Synthesis and structures of cross-conjugated bis-dehydroannulenes with a Y-enediyne motif and different π topologies

Article abstract of DOI:10.1002/chem.200601512

A synthesis of cross-conjugated bis-dehydroannulenes with different topologies of the it electrons by Cu^II-mediated oxidative coupling of the corresponding terminal acetylenic precursors is reported. In general, of the two possible modes of cyc

Full text of DOI:10.1002/chem.200601512

FULL PAPER
DOI: 10.1002/chem.200601512
Synthesis and Structures of Cross-Conjugated Bis-dehydroannulenes with a
Y-Enediyne Motif and Different p Topologies
Arkasish Bandyopadhyay,[a] Babu Varghese,[b] Henning Hopf,[c] and
Sethuraman Sankararaman*[a]
Dedicated to Professor Dr. Rolf Gleiter on the occasion of his 70th birthday
Abstract: A synthesis of cross-conju-
gated bis-dehydroannulenes with differ-
ent topologies of the p electrons by
CuII-mediated oxidative coupling of the
corresponding terminal acetylenic pre-
cursors is reported. In general, of the
two possible modes of cyclization,
which would yield either a [13]an-
nulene or an [18]annulene, the precur-
sors yielded exclusively the bis-dehy-
dro[13]annulenes. However, one exam-
ple of the formation of a bis-dehy-
dro[18]annulene is also reported. The
mode of cyclization to form either the
[13]annulene or the [18]annulene is ex-
plained on the basis of the conforma-
tional preference of the core unit bear-
ing the Y-enediyne moieties. The struc-
tures of the two types of bis-annulenes
have been unequivocally established by
means of single-crystal X-ray crystallo-
graphic analysis.
Keywords: alkynes
cross-conjugation
macrocycles
·
annulenes
enediynes
·
·
·
Introduction
ganometallic moieties, and these exhibit interesting electron-
ic properties.[5] The majority of the dehydrobenzoannulenes
reported to date belong to the family of linearly conjugated
annulenes. The oxidative coupling of cis-1,2-diethynylethene
and its derivatives gives dehydroannulenes with linear con-
jugation of the p electrons.[6] On the other hand, oxidative
coupling of 1,1-diethynylethene and its derivatives results in
the formation of expanded radialenes, which may be consid-
ered as cross-conjugated dehydroannulenes[7] (Scheme 1).
Cross-conjugated molecules exhibit unusual and interest-
ing electronic properties[8] that result from an alternative
mode of p-electron communication, and those incorporating
Y-enediyne units have been shown to exhibit interesting sol-
vatochromic, electrochromic, and nonlinear optical (NLO)
properties.[9] Such molecules are potentially useful as organ-
ic materials in molecular electronics and photonics. Herein,
Extensively conjugated organic molecules are potentially
useful in molecular electronics and photonics applications.[1]
The discovery of fullerenes and carbon nanotubes has creat-
ed a flurry of activity in the design and synthesis of other
forms of carbon allotropes, such as graphyne, graphadiyne,
cyclo[n]carbons, and so on.[2] Several model compounds
bearing graphadiyne structural units have been reported.[3]
These molecules are fully conjugated dehydrobenzoannu-
lenes that incorporate several acetylenic bridges.[4] Dehydro-
benzoannulenes of various sizes, shapes, and symmetries
have been synthesized, as well as some bearing various or-
[a] A. Bandyopadhyay, Prof. Dr. S. Sankararaman
Department of Chemistry
Indian Institute of Technology Madras
Chennai 600036 (India)
Fax : (+91)44-2257-0545
E-mail: sanka@iitm.ac.in
[b] Dr. B. Varghese
Sophisticated Analytical Instrument Facility
Indian Institute of Technology Madras
Chennai 600036 (India)
[c] Prof. Dr. H. Hopf
Institute of Organic Chemistry
University of Braunschweig
38106 Braunschweig (Germany)
Scheme 1. Oxidative coupling of cis-enediynes and Y-enediynes to give
linear and cross-conjugated dehydroannulenes, respectively.
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we describe the synthesis of several cross-conjugated bis-de-
hydroannulenes. We have chosen two known bis-Y-ene-
diynes as core units, namely 1,4-bis(1-ethynyl-2-propynylide-
ne)cyclohexane (1b)[10] and 9,10-bis(1-ethynyl-2-propynyli-
dene)-9,10-dihydroanthracene (2b)[11] (Scheme 2).
core. Tetrayne 1b is reasonably stable and is amenable to
cross-coupling reactions with iodoarenes. Sonogashira cou-
pling of 1b, prepared in situ from 1a, with suitably substitut-
ed iodoarenes (4a–c) gave the corresponding tetrasubstitut-
ed derivatives (5a–c) in yields of 35–40% as yellow to red-
orange solids (Scheme 4).
Scheme 2. Structures of bis-Y-enediyne core units. It should be noted that
3b has yet to be synthesized. R=trimethylsilyl (TMS) for 1a, 2a, and
3a; R=H for 1b, 2b, and 3b.
The synthesis of fully cross-conjugated bis-Y-enediynes 3b
has yet to be accomplished.[12] The general structures of the
precursors derived from these core units are shown in
Scheme 3, along with the two possible modes of cyclization
to yield bis-dehydro[13]annulene and bis-dehydro[18]annu-
lene, respectively.
Scheme 4. Synthesis of precursors of bis-dehydro[13]annulenes with a
1,4-cyclohexylidene core. a) K2CO3, MeOH, Et2O, RT, 3 h; b) [Pd-
AHCTER(UNG PPh3)4], CuI, iPr2NH, THF, 608C.
The tetrasubstituted derivatives 5a–c were characterized
1
by spectroscopic techniques. In the H NMR spectra of 5a–
c, all eight protons of the cyclohexyl ring appeared as a
sharp singlet at around d=2.90 to 2.96 ppm, indicating the
high conformational fluxionality of this ring. This signal re-
mained as a sharp singlet even at À608C. Removal of the
TMSgroups from 5b,c and oxidative coupling of the result-
ing terminal acetylenes were accomplished in a single step
to yield the corresponding bis-dehydro[13]annulenes 6b,c in
moderate yields (Scheme 5).
