Photochromism of dihydroindolizines. Part XVI: first attempts toward molecular wires comprising photochromic dihydro 5-azaindolizines and π-extended ethynyl and butadiynyl oxadiazole derivatives

Article abstract of DOI:10.1016/j.tetlet.2009.11.117

Eight new photochromic dihydro 5-azaindolizines (DHAIs) linked with 2,5-diaryl-1,3,4-oxadiazole (OXD) derivatives containing terminal ethynes and butadiyne substituents on the fluorene part of the DHAI skeleton are synthesized via palladium-mediated coupl

Full text of DOI:10.1016/j.tetlet.2009.11.117

Tetrahedron Letters 51 (2010) 730–733
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Tetrahedron Letters
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Photochromism of dihydroindolizines. Part XVI: first attempts toward
molecular wires comprising photochromic dihydro 5-azaindolizines and
p-extended ethynyl and butadiynyl oxadiazole derivatives
Saleh Abdel-Mgeed Ahmed a,b,
*
a Chemistry Department, Faculty of Science, Assiut University, 71516 Assiut, Egypt
b Chemistry Department, Faculty of Science, Taibah University, 30002, Al-Madena Al-Mounawara, Saudi Arabia
a r t i c l e i n f o
a b s t r a c t
Article history:
Eight new photochromic dihydro 5-azaindolizines (DHAIs) linked with 2,5-diaryl-1,3,4-oxadiazole (OXD)
derivatives containing terminal ethynes and butadiyne substituents on the fluorene part of the DHAI
skeleton are synthesized via palladium-mediated coupling reaction pathways. Irradiation of the DHAI-
OXD derivatives with polychromatic light affords red- and green-colored betaines.
Ó 2009 Elsevier Ltd. All rights reserved.
Received 8 September 2009
Revised 21 November 2009
Accepted 27 November 2009
Available online 1 December 2009
Keywords:
Photochromism
Dihydro 5-azaindolizines (DHAI)
Molecular wires
Oxadiazole (OXD)
Sonogashira coupling
Recently, there has been a renaissance in the chemistry of aro-
matic alkynes utilizing the palladium-catalyzed cross-coupling
reactions of terminal alkynes with aryl halides (the Sonogashira
reaction).1 This protocol provides an efficient and versatile means
materials, based on the 1,5-electrocyclization between two distinct
isomeric states: ring-opened form (betaine-form) and ring-closed
form (DHI or DHAI forms), are promising candidates for optical
storage media and electronic devices.9 In continuation of our re-
search on the synthesis and photochromic properties of active
functionalized systems suitable for molecular wires and electronic
device applications, the aim of the present work was to develop the
synthetic chemistry of photochromic dihydro 5-azaindolizines
bearing OXD-derivatives possessing terminal ethynyl and but-
adiyne substituents on the fluorene part of the DHAI skeleton.6,9
The synthesis of the target molecules through various Sonogash-
ira-mediated coupling reactions is described.
2,5-Diaryl-1,3,4-oxadiazole derivatives 1 and 2a,b were synthe-
sized according to the established procedures.6 The synthesis of
spirocyclopropenes 8a–d was accomplished in five steps, starting
with the previously known conversion of fluorenone into 2-mono
and 2,7-diiodo-9H-fluoren-9-one in 54% yield in three steps9
(Scheme 1). The Sonogashira coupling of 2-mono and 2,7-diiodo-
9H-fluoren-9-ones 3a,b with 2-(4-tert-butylphenyl)-5-(4-ethynyl-
phenyl)-1,3,4-oxadiazole 2a (OXD1) and 2-[4-(buta-1,3-diy-
nyl)phenyl]-5-(4-tert-butylphenyl)-1,3,4-oxadiazole 2b (OXD2) in
the presence of Pd(PPh3)2Cl2/Cu2I2/Et3N in THF at 45 °C for 12 h
afforded the coupled products 4a–d in moderate yields (46–59%).
Condensation of 4a–d with anhydrous hydrazine in boiling eth-
anol for 4 h led to the formation of the corresponding hydrazones
5a–d in excellent yields (86–94%). The evidence for the formation
of compounds 5 was provided by 1H NMR spectra which showed a
of extending
p–p conjugation in organic compounds, affording
simplified molecular structures compared with their alkene ana-
logues which exhibit Z/E isomerism. Ethynylene-extended arenes
and heteroarenes have been shown to function as nanoscale
‘molecular wires’, with the extent of intramolecular conjugation
dependent upon the topology of the
p-system and the molecular
length.2 Synthetic advances have led to remarkable polyene sys-
tems of nanoscale lengths end-capped with organometallic3 or silyl
substituents.4 However, ethyne and butadiyne derivatives remain
fundamentally important targets for studies on optoelectronic
properties of carbon-rich backbones which have not been well-
established experimentally. In this context, Wang et al.5,6 studied
2,5-diaryl-1,3,4-oxadiazole (OXD) derivatives due to their good
thermal and chemical stabilities and their high photoluminescence
quantum yields. Photochromic dihydroindolizines (DHIs) and
dihydro 5-azaindolizines (DHAIs), which were discovered and
developed by Dürr,7 are well-known photochromic materials that
have attracted significant interest from the viewpoints of both fun-
damental elucidation of electrocyclization reactions and their po-
tential applications to optical memories and switches.8 These
* Tel.: +20 882332200; fax: +20 882342708.
E-mail address: saleh_63@hotmail.com.
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.11.117

