Perfluoroalkylated Main-Group Element Lewis Acids as Catalysts in Transfer Hydrogenation

Article abstract of DOI:10.1002/ejic.201700524

Transfer hydrogenation plays an important part in organic chemistry. Recently, strong Lewis acids like B(C₆F₅)₃ have been introduced as a catalyst for these reactions. We successfully employed the Lewis acid (C₂F₅)₃PF₂ as a catalyst in the transfer hydrogenation between 1,3,5-trimethylcyclohexa-1,4-diene and 1,1-diphenylethylene. Surprisingly, the treatment of the diene alone with a catalytic amount of (C₂F₅)₃PF₂ led to a quantitative dismutation to mesitylene and 1,3,5-trimethylcyclohexane. With B(C₆F₅)₃, there was a solvent-dependency: in CH₂Cl₂ mainly the dismutation products were obtained, while in toluene the evolution of H₂ was observed. Additionally, the catalytic activity of various perfluoroalkylated germanes and silanes was tested.

Full text of DOI:10.1002/ejic.201700524

A Journal of
Accepted Article
Title: Perfluoroalkylated main-group element Lewis acids as catalysts
in transfer hydrogenation
Authors: Julia Bader, Alexander Maier, Jan Paradies, and Berthold
Hoge
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
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201700524
Link to VoR: http://dx.doi.org/10.1002/ejic.201700524

10.1002/ejic.201700524
European Journal of Inorganic Chemistry
COMMUNICATION
Perfluoroalkylated main-group element Lewis acids as catalysts
in transfer hydrogenation
Julia Bader,[a] Alexander F. G. Maier,[b] Jan Paradies,[b] and Berthold Hoge*[a]
Dedication ((optional))
Abstract: Transfer hydrogenation plays an important part in organic
chemistry. Recently, strong Lewis acids like B(C6F5)3 have been
introduced as a catalyst for these reactions. We successfully
employed the Lewis acid (C2F5)3PF2 as a catalyst in the transfer
hydrogenation between 1,3,5-trimethylcyclohexa-1,4-diene and 1,1-
diphenylethylene. Surprisingly, the treatment of the diene alone with
a catalytic amount of (C2F5)3PF2 led to a quantitative dismutation to
hydrogen disproportionation reactions.5 He also predicted a
smooth dehydrogenation for “dihydrobenzenes”, which was
confirmed by Böeseken in 19186 as well as by Zelinsky and
Pawlow7 and by Corson and Ipatieff8 in the 1930s.
Transfer hydrogenation reactions can also be divided according
to the type of catalyst used. In the subfield of organocatalysts,
chiral phosphoric acids are among the most employed ones.9
Recently, the range of suitable catalysts has been extended to
include strong Lewis acids. Different Lewis acids as catalysts
have been screened in the reaction of a cyclohexene-trione
derivative with cyclopentadiene.10 A diamidocarbene derivative
has been shown to facilitate transfer hydrogenation of 1,4-
mesitylene and 1,3,5-trimethylcyclohexane. Using B(C6F5)3,
a
solvent-dependency was observed, with CH2Cl2 yielding mainly the
dismutation products, while in toluene the evolution of H2 was
observed. Additionally, the catalytic activity of various
perfluoroalkylated germanes and silanes was tested.
cyclohexadiene
and
9,10-dihydrophenanthrene.11
The
electrophilic phosphonium cation [P(C6F5)3F]+ catalyses
dehydrocoupling and simultaneous hydrogenation of olefins.12
B(C6F5)3, known for its value in FLP chemistry, has been used
by the Stephan group for transfer hydrogenation of imines,
enamines and N-heterocycles using iPr2NH as a hydrogen
source,13 while Oestreich et al. used B(C6F5)3 for the transfer
hydrogenation of imines and 1,1-disubstituted alkenes with
cyclohexa-1,4-dienes as the source of hydrogen.14,15 The first
step in the proposed catalytic cycle is a hydride abstraction from
the diene and formation of the [B(C6F5)3H]- anion. Only recently
such reactivity has been exploited for the FLP catalysed
oxidation of heterocycles in the absence of hydrogen
acceptors.16,17
Introduction
Transfer hydrogenation plays an important part in industrial and
academic organic chemistry. Nowadays, the term is mostly used
for a catalysed hydrogen transfer between unlike donor and
acceptor units in homogeneous systems. The subject
experiences a recent increase in interest, as shown by nearly
1700 publications in the last five years, 60 of which were
reviews.1
A special case of transfer hydrogenation was classified by
Braude and Linstead as “hydrogen disproportionation” in which
hydrogen is transferred between identical donor and acceptor
units.2 One of the first examples was described as early as 1902
by Knoevenagel and Fuchs who discovered the palladium or
Over the last years, we have established the phosphorane
(C2F5)3PF2 as a strong Lewis acid in a broad variety of
applications.18 (C2F5)3PF2 is produced on a multiton scale. As a
strong Lewis acid, it forms stable adducts with neutral Lewis
bases such as dimethylaminopyridine or imines, but also with a
Brønsted
acid
catalysed
disproportionation
of
a
dihydrolutidinedicarboxylate (Eq. (1)).3
multitude of anionic Lewis bases such as F-, CN-, OH-,
20
alcoholates,19 or even H-
.
The hydridophosphate
[P(C2F5)3F2H]- can be synthesized on a multi-gram scale in
combination with different cations to render solid or liquid salts. It
is stable at room temperature for years. The stability of the
hydridophosphate [P(C2F5)3F2H]- inspired us to investigate the
(1)
use of (C2F5)3PF2 as
a Lewis acidic catalyst in transfer
One year later, Knoevenagel described the same reaction with
dimethyl-1,4-dihydroterephthalate.4 Wieland added in 1912
hydrazobenzene and dihydronaphthalene to the portfolio of
hydrogenation reactions. In this contribution, we will discuss the
suitability of (C2F5)3PF2 in different types of hydrogen transfer
reactions, as well as compare it to various pentafluoroethylated
silanes and germanes.
[a]
Dr. J. Bader, Prof. Dr. B. Hoge
Centrum für Molekulare Materialien
Fakultät für Chemie
Universität Bielefeld
Results and Discussion
Universitätsstraße 25, 33615 Bielefeld (Germany)
http://www.uni-bielefeld.de/chemie/acii/hoge/
E-mail: b.hoge@uni-bielefeld.de
A. F. G. Maier, Prof. Dr. J. Paradies
Institut für Organische Chemie
The most promising model system was one that also yielded
excellent results with B(C6F5)3 in the work of Oestreich and
coworkers: the transfer of dihydrogen from one unsaturated
hydrocarbon to another. With a catalytic amount of B(C6F5)3 (10
mol-%), the hydrogen transfer from several cyclohexa-1,4-diene
[b]
Universität Paderborn
Warburger Straße 100, 33098 Paderborn (Germany)
This article is protected by copyright. All rights reserved.

10.1002/ejic.201700524
European Journal of Inorganic Chemistry
COMMUNICATION
derivatives onto 1,1-diphenylethylene and other 1,1-disubstituted
alkenes proceeded smoothly at room temperature under
formation of the corresponding aromatic system and an
alkane.15
Accordingly, our first test was the reaction of 1,3,5-
trimethylcyclohexa-1,4-diene with 1,1-diphenylethylene, adding
10 mol-% (C2F5)3PF2 as a catalyst. Within 10 minutes, we
observed NMR spectroscopically a quantitative conversion to
mesitylene and 1,1-diphenylethane (Eq. (2)).
Going
from
1,3,5-trimethylcyclohexa-1,4-diene
to
1,3-
cyclohexadiene, we were curious whether (C2F5)3PF2 would also
catalyse the hydrogen transfer. We did observe a reaction,
however, most of it was ascribed to polymerisation, as indicated
by the broad signals in the 1H NMR spectrum as well as the
consistency of the reaction mixture.
Another hydrogen transfer reaction where B(C6F5)3 proved its
worth as a catalyst is the reaction of imine derivatives and
cyclohexa-1,4-dienes which led under reflux in toluene to an
amine and aromatic cyclohexatriene derivatives.14 But even after
a prolonged reaction time, (C2F5)3PF2 did not initiate the
hydrogen transfer.
To test a broader variety of Lewis acids, we employed different
perfluoroalkylated silicon and germanium compounds in the
dismutation reaction of 1,3,5-trimethylcyclohexa-1,4-diene (see
Table 1). The corresponding silicon and germanium compounds
had been synthesized in our group and have already been
established as strong Lewis acids.21 While the germanes did not
exhibit any catalytic activity, with amounts between 12 and 35
mol-%, the results for the silanes showed some variety. With
(C2F5)3SiH no conversion was observed even after 5 days.
(C2F5)2SiH2 did achieve some conversion, however, it was far
from complete even after 5 days. (C2F5)3SiF and (C2F5)2SiF2
both catalysed the reaction very efficiently with a full conversion
after less than 20 minutes. Tetrakis(pentafluoroethyl)silane,
(2)
To confirm the role of the catalyst, we conducted a blind test
without the addition of (C2F5)3PF2, and even after 3 days no
conversion could be observed. Treatment of 1,3,5-
trimethylcyclohexa-1,4-diene alone with a catalytic amount of
(C2F5)3PF2 (1 mol-%) in dichloromethane, however, led to a full
conversion of the diene to mesitylene and another product in a
2:1 ratio. Further NMR spectroscopic investigations of the
concentrated product mixture and more significantly, a GC-MS
investigation led to the identification of the second product as
stereoisomers of 1,3,5-trimethylcyclohexane (Eq. (3)).
(C2F5)4Si, showed
a slight delay, but NMR spectroscopic
investigations showed a full conversion after 20 minutes as well.
It seemed a promising candidate to reduce the catalytic amount
from 15 to 1 mol-%. While full conversion was ultimately
observed, the reaction was noticeably slower than for (C2F5)3PF2,
with a conversion of only 20 % after one hour.
(3)
As this stood in contrast to the results of Oestreich et al. who
observed
a
quantitative formation of mesitylene and
dihydrogen15, we treated 1,3,5-trimethylcyclohexa-1,4-diene in
dichloromethane with 4 mol-% of freshly prepared B(C6F5)3. We
did observe a slight gas evolution, however, NMR spectroscopic
investigations clearly also showed the formation of 1,3,5-
trimethylcyclohexane beside mesitylene. We were able to fully
reproduce the results of Oestreich et al., however, by changing
the solvent to toluene, with the product mixture exhibiting only
signals for mesitylene (Eq. (4)).
The transfer of these reaction conditions to our own catalyst,
however, only led to a slight delay of the reaction. After 15
minutes the reaction was completed and showed the same
products and ratio as in dichloromethane.
(4)
This article is protected by copyright. All rights reserved.

10.1002/ejic.201700524
European Journal of Inorganic Chemistry
COMMUNICATION
Table 1. Screening of perfluoroalkylated Lewis acids in the dismutation of
1,3,5-trimethylcyclohexa-1,4-diene.
Conclusions
In conclusion, the Lewis acid (C2F5)3PF2 was successfully
employed as a catalyst in the transfer hydrogenation between
1,3,5-trimethylcyclohexa-1,4-diene and 1,1-diphenylethylene.
The treatment of 1,3,5-trimethylcyclohexa-1,4-diene alone with a
catalytic amount of (C2F5)3PF2 led to the quantitative dismutation
to mesitylene and 1,3,5-trimethylcyclohexane. Using B(C6F5)3, a
solvent-dependency was observed, with CH2Cl2 yielding mainly
the dismutation products, while in toluene the evolution of H2
was observed. The screening of various perfluoroalkylated
germanes and silanes distinguished the more Lewis acidic
silanes as suitable catalysts, while the germanes showed no
catalytic activity.
amount
(mol-%)
conversion (%),
1 h[a]
conversion (%),
7 d[a]
[cat]
(C2F5)3PF2
(C2F5)3PF2
(C2F5)4Ge
(CF3)3GeH
(C2F5)3GeCl
(C2F5)2GeH2
(C2F5)4Si
10
1
100
100[b]
12
26
36
25
15
1
-
-
-
-
-
-
-
Experimental Section
-
100
20
100
100
-
Disproportionation of 1,3,5-trimethylcyclohexa-1,4-diene: 1,3,5-
Trimethylcyclohexa-1,4-diene (1.493 g, 12.22 mmol) was dissolved in
dichloromethane (10 mL) and (C2F5)3PF2 (0.12 mmol) was condensed
onto the mixture. The mixture was warmed to room temperature and
stirred for 10 min, after which full conversion was confirmed by NMR
spectroscopy. The mixture was carefully concentrated in vacuum by
removing the solvent. The remaining residue, a red oil, was investigated
by NMR spectroscopy and GC-MS. 1H NMR (500 MHz, CDCl3, 25 °C): δ
= 0.5 – 2.1 (m, 18H, 1,3,5-trimethylcyclohexane), 2.4 (s, 18H, CH3
(mesitylene)), 6.9 ppm (s, 6H, CH (mesitylene)); 13C{1H} NMR (500 MHz,
CDCl3, 25 °C): δ = 18.9, 21.2 (s, CH3 (mesitylene)), 22.8, 23.1, 26.5, 28.5,
32.5, 40.7 (s, CH2), 44.0 (s, CH2), 44.9 (s, CH2), 127.0 (s, Car
(mesitylene)), 128.8, 137.8 (s, Cquart (mesitylene)). GC-MS (m/z (%)):
4.180 min, area 19.247.454 (126 (5) [C6H9(CH3)3]+., 111 (84), 95 (2), 89
(1), 83 (3), 77 (2), 69 (100), 61 (4), 55 (44), 41 (52)); 4.815 min, area
20.652.078 (126 (8) [C6H9(CH3)3]+., 111 (73), 95 (2), 89 (1), 83 (6), 77 (2),
69 (100), 61 (2), 55 (48), 41 (55)); 6.249 min, area 85.332.304 (120 (44)
[C6H3(CH3)3]+., 115 (3), 105 (100) [C6H3(CH3)2]+, 91 (12), 77 (12), 63 (5),
58 (1), 51 (8), 39 (15)).
(C2F5)4Si
100
(C2F5)3SiF
(C2F5)2SiF2
(C2F5)3SiH
(C2F5)2SiH2
[CPh3][BF4]
B(C6F5)3
30
27
59
33
13
4
-
9
15
100[c]
Determined by 1H NMR spectroscopy with regard to the diene (starting
[a]
[b]
material).
Determined by 1H NMR spectroscopy with 1,2-dimethoxyethane
[c]
as the internal standard which was added after the reaction.
mesitylene/1,3,5-trimethylcyclohexane (3:2) deviated from the theoretical
value (2:1).
The ratio
Reaction of 1,3,5-Trimethylcyclohexa-1,4-diene and 1,1-diphenyl-
ethylene in the presence of (C2F5)3PF2: 1,3,5-Trimethylcyclohexa-1,4-
diene (0.246 g, 2.01 mmol) was dissolved in dichloromethane (4 mL).
1,1-Diphenylethylene (0.343 g, 1.90 mmol) and (C2F5)3PF2 (0.081 g, 0.19
mmol) were added and the mixture was stirred at room temperature for
10 min, after which complete conversion was confirmed by NMR
spectroscopy. The mixture was carefully concentrated in vacuum until a
Finally, the catalytic activity of the Lewis acidic trityl cation was
investigated treating 1,3,5-trimethylcyclohexa-1,4-diene with
[CPh3][BF4] (Eq. (5)). While the abstraction of “hydrogen” could
be proven by NMR spectroscopically detecting signals for
mesitylene, no signals for 1,3,5-trimethylcyclohexane were
observed. Instead, the formation of CPh3H was detected,
showing that while [CPh3]+ is indeed able to abstract a hydride
and thus initiate the elimination of a proton from the diene, the
transfer of hydrogen back onto a second diene molecule does
not happen. The addition of 1,1-diphenylethylene as a hydrogen
acceptor did not effect any change.
