Amino-substituted diazocines as pincer-type photochromic switches

Abstract

Azobenzenes are robust, reliable, and easy to synthesize photochromic switches. However, their high conformational flexibility is a disadvantage in machine-like applications. The almost free rotation of the phenyl groups can be restricted by bridging two ortho positions with a CH(2)CH(2) group, as realized in the dihydrodibenzo diazocine framework. We present the synthesis and properties of 3,3'-amino- and 3,3'-acetamido substituted diazocines. Upon irradiation with light of 405 and 530 nm they isomerize from the cis to the trans configuration and back, and thereby perform a pincer-like motion. In the thermodynamically more stable cis isomer the lone pairs of the amino nitrogen atoms point towards each other, and in the trans form they point in opposite directions. The distance between the amino nitrogen atoms changes between 8 Å (cis) and 11 Å (trans isomer).

Full text

1
Amino-substituted diazocines as pincer-type
photochromic switches
Hanno Sell1, Christian Näther2 and Rainer Herges*1
Full Research Paper Open Access
Address:
1Otto-Diels Institut für Organische Chemie,
Christian-Albrechts-Universität zu Kiel, Otto-Hahn-Platz 4, 24418 Kiel,
Germany and 2Institut für Anorganische Chemie,
Christian-Albrechts-Universität zu Kiel, Max-Eyth-Str. 2, 24418 Kiel,
Germany
Email:
Rainer Herges* - rherges@oc.uni-kiel.de
* Corresponding author
Keywords:
azobenzene; diazocine; molecular pincer; molecular switches;
photochromic compound
Beilstein J. Org. Chem. 2013, 9, 1–7.
doi:10.3762/bjoc.9.1
Received: 04 September 2012
Accepted: 26 November 2012
Published: 02 January 2013
This article is part of the Thematic Series "Molecular switches and cages".
Guest Editor: D. Trauner
© 2013 Sell et al; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
Azobenzenes are robust, reliable, and easy to synthesize photochromic switches. However, their high conformational flexibility is a
disadvantage in machine-like applications. The almost free rotation of the phenyl groups can be restricted by bridging two ortho
positions with a CH2CH2 group, as realized in the dihydrodibenzo diazocine framework. We present the synthesis and properties of
3,3’-amino- and 3,3’-acetamido substituted diazocines. Upon irradiation with light of 405 and 530 nm they isomerize from the cis
to the trans configuration and back, and thereby perform a pincer-like motion. In the thermodynamically more stable cis isomer the
lone pairs of the amino nitrogen atoms point towards each other, and in the trans form they point in opposite directions. The dis-
tance between the amino nitrogen atoms changes between 8 Å (cis) and 11 Å (trans isomer).
1
Introduction
Azobenzenes probably are the most frequently used
photochromic switches in chemistry. They are employed as
molecular actuators to drive a number of dynamic machine-like
functions [1]. To achieve sophisticated engineering tasks such
as directed motion at the molecular level [2,3], the geometry
change and the force induced during cis–trans isomerization has
to be coupled to the environment. In the macroscopic world,
therefore, machines are made from stiff materials. Azobenzene,
however, is a rather floppy molecule. Both phenyl rings can
rotate with a low activation barrier, and isomerization of the
trans form can occur in two different directions, forming two
different isomers (enantiomers in the parent system) [4]. Power
transmission to neighbouring molecules is inefficient because of
energy transfer to internal conformational motion. The first and
probably most simple measure to make azobenzene stiffer
would be to prevent the phenyl groups from rotating.

Beilstein J. Org. Chem. 2013, 9, 1–7.
2
Scheme 1: Synthesis of 3,3’-diamino-EBAB 4 and its acetamide derivative 5.
Connecting both rings with each other via an alkane bridge is
probably the most straightforward way to achieve that. Such a
molecule, 5,6-dihydrodibenzo[c,g][1,2]diazocine (1), has been
known for more than a hundred years [5]. However, only
recently we discovered that the photophysical properties of 1
(quantum yields, photostationary states) are superior to those of
parent azobenzene, and most of its derivatives [6-8]. In contrast
to azobenzene, diazocine 1 is most stable in its cis configur-
ation, which has a boat conformation. The trans isomer comes
in two conformations: a chair and a twist form, of which the
twist is more stable (Figure 1).
Figure 1: Configurations and conformations of 5,6-
dihydrodibenzo[c,g][1,2]diazocine (1), and DFT (B3LYP/6-31G*) calcu-
lated energies (kcal mol−1) relative to the most stable cis boat form.
