Synthesis of dehydrobenzoannulenes with pyrene core

Article abstract of DOI:10.1007/s12039-012-0263-z

Synthesis of dehydrobenzoannulenes (DBAs) with pyrene core from 1,8-diethynylpyrene and 1,3,6,8-tetraethynylpyrene as building blocks is reported. A sequence involving Sonogashira coupling, Corey- Fuchs reaction and oxidative coupling (Eglinton coupling) is used for the synthesis of pyrene-based dehydrobenzoannulenes. Due to the presence of pyrenechromophore these DBAs and their precursors are highly fluorescent and emit in the visible region, due to extended conjugation of the acetylenic units with the pyrene core. Indian Academy of Sciences.

Full text of DOI:10.1007/s12039-012-0263-z

c
J. Chem. Sci. Vol. 124, No. 3, May 2012, pp. 597–607. ꢀ Indian Academy of Sciences.
Synthesis of dehydrobenzoannulenes with pyrene core
ANTONY JOSEPH, GANDIKOTA VENKATARAMANA
and SETHURAMAN SANKARARAMAN∗
Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036, India
e-mail: sanka@iitm.ac.in
MS received 9 January 2012; revised 11 February 2012; accepted 8 March 2012
Abstract. Synthesis of dehydrobenzoannulenes (DBAs) with pyrene core from 1,8-diethynylpyrene and
1,3,6,8-tetraethynylpyrene as building blocks is reported. A sequence involving Sonogashira coupling, Corey–
Fuchs reaction and oxidative coupling (Eglinton coupling) is used for the synthesis of pyrene-based dehy-
drobenzoannulenes. Due to the presence of pyrenechromophore these DBAs and their precursors are highly
fluorescent and emit in the visible region, due to extended conjugation of the acetylenic units with the
pyrene core.
Keywords. Pyrene; dehydrobenzoannulene; acetylene; fluorescence.
1. Introduction
hybrids9 and [2.2]paracyclophane-DBA hybrids have
been reported.10 Recently DBAs with triphenylene core
have been reported.11
Dehydrobenzoannulenes (DBAs), also known as ben-
zocyclynes have been known for a long time.1 They
are benzoannulenes in which the olefinic bonds are
replaced by acetylenic bonds and hence the name
dehydrobenzoannulenes. Initial interest in this class of
compounds emerged from the stabilization of dehy-
droannulenes by fusing an aromatic ring and the study
of tropicity (diamagnetic and paramagnetic) of the
annulenes. Recently DBAs have attracted great deal of
interest as a result of their applications as precursors
for new forms of carbon allotropes,2 as building blocks
and model systems for 2D carbon networks3 and their
potential application as optoelectronic materials.4 Inte-
rest in their aromatic/antiaromatic properties also con-
tinues as some of the DBAs serve as sensitive probes
for the study of tropicity of the annulene ring.5 The
synthesis of DBAs has been made facile with the deve-
lopment of Pd(0) catalysed C–C bond forming reac-
tions, especially those involving connection of sp2-sp
and sp-sp carbons.6 DBAs of different π-topologies
have been synthesized in an effort to understand the
structure property relationships.7 We have reported the
syntheses and structures of cross-conjugated DBAs
bearing Y-enediyne motifs.8 Most of the DBAs known
till date have a benzene or thiophene ring fused to the
dehydroannulene ring. Reports of DBAs with fusion
of higher polycyclic aromatic hydrocarbons are scarce.
Synthesis and tropicity of dimethyldihydropyrene-DBA
We report here the synthesis and spectroscopic pro-
perties of DBAs with a pyrene ring. Pyrene is a proto-
typical fluorescent polycyclic aromatic hydrocarbon.12
We have studied the effect of acetylenic conjugation
on the fluorescence emission of ethynylpyrenes.13 For
example 1,3,6,8-tetraethynylpyrene has been reported
for the first time from our laboratory,13 which can be
used as a building block in acetylenic scaffolds of larger
molecular systems.
2. Experimental
1,8-Diethynylpyrene (2)14 and iodides 3a–c15 were syn-
thesized by reported procedures and were characterized
by spectroscopic techniques. Reactions were carried out
under nitrogen atmosphere unless stated otherwise.
2.1 General procedure for the preparation of 4a–c
3a–c (2.4 mmol), [PdCl2(PPh3)2] (0.06 eq, 0.14 mmol)),
CuI (0.03 eq, 0.07 mmol), PPh3 (0.06 eq, 0.14 mmol)
were added to a degassed solution of diisopropylamine
(25 mL) and THF (20 mL). The reaction mixture was
heated at 50◦C for 30 min. The reaction mixture was
cooled to room temperature and then a solution of 1,8-
ethynylpyrene (2) (1.0 mmol) dissolved in degassed
∗For correspondence
597

598
Antony Joseph et al.
THF was added drop-wise using a syringe. After com- 1595, 1507, 1465, 1260 cm−1; UV-Vis (CH2Cl2) λmax
plete addition, the mixture was heated at 50◦C. Reac- (log ε) 440 (4.48), 422 (4.54), 365 (4.31), 341 (4.42),
tion was monitored by TLC. After completion of the 328 (4.45), 295 (4.56), 275 (5.86) nm;1H NMR
reaction, the mixture was cooled to room temperature (400 MHz, CDCl3) δ = 8.87 (s, 2H), 8.12 (q, J =
and poured into ice cold 2 N HCl (25 mL). The reac- 8.0 Hz, 4H), 8.05 (s, 2H), 7.13 (s, 2H), 7.04 (s, 2H),
tion mixture was extracted with CH2Cl2 (2 × 20 mL). 4.02 (m, 8H), 1.83 (m, 8H), 1.24 (m, 104H), 0.85 (m,
The organic layer was further washed with brine solu- 12H), 0.28 (s, 18H); 13C NMR (100 MHz, CDCl3):
tion and dried over anhydrous Na2SO4 and evaporated 149.1, 148.9, 131.4, 131.0, 127.6, 126.5, 124.7, 124.0,
the solvent under reduced pressure. Column chromato- 118.8, 118.6, 118.2, 116.3, 115.6, 103.9, 69.6, 94.4,
graphic purification of the crude product on silica gel 90.7, 69.0, 68.9, 31.6, 29.6, 29.5, 29.4, 29.4, 29.3,
with hexane/dichloromethane as eluent furnished pure 29.2, 29.1, 28.9, 28.8, 25.7, 25.6, 22.4, 13.8, 0.1;
4a–c.
