Dehydrogenative Coupling Reactions with Oxidized Guanidino-Functionalized Aromatic Compounds: Novel Options for σ-Bond Activation

Article abstract of DOI:10.1002/chem.201602236

We present a new option for metal-free σ-bond activation, making use of oxidized, guanidino-functionalized aromatic compounds (GFAs). We demonstrate this new option by the homocoupling reactions of thiols and phosphines. The kinetics and the reaction pathway were studied by a number of experiments (including heterocoupling of thiols and phosphines), supported by quantum-chemical computations. Reaction of the oxidized GFA with p-dihydrobenzoquinone to give p-benzoquinone shows that typical proton-coupled electron-transfer reactions are also possible.

Full text of DOI:10.1002/chem.201602236

DOI: 10.1002/chem.201602236
Communication
&
Bond Activation
Dehydrogenative Coupling Reactions with Oxidized Guanidino-
Functionalized Aromatic Compounds: Novel Options for s-Bond
Activation
[
a]
Ute Wild, Stefanie Federle, Arne Wagner, Elisabeth Kaifer, and Hans-Jçrg Himmel*
Abstract: We present a new option for metal-free s-bond
activation, making use of oxidized, guanidino-functional-
ized aromatic compounds (GFAs). We demonstrate this
new option by the homocoupling reactions of thiols and
phosphines. The kinetics and the reaction pathway were
studied by a number of experiments (including heterocou-
pling of thiols and phosphines), supported by quantum-
2
+
chemical computations. Reaction of the oxidized GFA with
p-dihydrobenzoquinone to give p-benzoquinone shows
that typical proton-coupled electron-transfer reactions are
also possible.
Scheme 1. Addition of EÀH to the tetrakisguanidine dication 1 (1=1,2,4,5-
tetrakis(tetramethylguanidino)benzene).
able from a strong blueshift of the electronic transitions) and
the strong proton affinity of neutral guanidines. Crucial param-
eters to anticipate the potential of element–hydrogen bond
2
+
2+
The activation of s-bonds is of key importance in synthetic
chemistry. Manifold recent work has shown impressively the
potential of small main-group element compounds for this
activation with 1 are the reduction potential of 1 (E1/2 =
+
À0.70 V vs. Fc/Fc (Fc=ferrocene) in CH Cl and À0.73 V vs
2
2
+
[11]
Fc/Fc in CH CN ) and the Brønsted basicity of 1 (estimated
3
[
1]
+
[10a]
purpose. Frustrated Lewis pairs (FLPs) heterolytically split di-
pK value of 25.3 for 1-H in CH CN ). The products of the
A 3
[2]
hydrogen and activate other s-bonds. With some combina-
tions of Lewis acids and bases these reactions are reversible, al-
lowing their use in metal-free catalytic hydrogenation and
other useful reactions. Oxidative addition to unsaturated main-
group-element molecules was intensively studied, and from
the results it appears now possible to replace common transi-
addition reaction (see Scheme 1) are more or less electrophilic,
depending on the capability of E to accommodate positive
charge, and are likely to react further, for example, with anoth-
er EÀH. In the examples presented in this work, the activation
of the ArSÀH bond in thiols and the Ar PÀH bond in phos-
2
phines (in which Ar denotes phenyl or p-substituted phenyl)
[3]
tion-metal catalysts. Especially, carbenes and carbene-ana-
are the first steps in a proton-coupled electron-transfer process
I
[4,5]
[12]
logue species such as monomeric Al compounds,
silylenes,
(PCET) that eventually leads to dehydrogenative coupling to
[
6,7]
or germylenes
insert (thermally or with light) into a variety
disulfides or diphosphines, respectively (see Scheme 2).
of different EÀH bonds. A remaining general challenge for cat-
alytic applications is the optimization of the thermodynamic
and kinetic properties to enable both oxidative-addition and
The formation of disulfides from thiols is of high importance
[
13]
in biology (proteins) and in industrial processes (e.g., vulcan-
ization of natural rubber). In nature, enzymes catalyze disulfide
[8,9]
reductive-elimination reactions.
Herein we report on a novel option for the metal-free activa-
tion of elemental-hydrogen s-bonds, making use of the salt
1
(PF6)2 of the guanidino-functionalized aromatic (GFA) com-
pound 1,2,4,5-tetrakis(tetramethylguanidino)benzene (1, see
[
10]
Scheme 1). In contrast to oxidative-addition reactions to car-
2
+
benes and related species, the addition of EÀH to 1 leads to
reduction of the tetrakisguanidine compound, taking advant-
age of the creation of a 6p-aromatic system (becoming notice-
[
a] U. Wild, S. Federle, Dr. A. Wagner, Dr. E. Kaifer, Prof. Dr. H.-J. Himmel
Anorganisch-Chemisches Institut, Ruprecht–Karls–Universität Heidelberg
Im Neuenheimer Feld 270, 69120 Heidelberg (Germany)
E-mail: hans-jorg.himmel@aci.uni-heidelberg.de
Scheme 2. Dehydrogenative coupling of a) two thiols to disulfides (X=H,
OMe or Cl) and b) two phosphines to diphosphines (X=H or OMe) with
1(PF ) .
6 2
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201602236.
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Communication
formation in the endoplasmic reticulum. Several catalytic
methods were developed for the oxidation of thiols with di-
[
14,15]
oxygen.
One of the great challenges of this approach is to
circumvent over-oxidation (leading to sulfoxides, sulfones, thio-
sulfinates, and thiosulfonate byproducts). Catalytic dehydro-
coupling reactions under an inert gas atmosphere were also
[
16,17]
[18]
developed,
some of which require UV light. In general,
dehydrocoupling reactions are highly interesting, as they are
exceptionally atom-efficient and only produce H as a byprod-
2
[19]
uct. However, most of these reactions are limited to certain
thiols, require larger amount of catalyst, and/or are slow. One
problem of any catalytic approach certainly is the possible
blocking of the active catalyst by reaction with the thiol or the
disulfide. The formation of PÀP bonds also is of considerable
[
20]
interest.
The common approach is the reduction of R PX
2
(
X=halide) by alkali metals or other reducing agents. Such re-
actions are costly and atom-economically disadvantageous.
Several catalytic PÀP dehydrocoupling reactions were reported
[
21,22]
in the literature.
Again, a challenge for the general use of
such methods is the strong interaction of phosphines with co-
ordinatively unsaturated complexes, which could lead to effec-
tive blocking of the active catalyst. Radius et al. recently report-
ed the metal-free dehydrogenative coupling of phosphines
[23]
with stoichiometric amounts of a carbene and Gessner et al.
reported dehydrogenative coupling of phosphines with stoi-
[24]
chiometric amounts of lithium chloride carbenoids.
In the following, we report the results of the s-bond activa-
2
+
tion experiments with 1 . Dehydrogenative coupling of thiols
2
+
with 1 (see Scheme 2a) proceeds quickly. Discoloration of
the initially intensely green solutions, arising from the conver-
2
+
sion of 1 by proton-coupled electron-transfer to pale beige
2
+
(
1-H2) , occurred within seconds in CH Cl or CH CN, when
2 2 3
À1
concentrations of around 0.1 molL or higher are used. The
salt (1-H )(PF ) could be completely removed, and the disul-
2
6 2
fides were isolated in high yield (see the Supporting Informa-
tion). Further experiments showed that the reaction is slowed
down in dilute solutions, allowing the study of the kinetics of
this reaction (see the Supporting Information, Figures S1–S13).
Figure 1a displays the UV/Vis spectra measured for the reac-
Figure 1. a) UV/Vis spectra recorded for the reactions of 1(PF
6
)
2
with p-MeO-
À5
À1
PhSH in dilute solution in CH
of the concentration c of 1 and its initial concentration c (from absorp-
0
2
Cl
2
(concentration 4.0”10 molL ). b) Ratio
2
+
tions at 427 nm) versus time for three different thiols.
2
+
tion between 1 and p-methoxyphenylthiol in solution with
À5
À1
c (thiol)=2·c (1(PF ) )=4.0”10 molL at a temperature of
the phenyl group, but also with the applied solvent. Hence,
the reaction is much faster in CH CN than in CH Cl (see
0
0
6 2
3
58C in CH Cl . Under these conditions, the strong absorption
2 2
3
2
2
2
+
at 427 nm that is characteristic for 1 and responsible for the
green color of the solutions, completely vanishes within 1.5 h.
The presence of an isosbestic point at 354 nm indicates that
no stable intermediate is formed, and that the absorptions
below).
Dehydrogenative PÀP coupling reactions proceed much
slower. The conversion was therefore followed in CH CN at
3
higher concentrations c (phosphine)=2·c (1(PF ) =0.107 or
0
0
6 2
2
+
2+
À3
only arise from the 1 reactant and (1-H2) product mole-
cules. Exchange of the methoxy group by less electron-donat-
ing or -withdrawing groups decelerates the reaction. Hence,
the reaction rate (see Figure 2b) decreases in the order X=
OMe (t1/2 =22.2 min)>H (t1/2 =40.7 min)> Cl (t1/2 =121.1 min).
In the first period of the homocoupling reactions with PhSH
and p-ClPhSH, the spectra again reveal the presence of an isos-
bestic point. However, the spectra measured at the end of the
reaction do not run through this point (see the spectra in the
Supporting Information), possibly indicating protonation equili-
bria. The reaction rate varies not only with the substituents at
0.081 moldm (for diphenylphosphine and di(p-methoxyphe-
nyl)phosphine, respectively) and at a temperature of 808C
31
1
31
using P{ H} NMR spectroscopy. In the P NMR spectra, it is
easy to differentiate between the Ph PH and (MeOPh) PH reac-
2
2
tant signals at d=À40.8 and À45.6 ppm, and the correspond-
ing diphosphine product signals at d=À16.7 and À20.7 ppm
(see Figure 3a). In Figure 3b, the NMR yields of the coupling
products for diphenylphosphine and di(p-methoxyphenyl)-
phosphine are plotted as function of the reaction time; yields
of about 25% and 65%, respectively, were obtained after 5 h.
The coupling product (MeOPhP) crystallized out in the NMR
2
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31
Figure 2. a) P NMR spectra (CD
3
CN, 81.01 MHz) of 1(PF
6
)
2
and (MeOPh)
2
PH (1:2 molar ratio). b) Comparison between the reaction rates observed for the reac-
tions of 1(PF with Ph PH and (p-MeOPh)
6
)
2
2
2
PH in CH CN at a temperature of 808C for two different stoichiometries (1:2 or 2:2).
3
2
+
tube. Further experiments showed that the yield of coupling
product could be increased to almost 50% for diphenylphos-
phine and more than 70% for di(p-methoxyphenyl)phosphine
and 1 , as sketched in Figure 3. The reaction energy is posi-
À1
tive (D E= +43 kJmol ) for the addition of PhSH and almost
R
À1
2+
zero (D E=À4 kJmol ) for the addition of Ph PH to 1 . This
R
2
if the two reactants are applied in equimolar ratios
first step is disfavored by a negative entropy change causing
À1
(
c (phosphine)=c (1(PF ) =0.107 and 0.081 molL ).
