complex and
98 equal to HBr elimination from 1, and the proton is
99 reduced as 0.5 equiv of H2 are evolved
100 simultaneously (Scheme S3).14,27 Subsequently, B is
101 formed (red trace). B exhibits a characteristic CO
102 vibration at a frequency of 1982 cm−1. The shift of
103 the A’ CO vibrational mode of B with regard to the
104 one of 1 is characteristic for a 1e− reduction at each
28 Isotope labelling studies employing [D6]-phenol
29 showed, that the maximum current of the catalytic
30 wave does not change with regard to the same
31 experiment with [H6]-phenol, though, catalysis is not
32 under kinetic control in these experiments (Figure
33 S33, Figure S35).23 When deuterated phenol was
34 used in a CPE experiment, the current was
35 considerably lower (Figure S34), indicating a normal
36 H/D isotope effect. This tentatively excludes an H-
37 atom transfer of the putative Mn−H species to the
38 ketone as such reversible HAT to carbon skeletons
39 usually show an inverse isotope effect due to the low
40 initial frequency of the Mn−H/Mn−D bond relative
41 to that of the C−H/C−D bond.24 CV experiments
42 with varying concentrations of catalyst revealed that
43 the current of the catalytic wave increases linearly
44 with increasing concentrations of 1 (Figure S37).
45 Since the catalytic current is proportional to
46 catalyst’s concentration according to eq (1),25 this
47 means that the reaction is first order in 1.
48 푖cat = 푛cat ∙ F ∙ 퐴 ∙ 푐cat
∙
퐷
cat ∙ 푘obs
(1)
(2)
√
49 푘표푏푠 = 푘푐푎푡 ∙ 푐푃푥ℎ푂퐻 ∙ 푐푦
푎푐푒푡표푛푒
50 ncat = number of transferred electrons, F = Faraday
51 constant, kobs = observed or apparent catalytic rate
52 constant, ccat = catalyst concentration, cPhOH = phenol
105 α-diimine unit and
a five-coordinated metal
106 centre.14,28 Thus, B has been assigned previously to
107 [Mn2(H−1L)(CO)6]−.14, We tentatively assume that
108 the ligand is still deprotonated in B as this species is
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formed in the absence of phenol. Upon further
reduction a further species C appears with CO
stretching frequencies at 1902 and 1865 cm−1 (Figure
S46, green trace). C has been observed previously
and assigned to [Mn(H−1L3)(CO)3]3−, that is a
symmetric Mn compound in which each Mn-entity
is reduced twice.14,28
44 Subsequently, we investigated the influence of the
45 proton relay on catalysis and utilised 2 as catalyst
46 (Figure S1).14 2 has a −OMe function instead of the
47 −OH function. It exhibits an irreversible, multiple
48 electron redox process at −1.93 V in thf (v = 0.1
49 Vs−1) and further reduction processes at −2.35 V and
50 −2.60 V (Figure 4), similar as previously described
51 in dmf.14 Upon adding phenol to 2, a new multiple
52 electron reduction wave appears at −2.60 V,
53 however, it appears at considerably lower potential
54 and less pronounced as in the otherwise same
55 experiment with 1. Upon adding acetone, the CV
56 trace is very similar to the one without acetone and
57 no new redox processes appear (Figure 4). This
58 indicates that 2 is not an efficient catalyst for
59 hydrogenation of acetone (ν = 0.1 Vs−1). Indeed,
60 when we run a CPE experiment with 2, acetone, and
61 phenol in thf, considerably less charge was passed
62 than with 1 and considerably smaller amounts of iso-
63 propanol were formed (Figure S5, Figure S43). Iso-
64 propanol formation can be rationalised by the large
65 excess of external phenol, which also leads to
66 protonation of the metal centre in an equilibrium
67 reaction, however the external proton source is less
68 efficient and thus, leads to a low reaction rate for
69 catalysis, which highlights the importance of an
70 internal proton relay for catalysis.29
8
9
The same experiment in the presence of phenol and
acetone revealed slight differences in the species
10 formation (Figure 3). Upon reduction of 1 (black
11 trace), A is formed (blue trace), and subsequently B
12 as major species (red trace, νCO = 1982, 1902, and
13 1890 cm−1). The characteristic C=O vibration of
14 acetone at a frequency of 1715 cm−1 is present at the
15 beginning of the experiment and decreases after
16 formation of B under applied potential. Thus, B
17 represents the steady state species under catalytic
18 conditions. Towards the end of the experiment, after
19 the C=O band of acetone is vanished, CO stretching
20 vibrations characteristic for C appear (Figure S47,
21 green trace).