Dipol moments are given in Debye.
Proper substitution of the diazocine molecular framework is
necessary to control the interaction with the environment or
with other molecules. Therefore, we explore different
approaches to prepare diazocine derivatives. Since the nomen-
clature is not unambiguous and, hence, potentially confusing,
we refer to 5,6-dihydrodibenzo[c,g][1,2]diazocine derivatives as
2,2’-ethylene-bridged azobenzenes (EBABs).
Results and Discussion
Synthesis
Woolley et al. very recently published the synthesis of a 4,4’-
diamino-2,2’-ethylene-bridged azobenzene (4,4’-diamino-
EBAB), which exhibits excellent photophysical properties [9].
In planning our synthesis (and not yet being aware of the results
of the above authors) we were concerned about the fact that
amino substituents in the 4-position with respect to the azo
group would impair photochemical conversion, as this is known
for 4-amino-substituted azobenzenes [10]. We therefore set out
to synthesize 3,3’-diamino-EBAB 4 and derivatives thereof.
Key steps of the synthesis are the introduction of the
substituents and formation of the azo bond. Several approaches
were evaluated, changing the order of the steps and the groups
from which the azo unit was generated. The preferred proce-
dure starts from commercially available 1,2-bis(4-amino-
phenyl)ethane (2). Nitration proceeds almost quantitatively in
ortho position to the ethylene bridge, forming 1,2-bis(2-nitro-4-
aminophenyl)ethane (3) [11]. Intramolecular reductive coupling
of the nitro groups to form the azo unit proceeds with notori-
ously low yields. The most frequently used procedure using Zn
as the reducing agent in Ba(OH)2 [12] or NaOH [13] gives irre-
producible and low yields varying from 2% to not more than
19%. Glucose, however, in basic ethanolic solution turned out
to furnish the azo compound 4 reproducibly in more than 20%
yield. The acetamide derivative 5 is formed by treatment of
3,3’-EBAB 4 with acetic anhydride (Scheme 1).

Beilstein J. Org. Chem. 2013, 9, 1–7.
3
Figure 2: Crystal structures of the cis isomers of 3,3’-diamino-EBAB 4 and its acetamide derivative 5. The atoms of the ethylene bridge are disor-
dered.
The structures of the products were confirmed by 1H and
13C NMR spectroscopy, as well as X-ray crystallography
(Figure 2). As in the parent system 1 the amino and acetamido
derivates 4 and 5 are thermodynamically most stable in their cis
form.
Photochromic Properties
trans-Azobenzene exhibits a high-intensity π–π* band at
λmax = 316 nm (ε = 22,000 L∙M−1∙cm−1) and a symmetry-
forbidden n–π* band at 444 nm with a very low extinction coef-
ficient (ε = 440 L∙M−1∙cm−1) [10]. Irradiation with UV-light of
365 nm converts the trans to the cis isomer. The n–π* absorp-
tion of the nonplanar cis-azobenzene is not formally symmetry
forbidden. Even though it has a rather low extinction coeffi-
cient (1250 L∙M−1∙cm−1), irradiation into the n–π* band leads to
complete conversion back to the trans isomer [14]. Photo-
switching of 5,6-dihydrodibenzo[c,g][1,2]diazocine (unsubsti-
tuted EBAB 1), however, is performed differently [6]. Conver-
sion of cis to trans, as well as isomerization of trans to cis, is
achieved by irradiation into the corresponding n–π* bands. This
is possible because the n–π* bands appear well separated at
different wavelengths (cis: 404 nm, trans: 490 nm) and the tran-
sitions are allowed (albeit weak) in both isomers. Since
substituents in the meta position are known to interact less effi-
ciently with each other than those in the ortho or para positions
we decided to examine EBABs that are 3,3’-substituted, hoping
that the excellent switching properties of the parent system
would be retained. For determination of the ratio of isomers in
photostationary states we used 1H NMR (for details see
Supporting Information File 1).
The UV spectrum of cis-3,3’-diamino-EBAB 4 exhibits a π–π*
transition at 350 nm, and a shoulder at approximately 400 nm,
which arises from the n–π* transition of the azobenzene
Figure 3: UV–vis spectra of the diazocine derivatives 3,3’-diamino-
EBAB 4 and its bisamide derivative 5 in acetonitrile.
substructure (Figure 3). Irradiation of a solution of 4 in acetoni-
trile with light of 405 nm leads to isomerization to the trans
isomer with 34% trans form in the photostationary state (deter-
mined by 1H NMR). The rather low conversion rate is probably

Beilstein J. Org. Chem. 2013, 9, 1–7.
