Synthesis and Physical Properties of Strained Doubly Phosphorus-Bridged Biaryls and Viologens

Article abstract of DOI:10.1002/chem.201605272

New P/N-containing π-electron systems comprising fully planar biaryl arrays are synthesized by multiple radical phosphanylation. The biaryl moiety in these highly strained planar π-systems is rigidified by double P-bridging. The electronic properties of the core biaryl entity are varied by introducing N-donor substituents or by installing N-atoms within the π-system, thereby moving to the viologen core structure. The electrochemical and photophysical properties of these compounds are discussed and compared with those of related systems.

Full text of DOI:10.1002/chem.201605272

A Journal of
Accepted Article
Title: Synthesis and Physical Properties of Strained Doubly
Phosphorus-Bridged Biaryls and Viologens
Authors: Tobias Greulich, Eriko Yamaguchi, Carsten Doerenkamp,
Maximilian Lübbesmeyer, Constantin Gabriel Daniliuc, Aiko
Fukazawa, Hellmut Eckert, Shigehiro Yamaguchi, and Armido
Studer
This manuscript has been accepted after peer review and appears as an
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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 ensure accuracy of information. The authors are responsible for the
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To be cited as: Chem. Eur. J. 10.1002/chem.201605272
Link to VoR: http://dx.doi.org/10.1002/chem.201605272
Supported by

Chemistry - A European Journal
10.1002/chem.201605272
FULL PAPER
Synthesis and Physical Properties of Strained Doubly
Phosphorus-Bridged Biaryls and Viologens
[a]
[b]
[c]
[a]
Tobias W. Greulich, Eriko Yamaguchi, Carsten Doerenkamp, Maximilian Lübbesmeyer,
[a]
[b]
,[c,d]
Shigehiro Yamaguchi*,[b] and Armido
Constantin G. Daniliuc, Aiko Fukazawa, Hellmut Eckert,*
Studer*,[a]
New P/N-containing -electron systems comprising fully planar biaryl
arrays are synthesized by multiple radical phosphanylation. The biaryl
moiety in these highly strained planar -systems is rigidified by double
P-bridging. Electronic properties of the core biaryl entity are varied by
introducing N-donor substituents or by installing N-atoms within the -
system thereby moving to the viologen core structure.
Electrochemical and photophysical properties of these compounds
are discussed and compared with those of related systems.
by two phosphine sulfide moieties. We show that this P/N-
containing polycyclic compound can be transformed to the
corresponding doubly P-bridged viologen.[11] Recently,
Baumgartner and co-workers have successfully synthesized a
mono-P-bridged viologen with highly electron-accepting
character.[ We envisioned that the doubly P-bridged structure
would enable us to gain further enhanced electron-accepting
character. Its physical properties will be discussed by comparing
with those of analogues that lack the viologen core structure or
the P-bridging moieties. The synthesis of these analogues and
their structures will also be discussed.