MALDI-TOF MS (C106H162O4Si2) : 1578 [M++Na].
2.2 General procedure for the preparation of
dehydrobenzoannulenes 1a–c
2.1a 1,8-Bis-((4,5-bis(decyloxy)-2-((trimethylsilyl)-
ethynyl)phenyl)ethynyl)pyrene (4a): Yield 22% (0.27 g,
0.20 mmol from 1.46 g, 2.40 mmol of 3a), Waxy solid,
IR (neat): 2961, 2922, 2853, 2372, 2339, 2147, 1507,
1465, 1260 cm−1; UV-Vis (CH2Cl2) λmax (log ε) 440
(4.60), 420 (4.63), 365 (4.27), 341 (4.46), 327 (4.45),
275 (4.67), 251 (4.92) nm; 1H NMR (400 MHz, CDCl3)
δ = 8.89 (s, 2H), 8.23 (d, J = 8.0 Hz, 2H), 8.08 (d, J =
8.0 Hz, 2H), 7.99 (s, 2H), 7.14 (s, 2H), 4.01 (m, 8H), 1.8
(m, 8H), 1.27 (m, 56H), 0.85 (m, 12H), 0.31 (s, 18H);
13C NMR (100 MHz, CDCl3): 149.2, 149.1, 131.1,
130.9, 129.7, 127.5, 126.5, 124.6, 123.9, 118.8, 118.5,
118.3, 116.3, 115.7, 103.9, 96.6, 94.4, 90.7, 68.9, 31.6,
29.3, 29.3, 29.2, 29.1, 29.0, 28.9, 28.8, 25.7, 25.6,
22.4, 13.8, 0.7; MALDI-TOF MS (C82H114O4Si2) :
1218 [M+].
A solution of 4a–c (0.13 mmol) dissolved in degassed
THF (15 mL) was treated with n-Bu4NF (0.1 eq.) and
the resulting mixture was stirred at room temperature
for 30 min. Progress of the reaction was monitored by
TLC. After the reaction was complete the reaction mix-
ture was transferred to another round bottom flask that
contained Cu(OAc)2.H2O (3 eq.) dissolved in 10 mL
acetonitrile/pyridine (4:1) mixture. The reaction mix-
ture was stirred at room temperature for 6–8 h. Dur-
ing this period a yellow precipitate appeared. The reac-
tion mixture was neutralized with 5% aqueous HCl
and extracted with dichloromethane (20 mL ×2). The
organic layer was washed with water (15 mL × 2), dried
over Na2SO4 and solvent was evaporated to dryness.
Crude product was purified by column chromatogra-
phy on silica gel using hexane and dichloromethane as
eluent to yield 1a–c.
2.1b 1,8-Bis-((4,5-bis(dodecyloxy)-2-((trimethylsilyl)-
ethynyl)phenyl)ethynyl)pyrene (4b): Yield 19% (0.25 g,
0.19 mmol from 1.60 g, 2.40 mmol of 3b), waxy solid,
IR (neat): 2960, 2922, 2853, 2372, 2340, 2147, 1594,
1507, 1465, 1260, 1021 cm−1; UV-Vis (CH2Cl2) λmax
(log ε) 440 (4.66), 420 (4.71), 366 (4.42), 342 (4.57),
327 (4.57), 275 (4.75), 242 (5.01) nm; 1H NMR
(400 MHz, CDCl3) δ = 8.88 (s, 2H), 8.13 (q, J =
8.0 Hz, 4H), 8.06 (s, 2H), 7.13 (s, 2H), 7.04 (s, 2H),
4.03 (m, 8H), 1.80 (m, 8H), 1.27 (m, 72H), 0.85 (m,
12H), 0.28 (s, 18H); 13C NMR (100 MHz, CDCl3):
149.0, 148.9, 131.4, 130.9, 129.8, 127.6, 126.5, 124.7,
124.0, 118.7, 118.5, 118.2, 116.2, 115.5, 103.8, 96.6,
94.3, 90.7, 68.8, 68.9, 316, 29.5, 29.4, 29.3, 29.1,
28.9, 28.8, 25.7, 22.4, 13.8, 0.7; MALDI-TOF MS
(C90H130O4Si2) : 1330 [M+].
2.2a Dehydrobenzoannulene 1a: Yield 20% (0.03 g,
0.03 mmol from 0.15 g, 0.13 mmol of 4a), yellow
colour solid, mp 67◦C, IR (neat): 2923, 2852, 2373,
2339, 1591, 1506, 1466, 1249, 1206 cm−1; UV-Vis
(CH2Cl2) λmax (log ε) 450 (4.47), 425 (4.35), 410
(4.45), 370 (5.25), 307 (4.80), 295 (4.70) nm;1H NMR
(400 MHz, CDCl3) δ = 8.26 (s, 2H), 8.10 (AB, J =
8.0 Hz, 4H), 8.05 (s, 2H), 7.06 (s, 4H), 3.97 (t, J =
4.0 Hz, 4H), 3.87 (t, J = 4.0 Hz, 4H), 1.80 (m, 8H), 1.24
(m, 56H), 0.87 (m, 12 H); 13C NMR (100 MHz, CDCl3):
149.4, 148.7, 132.0, 130.6, 127.7, 127.3, 126.5, 124.1,
122.8, 118.3, 110.1, 117.0, 115.3, 97.1, 91.7, 82.9, 78.3,
69.1, 31.9, 29.7, 29.7, 29.6, 29.5, 29.4, 29.2, 26.0, 22.7,
14.1; MALDI-TOF MS (C76H96O4) : 1074 [M+].