S. Abdel-Mgeed Ahmed / Tetrahedron Letters 51 (2010) 730–733
731
N
N
N
N
H
Br
n
O
O
OXD- derivatives
1
2a, n= 1
2b, n= 2
N
N
O
O
N
N
I
N
N
Pd(PPh3)2Cl2
Cu2I2 / Et3N
O
NH2NH2
EtOH
+
O
n
n
X
N-NH2
O
3a, X= H
3b, X= I
X
X
n
5a, n= 1, X =H
5b, n= 1, X = OXD (88%)
5c, n= 2, X = H (92%)
(94%)
4a, n= 1, X =H
4b, n= 1, X = OXD (50%)
4c, n= 2, X = H (54%)
(59%)
H
5d, n= 2, X= OXD (86%)
4d, n= 2, X= OXD (46%)
2a, n= 1
2b, n= 2
N
N
N
O
O
N
O
N
N
MnO2
Et2O
DMAD
Et2O
hν
Et2O
n
n
COOCH3
n
N
N2
N
COOCH3
COOCH3
COOCH3
7a, n= 1, X =H (44%)
7b, n= 1, X = OXD (40%)
7c, n= 2, X = H (39%)
7d, n= 2, X= OXD (37%)
X
X
X
6a, n= 1, X =H
6b, n= 1, X = OXD (29%)
6c, n= 2, X = H (24%)
6d, n= 2, X= OXD (20%)
(34%)
8a, n= 1, X =H
(44%)
8b, n= 1, X = OXD (23%)
8c, n= 2, X = H (19%)
8d, n= 2, X= OXD (15%)
Scheme 1. Preparation of mono- and disubstituted spirocyclopropene-OXD precursors 8a–d.
broad singlet at 4.98–5.08 ppm corresponding to the NH2 protons
which disappeared upon treatment with deuterium oxide. Also,
the IR spectra showed that the carbonyl absorptions present in
compounds 4 at 1712–1727 cmꢀ1 had disappeared and instead,
amino group absorptions at 3200–3220 cmꢀ1 were detected. Oxi-
dation of the hydrazones 5a–d with manganese dioxide in dry
ether at room temperature in the absence of light for 9 h afforded
the 9-diazo-derivatives 6a–d in poor yields (20–34%). Reaction of
dimethyl acetylenedicarboxylate (DMAD) with the 9-diazofluorene
derivatives 6a–d in dry ether in the dark for 12 h led to the forma-
tion of pyrazole cycloadducts 7a–d in 37–44% yields. The spirocy-
clopropene precursors 8a–d were obtained via photolysis of
pyrazoles 7a–c. The photolysis was carried out using a high pres-
sure mercury lamp (125 W) in dry ether solution for 3 h under a
nitrogen atmosphere.
Pure spirocyclopropene derivatives 8a–d were obtained in low
yields (15–28%) after flash chromatography purifications. The
low yields can be attributed to the formation of unidentified by-
products which may be derived from addition polymerization on
the acetylenic groups in the fluorene part. The structures of the
newly synthesized spirocyclopropenes 8a–d (Scheme 1) were
established by spectroscopy (NMR, IR, and mass spectrometry)

