Photoisomerizable azobenzene-containing oxacalixarenes

Article abstract of DOI:10.1016/j.tetlet.2011.11.114

The first examples of photo-responsive azobenzene-containing oxacalixarenes have been synthesized via a 3+1 macrocyclization approach. Introduction of the photoresponsive unit was achieved by using 4-phenylazoresorcinol or (E)-4-(4′-nitrophenylazo)resorci

Full text of DOI:10.1016/j.tetlet.2011.11.114

Tetrahedron Letters 53 (2012) 616–619
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Photoisomerizable azobenzene-containing oxacalixarenes
Claudia Gargiulli a, Giuseppe Gattuso a, , Anna Notti a, Sebastiano Pappalardo b, Melchiorre F. Parisi a,
⇑
Fausto Puntoriero c
a Dipartimento di Chimica Organica e Biologica, Università di Messina, Viale F. Stagno d’Alcontres 31, 98166 Messina, Italy
b Dipartimento di Scienze Chimiche, Università di Catania, Viale A. Doria 6, 95125 Catania, Italy
c Dipartimento di Chimica Inorganica, Analitica e Chimica Fisica, Università di Messina, Viale F. Stagno d’Alcontres 31, 98166 Messina, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
The first examples of photo-responsive azobenzene-containing oxacalixarenes have been synthesized via
a 3+1 macrocyclization approach. Introduction of the photoresponsive unit was achieved by using 4-
phenylazoresorcinol or (E)-4-(40-nitrophenylazo)resorcinol as the nucleophilic component in the macro-
cyclization reaction. These novel macrocycles have been characterized by means of 1D and 2D NMR spec-
troscopy and DFT calculations (B3LYP/6-31G(d)). According to thermal and photoisomerization studies,
tetranitro-oxacalix[4]arenes are less prone to E?Z isomerization than oxacalix[2]arene[2]triazines and,
within the two series, p-nitrophenylazo derivatives are more unwilling to isomerize than their phenylazo
analogues.
Received 22 October 2011
Revised 21 November 2011
Accepted 23 November 2011
Available online 28 November 2011
Keywords:
Oxacalixarenes
Metacyclophanes
Azobenzene
Ó 2011 Elsevier Ltd. All rights reserved.
Photoisomerization
Photoresponsive heteracalixarenes
The synthesis of the earliest crown ethers1 and the discovery of
their ability to act as molecular receptors have been the prime
movers in the development of host–guest chemistry and, from
there, of the broader multidisciplinary field of supramolecular
chemistry. Since then, a wide variety of different macrocyclic com-
pounds—cyclodextrins2 and calixarenes,3 to name the most rele-
vant examples—have been shown to possess analogous or even
better molecular recognition properties. Others such as heteraca-
lixarenes4 (and in particular aza- and oxacalixarenes), although ex-
tremely appealing for the purpose of molecular recognition, have
not yet been widely investigated, primarily because of the lack of
convenient procedures for their synthesis and/or further
derivatization.
Oxacalixarenes, for instance, have only recently started to gain
increasing attention, despite the fact that their discovery goes back
to 1966.5 Differently from the parent calixarenes, these
oxa[14]metacyclophanes are commonly prepared by SNAr one-
pot reaction of aromatic diols with electron deficient dihaloaro-
matic (or heteroaromatic) compounds.4a–d This method provides
access to a large number of different oxacalix[4]arenes in moderate
to very good yields, by making use of a variety of easily available
nucleophilic and electrophilic components.6 However, no matter
how efficient this synthetic approach is, it does not yield macrocy-
cles with sequences of (hetero)aromatic units different from the
simplest ABAB one.
Wang et al.7 have lately reported a new and convenient syn-
thetic route to oxacalix[2]arene[2]triazines, reliant on a 3+1 frag-
ment coupling two-step synthesis, which takes advantage of the
relative reactivity of the halogen substituents present on the
2,4,6-trichloro-1,3,5-triazine ring. Such an approach, which can
easily be applied to analogous systems,8 has paved the way for
the preparation of oxacalix[n]arenes composed of three or more
different (hetero)aromatic units.9
Given the potential of the above-mentioned method, the syn-
thesis of azobenzene-containing oxacalixarenes was considered
on the basis of the key role played by photoresponsive moieties
in the development of powerful molecular receptors.10 Azobenzene
moieties—because of their tendency to undergo reversible E/Z
photoisomerization11—are well known to induce conformational
modification on the macrocyclic framework they are attached to
and, as a result, may prompt selective capture and/or controlled re-
lease of a target substrate.10 Herein, as a follow-up of our recent
studies on tetraureido-12 and enantiomerically pure BINOL-con-
taining oxacalix[4]arenes,13 we wish to report on the synthesis of
the first oxacalixarenes bearing azobenzene groups along with a
preliminary study on their light-induced and thermal E/Z
isomerization.
Oxacalix[4]arenes (E)-1a,b and oxacalix[2]arene[2]triazines (E)-
2a,b were prepared by 3+1 fragment condensation starting from
the known precursors 314 and 4,7 respectively. These triaryl com-
pounds were in turn reacted with either (E)-4-phenylazoresorcinol
(Sudan Orange G) or (E)-4-(40-nitrophenylazo)resorcinol according
to Scheme 1. Difluoro derivative 3 was treated with equimolar
⇑
Corresponding author. Tel.: +39 090 6765241; fax: +39 090 392840.
E-mail address: ggattuso@unime.it (G. Gattuso).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.11.114

C. Gargiulli et al. / Tetrahedron Letters 53 (2012) 616–619
617
dinitroaromatic units (d = 6.05–6.21 ppm, Fig. 1) which, in analo-
gous systems,4a–c,12 has been attributed to the anisotropic shield-
ing induced by the flanking resorcinol rings. In addition, in both
macrocycle pairs, the peaks for the hydrogen atom of the unsubsti-
tuted resorcinol unit are shifted toward higher fields with respect
to the same signals of the relevant precursors (Dd = 0.09–0.17 and
0.29–0.43 ppm for (E)-1a,b and (E)-2a,b, respectively), most prob-
ably as the result of a mutual shielding between the resorcinol
moieties.
Additional evidence in favor of a 1,3-alternate saddle-shaped
conformation of derivatives (E)-1a,b and (E)-2a,b was also col-
lected by carrying out molecular modeling calculations16 at the
density functional level of theory (B3LYP/6-31G(d),17 see
Table S1 and Figs. S5–S8 in the Supplementary data). This preferen-
tial conformation arrangement, which has very often been ob-
served
in
the
solid
state
structure
of
related
oxacalix[4](hetero)arenes,4a–c,18,19 derives from the tendency to
ensure conjugation between the bridging oxygen atoms and the
electron-poor nitroaromatic rings.4a–c
As far as the absorption spectra are concerned, oxacalixarenes
(E)-1a,b and (E)-2a,b consistently show a weak band in the visible
and an intense one in the near UV region, typical of the azobenzene
moiety (Fig. 2).11a,20 In addition, derivatives (E)-1a,b display a high-
intensity band (centered at 267 nm), likely due to a
p?
p⁄ transi-
tion involving the dinitroaryl moieties of the oxacalix[4]arene
framework.21 The spectral differences between the pairs of pheny-
lazo and p-nitrophenylazo-substituted macrocycles (1a/2a and 1b/
2b, respectively) can be attributed to the presence of the nitro-sub-
stituent on the azobenzene subunit. Such an electron-withdrawing
Scheme 1. Synthesis of oxacalixarenes (E)-1a,b and (E)-2a,b.
amounts of the relevant azo dye in MeCN (60 °C) in the presence of
Et3N to give oxacalixarenes (E)-1a and (E)-1b in a 61% and a 91%
yield, respectively. The reaction with tetrachloro intermediate 4,
on the other hand, was carried out in acetone at room temperature
using i-Pr2NEt as the base. Macrocyclizations in this case produced,
compounds (E)-2a and (E)-2b in an 17%14 and a 87% isolated yield.
Oxacalixarenes (E)-1a,b and (E)-2a,b were characterized by 1H
and 13C NMR and UV–vis spectroscopy. 1H NMR spectra of com-
pounds (E)-1a,b suggest that these compounds adopt in solution
a 1,3-alternate saddle-shaped conformation. This type of structural
arrangement is consistent with the significant highfield shift of the
resonances assigned15 to the intra-annular hydrogen atoms of the
group induces a lower energy shift (
⁄ transitions of the latter pair, perturbing at the same time
their S1 n?
Dkmax ca. 10–14 nm) on the S2
p?p
p
⁄ transitions (broadening of the bands in the 400–
550 nm region).
With a view to subsequent use of these oxacalixarene deriva-
tives as potential photoresponsive molecular receptors, prelimin-
ary isomerization experiments (k = 365 nm 16 h, rt, CDCl3) were
carried out. Analysis of the 1H NMR spectra of the irradiated sam-
ples (Fig. S13) revealed that all compounds undergo photoisomer-
ization, producing mixtures of the E- and Z-isomers—albeit with
Figure 1. 1H NMR spectra (500 MHz, CDCl3, 25 °C, 5 mM) of oxacalix[4]arenes (E)-1a,b and oxacalix[2]arene[2]triazines (E)-2a,b.

618
C. Gargiulli et al. / Tetrahedron Letters 53 (2012) 616–619
Figure 2. Absorption spectra (5.5 ꢀ 10ꢁ5 M, CH2Cl2, 25 °C) of 1a,b and 2a,b in the E-form (solid line) and in the photo-stationary state (dashed line) reached after 1 h
irradiation at k = 334 nm. Insets show the fatigue-cycle plots.
very different ratios—2a being the most prone to isomerization and
1b the most resistant (see below, Table 1). Spectra of the Z-isomers
differ significantly from those of the parent E-isomers as a result of
the shielding effect produced by the isomerized exocyclic ring. Fur-
thermore, molecular modeling (Fig. S9–S12) suggests that the
change in the configuration of the N@N double bond affects the
conformation of the macrocycle, determining slight modifications
of the aryl–aryl interplanar angles (Table S1).
present on the azobenzene moiety. Accordingly, the quantum
yields decrease from 0.13 to 0.10 for 1a and 1b, respectively, and
from 0.15 to 0.12 for 2a and 2b. Both composition of the photosta-
tionary states and the quantum yield values indicate that the p-ni-
tro substituent in 1b and 2b significantly lowers the efficiency of
the photoisomerization process. In addition, in the case of 1b isom-
erization is further inhibited by the presence of the four nitro
groups on the oxacalixarene macrocycle.
The dependence of the azobenzene photoisomerization process
(E¢Z) on the nature of the macrocyclic frameworks was subse-
quently investigated via UV–vis spectroscopy, by selectively irradi-
ating (k = 334 and 432 nm) CH2Cl2 solutions of 1a,b and 2a,b. (E)-
Diphenyldiazene was chosen as a reference compound for these
experiments.22 In all instances, the absorption spectra of the E-
and Z-isomers were found to partially overlap (Fig. 2) and, as a con-
sequence, upon irradiation at k = 334 nm a stationary state was
reached.23 The changes in shape and energy of the absorption
bands are similar to those observed for the reference (E)-diphenyl-
diazene (Fig. S14). At equilibrium, the S1 band of each macrocycle
undergoes a small hyperchromic effect whereas the S2 one experi-
ences a hypsochromic shift and a substantial hypochromic effect.
Irradiation at k = 432 nm induces the quantitative reverse isom-
erization to the E-isomer. The photoequilibrium conditions can
repeatedly be reached in a cyclic manner with no evidence of prod-
uct degradation (see insets in Fig. 2).
In line with the known process of thermal reverse isomerization
of (Z)-azobenzene derivatives24 to the corresponding E-isomers in
the dark, oxacalixarenes (Z)-1a,b and (Z)-2a,b (CH2Cl2,25 25 °C)
were found to revert back to their E-isomers with first-order kinet-
ics (Fig. 3).
Close inspection of the first-order rate constants (kZ?E) reported
in Table 1 confirms that the p-nitro group also plays a significant
role in the reverse isomerization process, by way of stabilizing
more efficiently the E- rather than the Z-isomers. As a result, p-
nitrophenylazo substituted oxacalixarenes 1b and 2b display high-
er kZ?E values than the corresponding phenylazo derivatives 1a
Data in Table 1 also indicate that the E?Z photoisomerization
process becomes less efficient when the p-nitro-substituent is
Table 1
Selected parameters for the isomerization of 1a,b and 2a,b
kZ?E/sꢁ1c
a
b
UE?Z
(E/Zratio)
1a
1b
2a
2b
0.13
0.10
0.15
0.12
(44:56)
(71:29)
(8:92)
4.0 ꢀ 10ꢁ4
6.2 ꢀ 10ꢁ4
1.3 ꢀ 10ꢁ5
2.5 ꢀ 10ꢁ5
(51:49)
a
Irradiation at 334 nm.
Obtained by direct integration of relevant peaks present in the 1H NMR spectra
b
(Fig. S13).