2.1c 1,8-Bis-((4,5-bis(hexadecyloxy)-2-((trimethylsilyl)-
ethynyl)phenyl)ethynyl)pyrene (4c): Yield 18% (0.25 g,
0.16 mmol from 1.87 g, 2.40 mmol of 3c) waxy solid,
IR (neat): 2959, 2922, 2853, 2373, 2340, 2275, 2147,
2.2b Dehydrobenzoannulene 1b: Yield 12% (0.02 g,
0.01 mmol from 0.16 g, 0.13 mmol of 4b), yellow
colour solid, mp 72–74◦C, IR (neat): 2960, 2922,

Synthesis of dehydrobenzoannulenes with pyrene core
599
2852, 2372, 2339, 1592, 1506, 1466, 1249, 1206 cm−1; 94.8, 92.1, 35.6, 31.3 ppm, MS (70 eV, EI): m/z (%) =
UV-Vis (CH2Cl2) λmax (log ε) 450 (4.40), 425 (4.30), 570 (100) [M+], 277 (20).
410 (4.39), 371 (5.16), 307 (4.73), 295 (4.63) nm;
1H NMR (400 MHz, CDCl3) δ = 8.61 (s, 2H), 8.32
(AB, J = 8.0 Hz, 4H), 8.26 (s, 2H), 4.09 (m, 8H),
2.2e 1,8-Bis((2-(2,2-dibromovinyl)-5-t-butylphenyl)
1.90 (m, 8H), 1.25 (m, 72H), 0.86 (m, 12H); 13C NMR
ethynyl)pyrene (7): To a solution of PPh3 (1.84 g,
(100 MHz, CDCl3): 148.6, 148.2, 130.9, 130.3, 128.7,
126.8, 125.8, 123.9, 118.5, 117.6, 116.4, 115.8, 114.9,
93.4, 90.2, 82.2, 78.8, 68.3, 52.3, 30.9, 28.7, 28.6,
28.6, 28.5, 28.4, 28.3, 28.2, 28.1, 25.0, 21.6, 13.1;
MALDI-TOF MS (C84H112O4) : 1187 [M+].
7.0 mmol) in dry CH2Cl2 (35 mL) powdered zinc (0.5 g,
7.0 mmol) was added and cooled to 0◦C. CBr4 (2.33 g,
7.0 mmol) was added in small portions over 30 min.
The reaction mixture was brought to rt and contin-
ued the stirring for 1 h. It was again cooled to 0◦C
and 6 (0.5 g, 0.9 mmol) was added in small portions
over 30 min. Stirring was continued at rt for 8 h. The
2.2c Dehydrobenzoannulene 1c: Yield 8% (0.02 g, reaction mixture was filtered and solvent was evapo-
0.01 mmol from 0.19 g, 0.13 mmol of 4c), yellow colour rated to dryness. The crude product was purified by
solid, mp 74–76◦C, IR (neat): 2960, 2922, 2852, column chromatography on a silica gel using hexane
2372, 2339, 1592, 1506, 1466, 1249, 1206 cm−1; UV- to yield 7 (0.66 g, 85 %) as a pale yellow solid. Mp
Vis (CH2Cl2) λmax (log ε) 450 (4.25), 425 (4.16), 165◦C (dec), IR (neat): ν 2210 (C≡C) cm−1,1H NMR
410 (4.25), 370 (5.01), 310 (4.55), 295 (4.49) nm; (400 MHz, CDCl3): δ 8.79 (s, 2H), 8.22 (AB quartet,
1H NMR (400 MHz, CDCl3) δ = 8.59 (s, 2H), 8.27 (q, J = 7.84 Hz, 2H), 8.14 (AB quartet, J = 8.08 Hz,
J = 8.0 Hz, 4H), 8.20 (s, 2H), 7.25 (s, 2H), 7.33 (s, 2H), 8.08 (s, 2H), 8.06 (s, 2H), 7.81 (d, J = 8.3, 2H),
2H), 4.04 (m, 8H), 1.85 (m, 8H), 1.19 (m, 104H), 0.78 7.74 (d, J = 2.0 Hz, 2H), 7.45 (dd, J = 8.3, 2.0 Hz,
(m, 12H); 13C NMR (100 MHz, CDCl3): 149.9, 149.1, 2H), 1.4 (s, 18H) ppm, 13C NMR (100 MHz, CDCl3):
132.4, 131.0, 127.9, 127.5, 126.9, 124.6, 123.3, 118.8, δ 151.7, 135.9, 134.5, 131.9, 131.5, 130.6, 129.1, 128.1,
118.5, 118.4, 117.2, 115.7, 97.3, 91.9, 83.2, 78.4, 69.4, 127.6, 126.8, 125.6, 124.9, 124.2, 122.6, 118.3, 94.0,
32.1, 29.9, 29.8, 29.6, 29.5, 29.3, 29.2, 26.2, 22.8, 14.3; 93.6, 91.1, 34.8, 31.1 ppm, MS (70 eV, EI): m/z (%) =
MALDI-TOF MS (C100H144O4) : 1408 [M+].
878 (13), 880 (52), 882 (80), 884 (52), 886 (13) [Br
isotope peaks, M+], 800 (20), 802 (60), 804 (60), 806
(20) [M+-Br], 720 (10), 722 (20), 724 (10) [M+-2Br].
