Oligomerization of the Thiophene-Based p-Quinodimethanes 2,5-dihydrothiophene and 2-Ethylidene-5-methylene-2,5-dihydrothiophene

Article abstract of DOI:10.1021/jo961528i

Flash vacuum pyrolysis (FVP) of (5-methyl-2-thiophene-yl)methyl benzoate (8) produces in ca. 75% yield 2,5-dimethylene-2,3-dihydrothiophene, S-monomer (3). S-Monomer 3 is relatively stable dissolved in carbon disulfide-chloroform at -78°C. The structure of 3 is confirmed by its spectral properties. When a 0.17 M solution of S-monomer 3 was allowed to warm to room temperature, SS-dimer 5 ([2,2](2,5)thiophenophane, 14.7%), SSS-trimer 7 ([2,2,2](2,5)thiophenophane, 44.3%), and polymer were produced. A small amount (< 1%) of an SSSS-tetramer was detected by GC/MS. The mechanism proposed for the formation of these oligomers involves the combination of two molecules of 3 to give an intermediate diradical (11) that can close to form dimer 5 or react with additional molecules of 3 to form the higher oligomers. Evidence for the trapping of diradical 11 by 2,5-dimethylene-2,5-dihydrofuran (O-monomer 2) was obtained. Co-oligomerization of S-monomer 3 and O-monomer 2 gave four compounds containing the thiophene moiety: OS-dimer 16, SS-dimer 5, OSS-trimer 17, and SSS-trimer 7. Some OO-dimer 4 was produced but no OOO-trimer 6 was observed and only a trace of OOS-trimer 18 was detected. Additional support for the diradical mechanism was obtained from the study of the oligomerization of the methyl derivatives of 3, 2-ethylidene-5-methylene-2,5-dihydrothiophene (10, E and Z isomers), prepared by the FVP of (5-ethyl-2-thiophene-yl)methyl benzoate (9). Oligomerization of 10 gave several dimers and trimers including two acyclic dimers that are accounted for by intramolecular disproportionation.

Full text of DOI:10.1021/jo961528i

8980
J . Org. Chem. 1997, 62, 8980-8986
Articles
Oligom er iza tion of th e Th iop h en e-Ba sed p-Qu in od im eth a n es
2,5-Dim eth ylen e-2,5-d ih yd r oth iop h en e a n d
2-Eth ylid en e-5-m eth ylen e-2,5-d ih yd r oth iop h en e
Walter S. Trahanovsky,* Deborah Louise Miller, and Yili Wang
Department of Chemistry, Iowa State University, and Ames Laboratory, Ames, Iowa 50011-3111
Received August 6, 1996X
Flash vacuum pyrolysis (FVP) of (5-methyl-2-thiophene-yl)methyl benzoate (8) produces in ca. 75%
yield 2,5-dimethylene-2,3-dihydrothiophene, S-monomer (3). S-Monomer 3 is relatively stable
dissolved in carbon disulfide-chloroform at -78 °C. The structure of 3 is confirmed by its spectral
properties. When a 0.17 M solution of S-monomer 3 was allowed to warm to room temperature,
SS-dimer 5 ([2,2](2,5)thiophenophane, 14.7%), SSS-trimer 7 ([2,2,2](2,5)thiophenophane, 44.3%),
and polymer were produced. A small amount (<1%) of an SSSS-tetramer was detected by GC/MS.
The mechanism proposed for the formation of these oligomers involves the combination of two
molecules of 3 to give an intermediate diradical (11) that can close to form dimer 5 or react with
additional molecules of 3 to form the higher oligomers. Evidence for the trapping of diradical 11
by 2,5-dimethylene-2,5-dihydrofuran (O-monomer 2) was obtained. Co-oligomerization of S-
monomer 3 and O-monomer 2 gave four compounds containing the thiophene moiety: OS-dimer
16, SS-dimer 5, OSS-trimer 17, and SSS-trimer 7. Some OO-dimer 4 was produced but no OOO-
trimer 6 was observed and only a trace of OOS-trimer 18 was detected. Additional support for the
diradical mechanism was obtained from the study of the oligomerization of the methyl derivatives
of 3, 2-ethylidene-5-methylene-2,5-dihydrothiophene (10, E and Z isomers), prepared by the FVP
of (5-ethyl-2-thiophene-yl)methyl benzoate (9). Oligomerization of 10 gave several dimers and
trimers including two acyclic dimers that are accounted for by intramolecular disproportionation.
In tr od u ction
The heterocyclic p-QDMs based on furan (O-monomer
2) and thiophene (S-monomer 3) have also been prepared
by several different methods. Each has been fully
characterized and is known to give the cyclic dimer (4,
5), the cyclic trimer (6, 7), and polymer at moderate
temperatures.4-17
The S-monomer 3 can be prepared in good yield by the
flash vacuum pyrolysis (FVP) of (5-methyl-2-thiophene-
yl)methyl benzoate (8).13b,16 We have prepared 3 by the
FVP of 8 and studied its oligomerization. Also, in an
attempt to more fully understand the mechanism of the
dimerization and trimerization of 3, the oligomerization
During the past 50 years, considerable attention has
been focused on p-quinodimethanes (p-QDMs), a large
and important class of reactive molecules. In 1947
Szwarc reported for the first time that the pyrolysis of
p-xylene at low pressure leads to the formation of a white
polymeric material, and he proposed that this material
was formed by polymerization of the reactive intermedi-
ate p-xylylene (1), the benzene-based p-QDM.1 Ten years
later Errede and co-workers showed that solutions of
p-xylylene (1) could be prepared at low temperatures
(-78 °C).2
(4) Anderson, J . L. U.S. Patent 2,756,239, 1956.
(5) Winberg, H. E.; Fawcett, F. S.; Mochel, W. E.; Theobald, C. W.
J . Am. Chem. Soc. 1960, 82, 1428-1435.
(6) Kamenar, B.; Prout, C. K. J . Chem. Soc. 1965, 4838-4851.
(7) Gault, I.; Price, B. J .; Sutherland, I. O. J . Chem. Soc., Chem.
Commun. 1967, 540-541.
(8) Mizogami, S.; Osaka, N.; Otsubo, T.; Sakata, Y.; Misumi, S.
Tetrahedron Lett. 1974, 10, 799-802.
p-Xylylene (1), substituted p-xylylenes, and p-QDMs
based on other benzenoid aromatic systems including
naphthalene and anthracene have been prepared by a
number of different methods. These p-QDMs have been
fully characterized by NMR, IR, and UV-visible spec-
troscopic techniques, and their reactions have been
studied in detail.3
(9) Trahanovsky, W. S.; Park, M.-G. J . Org. Chem. 1974, 39, 1448-
1449.
(10) Kaupp, G. Angew. Chem., Int. Ed. Engl. 1976, 15, 442-443.
(11) