Arylethynyl derivatives of the dihydroazulene/vinylheptafulvene photo/thermoswitch: Tuning the switching event

Article abstract of DOI:10.1021/ja103235g

A selection of dihydroazulene (DHA) photoswitches incorporating an arylethynyl-substituent in the seven-membered ring was prepared by palladium-catalyzed Sonogashira cross-coupling reactions employing a suitable bromo-functionalized DHA. Shielding of the

Full text of DOI:10.1021/ja103235g

Published on Web 06/16/2010
Arylethynyl Derivatives of the Dihydroazulene/
Vinylheptafulvene Photo/Thermoswitch: Tuning the
Switching Event
Søren Lindbæk Broman, Michael Åxman Petersen, Christian G. Tortzen,
Anders Kadziola, Kristine Kilså, and Mogens Brøndsted Nielsen*
Department of Chemistry, UniVersity of Copenhagen, UniVersitetsparken 5,
DK-2100 Copenhagen Ø, Denmark
Received April 23, 2010; E-mail: mbn@kiku.dk
Abstract: A selection of dihydroazulene (DHA) photoswitches incorporating an arylethynyl-substituent in
the seven-membered ring was prepared by palladium-catalyzed Sonogashira cross-coupling reactions
employing a suitable bromo-functionalized DHA. Shielding of the alkyne bridge and separating the aryl
and DHA units, by sterically demanding groups, was required to obtain stable compounds. The DHAs
underwent a light-induced ring-opening to vinylheptafulvenes (VHFs) which were thermally converted to a
mixture of two DHA regioisomers, one of which was the original one. The influence of the aryl groups on
the DHA and VHF absorptions and on their interconversion was investigated in detail. The rates of the
switching events were finely tuned by the donor or acceptor strength of the aryl group. The thermal ring
closure was found to proceed most readily in the presence of an electron-donating group on the seven-
membered ring. The rate constant was found to follow a Hammett linear free energy correlation, which
signals that stabilization of a positive charge in the seven-membered ring plays a crucial role in the ring-
closure reaction. In view of these findings, it was possible to control the switching event by protonation/
deprotonation of an anilino-substituted DHA. Also, the light-induced ring opening reaction was strongly
controlled by acid/base. In addition to the mesomeric effects exerted by an arylethynyl group, the inductive
effects exerted by different groups on the thermal ring closure were elucidated. Although the alkyne bridge
transmits the electronic character of the aryl group, the ring-closure is retarded for all the ethynylated
compounds relative to the parent unsubstituted compound. Along with our synthesis of suitable arylalkynes,
we discovered an interesting byproduct in a Sonogashira cross-coupling reaction involving a nitrophenyl
group, namely a diaryl azoxy compound. Its structure was confirmed by X-ray crystallographic analysis.
Introduction
to photoswitches, such as azobenzenes and dithienylethenes, that
can be interconverted between two states by light.2 The term
Molecular switches possess at least two reversibly intercon-
vertible molecular states and are important within the field of
molecular electronics, photonic devices, and advanced molecular
and supramolecular materials.1 Particular attention has been paid
photochromism is used to characterize the properties of such
compounds that change their optical absorption upon light-
induced isomerization. Synthetic protocols are available for
ready functionalization of both rings in azobenzenes and
dithienylethene compounds with suitable groups, such as
electron donor and acceptor units (push-pull photochro-
mophores), anchoring groups for adhesion to electrodes, func-
tional groups that provide liquid crystallinity, or groups for
incorporation of the photoswitch into biological systems.2 One
photoswitch that has been less explored in advanced systems is
the dihydroazulene/vinylheptafulvene (DHA/VHF) system in-
corporating two cyano groups at C-1 (Scheme 1).3,4 DHA
undergoes a photochemically induced ring-opening to VHF,
usually with a notably higher quantum yield than that of the
trans to cis isomerization of azobenzenes. VHF in turn
(1) (a) Balzani, V.; Go´mez-Lo´pez, M.; Stoddart, J. F. Acc. Chem. Res.
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Kno¨chel, T.; Maas, G.; Seitz, U. Chem. Ber. 1986, 119, 2631–2646.
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J. AM. CHEM. SOC. 2010, 132, 9165–9174 9165

A R T I C L E S
Broman et al.
Scheme 1. Dihydroazulene (DHA)-Vinylheptafulvene (VHF)
VHF system has been less explored in advanced systems. The
conjugated part of DHA has seven possible positions (C2, C3,
C4, C5, C6, C7, and C8; for numbering, see Scheme 1) at which
its properties can be potentially tuned by substituent groups and
at which it can be incorporated into larger systems. We have
very recently taken the first step toward advancing the chemistry
of the seven-membered ring.7,8 Thus, an efficient regioselective
bromination-elimination protocol was devised for preparing the
bromo-substituted DHA 2a from DHA 1a (Chart 1). Special
care was required in the elimination step to avoid elimination
of HCN along with HBr, which would furnish the corresponding
10π-aromatic azulene. Indeed, the choice of base was rather
delicate, but it was found that LiN(SiMe3)2 provided the desired
elimination product 2a in a yield of >90%.7b Subsequently a
Sonogashira cross-coupling reaction9 with triisopropylsilylacety-
lene provided the product 3a that after a ring-opening/closure
cycle provided a mixture of the two regioisomers 3a and 4a.
The ratio was controlled by both the wavelength of irradation
and the solvent polarity (3a:4a ratio of 1:2 in MeCN and 1:0 in
cyclohexane after one light (353 nm)-heat cycle). Here we
present the further scope of compounds 2a and 3a for Sono-
gashira cross-coupling reactions with the aim to functionalize
DHA with electron-donating and electron-withdrawing aryl
groups. The ultimate goal was to determine to which extent the
acetylenic spacer transmits the donor or acceptor property of
the aryl group, which is expected to influence the switching
event.
Photoswitch
Chart 1. Functionalized DHAs
Results and Discussion
Our first objective was to subject compound 3a to further
acetylenic scaffolding. This required removal of the silyl
protecting group to provide the terminal alkyne. First, desily-
lation of 3a was attempted by a fluoride source (Bu4NF) in wet
THF. However, these conditions caused elimination of HCN
to provide a mixture of azulenes. For this reason we decided to
incorporate the more labile trimethylsilyl-protecting group by
treating the bromide 2a with trimethylsilylacetylene according
to the conditions provided in Scheme 2. Careful treatment of
the resulting DHA 5a with Bu4NF again resulted in elimination
of HCN as did treatment with K2CO3 in a mixture of THF and
MeOH. Yet, treating DHA 5a with acetic acid before addition
of the fluoride source gave, gratifyingly, the terminal alkyne
6a in high yield. Next, this alkyne was treated with iodobenzene
under Sonogashira cross-coupling conditions. We were able to
isolate DHA 7a from this reaction. A spot of the species on a
TLC plate changed color from yellow to violet after exposure
undergoes a thermally induced ring-closure back to DHA. The
initially formed s-cis-VHF is in equilibrium with the generally
more stable s-trans-VHF. The large structural difference
between the DHA and VHF isomers, both in regard to
conjugation, spatial shape, and dipole moment,5 is reflected by
significantly different absorption maxima of the two species.
Thus, the phenyl-substituted DHA 1a (Chart 1) exhibits an
absorption maximum (λmax) at 354 nm in MeCN, while the
related VHF 1b absorbs at λmax ) 471 nm.6 The letters a and
b will be used in general to designate a pair of DHA (a) and
VHF (b) compounds that are interconvertible. Another attractive
property of the system is that both the light-induced DHAfVHF
and the thermal VHFfDHA conversion proceeds quantitatively,
while cis-trans isomerizations are often characterized by a
photostationary equilibrium.