2.2d 1,8-Bis(2-formyl-5-t-butylphenylethynyl)pyrene
(6): A Schlenk flask was charged with 2-bromo-4-t-
butylbenzaldehyde (5) (2.2 g, 9.2 mmol), Pd(PPh3)2Cl2 2.2f 1,8-Bis((2-ethynyl-5-t-butylphenyl)ethynyl)pyrene
(0.26 g, 0.37 mmol), PPh3 (0.19 g, 0.8 mmol), CuI (8): An oven dried Schlenk flask was charged
(0.14 g, 0.8 mmol) and degassed Et3N (50 mL). The with 7 (0.4 g, 0.45 mmol) and dry hexane/toluene
mixture was stirred at room temperature (rt) for 15 min (2:1 v/v, 30 mL) and cooled to −50◦C. n-BuLi
and solid 1,8-diethynylpyrene (2) (1.0 g, 4.0 mmol) (1.6 M in hexane, 2.26 mL, 3.63 mmol) was added
was added. Stirring was continued at 55◦C for 3 h. The with stirring. After stirring at −50◦C for 30 min
reaction mixture was cooled to room temperature and the reaction was complete. It was quenched with
filtered. The filtrate was neutralized with 5% aqueous saturated NH4Cl at −50◦C extracted with CH2Cl2
HCl and extracted with CH2Cl2 (50 mL).The organic (30 mL). The organic layer was washed with water
layer was washed with water (2 × 30 mL) and dried (2 × 40 mL), dried over Na2SO4 and then solvent was
over Na2SO4 and then solvent was removed. The crude evaporated to dryness. The crude product was purified
product was purified by column chromatography on by column chromatography on silica gel using hexane
silica gel using CH2Cl2-hexane (20:80 v/v) to yield 6 to yield 8 (0.22 g, 85%) as a pale yellow solid. Mp
(1.92 g, 84%) as a yellow solid. Mp 228–230◦C, IR 178–180◦C (dec), IR (neat): ν 2200 (C≡C) cm−1, UV-
1
(neat): ν 2195 (C≡C), 1693 (C=O) cm−1, H NMR Vis (CH2Cl2): λmax (log ε) 432 (4.72), 408 (4.72), 332
(400 MHz, CDCl3): δ 10.81 (s, 2H), 8.80 (s, 2H), 8.28 (4.89), 316 (4.72), 231 (5.01) nm, 1H NMR (400 MHz,
(AB quartet, J = 8.0 Hz, 2H), 8.19 (AB quartet, J = CDCl3): δ 9.00 (s, 2H), 8.23 (AB quartet, J = 8.0 Hz,
8.0 Hz, 2H), 8.12 (s, 2H), 7.97 (d, J = 8.0 Hz, 2H), 2H), 8.13 (AB quartet, J = 8.0 Hz, 2H), 8.05 (s, 2H),
7.84 (d, J = 1.6 Hz, 2H), 7.57 (dd, J = 8.0, 1.6 Hz, 7.66 (d, J = 2.4 Hz, 2H), 7.65 (d, J = 7.6 Hz, 2H),
2H), 1.43 (s, 18H) ppm, 13C NMR (100 MHz, CDCl3): 7.0 (dd, J = 8.2, 2.0 Hz, 2H), 3.55 (s, 2H), 1.39 (s,
δ 191.6, 158.2, 133.9, 132.2, 132.0, 130.6, 128.7, 18H) ppm, 13C NMR (100 MHz, CDCl3): δ 151.6,
128.5, 128.0, 126.9, 126.8, 126.6, 125.6, 124.4, 118.1, 132.0, 131.7, 131.4, 129.9, 129.8, 127.9, 126.8, 126.1,

600
Antony Joseph et al.
125.0, 124.1, 123.6, 118.6, 94.1, 92.0, 83.4, 80.8, 34.8, ν 2185 (ν C≡C) cm-1, MALDI-TOF MS :m/z = 694
31.0 ppm.
[M+, C56H22], 1388 (2M+, π-dimer).
2.2g Dehydrobenzoannulene
1d: Cu(OAc)2.H2O 2.2j 1,3,6,8-Tetrakis(2-formyl-5-t-butylphenylethynyl)
(0.14 g, 0.72 mmol) was dissolved in a mixture of ace- pyrene (14): A mixture of 5 (0.40 g, 1.67 mmol),
tonitrile and pyridine (4:1 v/v, 7.5 mL) and 8 (0.1 g, Pd(PPh3)4 (0.077 g, 0.067 mmol), CuI (0.025 g,
0.2 mmol) was added. The reaction mixture was stirred 0.13 mmol) and Et3N (8.0 mL) was stirred at room
at rt for 4 h. and then neutralized with 5% aqueous temperature for 15 min. 1,3,6,8-tetraethynylpyrene
HCl followed by extraction with CH2Cl2 (20 mL). The (12) (0.1 g, 0.33 mmol) was added and stirring was
organic layer was washed with water (30 mL), dried continued at 50◦C for 4 h during which the reaction
over Na2SO4 and solvent was evaporated to dryness. mixture turned orange red. Solvent was removed under
The crude product was purified by column chromatog- reduced pressure and the crude product was purified
raphy on silica gel using hexane to yield 1d (0.055 g, by column chromatography on silica gel and eluted
55%) as a yellow solid. Mp >290◦C, UV-Vis (CH2Cl2): with a mixture of CH2Cl2 and hexane (1:2 v/v) to
λmax (log ε) 440 (4.74), 414 (4.54), 401 (4.46), 353 yield tetraaldehyde 15 (0.25 g, 81%) as an orange
(5.25), 304 (4.96) nm, IR (neat): ν 2180 (C≡C) cm−1, red fluorescent solid. Mp 210◦C (dec), IR (neat):
1H NMR (400 MHz, CDCl3): δ 8.87 (s, 2H), 8.31 ν 1683 (C=O), 2190 (C≡C) cm−1, 1H NMR (400 MHz,
(AB quartet, J = 8.0 Hz, 2H), 8.24 (AB quartet, CDCl3): δ 10.77 (s, 4H), 8.78 (s, 4H), 8.52 (s, 2H),
J = 8.0 Hz, 2H), 8.20 (s, 2H), 7.88 (d, J = 8.4 Hz, 7.92 (d, J = 8.4, 4H), 7.85 (d, J = 1.6 Hz, 4H),
2H), 7.86 (d, J = 1.9 Hz, 2H), 7.49 (dd, J = 8.4, 7.56 (dd, J = 8.4, 1.6 Hz, 4H) 1.45 (s, 36 H) ppm,
1.9 Hz, 2H), 1.43 (s, 18H) ppm, 13C NMR (100 MHz, 13C NMR (100 MHz, CDCl3): δ 190.5, 158.0, 135.3,
CDCl3): δ 152.5, 135.5, 132.8, 131.3, 129.2, 128.1, 134.2, 132.3, 130.0, 129.7, 128.5, 127.8, 126.6, 125.8,
127.8, 127.2, 125.4, 125.1, 124.6, 123.4, 121.5, 118.4, 118.8, 93.3, 92.9, 35.4, 31.0 ppm, MS (70 eV, EI): m/z
97.4, 92.7, 82.9, 79.1, 35.1, 31.1 ppm, MS (70 eV, EI): (%) = 938 (35%) [M+], 920 (40), 882 (95), 854 (100),
m/z (%) = 560 (100) [M+], HRMS: Calcd. for C44H34 826 (90).
