Ruthenium-Catalyzed E-Selective Partial Hydrogenation of Alkynes under Transfer-Hydrogenation Conditions using Paraformaldehyde as Hydrogen Source

Article abstract of DOI:10.1002/cctc.202001411

E-alkenes were synthesized with up to 100 % E/Z selectivity via ruthenium-catalyzed partial hydrogenation of different aliphatic and aromatic alkynes under transfer-hydrogenation conditions. Paraformaldehyde as a safe, cheap and easily available solid hydrogen carrier was used for the first time as hydrogen source in the presence of water for transfer-hydrogenation of alkynes. Optimization reactions showed the best results for the commercially available binuclear [Ru(p-cymene)Cl₂]₂ complex as pre-catalyst in combination with 2,2-bis(diphenylphosphino)-1,1-binaphthyl (BINAP) as ligand (1 : 1 ratio per Ru monomer to ligand). Mechanistic investigations showed that the origin of E-selectivity in this reaction is the fast Z to E isomerization of the formed alkenes. Mild reaction conditions plus the use of cheap, easily available and safe materials as well as simple setup and inexpensive catalyst turn this protocol into a feasible and promising stereo complementary procedure to the well-known Z-selective Lindlar reduction in late-stage syntheses. This procedure can also be used for the production of deuterated alkenes simply using d₂-paraformaldehyde and D₂O mixtures.

Full text of DOI:10.1002/cctc.202001411

Full Papers
doi.org/10.1002/cctc.202001411
ChemCatChem
1
2
3
4
5
6
7
8
9
Ruthenium-Catalyzed E-Selective Partial Hydrogenation of
Alkynes under Transfer-Hydrogenation Conditions using
Paraformaldehyde as Hydrogen Source
Marcus N. A. Fetzer+,[a] Ghazal Tavakoli+,[a] Axel Klein,[a] and Martin H. G. Prechtl*[a, b]
Dedicated to Prof. Dr. Luis A. Oro on the occasion of his 75th anniversary in 2020.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
E-alkenes were synthesized with up to 100% E/Z selectivity via
ruthenium-catalyzed partial hydrogenation of different aliphatic
and aromatic alkynes under transfer-hydrogenation conditions.
Paraformaldehyde as a safe, cheap and easily available solid
hydrogen carrier was used for the first time as hydrogen source
in the presence of water for transfer-hydrogenation of alkynes.
Optimization reactions showed the best results for the
commercially available binuclear [Ru(p-cymene)Cl2]2 complex as
pre-catalyst in combination with 2,2-bis(diphenylphosphino)-
1,1-binaphthyl (BINAP) as ligand (1:1 ratio per Ru monomer to
ligand). Mechanistic investigations showed that the origin of E-
selectivity in this reaction is the fast Z to E isomerization of the
formed alkenes. Mild reaction conditions plus the use of cheap,
easily available and safe materials as well as simple setup and
inexpensive catalyst turn this protocol into a feasible and
promising stereo complementary procedure to the well-known
Z-selective Lindlar reduction in late-stage syntheses. This
procedure can also be used for the production of deuterated
alkenes simply using d2-paraformaldehyde and D2O mixtures.
1. Introduction
catalyst,[18] and even base metal catalysts.[19–20] Various transition
metal catalysts[5,10,13,15,36–51] have been developed so far which in
combination with different transfer hydrogenation sources[21–31]
or dihydrogen gas[32–35] can be used for the stereoselective
semi-hydrogenation (reduction) of alkynes to olefins. In this
regard, selective formation of E-alkenes is more challenging and
less well-studied.[36–40] Traditionally, E-alkenes obtain from the
Birch reduction using dissolved metals which precludes the use
of base-labile and sensitive substrates.[41–45] To date, there is a
handful of widely applicable direct procedures for the selective
reduction of alkynes to E-alkenes by means of catalytic semi-
(transfer)hydrogenation.[30,46–49] Scheme 1a shows some of the
successful E-selective alkene synthesis procedures using H2 gas
as the hydrogenation source.[39–40,50–53] Application of H2 gas in
these examples, however, requires a more expensive setup and
increases the safety issues. Transfer semi-hydrogenation could
be therefore, a good solution for this problem. In 2016,
Lindhardt et al. reported Ru-catalyzed E-selective alkyne transfer
semi-hydrogenation at low temperatures and low H2 pressure
(Scheme 1b) on a large substrate scope of alkynes.[17] The
reactions were carried out in a special two-chamber reactor
(COwareTM) or in a continuous flow reactor. Different substrates
containing various functional groups were reduced under these
conditions with very high yields and chemo- and stereo-
selectivity. However, simple and easily scalable protocols,
avoiding the use of sophisticated equipment, for sustainable E-
selective transfer hydrogenation of alkynes complementing the
Lindlar-type Z-selective reactions are so far not at hand.
Semi-hydrogenation of alkynes is a very attractive and challeng-
ing reaction and a wide spectrum of homogeneous and
heterogeneous catalytic approaches has been developed in this
context.[1–8] This transformation is especially interesting from the
point of view that alkynes are readily accessible from the well-
known and reliable Sonogashira reaction.[9–13] Alkenes are most
versatile building blocks which can be converted easily into
many other functional groups.[14–16] Olefins can be found also in
technically or medically important natural products, bulk
chemicals, fragrances, pharmaceuticals, agrochemicals, and
polymers[14–16] In all these cases, the control over the stereo-
chemistry is crucial.[17] Reduction of alkynes selectively to Z-
alkenes can be carried out using the well-known Lindlar
[a] M. N. A. Fetzer,+ Dr. G. Tavakoli,+ Prof. A. Klein, Priv.-Doz. Dr. M. H. G. Prechtl
Department of Chemistry
University of Cologne
Greinstr. 6
D-50939 Köln (Germany)
E-mail: martin.prechtl@uni-koeln.de
Homepage: www.h2.bio
[b] Priv.-Doz. Dr. M. H. G. Prechtl
Instituto Superior Técnico
Universidade de Lisboa
Av. Rovisco Pais 1
1049-001 Lisboa (Portugal)
E-mail: martin.prechtl@tecnico.ulisboa.pt
[+] These authors contributed equally to this work.