1
to UV light. Also, the H NMR spectrum indicated formation
The pioneering work by Daub and co-workers3 has allowed
ready incorporation of a variety of aryl groups in the five-
membered ring of DHA. The aryl group is conveniently included
early in the synthesis before the final DHA is formed. One major
obstacle has been the incorporation of functional groups in the
seven-membered ring of DHA. Indeed, we reckon that the lack
of synthetic protocols for functionalizing both the five- and
seven-membered rings is one of the main reasons why the DHA/
of the right product. Yet, the compound was not stable in
solution and degraded to an unidentified species that was no
longer photochromic. A simple TLC inspection readily allows
one to identify the presence or absence of a DHA species as
the spot changes color from yellow to red or violet (depending
(7) (a) Petersen, M. Å.; Kilså, K.; Kadziola, A.; Nielsen, M. B. Eur. J.
Org. Chem. 2007, 1415–1418. (b) Petersen, M. Å.; Broman, S. L.;
Kadziola, A.; Kilså, K.; Nielsen, M. B. Eur. J. Org. Chem. 2009, 2733–
2736.
(4) (a) Gobbi, L.; Seiler, P.; Diederich, F. Angew. Chem., Int. Ed. 1999,
38, 674–678. (b) Petersen, M. Å.; Zhu, L.; Jensen, S. H.; Andersson,
A. S.; Kadziola, A.; Kilsa˚, K.; Nielsen, M. B. AdV. Funct. Mater. 2007,
17, 797–804. (c) Petersen, M. Å.; Andersson, A. S.; Kilsa˚, K.; Nielsen,
M. B. Eur. J. Org. Chem. 2009, 1855–1858.
(8) We note that other DHA derivatives functionalized in the seven-
membered ring are known, but these derivatives do not undergo light-
induced conversion to VHFs; see for example: (a) Prinzbach, H.; Herr,
H.-J. Angew. Chem., Int. Ed. Engl. 1972, 11, 135–136. (b) Sugihara,
Y.; Yamamoto, H.; Mizoue, K.; Murata, I. Angew. Chem., Int. Ed.
Engl. 1987, 26, 1247–1249. (c) Yu, F.-F.; Yang, W.-G.; Shi, M. Chem.
Commun. 2009, 1392–1394.
(5) Plaquet, A.; Champagne, B.; Castet, F.; Ducasse, L.; Bogdan, E.;
Rodriguez, V.; Pozzo, J.-L. New J. Chem. 2009, 33, 1349–1356.
(6) Go¨rner, H.; Fischer, C.; Gierisch, S.; Daub, J. J. Phys. Chem. 1993,
97, 4110–4117.
(9) Sonogashira, K.; Tohda, Y.; Hagihara, N. Tetrahedron Lett. 1975,
4467–4470.
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Tuning the Switching Event
A R T I C L E S
Scheme 2. Synthesis of Arylethynyl-DHAs that Decomposed after Isolation
Scheme 3. Synthesis of Arylacetylenes with Methyl Groups
Shielding the Alkyne
Chart 2. By-Product (Azoxy Compound) Formed from 15 under the
Sonogashira Conditions
upon hydrogenation of nitrobenzene in the presence of
[Pd(PhCN)2Cl2] in DMF.15 We have done some preliminary
experiments to optimize the azoxy formation. First, formation
of azoxybenzene was observed as well (according to GC-MS)
by refluxing nitrobenzene, [PdCl2(PPh3)2], and CuI in THF. The
presence of CuI was required for any conversion to occur, while
the presence of an amine was found to retard the reaction. Next,
we subjected compound 15 to the conditions ([PdCl2(PPh3)2]
and CuI in THF, reflux), and found that 50 mol % of both
on the substitution) upon irradiation with UV-light within the
region 350-400 nm. In another reaction, the bromide 2a was
subjected to a cross-coupling reaction with p-nitrophenylacety-
lene. Once again, we managed to isolate what seemed to be a
DHA product (8a), but this compound as well underwent
conversion to a yet unidentified non-DHA species.
Likely, the alkyne unit in 7a and 8a takes part in cycloaddition
reactions. At the moment, it is unclear which product(s) are
formed. No reaction was in fact observed by simply stirring
parent compound 1a and tolane in THF at 50 °C overnight
(reflux in CH2Cl2 gave no conversion either). Moreover, alkyne
6a could be isolated with no apparent decomposition. Despite
these somewhat ambiguous results, we decided to shield the
alkyne unit of the arylethynyl DHAs by bulky groups at the
ortho positions on the phenyl ring. From the known iodides
9,10 10,11 and 11,12 the ethynylated aromatics 12,13 13, 14, and
15 were prepared (Scheme 3).
Interestingly, the cross-coupling conditions responsible for
formation of 14 and 15 also provided a byproduct (10%) that
in the latter case turned out to be the azoxybenzene 16 (Chart
2) as confirmed by X-ray crystallographic analysis (Figure 1).14
Thus, two molecules of 15 have been reductively dimerized.
This is to our knowledge the first time such a species has been
identified as a byproduct in the Sonogashira reaction of
nitrobenzenes. However, an azoxy byproduct has been ob-
tained previously in the presence of palladium; for example,
(10) Ohno, A.; Tsutsumi, A.; Yamazaki, N.; Okamura, M.; Mikata, Y.;
Fujii, M. Bull. Chem. Soc. Jpn. 1996, 69, 1679–1685.
Figure 1. (a) X-ray crystal structure of the azoxy compound 16. (b) The
molecules are packed in layers dislocated in two dimensions (the figure
was made with the program Mercury from the Cambridge Crystallographic
Data Centre) and no π-π stacking is observed. The two phenyl moieties
are virtually coplanar. Projected down the Si-Si line, the NO bond makes
an angle of 18° with the remaining π-conjugated plane, presumably due to
steric effects. The N-N and N-O distances are observed to be 1.24 and
1.49 Å, respectively.
(11) Kajigaeshi, S.; Kakinami, T.; Yamasaki, H.; Fujisaki, S.; Okamoto,
T. Bull. Chem. Soc. Jpn. 1988, 61, 600–602.
(12) (a) Bartoli, G.; Ciminale, F.; Todesco, P. E. J. Org. Chem. 1975, 40,
872–874. (b) Van Berk, P.; Van Langen, J. O. M.; Verkade, P. E.;
Webster, B. M. Recl. TraV. Pays-Bas 1956, 75, 1137–1154. (c)
Hodgson, H. H.; Walker, J. J. Chem. Soc. 1933, 1620.
(13) Saiki, T.; Akine, S.; Goto, K.; Tokitoh, N.; Kawashima, T.; Okazaki,
R. Bull. Chem. Soc. Jpn. 2000, 73, 1893–1902.
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Scheme 4. Synthesis of Stable Arylethynyl-DHAs
Figure 2. Irradiation of DHA 20a (1.2 × 10-5 M) in MeCN with light
(354 nm) during 0-140 s (10-s steps). The final VHF (20b) absorption
spectrum is indicated by the thick curve.
palladium and copper catalysts provided complete conversion
to 16 (within 1 h as judged by TLC inspection).
The silyl protecting groups of 12-15 were removed under
standard conditions, by either K2CO3 in MeOH/THF (removal
of SiMe3) or Bu4NF in THF (removal of Sii-Pr3) to furnish
alkynes 17,16 18, and 19 in high yields.