22
23 Figure 3. Linear sweep IR-SEC of 1, acetone and
24 phenol. The black spectrum is the start spectrum, the
25 blue one that of the species after initial reduction and
26 the red one during catalysis. c1 = 3 mM, cphenol = 30
n
27 mM, cacetone = 30 mM, 0.2 M Bu4NPF6, ν = 0.0025
28 Vs−1, thf.
71
72 Figure 4.CV data of 2 in thf, c2 = 1 mM, cphenol = 10
73 mM, cacetone
74 ν = 0.1 Vs−1.
= 100 mM, 0.2 M
nBu4NPF6,
29 Since 1 and A appear as intermediates during
30 catalysis according to IR-SEC studies, they were re-
31 investigated by CV measurements in thf. Scan rate
32 dependent CV data of 1 showed that the peak
33 potentials shift with increasing scan rates indicating
34 coupled chemical reactions after reductions such as
35 OH-bond breaking and bromide loss as previously
36 observed in dmf (Figure S40).14 A exhibits four
37 reduction processes in thf (Ep,c of −2.07 V, −2.21 V,
38 −2.66 −2.93 V ν = 0.1 Vs−1, Figure S42). The first
39 reduction process of 1 is not present in A supporting
40 the hypothesis that initial reduction of 1 leads indeed
41 to O−H bond breaking and formation of the
42 phenolate complex (Scheme 1, Scheme S3, Figure
43 S42).14
75 Taking all measurements together, a preliminary
76 mechanism is proposed (Scheme 1). IR-SEC studies
77 revealed that the complex keeps the facial tri-
78 carbonyl motive during reduction. B represents the
79 steady state species under catalysis according to IR-
80 SEC. In line with this, catalysis appears at the
81 potential of the third reduction of 1, which is equal
82 to the second reduction of A. The reduction of 1
83 forming A and 0.5 equiv of H2 requires one electron
84 and further reduction forming B requires two
85 electrons. Indeed, electrolysis of 1 at a potential of
86 −2.2 V confirmed that 3 electrons are consumed
87 upon forming
B
and that H2 is evolved
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simultaneously (Figure S45).14,30 Subsequently, we
suggest protonation of B in an equilibrium reaction
with external phenol. Further reduction leads to the
transient formation of a MnI−H entity due to the
internal proton shift from the phenol-OH unit to the
metal ion. Based on the calculated pKa value of 27
34 Although, the investigation indicate that catalysis
35 proceeds at only one Mn ion, the reduction of the
36 second Mn(CO)3 unit is crucial for catalysis. The
37 corresponding mononuclear complex, 3 (Figure S1),
38 is not active in the electrochemical hydrogenation
39 reaction of ketones. Only a small increase in current
40 was observed in the CV experiment of 3 with phenol
41 at very negative potential, i.e. Ep = −2.6 V (v = 0.1
42 Vs−1) and it does not change, when acetone is added
43 (Figure S44).