Bis-dehydro[13]annulenes 6b,c were thoroughly charac-
terized by using various spectroscopic techniques. Careful
Scheme 3. Oxidative coupling of the precursor in two different modes to
yield bis-dehydroannulenes of different p topologies.
Results and Discussion
Scheme 5. Synthesis of bis-dehydroannulenes with a cyclohexane core.
Synthesis: Bis-Y-enediyne 1b was chosen as the precursor
for the synthesis of bis-annulenes with a 1,4-cyclohexylidene
a) K2CO3, MeOH, CH2Cl2, RT, 24 h; b) Cu
RT, 4 days.
ACHTRE(UNG OAc)2, pyridine (py), MeCN,
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Cross-Conjugated Bis-dehydroannulenes
FULL PAPER
1
examination of the H NMR spectra of the crude products
of cyclization of 5b,c revealed that only one type of annu-
lene had been formed in the reaction. Only one sharp sin-
glet was observed for all eight protons of the cyclohexyl ring
in 6b,c. Furthermore, only one set of aromatic multiplets
was observed. That the oxidative cyclization of 5b,c had
yielded only [13]annulenes and not [18]annulenes
(Scheme 3) was confirmed by comparing the 1H NMR
chemical shifts of the cyclohexyl protons in 5b,c and 6b,c.
The cyclohexyl protons appeared as a sharp singlet at d=
2.96 (5b), 2.92 (5c), 2.93 (6b), and 2.92 ppm (6c), respec-
tively. The chemical shift of the cyclohexyl protons thus re-
mained the same before and after the cyclization. Had the
cyclization proceeded to yield [18]annulenes, the cyclohexyl
protons would have been relatively more shielded in 6b,c
relative to 5b,c owing to the anisotropic effect of the ring
current of the annulene ring. Only in the case of [13]an-
nulene would the chemical shift of the cyclohexyl protons
be expected to remain unaffected, in accordance with the
experimental findings. Further support for the formation of
[13]annulenes in the cyclization was obtained from single-
crystal X-ray crystallographic analysis of 6c,[13] which un-
equivocally established the mode of cyclization (Figure 1).
Oxidative cyclization of 5a yielded a very insoluble materi-
al, which could not be purified.
Scheme 6. Synthesis of precursors of bis-dehydro[13]annulenes with a
9,10-dihydroanthracene core. a) [PdACHTRE(UNG PPh3)4], CuI, iPr2NH, THF, toluene,
608C.
Figure 1. PLUTO representation of the structure of 6c in the crystal,
showing the chair conformation of the cyclohexylidene ring. The nitrogen
atoms are labeled.
Scheme 7. Synthesis of bis-dehydro[13]annulenes with a 9,10-dihydroan-
thracene core. a) K2CO3, MeOH, CH2Cl2, RT for 9a; tetrabutyl ammoni-
um fluoride (TBAF), MeOH, THF, RT for 9b; b) CuCAHTRE(UNG OAc)2, py, MeCN,
CH2Cl2, RT.
Attempted dehydrogenation of 6b,c using 2,3-dichloro-
5,6-dicyano-1,4-benzoquinone (DDQ) in refluxing toluene
did not yield the desired fully cross-conjugated bis-annu-
lenes with 3 as the core unit. Instead, only a black intracta-
ble material was obtained from these reactions.
1
as in the case of bis-[13]annulenes 6b,c, the H NMR chemi-
cal shifts of the eight anthracene ring protons remained
almost the same before and after cyclization in 9a,b and
10a,b, respectively. For example, the anthracene ring pro-
tons appeared at d=8.60 and 7.30 ppm in 9a and at d=8.49
and 7.3 ppm in 9b, whereas the same protons appeared at
d=8.47 and 7.43 in 10a and at d=8.55 and 7.45 ppm in 10b
(Figure 2). It should also be noted that the introduction of
bulky tert-butyl groups at different positions on the benzene
rings in 9a and 9b did not alter the mode of cyclization.
Fourfold coupling of cis-enediyne 11 with 7 yielded pre-
The precursors 9a,b for the synthesis of bis-dehy-
dro[13]annulenes (10a,b) containing a 9,10-dehydroanthra-
cene core were obtained by fourfold Sonogashira coupling
of tetrabromide 7 with 8a and 8b, respectively (Scheme 6).
Removal of the silyl protecting groups from 9a,b followed
by oxidative coupling was carried out in one pot to yield
bis-dehydro[13]annulenes 10a,b in good yields as red solids
(Scheme 7).
In 9a,b and 10a,b, the protons on the anthracene ring ap-
peared as a single AA’BB’ pattern, indicating the symmetri-
cal tetrasubstitution pattern in these molecules. Moreover,
1
cursor 12 in 24% yield (Scheme 8). The H NMR spectrum
of 12 displayed a single AA’BB’ pattern due to the aromatic
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S. Sankararaman et al.
protons of 13 appeared at d=7.65 and 7.08 ppm, while those
of 12 appeared at d=8.34 and 7.28 ppm, in both cases as an
AA’BB’ pattern (Figure 3). This clearly suggests that the
Figure 2. 1H NMR spectra (aromatic region) of 9a (top) and 10a
(bottom). Ha and Hb correspond to the protons on the 9,10-dihydroan-
thracene ring.
Figure 3. 1H NMR spectra (aromatic region) of 12 (top) and 13 (bottom).
Ha and Hb correspond to the protons on the 9,10-dihydroanthracene ring;
Hc and Hd correspond to the olefinic protons.
cyclization had yielded a bis-dehydro[18]annulene and not a
bis-dehydro[13]annulene. The shielding of the aromatic pro-
tons in 13 is due to the anisotropy effect of the ring current,
which is absent in 12. The olefinic protons of 13 appeared at
d=6.41 and 6.14 ppm while those of 12 appeared at d=5.89
and 5.78 ppm, in both cases as an AB pattern (Figure 3).
Finally, bis-annulene 13 was structurally characterized by
means of single-crystal X-ray diffraction analysis, which
clearly showed it to be a bis-dehydro[18]annulene and not a
bis-dehydro[13]annulene (Figure 4).[13] The structure of 13 is
nonplanar. The plane of the benzenoid ring lies at a distance
of 4.5 Š from the plane of the annulene ring. The structure
of 13 resembles that of a recliner-chair. There are no unusu-
al bond lengths or strained bond angles in 13, with all pa-
Scheme 8. Synthesis of bis-dehydro[18]annulene 13. a) [Pd
iPr2NH, THF, toluene, 608C; b) TBAF, MeOH, THF, RT, 24 h; c) Cu-
(OAc)2, py, MeCN, CH2Cl2, RT. (TIPS: triisopropylsilyl)
ACHTRE(UNG PPh3)4], CuI,
ACHTREUNG
protons at d=8.34 and 7.28 ppm and a single AB pattern
due to the olefinic protons at d=5.96 and 5.87 ppm. In situ
desilylation followed by oxidative coupling using CuII yield-
ed 13 in 40% yield (Scheme 8).
Careful examination of the 1H NMR spectrum of the
crude product clearly showed the formation of only one
type of annulene, as evident from a single AA’BB’ pattern
due to the aromatic protons and a single AB pattern due to
the olefinic protons. Furthermore, the chemical shifts of the
aromatic protons showed these to be considerably more
shielded in 13 than in the open precursor 12. The aromatic
Figure 4. Structure of 13 in the crystal: A) front view in ORTEP repre-
sentation; B) side view in PLUTO representation, showing the recliner-
chair-like structure. Distances: a=3.839, b=3.847, c=4.606 Š.
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Cross-Conjugated Bis-dehydroannulenes
FULL PAPER
rameters lying within the expected ranges. The distances a
and c indicated in Figure 4 clearly show that the aromatic
protons Ha and Hb are located at 3.8 and 4.6 Š, respectively,
from the annulene p system. At these distances, which are
much larger than the optimal distance for p–p interaction,
the anisotropic effect of the ring current would be expected
to be minimal. In fact, the distance dependence of the aniso-
tropic effect is clearly evident, in that Ha, which lies closer
to the annulene p electrons, is more shielded (d=0.69 ppm)
than Hb (d0.20 ppm). Alternatively, Hb protons that lie di-
rectly above the triple bonds could be under the influence
of the deshielding effect of the anisotropy of the triple bond
and the shielding effect of the ring current of the annulene
p system, such that the chemical shift of Hb is the net result
of these two opposing effects.
Tetrayne 2b has been structurally characterized by single-
crystal X-ray diffraction analysis, which clearly showed that
the middle ring of the dihydroanthracene moiety has a boat-
like conformation.[15] The crystal structure of 13 also clearly
revealed the boatlike geometry of the core unit. This would
imply that the precursor, namely the desilylated derivative
of 12, also has a boatlike geometry of the core unit. Forma-
tion of the bis-dehydro[18]annulene 13 rather than the iso-
meric [13]annulene is perhaps dictated by the geometry of
the core unit and the conformation of the pendant ethynyl
groups in the precursor. We presume that derivatives 9a,b
also have the same geometry with respect to the dihydroan-
thracene ring. However, 9a,b underwent oxidative cycliza-
tion to preferentially afford only the bis-dehydro[13]annu-
lenes and not the corresponding bis-dehydro[18]annulenes.
This would imply that the mode of cyclization of 9a and 9b
to yield 10a and 10b, respectively, is dictated by the confor-
mation of the Y-enediyne side chain rather than that of the
central core anthracene ring.
Haley and co-workers[14] have shown that the mode of
cyclization to form either bis-dehydro[14]annulenobenzene
or bis-dehydro[15]annulenobenzene can be switched by
changing the conditions of the oxidative cyclization. When
the reactions were carried out under typical Glaser–Eglinton
conditions using CuII, only the bis-[15]annulenes were
formed. On the other hand, when the oxidative couplings
were carried out using PdII bearing bidentate phosphane li-
gands, only bis-[14]annulenes were formed. The formation
of these two isomeric bis-annulenes can be rationalized in
terms of the geometries of the intermediate metal acetylides.
In the case of Cu-mediated cyclization, a pseudo-trans con-
figuration of an intermediate
Electronic absorption and emission spectroscopy: Absorp-
tion and emission spectra of the annulenes and their corre-
sponding uncyclized precursors were recorded for sample
solutions in dichloromethane (Table 1). In the cyclohexane
series, the absorption maxima of the annulenes (6b,c) were
found to be redshifted by l=50–55 nm relative to those of
the uncyclized precursors (5b,c) (Figure 5). This observation
involving a dimeric CuI acety-
Table 1. Absorption and emission data for bis-dehydroannulenes and their precursors.
Absorption lmax [nm] (loge) in CH2Cl2
lide is proposed, whereas in
the case of Pd-mediated cycli-
zation, a cis-like geometry of
the PdII acetylide is proposed.
When oxidative palladium
Emission lmax [nm]
in CH2Cl2
5a
5b
5c
6b
6c
9a
9b
330 (sh, 4.54), 300 (4.65), 260 (4.90), 254 (4.88), 238 (4.92)
330 (sh, 4.72), 302 (4.83), 261 (5.06), 241 (5.09)
370 (4.84), 347 (4.87), 316 (4.83), 264 (5.01), 256 (5.01)
379 (4.49), 277 (4.95), 230 (4.83)
446 (4.58), 415 (sh, 447), 343 (sh, 4.87), 328 (5.13), 318 (5.13), 258 (4.61), 231 (4.88) 486
423 (3.74), 359 (3.69), 237 (4.34)
417 (3.22), 348 (3.19), 241 (3.96)
not measured
405
417
408, 428
chemistry using [PdCl2ACHTRE(UNG dppe)]
(dppe: 1,2-bis(diphenylphos-
phino)ethane) was employed
for the cyclization of desilylat-
ed 9a,b, under the reaction
conditions reported by Haley
and co-workers, we again ob-
served the exclusive formation
of 10a,b, and none of the bis-
447
496
10a 454 (1.82), 286 (2.47)
10b 461 (3.12), 386 (3.11), 288 (3.63), 243 (3.52)
nonfluorescent
nonfluorescent
449
12
13
426 (2.83), 366 (2.79), 348 (2.96), 325 (3.00), 293 (3.18), 277 (3.21), 264 (3.20)
376 (2.66), 334 (2.76)
nonfluorescent
ACHTREUNG[18]annulene. Therefore, we conclude that with these sub-
strates the mode of cyclization is unaffected by the metal
ion used. In Haleyꢁs bis-dehydroannulenes, the core ring is
planar. In the present study, however, the core ring is a non-
planar cyclohexane. In 1a, the cyclohexane ring has a chair
conformation,[12] and this is also evident from the X-ray
structure of 6c. Molecular models show that in order to
form the bis-dehydrobenzo[18]annulene, the cyclohexane
ring must adopt either a boat or a twist-boat conformation.
Furthermore, in this mode of cyclization, the conjugation is
incomplete due to the methylene groups of the core ring.
These factors perhaps dictate the mode of cyclization to
form the bis-dehydro[13]annulene.
Figure 5. Comparison of the UV/Vis absorption spectra of the open pre-
cursors 5b,c and the annulenes 6b,c in CH2Cl2 (10À5 m).
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S. Sankararaman et al.
conforms to the expected trend for annulenes in which the
conjugation is complete. Among the studied annulenes, 6c,
bearing electron-donating di-n-butylamino substituents,
shows a redshift of its absorption bands relative to those of
6b, which bears tert-butyl groups. A similar trend is ob-
served for 5b and 5c. Compared with the substrates with a
cyclohexane core (5b,c and 6b,c), those with an anthracene
core (9a,b and 10a,b) show a redshift of the absorption
bands. In contrast to the cyclohexane series, the anthracene
series has complete cross-conjugation of the two annulene
rings through the anthracene unit. Therefore, the absorption
bands are more redshifted in the anthracene series. More-
over, within the anthracene series, the absorption bands of
the annulenes (10a,b) are more redshifted relative to those
of the corresponding precursors (9a,b) (Figure 6).
the substrates bearing electron-donating N,N-dibutylamino
groups (5c and 6c) showed bathochromic shifts in compari-
son to those of substrates bearing only tert-butyl groups (5b
and 6b). The bathochromic shift is more pronounced in the
case of the annulenes than in the case of the precursors.
Specifically, there is a bathochromic shift of 55 nm between
6b (lmax =431 nm) and 6c (lmax =486 nm), whereas the shift
is only 12 nm between 5b (lmax =405 nm) and 5c (lmax
=
417 nm). Comparison of the precursors (5b and 5c) and the
annulenes (6b and 6c) also clearly shows that the emission
band of the annulenes is more redshifted than that of the
precursors. Specifically, the bathochromic shift is 26 nm be-
tween 5b and 6b and 69 nm between 5c and 6c. In the case
of the precursors, the phenylethynyl substituents are likely
to be nonplanar. However, cyclization results in the [13]an-
nulene ring becoming planar, thus allowing for better delo-
AHCTREcUNG alization of the p electrons. Therefore, the annulenes show
a larger bathochromic shift relative to the corresponding
precursors. These observations are in line with the findings
reported by Haley[13] for linearly conjugated dehydroben-
zoannulenes. The precursors 9a (lmax =447 nm) and 9b
(lmax =496 nm) are fluorescent, and the emission of 9b is
redshifted by 49 nm in comparison with that of 9a. In con-
trast, the fully cross-conjugated annulenes (10a,b and 13)
are not fluorescent. In fully cross-conjugated annulenes such
as 10a,b and 13, nonradiative deactivation of the S1 state
competes effectively with fluorescence emission.[16]
Conclusion
Figure 6. Comparison of the UV/Vis absorption spectra of 9b (10À5 m),
10b (10À4 m), 12 (5”10À4 m), and 13 (1.2”10À4 m) (in CH2Cl2).
We have synthesized a series of cross-conjugated dehy-
droannulenes with either a 1,4-cyclohexane core or a 9,10-
dihydroanthracene core from the corresponding terminal
acetylene precursors by means of Eglinton coupling. The ox-
idative coupling reaction generally yielded bis-dehy-
dro[13]annulenes as exclusive products in both series. In one
case, however, it yielded a bis-dehydro[18]annulene. The
[13]annulene and the [18]annulene have different p topolo-
gies. One example of each of these classes of annulenes has
been structurally characterized by means of single-crystal X-
ray crystallographic analysis. The electronic absorption and
emission properties of this class of annulenes have been
studied.
Fluorescence emission spectra were measured for sample
solutions in deoxygenated CH2Cl2 (Table 1). The fluores-
cence spectra of the precursors (5b,c) and the annulenes
(6b,c and 9a,b) showed a single, broad, featureless band in
the region l=350–600 nm (Figure 7). The emission bands of
Experimental Section
General procedure for the synthesis of 5a–c: Tetrayne 1b[10] was pre-
pared in situ by removal of the trimethylsilyl groups from 7,7,8,8-tetra-
kis(trimethylsilylethynyl)-p-quinodimethane (1a).[17] K2CO3 (2.81 g,
20 mmol) was added to a degassed solution of 1a (1.0 g, 2 mmol) in
MeOH (50 mL) and diethyl ether (50 mL). The mixture was stirred
under an argon atmosphere for 3 h at 25–308C. The solvents were then
removed under reduced pressure at room temperature and the residue
was added to ice-cold water (100 mL) and extracted with diethyl ether
(3”50 mL). The combined organic extracts were washed thoroughly with
saturated brine (2”100 mL). After drying over anhydrous Na2SO4, the
Figure 7. Fluorescence spectra in CH2Cl2 (10À4 m) of 5b (lex =302 nm), 5c
(lex =316 nm), 6b (lex =350 nm), 6c (lex =420 nm), 9a (lex =356 nm), 9b
(lex =357 nm), and 12 (lex =322 nm).
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Cross-Conjugated Bis-dehydroannulenes
FULL PAPER
solvent was removed under reduced pressure at room temperature to
afford the crude tetrayne 1b, which was redissolved in THF (20 mL). In
a separate Schlenk flask, a mixture of iodoarene (12 mmol) and diisopro-
pylamine (50 mL) was degassed by bubbling argon through it for 10 min.
