The photochemical synthesis of naphtho[2,3-g]thiopheno[3,2- e]benzo[b]thiophene: A new heterocyclic system

Article abstract of DOI:10.1055/s-1999-3557

The synthesis is described of the hitherto unknown 2,3-bis(2'- thienyl)naphthalene (1). Photochemical cyclisation of 1 affords naphtho[2,3- g]thiopheno[3,2-e]benzo[b]thiophene (2), the parent compound of a new aromatic heterocyclic series.

Full text of DOI:10.1055/s-1999-3557

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Peter Brooks,a Donato Donati,b Andrew Pelter,*a Fabio Poticellib
a Department of Chemistry, University of Wales Swansea, Singleton Park, Swansea, SA2 8PP, UK
Fax+44(1792)295747; Eꢀmail: A.Pelter@swansea.ac.uk
b Istituto di Chimica Organica, Pian dei Mantellini, 44-Siena, Italy
Fax+39(577)298037; Eꢀmail: donati@unisi.it
5HFHLYHGꢁꢂꢃꢁ0DUFKꢁꢄꢅꢅꢅ
tween ꢋ and 2-thienylcadmium chloride or 2-thienylzinc
chloride. Our experience in the synthesis of 1,2-bis(2’-
thienyl)benzene4 led us to predict that, due to steric fac-
tors, the introduction of the second thienyl unit to give the
hitherto unknown ꢁ might be difficult, and this proved to
be the case.1 After much experimentation it was found that
10 equivalents of 2-thienylzinc chloride reacted with ꢋ in
refluxing glyme with 5 mol% of (Ph3P)4Pd catalyst for 18
hours to give ꢁ in 74% yield, together with unreacted ꢋ
(19%). Thus ꢁ is now readily accessible for both electro-
chemical and photochemical investigations. It is interest-
ing to note that in the mass spectrum of ꢁ there is a large
M-2 peak, possibly due to the conversion of ꢁ to ꢄ.
$EVWUDFWꢇ The synthesis is described of the hitherto unknown 2,3-
bis(2´-thienyl)naphthalene ꢈꢁꢉ. Photochemical cyclisation of ꢁ af-
fords naphtho[2,3-J]thiopheno[3,2-H]benzo[E]thiophene ꢈꢄꢉ, the
parent compound of a new aromatic heterocyclic series.
.H\ꢀZRUGVꢇ dithienylation, palladium catalysis, photochemical cy-
clisation, novel symmetrical heterocyclic system
As part of an ongoing electrochemical investigation of the
effects of structure on electrochromism, a series of bis(2-
thienyl)naphthalenes have been prepared.1 Amongst these
is 2,3-bis(2´-thienyl)naphthalene ꢈꢁꢉ, a previously un-
known compound, the synthesis of which is outlined in
the Scheme. Commercially available 2-bromonaphtha-
lene-bis(hexachlorocyclopentadiene) adduct ꢂ was bro-
minated by a known procedure2 to the 2,3-dibromo adduct
ꢊ in 78% yield. The retro Diels–Alder reaction of ꢊ to give
2,3-dibromonaphthalene ꢋ is a known reaction2,3 but the
procedure outlined below affords an 86% yield (23%
more than the literature process) and is operationally
much simpler.
Irradiation of a solution of ꢁ and iodine in benzene gave
compound ꢄ, in accord with a known mechanism of aryl
photocoupling.5 The structure of highly symmetrical ꢄ
was initially assigned on the basis of NMR and mass spec-
troscopic data and it has now been confirmed by an X-ray
analysis, as shown in the Figure.