560.25043; found 560.24924.
2.2k 1,3,6,8-Tetrakis((2-(2,2-dibromovinyl)-5-t-butyl-
2.2h 1,3,6,8-Tetrakis((2-ethynylphenyl)ethynyl)pyrene phenyl)ethynyl)pyrene (16): A mixture of 15 (1.0 g,
(14): A red solution of 13 (0.5 g, 0.5 mmol) in 1.0 mmol), CBr4 (5.67 g, 17.0 mmol) and PPh3 (4.5 g,
degassed THF (25 mL) was treated with n-Bu4NF 17.0 mmol) in 60 mL of benzene was refluxed for 8 h.
(0.013 g, 0.05 mmol) and the resulting mixture was The reaction mixture was brought to room temperature
stirred at rt for 15 min. The reaction mixture was then and filtered and the filtrate was passed through a short
poured into ice cold water (100 mL) and the red colour pad of silica gel using benzene as an eluent. Solvent
solid was filtered and repeatedly washed with water was evaporated to dryness and the solid obtained was
(2 × 100 mL) and dried. Due to its poor solubility washed with excess of acetone to get pure 16 (0.9 g,
NMR of 14 could not be obtained. Yield: 0.33 g (95%), 55%) as an orange red solid. Mp 262◦C (dec), IR
IR (neat): ν 2196 (C≡C) cm−1, MALDI-TOF MS: m/z (neat): ν 2195 (C≡C) cm−1, UV-Vis (CH2Cl2): λmax
698 [C56H26, M+].
(log ε) 482 (4.81), 453 (4.69), 353 (4.92) 250 (5.01)
nm, H NMR (400 MHz, CDCl3): δ 8.79 (s, 4H), 8.47
1
(s, 2H), 8.07 (s, 4H), 7.80 (d, J = 8.3 Hz, 4H), 7.75 (d,
J = 1.9 Hz, 4H), 7.47 (dd, J = 8.3, 1.9 Hz, 4H), 1.41
(s, 36H) ppm, 13C NMR (100 MHz, CDCl3): δ 151.9,
136.0, 134.9, 134.0, 132.7, 132.1, 129.4, 128.0, 127.5,
125.9, 122.5, 119.1, 94.9, 92.7, 91.4, 34.9, 31.2 ppm.
2.2i Bis-dehydrobenzoannulene 9: Cu(OAc)2.H2O
(0.23 g, 1.15 mmol) was dissolved in a mixture of
pyridine and ether (3:1 v/v, 100 mL). Compound 14
(0.1 g, 0.15 mmol) was finely suspended in 20 mL of
pyridine/ether (3:1 v/v) and added to the reaction mix-
ture using a dropping funnel over 3 h with continuous
stirring at rt. After complete addition, the reaction mix- 2.2l 1,3,6,8-Tetrakis((2,6-bis(2,2-dibromovinyl)-4-t-
ture was stirred at rt for 24 h. The red solid precipitated butylphenyl)ethynyl)pyrene(19): To a solution of PPh3
during course of the reaction was filtered and washed (2.4 g, 9.14 mmol) in dry CH2Cl2 (50 mL) powdered
with dil HCl (50 mL). The solid obtained was com- zinc (0.6 g, 9.14 mmol) was added and the mixture was
pletely insoluble and hence solution characterization cooled to 0◦C. CBr4 (3.03 g, 9.14 mmol) was added
could not be done. DBA 9 (0.10 g, 10%), IR (neat): portion-wise over 30 min. The reaction mixture was

Synthesis of dehydrobenzoannulenes with pyrene core
601
brought to rt and stirring was continued for 1 h. It was followed by oxidative coupling of the resulting termi-
cooled to 0◦C and 18 (0.3 g, 0.3 mmol) was added nal acetylenes using Cu(OAc)2 and pyridine was carried
portion-wise over 30 min. Stirring was continued at rt out in one-pot to yield DBAs 1a–c (scheme 2).
for 12 h. Reaction mixture was filtered and then solvent
was evaporated to dryness. The crude product was
purified by column chromatography on silica gel using
3.2 Synthesis of DBA 1d
THF/hexane (5:95, v/v) as eluent to yield 19 as a red
solid. Yield 60%; Mp: 245◦C (decomp), IR (neat): ν
Synthesis of 1d bearing tert-butyl group was accom-
2220 (C≡C) cm−1, UV-Vis (CH2Cl2): λmax (log ε) 460
plished following a slightly different route. Sono-
gashira coupling of diyne 2 with 2-bromo-4-t-
(4.94), 360 (5.10), 254 (5.44) nm, 1H NMR (400 MHz,
CDCl3): δ 8.81 (s, 4H), 8.48 (s, 2H), 8.07 (s, 8H),
7.85 (s, 8H), 1.40 (s, 36H) ppm, 13C NMR (100 MHz,
CDCl3): δ 151.4, 137.6, 136.1, 134.1, 131.9, 128.0,
127.5, 125.9, 124.0, 118.8, 98.3, 92.6, 92.2, 35.2,
31.0 ppm.
butylbenzaldehyde (5) under standard conditions pro-
ceeded smoothly and gave the desired dialdeyhde 6 in
84% yield as an yellow fluorescent solid (scheme 3). In
comparison to 3a–c bromoaldehyde 5 underwent cou-
pling in a facile manner and polymerization of 2 under
these conditions was minimum and the desired coupling
product was obtained in good yield. The aldehyde pro-
tons in 6 appeared as a singlet at δ 10.81 ppm in the
1H NMR spectrum. Dialdehyde 6 was converted to bis-
dibromovinyl derivative 7 by treating with CBr4, Ph3P
and Zn in DCM in 85% yield. The vinylic protons in
3. Results and discussion
The structures of the DBAs (1a–d) with pyrene
core, derived from 1,8-diethynylpyrene (2), are shown
in scheme 1. DBAs 1a–c with long alkoxy chains
were expected to exhibit discotic liquid crystalline
behaviour16 and hence were investigated.