19]
Organic -electron systems belong to a promising class of
functional materials for the development of organic electronic
devices such as field-effect transistors, organic light-emitting
Tetrabromobipyridine 1, used as a precursor for the key
phosphanylation step towards 2, was prepared from 3,5-
diodes or solar collecting modules.[1-3]
A
great variety of
[12]
[13,18a]
dibromopyridine by a modified Ullmann coupling
yield (Scheme 1). Radical phosphanylation was achieved upon
treatment of 1 with PhP(SnMe in PhCF in the presence of 1,1´-
in 71%
compounds and -materials have been investigated during the
past decades including hydrocarbon-based polycyclic arenes, -
3
)
2
3
conjugated oligomers and polymers or ladder-type -electron
azobis(cyclohexanecarbonitrile) (V-40) at 125 °C. Oxidation of the
bisphosphole intermediate with hydrogen peroxide led to partial
N-oxide formation and we therefore had to modify the P-oxidation
step. After intensive experimentation we found that elemental
sulfur selectively oxidizes the P-atoms in the intermediate
phosphole obtained after the fourfold radical cyclization to provide
the P-phenylated phosphine sulfide trans-2a. A related side
product was isolated in traces and characterized by X-ray crystal
structure analysis. In this mono-bridged compound methanol
inserted into one of the endocyclic C-P bonds in 2a (methanolyis
of a C-P bond) while partially releasing ring strain and the
backbone is substantially distorted compared to trans-2a (see
Supporting Information).[14] In order to overcome the observed
poor solubility of 2a (even in chlorinated organic solvents), we
systems.[
4-6]
Although these materials have been successfully
employed as components in organic electronics, there is room at
the top to improve their performance. A valuable strategy for the
fine-tuning of structure, properties and functions is offered by the
incorporation of main group elements into a -conjugated
backbone.[ Especially phosphorus as a bridging unit results in
increased -conjugation due to a flattening of the parent polycyclic
structure.[
6b]
7,8]
We have recently shown that the radical phosphanylation is
a powerful method for the synthesis of bisphosphoryl-bridged
biaryls.[ The key step towards such arylphosphanes is a fourfold,
sequential trapping of aryl radicals by bisstannylated phosphanes
using 2,2’,6,6’-tetrabromo-biaryls as substrates.[9,10]
9]
In this full paper, we present the synthesis, photophysical
and electronic properties of multifunctional organic -electron
systems based on a bipyridine scaffold that is doubly P-bridged
switched from PhP(SnMe
3
)
2
to n-octylP(SnMe
3
)
2
as the
phosphanylation reagent and successfully synthesized both
diastereoisomers of the n-octyl-substituted bis(phosphine
sulfide)-bridged bipyridines, trans-2b and cis-2b, that showed
enhanced solubility as compared to trans-2a. Tetrabromobiphenyl
[
[
a]
b]
Dr. Tobias Greulich, Maximilian Lübbesmeyer, Dr. Constantin
Gabriel Daniliuc, Prof. Dr. Armido Studer, Westfälische Wilhelms
Universität, Organisch-Chemisches Institut, Corrensstrasse 40,
3
was obtained from 1,3-dibromo-1-iodobenzene according to a
literature procedure[15] via chemoselective iodine-lithium
exchange with n-butyllithium and subsequent CuCl -mediated
oxidative homocoupling (48% yield). Fourfold radical
phosphanylation with PhP(SnMe and subsequent S -oxidation
afforded the trans-bisphosphole sulfide 4 (21%).
48149 Münster (Germany), E-mail: Studer@uni-muenster.de
2
Dr. Eriko Yamaguchi, Dr. Aiko Fukazawa, Prof. Dr. Shigehiro
Yamaguchi, Department of Chemistry, Graduate School of Science
and Institute of Transformative Bio-Molecules (WPI-ITbM), Nagoya
University, Furo, Chikusa, Nagoya 464-8602, Japan.
Email: yamaguchi@chem.nagoya-u.ac.jp
3
)
2
8
N,N-Dibenzyl-3,5-dibromoaniline was used as a starting
material for the preparation of 6. Known[16] homocoupling of this
[
[
c]
e]
Carsten Doerenkamp, Prof. Dr. Hellmut Eckert, Westfälische
Wilhelms Universität, Physikalisch-Chemisches Institut,
Corrensstrasse 28/30, 48149 Münster (Germany).
Prof. Dr. Hellmut Eckert, Instituto de Física em Sao Paulo,
Universidade de Sade Sao Paulo, Av. Trabalhador Saocarlense
aniline with FeCl
in 44% yield. Radical phosphanylation in analogy to the
preparation of 4 gave the corresponding bisphosphole that was in
situ oxidized (S ) to 6 (19%). Notably, for 6 and 4 only the trans-
isomer could be isolated after oxidation.
3
in toluene at 125 °C provided the tetrabromide
5
400, Sao Carlos, S.P. 13560-590 (Brazil).
8
Supporting information for this article is given via a link at the end of
the document.