1
brownish liquid remained. H NMR (500 MHz, CDCl3, 25 °C): δ = 1.8 (d,
3J(HH) = 7, 3H, CH3 (diphenylethane)), 2.4 (s, 9H, CH3 (mesitylene)), 4.3
3
(q, J(HH) = 7, 1H, CH (diphenylethane)), 6.9 (s, 3H, CHar (mesitylene)),
7.3-7.4 ppm (m, 10H, CHar (diphenylethane)); 13C{1H} NMR (500 MHz,
CDCl3, 25 °C):
δ = 21.3 (s, CH3 (mesitylene)), 22.0 (s, CH3
(diphenylethane)), 44.9 (s, CH (diphenylethane)), 126.1 (s, Car
(diphenylethane)), 127.0 (s, Car (mesitylene)), 127.7 (s, Car
(diphenylethane)), 128.5 (s, Car (diphenylethane)), 137.8 (s, Cquart
(mesitylene)), 146.5 (s, Cquart (diphenylethane)).
(5)
General procedure for the reaction of 1,3,5-trimethylcyclohexa-1,4-
diene with a Lewis acid: In an NMR tube equipped with a Young valve,
1,3,5-trimethylcyclohexa-1,4-diene was dissolved in dichloromethane and
the respective Lewis acid was condensed onto the solution. The reaction
mixture was investigated by NMR spectroscopy over 7 days.
This article is protected by copyright. All rights reserved.

10.1002/ejic.201700524
European Journal of Inorganic Chemistry
COMMUNICATION
Reaction of [CPh3][BF4] with 1,3,5-trimethylcyclohexa-1,4-diene:
[CPh3][BF4] (0.047 g, 0.14 mmol) was dissolved in CH2Cl2 (5 mL) and
treated with 1,3,5-trimethylcyclohexa-1,4-diene (0.132 g, 1.08 mmol). A
sudden decolouration of the previously yellow solution occurred. The
reaction mixture was investigated by NMR spectroscopy. Another 0.273
g (0.828 mmol) of [CPh3][BF4] were added. 1H NMR (300 MHz, CH2Cl2,
25 °C): δ = 2.2 (s, 9H, CH3 (mesitylene)), 5.5 (s, 1H, CH (Ph3CH)), 6.7 (s,
3H, CHar (mesitylene)), 7.0 – 7.2 ppm (m, CHar (Ph3CH)); 13C{1H} NMR
(300 MHz, CH2Cl2, 25 °C): δ = 20.8 (s, CH3 (mesitylene)), 56.7 (s, CH
(Ph3CH)), 126.2 (s, Cpara (Ph3CH)), 126.7 (s, Car (mesitylene)), 128.2 (s,
Cmeta (Ph3CH)), 129.2 (s, Cortho (Ph3CH)), 143.9 ppm (s, Cipso (Ph3CH)).
[5]
[6]
[7]
[8]
[9]
H. Wieland, Chem. Ber. 1912, 45, 484 – 493.
J. Böeseken, Rec. Trav. Chim. 1918, 37, 255 – 265.
N. D. Zelinsky, G. S. Pawlow, Chem. Ber. 1933, 66, 1420 – 1422.
B. B. Corson, V. N Ipatieff, J. Am. Chem. Soc. 1939, 61, 1056 – 1057.
for example: M. Rueping, A. Kuenkel, I. Atodiresei, Chem. Soc. Rev.
2011, 40, 4539 – 4549.
[10] N. A. Eddy, J. J. Richardson, G. Fenteany, Eur. J. Org. Chem. 2013,
5041 – 5044.
[11] J. P. Moerdyk, C. W. Bielawski, Chem. Eur. J. 2013, 19, 14773 – 14776.
[12] M. Pérez, C. B. Caputo, R. Dobrovetsky, D. W. Stephan, Proc. Natl.
Acad. Sci. U.S.A. 2014, 111, 10917 – 10921.
[13] J. M. Farrell, Z. M. Heiden, D. W. Stephan, Organometallics 2011, 30,
4497 – 4500.
[14] I. Chatterjee, M. Oestreich, Angew. Chem. Int. Ed. 2015, 54, 1965 –
1968.
[15] I. Chatterjee, Z.-W. Qu, S. Grimme, M. Oestreich, Angew. Chem. Int.
Ed. 2015, 54, 12158 – 12162.
Acknowledgements
[16] A. F. G. Maier, S. Tussing, T. Schneider, U. Flörke, Z.-W. Qu, S.
Grimme, J. Paradies, Angew. Chem. 2016, 128, 12407–12411; Angew.