4
Figure 4: Absorbances of solutions of 4 and 5 in acetonitrile at 405 nm (red) and 485 nm (blue) in the corresponding photostationary states upon
alternating irradiation at 405 and 530 nm.
Table 1: Half-life and photostationary states of EBAB 1 and derivatives.
compound half life [h] PSS 405 nm
% trans
PSS 520 nm
% trans
EBAB 14.5a92% <1%
3,3’-diamino-EBAB 474b34% <1%
3,3’-diacetamido-EBAB 546b54% <1%
4,4’-diacetamido-EBAB 4.8c70% <1%
a300 K, n-hexane [6]; b300 K, MeCN; c293 K, DMSO [9].
due to the overlap of the π–π* and n–π* transitions in the cis
form. In the trans isomer the n–π* transition is shifted to longer
wavelengths (~500 nm) and is clearly separated from any other
absorption. Irradiation with light at 450–600 nm therefore
converts the trans isomer completely back to the cis form. As
compared to the bisamino-substituted EBAB 4 the π–π* and the
n–π* bands of the cis isomer of the acetamido derivate 5 are
better separated (shoulder at 320 nm and 400 nm) (Figure 3).
The photostationary state upon irradiation with 405 nm there-
fore rises to 54% trans form. As in the bisamino-substituted
system 4, conversion of 5 back to the cis isomer is quantitative
within the error limits of 1H NMR and UV–vis spectroscopy
(Figure 4).
Thermal stability of the trans isomers
Amino and alkylamino substituents in para position to the azo
group reduce the lifetime of the cis states of azobenzenes
[15,16]. A dramatically shortened half-life of an ortho-dimeth-
ylamino-substituted cis-phenylazopyridine (cis-4-N,N-dimethyl-
amino(3-phenylazo)pyridine) has also been observed [17]. The
reverse effect was observed in our meta-substituted trans-3,3’-
diamino-EBAB 4. While the unsubstituted trans-EBAB 1
exhibits a half-life of 4.5 h in n-hexane solution at room
temperature, trans-3,3’-EBAB 4 isomerizes to the more stable
cis form with a half-life of 74 h. The corresponding half-life of
trans-3,3’-acetamido-EBAB 5 is 46 h (Table 1).
Application as a molecular pincer
In the 1H NMR spectra of cis-4 and cis-5 the four protons of the
ethylene bridge yield a centred multiplet. This symmetry of the
fine structure shows that they divide into two chemically
unequal groups of two chemically equal protons. Hence, there is
no boat inversion at room temperature on the NMR time scale.
According to the X-ray crystal structures the amino nitrogen
lone pairs of cis-3,3’-diamino EBAB 4 point towards each other
and (if extended to more than 6.5 Å) would intersect with an
angle of about 65°. If the acetamido groups in 5 are rotated
appropriately, the N–H bonds (extended to 10.5 Å) intersect
with an angle of about 45°. Thus, the EBAB derivatives 4 and 5
should be suitable as molecular pincers.
To demonstrate this property we studied the binding of ethyl-
enediamine to the two isomers of the acetamido derivative 5
(Figure 5).
We carried out 1H NMR titrations of cis-5 as well as of the
photostationary mixture of cis-5 and trans-5 upon irradiation
with light of a wavelength of 405 nm with ethylenediamine in
acetonitrile. The spectra showed a significant chemically
induced shift (CIS) of the acetamide protons of both isomers
upon addition of ethylenediamine. Equilibrium analysis with
respect to the CIS binding isotherms by means of nonlinear
least-squares methods (for details see Supporting Information

Beilstein J. Org. Chem. 2013, 9, 1–7.
5
Figure 5: DFT-calculated structure (B3LYP/6-31+G**) of a complex of
5 with ethylenediamine as a conceivable model of the binding mode of
3,3’-diacetamido-EBAB 5.
File 1) [18,19] yielded a binding constant for the 1:1 ethylenedi-
amine complex of the cis isomer of Ka,cis = 0.88 ± 0.03 M−1.
For the corresponding complex of the trans isomer a slightly
lower binding constant of Ka,trans = 0.61 ± 0.05 M−1 was deter-
mined in acetonitrile-d3 at 25 °C.