The terminal arylalkynes were then subjected to palladium-
catalyzed cross-coupling reactions with the DHA-bromide 2a
(Scheme 4). The resulting DHAs 20a, 21a, and 22a were
isolated in reasonable yields, and, gratifyingly, they were all
resistant toward degradation upon standing in solution after
isolation. Thus, our design strategy turned out successful; the
two methyl groups are indeed able to shield the alkyne unit
suffiently to avoid degradation. Yet, we are not currently able
to state whether the stabilizing effect is of steric or electronic
character. Synthesis of the nitrophenyl-substituted DHA 22a was
the most difficult; it required ultrasound (using a sonication bath)
to proceed. We have previously17 found ultrasound advanta-
geous for the Sonogashira cross-coupling involving compounds
that do not withstand prolonged heating. While compound 22a
is not stable under the Sonogashira conditions, it is perfectly
stable when isolated pure. We found that microwave heating
could also be employed for the reaction. DHA 22a was also
Figure 3. Thermal back-reaction of VHF 20b (1.2 × 10-5 M) in MeCN
monitored at 70 °C (2-min steps). The final DHA (20a/26a) absorption
spectrum is indicated by the thick curve.
Chart 3. Terminal Alkynes
Chart 4. E/Z-VHFs
(14) Compound 16 crystallizes in the triclinic crystal system, with the sum
formula C38H58N2OSi2 and a unit cell volume of 934.5 Å3. This means
that there is only one molecule in the unit cell. Because the molecule
itself does not possess an inversion center (the oxygen atom breaks
the symmetry), the natural conclusion is that the space group must be
P1. However, when solved in P1, peaks in the initial electron density
map clearly display inversion symmetry. So albeit the space group
must be P1, the molecule is disordered with the oxygen atom having
two possible positions depending on the microscopic cell (a twin
hypothesis was also tested but did not seem to be appropriate). On
Chart 5. DHAs Substituted at Position 6
j
average, the space group cannot be distinguished from P1 and the
structure refinement performs much better in this space group. First
of all this description constrains all atoms with an inversion center
and the oxygen population to 0.5. Second, the thermal ellipsoids appear
more reasonable and the resulting difference Fourier reveals many of
the hydrogen atoms which was not the case when attempted refined
in P1.
prepared by an alternative route, namely from the DHA-alkyne
6a and the iodide 11 (Scheme 4). In effect, both DHAs 2a and
6a present useful building blocks for acetylenic scaffolding.
The selection of functionalized DHAs (designated by a) was
subjected to switching studies in MeCN. They all underwent
light-induced ring-opening to the corresponding VHFs (b),
which underwent thermal back-reactions. The absorption maxi-
mum of each individual DHA was used as excitation wave-
length. The conversions are characterized by isosbestic points
(15) Mondal, T. K.; Banerjee, T. K.; Sen, D. Indian J. Chem., Sect. A 1980,
19, 846–848.
(16) Panda, J.; Virkler, P. R.; Detty, M. R. J. Org. Chem. 2003, 68, 1804–
1809.
(17) (a) Qvortrup, K.; Andersson, A. S.; Mayer, J.-P.; Jepsen, A. S.; Nielsen,
M. B. Synlett 2004, 2818–2820. (b) Andersson, A. S.; Qvortrup, K.;
Torbensen, E. R.; Mayer, J.-P.; Gisselbrecht, J.-P.; Boudon, C.; Gross,
M.; Kadziola, A.; Kilså, K.; Nielsen, M. B. Eur. J. Org. Chem. 2005,
3660–3671. (c) Andersson, A. S.; Kerndrup, L.; Madsen, A. Ø.; Kilså,
K.; Nielsen, M. B.; La Porta, P. R.; Biaggio, I. J. Org. Chem. 2009,
74, 375–382.
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Tuning the Switching Event
A R T I C L E S
Table 1. Absorption Maxima (λmax/nm) of DHAs and VHFs Measured in MeCN
substituent
7-substituted DHA
7-substituted/6-substituted DHA (ratio)a
6-substituted DHAb
E/Z-VHF
H
Br
354 (1a)
356 (2a)
355 (3a)
355 (5a)
-
-
471 (1b)
461 (2b)
476 (3b)
476 (5b)
469 (6b)
490 (20b)
527c (21b)
482 (22b)
(7:3)
360 (23a)
381 (4a)
380 (24a)
379 (25a)
400 (26a)
ca. 420d (27a)
411 (28a)
CtC-Sii-Pr3
CtC-SiMe3
CtC-H
CtC-Ar-H
CtC-Ar-NH2
CtC-Ar-NO2
376 (1:2)
372 (5:7)
369 (7:4)
385 (1:1)
330/420d
407 (5:3)
355 (6a)
354 (20a)
330, 350 (shc) (21a)
344 (22a)
a After one light-heat (ring-opening/closure) cycle, starting from the neat 7-substituted isomer; the ratios were calculated from the 1H NMR spectra.
b Absorption maximum was calculated by subtracting the absorption spectrum of the neat 7-susbtituted DHA from the absorption spectrum of the
mixture using the isomer ratio obtained from H NMR spectroscopy. c Sh ) shoulder. d See text.
1
Figure 5. Summary of absorption maxima for the aryl-substituted DHAs
and VHFs. The shoulder at ca. 350 nm was plotted for 21a.
new cross-peak is seen for 26a, namely between protons H-8
and H-7, which signals that the substituent group is no longer
placed at H-7 but at H-6. Moreover, the H-7 proton shows no
further coupling. All of the three arylethynyl-DHAs are stable
up to at least 70 °C in nondegassed solution, and no bleaching
was observed after five repetitive light-heat cycles (DHA f
VHF f DHA).
A summary of absorption maxima for the compounds in
MeCN in comparison to those previously studied (1a,b6 and
3a,b/4a7b) is provided in Table 1 together with DHA isomer
ratios obtained after one light-heat cycle (the ratios did not
change, however, after more cycles). Absorption maxima for
the aryl-substituted derivatives are also summarized in Figure
5. First, we note that the 6-substituted DHA isomers exhibit
Figure 4. 1H NMR Spectrum (CD3CN) of 20a and the COSY spectrum
after one light-heat cycle, after which a mixture of 20a and 26a was formed.
For the proton labeling, see Scheme 1. The numbers in blue refer to 20a,
while the primed numbers in red refer to 26a. The symbol * indicates a
signal that originates from either H-4, H-5, or H-6.