(%)=847 (100) [M+Na, C64H56Na], 848 (76), 849 (32); 931 (92), [M+Ag,
C64H56Ag], 932 (62), 933 (100), 934 (61), 935 (23); HRMS: m/z calcd for
C64H56Ag: 931.3433, found: 931.3407.
Compound 6c: Yield: 0.22 g (28%), dark-purple solid; m.p. 104–1068C;
1H NMR (400 MHz, CDCl3): d=7.36 (d, 4H, J=8.8 Hz), 6.61 (d, 4H, J=
2.7 Hz), 6.58 (dd, 4H, J=8.8, 2.7 Hz), 3.26 (t, 16H, J=7.2 Hz), 2.92 (s,
8H), 1.56 (q, 16H, J=7.2 Hz), 1.36 (sextet, 16H, J=7.2 Hz), 0.96 ppm (t,
24H, J=7.2 Hz); 13C NMR (100 MHz, CDCl3): d=154.5, 147.2, 131.9,
125.5, 116.2, 112.0, 111.3, 100.3, 91.7, 88.9, 87.7, 80.0, 50.7, 32.6, 29.3, 20.3,
13.9 ppm; IR (neat): n˜ =2185 cmÀ1; UV/Vis (CH2Cl2): lmax (loge)=446
(4.58), 415 (sh, 447), 343 (sh, 4.87), 328 (5.13), 318 (5.13), 258 (4.61),
231 nm (4.88); ESI-MS: m/z (%)=1215 (77) [M+Ag, C80H92N4Ag], 1216
(68), 1217 (100), 1218 (70), 1219 (35), 1220 (12); HRMS: m/z calcd for
C80H92N4Ag: 1215.6373, found: 1215.6350.
[PdACHTREUNG(PPh3)4] (0.23 g, 0.2 mmol) and CuI (77 mg, 0.4 mmol) were then
added and the mixture was stirred at room temperature for 10 min. The
solution of tetrayne 1b in THF was then added dropwise and the mixture
was heated to 608C and stirred for 48 h. After cooling to room tempera-
ture, the contents of the flask were diluted with CH2Cl2 (150 mL). The
resulting solution was washed first with ice-cold 2n HCl (3”100 mL) and
then with saturated NH4Cl solution (3”100 mL). The organic layer was
dried over anhydrous Na2SO4 and the solvent was removed under re-
duced pressure. The crude product thus obtained was purified by column
chromatography on silica gel, eluting with diethyl ether/hexane (5:95,
v/v).
General procedure for the synthesis of 9a,b and 12: A solution of the tet-
Compound 5a: Yield: 0.72 g (40%) as a yellow solid from 1a (1.0 g,
2.0 mmol) and 4a[18,19,22] (3.36 g, 12.0 mmol); m.p. 180–1828C; 1H NMR
(400 MHz, CDCl3): d=7.31 (m, 8H), 7.05 (m, 8H), 2.78 (s, 8H),
0.04 ppm (s, 36H); 13C NMR (100 MHz, CDCl3): d=158.4, 132.5, 132.0,
127.9, 127.7, 125.9, 125.3, 103.6, 100.4, 98.6, 90.5, 89.4, 32.2, À0.02 ppm;
IR (neat): n˜ =2158 cmÀ1; UV/Vis (CH2Cl2): lmax (loge)=330 (sh, 4.54),
300 (4.65), 260 (4.90), 254 (4.88), 238 nm (4.92); ESI-MS: m/z (%)=999
(65) [M+Ag, C60H60Si4Ag], 1000 (40), 1001 (100), 1002 (48), 1003 (15);
HRMS: m/z calcd for C60H60Si4Ag: 999.2823, found: 999.2795; elemental
analysis calcd (%) for C60H60Si4: C 80.66, H 6.77; found: C 79.23, H 6.82.
rabromide
(50 mL), and THF (30 mL) was degassed by bubbling argon through it
for 10 min. [Pd(PPh3)4] (0.23 g, 0.2 mmol) and CuI (0.09 g, 0.46 mmol)
7 (1.0 g, 2.0 mmol) in toluene (50 mL), diisopropylamine
AHCTREUNG
were added and the resulting mixture was stirred for 10 min. A degassed
solution of the terminal acetylene (8a,b or 11; 10.0 mmol) in THF
(20 mL) was then added dropwise. The reaction mixture was stirred for
the specified duration at 608C under an argon atmosphere. After cooling
to room temperature, the mixture was diluted with CH2Cl2 (150 mL) and
washed successively with 2n HCl (3”200 mL) and saturated aqueous
NH4Cl solution (3”100 mL). The organic layer was dried over anhydrous
Na2SO4 and the solvents were removed under reduced pressure at room
temperature. The crude product was purified by column chromatography
on silica gel, eluting with CH2Cl2/hexane (1:19, v/v).
Compound 5b: Yield: 2.3 g (40.5%) as a yellow solid from 1a (2.5 g,
5.0 mmol) and 4b[6c,18] (10.85 g, 30.4 mmol); m.p. 140–1428C; 1H NMR
(400 MHz, CDCl3): d=7.52 (d, 4H, J=2 Hz), 7.44 (d, 4H, J=8.2 Hz),
7.32 (dd, 4H, J=8.2, 2 Hz), 2.96 (s, 8H), 1.34 (s, 36H), 0.25 ppm (s,
36H); 13C NMR (100 MHz, CDCl3): d=157.9, 151.1, 131.8, 129.3, 125.4,
124.8, 123.2, 104.2, 100.5, 97.7, 90.5, 88.8, 34.7, 32.1, 31.0, 0.04 ppm; IR
(neat): n˜ =2157 cmÀ1; UV/Vis (CH2Cl2): lmax (loge)=330 (sh, 4.72), 302
(4.83), 261 (5.06), 241 nm (5.09); ESI-MS: m/z (%)=1139 (97) [M+Na,
C76H92Si4Na], 1140 (100), 1141 (67), 1142 (33), 1143 (17); 1223 (63)
[M+Ag, C76H92Si4Ag], 1224 (65), 1225 (100), 1226 (80), 1227 (50), 1228
(25); HRMS: m/z calcd for C76H92Si4Ag: 1223.5327, found: 1223.5328;
elemental analysis calcd (%) for C76H92Si4: C 81.66, H 8.30; found: C
81.71, H 8.26.
