Gaining control: Direct Suzuki arylation of dihydroazulenes and tuning of photo- and thermochromism

Article abstract of DOI:10.1002/ejoc.201001554

A protocol for functionalizing the seven-membered ring of the DHA/VHF photochromic system via a Suzuki cross-coupling was developed. Kinetic studies reveal that direct attachment of an aromatic moiety on the seven-membered ring strongly affects the thermal back reaction. By the introduction of an anilino group, pH control of both the photochromic and thermochromic properties was obtained. Furthermore, it was possible to switch between three different states: two photochromic and one thermochromic.

Full text of DOI:10.1002/ejoc.201001554

FULL PAPER
DOI: 10.1002/ejoc.201001554
Gaining Control: Direct Suzuki Arylation of Dihydroazulenes and Tuning of
Photo- and Thermochromism
Michael Åxman Petersen,*[a] Søren Lindbæk Broman,[a] Kristine Kilså,[a] Anders Kadziola,[a]
and Mogens Brøndsted Nielsen*[a]
Keywords: Linear free energy relationships / Photochromism / Thermochromism / Molecular switches / Photoswitches /
Cross-coupling
A protocol for functionalizing the seven-membered ring of
the DHA/VHF photochromic system via a Suzuki cross-cou-
pling was developed. Kinetic studies reveal that direct at-
tachment of an aromatic moiety on the seven-membered ring
strongly affects the thermal back reaction. By the introduc-
tion of an anilino group, pH control of both the photochromic
and thermochromic properties was obtained. Furthermore, it
was possible to switch between three different states: two
photochromic and one thermochromic.
color and dipole moment.[4] Despite the attractive proper-
ties, reports of incorporation of this photochromic system
in larger molecular systems and devices have been limited.
Recently we reported a method for introducing an acetyl-
ene functionality in the seven-membered ring of DHA via a
Sonogashira cross-coupling reaction[5] and obtained a small
selection of acetylenic DHAs 1a–3a (Figure 1).[6] It was
found that the electronic properties of the substituent on
the phenyl ring (R = NH2, H, or NO2) was transmitted
through the alkyne bridge. Property tuning is a desirable
feature of photochromic systems, and by moving the sub-
stituent closer to the seven-membered ring, the influence of
the substituent should be more pronounced. Here we report
the direct arylation of the seven-membered ring of DHA
using the Suzuki cross-coupling.[7]
Introduction
Photochromic compounds have the ability to reversibly
change absorption properties upon irradiation with light in
at least one direction. Switching between states are ac-
companied by changes in electronic, structural, and/or
chemical properties which makes photoswitches potential
building blocks for materials science, supramolecular chem-
istry, and biotechnology.[1]
The dihydroazulene (DHA)/vinylheptafulvene (VHF)
system, discovered by Daub and co-workers,[2] is photo-
chromic in one direction and thermochromic in the other
(Scheme 1).
Scheme 1. Photochromic DHA/VHF.
Quantum yields of the photochemical conversion of
DHAǞVHF are in general quite high for many DHAs, and
the rates of the thermal conversion VHFǞDHA are rela-
tively slow,[3] which makes it possible to switch cleanly be-
tween the two states. Different electronic and structural
properties of the two states result in significant changes in
Figure 1. Previously synthesized DHAs and corresponding VHFs.
Results and Discussion
The synthesis starts with selective bromination of parent
unsubstituted DHA 4 to give dibromide 5,[8] followed by
elimination of HBr to give the 7-bromo DHA 6.[5] For in-
troducing phenyl derivatives directly on the seven-mem-
bered ring, the Suzuki cross-coupling reaction between
DHA 6 and a suitable boronic acid was chosen. Standard
alkaline (Na2CO3 or NaOH) conditions were deliberately
[a] Department of Chemistry, University of Copenhagen,
Universitetsparken 5, 2100 Copenhagen Ø, Denmark
Fax: +45-35320212
E-mail: aaxman@kiku.dk
mbn@kiku.dk
http://mbn.kiku.dk/mbn_home.htm
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201001554.
Eur. J. Org. Chem. 2011, 1033–1039
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M. Å. Petersen, S. L. Broman, K. Kilså, A. Kadziola, M. B. Nielsen
FULL PAPER
avoided on account of the possibility of formation of azu-
lenes from elimination of HCN, known to occur read-
ily.[5,6,8] Instead the Suzuki cross-coupling was carried out
using fluoride to activate the aryl boronic acid.[9] The
choice of fluoride source proved to be important; the use of
Bu4NF was accompanied by substantial azulene formation.