Conclusion
We prepared ethylene-bridged azobenzene (EBAB) derivatives
with amino and acetamido substituents in the meta position with
respect to the azo group (3,3’-diamino-EBAB 4, and 3,3’-di-
acetamido-EBAB 5). In contrast to azobenzene, and in agree-
ment with the parent EBAB, the cis isomer is more stable than
the trans form. Compared to the parent EBAB, which is a very
efficient photoswitch, the conversion from the cis to the trans
isomer upon irradiation with 405 nm is reduced to 34%
(diamino derivative 4) and 54% (diacetamido derivative 5, cf.
92% in the parent compound). The thermal half-life of the trans
isomer, however, is drastically increased (3,3’-diamino-EBAB
4: 74 h, 3,3’-diacetamido-EBAB 5: 46 h). The EBAB deriva-
tives upon photoisomerization perform a pincer-like motion. Di-
acetamido derivative 5 binds ethylenediamine better in its cis
(closed) form than in its trans configuration (open form).
Experimental
General remarks
All chemicals were purchased from commercial sources and
used without further purification. All NMR spectra were
recorded with instruments of the company Bruker (AC 200,
DRX 500, and AV 600). The assignments of the NMR signals
were confirmed by the evaluation of COSY, HSQC and HMBC
spectra. The chemical shifts of the signals were all referenced to
residual solvent peaks. The mass spectra were recorded on a
Finnigan MAT 8230 instrument. IR spectra were recorded with
a Spectrum 100 instrument from Perkin-Elmer equipped with an
ATR unit from the company Loriot-Oriel. Melting points were
taken without correction. UV–vis spectra were recorded on a
Perkin-Elmer Lambda 14 spectrometer.
Synthesis
1,2-Bis(2-nitro-4-aminophenyl)ethane (3): A solution of 5.0 g
1,2-bis(4-aminophenyl)ethane (24.0 mmol) in 40 mL of sulfuric
acid was warmed up to 60 °C and a solution of 4.4 g
(52.0 mmol) finely grounded sodium nitrate in 45 mL of
sulfuric acid was added dropwise. The mixture was stirred at
60 °C for 6 h and afterwards poured into 200 mL of ice–water.
The resulting suspension was neutralised by the addition of an
aqueous ammonia solution (32%). The red precipitate was
filtered off, washed with water and dried in vacuum over CaCl2.
Yield: 7.2 g (23.6 mmol, 99%). Mp 247–249 °C; 1H NMR
(500 MHz, DMSO-d6) δ 7.06 (d, 4J6,2 = 2.0 Hz, 2H, 6-H), 6.99
(d, 3J3,2 = 8.3 Hz, 2H, 3-H), 6.78 (dd, 3J2,3 = 8.3 Hz, 4J2,6 =
2.1 Hz, 2H, 2-H), 5.58 (s, 4H, 1-NH2), 2.86 (s, 4H, 7-H);
13C NMR (126 MHz, DMSO-d6) δ 149.90 (Cq, C-5), 148.58
(Cq, C-1), 132.85 (d, C-3), 121.85 (Cq, C-4), 119.21 (d, C-2),
108.48 (d, C-6), 33.25 (t, C-7); IR (ATR): 3444 (m), 3363 (s),
3234 (w), 3061 (w), 2947 (w), 2877 (w), 1622 (s), 1513 (vs),
1495 (vs), 1324 (vs), 1272 (s), 1263 (s), 829 (s), 818 (s) cm−1;
EIMS (70 eV) m/z (% relative intensity): 302 (16) [M]+, 151
(100); CIMS (isobutane) m/z (% relative intensity): 303 (100)
[M + H]+.