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Chemistry - A European Journal
10.1002/chem.201605272
FULL PAPER
structural features were found. Typical key structural
characteristics of this class of doubly P-bridged biaryl-systems
can be summarized as follows: prolonged endocyclic C-P bonds,
significantly shortened central C-C bonds, tetrahedrally distorted
conformation of the P-atoms and a high degree of coplanarity of
the biaryl backbone.
Scheme 1. Preparation of bis(phosphine sulfide)-bridged bipyridines 2 and the
doubly P-bridged biphenyls 5, 6 by fourfold radical phosphanylation and
8
subsequent oxidation with S .
Trans-2a, trans-2b, 4 and 6 were characterized by X-ray
crystal structure analysis (Figure 1, Table 1). Due to the fact that
all four compounds contain a crystallographic inversion center,
the asymmetric unit of these systems shows only half part of the
molecule. For P-phenylated or P-alkylated phosphine sulfides the
incorporation of two phosphine sulfide moieties forced the P/N-
heterocyclic -system in 2a and 2b into a perfect coplanar
orientation with a torsion angle of 0°. Similarly, the biphenyl based
doubly P-bridged phosphine sulfides in 4 and 6 appear in a planar
conformation (torsion angle = 0°). It was further found that in each
of the four structures, both P-atoms appear in a tetrahedrally
distorted fashion showing an endocyclic C-P-C angle of 92.2-
Figure 1. X-ray crystal structures of trans-bis(phosphine sulfide)-bridged
bipyridines trans-2a (top left), trans-2b (top right),
bottom right); trans-2a thermal ellipsoids are shown with 30% and
trans-2b with 50% probability.
4 (bottom left) and 6
(
, 4
,
6
Table 1. Selected structural key features of phosphine sulfides trans-2a, trans-
2a, 4 and 6.
Compound
C–Pendocycl. [Å]
C–Ccentral [Å]
αCPC [°]
trans-2a
1.848(2), 1.849(2)
1.850(2), 1.853(2)
1.841(4), 1.853(4)
1.844(3), 1.851(3)
1.425(3)
1.439(4)
1.440(7)
1.426(5)
92.4(1)
92.2(1)
92.6(2)
93.5(1)
trans-2b
4
6
93.5° with imbedded, remarkably prolonged endocyclic C-P
bonds (1.841(4)–1.853(4) Å) diminishing the incurred ring strain.
Notably, in non-strained triarylphosphanes the C-P bond length is
Compounds trans-2b and cis-2b were further transformed to
1.801-1.833 Å and in all compounds studied herein the endocyclic
the corresponding viologens (trans-7 and cis-7) by double N-
alkylation with methyl triflate, which were isolated after
crystallization in good yields (Scheme 2).
C-P bond is significant longer. Another clear indication for a
strained -system is the considerably shortened central C-C bond
of the (hetero)biaryl moieties in 2a, 2b, 4 and 6 which arises from
the annulation of the two phosphine sulfide bridges ending up in
a four ring-fused P/N-heterocyclic scaffold. In the series, trans-2a
(
1.425(3) Å) showed the shortest central C-C bond. In recent
studies similar compounds were investigated[9] and comparable
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Table 2. Electrochemical and photophysical properties of trans-7, cis-7, 4, 6 and
MV2+
.
UV/Vis Absorption
abs [nm] ( [M–1cm–1])
Compound
E
Red1,1/2 [V]a
E
Red2,1/2 [V]a
λ
trans-7b
cis-7b
4d
–0.41
(–0.53)c
–0.72
–0.91
(–0.87)c
386 (200)
387 (180)
318 (620)
475 (1030)
260 (200)
-
-
6d
–0.86
Scheme 2. Synthesis of bis(phosphine sulfide)-bridged viologens trans-7 and
cis-7.
MV2+[15]
–1.09
–1.52
a
+
–
Determined by cyclic voltammetry (Sample 1 mM; Bu
4
N PF
6
0.1 M; scan rate
–1
1
00 mV s ). Potentials were calibrated with a ferrocene/ferrocenium ion couple
+
b
c
d
2 2
(Fc/Fc ). In MeCN. Cathodic peak potential. In CH Cl .