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/cctc.202001411
Recently, the number of reports on the use of C1 molecules
such as formic acid,[21–22,24,26,28,30, 37–38,54–57] formaldehyde,[58–61] and
methanol[62] as hydrogen sources in transfer-hydrogenation
reactions are increasing. Although it has been considered
frequently as an inexpensive and safer substitute for pressurized
© 2021 The Authors. ChemCatChem published by Wiley-VCH GmbH. This is
an open access article under the terms of the Creative Commons Attribution
Non-Commercial NoDerivs License, which permits use and distribution in
any medium, provided the original work is properly cited, the use is non-
commercial and no modifications or adaptations are made.
ChemCatChem 2021, 13, 1317–1325
1317
© 2021 The Authors. ChemCatChem published by Wiley-VCH GmbH

Full Papers
doi.org/10.1002/cctc.202001411
ChemCatChem
water),[67] for reductive methylation (with pFA in water)[61] and
1
2
3
4
5
6
7
8
9
most recently for reductive deamination of nitriles to alcohols
(with pFA in water).[58] In terms of practical aspects, it turned
out that the easy-to-handle non-volatile paraformaldehyde
(oligomeric solid powder) is very convenient in comparison to
the volatile C1-molecule competitors, methanol, aqueous
formaldehyde and formic acid, while the hydrogen content of
pFA is equal to formaldehyde referring to the monomer. Thus,
pFA has a high potential to be used as liquid organic hydrogen
carrier. Upon addition of water, pFA converts to methanediol
with a hydrogen content of 8.4% wt. which under the
appropriate reaction conditions can be degraded to H2 and CO2
through an exothermic pathway.[60,63] On the other hand, pFA is
a very cheap, stable and readily available solid which can be
handled easily in industrial applications and in the lab and we
are aiming to test its activity in other challenging hydro-
genations or reductions.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
Herein, we report, on a highly efficient E-selective, environ-
mentally benign, base-free, operationally simple, mild and safe
semi-hydrogenation of alkynes using a commercially available
Ru catalyst and pFA as the hydrogen source (Scheme 1c). The
presence
of
the
commercially
available
2,2-bis
(diphenylphosphino)-1,1-binaphthyl (BINAP) proved to be nec-
essary for reaching high conversions and selectivities. The
reactions did not require high temperatures, long reaction
times, inert conditions, degassed/dried solvents, or special set-
ups. The use of safe, stable, cheap and solid pFA as hydrogen
source is another promising feature of this procedure which
makes it also feasible for industrial applications.
2. Results and Discussion
Inspired by our recent publication on reductive deamination of
nitrile to alcohols,[58] semi transfer hydrogenation of diphenyla-
cetylene as the model substrate using aqueous pFA as hydro-
gen source in presence of the [Ru(p-cymene)Cl2]2 complex 1 as
catalyst was tested initially (Table 1). In first experiments, adding
no ligand (Entries 1 to 7) conversions up to about 30% were
Scheme 1. Previously reported catalytic reactions for E-selective semi-hydro-
genation of alkynes compared to this work.[17,40,50–52]
hydrogen gas[58,63] in transfer hydrogenations and reductions,
formic acid suffers from some drawbacks. High temperatures
1
obtained based on product formation determined by H-NMR
°
(up to 150 C) often seem to be necessary in most of these
through variation of the amount of pFA, temperature, and
reaction time, although no stereo-selectivity was obtained
under these conditions. This is remarkable in view of the report
by Lindhardt et al. that semi transfer hydrogenation of
diphenylacetylene with this catalyst using in-situ produced
hydrogen gas or formic acid as the transfer hydrogenation
source is not possible (Scheme 1).[17] Reducing the reaction
times did not enhance selectivity (Table 1, Entries 2–4).
reactions..[37–38,54–57] And, its acidic nature causes some restric-
tions for acid sensitive substrates and thereby lowers functional
group tolerance.[37–38,54–57] To adjust the pH of the reaction
media, addition of bases, such as amines is frequently
inevitable. To overcome the problem of acidity of formic acid in
such reactions, application of methanediol produced from
formaldehyde in aqueous solutions could be suitable since it is
less acidic and therefore, applicable for acid sensitive substrates
and reactions under base-free conditions.[58–60,62–64]
°
Increasing the temperature to 70 C resulted in higher con-
versions but partial over-reduction occurred (Table 1, Entry 5).
Decreasing the concentration of pFA was not beneficial
(Table 1, Entries 6–7 and Table S3, SI).
In continuation of our work to generate dihydrogen from
aqueous formaldehyde,[65–66] or in situ generated aqueous
formaldehyde from methanol[62] and paraformaldehyde (pFA)[66]
for hydrogenations or as energy carrier, we have also used
these hydrogen sources in combination with [Ru(p-cymene)Cl2]
as pre-catalyst for transfer hydrogenation of alkenes (with
methanol),[62] for formaldehyde dismutation (with pFA in
To investigate effects of the reaction medium on the
reactions, different organic solvents were used in combination
with water which resulted in the conclusion that a water
toluene mixture, as well as 1,2-dichloroethane (DCE) are the
best reaction media (Table S1, SI). Application of pure water
ChemCatChem 2021, 13, 1317–1325
www.chemcatchem.org
1318
© 2021 The Authors. ChemCatChem published by Wiley-VCH GmbH

Full Papers
doi.org/10.1002/cctc.202001411
ChemCatChem
detected. Further decreasing the reaction time resulted in both
lower conversion and lower selectivity.
Table 1. Optimization of the reaction parameters for the partial transfer
hydrogenation of diphenylacetylene (DPA).
1
2
Following these observations, we tried to test the catalytic
activity and selectivity of the other Ru-based catalysts with
structures similar to complex 1. The binuclear complexes 2–8
(close derivatives of 1) (Figure 1) were synthesized following
our literature procedures[61,65,67] and were employed in the
reaction (Table 2). As we expected, most of these catalysts
showed high activity in the model reaction. Similar results were
obtained using complexes 2–4 compared with 1. As they were
not significantly better, we focused on the use of 1 as a
commercially available catalyst and Entry 20 (Table 1) was
selected finally as the set optimized parameters resulting in full
conversion and high selectivity after 2 hours reaction time,
while a shorter reaction time of 1.5 hours gave lower conversion
(91%).