significantly red-shifted absorption maxima relative to those of
the 7-substituted ones; the absorption maxima of the 6-substi-
tuted DHAs were estimated from the maxima of the isomeric
mixtures and the neat spectra of 7-substituted DHAs (by
subtracting the spectra, employing the isomer ratios determined
by NMR). These redshifts are explained by the fact that DHA
takes a boat-like conformation,7b and placement of the sub-
stituent group at position 6 instead of position 7 renders it part
of a larger planar π-system comprised of C2-C6 and the phenyl
group. The more efficient conjugation between DHA and the
substituent when placed in position 6 is clearly realized by
looking at the geometry-optimized structures of DHAs 20a and
26a (B3LYP/6-31G(d) level) obtained using the Gaussian 03
program package18 (Figure 6). The different absorption char-
acteristics is most evident for the donor-substituted compounds
21a and 27a. This is not surprising, inasmuch as the transition
likely has charge-transfer character. It was not possible to
convert 21a quantitatively to 21b in MeCN (low quantum yield,
Vide infra). However, changing the solvent to cyclohexane
allowed quantitative ring-opening. Transferring the resulting
in the UV-vis absorption spectra. The change in the UV-vis
absorption spectra for the light-induced opening of 20a to 20b
is shown in Figure 2, while the thermal ring-closure of 20b is
shown in Figure 3. As observed previously,7b the ring-opening
reaction provided VHF E/Z-isomers (configuration of fulvene
double bond; for simplicity, both VHF isomers are designated
by the symbol b, Chart 4), while the thermal ring-closure
reaction gave a mixture of 7- and 6-substituted DHAs (23a-28a,
Chart 5). The ratio of E and Z isomers could be determined by
1H NMR spectroscopy for the VHFs 3b, 5b, 6b, 20b, and 22b
(by virtue of nonoverlapping signals for at least one group,
allowing for determination of the integral ratio) and were in all
cases ca. 1:1. The two DHA isomers are more readily distin-
guished by 1H NMR spectroscopy, and a COSY spectrum
supported the assignment of the new DHA species to the
6-substituted isomer. The COSY spectrum of 20a/26a is shown
in Figure 4. Coupling between protons H-8a and H-8 are seen
for 20a, but as expected no further coupling is seen for H-8. A
(18) Frisch, M. J.; et al. Gaussian 03, ReVision B.03; Gaussian, Inc.:
Wallingford, CT, 2004.
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A R T I C L E S
Broman et al.
Figure 7. Linear free energy correlation between ln (k25 °C) and Hammett
+
constants σp (O) and σp (b). σp(NH2) ) -0.66, σp(H) ) 0, σp(NO2) )
0.78; σp+(NH2) ) -1.30, σp+(H) ) 0, σp+(NO2) ) 0.79. The points
corresponding to H and NO2 are overlapping.
conjugation; NH2: -1.30, H: 0, NO2: 0.79).20 A straight line
with a negative slope is obtained, which indicates that the ring-
closure is enhanced by the ability of electron-donating substit-
uents to stabilize a polar, zwitterionic transition state structure
(Figure 8). This polar structure of the transition state structure
was previously suggested by Daub3e based on the fact that the
ring-closure occurs faster in polar solvents and when an electron-
withdrawing substituent is present on the five-membered ring.3e,6
Here we have unequivocally shown that an electron-withdrawing
(EW) group in the heptafulvene ring slows the ring-closure,
while an electron-donating (ED) one accelerate the ring-closure.
Plotting ln k against the Hammett constant σp (NH2: -0.66, H:
0, NO2: 0.78)21 provides a poorer linear correlation than the
Figure 6. Geometry-optimized structures of (a) 20a and (b) 26a. Calcula-
tions were performed at the B3LYP/6-31G(d) level.
VHF 21b (after removal of the solvent in vacuo) to MeCN,
followed by thermal ring-closure to 21a and 27a, allowed
estimation of the absorption maximum of 27a. The value was
in agreement with the one obtained by another experiment
performed in MeCN, namely a cycle involving (i) protonation
of 21a, (ii) quantitative ring-opening, (iii) deprotonation, and
(iv) ring-closure to 21a/27a (Vide infra). Significant tuning of
the VHF absorption by virtue of the aryl group is also observed.
Thus, the p-anilino derivative 21b exhibits a considerably red-
shifted absorption maximum relative to that of the parent VHF
1b, which is again explained by a charge-transfer character of
the transition. The change is accompanied by a visible color
change from red (1b) to violet (21b).
Rate constants (k) for the thermal back-reaction were
measured at different temperatures (T) for each VHF. From
Arrhenius plots, the activation energy Ea and pre-exponential
factor A were obtained. A summary of kinetic data is provided
in Table 2. First, we note that direct attachment of the electron-
withdrawing bromo-substituent in compound 2b results in a very
slow ring-closure characterized by the longest half-life (t1/2) in
the series. Introducing an alkyne unit (6b) also retards the ring-
closure in comparison to the parent compound 1b. Comparison
of the ethynylated compounds 3b, 5b, and 6b reveals the effect
of changing a trialkylsilyl for a hydrogen substituent: Inductive
electron-donation by the silyl group enhances the rate of ring-
closure. For the arylated compounds the rate constant depends
on the donor- and acceptor-substitution at the para position of
the aryl group. In fact, the data nicely fit a linear free energy
correlation19 as revealed in Figure 7 in which ln k (25 °C) is
plotted against the Hammett constant σp+ (that includes through-
+
one against σp (Figure 7), which signals the importance of
through-conjugation as depicted in Figure 8. It should be
emphasized that while the effect of the aryl group is obviously
transmitted via the alkyne bridge, the inherent property of the
alkyne unit is to slow down the ring-closure, and this effect is
not even overcome by an electron-donating anilino group as
revealed by comparing 21b and 1b. Finally, we would like to
stress the high stability of the arylethynyl-substituted DHAs/
VHFs; no apparent decomposition occurs after five light-heat
cycles as judged from the UV-vis absorption spectra.
The strong dependence on the character of the aryl substituent
suggests that it should be possible to tune both the optical
properties and the switching event by protonation/deprotonation
of the anilino group in 21a. As a matter of fact, protonation of
the neutral DHA and VHF species results in significantly altered
absorption properties as shown in Figure 9. As expected, the
absorption of VHF 21b is significantly blueshifted from λmax
527 nm to λmax 492 upon protonation. In addition, ring-closure
of the protonated VHF is hampered as the rate constant is
lowered by a factor of ca. 2, and the Arrhenius plot coincides
with that of 20b (cf., Supporting Information S38). The different
ring-closure rate of neutral 21b and protonated 21b (21b·H+)
Table 2. Kinetic Data for the Thermal Ring-Closure of E/Z-VHFs
substituent
E/Z-VHF
Ea (kJ/mol)
A (s-1
)
t1/2 °C (min)
50
t1/2 °C (min)
25
k 25 °C (s-1
)
H
Br
1ba
2b
84.9
4.7 × 1011
2.3 × 1011
3.0 × 1010
6.4 × 1011
4.1 × 1011
2.9 × 1011
4.5 × 1011
4.6 × 1011
-
165
909
670
658
893
407
182
654
7.0 × 10-5
1.27 × 10-5
1.7 × 10-5
1.76 × 10-5
1.29 × 10-5
2.84 × 10-5
6.42 × 105
1.76 × 10-5
93.7 ( 0.4
93.8 ( 0.5
94.6 ( 0.6
94.2 ( 0.6
91.4 ( 0.8
90.4 ( 0.7
93.7 ( 0.7
49.0
35.0
33.9
46.8
23.2
10.4
34.7
C≡C-Sii-Pr3
C≡C-SiMe3
C≡C-H
3bb
5b
6b
C≡C-Ar-H
C≡C-Ar-NH2
C≡C-Ar-NO2
20b
21b
22b
a Reference 6. b Reference 7b.
9
9170 J. AM. CHEM. SOC. VOL. 132, NO. 26, 2010

Tuning the Switching Event
A R T I C L E S
Figure 8. Ring-closure of VHF is strongly controlled by the electronic properties of the substituent, which implies the involvement of a zwitterionic
structure for the transition state. An electron-donating (ED) p-anilino group will exert a through-conjugation stabilizing effect on the transition state.
Figure 10. Control of the switching events by acid/base. The terms fast
and slow refer to the same switching event, either ring-opening or ring-
closure.