Cl6
Cl6
8
1
4
Br
Br
Br
8a
Br
Br
2
3
7
a
b
76%
86%
6
4a
5
5
Cl6
Cl6
4
3
5'
3'
1''
S
1'
1
2
S
S
4'
8
1
4
12b
12
11
8
3
4
8a
11a
2
3
12a
7
6
10
9
2'
3a
c
d
3b
74%
54%
2''
7a
6b
6a
4a
7
5
Ortep drawing of the structure of compound , showing
50% thermal ellipsoids for non-hydrogen atoms
5''
1
2
S
3''
6
5
4''
Reagents and conditions: (a) Br2 (2 equiv)/Fe; (b) 300 °C; (c) 2-thi-
enylzinc chloride (10 equiv)/(Ph3P)4Pd (0.1 equiv)/glyme, reflux; (d)
hn, I2/benzene
1
The assignment of H and 13C NMR spectra of this new
heterocyclic system was achieved on the basis of the fol-
lowing considerations. The number of signals in both the
13C and 1H NMR spectra indicated a two fold axis of sym-
metry in the molecule. Irradiation of the singlet at
d = 8.64, attributable to H-7(12), gave a NOE on the mul-
tiplet at d = 8.04, allowing us to assign this signal to H-
It was envisaged that the two thienyl units in ꢁ would be
introduced via a palladium catalysed cross coupling be-
Synthesis 1999, No. 8, 1303–1305 ISSN 0039-7881 © Thieme Stuttgart · New York

ꢁꢂꢃꢊ
P. Brooks et al.
6+257ꢀ3$3(5
300ºC for 10 min during which a large quantity of brown fumes
were evolved. The reaction mixture was removed from the Woods
metal bath, cooled under N2 and the damp solid produced was heat-
ed at 100ºC/1Torr using a Kugelrohr apparatus. A quantity of yel-
low liquid was removed and the dry, brown solid remaining was
heated in a sublimation apparatus at 140ºC/1Torr. This yielded ꢋ as
a white solid (2.930g, 86%); mp 138–140ºC (Lit3 mp 137–139ºC).
8(11) and the multiplet at d = 7.53 to H-9(10). Afterwards,
1
the H-13C heterocorrelation spectrum allowed signal at-
tribution of the secondary carbons of the thienyl moieties.
The remaining two CH signals (and consequently 1H sig-
nals through the heterocorrelation spectrum) were attrib-
1
uted on the basis of the -(CH) values:167.2 Hz for the
signal at d = 123.3 (C-3(4)), and 185.1 Hz for the signal at
d = 125.3 (C-2(5)), in agreement with the reported data of
several thiophene derivatives6.
HPLC:5.48 min in 95:5 MeOH/H2O on an Apex II ODS column at
a rate of 1.00 mL/min at 254 nm.
1H NMR: d = 7.49 [dd, 2 H, - = 6.30, 3.19, H-6(7)], 7.69 [dd, 2 H,
- = 6.17, 3.13, H-5(8)], 8.10 [s, 2 H, s, H-1(4)].
13C NMR: d = 121.9 [C-2(3)], 126.8, 127.2 [C-5(8), C-6(7)], 132.2
[C-1(4)], 133.0 [C-4a(8a)].
Compound ꢄ is the parent compound of a new aromatic
heterocyclic series, the naphtho[2,3-J]thiopheno[3,2-
H]benzo[E]thiophenes. There is an obvious similarity be-
tween ꢄ and ꢁ, but in ꢄ the thiophene rings are held rigidly
in the plane of the anthracene ring system, whereas in ꢁ
they can be rotated. We are currently examining both ꢁ
and ꢄ for their electrochromic properties and the compar-
ison should prove instructive.
IR (KBr): n = 3059w (C–H), 1486m (C = C arom), 1426m (C = C
arom), 888s (C–H b-naph), 756vs cm–1 (C–H RUWKR disubstituted
arom).
UV/Vis (MeOH): lmax (log10 e) = 231 (4.70), 275 (3.67), 286 nm
(3.64).
MS: m/z = 288 (M+, 31), 286 (M+, 69), 284 (M+, 34), 207 (25), 205
(25), 126 (100).