To investigate the scope of our reaction, different kinds of
aliphatic and aromatic alkynes were employed under the
obtained optimized conditions (Figure 2). While other Ru-
catalyzed systems reduced aliphatic or aryl-alkyl alkynes only in
low yields (<25%) and unselectively,[68] semi transfer hydro-
genation of dialkyl substituted alkynes as well as unsymmetrical
aryl-alkyl substituted and sole aromatic ones was done
successfully and with good to excellent E-selectivity using our
catalytic system. The reaction times were also noticeably short.
Functional group tolerance of the defined protocol is also
appreciable. The presence of both electron-donating and
electron-withdrawing groups on the aromatic ring can be
tolerated very well although the reaction rates are higher using
substrates bearing electron-donating groups. While proto-
3
4
5
6
7
8
9
Entry pFA
(eq)
T
time
[h]
ligand
[mol%]
conv.
[%]
E:Z:alkane
°
[ C]
1
2
3
4
5
6
7
8
8
8
8
8
8
7
5
8
8
8
60
60
60
60
70
70
70
70
70
70
18
3
6
12
18
18
18
18
18
18
–
–
–
–
–
–
–
29
trace
10
18
36
21
<5
30
–
–
–
–
–
–
–
–
–
–
–
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
PhCN (10)
phen[b] (5)
NH2-PhCN[c]
(10)
9
10
11
11
12
8
8
8
8
70
70
80
80
18
18
18
18
Br-PhCN[d] (10) 32
–
–
Ph-Py[e] (5)
Ph-Py[e] (5)
PPh3 (4)
37
46
9
13
14[a]
trace:0:
>99
15[a]
16[a]
17[a]
18[a]
19[a]
20[a]
8
8
8
8
8
8
80
80
80
80
80
80
18
18
18
12
5
(RO)3PO[f] (4)
BINAP[g] (8)
BINAP (4)
BINAP (4)
BINAP (4)
BINAP (4)
68
28:27:45
71:0:29
46:0:54
54:0:46
81:0:19
>99:0:0
100
100
100
100
100
2
Reaction conditions: diphenylacetylene (1 mmol), 1 (1 mol%), pFA, ligand,
toluene (2 mL), H2O (2 mL), T, time. [a] 2 mol% of 1 was used, [b] 1,10-
Phenantroline, [c] 2-Aminobenzonitrile, [d] 4-Bromobenzonitrile, [e] 2-
Phenylpyridine, [f] Tris(2-ethylhexyl) phosphate, [g] 2,2‘-bis(diphenylpho-
sphino)-1,1‘-binaphthyl.
resulted in only 57% conversion while E/Z selectivity was 32:25
(Table S1). Toluene was selected for further experiments, while
the toxic DCE was discarded. The ratio of toluene and water
was varied in the next step and it was found that the best
results can be obtained using a 1:1 ratio.
In the next steps, other first- and second- row transition
metal catalysts were used instead of 1 in this reaction but in all
cases, no better results were observed compared to the model
reaction (Table S2) and therefore, optimization was continued
using 1.
Different types of ligands were employed in the next step
to increase the selectivity. Nitrogen-based ligands in most cases
improved the conversions but had no significant effect on the
selectivity (Table 1, Entries 8–13). To our delight, introduction of
phosphorous-based ligands to the reaction medium had a
significant effect on both the selectivity and/or conversion of
the reaction (Entries 14–16). The best ligand in this series was
BINAP with quantitative conversion and >99% E-selectivity
using 2 mol% of complex 1 and 8 mol% of BINAP (2,2‘-bis
(diphenylphosphino)-1,1‘-binaphthyl). Decreasing the concen-
tration of BINAP to 4 mol% resulted in higher reaction rates but
also formation of diphenylethane as side-product (Table 1,
Entry 17). In order to prevent over-reduction, the reaction time
was reduced in the next step. The best results were obtained
after 2 hours while no Z-stilbene or 1,2-diphenylethane was
Figure 1. The binuclear Ru complexes 1–8 used as pre-catalysts in this study.
ChemCatChem 2021, 13, 1317–1325
www.chemcatchem.org
1319
© 2021 The Authors. ChemCatChem published by Wiley-VCH GmbH

Full Papers
doi.org/10.1002/cctc.202001411
ChemCatChem
Table 2. Comparison of the catalytic activity and selectivity of pre-catalysts 1–8 in the partial transfer hydrogenation of diphenylacetylene.
1
2
3
4
5
Entry
pre-catalyst
Conv. [%]
E:Z:alkane
6
7
8
9
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7
8
100
100
100
100
84
58
10
33
100:0:0
100:0:0
99:0:1
94:6:0
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
66:21:13
59:41:0
8:2:0
17:16:0
°
Reaction conditions: diphenylacetylene (1 mmol), pFA (8 mmol, 240 mg), pre-catalyst (2 mol%), BINAP (4 mol%), H2O (2 mL), toluene (2 mL) at 80 C for
2 hours.
dehalogenation of halogen-containing alkynes is a known side-
reaction in semi-hydrogenation using metal-based catalysts
incl. Lindlar-type catalysts,[19,69] no sign of proto-dechlorination
of substrate 9b was observed under our conditions offering
additional flexibilities. Furthermore, reducible functional groups
such as ester and nitro groups were tolerated successfully.
However, for the nitro compound 9q yielding 10q around 10%
of the overreduction side-product was also detected in the
mixture. It is noteworthy that the other catalytic systems used
for the semi-hydrogenation of alkynes suffer from low chemo-
selectivity in regard to nitro containing substrates.[50]
The reaction of heterocyclic compound 9o to 10o resulted
in good E:Z selectivity albeit it required longer reaction time.
Low stereo-selectivity was observed, however, for the conver-
sion of other heterocyclic compounds. For example, coordina-
tion of the pyridine group to the Ru center might deactivate
the catalyst by the formation of RuÀ N bonds. This assumption
was confirmed via ESI MS analysis of the crude reaction mixture
(Figure S67 SI). As the triple bond in this molecule is in para
position of the pyridine ring, interaction of the triple bond with
the active sites of the catalyst is probably hampered resulting in
poor stereo-selectivity.
occurs faster than the reduction of triple bond, in line with the
coordination of the CN moiety to the Ru center preventing the
approach of the triple bond to the Ru and subsequent
hydrogenation. In addition, we could synthesize cinnamyl
alcohol, nitro cinnamic acid, carbamates and thio carbamates
(Figure 2).