Using KF in a two-phase solvent system of toluene and
water minimized the formation of azulenes. DHAs 7a–10a
were obtained in 17–37% yield (3 steps from 4) using these
conditions (Scheme 2). It is noted that coupling of boronic
esters failed. Methylation of 7a finally provided DHA 11a.
Scheme 2. a: Br2, CH2Cl2. b: LiHMDS, THF. c: Pd(PPh3)4,
KF·2H2O, toluene/H2O. d: CH3I.
Crystals suitable for X-ray crystallography (grown from
CH2Cl2/heptanes) were obtained of DHA 10a. The crystal
structure (Figure 2) reveals, as expected, a twisting of the
seven-membered ring; the plane defined by C6–C7–C8 is
twisted 52° out of the plane defined by C2–C3–C3a–C4.
Furthermore the plane defined by the aromatic ring in the
7-position is twisted by 35.1° in respect to the C6–C7–C8
plane.
Figure 3. UV/Vis spectra in MeCN of DHAs 7a–11a (solid), VHFs
7b–11b (dashed), and DHAs 7a/c–11a/c after one full cycle of
switching (dots). VHF 7b was generated by i. protonation
(500 equiv. TFA), ii. irradiation, iii. neutralization (500 equiv.
Et3N). Relative absorbances are shown by setting the VHF absorp-
tion to one at its longest wavelength absorption maximum.
Upon irradiation with light (353 nm), DHAs 8a–11a are
completely converted into their corresponding VHFs 8b–
11b within minutes with isosbestic points in the evolution
of absorption spectra (see Supporting Information). How-
ever, as observed in our previous work with DHAs substi-
tuted in the seven-membered ring,[5a,6] partial isomerization
to the 6-substituted isomer (designated by c, Scheme 3) oc-
curs during switching, resulting in an apparent redshifted
DHA absorption for the whole selection of compounds 7–
11 (Figure 3, dotted lines). Photo-induced ring opening of
Figure 2. X-ray crystal structure of 10a.
The absorption properties of DHAs 7a–11a and the cor-
responding VHFs 7b–11b (obtained by light-induced ring-
opening, vide infra) were investigated in MeCN (Figure 3).
The longest wavelength absorptions of the VHFs are all
redshifted compared to the parent VHF which lacks a sub-
stituent in the heptafulvene ring (λmax = 471 nm). Dimeth-
ylanilino substituted VHF 7b exhibits an absorption maxi-
mum at 554 nm, which corresponds to an impressive
10768 cm–1 redshift in absorbance compared to the corre-
sponding DHA 7a.
Scheme 3.
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Direct Suzuki Arylation of Dihydroazulenes
DHAs 8a–11a resulted in VHFs 8b–11b, each formed in a comes evident, when comparing the ratio of the formed 6-/
1:1 E/Z ratio, and after one full cycle of switching, a mix- 7-anilino substituted DHAs 7a and 7c, under neutral and
ture of 6-/7-DHAs 8a/c–11a/c formed (the ratios were deter- acidic conditions: When neutral VHF 7b is thermally
1
mined by H-NMR and are given in Table 1). Interestingly, closed, the content of 7-substituted DHA is higher than
whereas the E/Z ratio is unaffected by the nature of the that obtained after thermal ring closure and deprotonation
substituent, the formation of 7-substituted DHA is favored (Et3N) of 7b·H+ (Figure 6, inset, vide infra).
when bearing electron donating substituents. This also be-
The mixture of 6- and 7-substituted DHA-isomers 8a/c–
11a/c formed after one full of switching is still photochro-
mic, and can be converted fully to VHF, but not with clean
isosbestic points, due to the different absorption properties
of the 6- and 7-isomers. Continued switching does not cause
any further change in the isomer ratios and after 5 full light-
heat cycles no bleaching was observed.
Table 1. Quantum yields for DHAǞVHF, absorption maxima of
DHAs 7a–11a, VHFs 7b–11b, DHAs 7c–11c in MeCN at room
temp. and isomer ratios formed during switching.
λ7–DHA
nm
/
λVHF
nm
/
λ6–DHA /
[a]
ΦDHA–VHF
E/Z[d]
7/6[d]
nm[c]
The quantum yields of the photochemical conversion of
selected 7-substituted DHAs in MeCN are given in Table 1.