both addition reactions to be endergonic (D G= +53 and
0
0
6 2
R
À1
According to DFT computations (B3LYP/TZVP energies at the
minima found with BP86/SVP), the overall reaction between
+96 kJmol for Ph PH and PhSH, respectively). Certainly, the
2
addition step benefits from solvent stabilization, as shown ex-
2
+
À1
Ph PH and 1 is exothermic (D E=À79 kJmol ), in contrast
plicitly for CH CN below. Tautomerization in the second step is
2
R
3
2
+
À1
to dehydrocoupling of Ph PH in the absence of 1 yielding di-
phosphine and H (D E= +23 kJmol ). Similarly, the reaction
energetically favored (D E=À55 and À47 kJmol
for the
2
R
À1
PhSH and Ph PH reactions, respectively), and creates a nucleo-
2
R
2
2
+
À1
between PhSH and and 1 is exothermic (D E=À71 kJmol ),
philic sulfur (or phosphorus) center in close proximity to an
electrophilic nitrogen. Hence a frustrated Lewis pair (FLP) inter-
mediate is created. According to our calculations, S···N and
P···N separations of 3.250 and 2.989 Š are adopted (in which N
is the imino nitrogen atom of the adjacent guanidino group).
The last, EÀE bond-forming step is exergonic for both the
R
again in contrast to dehydrocoupling of PhSH in the absence
2
+
À1
of 1 yielding disulfide and H (D E= +31 kJmol ). The over-
2
R
all thermodynamics is only slightly influenced by substitution
of the hydrogen in para position of the phenyl groups by me-
thoxy (see the Supporting Information).
À1
On the basis of the experimental results, we suggest
a three-step pathway (EÀH addition, tautomerization, and EÀE
PhSH and the Ph PH reaction (D G=À63 and À39 kJmol , re-
2
R
spectively).
bond formation) for the reactions between Ar EH (E=S and
There is strong experimental indication that the first step is
the rate-determining step. First, the reaction rate increases for
n
n=1, or E=P and n=2, Ar=phenyl or substituted phenyl)
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Figure 3. Formal division of the dehydrogenative EÀE coupling reactions (E=P or S) into three steps, as well as reaction energies including ZPE (D
R
E) and (in
0
brackets) reaction Gibbs free energies (D
R
G at 273.15 K, 0.1 MPa) as calculated at the B3LYP/TZVP//BP86/SV(P) level of theory.
0
À1
electron-donating substituents in para position of the phenyl
groups. In the first step, an imino-base-stabilized sulfenium
and phosphonium cations are created, species that are evi-
dently stabilized by electron-rich substituents. Indeed, quan-
tum chemical computations predict the first step to be the
only step that is favored by aromatic substitution with the
electron-donating methoxy group. For addition of MeOPhSH
endergonic (D E and D G values of +4 and +45 kJmol , re-
R
R
spectively.) In any case, the experimental results point to the
direct engagement of the coordinating solvent molecules in
the rate-determining step.
We also carried out experiments with different stoichiomet-
ric ratios of the two reactants in CH Cl . If the concentration of
2
2
2
+
PhSH is doubled (ratio 1 :PhSH=1:4), the t1/2 value decreases
to 26.1 min (compared to 40.7 min for the 1:2 ratio). If both
concentrations are halved (keeping the 1:2 ratio), the t1/2 value
slightly decreases (t1/2 =40.4 min). Moreover, for ClPhSH, t1/2
values of 121.1, 70.3, 16.7, and 12.4 min were found for
2
+
À1
or (MeOPh) PH to 1 , D E values of +25 and À31 kJmol , re-
2
R
À1
spectively, result, which are 18 and 27 kJmol , respectively,
lower than the values calculated for PhSH and Ph PH. The
2
second step is only slightly affected by aromatic substitution,
2
+
and the third step is disfavored upon methoxy substitution by
ClPhSH:1 =2, 4, 10, and 20, respectively. From all experi-
ments, it can be concluded that the reaction rate depends on
the concentrations of both reactants, but an accurate determi-
nation of the reaction order proved to be difficult.
À1
1
2 and 14 kJmol , respectively (see Figure 3).