In order to get some mechanistic insight, the model
substrate, diphenylacetylene, was reacted under the optimized
conditions with H2O replaced by D2O and pFA replaced by d2-
pFA. After 3 hours, the deuterated product 10u was obtained
with >99% E-selectivity and full conversion (Scheme 3a). In
another experiment, normal pFA was used in combination with
The hydrogenation of the nitrile-containing substrate 9p
was found problematic as only 16% of the corresponding E-
alkene was formed alongside with the phenylmethanol deriva-
tive 10p’ (Scheme 2). This is in line with our previously report
on reductive deamination of nitriles to alcohols.[58] The product
ratio in Scheme 2 probably means that the nitrile reduction
Scheme 3. Isotope labelling experiments. Semi transfer hydrogenation of
diphenylacetylene under standard conditions using a) D2O and d2-pFA; b)
D2O and normal pFA; and c) d2-pFA and H2O. The degree of deuteration was
determined by comparison of 1H NMR and 2H NMR as well as GC-MS
analyses. In parenthesis, the average isolated yields of two experiments have
been reported.
Scheme 2. Reduction of nitrile containing alkyne 9p under optimized
conditions.
ChemCatChem 2021, 13, 1317–1325
www.chemcatchem.org
1320
© 2021 The Authors. ChemCatChem published by Wiley-VCH GmbH

Full Papers
doi.org/10.1002/cctc.202001411
ChemCatChem
was formed as the main product. The use of D2O alongside with
pFA gave superior yields for the formation of deuterated
alkenes over the combination of H2O and d2-pFA.
1
2
3
Unfortunately, these experiments did not reveal the origin
of the E-selectivity in our reaction. One possibility could be the
initial formation of kinetically favorable Z-product followed by
its subsequent fast isomerization to the thermodynamically
stable E-alkene. To verify this assumption, the formation of all
possible products and the consumption of the substrate
diphenylacetylene with time in the model reaction were
examined by running several batch reactions under exactly the
same conditions but different reaction times. This should allow
to determine intermediates in the initial reaction period. We
found that indeed Z-stilbene forms initially and isomerizes over
time to the desired E-stilbene (Figure 3).
To figure out the factors affecting this isomerization, the
isomerization of the commercially available Z-stilbene was
examined under varying conditions (Table 3). Under the
standard optimized conditions obtained in this work, quantita-
tive conversion of Z-stilbene to E-stilbene (96%) and 1,2-
diphenyl ethane (4%) was achieved in 2 hours. In the absence
of pFA, the isomerization did not occur. Thus, pFA is either
involved in the structure of the catalyst which is responsible for
the isomerization or the reaction goes through the usual metal-
hydride pathway via intramolecular hydrogen transfer of the
Ru-coordinated olefin.[62] The third reaction was run without
[Ru(p-cymene)Cl2]2 and as expected, no isomerization occurred
after 2 hours. In the absence of the BINAP ligand, isomerization
was very slow and after 2 hours only 6% of the E-stilbene was
formed and more than 81% of the starting material was
detected. Thus, the BINAP ligand is vital for the transfer semi-
hydrogenation of alkynes to alkenes and for the isomerization
step. On this basis, one can say that BINAP is part of the
structure of the catalyst species involved in the rate determin-
ing step of the isomerization where fast Z to E isomerization is
enforced by the high bulkiness around the Ru center. The last
reaction was performed in toluene as solvent without water.
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
Figure 2. Partial transfer hydrogenation of aliphatic and aromatic alkynes
under optimized reaction conditions. a Reaction conditions: alkyne (1 mmol),
pFA (8 mmol, 240 mg), [Ru(p-cymene)Cl2]2 (1) (2 mol%), BINAP (4 mol%), H2O
°
(2 mL), toluene (2 mL) at 80 C for the appropriate time for full conversion if
not otherwise stated (isolated yields given in brackets or *NMR yields). The
ratios in second row in each case refer to E: Z: alkane ratios, respectively. E-
selectivity in % at quantitative NMR yields (>99%) if not otherwise stated;
*NMR yields for: 10 h 95%, 10k 96%, 10 m 69%, 10n 80%, 10o 89%, 10p
16%, 10q 73%, 10r 39%. The isolated yield of 10e was lower owing to the
lower boiling point of the product.
D2O as the hydrogen sources for this reaction (Scheme 3b).
71% E-selectivity was achieved in the production of deuterated
product under these conditions. A combination of d2-pFA and
H2O, however, showed the possibility of H/D exchange during
the catalytic cycle as it has been observed also in our previous
study (Scheme 3c).[58,66] In this case, non-deuterated E-stilbene
Figure 3. Conversion of diphenylacetylene to E- and Z-alkene over time. The
molecules were unequivocally detected using 300 MHz 1H NMR spectro-
scopy.
ChemCatChem 2021, 13, 1317–1325
www.chemcatchem.org
1321
© 2021 The Authors. ChemCatChem published by Wiley-VCH GmbH

Full Papers
doi.org/10.1002/cctc.202001411
ChemCatChem
Table 3. Z to E isomerization of Z stilbene.
1
2
3
4
5
Isomerization test conditions
Z-stilbene [%]
E-stilbene [%]
DPE*[%]
6
7
8
9
1+BINAP+pFA+H2O
0
96
0
0
6
40
4
0
0
13
1+BINAP+H2O (without pFA)
BINAP+pFA+H2O (without 1)
1+pFA+H2O (without BINAP)
1+BINAP+pFA (without H2O)
100
100
81
60
0
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
°
Reaction conditions: diphenylacetylene (1 mmol), pFA (8 mmol, 240 mg), catalyst 1 (2 mol%), BINAP (4 mol%), H2O (2 mL), toluene (2 mL) at 80 C for 2 hours.