Electron-donating groups are known to quench the photo-
chromic properties of the DHA/VHF-system,[2e,3,6,10] and
the photochemical conversion of DHA 7a to VHF 7b in
MeCN is indeed strongly retarded (ΦDHA–VHF Ͻ 0.5%),
but in cyclohexane DHA 7a can be fully converted to VHF
7b (ΦDHA–VHF = 83% Ϯ4).
7
8
9
Ͻ 0.5%
60% Ϯ4
–
347
348
355
318
356
554[b]
505
490
479
477
423
384
378
392
374
–
–
1:1
1:1
1:1
1:1
2:1
2:1
1:1
1:1
10 35% Ϯ5
11 50% Ϯ2
[a] The quantum yields were obtained by using ferrioxalate as acti-
nometer and a mercury lamp equipped with a 365 nm filter as the
light source. [b] Generated by i. protonation (500 equiv. TFA), ii. ir-
radiation iii. neutralization (500 equiv. Et3N). [c] Calculated from
absorption spectra obtained after on cycle of switching. [d] E/Z-
VHF and 7/6-DHA ratios determined by 1H-NMR spectroscopy
in CD3CN.
By monitoring the change of the VHF absorption with
time the rate constants for the thermal back reaction of
+
Figure 4. log(kR/kH) for the thermal back reaction VHFǞDHA vs. (᭺) and/or σp (+)[11] and linear fits to log(kR/kH) vs. σp (dotted
lines) and log(kR/kH) vs. σp (solid lines), a: of VHFs 7b–11b, b: of previously reported VHFs 1b–3b,[4] c: of previously reported 12b–
+
17b.[3] For more details see Supporting Information.
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M. Å. Petersen, S. L. Broman, K. Kilså, A. Kadziola, M. B. Nielsen
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VHFs 7b–11b to DHAs were calculated by exponential fit- magnitude of the slope may be explained by a cross-conju-
ting (Table 2). Our selection of VHFs 7b–11b exhibits rate gated pathway between the aryl group in 2-position and the
constants both faster and slower than the parent VHF (k25 negatively charged C(CN)2 unit in the TS (Figure 5). This
= 7.0ϫ10–5 s–1) and the reaction rate is nearly enhanced by less efficient degree of π-electron delocalization renders the
a factor 10 from nitrophenyl derivative 10b (k25
2.4ϫ10–5 s–1) to dimethylanilino derivative 7b (k25
2.3ϫ10–4 s–1).
=
=
rate of the VHFǞDHA conversion for the derivatives 12b–
17b less dependent on the nature of the aryl substituent
(smaller slope) compared to derivatives functionalized in
the seven-membered ring (7b–11b and 1b–3b).
Table 2. Activation energy (Ea), preexponential factor (A), rate con-
stant (k25) and half-life (t1/2,25) at 25 °C in MeCN for VHFǞDHA.
R
Ea (A)
/kJ
k25
t1/2,25
/s–1 ϫ10–5 /min
7[a]
8
–N(CH3)2 90.1Ϯ0.2 (1.42ϫ1012)
–OCH3
–H
23.1
7.7
5.6
2.4
2.7
50
91.6Ϯ0.2 (6.2ϫ1011)
89.9Ϯ0.8 (3.2ϫ1011)
95.4Ϯ1.0 (1.2ϫ1012)
150
206
490
425
9
10
11
–NO2
–N+(CH3)3 93.9Ϯ0.5 (7.7ϫ1011)
[a] Generated by i. protonation (500 equiv. TFA), ii. irradiation,
iii. neutralization (500 equiv. Et3N).
Figure 5. Suggested TS of the thermal back reaction VHFǞDHA.