Furthermore, the results obtained for the thiol reactions
show that the reaction proceeds much faster in CH CN than in
3
CH Cl . In the case of PhSH, the t value decreases from 40.7
The experiments showed that the homocoupling reaction of
phosphines requires higher temperatures (808C or higher),
2
2
1/2
to 20.5 min in CH Cl and CH CN, respectively. Similar to the
2
2
3
substitution with electron-donating groups, only the first step
is favored by the presence of a coordinating solvent. In further
whereas the homocoupling of thiols is fast at 358C in CH CN.
3
Hence it should be possible to achieve PÀS coupling by addi-
calculations, we estimated the stabilization of the sulfenium
tion of a thiol to a solution of 1(PF ) and a phosphine at ambi-
6
2
À1
cation by CH CN addition in the first step to be 34 kJmol (cf.
ent conditions. This was indeed observed. The reaction of the
3
À1
a binding energy of 119 kJmol was calculated for [PhS-
three reactants 1(PF ) , Ph PH, and MeOPhSH in a 1:1:1 stoi-
6
2
2
+
[25]
NCCH ] , without extra stabilization by the guanidine). If sol-
chiometric ratio gave the heterocoupling product
3
[
26]
vent coordination is included, the first step is associated by
Ph PSPhOMe in an NMR-based yield of 27%. Exchange of
2
À1
a reaction energy D E close to zero for PhSH (+9 kJmol ). As
Ph PH by (MeOPh) PH in these experiments led to an increase
R
2
2
can be seen from the plot in the Supporting Information, Fig-
of the yield of heterocoupled product ((MeOPh) PSPhOMe) to
2
ure S14, not only the t1/2 value, but also the form of the curve,
34% (see the NMR data in the Supporting Information). The
observation of heterocoupled products is consistent with the
reaction pathway shown in Scheme 3.
changes for reaction in CH CN. We cannot strictly exclude the
3
possibility that PhSH is activated by CH CN before addition to
3
2
+
the dication 1 . (Reaction between PhSh and CH CN to give
To see if oxidized GFA compounds could also be used in
3
[
12]
CH (PhS)C=NH is calculated to be also mildly endothermic and
standard PCET reactions, we reacted 1(PF ) with p-dihydro-
6 2
3
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be applied in typical proton-coupled electron-transfer reac-
tions. In this work we used the tetrakisguanidine 1, since salts
2
+
of the dication 1 could easily be prepared and are stable.
Certainly, such reactions are not limited to compound 1, but
also feasible for other GFA compounds, offering the possibility
to tune the two decisive reactivity parameters redox potential
and Brønsted basicity, to optimize the reaction conversion.
Scheme 3. Conversion of p-dihydrobenzoquinone to p-benzoquinone as typ-
ical example of a proton-coupled electron-transfer (PCET) reaction.
benzoquinone (see Scheme 3). In contrast to the hydrogena-
tive coupling reactions presented before, this reaction is ender-
À1
gonic (D G= +30 kJmol according to a B3LYP-D3/TZVP esti-
R
Acknowledgements
mate) in the absence of interactions with an environment. The
reaction might proceed via an hydrogen-bonded adduct be-
2
+
The authors thank the Deutsche Forschungsgemeinschaft for
financial support.
tween 1
and p-dihydrobenzoquinone, but UV/Vis spectra
showed no sign of such an adduct in acetonitrile at room tem-
2
+ [27]
perature (cf. known hydrogen-bonded aggregates of 1 ).
According to DFT calculations (see the Supporting Informa-
Keywords: bond activation · coupling reactions · guanidine ·
tion), adduct formation is accompanied by DE and DG values
redox chemistry
À1
of À45 and +15 kJmol , respectively. There also was no indi-
[
[
1] For a general comparison of main-group compounds and transition-
metal compounds, see: P. P. Power, Nature 2010, 463, 171–177.
cation of formation of quinhydrone (the adduct between the
p-benzoquinone product and p-dihydrobenzoquinone reac-
2] For recent review articles, see: a) D. W. Stephan, G. Erker, Angew. Chem.