(Z)-11,12-Dihydrodibenzo[c,g][1,2]diazocine-3,8-diamine
(4): A suspension of 1,2-bis(2-nitro-4-aminophenyl)ethane (3)
(1.059 g, 3.5 mmol) in a mixture of 140 mL ethanol and a solu-
tion of 8.8 g (220 mmol) sodium hydroxide in 35 mL water was
heated to 70 °C. A solution of 6.5 g (36 mmol) glucose in
20 mL water was added, and the reaction mixture was stirred
overnight. After cooling to room temperature, 500 mL water
was added, and the resulting mixture was extracted three times
with 100 mL of ethyl acetate. The organic phase was separated
and dried over sodium sulfate, and the solvent was evaporated
in vacuum. From the obtained residue the product was isolated
by flash chromatography (silica gel, cyclohexane/ethyl acetate
1:1) (254 mg, 1.1 mmol, 30%). Mp 193–196 °C; 1H NMR
(600 MHz, DMSO-d6) δ 6.67 (d, 3J6,5 = 8.2 Hz, 2H, 6-H), 6.24
(dd, 3J5,6 = 8.2 Hz, 4J5,3 = 2.3 Hz, 2H, 5-H), 5.97 (d, 4J3,5 =
2.3 Hz, 2H, 3-H), 5.15 (br.s, 4H, 4-NH2), 2.63 (mc, 4H, 7-Ha,
7-Hb,); 13C NMR (150 MHz, DMSO-d6) δ 155.87 (Cq, C-1),
146.76 (Cq, C-2), 130.15 (d, C-6), 115.18 (Cq, C-4), 112.88 (d,
C-5), 103.11 (d, C-3), 30.44 (t, C-7); IR (ATR): 3433 (m), 3344
(m), 3433 (m), 2952 (m), 2850 (m), 1703 (w), 1609 (vs), 1570
(m), 1497 (vs), 1455 (s), 1435 (m), 1303 (s), 1272 (s), 1170
(m), 1142 (m), 1094 (m), 1020 (m), 930 (w), 900 (m), 856 (m),
808 (vs) cm−1; EIMS (70 eV) m/z (% relative intensity): 238
(100) [M]+, 209 (92), 193 (46); CIMS (isobutane) m/z (% rela-
tive intensity): 239 (100) [M + H]+.

Beilstein J. Org. Chem. 2013, 9, 1–7.
6
(Z)-N,N'-(11,12-Dihydrodibenzo[c,g][1,2]diazocine-3,8-
diyl)diacetamide (5): In acetic acid anhydride (25 mL), (Z)-
11,12-dihydrodibenzo[c,g][1,2]diazocine-3,8-diamine (4)
(5 mg, 20 mmol) was dissolved. The solution was stirred at
room temperature overnight. Afterwards the solvent was evapo-
rated in vacuum, and the product was obtained as a pale yellow
solid (7 mg, 20 mmol, 100%). Mp 220–221 °C; 1H NMR
(500 MHz, MeCN-d3) δ 8.25 (s, 2H, 5-NH), 7.06 (d, 4J3,5 =
2.2 Hz, 2H, 3-H), 7.03 (dd, 3J5,6 = 8.2 Hz, 4J5,3 = 2.2 Hz, 2H,
5-H), 6.87 (d, 3J3,5 = 2.2 Hz, 2H, 3-H), 2.73 (m, 2H, 7-Ha,),
2.70 (mc, 4H, 7-Ha, 7-Hb), 2.00 (s, 6H, 9-H); 13C NMR
(125 MHz, MeCN-d3) δ 168.36 (Cq, C-8), 155.27 (Cq, C-1),
137.45 (Cq, C-2), 129.94 (d, C-6), 123.17 (Cq, C-4), 117.44 (d,
C-5), 108.59 (d, C-3), 30.29 (t, C-7), 22.96 (q, C-9); IR (ATR):
3253 (m), 3174 (m), 3102 (m), 3048 (m), 2924 (m), 1711 (m),
1680 (m), 1300 (s), 1260 (s), 1020 (s), 980 (m), 957 (m), 899
(m), 883 (m), 814 (s), 763 (m) cm−1. EIMS (70 eV) m/z (%
relative intensity): 322 (50) [M]+, 252 (92), 209 (100); CIMS
(isobutane): m/z (% relative intensity) 323 (100) [M + H]+.
Supporting Information
1H NMR spectra of 4 and 5 before and after the irradiation
with 405 nm, and 1H NMR binding study of
3,3-acetamido-EBAB (5) with ethylenediamine. cif-Files of
X-ray crystal structures of cis-4 and cis-5, and gaussian09
input file of the geometry optimization of the complex of
cis-5 and ethylenediamine (DFT B3LYP/6-31+G**).
Supporting Information File 1
Additional NMR spectra and 1H NMR binding study of
3,3-acetamido-EBAB (5) with ethylenediamine.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-9-1-S1.pdf]
Supporting Information File 2
Crystallographic information file of compound cis-4.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-9-1-S2.cif]
Supporting Information File 3
Crystallographic information file of compound cis-5.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-9-1-S3.cif]
Supporting Information File 4
Gaussian09 input file of the geometry optimization of the
complex of cis-5 and ethylenediamine.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-9-1-S4.gjf]
Acknowledgements
We would like to thank the Deutsche Forschungsgemeinschaft
(DFG) for funding through SFB 677 (Function by Switching).