1
7 appeared as a singlet at δ 8.06 ppm in the H NMR
spectrum. Treatment of 7 with LDA in THF yielded the
desired terminal acetylene 8 only in 10% yield. How-
ever, treatment of 7 directly with n-BuLi in a mixture
of hexane and toluene at −50◦C yielded diacetylene 8
in 85% as a pale yellow solid. The acetylenic protons
appeared as a singlet at δ 3.55 ppm in the 1H NMR spec-
trum of 8. Oxidative cyclization of 8 using Cu(OAc)2
and pyridine gave DBA 1d in good yield as a yellow
solid (scheme 3).
Pyrene protons, namely H-4/5 and H-9/10 appeared
as singlets in 4a–c, 8 and 1a–d. Peripheral protons H-
4/5 in DBAs 1a–d were deshielded by about 0.15 to
0.22 ppm when compared to the corresponding open
precursors 4a–c and 8, respectively. Intraannular pro-
tons H-9/10 in 1a–d were shielded by about 0.13 to
0.26 ppm when compared to that in 4a–c and 8, respec-
tively. Similarly, peripheral protons on the phenyl rings,
namely Ha and Hb (scheme 1), were deshielded in
DBAs 1a–d compared to those in open precursors
4a–c and 8. (table 1). These observations are consis-
tent with 1a–d being a closed loop of π−electrons in
comparison to 4a–c and 8. Compounds 1a–d can be
considered either as a dehydrobenzo[22]annulene or a
dehydrobenzo[18]- annulene, depending upon how the
π-electrons are counted on the pyrene ring.
3.1 Synthesis of DBA 1a–c
Synthesis of DBAs (1a–c) bearing dialkoxy groups
was accomplished by the Sonogashira coupling of 2
with 3a–c to yield 4a–c. Iodides 3a–c were sluggish
to undergo Sonogashira coupling and under the reac-
tion conditions 2 polymerized to yield an insoluble red
polymer which reduced the yields of the desired prod-
uct 4a–c. In situ deprotection of the silyl group in 4a–c
1
1
2
2
1
1
1
1
2
2
2
21
10
12 25
16 33
While 1d is a high melting solid, compounds 1a–c
are low melting due to the presence of long flexible
alkoxy chains. In fact, 1b and 1c are waxy solids. Struc-
tures 1a–c are isosceles trapezoid shaped that possess
a planar pyrene ring that is part of the rigid DBA
ring bearing long alkoxy chains. It was expected that
2
Scheme 1. Structure of DBAs (1a–d) synthesized from
1,8-diethynylpyrene (2)(systematic numbering of the pyrene
ring is shown).

602
Antony Joseph et al.
a
2
3a
3b
3c
10
12
16
21
25
33
4a−c
1a−c
Scheme 2. Synthesis of DBAs 1a–c. Reagents and reaction conditions: (a)
Pd(PPh3)2Cl2, Ph3P, CuI, i-Pr3N, 50◦C, 4–6 h, 4a 22%, 4b 19%, 4c 18%; (b) TBAF, THF,
rt, 1h; (c) Cu(OAc)2, py, CH3CN, rt, 12 h, 1a 10%, 1b 8%, 1c 8%.
a
2
b
6
7
c
t
d
d
8
Scheme 3. Synthesis of DBA 1d. Reagents and reaction conditions: (a) 2-bromo-4-t-
butylbenzaldehyde (5), Pd(PPh3)2Cl2, Ph3P, CuI, Et3N, 55◦C, 3 h, 84%; (b) CBr4, Ph3P, Zn,
CH2Cl2, rt, 8 h, 85%; (c) n-BuLi, hexane/toluene, –50◦C, 30 min, 85%; (d) Cu(OAc)2, py,
CH3CN, rt, 4 h, 55%.

Synthesis of dehydrobenzoannulenes with pyrene core
603
Table 1. Comparison of chemical shift (δH) values of various
protons of DBAs (1a–d) with their open precursors (4a–c and 8).
1a
4a
1b
4b
1c
4c
1d
8
H4/5 8.06 8.02 8.26 8.06 8.28 8.06 8.20 8.05
H9/10 8.28 8.93 8.62 8.88 8.66 8.87 8.87 9.0
Ha
Hb
7.06 7.09 7.30 7.05 7.32 7.04 7.88 7.65
7.07 7.17 7.37 7.14 7.40 7.13 7.86 7.66
such structural elements might impart discotic liquid solid (scheme 5). It was characterized by MALDI-TOF
crystalline properties to 1a–c. For example tripheny- MS which showed the molecular ion peak (M+) at m/z
lene derivatives bearing multiple long alkoxy chains are 698. It was immediately subjected to oxidative cycliza-
known to exhibit discotic liquid crystalline behaviour.14 tion under high dilution conditions using Cu(OAc)2
However, examination of 1a–c using a polarizing opti- and pyridine in ether gave 9 as a red solid, albeit in
cal microscope under both heating and cooling cycles poor yield. The solid was completely insoluble in all
did not exhibit any liquid crystalline state. These low the common organic solvents which hampered its char-
melting solids melted to a clear liquid state on heating acterization in solution. However, MALDI-TOF MS
and turned to a cloudy pasty mass on cooling.
clearly showed the molecular ion peak at m/z 694 along
with another peak at m/z 1388, presumably due to the
π-dimer of 9.