The viologens 7 contain four electron-accepting fragments.
The two reference compounds 4 and 6 with only two electron
accepting entities were included in these studies in order to
elucidate the effect of the two pyridinium fragments in 7 on the
photophysical and electrochemical properties of such doubly P-
bridged flat and rigid -systems. Bisphosphine sulfide 4 displays
a biphenyl-based analogue where the electronic character is
determined by the two phosphole sulfide units and in the
benzidine 6 the interaction of two electron-donating (para-
aminobiphenyls) and two withdrawing groups (phosphole sulfide)
affects these properties. Considering the electron densities of
these phosphine sulfides, the push-pull system 6 should be
reduced at the most negative potential, followed by biphenyl 4 and
both bis(phosphine sulfide)-bridged viologens trans-7 and cis-7.
Indeed, for compounds 4 and 6 the reduction takes place at
more negative values (Table 2). The twofold phosphine sulfide
bridged biphenyl 4 is reduced at ERed1,1/2 = −0.72 V. The different
substitution pattern of benzidine 6, while having the same
backbone as 4, leads to a reduction potential of ERed1,1/2 = −0.86
V in agreement with the above prediction.
-
1,2
-1
-0,8 -0,6 -0,4 -0,2
Potential [V]
0
Figure 2. Cyclic voltammogram of viologen trans-
versus ferrocene).
7
(1 m
M
4 6
in MeCN with Bu NPF
Viologens are well-known for their reversible redox
properties[11] and along these lines the bis(phosphine sulfide)-
bridged bipyridinium salts were analyzed by cyclic voltammetry.
For trans-7 we found two fully reversible redox processes (Figure
). The first one-electron reduction to the blue colored radical
cation was observed at a potential of ERed1,1/2 = –0.41 V vs. the
2
+
ferrocene/ferrocenium ion couple (Fc/Fc ) and the second one-
electron reduction was observed at ERed2,1/2 = –0.91 V (Table 2).
In comparison to the non-P-bridged parent methyl viologen
2+
-
ditriflate (MV /2TfO ) the reduction potential in trans-7 is
significantly shifted to more positive values by about
Red = 0.68 V.[17] This increased ability to accept electrons arises
The UV/Vis absorption properties of these phosphine sulfides
were next studied and we found that both viologen congeners,
trans-7 and cis-7, show similar absorption properties (Table 2).
These compounds have small and broad absorption bands in
acetonitrile with the maximum wavelengths (λabs) of 386 nm and
387 nm, respectively. Compared to the electroneutral 4,4´-
bipyridine and as already indicated by CV measurement the
Δ
E
from the incorporation of the two electron-withdrawing phosphine
sulfide moieties. For cis-7 the reduction steps are not reversible
and a degradation process is triggered upon one-electron
reduction, resulting in a flattened oxidation wave (see Supporting
Information). cis-7 is a slightly weaker oxidant as compared to its
trans congener in light of the cathodic peak potentials.
Notably, Baumgartner and coworkers introduced phosphorus-
containing N-alkyl viologen derivatives bearing only one
phosphine oxide-bridge, which showed the first reduction
potentials of ERed1,1/2 = –0.51 V in acetonitrile and ERed1,1/2 = –
absorption is considerably red shifted.[17] In CH
2 2
Cl the absorption
spectrum of the bisphosphoryl-bridged biphenyl 4 has a λabs of
318 nm. Due to the electron-donating effect of the amino groups
in a coplanar structure, the maximum wavelength of the
absorption maximum of benzidine 6 is significantly shifted to a
longer wavelength of λabs 475 nm.