Upon protonation of the anilino-substituted DHA 7a by
In our former work it was recognized that the thermal
back reaction (VHFǞDHA) exhibits a linear free energy addition of large excess of TFA (500 equiv.), the quantum
relationship when different arylethynyl substituents were in- yield of the photochemical conversion was improved signifi-
corporated.[6] Yet, the direct attachment of the aryl substit- cantly (ΦDHA–VHF = 45% Ϯ5). The absorption of 7a·H+
uent on the seven-membered ring displays a significantly resembles that of DHA 11a and the absorption of the corre-
stronger influence on the reaction. Employing the Hammett sponding protonated VHF 7b·H+ (λmax = 478 nm) re-
+
equation log(kR/kH) = σρ, using either σp or σp (through- sembles that of 11b. Addition of Et3N affords a significant
conjugation Hammett constant), the slope ρ was obtained redshift in absorption owing to formation of the neutral
by linear fitting: For VHFs 7b–11b, ρ = –0.55 (σ) and ρ = VHF 7b (λmax = 554 nm) (Figure 6). The rate of the thermal
–0.39 (σp+) (Figure 4, a), whereas the reaction of VHFs 1b– back reaction of 7b·H+ is significantly slower than that of
3b exhibits ρ = –0.39 (σ) and ρ = –0.27 (σp+) (Figure 4, the neutral VHF 7b. Within the boundaries of the neutral
b). The negative slope ρ indicates stabilization of a positive and the fully protonated species, the quantum yield and rate
charge. This is in good agreement with a zwitterionic or constants can be altered by the amount of acid present,
highly polarized transition state (TS) in the VHFǞDHA thus by pH control both the absorption properties of the
conversion (Figure 5). By employing the Hammett equation system can be tuned along with the switching event in both
on literature values of rate constants for derivatives with an directions (Scheme 4). It should be noted that after several
aryl substituent in position 2 of the DHA,[3] we obtained a cycles of switching, in the presence of acid, a weak bleach-
positive slope ρ = 0.29 (σ) (Figure 4, c). This change in sign ing of the DHA absorption accompanied by a slight evol-
reflects that now the ability of the aryl group to stabilize ution of an absorption band at 625 nm was observed as
a negative charge enhances the ring closure. The smaller result of degradation.
Figure 6. Absorption spectra of the neutral species DHA 7a (–) and VHF 7b (– · –) and the protonated species DHA 7a·H+ (···) and
VHF 7b·H+(– – –) (500 equiv. TFA) in MeCN. Inset: neutral species DHA 7a (–), mixture of 7a and 7c from VHF 7b (···), and mixture
of 7a and 7c from VHF 7b·H+ (– – –) after deprotonation in the final step. The percentages are estimated contents of the 7c isomer.
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Direct Suzuki Arylation of Dihydroazulenes
Scheme 4. pH-Controlled switching of anilino-substituted DHA/VHF in MeCN.
Scheme 5. Switching in cyclohexane.
Figure 7. Switching of 7a in cyclohexane. Cycle 0–10: irradiation with 353 nm-light (180 s), then heating. Cycle 10–11: irradiation with
420-nm light (3000 s). a: Selected absorptions vs. number of switching cycles. b: Selected UV/Vis absorption spectra.
Eur. J. Org. Chem. 2011, 1033–1039
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M. Å. Petersen, S. L. Broman, K. Kilså, A. Kadziola, M. B. Nielsen
FULL PAPER
the formed VHF species until no further changes in the absorption
spectrum were observed.
DHAs with a trialkylsilylethynyl substituent on the
seven-membered ring do not isomerize in apolar solvents
during a full switching cycle.[5a] Surprisingly, formation of
6-substituted DHA 7c (approx. 15%) from VHF 7b (t1/2,25
= 1750 min) was observed after one full switching cycle in
cyclohexane of DHA 7a (Scheme 5). The fact that isomer-
ization proceeds even in cyclohexane is believed to be a con-
sequence of a long-lived reaction intermediate. The aro-
The quantum yields were obtained by using ferrioxalate as acti-
nometer and a mercury lamp equipped with a 365 nm filter as the
light source.
7-[4-(Dimethylamino)phenyl]-2-phenyl-1,8a-dihydroazulene-1,1-di-
carbonitrile (7a): Bromide 6 (168 mg, 0.5 mmol), [4-(dimeth-
ylamino)phenyl]boronic acid (120 mg, 0.7 mmol), and KF·H2O
matic substituent exerts a more stabilizing effect on the re- (96 mg, 1.0 mmol) were dissolved in toluene (5 mL) and water
(5 mL). The mixture was degassed with argon under sonication be-
fore Pd(PPh3)4 (30 mg, 5 mol-%) was added. The reaction mixture
was stirred overnight at 60 °C, before saturated aqueous NH4Cl
(25 mL) was added, and the mixture was extracted with CH2Cl2
(2 ϫ 25 mL). The combined extracts were washed with brine
(25 mL), dried (Na2SO4), and evaporated in vacuo. Purification by
flash column chromatography (SiO2, CH2Cl2, Rf = 0.45), then flash
column chromatography (SiO2, EtOAc/heptanes, 1:4, v/v, Rf =
0.34) afforded 4a (70 mg, 37%) with minor impurities. An analyti-
cal pure sample was obtained by recrystallization from boiling
action intermediate than a trialkylsilylethynyl substituent.