*DPE=diphenylethane
After 2 hours, only 40% of the Z-stilbene was converted which
means that the presence of water does not seem to be crucial
for the isomerization, but the reaction proceeds markedly
slower in its absence. It is noteworthy that in this case, no
alkane was detected which means that water is one of the main
sources of hydrogen production under these conditions and no
hydrogenation can occur in its absence which is in agreement
with our previous studies for hydrogen production from pFA.[66]
In a further experiment, the model reaction was repeated in
a COware® reactor to see if the reaction goes through a
hydrogenation or a transfer hydrogenation pathway. Complex
1, pFA, H2O and toluene were added to one chamber where
hydrogen is supposed to be produced; while the second
chamber contained diphenylacetylene, precatalyst 1, BINAP,
H2O and toluene. In this setup, H2 produced in the first chamber
can be transferred to the second one to hydrogenate the alkyne
substrate. The results of this reaction show that after 3 hours,
only 49% of the starting material has been hydrogenated with
a Z:E ratio of 1:1 which implied no selectivity under these
conditions. Conducting this reaction for a longer time showed
that the reaction in COware does not reach the full conversion
even after 24 hours. The results showed that around 60%
conversion is obtained after 24 hours while the E: Z selectivity is
10:6, respectively, at this point. Therefore, it can be concluded
that although semi-hydrogenation of alkyne is possible via
hydrogenation using in situ produced H2 gas in a COware, the
obtained stereoselectivity is pretty much lower than the one
observed under the optimized conditions reported in this work
and the reaction needs a lot more times to be completed
Therefore, the presence of pFA and BINAP at the same time in
the reaction mixture is crucial for the semi-hydrogenation as
well as for the Z to E isomerization in agreement with the
product distribution in Table 3. One possible argument in this
experiments were done using aliquots of the sample reaction
mixture after 30 and 60 minutes. The presence of the hydride
species A and B (Figure S68 and S69, SI) in both reaction
mixtures according to the ESI-MS data shows that the reaction
follows the same catalytic pathway as reported in our previous
works for hydrogen generation from aqueous pFA solution.[58,61,
66] The existence of species C and D can also be confirmed from
the signal at 1057.064 and 1067.094, respectively (Figure S70
and S71, SI). These species can be formed via ligand exchange
between p-cymene ligands and BINAP respectively diphenylace-
tylene. It should be noted that no catalytic species could be
found in the ESI-MS data which can be assigned to any
monomeric species containing any combination of the catalyst
and the active reaction ingredients. Therefore, we assumed that
the formiato-bridge dimeric species are the active catalytic
species in this reaction.
context is that the coordination of BINAP, as
a strong
coordinating ligand, to the Ru center by replacing the p-cymene
ligands affects the electron density of the complex such that
the hydrogenation cycle can be facilitated. It can also increase
the steric hindrance around the metal center resulting in the
facile Z to E isomerization.
With the aim of revealing the nature of the catalytic species
involved in the reaction mechanism and to extend the utility of
this protocol in late-stage and natural product synthesis, extend
ESI-MS analyses were conducted in the next step. The ESI-MS
Figure 4. Proposed pathway for the transfer semi hydrogenation of alkynes.
ChemCatChem 2021, 13, 1317–1325
www.chemcatchem.org
1322
© 2021 The Authors. ChemCatChem published by Wiley-VCH GmbH

Full Papers
doi.org/10.1002/cctc.202001411
ChemCatChem
chemicals were used without further purification. Deionized water
Based on the results obtained from our ESI-MS analyses as
1
2
3
4
5
6
7
8
9
was used for all experiments. The hydrogenation reactions were
carried out without precautions against moisture or oxygen unless
otherwise stated.
well as the mechanistic experiments and our previous studies in
this context, a tentative pathway is proposed for the transfer
semi hydrogenation of alkynes (Figure 4). The mechanism
contains two different catalytic cycles. In the first catalytic cycle,
reforming pFA in water results in the formation of the formiato-
bridge complexes E and F which are responsible for the
production of hydrogen, carbon dioxide and formic acid over
the course of the reaction. In the presence of BINAP, alkyne, the
in situ formed formic acid and water, some of the molecules of
the catalyst precursor 1, respectively E, can also form G, the
active catalytic species for the second catalytic cycle where
alkyne is hydrogenated in the presence of the in situ formed
hydrogen via the intermediate species H to produce Z-alkene.
Fast isomerization of Z-alkene to its corresponding E-alkene in
the next step delivers the desired product while regenerating
the active catalytic species G.
Instrumentation
NMR spectra were recorded with Bruker Avance II 300 (1H NMR
2
300 MHz, 13C NMR 75 MHz) using TMS as reference. H NMR spectra
were recorded with Bruker Avance III 500 using TMS as reference.
Hexamethyldisilane was employed as the internal standard for
calculating the NMR conversions and yields. Data were reported as
chemical shifts multiplicity singlet (s), doublet (d), triplet (t),
multiplet (m). High resolution ESI-MS(+) was performed on a
Thermo Scientific LTQ Orbitrap XL. GC-MS measurements were
performed on Agilent Hewlett Packard 6890 Series Plus chromato-
graph. A HP 5973 Series was used as mass detector and hydrogen
employed as the carrier gas.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
Analytical methods
Electrospray ionization mass spectrometry (ESI-MS). High resolution
ESI-MS experiments were performed (resolution 30.000) using
THERMO Scientific LTQ Orbitrap XL mass spectrometer with a FTMS
analyzer. All measurements were done on positive ion mode.
Therefore, some species could appear as protonated (m/z [M+H]+),
diprotonated (m/z [M+2H]2+), sodium adduct (m/z [M+Na]+) or as
a combination (m/z [M+H+Na]2+). Some of the ESI-MS results
reveal that, during the reaction, free coordination sites could be
occupied by different ligands present in the reaction mixture.