0
.53 V in DMF vs. Fc/Fc+.[18] Moreover, a N-thienyl-substituted
phosphaviologen showing a reduction potential of ERed1,1/2 = –
.38 V in DMF was reported in the literature.[19] To our knowledge,
Methyl viologen has been used in photocatalytic applications
as an oxidative quencher for example in combination with
tris(bipyridine)ruthenium(II) (Ru(bpy)
Oxidative deactivation of the excited state leads to Ru(bpy)
which can eventually act as an oxidant.
Tris(bipyrazine)ruthenium(II) (Ru(bpz) ) bears more electron-
withdrawing ligands as compared to Ru(bpy)
3
2+) as a photocatalyst.
[19]
0
3+
the bis(octylphosphine sulfide)-bridged viologen trans-7 reported
herein represents the strongest electron acceptor in the series of
P-bridged N-alkyl viologens. The pronounced electron-accepting
ability should be caused by the two electron-withdrawing
phosphine sulfide moieties.
3
2
+
3
2
+
2+
3
and MV is not
2
+
able to deactivate the excited state of the Ru(bpz)
3
due to the
limited reduction potential.[19] Following this line, we investigated
the more powerful oxidizing agents trans-7 and cis-7 as possible
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Chemistry - A European Journal
10.1002/chem.201605272
FULL PAPER
quenchers of the excited state of the Ru(bpz) 2+-complex by
3
performing Stern-Volmer experiments (Figure 3). Interestingly, we
found that trans-7 acts as an efficient deactivator of the excited
2+
state of Ru(bpz)
3
but the corresponding cis-isomer does not
interact with the excited species leading to no quenching effect
(
Figure 3, right). By using trans-bridged phosphine sulfide trans-7
2
+
in combination with Ru(bpz)
3
representing a stronger oxidant
2
+
than Ru(bpy)
barriers of the standard MV /Ru(bpy)
should open new doors with respect to synthetic applications.[20]
3
it should be possible to overcome limiting redox
2
+
2+-couple and hence
3
Scheme 3. Reduction of bis(phosphine sulfide)-bridged viologen trans-7 to the
corresponding radical cation trans-7·+
.
The unpaired electron is delocalized through a heteroatoms-
substituted π-system and interacts with the P and N atoms
resulting in a complex hyperfine structure in the EPR spectrum
0,6
(
Figure 4). The signal is located at g = 2.0021, and is
0
0,4
퐼
푓
characterized by the isotropic hyperfine coupling to two 31P, two
−
1
퐼푓
1
4
N, four 1H and six 1H (two methyl groups) nuclei, with the
0,2
corresponding Aiso values of 7.9 G, 5.2 G, 0.4 G, and 1.4 G,
respectively. The 31P and 14N coupling constants are found to be
substantially larger than those extracted from the spectrum of the
phosphaviologen radical species published in reference 19.
0
520
560
600
640
680
0,0 0,5 1,0 1,5 2,0
[
푄]/10-5
M
Wavelength [nm]
a)
Figure 3. Decreasing emission intensities of Ru(bpz) 2+
concentrations of trans- (left); Stern-Volmer plots for trans-
(triangles, right).
by increasing
(squares) and cis-
3
3
7
7
Considering the reversible redox properties of viologens
passing through different colored intermediates and encouraged
by recent studies,[18b,19] we attempted the selective
monoreduction of trans-7 via SET in order to access the
corresponding intermediate, deep blue colored radical cation
·+
trans-7 . Based on the reduction potentials of trans-7 we had to
identify a reducing reagent for selective chemical monoreduction
with a potential between −0.41 V and −0.91 V vs. ferrocene. Zn
(
ERed = -0.76 V vs. SHE) seemed suitable and after mixing trans-
7
and zinc dust in CH CN in an inert atmosphere a selective
3
monoreduction was achieved (Scheme 3). The reaction mixture
showed a deep blue color, indicating the presence of the target
b)
·+
radical cation trans-7 . The UV spectrum depicted in Figure 4a of
the blue trans-7·+ was recorded using a spectroelectrochemistry
setup (see SI). In contrast to the parent viologen trans-7, that
shows only a weak absorbance in the UV region (maximum at 386
nm), the spectrum of trans-7·+ shows absorption maxima at 375
and 475 nm. Additionally, a broad absorption band from 500 to
700 nm including a distinct shoulder at 650 nm is observed. This
spectrum agrees well with a spectrum previously reported for a
[
18b]
mono P-bridged viologen derived radical cation.