The large redshift of the absorption of 6-DHA 7c com-
pared to that of 7-DHA 7a makes it possible to switch be-
tween three states: By irradiation with 353 nm-light 7-DHA
7a is converted to VHF 7b and heating will again result in
formation of both 6-DHA 7c and 7-DHA 7a. By repetition
of this cycle, 6-DHA 7c is accumulated. Upon irradiation
with 420 nm-light, the 6-DHA 7c is converted to VHF 7b
and by heating, the ratio of 7a/7c obtained is identical to
that after one full cycle of switching (Figure 7).
1
EtOAc as yellow greenish shinning crystals, m.p. 191.5–193 °C. H
3
NMR (300 MHz, CDCl3, 25 °C, TMS): δ = 7.76 (dd, JH,H = 7.9,
3
1.2 Hz, 2 H, ar), 7.64–7.37 (m, 3 H, ar), 7.30 (d, JH,H = 8.8 Hz, 2
H, ar), 6.89 (s, 1 H, =CH), 6.83–6.76 (m, 2 H, =CH), 6.71 (d, 3JH,H
Conclusions
3
= 8.8 Hz, 2 H, =CH), 6.44–6.22 (m, 1 H, =CH), 5.91 (d, JH,H
=
3
4
In conclusion, the first protocol for functionalizing the 4.6 Hz, 1 H, =CH), 3.81 (dd, JH,H = 4.6, JH,H = 1.4 Hz, 1 H,
CH), 2.97 ppm (s, 6 H, CH3). 13C NMR (75 MHz, CDCl3, 25 °C):
seven-membered ring of the DHA via a Suzuki cross-cou-
pling was developed. This direct aryl-functionalization af-
fects the thermal back reaction to an extent not previously
observed as revealed by a comparison of Hammett plots for
δ = 150.3, 140.8, 140.6, 139.3, 132.6, 132.1, 131.6, 130.6, 123.0,
129.2, 128.5, 128.1, 126.3, 120.1, 115.3, 113.3, 113.1, 112.2, 50.9,
45.0, 40.4 ppm. UV/Vis (MeCN): λmax (ε) = 269(15900), 347 nm
(14100). HRMS (ESP+): calcd. for C26H22N3: 376.1814; found
differently substituted VHFs. The anilino-bearing DHA 7a
376.1824.
proved to be a very versatile molecular switch; its photo-
chromic and thermochromic properties in MeCN could be
tuned by changing the pH. In cyclohexane 7a proved to act
7-(4-Methoxyphenyl)-2-phenyl-1,8a-dihydroazulene-1,1-dicarbo-
nitrile (8a): Bromide 6 (168 mg, 0.5 mmol), (4-methoxyphenyl)bo-
ronic acid (120 mg, 0.8 mmol), and KF·H2O (104 mg, 1.1 mmol)
as a three-way switch, with two photochromic states and
were dissolved in toluene (5 mL) and water (5 mL) under argon
one thermochromic.
atm. The mixture was degassed with argon under sonication before
Pd(PPh3)4 (30 mg, 5 mol-%) was added. The temperature was
raised to 60 °C and the reaction mixture was stirred overnight be-
fore saturated aqueous NH4Cl (25 mL) was added, and the mixture
Experimental Section
was extracted with CH2Cl2 (2ϫ25 mL). The combined extracts
General Methods: NMR Spectra were measured on a 300-MHz in-
were washed with brine (50 mL), dried (MgSO4), and evaporated
strument. All chemical shift values in the H- and 13C NMR spec-
in vacuo. Purification by flash column chromatography (SiO2, tolu-
tra are referenced to the solvent (δH = 7.26, δC = 77.00 ppm). Cou-
ene/heptanes, 1:9, v/v, Rf = 0.32) afforded 8a as a yellow solid
pling constants 3J and 4J refer to proton–proton 3-bond and 4-
(65 mg, 35 %) with traces of minor impurities. Recrystallization
bond couplings, respectively. Thin-layer chromatography (TLC)
from diethyl ether/heptanes gave 8a (50 mg) as a slightly yellow
was carried out on commercially available precoated plates (silica
solid, m.p. 112–113.5 °C. 1H NMR (300 MHz, CDCl3, 25 °C,
1
60) with fluorescence indicator; color change of the spot from yel-
low 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
3
TMS): δ = 7.76 (dd, JH,H = 8.0, 1.5 Hz, 2 H, ar), 7.54–7.38 (m, 3
H, ar), 7.38–7.29 (m, 2 H, ar), 6.99–6.85 (m, 3 H, ar, =CH), 6.85–
3
6.69 (m, 2 H, =CH), 6.36 (d, JH,H = 5.5 Hz, 1 H, =CH), 5.93 (d,
3
3JH,H = 4.5 Hz, 1 H, =CH), 3.82 (s, 3 H, CH3), 3.81 (d, JH,H
=
to exclude light; the isolated fractions were also kept in the dark.