3.3 Attempted synthesis of multiply bridged
In an effort to make a more soluble derivative
of 9, target 10 was chosen with tertiarybutyl groups
substituted around the periphery. Sonogashira cou-
pling of 2-bromo-4-t-butylbenzaldehyde 5 with 1,3,6,8-
tetraethynylpyrene (12) gave the corresponding tetra
coupled product 15 in 81% yield as an orange red
fluorescent solid (scheme 6). The aldehyde protons
appeared as a singlet at δ 10.77 ppm and the pyrene pro-
tons appeared as two singlet at δ 8.78 ppm (H-4/5/9/10)
and 8.52 ppm (H-2/7) in the integration ratio of 2:2:1,
respectively, which is consistent with the symmetry of
the 1,3,6,8-tetra substituted derivative of pyrene. We
have demonstrated earlier the successful utilization of
Corey–Fuchs reaction for the conversion of polyalde-
hydes to the corresponding terminal alkynes in the
dehydrobenzoannulenes with pyrene core
Having accomplished the synthesis of DBAs 1a–d
we investigated the synthesis of multiply bridged
DBAs 9–11 (scheme 4). It was envisaged that 1,3,6,8-
tetraethynylpyrene (12) could serve as the building
block for the synthesis of DBAs 9–11.
3.4 Synthesis of DBAs 9-11
Precursor 13, whose synthesis has been described from
our laboratory,17 was used for the synthesis of 9. Treat-
ment of 13 with n-Bu4NF in THF yielded octaacety-
lene 14 in near quantitative yield as a red insoluble
t
t
t
t
t
Scheme 4. Structure of multiply bridged DBAs 9–11 and 1,3,6,8-tetraethynylpyrene 12.

604
Antony Joseph et al.
a
b
Scheme 5. Synthesis of DBA 9. Reagents and reaction conditions: (a) n-Bu4NF, THF, rt,
15 min, 95%; (b) Cu(OAc)2, py, ether, rt, 24 h, 10%.
synthesis of polyalkynes.18 In our experience the use various solvents and reaction conditions. All these
of Bestmann–Ohira reagent, namely dimethyl (1-azo-2- attempts gave a fairly complicated mixture of products
oxopropyl)phosphonate, for the conversion of polyalde- along with 17 due to incomplete elimination. Isolation
hydes to polyalkynes gave only poor yields. There- of pure 17 from the mixture proved impractical.
fore, we used Corey–Fuchs methodology in the present
In a similar manner we attempted the synthesis of
study for the conversion of polyaldehyde precursors to the precursor to 20 from the readily available octaalde-
poly terminal alkynes. Tetraaldehyde 15 was readily hyde derivative 18 which could be easily prepared in
converted to the corresponding tetrakis(dibromovinyl) good yields from the Sonogashira coupling of 12 and
derivative 16 using CBr4, and Ph3P in 55% yield. The 2-bromo-5-t-butylisophthalaldehyde (21), as described
vinylic protons appeared as a singlet at δ 8.07 along by our group earlier.15 The octakis(dibromovinyl)
with two singlets due to the pyrene protons at 8.79 and derivative 19 was obtained from octaaldehyde 18 in
1
8.47 ppm in the H NMR spectrum of 16. Efforts to 60% yield as a red solid using CBr4 (scheme 7). The
convert the octabromo derivative 16 to the correspond- symmetrical tetra substitution pattern on the pyrene was
ing tetraalkyne 17 failed. Part of the problem was the confirmed by the observation of two singlets for the
poor solubility of 16, especially at low temperatures. pyrene protons at δ 8.79 and 8.47 ppm in the 2:1 ratio
The reaction was tried with LDA and n-BuLi using in the 1H NMR spectrum of 19. Once again, attempts to

Synthesis of dehydrobenzoannulenes with pyrene core
605
3
Scheme 6. Attempted synthesis of precursor 17 to DBA 10. Reagents and reaction condi-
tions: (a) 2-bromo-4-t-butylbenzaldehyde (5), Pd(PPh3)4, CuI, Et3N, 50◦C, 4 h, 81%; (b) CBr4,
Ph3P, benzene, reflux, 18 h, 55%; (c) n-BuLi, hexane/toluene, –50◦C, 1 h; (d) LDA, THF –78◦C,
30 min; (e) LDA, toluene/hexane, –50◦C, 1 h.
convert 19 to 20 under various conditions failed. There- in 8 the vibrational fine structure is lost in compar-
fore, the synthesis of targets 10 and 11 could not be ison to 2 and the lowest energy absorption bands of
accomplished using the present approach. DBA 11 is 8 appeared broad. In comparison to 8, the absorption
not a planar molecule. It must be emphasized that the bands of 1d are further red shifted by about 10 nm and
challenge in the synthesis of target 11 is due to highly the bands are also sharper (figure 1). This might be due
strained bowl-shaped structure of this molecule. Never- to the rigid nature of the dehydrobenzoannulene skele-
theless, an attempt has been made to synthesize 11 that ton. Extension of conjugation of pyrene chromophore
is described in this report.
with alkynyl groups results in the shifting of the fluo-
rescence emission to visible region.13,17 A comparison
of the absorption and fluorescence spectra of 1d, 2 and
8 clearly reveals that the absorption and emission maxi-
ma are shifted to longer wavelengths with increasing
conjugation. Otherwise the features of the absorption
and emission spectra in figures 1 and 2 are reminiscent
of the spectra of ethynylpyrene derivatives reported ear-
lier.13,17 Absorption and fluorescence emission spectra
of 4a–c and 1a–c were similar to that of 8 and 1d,
respectively.
3.5 Absorption and fluorescence emission spectra of
DBAs and their precursors
Absorption spectrum of 2 showed vibrational fine struc-
ture that is typical of substituted pyrene derivatives
and absorption is confined to the UV region. Extension
of conjugation in 8 and 1d resulted in bathochromic
shift by nearly 50 nm into the visible region. However,

606
Antony Joseph et al.
t
t
t
t
t
t
t
t
t
t
t
t
Scheme 7. Attempted synthesis of precursor 20 to DBA 11. Reagents and reaction conditions:
(a) CBr4, Ph3P, Zn, CH2Cl2, rt, 12 h, 60%; (b) n-BuLi, hexane/toluene, –50◦C, 1 h; (c) LDA,
THF –78◦C, 30 min; (d) LDA, toluene/hexane, −50◦C, 1 h.
4000
2
1.6
1.2
0.8
0.4
0
448, 471
8
1d
3000
2000
1000
0
400, 417
2
1d
2
455, 476
8
350
450
550
650
250
300
350
400
450
500
Wavelength (nm)
Wavelength (nm)
Figure 2. Fluorescence emission spectra (10−5 M) of 2
(λex 360 nm), 8 (λex 409 nm) and 1d (λex 405 nm) in CH2Cl2.