732
S. Abdel-Mgeed Ahmed / Tetrahedron Letters 51 (2010) 730–733
N
N
N
N
N
N
O
O
O
O
N
N
R
Et2O
R
Δ
+
R
N
24 h/RT
R
hν
R
N
COOCH3
COOCH3
COOCH3
n
n
n
+
n
N
R
N +
N
N
R
9a, R=H
9b, R=CH3
-
N
COOCH3
N
-
R
COOCH3
X
COOCH3
COOCH3
COOCH3
X
X
X
8a, n= 1, X =H
8b, n= 1, X = OXD
8c, n= 2, X = H
10a-h
11a-h
10`a-h
8d, n= 2, X= OXD
Scheme 2. Preparation of photochromic DHAIs 11a–h (method A) from spirocyclopropenes 8a–d (11a, R = X = H, n = 1; 11b, R = X = H, n = 2; 11c, R = H, X = OXD, n = 1; 11d,
R = H, X = OXD, n = 2; 11e, R = CH3, X = H, n = 1; 11f, R = CH3, X = H, n = 2; 11g, R = CH3, X = OXD, n = 1; 11h, R = CH3, X = OXD, n = 2).
and elemental analysis data. For example, the 1H NMR (400 MHz,
CDCl3) of spirocyclopropene precursor 8c showed the following
signals: d 8.45 (d, J = 7.8 Hz, 2H, CH-arom), 8.33 (d, J = 7.8 Hz, 1H,
CH-arom), 8.01 (d, J = 7.8 Hz, 1H, CH-arom), 7.92 (d, J = 7.8 Hz,
2H, CH-arom), 7.73 (s, 1H, CH-arom), 7.57 (d, J = 7.8 Hz, 2H, CH-
arom), 7.42 (d, J = 7.8 Hz, 2H, CH-arom), 7.34 (d, J = 7.8 Hz, 1H,
CH-arom), 7.30 (t, J = 6.7, 2H, CH-arom), 7.20 (t, J = 6.7, 1H, CH-
arom), 3.81 (s, 6H, 20, 30-CH3), 1.31 [s, 9H, C(CH3)3] ppm.
Electrophilic addition of spirocyclopropenes 8a–d to pyridazine
9a and 2,6-dimethylpyridazine 9b using the cyclopropene route8,9
(Scheme 2) in dry ether at room temperature under dry nitrogen in
the absence of light for 12 h (TLC monitoring using CH2Cl2 as elu-
ent) led to the formation of the photochromic dihydro 5-azaindoli-
zines (DHAIs) 11a–h in poor yields (18–29%, method A and Table 1
in the Supplementary data). The formation of the photochromic
DHAIs 11a–h occurs through electrophilic addition of the elec-
tron-deficient spirocyclopropenes 8a–d to the nitrogen of the
pyridazines 9a,b which led to the ring-opening via a cyclopropyl-
allyl conversion 100a–h to the colored betaines 10a–h. Subsequent
ring-closure to DHAIs 11a–h occurred via a slow thermal 1,5-elec-
trocyclization reaction which can be reversed upon exposure to
light.
Photochromic DHAIs 11a–h were obtained in a pure form by
column chromatography on silica gel using dichloromethane as
the eluent. An alternative successful method for the synthesis of
the target photochromic DHAIs 11a–h (Scheme 3) was achieved
through the palladium-mediated Sonogashira coupling of (50R)-di-
methyl 2-iodo- and 2,7-diiodo-4a0H-spiro[fluorene-9,50-pyrrol-
o[1,2-b]pyridazine]-60,70-dicarboxylates 12a,b and (50R)-dimethyl
2-iodo- and 2,7-diiodo-20,4a0-dimethyl-4a0H-spiro[fluorene-9,50-
pyrrolo[1,2-b]-pyridazine]-60,70-dicarboxylates 12c,d which were
previously prepared by us,9g,h with OXD-derivatives 2a,b. The pal-
ladium-catalyzed reaction (2.5% or 5%, for mono- and di-OXD
derivatives, respectively) in the presence of Cu2I2/Et3N in dry THF
for 8 h yielded the desired photochromic-OXD systems 11a–h in
moderate yields (38–53%) after purification by flash column chro-
matography on silica gel using CH2Cl2 as eluent (method B, Supple-
mentary data).
N
N
O
R
N
O
N
I
R
N
Pd(PPh3)2Cl2
Cu2I2 / Et3N
N
N
N
hν
O
+
Δ
COOCH3
COOCH3
R
X
R
n
R
COOCH3
12a, R = H, X = H
12b, R= H, X = I
12c, R= CH3, X =H
N
n
N +
N
-
N
12d, R = CH3, X = I
n
COOCH3
COOCH3
R
COOCH3
X
H
X
2a, n= 1
2b, n= 2
11a-h
10a-h
Scheme 3. An alternative pathway (method B) for the synthesis of photochromic DHAIs 11a–h (11a, R = X = H, n = 1; 11b, R = X = H, n = 2; 11c, R = H, X = OXD, n = 1; 11d,
R = H, X = OXD, n = 2; 11e, R = CH3, X = H, n = 1; 11f, R = CH3, X = H, n = 2; 11g, R = CH3, X = OXD, n = 1; 11h, R = CH3, X = OXD, n = 2).