Compound 9a: The reaction was carried out for 48 h. The product 9a
was obtained as
a dark-brown solid (1.18 g, 42%) from 7 (1.2 g,
2.3 mmol) and 8a[21] (2.83 g, 11.1 mmol); m.p. 152–1548C; 1H NMR
(400 MHz, CDCl3): d=8.59 (m, 4H), 7.43 (d, 4H, J=1.7 Hz), 7.31 (m,
8H), 7.21 (dd, 4H, J=8.2, 1.7 Hz), 1.23 (s, 36H), 0.11 ppm (s, 36H); 13C
NMR (100 MHz, CDCl3): d=151.3, 145.9, 134.3, 131.9, 129.2, 127.7,
127.3, 125.4, 125.1, 123.3, 104.1, 100.9, 98.0, 92.5, 91.8, 34.7, 31.0, 0.0 ppm;
IR (neat): n˜ =2160 cmÀ1; UV/Vis (CH2Cl2): lmax (loge)=423 (3.74), 359
(3.69), 237 nm (4.34); HRMS: ESI-MS m/z calcd for C84H92Si4Ag:
1319.5327, found: 1319.5317.
Compound 5c: Yield: 2.5 g (35%) as a reddish-orange solid from 1a
(2.5 g, 5.0 mmol) and 4c[18,20] (13.0 g, 30.4 mmol); m.p. 142–1448C; 1H
NMR (400 MHz, CDCl3): d=7.31 (d, 4H, J=9 Hz), 6.71 (d, 4H, J=
2.5 Hz), 6.55 (dd, 4H, J=9, 2.5 Hz), 3.29 (t, 16H, J=7.3 Hz), 2.92 (s,
8H), 1.59 (q, 16H, J=7.3 Hz), 1.37 (sextet, 16H, J=7.3 Hz), 0.99 (t,
24H, J=7.3 Hz), 0.25 ppm (s, 36H); 13C NMR (100 MHz, CDCl3): d=
147.2, 133.2, 125.9, 114.8, 112.8, 111.9, 104.9, 100.9, 96.7, 90.9, 87.4, 50.5,
32.2, 29.3, 20.2, 13.9, 0.11 ppm; IR (neat): n˜ =2193, 2157 cmÀ1; UV/Vis
(CH2Cl2): lmax (loge)=370 (4.84), 347 (4.87), 316 (4.83), 264 (5.01),
256 nm (5.01); ESI-MS: m/z (%)=1508 (55) [M+Ag, C92H128N4Si4Ag],
1509 (68), 1510 (100), 1511 (87), 1512 (70), 1513 (35); HRMS: m/z calcd
for C92H128N4Si4Ag: 1507.8267, found: 1507.8231.
Compound 9b: The reaction was carried out for 15 h. The product 9b
was obtained as a brown solid (1.2 g, 30%) from 7 (1.0 g, 1.93 mmol) and
8b (3.93 g, 11.6 mmol); m.p. 152–1558C; 1H NMR (400 MHz, CDCl3):
d=8.49 (m, 4H), 7.35 (d, 4H, J=8.1 Hz), 7.34 (s, 4H), 7.23 (m, 4H), 7.18
(dd, 4H, J=8.1, 2.0 Hz), 1.19 (s, 36H), 0.96 ppm (s, 84H); 13C NMR
(100 MHz, CDCl3): d=151.0, 146.2, 134.7, 132.4, 129.1, 127.6, 127.1,
125.6, 125.4, 123.1, 105.4, 101.1, 94.4, 93.1, 91.5, 34.8, 31.1, 18.7, 11.3 ppm;
IR (neat): n˜ =2153 cmÀ1; UV/Vis (CH2Cl2): lmax (loge)=417 (3.22), 348
(3.19), 241 nm (3.96); ESI-MS: m/z (%)=1655 (25) [M+Ag,
C108H140Si4Ag], 1656 (62), 1657 (100), 1658 (90), 1659 (45); HRMS: m/z
calcd for C108H140Si4Ag: 1655.9083, found: 1655.9037.
Compound 12: The reaction was carried out for 24 h. The product 12 was
obtained as a red solid (0.52 g, 24%) from 7 (1.0 g, 1.94 mmol) and 11[22]
(2.72 g, 11.6 mmol); m.p. 125–1288C; 1H NMR (400 MHz, CDCl3): d=
8.34 and 7.28 (AA’BB’, 8H), 5.89 (d, 4H, J=11.0 Hz), 5.78 (d, 4H, J=
11.0 Hz), 1.10 ppm (s, 84H); 13C NMR (100 MHz, CDCl3): d=144.8,
133.1, 126.3, 126.2, 119.0, 118.7, 102.8, 99.5, 99.3, 94.7, 90.6, 17.6,
10.2 ppm; IR (neat): n˜ =2140 cmÀ1; UV/Vis (CH2Cl2): lmax (loge)=426
(2.83), 366 (2.79), 348 (2.96), 325 (3.00), 293 (3.18), 277 (3.21), 264 nm
(3.20); ESI-MS: m/z (%)=1231 (50) [M+Ag, C76H100Si4Ag], 1232 (58),
1233 (100), 1234 (70), 1235 (35); HRMS: m/z calcd for C76H100Si4Ag:
1231.5953, found: 1231.5970.
General procedure for the synthesis of 6b,c: Solid K2CO3 (20 equiv) was
added to a solution of the precursor (5b or 5c; 1 g) in degassed MeOH
(50 mL) and CH2Cl2 (50 mL) and the mixture was stirred for 24 h at
room temperature under an argon atmosphere. A mixture of CH3CN
(75 mL) and pyridine (25 mL) was then added, followed by Cu-
ACHTREUNG(OAc)2·H2O (40 equiv). The reaction mixture was stirred at room tem-
perature for 3 days. It was then diluted with CH2Cl2 (200 mL) and
washed successively with 2n HCl (3”200 mL) and saturated NH4Cl solu-
tion (3”100 mL). The organic layer was dried over anhydrous Na2SO4
and the solvents were removed under reduced pressure. The crude prod-
uct was purified by column chromatography on silica gel, eluting with
CH2Cl2/hexane (1:9, v/v).