Figure 1. Absorption spectra (10−5 M) of 2, 8 and 1d in
CH2Cl2.

Synthesis of dehydrobenzoannulenes with pyrene core
607
4. Conclusion
and Rubin Y 1992 Angew. Chem. Int. Ed. Engl. 31 1101;
(c) Diederich F 1994 Nature 368 199
3. For reviews (a) Haley M M 1998 Synlett 557;
(b) Marsden J A, Palmer G J and Haley M M 2003 Eur.
J. Org. Chem. 2355; (c) Bunz U H F, Rubin Y and Tobe
Y 1999 Chem. Soc. Rev. 28 107
4. (a) Baughman R H, Eckhardt H and Kertesz M 1987
J. Chem. Phys. 87 6687; (b) Marsden J A and Haley M M
2005 J. Org. Chem. 70 10213; (c) Sarkar A, Pak J J,
Rayfield G W and Haley M M 2001 J. Mater. Chem. 11
2943
5. (a) For a review, Mitchell R H 2001 Chem. Rev. 101
1301; (b) Laskoski M, Steffen W, Smith M D and Bunz
U H F 2001 Chem. Commun. 691; (c) Boydson A J,
Haley M M, Williams R V and Armantrout J R 2002
J. Org. Chem. 67 8812; (d) Boydson A J and Haley
M M 2001 Org. Lett. 3 3599
6. (a) Brandsma L, Vasilevsky S F and Verkruijsse H D
1999 Application of transition metal catalysts in organic
synthesis (eds) A de Meijere and F Diederich, Berlin:
Springer, 67, 179; (b) de Meijere A and Diederich F
2004 Metal-catalyzed cross-coupling reactions, Wein-
heim: Wiley-VCH
DBAs 1a–d with a pyrenecore have been synthesized
for the first time using a sequence of Sonogashira cou-
pling, Corey–Fuchs reaction and Eglinton coupling.
A comparison of the chemical shift values of vari-
ous pyrene protons of DBAs with that of their open
precursors clearly showed that the peripheral protons
H-4/5 were deshielded and the internal protons
H-9/10 were shielded in the DBAs, consistent with the
closed loop of π–electrons. DBAs 1a–c, bearing long
alkoxy groups did not exhibit any liquid crystalline
behaviour. In comparison to 2 the absorption and emis-
sion spectra of 1a–d, 4a–c and 8 were red shifted due to
extension of conjugation. DBA 9 was synthesized and
characterized by only mass spectrometry. MALDI-MS
data showed molecular ion peak corresponding to 9
along with a peak with double the m/z value of the
molecular ion, presumably due to the corresponding
π-dimer. Solution phase characterization could not be
done due to its insoluble nature in most common
organic solvents. Synthesis of polyacetylene precursors
17 and 20 for DBAs 10 and 11, respectively, was ham-
pered due to incomplete elimination of 16 and 19 under
a variety of conditions and also presumably due to the
instability of polyacetylenic derivatives 17 and 20 under
reaction and ambient conditions.
7. (a) For a recent review, Tobe Y and Sonoda M 2004
Modern cyclophane chemistry 1 1 R Gleiter and H Hopf,
(Eds), Weinheim: Wiley-VCH; (b) Haley M M, Brand S
C and Pak J J 1997 Angew. Chem. Int. Ed. Engl. 36 836;
(c) Wan W B and Haley M M 2001 J. Org. Chem. 66
3893
8. Bandyopadhyay A, Varghese B, Hopf
H
and
Sankararaman S 2007 Chem. Eur. J. 13 3813
9. (a) Kimball D B, Haley M M, Mitchell R H and Ward
T R 2001 Org. Lett. 3 1709; (b) Kimball D B, Haley
M M, Mitchell R H, Ward T R, Bandyopadhyay S,
Williams R V and Armantrout J R 2002 J. Org. Chem.
67 8798
Acknowledgements
We thank the Department of Science and Techno-
logy (DST) and the Council of Scientific and Indus-
trial Research (CSIR), New Delhi for financial support.
We also thank the Department of Chemistry and SAIF,
Indian Institute of Technology Madras for spectral
data.
10. (a) Boydston A J, Bondarenko L, Dix I, Weakley T J R,
Hopf H and Haley M M 2001 Angew. Chem. Int. Ed. 40
2986; (b) Heinrichs H, Fischer A K, Jones P G, Hopf H
and Haley M M 2005 Org. Lett. 7 3793
11. Takeda T, Fix A G and Haley M M 2010 Org. Lett. 12
3824
12. Winnik F M 1993 Chem. Rev. 93 587
13. Venkataramana G and Sankararaman S 2005 Eur. J. Org.
Chem. 4162
14. Leroy-Lhez S and Fages F 2005 Eur. J. Org. Chem. 2684
15. Johnson II C A, Lu Y and Haley M M 2007 Org. Lett. 9
3725
References
1. (a) Staab H A and Graf F 1966 Tetrahedron Lett. 16. (a) Chandrasekar S and Raganath G S 1990 Rep. Prog.
751; (b) Campbell J D, Eglinton G, Henderson W and
Raphael R A 1966 J. Chem. Soc. Chem. Commun. 87;
(c) Darby N, Cresp T M and Sondheimer F 1977 J. Org.
Phys. 53 57; (b) Boden N, Bushby R J, Cammidge A N,
El-Mansoury A, Martin P S and Lu Z 1999 J. Mater.
Chem. 9 1391
Chem. 42 1960; for a recent review Spitler E L, Johnson 17. Venkataramana G and Sankararaman S 2006 Org. Lett.
II C A and Haley M M 2006 Chem. Rev. 106 5344 8 2739
2. (a) Boese R, Matzger A J and Vollhardt K P C 1997 J. 18. Narayanan V, Sankararaman S and Hopf H 2005 Eur. J.
Am. Chem. Soc. 119 2052; for reviews (b) Diederich F Org. Chem. 2740