3. Conclusion
Highly selective partial transfer-hydrogenation of both aliphatic
and aromatic substituted alkynes was achieved using the
commercially available [Ru(p-cymene)Cl2]2 (1) complex as pre-
catalyst alongside with the ligand 2,2-bis(diphenylphosphino)-
1,1’-binaphthyl (BINAP) (1:1 ratio per Ru atom and BINAP) in a
1:1 mixture of water and toluene. Paraformaldehyde (pFA), as a
safe, stable, easy to handle, cheap and commercially available
reagent has been used for the first time as the hydrogen source
in this reaction. This protocol is complementary to the tradi-
tional Lindlar reduction resulting in the production of Z-alkenes
with very high chemo and stereo-selectivity. Mechanistic
investigations showed that E-selectivity in our reactions is a
result of Z to E isomerization of the formed alkenes occurring
only in the presence of the catalyst 1 and pFA while the
presence of BINAP is not crucial but markedly speeds up the
isomerization. Through our procedure, E-alkenes can be synthe-
sized without the use of inert conditions, dry solvents and
special setups with up to 100% selectivity. This work also
highlights the importance of ligand-metal interactions in
catalytic procedures such that, the reaction cannot be done
selectively in the absence of BINAP and any of the other tested
ligands (N and P donors). The procedure can be also used for
the production of deuterated alkenes using D2O instead of H2O.
Due to the use of pFA as a neutral reagent as well as base-free
conditions, the functional group tolerance of the presented
method is high and many different substrates can be exposed
to these conditions. All in all, we believe that this method holds
great promises for future application in late-stage synthesis.
Gas chromatography- mass spectroscopy (GC-MS). GC-MS experi-
ments were performed using an Agilent HP6890 system coupled
with a mass detector (MSD) 5937 N. Dihydrogen is used as carrier
gas with a flow rate of 14 mL/min and a pressure of 0.3 bar. An
Agilent 19091S-4335 HP-5 MS (30 m ×0.25 mm) was used as capillary
tube.
Syntheses and reactions
The complexes 2–8 were synthesized and characterized according
to our previous reports.[61,65]
Syntheses of alkynes via Sonogashira reaction. The alkyne starting
materials were synthesized based on a literature procedure[70] with
slight modifications as shown below (detailed information in the
Supporting Information):
General Procedure 1 (GP1)
[Pd(PPh3)2Cl2] (72.0 mg, 0.10 mmol, 2 mol%) was added to an oven
dried Schlenk tube under an Ar atmosphere followed by
CuI (26 mg, 0.135 mmol, 3 mol%), 20 mL of dry and degassed THF,
5 mL of dry NEt3 and 4.4 mmol of the halogen containing
compound. Then, 4 mmol of the corresponding terminal alkyne
were added. The resulting reaction mixture was further degassed
and stirred at room temperature for 48 h under Ar. The progress of
the reaction was controlled using TLC. Upon completion of the
reaction, 15 mL of distilled water was added to the mixture and the
reaction was extracted with ethyl acetate (4×8 mL). The combined
organic layers were dried over Na2SO4, filtered and evaporated
under high vacuum. The crude material was purified using silica gel
column chromatography (silica gel from ACROS 60 Å (0.035–
0.070 mm)) with appropriate eluents (refer SI) to give the expected
product.
Experimental Section
Materials
Paraformaldehyde (pFA) was purchased from Alfa Aesar. d2-pFA
and [RuCl2(p-cymene)]2 were purchased from Sigma Aldrich. All
ChemCatChem 2021, 13, 1317–1325
www.chemcatchem.org
1323
© 2021 The Authors. ChemCatChem published by Wiley-VCH GmbH

Full Papers
doi.org/10.1002/cctc.202001411
ChemCatChem
General Procedure 2 (GP2)
performed syntheses of Ru complexes. Open access funding
enabled and organized by Projekt DEAL.
1
2
3
4
5
6
7
8
9
[Pd(PPh3)2Cl2] (72.0 mg, 0.10 mmol, 2 mol%) was added to an oven-
dried microwave vial under an Ar atmosphere followed by
CuI (26 mg, 0.135 mmol, 3 mol%), 10 mL of dry and degassed THF,
3 mL of dry NEt3 and 4.4 mmol of the corresponding aryl/alkyl
halide. The mixture was stirred for 10 min under Ar. To this mixture,
4 mmol of the terminal alkyne was added. The resulting reaction
Conflict of Interest
The authors declare no conflict of interest.
°
mixture was heated to 60 C for 3 h in an Anton Paar Monowave 300
microwave. Then, 10 mL of distilled water was added and the
reaction mixture was extracted with EtOAc. The combined organic
layers were dried over Na2SO4, filtered and evaporated under
reduced pressure. The crude material was purified using silica gel
column chromatography to give the expected product.
Keywords: Partial transfer hydrogenation · E-selective alkyne
hydrogenation
·
deuterated alkenes
·
ruthenium
·
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
paraformaldehyde
General Procedure for the semi-hydrogenation of alkynes
[1] C. Deraedt, M. d’Halluin, D. Astruc, Eur. J. Inorg. Chem. 2013, 2013, 4881–
4908.
[2] A. H. Hoveyda, J. Org. Chem. 2014, 79, 4763–4792.
[3] C. Oger, L. Balas, T. Durand, J. M. Galano, Chem. Rev. 2013, 113, 1313–
1350.
A 20 mL crimp-top headspace-vial equipped with a magnetic stir
bar was loaded with pFA (240 mg, 8.00 mmol), [Ru(p-cymene)Cl2]2
complex (12.2 mg, 0.02 mmol, 2 mol%), BINAP (24.9 mg, 0.04 mmol,
4 mol%) and 1 eq. (1 mmol) of an appropriate alkyne. A mixture of
2 mL water and 2 mL toluene was used as solvent. The reaction
[4] M. Crespo-Quesada, F. Cardenas-Lizana, A. L. Dessimoz, L. Kiwi-Minsker,
ACS Catal. 2012, 2, 1773–1786.
[5] A. Molnar, A. Sarkany, M. Varga, J. Mol. Catal. A 2001, 173, 185–221.
[6] J. F. Teichert, in Homogeneous Hydrogenation with Non-Precious Cata-
lysts (Ed.: J. F. Teichert), Wiley VCH, Weinheim, 2019, pp. i-x.
[7] M. E. Moret, R. J. M. Klein Gebbink, in Non-Noble Metal Catalysis (Eds.:
M. E. Moret, R. J. M. Klein Gebbink), Wiley VCH, Weinheim, 2019, pp. i-
xix.