Int. Ed. 2015, 54, 6400–6441; Angew. Chem. 2015, 127, 6498–6541;
b) D. W. Stephan, Acc. Chem. Res. 2015, 48, 306–316; c) D. W. Stephan,
J. Am. Chem. Soc. 2015, 137, 10018–10032; d) A. E. Ashley, D. O’Hare,
Top. Curr. Chem. 2013, 334, 191–218; e) D. W. Stephan, G. Erker, Angew.
Chem. Int. Ed. 2010, 49, 46–76; Angew. Chem. 2010, 122, 50–81.
À1 [28]
tant), which has a lower binding energy (À21 kJmol ). The
1
kinetics was followed by H NMR spectroscopy. Figure 4 shows
[
3] a) G. D. Frey, V. Lavallo, B. Donnadieu, W. W. Schoeller, G. Bertrand, Sci-
ence 2007, 316, 439–441; b) J. D. Masuda, W. W. Schoeller, B. Donna-
dieu, G. Bertrand, Angew. Chem. Int. Ed. 2007, 46, 7052–7055; Angew.
Chem. 2007, 119, 7182–7185; c) J. D. Masuda, W. W. Schoeller, B. Donna-
dieu, G. Bertrand, J. Am. Chem. Soc. 2007, 129, 14180–14181; d) B. Rao,
H. Tang, X. Zeng, L. Liu, M. Melaimi, G. Bertrand, Angew. Chem. Int. Ed.
2015, 54, 14915–14919; Angew. Chem. 2015, 127, 15128–15132.
[4] a) H.-J. Himmel, Z. Anorg. Allg. Chem. 2005, 631, 1551–1564; b) M. R.
Crimmin, M. J. Butler, A. J. P. White, Chem. Commun. 2015, 51, 15994–
15996.
[
[
5] M. Asay, C. Jones, M. Driess, Chem. Rev. 2011, 111, 354–396.
6] a) M. Kira, S. Ishida, T. Iwamoto, Chem. Rec. 2004, 4, 243–253; b) T. J. Ha-
dlington, M. Hermann, G. Frenking, C. Jones, Chem. Sci. 2015, 6, 7249–
7
2
257; c) M. Kira, S. Ishida, T. Iwamoto, C. Kabuto, J. Am. Chem. Soc.
002, 124, 3830–3831; d) J. W. Dube, VC. M. E. Craham, C. L. B. Macdon-
ald, Z. D. Brown, P. P. Power, P. J. Ragogna, Chem. Eur. J. 2014, 20, 6739–
744.
6
[
[
7] S. Ishida, T. Iwamoto, C. Kabuto, M. Kira, Nature 2003, 421, 725–727.
8] J. P. Moerdyk, G. A. Blake, D. T. Chase, C. W. Bielawski, J. Am. Chem. Soc.
2013, 135, 18798–18801.
Figure 4. Conversion of p-dihydrobenzoquinone to p-benzoquinone fol-
1
lowed by H NMR spectroscopy (the quotient of the reactant concentration
2
+
[9] See examples and discussion in: A. V. Protchenko, J. I. Bates, L. M. A.
Saleh, M. P. Blake, A. D. Schwarz, E. L. Kolychev, A. L. Thompson, C.
Jones, P. Mountford, S. Aldridge, J. Am. Chem. Soc. 2016, 138, 4555–
c (either 1 or p-dihydrobenzoquinone) and its initial concentration c
0
versus reaction time).
4564.
[
10] a) A. Peters, E. Kaifer, H.-J. Himmel, Eur. J. Org. Chem. 2008, 5907–5914;
b) A. Peters, C. Trumm, M. Reinmuth, D. Emeljanenko, E. Kaifer, H.-J.
Himmel, Eur. J. Inorg. Chem. 2009, 3791–3800.
the conversion as a function of the reaction time; the corre-
1
sponding H NMR spectra are displayed in the Supporting In-
[
[
11] B. Eberle, O. Hübner, A. Ziesak, E. Kaifer, H.-J. Himmel, Chem. Eur. J.