All spectroscopic measurements (including photolysis) were per-
formed 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; irradiation was per-
formed with 353-nm light (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 (any minor photochemical
conversion as result of irradiation from the spectrophotometer was
ignored). Light/heat cycles described above correspond to the fol-
lowing two steps: i. irradiation of the DHA species until no further
changes in the absorption spectrum were observed, ii. heating of
1.5 Hz, 1 H, CH) ppm. 13C NMR (75 MHz, CDCl3, 25 °C): δ =
159.6, 141.1, 140.6, 139.2, 132.7, 132.3, 132.2, 131.6, 130.5, 130.1,
129.3, 128.9, 126.3, 120.1, 115.2, 114.7, 113.9, 113.0, 55.3, 50.9,
45.0 ppm. UV/Vis (MeCN): λmax (ε) = 266 (21448), 348 nm (14400).
HRMS (ESP+): calcd. for C25H19N2O: 363.1497; found 363.1497.
2,7-Diphenyl-1,8a-dihydroazulene-1,1-dicarbonitrile (9a): Bromide 6
(168 mg, 0.5 mmol), phenylboronic acid (101 mg, 0.83 mmol), and
KF·H2O (94 mg, 1.0 mmol) were dissolved in toluene (5 mL) and
water (5 mL) under argon atm. The mixture was degassed with
argon under sonication before Pd(PPh3)4 (30 mg, 5 mol-%) was
added, and the mixture was stirred at 60 °C for 3 days. The reaction
mixture was quenched by addition of saturated aqueous NH4Cl
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Direct Suzuki Arylation of Dihydroazulenes
(25 mL) and extracted with CH2Cl2 (2ϫ25 mL). The combined or-
ganic extracts were washed with brine (25 mL), dried (MgSO4), and
evaporated in vacuo. Purification by flash column chromatography
(SiO2, ether/heptanes, 1:2, v/v, Rf = 0.34) followed by flash column
chromatography (SiO2, CH2Cl2/heptanes, 1:1, v/v, Rf = 0.53) af-
forded the phenyl-substituted DHA 9a as an orange foam (50 mg,
30%). 1H NMR (300 MHz, CDCl3, 25 °C, TMS): δ = 7.82–7.70
140.6, 137.6, 133.4, 131.6, 130.5, 130.3, 130.1, 129.7, 129.3, 126.3,
120.4, 117.8, 114.8, 112.9, 57.9, 50.7, 44.9 ppm; one signal overlap-
ping. UV/Vis (MeCN): λmax (ε) = 247 (20400), 277 (16300), 256 nm
(10700). HRMS (ESP+): calcd. for C27H24N3+: 390.1965; found
390.1942.
Supporting Information (see footnote on the first page of this arti-
cle): NMR spectra of DHAs 7a–11a, summarized UV/Vis data of
respective DHAs and VHFs, photochemical conversion of DHAs
to VHFs (first and second cycle) and kinetic data for the thermal
conversion of VHFs to DHAs.
3
(m, 2 H, ar), 7.57–7.28 (m, 8 H, ar), 6.91 (s, JH,H = 12 Hz, 1 H,
=CH), 6.87–6.71 (m, 2 H, =CH), 6.37 (d, 3JH,H = 5 Hz, 1 H, =CH),
3
3
4
6.00 (d, JH,H = 5 Hz, 1 H, =CH), 3.85 (dd, JH,H = 5, JH,H
=
2 Hz, 1 H, CH) ppm. 13C NMR (75 MHz, CDCl3, 25 °C): δ =
141.1, 140.5, 140.0, 139.8, 132.3, 132.0, 131.6, 130.4, 130.1, 129.3,
128.6, 128.0, 127.7, 126.3, 120.2, 116.2, 115.2, 112.9, 50.9,
45.0 ppm. UV/Vis (MeCN): λmax (ε) = 240 (16400), 273 (20000),
355 nm (15000). HRMS (ESP+): calcd. for C24H17N2: 333.1392
found 333.1380.