To further
·+
support generation of trans-7 , EPR spectroscopy was
conducted on the reaction mixture in anhydrous degassed
acetonitrile, proving formation of a radical intermediate (Figure
4b).
to the radical cation trans- ·
7
+
at constant
Figure 4. a) Reduction of trans-
7
potential of -580 mV versus ferrocene/ferrocenium ion couple tracked by
UV/vis spectroscopy. b) Experimental (black) and simulated (red) X-band
EPR spectrum of the radical cation trans-7·+ derived from one electron-
reduction of trans-7.
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Chemistry - A European Journal
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[
[
2]
3]
a) A. C. Arias in Organic Field-Effect Transistors, CRC Press, 2007, pp.
419-432; b) M. C. T. L. Torsi, B. Crone, A. Dodabalapur, L. Wang in
Compounds that undergo a color change upon a specific
stimulus are used in electronic applications like electrochromic
switches.[ Assuming sufficient stability of both involved species
and reversibility of their reduction/oxidation the bis(phosphine
sulfide)-bridged viologen trans-7 displays a potential organic
material for electrochromic devices.
Organic Field-Effect Transistors, CRC Press, 2007, pp. 507-528.
a) X. Gong, D. Moses, A. J. Heeger in Organic Light Emitting Diodes,
Wiley-VCH, Weinheim , 2006, pp. 151-180; b) A. C. Grimsdale in Organic
Light Emitting Diodes, Wiley-VCH, Weinheim , 2006, pp. 215-243; c) K.
Meerholz,C.-D. Müller, O. Nuyken in Organic Light Emitting Diodes,
Wiley-VCH, Weinheim , 2006, pp. 293-318.
21]
In summary, we have synthesized electron-deficient P/N-
[4]
J. E. Anthony, Angew. Chem. Int. Ed. 2008, 47, 452-483; b) Z. Sum, Q.
Ye, C. Chi, J. Wu, Chem. Soc. Rev. 2012, 41, 7857-7889.
doped organic π-systems derived from
a tetrabrominated
[
[
5]
6]
J. Roncali, Chem. Rev. 1997, 97, 173-206.
bipyridine. The photophysical as well as electrochemical
characteristics of these compounds have been measured and
discussed in comparison to related all-carbon biaryl systems. X-
ray structure analysis revealed that due to the two rigid P-bridges,
the viologen core structure is in perfect coplanar conformation
with a torsion angle of 0°. trans-bridged bis(phosphine sulfide)
trans-7 belongs to a special family of phosphorus-containing
viologens and represents to the best of our knowledge the most
electron-accepting member of this particular class of compounds.
One electron reduction of trans-7 with Zn leads to the
corresponding stable blue radical cation that was characterized
by EPR spectroscopy. The demonstration of the ready tuning of
physical properties of these multifunctional π-materials underlines
the great potential of such phosphorus-containing organic π-
systems in materials science.
a) U. Scherf, J. Mater. Chem. 1999, 9, 1853-1864; b) A. Fukazawa, S.
Yamaguchi, Chem. Asian J. 2009, 4, 1386-1400.
[
7]
a) T. Baumgartner, R. Réau, Chem. Rev. 2006, 106, 4681-4727; b) Y.
Ren, T. Baumgartner, Dalton Trans. 2012, 41,7792-7800; c) D. Joly, P.-
A. Bouit, M. Hissler, J. Mater. Chem. C, 2016, 4, 3686-3698.
a) H. Chen, W. Delaunay, L. Yu, D. Joly, Z. Wang, J. Li, Z. Wang, C.
Lescop, D. Tondelier, B. Geffroy, Z. Duan, M. Hissler, F. Mathey, R.