Figure 9. Changes in absorption spectra upon protonation of 21a (black)
and 21b (blue) in MeCN. Protonation of 21a causes a small redshift (red)
and of 21b a blueshift (green). Thermal ring-closure of 21b results in a
mixture of 21a and 27a (purple).
tion-cross-coupling strategy for directly functionalizing DHA
regioselectively on the seven-membered ring. The donor or
acceptor character of the aryl group is transmitted through the
alkyne bridge. Thus, studies on the thermal ring-closure of the
corresponding VHF compounds provide a Hammett linear free
energy relationship. The slope of the Hammett plot indicates a
zwitterionic structure of the transition state. In accordance with
this picture, inductive electron withdrawal was found to hamper
the ring-closure; in fact, a bromo-substituted VHF exhibited the
slowest back-reaction among the compounds investigated. The
quantum yield for the ring-opening was significantly decreased
for the p-anilino-substituted DHA relative to the parent DHA
1a. Yet, upon protonation of the NH2 group, a similar quantum
yield as that of 1a was obtained. The opposing effects exerted
by donor and acceptor groups were employed in a pH-controlled
switch based on this p-anilino-substituted DHA. In addition,
protonation resulted in significantly changed absorption proper-
ties of both the DHA and VHF species. In particular, the VHF
absorption maximum is strongly altered by the functional group
in the heptafulvene ring. The different absorption properties of
6- and 7-substituted DHAs, reflecting different degrees of
conjugation efficiencies, also deserve mention. Altogether, the
different absorption profiles make the system particularly
attractive for advanced, photochromic materials.
offers the possibility to control the switching event by acid/
base. Thus, irradiation with light (330 nm) cannot fully convert
DHA 21a to VHF 21b, but upon protonation (large excess of
trifluoroacetic acid or 1 mol equiv of p-toluenesulfonic acid),
DHA 21a·H+ is fully converted to VHF 21b•H+ after 1 min of
irradiation (356 nm). Upon addition of NEt3, (large excess), the
neutral VHF 21b is obtained. A test experiment showed that
the ring-closure of 20b was not affected by the presence of acid.
Quantum yields for the light-induced ring-opening of 21a and
of the protonated species hereof were measured at an excitation
wavelength of 355 nm relative to that of the parent DHA 1a
for which φDHA-VHF ) 0.55.6 Thus, experiments were performed
under similar conditions (same lamp, time of irradiation,
excitation wavelength and absorbance at this wavelength). For
21a we obtain in this way a quantum yield of only 1.4%, while
protonation increases the quantum yield of ring-opening to 60%
for 21a·H+. The results are summarized in Figure 10.
Conclusions
In conclusion, novel DHA derivatives funtionalized with an
arylethynyl in the seven-membered ring were successfully
prepared. The synthesis, however, required shielding of the
alkyne unit by two methyl groups at the ortho positions of the
aryl. These compounds are stable in solution in contrast to those
without the two methyl groups, and they were subjected to both
light-induced ring-opening and thermal ring-closure reactions.
The synthesis takes advantage of our bromination-elimina-
In parallel work, we are exploring the possibility of using
the DHA-bromide for other metal-catalyzed cross-coupling
reactions. We reckon that direct attachment of the aryl donor/
acceptor group on the seven-membered ring will influence the
VHF ring-closure more strongly. Thus, while an alkyne bridge
does transmit the electron-donating or -attracting character of
the aryl group, it also hampers the ring-closure event. Moreover,
functionalization at the remaining positions on the seven-
membered ring of DHA may allow for further fine-tuning of
the switching event in the future. We are currently working on
(19) For examples of other photo/thermoswitches, for which Hammett
correlations were established, see: (a) Swansburg, S.; Buncel, E.;
Lemieux, R. J. Am. Chem. Soc. 2000, 122, 6594–6600. (b) Lu, N. T.;
Nguyen, V. N.; Kumar, S.; McCurdy, A. J. Org. Chem. Chem. 2005,
70, 9067–9070. (c) Cordes, T.; Schadendorf, T.; Priewisch, B.; Ru¨ck-
Braun, K.; Zinth, W. J. Phys. Chem. A 2008, 112, 581–588.
(20) Brown, H. C.; Okamoto, Y. J. Am. Chem. Soc. 1958, 80, 4979–4987.
(21) McDaniel, D. H.; Brown, H. C. J. Org. Chem. 1958, 23, 420–427.
9
J. AM. CHEM. SOC. VOL. 132, NO. 26, 2010 9171

A R T I C L E S
Broman et al.
131.8, 131.6, 130.5, 130.4, 129.5, 126.6, 125.4, 123.0, 120.7, 115.0,
112.8, 103.2, 94.9, 51.1, 45.0, 0.1 ppm. Calcd for C23H20N2Si
(352.5): C 78.37, H 5.72, N 7.95; found: C 78.17, H 5.74, N 7.90%.
MS (ESP+): m/z ) 353.3 [MH+].
the development of other synthetic protocols for obtaining these
remaining regioisomers.
Experimental Section
7-Ethynyl-2-phenyl-1,8a-dihydroazulene-1,1-dicarbonitrile (6a).
The TMS-protected acetylene, 5a (190 mg, 0.54 mmol), was
dissolved in THF (20 mL). MeOH (20 mL), acetic acid (8.0 mL),
and a solution of tetrabutylammonium fluoride (24.0 mL, 24.0
mmol, 1 M) in THF were successively added. The mixture was
stirred at rt overnight. The solution was diluted with Et2O (50 mL)
and washed with saturated aqueous NH4Cl (2 × 50 mL). The
organic phase was dried with MgSO4, filtered, and the solvent
evaporated in vacuo. Purification by column chromatography (SiO2,
toluene, Rf ) 0.61) yielded 6a (126 mg, 83%) as a yellow to red
solid. Mp 111-113 °C. 1H NMR (500 MHz, CDCl3): δ )
7.71-7.76 (m, 2H), 7.41-7.51 (m, 3 H), 6.87 (s, 1 H), 6.61 (dd,
3J ) 11.4 Hz, 3J ) 6.2 Hz, 1 H), 6.50 (d, 3J ) 11.4 Hz, 1 H), 6.30
General Methods. All solvents used for Sonogashira cross-
coupling reactions were thoroughly degassed at a sonication bath
with argon bobbling through. NMR Spectra were measured on 300
1
or 500 MHz instruments. All chemical shift values in the H- and
13C NMR spectra are referenced to the solvent (δH ) 7.26 ppm, δC
) 77.26 ppm). Coupling constants 3J and 4J refer to proton-proton
3-bond and 4-bond couplings, respectively. Thin Layer Chroma-
tography (TLC) was carried out on commercially available pre-
coated plates (silica 60) with fluorescence indicator; color change
of the spot from yellow to red or in some cases violet upon
irradiation by UV-light indicated the presence of a DHA. For
column chromatographic purification of DHAs, the column was
covered by an alumina foil to exclude light; the isolated fractions
were also kept in the dark. All melting points are uncorrected. All
spectroscopic measurements (including photolysis) were performed
in a 1-cm path length cuvette. UV-vis absorption spectra were
obtained by scanning the wavelength from 800 to 200 nm.
Photoswitching experiments were performed using a 150 W xenon
arc lamp equipped with a monochromator; the DHA absorption
maximum for each individual species was chosen as the wavelength
of irradiation (line width (2.5 nm). The thermal ring-closure was
performed by heating the sample (cuvette) by a Peltier unit in the
UV-vis spectrophotometer. Light s heat cycles described above
correspond to the following two steps: i) irradiation of the DHA
species until no further changes in the absorption spectrum were
observed, ii) heating of the formed VHF species until no further
changes in the absorption spectrum were observed.