2-Bromonaphthalene-bis(hexachlorocyclopentadiene) adduct ꢂ,
Br2, iron powder, 1,1,2,2-tetrachloroethane, and (Ph3P)4Pd were
used as received from the suppliers. Thiophene was stirred over
CaH2 under argon for 3 d, distilled, and stored over 4Å molecular
sieves under argon. THF and glyme were stirred over CaH2 under
argon for 3 d, distilled, and then refluxed over sodium/benzophe-
none and used immediately upon collection. ZnCl2 was stored over-
night in an oven at 260ºC and fused using a Bunsen burner in a
porcelain crucible. All reagents used in air sensitive reactions were
dried by storing under vacuum over P2O5. All procedures involving
air sensitive reagents were performed in oven dried glassware as-
sembled whilst hot, sealed with septa, cooled in a slow stream of dry
argon, flame dried and cooled again. Transfer of reagents between
flasks was achieved via double ended needles using argon pressure,
or by graduated syringes, all reaction vessels being under a positive
pressure of argon. BuLi was purchased as a 2.5 M solution in hex-
anes and estimated prior to use.7 Chromatographic purifications
were carried out on columns of silica gel (30–70 micron mesh) un-
der medium pressure, using AnalaR petroleum ether (bp 40–60ºC)
or toluene as eluent. Melting points were recorded on a Gallenkamp
microscope block electrothermal apparatus and are uncorrected. 1H
and 13C NMR spectra were recorded on a 400 Bruker instrument at
400.1 MHz and 100.6 MHz respectively. NMR spectra were record-
ed in CDCl3 unless otherwise stated and are reported relative to
TMS with - values quoted in Hz. IR spectra were recorded on a Per-
kin Elmer 1725X FT-IR as KBr discs with relative absorbances de-
scribed as weak w, medium m, strong s or very strong vs. UV/
visible spectra were recorded on either a Perkin Elmer Lambda 9 or
a Philips PU8720 with absorption maxima and intensities reported
in nm and (log10 e). Mass spectra were obtained using either a VG
12-253 instrument operating in alternating chemical/electron im-
pact ionisation (ACE) conditions or a VG Quattro II instrument op-
erating in chemical ionisation or electron impact ionisation modes.
Accurate masses were recorded on a VG ZAB-E instrument operat-
ing in electron impact ionisation mode using manual peak matching
techniques.
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To a 100 mL round bottomed flask under argon and containing a
magnetic follower were added glyme (19 mL) and thiophene (1.0
mL, 1.05 g, 12.5 mmol) via dry syringes. The mixture was cooled
to 0ºC and BuLi (5.0 mL, 2.5 M, 12.5 mmol) in hexanes was added.
The cooling bath was removed and the reaction mixture allowed to
stir for 3.5 h at r.t. Freshly fused ground ZnCl2 (1.77 g, 13.0 mmol)
was placed under argon in a 100mL 2-necked round bottomed flask
containing a magnetic follower and connected to a spiral water con-
denser and a ground glass tap. Glyme (5 mL) was added via a dry
syringe and then the 2-thienyllithium solution was added via a can-
nula. The mixture was stirred at r.t. for 1h. A solution of (Ph3P)4Pd
(0.1429 g, 0.124 mmol) and ꢋ (0.3488 g, 1.22 mmol) dissolved in
glyme (15 mL) was added via a cannula to the 2-thienylzinc chlo-
ride solution and the mixture heated under reflux for 18 h, then al-
lowed to cool to r.t. and quenched with 3 M HCl (20 mL). Petroleum
ether (30 mL) was added and the two layers were separated. The
aqueous layer was washed with petroleum ether (2 x 30 mL) and
CH2Cl2 (4 x 30 mL). The combined organic layers were dried
(MgSO4), filtered and the solvent removed in vacuo to yield a dark
brown oil (1.3 g). The oil was pre-adsorbed onto silica gel(CH2Cl2),
placed on a silica gel column and eluted with toluene to yield an or-
ange oil, which in turn was pre-adsorbed onto silica gel (CH2Cl2),
placed on a silica column and eluted with petroleum ether to yield ꢋ
as a white solid (0.0673 g, 19% recovery) followed by ꢁ as a white
crystalline solid (0.2641g, 74.0%); mp 111ºC.
HPLC:13.0 min in 85:15 MeOH/H2O on an Apex II ODS column at
a rate of 1.00 mL/min at 280 nm.
1H NMR: d = 6.90 [dd, 2 H, - = 3.53, 1.12 Hz, H-3’(3’’)], 6.95 [dd,
2 H, - = 5.08, 3.54 Hz, H-4’(4’’)], 7.25 [dd, 2 H, - = 5.09, 1.15 Hz,
H-5’(5’’)], 7.44 [dd, 2 H, - = 6.30, 3.19 Hz, H-5(8) or 6(7)], 7.78
[dd, 2 H, - = 6.17, 3.13 Hz, H-6(7) or 5(8)], 7.94 [s, 2 H, H-1(4)].
13C NMR: d = 126.1, 126.9, 127.1, 127.8, 128.1, 130.2 [C-3’(3’’),
C-4’(4’’), C-5’(5’’), C-1(4), C-5(8), C-6(7)], 132.0, 132.7 [C-2(3),
C-4a(8a)], 142.7 [C-2’(2’’)].
The 2,3-dibromonaphthalene-bis(hexachlorocyclopentadiene) ad-
duct ꢊ was prepared by bromination of the 2-bromoadduct ꢂ accord-
ing to the literature procedure.2
IR (KBr): n = 3100w (C–H), 3057w (C–H), 1590m (C = C arom),
1488m (C = C arom), 889m (C–H b-naph), 750s (C–H RUWKR disub-
stituted arom), 703vs cm–1 (C–S).
ꢄꢅꢂꢆ'LEURPRQDSKWKDOHQHꢀꢈꢋꢉ
2,3-Dibromonaphthalene-bis(hexachlorocyclopentadiene) adduct
ꢊ2 (10.239g, 12.3mmol) was placed in a 25mL round bottomed flask
together with anti-bumping granules. The flask was flushed with
N2, heated from 100 to 300ºC in a Woods metal bath, and held at
UV/Vis (MeOH): lmax (log10 e) = 222 (4.28), 250 (4.38), 267 (4.47),
277 nm (4.54).
MS: m/z = 292 (M+, 100), 290 (21), 258 (37).
Synthesis 1999, No. 8, 1303–1305 ISSN 0039-7881 © Thieme Stuttgart · New York

6+257ꢀ3$3(5
The Photochemical Synthesis of Naphtho[2,3-J]thiopheno[3,2-H]benzo[E]thiophene
ꢁꢂꢃꢋ
HRMS: m/z found 292.0380 (M+), C18H12S2 requires 292.0380
(M+).
b = 105.010(4)°,
V = 1329.6(3)
Å3,
Dc = 1.451
gcm–3,
F(000) = 600, Mo-Ka radiation, l = 0.71073 Å. Intensity data were
collected with Siemens R3m/V. 2339 independent reflections mea-
sured, 1325 observed with I > 2 s(I). The structure was solved by a
direct method using SHELXTL PLUS. Anisotropic temperature
factors were used for non-hydrogen atoms. The positions of
hydrogen atoms were found by Difference Fourier map, and
refined isotropically. Final R = 0.0561 and wR2 = 0.1031
Anal. calcd for C18H12S2: C, 73.93; H, 4.14. Found C, 74.43; H,
4.34.
1DSKWKR>ꢄꢅꢂꢆJ@WKLRSKHQR>ꢂꢅꢄꢆH@EHQ]R>E@WKLRSKHQHꢀꢈꢄꢉ
A solution of ꢁ (0.130 g, 0.445 mmol) and I2 (0.120 g, 0.473 mmol)
dissolved in benzene (45 mL) was irradiated for 24 h in a quartz
tube with a water cooled 400 watt medium pressure mercury lamp
(Photochemical Reactors Ltd.). The mixture was washed with 0.1 N
Na2S2O4 solution and with H2O, then dried (Na2SO4), filtered and
evaporated in vacuo. The solid residue was fractionally sublimed
first at 45ºC/0.05 Torr to give the starting material (55 mg, 42%)
and then at 110 °C to give ꢄ after crystallization from cyclohexane
as yellow needles; mp 235–237 °C (40 mg, 54% yield based on the
unrecovered starting material).
[w–1 = s2(F)+(0.0496 P)2] where P = Max (Fo +2 * Fc2) / 3.
2
Lists of the fractional atomic co-ordinates, bond lengths and angles,
thermal parameters have been deposited at the Cambridge Crystal-
lographic Data Centre, UK as supplementary material.
$FNQRZOHGJHPHQW
We thank the EPSRC and DERA for financial support. The collec-
tion of X-Ray crystallographic data by the "Centro di Analisi e De-
terminazioni Strutturali" of the University of Siena is gratefully
acknowledged.
HPLC:5.68 min in MeCN on a HPLC-Technology columm packed
with Hypersil 5-ODS at a rate of 1.0 mL/min at 254 nm.
1H NMR: d = 7.53 (dd, 2 H, J = 6.50, 3.26, H-9(10)), 7.58 (d, 2 H,
J = 5.16, H-2(5)), 7.71 (d, 2 H, J = 5.16, H-3(4)), 8.04 (dd, 2 H,
J = 6.50, 3.26, H-8(11)), 8.64 (s, 2 H, H-7(12)).
13C NMR: d = 122.4 (C-7(12)), 123.3 (C-3(4)), 125.3 (C-2(5)),
125.8 (C-9(10)), 127.9 (C-8(11)), 126.0, 131.8, 133.3, 135.4 (C-
3a(3b), C-6a(12b), C-6b(12a), C-7a(11a)).
5HIHUHQFHV
(1) Brooks, P.; Pelter, A. Unpublished observations.
(2) LeHoullier, C. S.; Gribble, G. W. -. 2UJ. &KHP. ꢁꢌꢍꢂ, ꢆꢇ,
2364.
(3) Danish, A. A.; Silverman, M.; Tajima, Y. A. -.ꢁ$P.ꢁ&KHP.ꢁ
6RF. ꢁꢌꢋꢊ, ꢈꢉ, 6144.
IR (KBr): n = 3100w (C–H), 3057w (C–H), 1550m (C = C arom),
1485w (C = C arom), 875m (C–H), 750m, 725s cm–1.
(4) Pelter, A.; Jenkins, I.; Jones, D. E. 7HWUDKHGURQ ꢁꢌꢌꢎ, ꢊꢂ,
10357.
(5) Kellogg, R. M.; Groen, M. B.; Wynberg, H. -ꢋꢁ2UJꢋꢁ&KHP.
ꢁꢌꢏꢎ, ꢂꢌ, 3093.
(6) Gronowitz, S.; Johnson, I.; Hornfeldt, A. B. &KHPꢋꢁ6FU. ꢁꢌꢎꢋ,
ꢈ, 76.
(7) Furniss, B. S.; Hannaford, A. J.; Smith, P. W. G.; Tatchell,
A.R. 9RJHO¶Vꢁ7H[WERRNꢁRIꢁ3UDFWLFDOꢁ2UJDQLFꢁ&KHPLVWU\, 5th
Ed.; Longman: London, 1996.
UV/Vis (MeCN): lmax (log10 e) = 392 (3.54), 370 (3.70), 351 (3.64),
333 (3.54), 303 (4.87), 292 (4.80), 282sh (4.54), 223sh (4.53), 217
nm (4.62).
MS: m/z = 290 (M+,100), 245 (28), 145 (38)
HRMS: m/z Found 290.0230(M+), C18H10S2 requires 290.0224
(M+).
Anal. calcd for C18H10S2: C, 74.50; H, 3.40. Found C, 74.17; H,
3.63.
&U\VWDOꢀ'DWDꢀDQGꢀ6WUXFWXUHꢀ5HILQHPHQWꢀRIꢀ&RPSRXQGꢀꢄ
Crystal data: C18H10S2, Mw = 290.38, monoclinic, space group P21/
n, Z = 4, a = 15.944(1), b = 5.300(1), c = 16.290(1) Å,
Article Identifier:
1437-210X,E;1999,0,08,1303,1305,ftx,en;P01999SS.pdf
Synthesis 1999, No. 8, 1303–1305 ISSN 0039-7881 © Thieme Stuttgart · New York