Figure 3. Absorption variations at 340 nm for the thermal Z?E reactions in the
dark at 25 °C in CH2Cl2.
c
Measured at 25 °C.

C. Gargiulli et al. / Tetrahedron Letters 53 (2012) 616–619
619
and 2a (6.2 ꢀ 10ꢁ4 and 2.5 ꢀ 10ꢁ5 sꢁ1 vs 4.0 ꢀ 10ꢁ4 and
1.3 ꢀ 10ꢁ5 sꢁ1, respectively). Similar rate constant comparison be-
tween oxacalix[4]arenes 1a,b and oxacalix[2]arene[2]triazine 2a,b
reveals that the former pair undergoes thermal Z?E isomerization
faster than the latter one. According to this cross-comparison, the
nature of the oxacalixarene framework affects the excited states
involved in the thermal isomerization process of the azobenzene
moieties, with the dinitroaryl rings—more than the chlorotriazine
ones—exerting a stabilizing effect on the E-isomers. This observa-
tion is in agreement with the results obtained from the photoiso-
merization experiments.
In conclusion, we have synthesized the first family of azoben-
zene-containing photoisomerizable oxacalixarenes and investi-
gated their photoinduced and thermal E/Z isomerization.
Additional studies aiming to elucidate the molecular recognition
properties of these and other structurally-related oxacalixarenes,
along with in-depth investigations on their photoswitching behav-
ior, are currently in progress.
10. (a) Reuter, R.; Wegner, H. A. Chem. Commun. 2011, 12267–12276; (b)
Brancatelli, G.; Capici, C.; Gattuso, G.; Geremia, S.; Notti, A.; Pappalardo, S.;
Parisi, M. F.; Sortino, S.; Vittorino, E. Chem.–Asian J., doi: 10.1002/
asia.201100738.; (c) Liu, M.; Yan, X.; Hu, M.; Chen, X.; Zhang, M.; Zheng, B.;
Hu, X.; Shao, S.; Huang, F. Org. Lett. 2010, 12, 2558–2561; (d) Muraoka, T.;
Kinbara, K.; Aida, T. Nature 2006, 440, 512–515; (e) Balzani, V.; Ceroni, P.;
Giansante, C.; Vicinelli, V.; Klärner, F.-G.; Verhaelen, C.; Vögtle, F.; Hahn, U.
Angew. Chem., Int. Ed. 2005, 44, 4574–4578; (f) Balzani, V.; Venturi, M.; Credi, A.
Molecular Devices and Machines-A Journey into the Nanoworld; Wiley-VCH:
Weinheim, 2003; (g) Molecular Switches; Feringa, B. L., Ed.; Wiley-VCH:
Weinheim, 2001; (h) Shinkai, S. In Comprehensive Supramolecular Chemistry;
Atwood, J. L., Davies, J. E. D., MacNicol, D. D., Vögtle, F., Eds.; Pergamon: Oxford,
1996; Vol. 1, p 671.
11. (a) Bandara, H. M. D.; Burdette, S. C. Chem. Soc. Rev., doi: 10.1039/c1cs15179g;
(b) Puntoriero, F.; Ceroni, P.; Balzani, V.; Bergamini, G.; Vögtle, F. J. Am. Chem.
Soc. 2007, 129, 10714–10719; (c) Jousselme, B.; Blanchard, P.; Gallego-Planas,
N.; Delaunay, J.; Allain, M.; Richomme, P.; Levillain, E.; Roncalli, J. J. Am. Chem.
Soc. 2003, 125, 2888–2889; (d) Norikane, Y.; Kitamoto, K.; Tamaoki, N. J. Org.
Chem. 2003, 68, 8291–8304.
12. Capici, C.; Garozzo, D.; Gattuso, G.; Messina, A.; Notti, A.; Parisi, M. F.; Pisagatti,
I.; Pappalardo, S. ARKIVOC 2009, viii, 199–211.
13. Capici, C.; Gattuso, G.; Notti, A.; Parisi, M. F.; Bruno, G.; Nicolò, F.; Pappalardo, S.
Tetrahedron Lett. 2011, 52, 1351–1353.
14. It is worth mentioning that in the case of (E)-2a 1H NMR inspection of the
reaction mixture—before column chromatography—indicated a very high yield
of the cyclic product.
Acknowledgment
15. COSY NMR spectra for 1a,b and 2a,b are reported in the Supplementary data
(Figs. S1–S4).