for
the
similar
manganese
compound
[(Lbpy)Mn(CO)3H] (Lbpy = 2,2’-bipyridine)31 and the
experimentally determined pKa of phenol of 29.14 in
10 MeCN,32 the proton shift may be best described as
11 an equilibrium reaction between the two species. We
12 assume that such a Mn(I)−H species represents the
13 active species, as such motives were proposed
14 previously in similar thermal protocols with H2,
15 though further reduction of the putative MnH species
16 prior transfer cannot be excluded.2g,16,34,33 However,
17 since the coordination sphere of such a species is 6,
18 pre-coordination of acetone to the Mn−H unit at the
19 same metal centre seems unlikely, and thus we
20 assume H− transfer from the Mn−H unit and of H+
21 from phenol to the ketone.34 Similar scenario has
22 been suggested recently for the hydrogenation of
23 imines and ketones with manganese(I) complexes
24 and is in line with the observed reaction order of one
25 in phenol and in 1.2g,16,34,34 Mezzetti et al showed in
26 a stochiometric reaction that acetophenone indeed
27 inserts into a Mn−H bond for LPNP(CO)2Mn−H
44 Conclusion
45 We have developed an electrochemical method for
46 the chemoselective hydrogenation of ketones and
47 aldehydes using a base-metal catalyst. In a proof of
48 principle study we showed, that the in-situ generated
49 hydride species hydrogenates C=O bonds over C=C
50 bonds. The competing hydrogen evolution reaction
51 was supressed successfully as the reactions proceed
52 with moderate to high FE. Detailed mechanistic
53 investigation suggest [Mn2(H−1L)(CO)6]−14 as steady
54 state species during catalysis. The internal proton
55 relay represents the key feature to form the putative
56 hydride species as it facilitates proton transfer to the
57 metal centre, which then reacts with the ketone
58 forming the alcohol. 2, which exhibits an –OMe
59 group instead of –OH is barely active and the same
60 is to say for the mononuclear manganese complex 3
61 highlighting the importance of the internal proton
62 relay and the two metal centres. The electrochemical
63 protocol provides a safer and green method for the
64 hydrogenation of C=O bonds compared to traditional
65 protocols, as a more redox and energy economic way
66 to the substrate H2 is explored and a base metal
67 catalyst was utilised.
28 forming the alcoholate (LPNP
29 (cyclohexyl(methyl)phosphanyl)ethyl)amine).34
=
bis(2-
68 Acknowledgement
69 This work was supported by funding from the DFG
70 [SI 1577/2 (I.S.)] and the Georg-August-Universität
71 Göttingen. We thank Marc Neben for assisting the
72 experiments employing 3. IS thanks Prof. Sven
73 Schneider for helpful discussion.
74 Experimental
75 1, 2 and 3 were synthesised as previously
76 described.14
77 General procedure for the hydrogenation of the
78 ketones and aldehydes: All CPE experiments were
79 carried out inside a glovebox with a closed, divided
80 cell. The counter electrode was separated from the
81 bulk solution by a porous glass frit (P3). Carbon
82 foam was used as working electrode, a Pt spiral as
83 counter electrode, and a Ag/AgNO3 electrode as
84 reference electrode. The cathodic compartment was
85 filled with a solution of 1 (1mM), substrate (20 mM),
86 phenol (150 mM), and benzene (8.2 µL) as internal
87 standard in [D8]-thf (3 mL). The anodic
30
31 Scheme 1. Proposed mechanism for the
32 hydrogenation of acetone using 1. H−1L denotes the
33 ligand deprotonated at the phenol unit.
6
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1
2
3
4
5
6
7
8
compartment was filled with a solution of ferrocene
(50 mg) in [D8]-thf (3 mL) as sacrificial reagent.
After electrolysis the liquid phase of both
compartments of the cell was transferred into a
Schlenk flask, which was frozen in liquid nitrogen.
The Schlenk flask was connected via a glassbridge
to a second flask. A static, high vacuum was applied
on the whole system and the frozen electrolysis
9 solution was slowly warmed up, whereas the second
10 flask was cooled with liquid nitrogen in order to
11 condense all volatile compounds. The mixture was
1
12 analysed by H-NMR spectroscopy afterwards. All
13 alcohols are literature known and thus, the products
14 were identified by comparison with the original data.
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8
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TOC
Herein, we present an electrochemical protocol for the hydrogenation of ketones forming selectively the
corresponding alcohols using protons and electric current. A dinuclear manganese catalyst is used with an internal
proton relay to facilitate the reaction. Investigation by means of electrochemical methods and IR-
spectroelectrochemistry shed light on the catalytic cycle.
1
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