3
3
3
(d, J ) 6.2 Hz, 1 H), 6.21 (d, J ) 4.6 Hz, 1 H), 3.81 (dd, J )
4.6 Hz, 4J ) 1.4 Hz, 1 H), 2.94 (s, 1 H) ppm. 13C NMR (125 MHz,
CDCl3): δ ) 141.5, 140.3, 132.0, 131.9, 131.8, 130.6, 130.4, 129.6,
126.6, 126.0, 122.1, 120.7, 115.0, 112.7, 82.3, 51.0, 45.1 ppm; one
signal missing.
3,5-Dimethyl-4-(trimethylsilylethynyl)aniline (13). The iodoa-
niline 10 (3.46 g, 14.0 mmol), was dissolved in dry, degassed THF
(75 mL) under an argon atmosphere. Diisopropylamine (6.0 mL),
CuI (26 mg, 0.14 mmol), and [PdCl2(PPh3)2] (490 mg, 0.70 mmol)
were added, and the solution was degassed by sonication under
argon atmosphere for 20 min. Then trimethylsilylacetylene (6.0 mL,
42 mmol) was added, and the mixture was stirred at 40 °C for 2
days. The solution was diluted with Et2O (200 mL) and washed
with saturated aqueous NH4Cl (2 × 150 mL). The organic phase
was dried with MgSO4, filtered, and the solvent evaporated in vacuo.
Purification by column chromatography (SiO2, CH2Cl2/hexanes 7:3
7-Bromo-2-phenyl-1,8a-dihydroazulene-1,1-dicarbonitrile (2a)s
General Procedure. Freshly prepared 7,8-dibromo-2-phenyl-
1,7,8,8a-tetrahydroazulene-1,1-dicarbonitrile (according to literature
protocol7a) (416 mg, 1.00 mmol) was dissolved in dry THF (20
mL) at 0 °C. A solution of LiHMDS (1.10 mL, 1.10 mmol, 1 M)
in toluene was added and the solution was stirred for 2 h while the
temperature was allowed to rise to rt. The solution was diluted with
Et2O (100 mL) and washed with saturated aqueous NH4Cl (2 ×
100 mL). The organic phase was dried with MgSO4, filtered, and
the solvent evaporated in vacuo. The crude product can be employed
directly for palladium-catalyzed cross-coupling reactions. An
analytically pure sample was obtained (with loss of product) by
column chromatography (SiO2, 30% Et2O/heptanes) followed by
1
v/v), Rf ) 0.33) yielded 13 (2.25 g, 65%) as a brownish oil. H
NMR (300 MHz, CDCl3): δ ) 6.33 (s, 2 H), 3.68 (br s, 2 H), 2.38
(s, 6 H), 0.30 (s, 9 H) ppm. 13C NMR (75 MHz, CDCl3): δ )
146.4, 142.3, 113.4, 112.9, 104.0, 100.0, 21.2, 0.5 ppm. HR-MS
(ESP+): m/z ) 240.1188 [MNa+]; calcd for C13H19NSiNa: m/z )
240.1184.
3,5-Dimethyl-4-(trimethylsilylethynyl)nitrobenzene (14). The
iodonitrobenzene 11 (3.84 g, 13.9 mmol), was dissolved in dry,
degassed THF (35 mL) under an argon atmosphere. Diisopropy-
lamine (2.5 mL), CuI (25 mg, 0.13 mmol), and [PdCl2(PPh3)2] (240
mg, 0.34 mmol) were added and the solution was degassed by
sonication for 10 min under argon. Then trimethylsilylacetylene
(4.2 mL, 30 mmol) was added, and the mixture was stirred at 40
°C overnight. The solution was diluted with Et2O (200 mL) and
washed with saturated aqueous NH4Cl (2 × 150 mL). The organic
phase was dried with MgSO4, filtered, and the solvent evaporated
in vacuo. Purification by column chromatography (SiO2, CH2Cl2/
hexane 7:3 v/v, Rf ) 0.51) yielded 14 (2.42 g, 70%) as a light
1
recrystallization from CH2Cl2/heptanes (1:10 v/v). H NMR (300
MHz, CDCl3): δ ) 7.74 (m, 2 H), 7.48 (m, 3 H), 6.89 (s, 1 H),
6.53 (m, 2 H), 6.31 (m, 1 H), 6.13 (d, 3J ) 4.4 Hz, 1 H), 3.80 (dd,
3J ) 4.4 Hz, 4J ) 1.7 Hz, 1 H) ppm. 13C NMR (75 MHz, CDCl3):
δ ) 141.9, 141.2, 132.9, 132.3, 131.8, 130.7, 130.1, 129.5, 126.6,
120.3, 120.2, 120.0, 114.8, 112.6, 51.2, 44.8 ppm. HR-MS: m/z )
357.0003 [MNa+]; calcd for C18H11N2BrNa: m/z ) 357.0003.
2-Phenyl-7-(trimethylsilylethynyl)-1,8a-dihydroazulene-1,1-
dicarbonitrile (5a). A crude batch of 2a synthesized from 7,8-
dibromo-2-phenyl-1,7,8,8a-tetrahydroazulene-1,1-dicarbonitrile (416
mg, 1.00 mmol), was dissolved in dry THF (10 mL). Diisopropy-
lamine (0.5 mL), CuI (3.8 mg, 0.02 mmol), and [PdCl2(PPh3)2] (70
mg, 0.10 mmol) were added and the solution was degassed by
sonication for 10 min under an argon atmosphere. Trimethylsily-
lacetylene (0.3 mL, 2.1 mmol) was added, and the reaction mixture
was stirred at 35-40 °C overnight. The solution was diluted with
Et2O (100 mL) and washed with saturated aqueous NH4Cl (2 ×
100 mL). The organic phase was dried with MgSO4, filtered, and
the solvent evaporated in vacuo. Purification by column chroma-
tography (SiO2, toluene, Rf ) 0.70) yielded 5a (156 mg, 44%) as
a yellow to green solid. Mp 134.0-135.0 °C. 1H NMR (300 MHz,
CDCl3): δ ) 7.72-7.75 (m, 2 H), 7.52-7.44 (m, 3 H), 6.87 (s, 1
H), 6.51-6.63 (m, 2 H), 6.29 (d, 3J ) 5.9 Hz, 1 H), 6.18 (d, 3J )
1
yellow solid. Mp 65-66 °C. H NMR (300 MHz, CDCl3): δ )
7.90 (s, 2 H), 2.51 (s, 6 H), 0.29 (s, 9 H) ppm. 13C NMR (75 MHz,
CDCl3): δ ) 146.6, 142.4, 130.0, 121.6, 109.2, 101.0, 21.4, 0.1
ppm. Calcd for C13H17NO2Si (247.37): C 63.12, H 6.93, N 5.66;
found: C 63.11, H 6.96, N 5.59%. MS (ESP+): m/z ) 248.0 [MH+].
3,5-Dimethyl-4-(triisopropylsilylethynyl)nitrobenzene (15).