MIUR (PRIN-2009A5Y3N9 project) is gratefully acknowledged
for the financial support.
16. Spartan’10, Wavefunction, Inc., Irvine, CA. See: Shao, Y.; Molnar, L. F.; Jung, Y.;
Kussmann, J.; Ochsenfeld, C.; Brown, S. T.; Gilbert, A. T. B.; Slipchenko, L. V.;
Levchenko, S. V.; O’Neill, D. P.; DiStasio, Jr., R. A.; Lochan, R. C.; Wang, T.; Beran,
G. J. O.; Besley, N. A.; Herbert, J. M.; Lin, C. Y.; Van Voorhis, T.; Chien, S. H.; Sodt,
A.; Steele, R. P.; Rassolov, V. A.; Maslen, P. E.; Korambath, P. P.; Adamson, R. D.;
Austin, B.; Baker, J.; Byrd, E. F. C.; Dachsel, H.; Doerksen, R. J.; Dreuw, A.;
Dunietz, B. D.; Dutoi, A. D.; Furlani, T. R.; Gwaltney, S. R.; Heyden, A.; Hirata, S.;
Hsu, C.-P.; Kedziora, G.; Khalliulin, R. Z.; Klunzinger, P.; Lee, A. M.; Lee, M. S.;
Liang, W. Z.; Lotan, I.; Nair, N.; Peters, B.; Proynov, E. I.; Pieniazek, P. A.; Rhee, Y.
M.; Ritchie, J.; Rosta, E.; Sherrill, C. D.; Simmonett, A. C.; Subotnik, J. E.;
Woodcock III, H. L.; Zhang, W.; Bell, A. T.; Chakraborty, A. K.; Chipman, D. M.;
Keil, F. J.; Warshel, A.; Hehre, W. J.; Schaefer III, H. F.; Kong, J.; Krylov, A. I.; Gilla,
P. M. W.; Head-Gordon, M. Phys. Chem. Chem. Phys. 2006, 8, 3172–3191.
17. (a) Gattuso, G.; Notti, A.; Parisi, M. F.; Pisagatti, I.; Amato, M. E.; Pappalardo, A.;
Pappalardo, S. Chem.-Eur. J. 2010, 16, 2381–2385; (b) Gargiulli, C.; Gattuso, G.;
Notti, A.; Pappalardo, S.; Parisi, M. F. Supramol. Chem. 2010, 22, 358–364.
18. (a) Yang, Y.; Xue, M.; Chen, C.-F. CrystEngComm 2010, 12, 3502–3505; (b)
Yasukawa, Y.; Kobayashi, K.; Konishi, H. Tetrahedron Lett. 2009, 50, 5130–5134;
(c) Wang, D.-X.; Zheng, Q.-Y.; Wang, Q.-Q.; Wang, M.-X. Angew. Chem., Int. Ed.
2008, 47, 7485–7488; (d) Konishi, H.; Mita, T.; Yasukawa, Y.; Morikawa, O.;
Kobayashi, K. Tetrahedron Lett. 2008, 49, 6831–6834.
19. Irrespective of the conformation adopted, oxacalixarenes 1a,b and 2a,b are
inherently chiral but, because of the absence of suitable probe resonances,
energy barriers for their racemization could not be determined by VT-NMR.
See: Boros, E. E.; Andrews, C. W.; Davis, A. O. J. Org. Chem. 1996, 61, 2553–2555.
and ref. 18b.
20. (a) Bonvalle, P. A.; Mullen, M. R.; Evans, P. J.; Stoltz, K. L.; Story, E. N.
Tetrahedron Lett. 2011, 52, 1117–1120. and references therein.; (b) Consoli, G.
M. L.; Geraci, C.; Neri, P.; Bergamini, G.; Balzani, V. Tetrahedron Lett. 2006, 47,
7809–7813.
Supplementary data
Supplementary data (general experimental methods, synthetic
procedures, COSY NMR spectra and calculated structures for oxaca-
lixarenes 1a,b and 2a,b) associated with this article can be found,
in the online version, at doi:10.1016/j.tetlet.2011.11.114.
References and notes
1. Pedersen, C. J. J. Am. Chem. Soc. 1967, 89, 7017–7036.
2. Comprehensive Supramolecular Chemistry; Szejtli, J., Osa, T., Eds.; Elsevier:
Oxford, 1996; Vol. 3,.