Réau, Angew. Chem. Int. Ed. 2012, 51, 214-217; b) M. Stolar, T.
Baumgartner, Chem. Asian J. 2014, 1212-1225; c) A. I. Arkhypchuk, M.-
P. Santoni, S. Ott, Angew. Chem. Int. Ed. 2012, 51, 7776-7780.
a) A. Bruch, A. Fukazawa, E. Yamaguchi, S. Yamaguchi, A. Studer,
Angew. Chem. Int. Ed. 2011, 50, 12094-12098; b) D. Hanifi, A. Pun, Y.
Liu, Chem. Asian J. 2012, 7, 2615-2620; c) T. W. Greulich, N. Suzuki, C.
G. Daniliuc, A. Fukazawa, E. Yamaguchi, A. Studer, S. Yamaguchi,
Chem. Commun. 2016, 52, 2374-2377.
[8]
[9]
[10] Other application of the radical phosphanylation: a) S. Vaillard, C. Mück-
Lichtenfeld, S. Grimme, A. Studer, Angew. Chem. Int. Ed. 2007, 46,
6
533-6536; b) A. Bruch, A. Ambrosius, R. Frohlich, A. Studer, D. B.
Guthrie, H. Zhang, D. P. Curran, J. Am. Chem. Soc. 2010, 132, 11452-
1454; c) M.-C. Lamas, A. Studer, Org. Lett. 2011, 13, 2236-2239.
Experimental Section
1
[
11] P. M. S. Monk in The Viologens: Physicochemical Properties, Synthesis
and Application of the Salts of 4,4´-Bipyridine, Wiley, Chichester, 1998.
Tetrabromide 1 (1.0 eq.), bisstannylated phosphine (3.0 eq.) and 1,1´-
azobis(cyclohexanecarbonitrile) (0.5 eq.) were placed under argon in an
oven-dried Schlenk-tube. Trifluorotoluene (0.1 M) was added and the
reaction mixture was stirred for 2-3 d at 125 °C. The reaction mixture was
[12] V. Cañibano, J. F. Rodríguez, M. Santos, M. A. Sanz-Tejedor, M. C.
Carreño, G. González, J. L. García-Ruano, Synthesis 2001, 2001, 2175-
2
179.
13] J. Hassan, M. Sévignon, C. Gozzi, E. Schulz, M. Lemaire, Chem. Rev.
002, 102, 1359-1470.
[
[
2
8
allowed to cool to rt, addition of S (5.0 eq.) was followed by stirring
14] A side product potentially derived from the cis-isomer via methanol
overnight. The crude product was purified by FC (pentane/CH
2
Cl
2
) and the
insertion into an endo-cyclic CP-bond was identified.
product was isolated as a solid.
[15] R. S. F. R. Leroux, N. Nicod, Lett Org. Chem. 2006, 3, 948-954.
[
[
[
16] X. Ling, Y. Xiong, R. Huang, X. Zhang, S. Zhang, C. Chen, J. Org. Chem.
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2
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Acknowledgements
We thank the SFB858 and the International Research Training
Group Münster/Nagoya for funding this work, as well as JSPS
Core-to-Core Program, A. Advanced Research Networks.
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Keywords: radical phosphanylation • phosphorous • -systems •
cyclovolammetry • EPR
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Chemistry - A European Journal
10.1002/chem.201605272
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COMMUNICATION
Coplanar and electron-poor
T. W. Greulich, E. Yamaguchi, C.
viologens are prepared by a fourfold
radical phosphanylation reaction.
Photophysical and electrochemical
properties of these novel rigid -
systems are reported and compared
with those of systems lacking the
rigidifying P-bridging moieties.
Doerenkamp, M. Lübbesmeyer, C. G.
Daniliuc, A. Fukazawa, H. Eckert, S.
Yamaguchi and A. Studer
Page No. – Page No.
Synthesis and Physical Properties of
Strained Doubly Phosphorus-Bridged
Biaryls and Viologens
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