[8] J. G. De Vries, C. J. Elsevier, in The Handbook of Homogeneous Hydro-
genation (Eds.: J. G. De Vries, C. J. Elsevier), Wiley VCH, Weinheim, 2006,
pp. I–XXXII.
[9] C. Deraedt, D. Astruc, Acc. Chem. Res. 2014, 47, 494–503.
[10] D. Gelman, S. L. Buchwald, Angew. Chem. Int. Ed. 2003, 42, 5993–5996;
Angew. Chem. 2003, 115, 6175–6178.
[11] D. Gelman, D. Tsvelikhovsky, G. A. Molander, J. Blum, J. Org. Chem.
2002, 67, 6287–6290.
°
mixture was stirred at 80 C in a heating block. The progress of
reaction was controlled either by TLC or NMR of the crude. After
the appropriate time, hexamethyldisilane as internal standard was
added and the mixture was stirred for 5 min at room temperature
prior to analysis. The mixture was diluted with ethyl acetate before
addition of water. The aqueous layer was then extracted three
times with EtOAc (3*10 mL). The organic layers were combined,
dried over Na2SO4 and filtered. If needed, the crude product was
purified over silica gel with cyclohexane, cyclohexane/dichloro-
methane or cyclohexane/ethyl acetate as eluents.
General procedure for determination of E: Z ratios
[12] T. Hundertmark, A. F. Littke, S. L. Buchwald, G. C. Fu, Org. Lett. 2000, 2,
1729–1731.
The reported E: Z ratios are based on the GC-MS analyses as well as
NMR spectra. In each case, after completion of the reaction,
hexamethyldisilane as internal standard was added to the crude
and the mixture was stirred for 5–10 min. A small portion of the
crude was then passed through a short silica column and washed
with ethyl acetate. This sample was then analyzed with GC-MS after
evaporation of solvent and dilution with absolute ethanol, meth-
anol or ethyl acetate. Comparing the integration of the GC-MS
signals and their retention times with the ones from the pure
products, the E: Z ratios were obtained. At the same time, NMR
spectra of the crude were recorded. As the chemical shift of the
olefinic protons as well as their coupling constants in E and Z
products are different, the integration of the signals of these
protons was used also as the reference for determination of E: Z
ratios. It should be noticed that the results of GC-MS and NMR
spectra were close which confirms the accuracy of the reported
values with two independent methods.
[13] A. Zapf, M. Beller, Top. Catal. 2002, 19, 101–109.
[14] S. G. Van Ornum, R. M. Champeau, R. Pariza, Chem. Rev. 2006, 106,
2990–3001.
[15] D. H. Woodmansee, A. Pfaltz, Chem. Commun. 2011, 47, 7912–7916.
[16] Y. Zhu, K. Burgess, Acc. Chem. Res. 2012, 45, 1623–1636.
[17] K. T. Neumann, S. Klimczyk, M. N. Burhardt, B. Bang-Andersen, T.
Skrydstrup, A. T. Lindhardt, ACS Catal. 2016, 6, 4710–4714.
[18] H. Lindlar, Helv. Chim. Acta 1952, 35, 446–450.
[19] H. Konnerth, M. H. G. Prechtl, Chem. Commun. 2016, 52, 9129–9132.
[20] T. N. Gieshoff, A. Welther, M. T. Kessler, M. H. G. Prechtl, A. Jaco-
bi von Wangelin, Chem. Commun. 2014, 50, 2261–2264.
[21] C. Belger, N. M. Neisius, B. Plietker, Chem. Eur. J. 2010, 16, 12214–12220.
[22] J. Broggi, V. Jurčík, O. Songis, A. Poater, L. Cavallo, A. M. Z. Slawin, C. S. J.
Cazin, J. Am. Chem. Soc. 2013, 135, 4588–4591.
[23] J. Choi, N. M. Yoon, Tetrahedron Lett. 1996, 37, 1057–1060.
[24] R. M. Drost, T. Bouwens, N. P. van Leest, B. de Bruin, C. J. Elsevier, ACS
Catal. 2014, 4, 1349–1357.
[25] S. Enthaler, M. Haberberger, E. Irran, Chem. Asian J. 2011, 6, 1613–1623.
[26] P. Hauwert, G. Maestri, J. W. Sprengers, M. Catellani, C. J. Elsevier,
Angew. Chem. Int. Ed. 2008, 47, 3223–3226; Angew. Chem. 2008, 120,
3267–3270.
[27] S. E. Lee, J. S. Vyle, D. M. Williams, J. A. Grasby, Tetrahedron Lett. 2000,
Acknowledgements
41, 267–270.
[28] J. Li, R. Hua, T. Liu, J. Org. Chem. 2010, 75, 2966–2970.
[29] K. Semba, T. Fujihara, T. Xu, J. Terao, Y. Tsuji, Adv. Synth. Catal. 2012,
354, 1542–1550.
[30] K. Tani, A. Iseki, T. Yamagata, Chem. Commun. 1999, 1821–1822.
[31] A. M. Whittaker, G. Lalic, Org. Lett. 2013, 15, 1112–1115.
[32] Y. Blum, D. Czarkie, Y. Rahamim, Y. Shvo, Organometallics 1985, 4, 1459–
1461.
[33] A. M. Kluwer, T. S. Koblenz, T. Jonischkeit, K. Woelk, C. J. Elsevier, J. Am.
Chem. Soc. 2005, 127, 15470–15480.
[34] R. R. Schrock, J. A. Osborn, J. Am. Chem. Soc. 1976, 98, 2143–2147.
[35] M. W. van Laren, C. J. Elsevier, Angew. Chem. Int. Ed. 1999, 38, 3715–
3717; Angew. Chem. 1999, 111, 3926–3929.
We gratefully acknowledge financial support provided by the
MIWF-NRW (NRW-returnee program), the Heisenberg-Program
(DFG), the COST Action “CÀ H Activation in Organic Synthesis
(CHAOS)”, the Ernst-Haage-Prize of the Max-Planck-Institute for
Chemical Energy Conversion. Moreover, we acknowledge the
Alexander-von-Humboldt Foundation (G.T.). In addition, we
acknowledge L. E. Heim, S. Vallazza and A. Thesing for previously
ChemCatChem 2021, 13, 1317–1325
www.chemcatchem.org
1324
© 2021 The Authors. ChemCatChem published by Wiley-VCH GmbH

Full Papers
doi.org/10.1002/cctc.202001411
ChemCatChem
[36] R. Kusy, K. Grela, Org. Lett. 2016, 18, 6196–6199.