The iodonitrobenzene 11 (4.92 g, 17.8 mmol) was dissolved in dry,
degassed THF (60 mL) under an argon atmosphere. Diisopropy-
lamine (10 mL), CuI (16 mg, 0.084 mmol), and [PdCl2(PPh3)2] (400
mg, 0.570 mmol) were added, and the solution was degassed by
sonication under argon atmosphere for 20 min. Then triisopropyl-
silylacetylene (8.0 mL, 36 mmol) was added, and the solution was
stirred at 45-50 °C for 2 days. The solution was diluted with Et2O
(200 mL) and washed with saturated aqueous NH4Cl (2 × 150 mL).
The organic phase was dried with MgSO4, filtered, and the solvent
evaporated in vacuo. Purification by column chromatography (SiO2,
toluene/heptanes 3:7 v/v, Rf ) 0.52) yielded 15 (4.27 g, 72%) as a
light yellow solid (and 16 as byproduct as a yellow powder, 0.55 g,
3
4
4.7 Hz, 1 H), 3.79 (dd, J ) 4.7 Hz, J ) 1.7 Hz, 1 H), 0.21 (s, 9
H) ppm. 13C NMR (125 MHz, CDCl3): δ ) 141.4, 140.3, 132.4,
9
9172 J. AM. CHEM. SOC. VOL. 132, NO. 26, 2010

Tuning the Switching Event
A R T I C L E S
1
10%). Mp 49-50 °C. H NMR (300 MHz, CDCl3): δ ) 7.92 (s,
degassed THF (10 mL) under argon atmosphere. Diisopropy-
lamine (0.5 mL), CuI (1.9 mg, 0.01 mmol), and [PdCl2(PPh3)2]
(35 mg, 0.05 mmol) were added, and the solution was subjected
to sonication under argon atmosphere for 3 h at 40 °C. The
reaction mixture was concentrated in vacuo. Purification by
column chromatography (SiO2, CH2Cl2, Rf ) 0.30) yielded 20a
(200 mg, 50%) as a yellow to brown solid. 1H NMR (300 MHz,
CDCl3): δ ) 7.74-7.77 (m, 2 H); 7.44-7.52 (m, 3 H); 6.89 (s,
2 H), 2.54 (s, 6 H), 1.16 (s, 21 H) ppm. 13C NMR (75 MHz, CDCl3,
delay )1.5 s, acquisition time )3.4 s): δ ) 146.5, 142.4, 130.4,
121.7, 106.1, 102.8, 21.7, 18.9, 11.5 ppm. Calcd for C19H29NO2Si
(331.52): C 68.83, H 8.82, N 4.22; found: C 68.89, H 8.94, N
4.22%. MS (ESP+): m/z ) 332.2 [MH+].
1,2-Bis(3,5-dimethyl-4-((triisopropylsilyl)ethynyl)phenyl)diazene
oxide (16) (byproduct). Yellow powder. Mp 156.0-156.5 °C. H
1
3
1 H); 6.61 (d, J ) 3.4 Hz, 2 H); 6.37 (s, 2 H); 6.33 (m, 1 H);
NMR (300 MHz, CDCl3): δ ) 7.98 (s, 2 H), 7.90 (s, 2 H), 2.55 (s,
6 H), 2.52 (s, 6 H), 1.17 (s, 21 H), 1.16 (s, 21 H) ppm. 13C NMR
(75 MHz. CDCl3): δ ) 147.0, 143.2, 141.9, 141.3, 127.2, 125.1,
123.9, 120.6, 104.6, 103.6, 103.4, 102.0, 21.8, 21.7, 19.0, 19.0,
11.6, 11.6 ppm. Calcd for C38H58N2OSi2 (615.05): C 74.21, H 9.50,
N 4.55; found: C 74.09, H 9.62, N 4.48%. MS (ESP+): m/z )
615.4 [MH+]. IR (KBr): 3436s, 2864s, 2890s, 2964s, 2942s, 2146m,
3
3
4
6.14 (d, J ) 4.6 Hz, 1 H); 3.88 (dd, J ) 4.6 Hz, J ) 1.7 Hz,
1 H); 3.69 (br s, 1 H), 2.36 (s, 6 H) ppm. 13C NMR (75 MHz,
CDCl3): δ ) 146.6; 142.3; 141.2; 140.6; 133.0; 132.0; 131.2;
130.6, 130.4; 129.5; 126.6; 124.0; 122.6; 120.7; 115.2; 113.6;
112.8; 112.5; 94.1; 88.5; 51.2; 45.2; 21.4 ppm. HR-MS (ESP+):
m/z ) 422.1629 [MNa+]; calcd for C26H21N3Na: m/z ) 422.1633.
7-[(4-Nitro-2,6-dimethylphenyl)ethynyl]-2-phenyl-1,8a-dihy-
droazulene-1,1-dicarbonitrile (22a). Method 1. A crude batch of
2a, synthesized from 7,8-dibromo-2-phenyl-1,7,8,8a-tetrahydroazulene-
1,1-dicarbonitrile (208 mg, 0.50 mmol), and the terminal acetylene
19 (175 mg, 1.00 mmol) were dissolved in dry, degassed THF (5 mL)
under argon atmosphere. Diisopropylamine (0.20 mL), CuI (1.9 mg,
0.010 mmol) and [PdCl2(PPh3)2] (35.1 mg, 0.05 mmol) were added,
and the solution was subjected to sonication under argon atmosphere
for 7 h at 40 °C. The reaction mixture was concentrated in vacuo.
Purification by column chromatography (SiO2, toluene, Rf ) 0.56)
yielded 22a (43 mg, 20%) as a yellow solid or foam.
1469s (N ) NO) cm-1
.
3,5-Dimethyl-4-ethynylaniline (18). Compound 13 (600 mg,
2.76 mmol) was dissolved in THF (10 mL). MeOH (10 mL) and
K2CO3 (546 mg, 5.52 mmol) were added, and the mixture was
stirred at rt overnight. The reaction mixture was diluted with Et2O
(50 mL), washed with saturated aqueous NH4Cl (2 × 50 mL), dried
with MgSO4 and the solvent evaporated in vacuo. Purification by
column chromatography (SiO2, EtOAc/heptanes 1:1 v/v, Rf ) 0.50)
yielded 18 (228 mg, 57%) as an orange solid. Mp 71.5-73.0 °C.
1H NMR (300 MHz, CDCl3): δ ) 6.36 (s, 2 H), 3.67 (br s, 2 H),
3.38 (s, 1 H), 2.36 (s, 6 H) ppm. 13C NMR (75 MHz, CDCl3): δ )
146.5, 142.7, 113.5, 112.0, 83.1, 82.1, 21.3 ppm. Calcd for C10H11N
(145.20): C 82.72, H 7.64, N 9.65; found: C 82.62, H 7.82, N 9.64.
MS (ESP+): m/z ) 146.1 [MH+].
Method 2. The ethynyl-DHA 6a (70 mg, 0.25 mmol) was
dissolved in dry degassed THF (5 mL) under argon atmosphere.
3,5-Dimethyl-4-iodonitrobenzene 11 (277 mg, 1.00 mmol), [Pd-
(PPh3)4] (17.5 mg, 0.025 mmol), CuI (0.9 mg, 0.005 mmol), and
diisopropylamine (0.1 mL) were added, and the solution was stirred
for 2 h at 40 °C. The reaction mixture was concentrated in vacuo.