2015, 21, 8578–8590.
formation, Figure S15. The reaction, completed after 35 min
À2
(
t1/2 =4.9 min) under the chosen conditions (c =3.25”10
0
12] a) R. I. Cukier, D. G. Nocera, Annu. Rev. Phys. Chem. 1998, 49, 337–369;
b) H. Decornez, S. Hammes-Schiffer, J. Phys. Chem. A 2000, 104, 9370–
9384; c) M. Hang, V. Huynh, T. Meyer, Chem. Rev. 2007, 107, 5004–5064.
13] a) D. Fass, Annu. Rev. Biophys. Bioeng. 2012, 41, 63–79; b) B. Zhou, I. B.
Baldus, W. Li, S. A. Edwards, F. Gräter, Biophys. J. 2014, 107, 672–681.
14] A. Dhakshinamoorthy, M. Alvaro, H. Garcia, Chem. Commun. 2010, 46,
À1
molL in CD CN), is faster than dehydrogenative PÀP cou-
3
pling, but slower than dehydrogenative SÀS coupling under
similar conditions. An NMR-based yield of 61% was obtained.
Again, the reaction is accelerated for an excess of p-dihydro-
benzoquinone.
[
[
6476–6478.
In summary, we present a new option for the metal-free acti-
vation of element–hydrogen s-bonds by addition of an EÀH
bond to an oxidized guanidino-functionalized aromatic com-
pound (GFA), and demonstrate its application in dehydrogena-
tive coupling reactions. We show that oxidized GFAs could also
[15] A. Corma, T. Ródenas, M. J. Sabater, Chem. Sci. 2012, 3, 398–404.
[
[
[
16] K. Tanaka, K. Ajiki, Tetrahedron Lett. 2004, 45, 25–27.
17] A. Saxena, A. Kumar, S. Mozumdar, J. Mol. Catal. A 2007, 269, 35–40.
18] a) K. Y. D. Tan, J. W. Kee, W. Y. Fan, Organometallics 2010, 29, 4459–
4463; b) K. Y. D. Tan, G. F. Teng, W. Y. Fan, Organometallics 2011, 30,
4136–4143.
Chem. Eur. J. 2016, 22, 11971 – 11976
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Communication
[
[
[
[
19] R. L. Melen, Chem. Soc. Rev. 2016, 775–788.
[26] T. Wada, A. Kondoh, H. Yorimitsu, K. Oshima, Org. Lett. 2008, 10, 1155–
1157.
[27] U. Wild, C. Neuhäuser, S. Wiesner, E. Kaifer, H. Wadepohl, H.-J. Himmel,
Chem. Eur. J. 2014, 20, 5914–5925.
20] E. M. Leitao, T. Jurca, I. Manners, Nat. Chem. 2013, 5, 817–829.
21] L.-B. Han, T. D. Tilley, J. Am. Chem. Soc. 2006, 128, 13698–13699.
22] A. K. King, A. Buchard, M. F. Mahon, R. L. Webster, Chem. Eur. J. 2015, 21,
1
5960–15963.
23] H. Schneider, D. Schmidt, U. Radius, Chem. Commun. 2015, 51, 10138–
0141.
24] S. Molitor, J. Becker, V. H. Gessner, J. Am. Chem. Soc. 2014, 136, 15517–
5520.
25] T. Brinck, P. Carlqvist, A. H. Holm, K. Daasbjerg, J. Phys. Chem. A 2002,
06, 8827–8833.
[28] a) F. Wçhler, Justus Liebigs Ann. Chem. 1844, 51, 145; b) V. Barone, I. Ca-
celli, M. d’Ischia, A. Ferretti, G. Prampolini, G. Villani, RSC Adv. 2014, 4,
876–885.
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Received: May 11, 2016
Published online on July 19, 2016
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