[37] T. G. Frihed, A. Fürstner, Bull. Chem. Soc. Jpn. 2016, 89, 135–160.
[38] M. Fuchs, A. Fürstner, Angew. Chem. Int. Ed. 2015, 54, 3978–3982;
Angew. Chem. 2015, 127, 4050–4054.
[39] M. Leutzsch, L. M. Wolf, P. Gupta, M. Fuchs, W. Thiel, C. Farès, A.
Fürstner, Angew. Chem. Int. Ed. 2015, 54, 12431–12436; Angew. Chem.
2015, 127, 12608–12613.
[40] K. Radkowski, B. Sundararaju, A. Fürstner, Angew. Chem. Int. Ed. 2013,
52, 355–360; Angew. Chem. 2013, 125, 373–378.
[41] R. A. Benkeser, G. Schroll, D. M. Sauve, J. Am. Chem. Soc. 1955, 77, 3378–
3379.
[42] K. N. Campbell, L. T. Eby, J. Am. Chem. Soc. 1941, 63, 216–219.
[43] E. M. Carreira, J. Du Bois, J. Am. Chem. Soc. 1995, 117, 8106–8125.
[44] C. E. Castro, R. D. Stephens, J. Am. Chem. Soc. 1964, 86, 4358–4363.
[45] I. Kováová, L. Streinz, Synth. Commun. 1993, 23, 2397–2404.
[46] P. Abley, F. J. McQuillin, J. Chem. Soc. D: Chem. Commun. 1969, 1503–
1504.
[57] R. Shen, T. Chen, Y. Zhao, R. Qiu, Y. Zhou, S. Yin, X. Wang, M. Goto, L.-B.
Han, J. Am. Chem. Soc. 2011, 133, 17037–17044.
[58] G. Tavakoli, M. H. G. Prechtl, Catal. Sci. Technol. 2019, 9, 6092–6101.
[59] G. Tavakoli, J. E. Armstrong, J. M. Naapuri, J. Deska, M. H. G. Prechtl,
Chem. Eur. J. 2019, 25, 6474–6481.
[60] L. E. Heim, H. Konnerth, M. H. G. Prechtl, Green Chem. 2017, 19, 2347–
2355.
[61] D. van der Waals, L. E. Heim, C. Gedig, F. Herbrik, S. Vallazza, M. H. G.
Prechtl, ChemSusChem 2016, 9, 2343–2347.
[62] L. E. Heim, D. Thiel, C. Gedig, J. Deska, M. H. G. Prechtl, Angew. Chem. Int.
Ed. 2015, 54, 10308–10312; Angew. Chem. 2015, 127, 10447–10451.
[63] M. Trincado, H. Grutzmacher, M. H. G. Prechtl, Phys. Sci. Rev. 2018, 3.
[64] B. Sam, B. Breit, M. J. Krische, Angew. Chem. Int. Ed. 2015, 54, 3267–3274;
Angew. Chem. 2015, 127, 3317–3325.
[65] L. E. Heim, S. Vallazza, D. van der Waals, M. H. G. Prechtl, Green Chem.
2016, 18, 1469–1474.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
[66] L. E. Heim, N. E. Schlorer, J. H. Choi, M. H. G. Prechtl, Nat. Commun. 2014,
[47] R. R. Burch, E. L. Muetterties, R. G. Teller, J. M. Williams, J. Am. Chem. Soc.
5.
1982, 104, 4257–4258.
[48] R. R. Burch, A. J. Shusterman, E. L. Muetterties, R. G. Teller, J. M. Williams,
J. Am. Chem. Soc. 1983, 105, 3546–3556.
[67] D. van der Waals, L. E. Heim, S. Vallazza, C. Gedig, J. Deska, M. H. G.
Prechtl, Chem. Eur. J. 2016, 22, 11568–11573.
[68] A. Ekebergh, R. Begon, N. Kann, J. Org. Chem. 2020, 85, 2966–2975.
[69] W. X. Niu, Y. J. Gao, W. Q. Zhang, N. Yan, X. M. Lu, Angew. Chem. Int. Ed.
2015, 54, 8271–8274; Angew. Chem. 2015, 127, 8389–8392.
[70] J. Dosso, J. Tasseroul, F. Fasano, D. Marinelli, N. Biot, A. Fermi, D.
Bonifazi, Angew. Chem. Int. Ed. 2017, 56, 4483–4487; Angew. Chem.
2017, 129, 4554–4558.
[49] N. O. Thiel, B. Kaewmee, T. T. Ngoc, J. F. Teichert, Chem. Eur. J. 2020, 26,
1597–1603.
[50] D. Srimani, Y. Diskin-Posner, Y. Ben-David, D. Milstein, Angew. Chem. Int.
Ed. 2013, 52, 14131–14134; Angew. Chem. 2013, 125, 14381–14384.
[51] R. Maazaoui, R. Abderrahim, F. Chemla, F. Ferreira, A. Perez-Luna, O.
Jackowski, Org. Lett. 2018, 20, 7544–7549.
[52] M. K. Karunananda, N. P. Mankad, J. Am. Chem. Soc. 2015, 137, 14598–
14601.
[53] S. A. Jagtap, B. M. Bhanage, J. Mol. Catal. 2018, 460, 1–6.
[54] J. Li, R. Hua, Chem. Eur. J. 2011, 17, 8462–8465.
[55] S. Musa, A. Ghosh, L. Vaccaro, L. Ackermann, D. Gelman, Adv. Synth.
Catal. 2015, 357, 2351–2357.
Manuscript received: August 30, 2020
Revised manuscript received: November 23, 2020
Version of record online: February 4, 2021
[56] E. Richmond, J. Moran, J. Org. Chem. 2015, 80, 6922–6929.
ChemCatChem 2021, 13, 1317–1325
www.chemcatchem.org
1325
© 2021 The Authors. ChemCatChem published by Wiley-VCH GmbH