CCDC-785071 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
7-(4-Nitrophenyl)-2-phenyl-1,8a-dihydroazulene-1,1-dicarbonitrile
(10a): Bromide 6 (168 mg, 0.5 mmol), (4-nitrophenyl)boronic acid
(95 mg, 0.57 mmol), and KF·2H2O (80 mg, 0.85 mmol) were dis-
solved in toluene (5 mL) and water (5 mL) under argon atmo-
sphere. The mixture was degassed with argon under sonication be-
fore Pd(PPh3)4 (30 mg, 5 mol-%) was added. The temperature was
raised to 60 °C and the reaction mixture was stirred for 4 days,
before the reaction was quenched by addition of saturated aqueous
NH4Cl (25 mL) and extracted with CH2Cl2 (2ϫ25 mL), washed
with brine (50 mL), dried (MgSO4), and evaporated in vacuo. Puri-
fication by flash column chromatography (SiO2, CH2Cl2/heptanes,
3:2, v/v, Rf = 0.20) followed by recrystallization from EtOAc/hep-
tanes afforded DHA 10a (33 mg, 17%) as orange crystals, m.p.
169.5–172 °C. Crystals suitable for X-ray analysis were grown from
CH2Cl2/heptanes. 1H NMR (300 MHz, CDCl3, 25 °C,TMS): δ =
Acknowledgments
The Danish Research Council for Independent Research, Natural
Sciences is gratefully acknowledged for financial support. In ad-
dition, 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”, grant
number 213609).
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3
3
8.24 (d, JH,H = 8.8 Hz, 2 H, ar), 7.77 (dd, JH,H = 8.0, 1.6 Hz, 2
3
H, ar), 7.57 (d, JH,H = 8.8 Hz, 2 H, ar), 7.48 (m, 3 H, ar), 6.93 (s,
3
1 H, =CH), 6.89 (m, 1 H, =CH), 6.73 (d, JH,H = 11.4 Hz, 1 H,
3
3
=CH), 6.41 (dd, JH,H = 6.3, 1.5 Hz, 1 H, =CH), 6.09 (d, JH,H
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3
4
4.7 Hz, 1 H, =CH), 3.87 ppm (dd, JH,H = 4.6, JH,H = 1.7 Hz, 1
H, CH). 13C NMR (75 MHz, CDCl3, 25 °C): δ = 147.5, 146.2,
141.7, 140.8, 138.0, 133.4, 131.4, 130.4, 130.1, 129.3, 128.41, 126.3,
123.8, 120.2, 118.7, 114.9, 112.8, 50.9, 44.9 ppm; one signal over-
lapping. UV/Vis (MeCN): λmax (ε) = 270(18500), 318 nm (18800).
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acetonitrile afforded the desired salt 11a (20 mg, 70%) as colorless
1
needles, m.p. 139–143 °C (decomp.). H NMR (300 MHz, CDCl3,
3
25 °C, TMS): δ = 8.04 (d, JH,H = 9.0 Hz, 2 H, ar), 7.74 (dd, 3JH,H
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= 7.9, 1.5 Hz, 2 H, ar), 7.58 (d, 3JH,H = 9.0 Hz, 2 H, ar), 7.53–7.36
3
(m, 3 H, ar), 6.94 (s, 1 H, =CH), 6.84 (dd, JH,H = 11.5, 6.1 Hz, 1
3
3
H, =CH), 6.64 (d, JH,H = 11.5 Hz, 1 H, =CH), 6.39 (dd, JH,H
=
3
6.1, 1.4 Hz, 1 H, =CH), 5.93 (d, JH,H = 4.6 Hz, 1 H, =CH), 4.01
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3
4
(s, 9 H, CH3), 3.82 (dd, JH,H = 4.6, JH,H = 1.4 Hz, 1 H, CH)
Received: November 17, 2010
Published Online: January 26, 2011
ppm. 13C NMR (75 MHz, CDCl3, 25 °C): δ = 146.5, 142.4, 141.3,
Eur. J. Org. Chem. 2011, 1033–1039
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
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