S. Abdel-Mgeed Ahmed / Tetrahedron Letters 51 (2010) 730–733
733
H
N
N
O
R
N
O
N
R
N
N
Pd(PPh3)2Cl2
Cu2I2 / Et3N
N
hν
O
+
N
Δ
COOCH3
COOCH3
R
X
R
COOCH3
N +
R
13a, R = H, X = H
13b, R= H, X =
13c, R= CH3, X =H
N
CH
N
C
Br
-
N
1
CH
C
13d, R = CH3, X =
COOCH3
COOCH3
R
COOCH3
X
X
11a,c,e,g
10a,c,e,g
Scheme 4. An alternative pathway (method C) for the synthesis of photochromic DHAIs 11a,c,e,g (11a, R = X = H, n = 1; 11c, R = H, X = OXD, n = 1; 11e, R = CH3, X = H, n = 1;
11g, R = CH3, X = OXD, n = 1).
In addition, photochromic DHAIs-OXD 11a,c,e,g were success-
fully synthesized via Sonogashira-mediated coupling of dimethyl
2- or 2,7-ethynyl-4a0H-spiro[fluorene-9,50-pyrrolo[1,2-b]pyrida-
zine]-60,70-dicarboxylates 13a,b and dimethyl 2- or 2,7-ethynyl-
Supplementary data
Supplementary data (synthetic procedures and analytical and
spectroscopic characterization) associated with this article can be
found, in the online version, at doi:10.1016/j.tetlet.2009.11.117.
20,4a0-dimethyl-4a0H-spiro[fluorene-9,50-pyrrolo[1,2-b]
pyrida-
zine]-60,70-dicarboxylates 13c,d with 2-(4-bromophenyl)-5-(4-
tert-butylphenyl)-1,3,4-oxadiazole 1. The palladium-catalyzed
reaction (2.5% or 5%, for mono- and di-OXD derivatives, respec-
tively) with Cu2I2/Et3N in dry THF for 5 h yielded the desired pho-
tochromic-OXDs in good yields (31–46%) after purification by flash
column chromatography on silica gel using CH2Cl2 as eluent
(Scheme 4, method C, Supplementary data). The obtained products
from the three different pathways gave the same analytical and
spectroscopic data as well as the same melting points (see Table 1
in the Supplementary data).
Irradiation of photochromic DHAI-OXD derivatives 11a–h with
polychromatic light in CH2Cl2 afforded red-colored betaines 10a–
d and green–blue colored betaines 11e–h. Details on the photo-
chromic properties, fluorescence, and photoluminescence will be
described in detail in a forthcoming paper.
In conclusion, photochromic dihydro 5-azaindolizines (DHAIs)
incorporating 2,5-diphenyl-1,3,4-oxadiazole (OXD) derivatives
containing terminal ethynyl- and butadiyne substituents on the
fluorene part of the DHAI skeleton have been synthesized using
three reaction pathways following Sonogashira-mediated coupling
methodologies. The three synthetic procedures gave identical
products as was proved by both analytical and spectroscopic meth-
ods. In addition, new spirocyclo-propenes containing OXD systems
have been synthesized via chemical and photochemical
procedures.
References and notes
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Eds.; Pergamon Press: Oxford, 1991; Vol. 3, p 521; (b) Sonogashira, K. In Metal
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Weinheim, 1998; p 203; (c) Sonogashira, K. J. Organomet. Chem. 2002, 653, 46.
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be considered as photochromic dihydropyrrolo[1,2-b]pyridazines.
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Acknowledgments
9. (a) Ahmed, S. A. Monatsh. Chem. 2004, 135, 1173; (b) Ahmed, S. A. J. Phys. Org.
Chem. 2002, 15, 392; (c) Ahmed, S. A. J. Phys. Org. Chem. 2006, 19, 402; (d)
Ahmed, S. A. J. Phys. Org. Chem. 2007, 20, 564; (e) Ahmed, S. A.; Pozzo, J. L. J.
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(g) Ahmed, S. A. Res. Lett. Org. Chem. 2008, Article ID 959372, p 5.; (h) Ahmed, S.
A. J. Phys. Org. Chem., accepted for publication.
The author is highly indebted to Professor H. Dürr, Germany;
Professor A. A. Abdel-Wahab, Egypt; and Professor H. Bouas-Lau-
rent, France, for their helpful discussions. Financial support of this
work from the Alexander von Humboldt Foundation (AvH) and Tai-
bah University (Project Numbers: 48/427 and 479/30) is
acknowledged.