Compound 10a: Precursor 9a (0.4 g, 0.33 mmol) was dissolved in de-
gassed MeOH (50 mL) and CH2Cl2 (50 mL). K2CO3 (0.91 g, 6.6 mmol)
was added and the mixture was stirred at room temperature under an
argon atmosphere for 24 h. CH3CN (75 mL), pyridine (25 mL), and Cu-
Compound 6b: Yield: 0.35 g (48%), purple solid; 1H NMR (400 MHz,
CDCl3): d=7.42 (d, 4H, J=8.2 Hz), 7.36 (d, 4H, J=2 Hz), 7.30 (dd, 4H,
J=8.2, 2 Hz), 2.93 (s, 8H), 1.24 ppm (s, 36H); 13C NMR (100 MHz,
CDCl3): d=159.1, 151.6, 130.9, 127.4, 126.4, 126.2, 124.6, 100.1, 91.5, 90.7,
AHCTRE(UNG OAc)2·H2O (2.63 g, 1.32 mmol) were then added and the resulting mix-
ture was stirred at room temperature for 4 days. It was then diluted with
CH2Cl2 (200 mL) and washed successively with ice-cold 2n HCl (3”
200 mL) and saturated aqueous NH4Cl solution (3”100 mL). The organic
87.7, 80.9, 35.2, 32.9, 31.3 ppm; IR (neat): n˜ =2195 cmÀ1
;
UV/Vis
(CH2Cl2): lmax (loge)=379 (4.49), 277 (4.95), 230 nm (4.83); ESI-MS: m/z
Chem. Eur. J. 2007, 13, 3813 – 3821
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
3819

S. Sankararaman et al.
layer was dried over anhydrous Na2SO4 and the solvents were removed
under reduced pressure at room temperature. The crude product was pu-
rified by column chromatography on silica gel, eluting with CH2Cl2/
hexane (1:9, v/v). The product 10a was obtained as an orange solid
(0.15 g, 48%); m.p.>2408C (decomp); 1H NMR (400 MHz, CDCl3): d=
8.46 and 7.43 (AA’BB’ pattern, 8H), 7.32 (m, 12H), 1.23 ppm (s, 36H);
13C NMR (100 MHz, CDCl3): d=150.7, 145.5, 133.8, 130.1, 127.1, 126.2,
126.1, 125.1, 124.8, 123.5, 100.1, 92.5, 91.8, 86.8, 79.9, 33.9, 29.9 ppm; IR
(neat): n˜ =2189 cmÀ1; UV/Vis (CH2Cl2): lmax (loge)=454 (1.82), 286 nm
(2.47); ESI-MS: m/z (%)=1027 (80) [M+Ag, C72H56Ag], 1028 (62), 1029
(100), 1030 (84); HRMS: m/z calcd for C72H56Ag: 1027.3433, found:
1027.3398.
K2CO3 (4.04 g, 29.2 mmol) was added to a stirred solution of 4-tert-butyl-
1-triisopropylsilylethynyl-2-trimethylsilylethynylbenzene
(6.0 g,
14.6 mmol) in degassed MeOH (50 mL) and diethyl ether (50 mL) under
an argon atmosphere at room temperature. The mixture was stirred for
1 h and then the solvents were removed under reduced pressure at room
temperature. The crude product was poured into ice-cold water (100 mL)
and extracted with CH2Cl2 (3”50 mL). The combined organic layers
were dried with Na2SO4 and the solvent was removed under reduced
pressure at room temperature. The crude product was purified by column
chromatography on silica gel using hexane as the eluent to yield 8b
(4.55 g, 92%) as a red liquid. 1H NMR (400 MHz, CDCl3): d=7.50 (d,
1H, J=2.0 Hz), 7.40 (dd, 1H, J=8.2, 2.0 Hz), 7.30 (d, 1H, J=8.2 Hz),
3.22 (s, 1H), 1.30 (s, 9H), 1.13 ppm (s, 21H); 13C NMR (100 MHz,
CDCl3): d=151.2, 132.7, 129.7, 125.3, 125.1, 123.1, 105.5, 104.1, 97.2, 93.8,
34.6, 30.9, 18.8, 11.4, 0.00 ppm; IR (neat): n˜ =2155 cmÀ1; ESI-MS: m/z
calcd for C23H35Si: 339.2508, found: 339.2515.
Compound 10b: Precursor 9b (0.3 g, 0.19 mmol) was dissolved in de-
gassed THF (30 mL), and nBu4NF (0.3 g, 1.16 mmol) and MeOH
(10 drops) were added. The mixture was stirred for 2 h at room tempera-
ture under an argon atmosphere. After removal of the solvent under re-
duced pressure, the concentrated reaction mixture was added to ice-cold
water (100 mL) and extracted with CH2Cl2 (3”30 mL). The combined ex-
tracts were washed thoroughly with saturated brine (2”250 mL), dried
over anhydrous Na2SO4, and concentrated under reduced pressure to a
volume of 20 mL. The concentrate was deoxygenated by bubbling argon
through it. It was then added to a Schlenk flask containing a solution of
Acknowledgements
We thank the DAAD, New Delhi, for financial support for A.B. to carry
out part of the research at the Institute of Organic Chemistry, TU-
Braunschweig, Germany. Financial support from the CSIR, New Delhi, is
gratefully acknowledged. We thank the SAIF, IIT Madras, for spectral
data and Mr. G. Venkataramana for a sample of 8a.
CuACHTREUNG(OAc)2·H2O (1.54 g, 7.74 mmol) in a deoxygenated mixture of CH3CN
(75 mL) and pyridine (25 mL). After stirring at room temperature for
10 h, the mixture was diluted with CH2Cl2 (200 mL) and washed succes-
sively with ice-cold 2n HCl (3”100 mL) and saturated NH4Cl solution
(3”100 mL). The organic layer was dried over Na2SO4 and the solvent
was removed under reduced pressure to afford the crude product. It was
purified by column chromatography on silica gel, eluting with CH2Cl2/
hexane (1:9, v/v). 10b was obtained as a red solid (0.13 g, 74%); m.p.