3. Gutsche, C. D. Calixarenes-An Introduction. In Monographs in Supramolecular
Chemistry; Stoddart, J. F., Ed., second ed.; The Royal Society of Chemistry:
Cambridge, UK, 2008. Vol. 6.
4. (a) For oxa- and azacalixarenes, see: Wang, M.-X. Acc. Chem. Res., doi: 10.1021/
ar200108c; (b) Wang, M.-X. Chem. Commun. 2008, 4541–4551; (c) Maes, W.;
Dehaen, W. Chem. Soc. Rev. 2008, 37, 2393–2402; (d) Parisi, M. F.; Pappalardo, S.
In Comprehensive Heterocyclic Chemistry III; Katritzky, A. R., Ramsden, C. A.,
Scriven, E. F. V., Taylor, R. J. K., Eds.; Elsevier: Oxford, UK, 2008; Vol. 14, pp 667–
750; (e) König, B.; Fonseca, M. H. Eur. J. Inorg. Chem. 2000, 2303–2310; For
thiacalixarenes, see: (f) Morohashi, N.; Narumi, F.; Iki, N.; Hattori, T.; Miyano, S.
Chem. Rev. 2006, 106, 5291–5316; (g) Lhoták, P. Eur. J. Org. Chem. 2004, 1675–
1692.
5. Sommer, N.; Staab, H. A. Tetrahedron Lett. 1966, 7, 2837–2841.
6. For some recent examples, of oxa- and azacalixarenes prepared by this
approach, see: (a) Touil, M.; Raimundo, J.-M.; Lachkar, M.; Marsal, P.; Siri, O.
Tetrahedron 2010, 66, 4377–4382; (b) Van Rossom, W.; Kishore, L.; Robeyns, K.;
Van Meervelt, L.; Dehaen, W.; Maes, W. Eur. J. Org. Chem. 2010, 4122–4129; (c)
Haddoub, R.; Touil, M.; Raimundo, J.-M.; Siri, O. Org. Lett. 2010, 12, 2722–2725;
(d) Xue, M.; Chen, C.-F. Org. Lett. 2009, 11, 5294–5297; (e) Katz, J. L.; Geller, B. J.;
Conry, R. R. Org. Lett. 2006, 8, 2755–2758; (f) Konishi, H.; Tanaka, K.; Teshima,
Y.; Mita, T.; Morikawa, O.; Kobayashi, K. Tetrahedron Lett. 2006, 47, 4041–4044;
(g) Katz, J. L.; Feldman, M. B.; Conry, R. R. Org. Lett. 2005, 7, 91–94.
7. Wang, M.-X.; Yang, H.-B. J. Am. Chem. Soc. 2004, 126, 15412–15422.
8. Konishi, H.; Mita, T.; Morikawa, O.; Kobayashi, K. Tetrahedron Lett. 2007, 48,
3029–3032.
21. This assignment was confirmed by comparison with the absorption spectrum
of an oxacalix[4]arene devoid of the azobenzene moiety, composed of
alternating dinitroaryl and resorcinol moieties (see ref. 12).
22. The tetranitro-oxacalix[4]arene framework of compounds 1a,b does not
significantly absorb above 320 nm, and therefore there is no overlap with
the relevant bands of the azobenzene moiety.
23. The E/Z concentration ratios depend on the molar absorption coefficients of the
two isomers at the excitation wavelength employed, as well as on the quantum
yields of the forward and reverse reactions.
24. Thermal Z?E isomerization of diphenyldiazene is a first-order process that
proceeds with an activation energy value close to 24 kcal mol–1 (Ciminelli, C.;
Granucci, G.; Persico, M. Chem.–Eur. J. 2004, 10, 2327–2341).
25. Solvent polarity is a key factor for this thermal process as demonstrated by the
results of Khayer and Sander, (Khayer, K.; Sander, W. J. Bangladesh Acad. Sci.
2008, 32, 111–116). Isomerization of diphenyldiazene in the dark is
accelerated by a factor of 1.2–1.7 as the solvent polarity decreases.
9. (a) Van Rossom, W.; Robeyns, K.; Ovaere, M.; Van Meervelt, L.; Dehaen, W.;
Maes, W. Org. Lett. 2011, 13, 26–129; (b) Van Rossom, W.; Ovaere, M.; Van
Meervelt, L.; Dehaen, W.; Maes, W. Org. Lett. 2009, 11, 1681–1684.