References
1. Merino, E.; Ribagorda, M. Beilstein J. Org. Chem. 2012, 8, 1071–1090.
doi:10.3762/bjoc.8.119
2. Purcell, E. M. Am. J. Phys. 1977, 45, 3–11. doi:10.1119/1.10903
3. Hänggi, P.; Marchesoni, F. Rev. Mod. Phys. 2009, 81, 387–442.
doi:10.1103/RevModPhys.81.387
4. Haberhauer, G.; Kallweit, C. Angew. Chem. 2010, 122, 2468–2471.
doi:10.1002/ange.200906731
Angew. Chem., Int. Ed. 2010, 49, 2418-2421.
doi:10.1002/anie.200906731
5. Duval, H. Bull. Soc. Chim. Fr. 1910, 7, 727.
6. Siewertsen, R.; Neumann, H.; Buchheim-Stehn, B.; Herges, R.;
Näther, C.; Renth, F.; Temps, F. J. Am. Chem. Soc. 2009, 131,
15594–15595. doi:10.1021/ja906547d
7. Jiang, C.-W.; Xie, R.-H.; Li, F.-L.; Allen, R. E. J. Phys. Chem. A 2011,
115, 244–249. doi:10.1021/jp107991a
8. Böckmann, M.; Doltsinis, N. L.; Marx, D. Angew. Chem., Int. Ed. 2010,
49, 3382–3384. doi:10.1002/anie.200907039
9. Samanta, S.; Qin, C.; Lough, A. J.; Woolley, G. A. Angew. Chem. 2012,
124, 6558–6561. doi:10.1002/ange.201202383
Angew. Chem., Int. Ed. 2012, 51, 6452–6455.
doi:10.1002/anie.201202383
10. Rau, H. In Azocompounds in Photochromism, Molecules and Systems;
Dürr, H.; Bouas-Laurent, H., Eds.; Elsevier: Amsterdam, 1990; p 165.
The p-Amino substitution leads to a bathochromic shift of the p–p*
transition and thus to an overlap of the p–p* and n–p* transitions.
11. Matei, S. Rev. Roum. Chim. 1966, 11, 843.
12. Paudler, W. W.; Zeiler, A. G. J. Org. Chem. 1969, 34, 3237–3239.
doi:10.1021/jo01263a004
13. Shine, H. J.; Chamness, J. T. J. Org. Chem. 1963, 28, 1232–1236.
doi:10.1021/jo01040a016
14. Knoll, H. Photoisomerism of Azobenzenes. CRC Handbook of Organic
Photochemistry and Photobiology, 2nd ed.; CRC Press LLC: Boca
Raton, 2004; 89-1–89-16.
15. Schulte-Frohlinde, D. Justus Liebigs Ann. Chem. 1958, 612, 138–152.
doi:10.1002/jlac.19586120115
16. Mourot, A.; Kienzler, M. A.; Banghart, M. R.; Fehrentz, T.;
Huber, F. M. E.; Stein, M.; Kramer, R. H.; Trauner, D.
ACS Chem. Neurosci. 2011, 2, 536–543. doi:10.1021/cn200037p
17. Thies, S.; Sell, H.; Bornholdt, C.; Schütt, C.; Köhler, F.; Tuczek, F.;
Herges, R. Chem.–Eur. J. doi:10.1002/chem.201201698.
18. del Piero, S.; Melchior, A.; Polese, P.; Portanova, R.; Tolazzi, M.
Ann. Chim. (Rome, Italy) 2006, 96, 29–49.
doi:10.1002/adic.200690005
19. de Levie, R. J. Chem. Educ. 1999, 76, 1594–1598.
doi:10.1021/ed076p1594

Beilstein J. Org. Chem. 2013, 9, 1–7.
7
License and Terms
This is an Open Access article under the terms of the
Creative Commons Attribution License
(http://creativecommons.org/licenses/by/2.0), which
permits unrestricted use, distribution, and reproduction in
any medium, provided the original work is properly cited.
The license is subject to the Beilstein Journal of Organic
Chemistry terms and conditions:
(http://www.beilstein-journals.org/bjoc)
The definitive version of this article is the electronic one
which can be found at:
doi:10.3762/bjoc.9.1