>2308C (decomp); 1H NMR (400 MHz, CDCl3): d=8.54 and 7.45
(AA’BB’ pattern, 4H), 7.41 (s, 4H), 7.23 (s, 8H), 1.25 ppm (s, 36H); 13C
NMR (100 MHz, CDCl3): d=152.0, 146.5, 134.8, 129.7, 128.5, 128.3,
127.0, 125.7, 121.9, 101.2, 93.5, 87.5, 80.9, 34.8, 30.9 ppm; IR (neat): n˜ =
2208 cmÀ1; UV/Vis (CH2Cl2): lmax (loge)=461 (3.12), 386 (3.11), 288
(3.63), 243 nm (3.52); ESI-MS: m/z (%)=1027 (56) [M+Ag, C72H56Ag],
1028 (73), 1029 (100), 1030 (52); HRMS: m/z calcd for C72H56Ag:
1027.3433, found: 1027.3440.
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Synthesis of 13: Compound 13 was synthesized from precursor 12 (0.4 g,
0.35 mmol) according to the procedure described for the synthesis of 10b
from 9b. 13 was obtained as a red solid (80 mg, 40%); m.p.>2108C
(decomp); 1H NMR (400 MHz, CDCl3): d=7.59 and 7.02 (AA’BB’ pat-
tern), 6.41 (d, 4H, J=10.5 Hz), 6.14 ppm (d, 4H, J=10.5 Hz); 13C NMR
(100 MHz, CDCl3): d=151.1, 132.6, 129.0, 127.3, 121.5, 118.5, 99.9, 96.5,
92.9, 80.9, 80.7 ppm; IR (neat): n˜ =2923, 2853, 1050, 794 cmÀ1; UV/Vis
(CH2Cl2): lmax (loge)=376 (2.66), 334 (2.76), 242 nm (3.03); ESI-MS: m/z
(%)=603 (80) [M+Ag, C40H16Ag], 604 (26), 605 (82), 606 (30); HRMS:
m/z calcd for C40H16Ag: 603.0303, found: 603.0306.
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Compound 8b: A solution of 8a[21] (6.0 g, 17.0 mmol) in diisopropyl-
ACHTREUNG
AHCTREUNG
0.34 mmol) were added, and the resulting mixture was stirred for 10 min
at room temperature. Triisopropylsilylacetylene (4.17 mL, 18.5 mmol)
was then added and the mixture was stirred at room temperature for
12 h. The reaction mixture was then diluted with CH2Cl2 (100 mL) and
washed successively with ice-cold 2n HCl (3”100 mL) and saturated
NH4Cl solution (3”100 mL). The organic layer was dried over Na2SO4
and the solvents were removed under reduced pressure at room tempera-
ture. The crude product was purified by column chromatography on
silica gel using hexane as the eluent to yield 4-tert-butyl-1-triisopropylsily-
lethynyl-2-trimethylsilylethynylbenzene (6.51 g, 94%) as a red liquid. 1H
NMR (400 MHz, CDCl3): d=7.36 (d, 1H, J=1.8 Hz), 7.27 (d, 1H, J=
8.2 Hz), 7.13 (dd, 1H, J=8.2, 1.8 Hz), 1.16 (s, 9H), 1.05 (s, 18H), 0.98 (s,
3H), 0.15 ppm (s, 9H); 13C NMR (100 MHz, CDCl3): d=151.2, 132.7,
129.7, 125.3, 125.1, 123.1, 105.5, 104.1, 97.2, 93.8, 34.6, 30.9, 18.8, 11.4,
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0.00 ppm; IR (neat): n˜ =2155, 2062 cmÀ1
; ESI-MS: m/z calcd for
C26H42Si2Na: 433.2723, found: 433.2733.
3820
www.chemeurj.org
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Chem. Eur. J. 2007, 13, 3813 – 3821

Cross-Conjugated Bis-dehydroannulenes
FULL PAPER
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on an Enraf-Nonius CAD4 diffractometer by using MoKa radiation
(l=0.71073 Š) at 293 K. Crystal data: monoclinic; space group
C2/c;
a=23.5607(6),
b=8.2705(2),
c=28.4402(8) Š;
V=
5384.3(2) Š3; a=g=908, b=103.695(3)8; Z=8. Data collection: a
crystal of approximate dimensions 0.3”0.2”0.2 mm was used to
record 6984 intensities, 2qmax =28.778, completeness to q=28.778
was 99.9%. Structure refinement: the structure was refined aniso-
tropically by full-matrix least-squares on F2 to wR2=0.1095, R1=
0.0434 for 362 parameters and zero restraints. CCDC-622536 (6c)
and CCDC-621595 (13) contain 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.
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[12] Attempted conversion of 1a to 3a using DDQ failed. Dehydrogena-
tion proceeded only partially to yield the corresponding dihydro de-
rivative. See ref. [10].
[13] X-ray crystallographic data for 6c: Data were collected on an
Enraf-Nonius CAD4 diffractometer by using MoKa radiation (l=
0.71073 Š) at 293 K. Crystal data: monoclinic; space group C2/c;
a=25.5231(19), b=9.2830(5), c=31.004(2) Š; V=7155.4(8) Š3; a=
g=90, b=103.074(4)8; Z=4. Data collection: a crystal of approxi-
mate dimensions 0.3”0.2”0.2 mm was used to record 6305 intensi-
ties, 2qmax =258, completeness to q=258 was 100%. Structure refine-
ment: the structure was refined anisotropically by full-matrix least-
squares on F2 to wR2=0.3342, R1=0.1336 for 400 parameters and
24 restraints. X-ray crystallographic data for 13: Data were collected
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[15] See the Supporting Information of ref. [11] for the crystal structure
data of 2a.
[16] N. J. Turro, Modern Molecular Photochemistry, The Benjamin/Cum-
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Received: October 24, 2006
Published online: January 16, 2007
Chem. Eur. J. 2007, 13, 3813 – 3821
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
3821