Purification by column chromatography (SiO2, toluene, Rf ) 0.56)
3,5-Dimethyl-4-ethynylnitrobenzene (19). Compound 15 (1.27
g, 3.83 mmol) was dissolved in wet THF (20 mL). A solution of
tetrabutylammonium fluoride in THF (1 mL, 1 mmol, 1 M) was
added and the solution was stirred for 10 min at rt. Water was
added to the solution until it became colorless. The reaction mixture
was diluted with Et2O (100 mL), washed with saturated aqueous
NH4Cl (2 × 100 mL), dried with MgSO4 and the solvent evaporated
in vacuo. Purification by column chromatography (SiO2, CH2Cl2/
heptanes 4:6 v/v, Rf ) 0.55) yielded 19 (610 mg, 91%) as a colorless
solid. Mp 170 °C. 1H NMR (300 MHz, CDCl3): δ ) 7.92 (s, 2 H),
3.78 (s, 1 H), 2.54 (s, 6 H) ppm. 13C NMR (75 MHz, CDCl3): δ )
147.0, 142.9, 128.9, 121.7, 90.5, 79.8, 21.4 ppm. Calcd for
C10H9NO2 (175.18): C 68.56, H 5.18, N 8.00; found: C 68.18, H
5.13, N 7.83%. MS (ESP+): m/z ) 176.1 [MH+].
1
yielded 22a (17 mg, 17%) as a yellow solid or foam. H NMR
(300 MHz, CDCl3): δ ) 7.93 (s, 1 H); 7.75-7.78 (m, 2 H);
7.42-7.53 (m, 3 H); 6.93 (s, 1 H); 6.59-6.73 (m, 2 H); 6.38 (d, 3J
) 5.9 Hz, 1 H); 6.27 (d, 3J ) 4.7 Hz, 1 H); 3.90 (dd, 3J ) 4.7 Hz,
4J ) 1.7 Hz, 1H); 2.54 (s, 6 H) ppm. 13C NMR (75 MHz, CDCl3):
δ ) 146.8; 142.1; 141.7; 140.6; 132.2; 131.8; 131.7; 130.7; 130.3;
129.6; 129.5; 126.6; 125.3; 122.7; 121.7; 120.7; 115.0; 112.8; 101.0;
85.8; 51.2; 45.1; 21.5 ppm. HR-MS (ESP+): m/z ) 452.1386
[MNa+]; calcd for C28H19N3O2Na: m/z ) 452.1375.
Crystallographic Details of Compound 16. Data were recorded
on a Nonius KappaCCD diffractometer using MoKR radiation
with a graphite monochromator. Data reduction was performed
with EvalCCD.22 The structure was determined and refined with
SHELXS and SHELXL, respectively,23,24 as implemented in
maXus.25 The isopropyl groups attached to the silicon atoms
display both static and dynamic disorder. One of the isopropyl
groups are described with two conformations almost equally
populated (0.54:0.46). The independent isopropyl groups were
restrained to be structurally alike (SHELXL97 SADI instruc-
tions). Also the SHELXL97 SIMU (restraining to isotropic
temperature parameters) and DELU (alike anisotropic parameters
for bonded atoms) instructions were applied to restrain the
refinement. Thermal ellipsoid plots were made with ORTEP26
as implemented in the program PLATON.27 The CIF-file was
checked with PLATON and the program enCIFer from the
7-[(2,6-Dimethylphenyl)ethynyl]-2-phenyl-1,8a-dihydroazu-
lene-1,1-dicarbonitrile (20a). A crude batch of 2a, synthesized
from 7,8-dibromo-2-phenyl-1,7,8,8a-tetrahydroazulene-1,1-di-
carbonitrile (416 mg, 1.00 mmol), and the terminal acetylene
17 (260 mg, 2.00 mmol) were dissolved in dry, degassed THF
(10 mL) under argon atmosphere. Diisopropylamine (0.5 mL),
CuI (3.8 mg, 0.020 mmol), and [PdCl2(PPh3)2] (70 mg, 0.10
mmol) were added, and the solution was subjected to sonication
under argon atmosphere for 2 h at 40 °C. The reaction mixture
was concentrated in vacuo. Purification by column chromatog-
raphy (SiO2, toluene, Rf ) 0.55) yielded 20a (138 mg, 36%) as
a yellow solid or foam. Mp 170.0-171.5 °C. 1H NMR (300
MHz, CDCl3): δ ) 7.73-7.76 (m, 2 H), 7.43-7.51 (m, 3 H),
7.10 -7.14 (m, 1 H), 7.02-7.07 (m, 2 H), 6.91 (s, 1 H),
3
3
6.62-6.63 (m, 2 H), 6.32 (d, J ) 5.2 Hz, 1 H), 6.20 (d, J )
3
4
4.6 Hz, 1 H), 3.88 (dd, J ) 4.6 Hz, J ) 1.7 Hz, 1 H), 2.45 (s,
6 H) ppm. 13C NMR (75 MHz, CDCl3): δ ) 141.3, 140.6, 140.5,
132.6, 131.9, 131.5, 130.5, 129.5, 128.3, 126.9, 126.6, 123.8,
123.5, 122.6, 120.7, 115.1, 112.8, 96.2, 87.5, 51.1, 45.1, 21.3
ppm; one signal missing. Calcd for C28H20N2 (384.47): C 87.47,
H 5.24, N 7.29; found: C 87.38, H 5.33, N 7.00%.
(22) Duisenberg, A. J. M.; Kroon-Batenburg, L. M. J.; Schreurs, A. M. M.
J. Appl. Crystallogr. 2003, 36, 220–229.
(23) Sheldrick, G. M. Acta Crystallogr. A 1990, 46, 467–473.
(24) Sheldrick, G. M. SHELXL97, Program for the refinement of crystal
structures; University of Go¨ttingen: Germany, 1997.
(25) Mackay, S.; Gilmore, C. J.; Edwards, C.; Stewart, N.; Shankland, K.
1999. maXus Computer program for the solution and refinement of
crystal structures; Bruker Nonius: The Netherlands, 1999.
(26) Johnson, C. K. ORTEP-II, A fortran thermal-ellipsoid plot program. Report
ORNL-5138; Oak Ridge National Laboratory: Oak Ridge, TN, 1976.
(27) Spek, A. L. PLATON, A Multipurpose Crystallographic Tool; Utrecht
University: Utrecht, The Netherlands, 2001.
7-[(4-Amino-2,6-dimethylphenyl)ethynyl]-2-phenyl-1,8a-dihy-
droazulene-1,1-dicarbonitrile (21a). A crude batch of 2a,
synthesized from 7,8-dibromo-2-phenyl-1,7,8,8a-tetrahydroazu-
lene-1,1-dicarbonitrile (208 mg, 0.50 mmol), and the terminal
acetylene 17 (290.4 mg, 1.00 mmol) were dissolved in dry,
9
J. AM. CHEM. SOC. VOL. 132, NO. 26, 2010 9173

A R T I C L E S
Broman et al.
Cambridge Crystallographic Data Centre (www.ccdc.cam.ac.uk),
from where the structure is available on demand with accession
code CCDC771508.
Supporting Information Available: NMR spectra, UV-vis
spectra showing the photo- and thermal switching events,
Arrhenius plots for the ring-closure reaction, complete ref
18, computational data, and X-ray data. This material is
available free of charge via the Internet at http://pubs.acs.
org.
Acknowledgment. The Danish Research Council for Indepen-
dent Research | Natural Sciences is gratefully acknowledged for
financial support. In addition, the research leading to these results
has received funding from the European Community’s Seventh
Framework Programme (FP7/2007-2013) under the grant agreement
“SINGLE” no 213609.
JA103235G
9
9174 J. AM. CHEM. SOC. VOL. 132, NO. 26, 2010