Chem. Int. Ed. 2016, 55, 12219 – 12223.
This work was financially supported by Merck KGaA (Darmstadt,
Germany).
[17] M. Kojima, M. Kanai, Angew. Chem. 2016, 128, 12412 – 12415; Angew.
Chem. Int. Ed., 2016, 55, 12224 – 12227.
[18] N. V. Ignat'ev, J. Bader, K. Koppe, B. Hoge, H. Willner, J. Fluorine
Chem. 2015, 171, 36–45.
[19] B. Hoge, J. Bader, N. Ignatyev, M. Schulte, EP 2 621 940 B1, Merck
Patent GmbH, Darmstadt; B. Hoge, J. Bader, N. Ignatyev, M. Schulte,
W. Hierse, W. Wiebe, H. Willner, EP 2 621 939 B1, Merck Patent
GmbH, Darmstadt; J. Bader, N. Ignat'ev, B. Hoge, Inorg. Chem. 2014,
53, 7547–7553.
Keywords: fluorine • Lewis acid • transfer hydrogenation •
perfluoroalkyl • phosphorus
[1]
for example: G. Brieger, T. J. Nestrick, Chem. Rev. 1974, 74, 567 –
580; D. Wang, D. Astruc, Chem. Rev. 2015, 115, 6621 – 6686.
E. A. Braude, R. P. Linstead, J. Chem. Soc. 1954, 3544 – 3547.
E. Knoevenagel, J. Fuchs, Chem. Ber. 1902, 35, 1788 – 1799; in this
report the structure of the Hantzsch ester was mistakenly drawn as 3,5-
dimethyl-1,4-dihydropyridine-2,6-dicarboxylate instead of 2,6-dimethyl-
1,4-dihydropyridine-3,5-dicarboxylate. The synthesis of the starting
materials was identical to reports by P. Griess, G. Harrow, Chem. Ber.
1888, 21, 2743 and R. Schiff, P. Prosio, Gazz. Chim. 1895, 25, 65.
E. Knoevenagel, B. Bergdoll, Chem. Ber. 1903, 36, 2857 – 2860.
[20] B. Hoge, J. Bader, N. Ignatyev, M. Schulte, EP 2 513 123 B1, Merck
Patent GmbH, Darmstadt.
[2]
[3]
[21] S. Steinhauer, J. Bader, H.-G. Stammler, N. Ignat'ev, B. Hoge, Angew.
Chem. Int. Ed. 2014, 53, 5206–5209; S. Pelzer, B. Neumann, H.-G.
Stammler, N. Ignat'ev, B. Hoge, Chem. Eur. J. 2016, 22, 3327–3332; S.
Pelzer, B. Neumann, H.-G. Stammler, N. Ignat'ev, B. Hoge, Chem. Eur.
J. 2016, 22, 4758–4763.
[4]
This article is protected by copyright. All rights reserved.

10.1002/ejic.201700524
European Journal of Inorganic Chemistry
COMMUNICATION
Entry for the Table of Contents (Please choose one layout)
Layout 1:
COMMUNICATION
Text for Table of Contents
Key Topic*
Author(s), Corresponding Author(s)*
Page No. – Page No.
Title
((Insert TOC Graphic here: max.
width: 5.5 cm; max. height: 5.0 cm;
NOTE: the final letter height should
not be less than 2 mm.))
*one or two words that highlight the emphasis of the paper or the field of the study
Layout 2:
COMMUNICATION
Transfer hydrogenation*
Julia Bader, Alexander F. G. Maier, Jan
Paradies, Berthold Hoge*
Page No. – Page No.
Perfluoroalkylated main-group
element Lewis acids as catalysts in
transfer hydrogenation
Surprising dismutation: The Lewis acid (C2F5)3PF2 catalyses the transfer
hydrogenation between 1,3,5-trimethylcyclohexa-1,4-diene and 1,1-
diphenylethylene. Surprisingly, the treatment of the diene alone with a catalytic
amount of (C2F5)3PF2 leads to a quantitative dismutation to mesitylene and 1,3,5-
trimethylcyclohexane. Additionally, the catalytic activity of various perfluoroalkylated
germanes and silanes was tested.
This article is protected by copyright. All rights reserved.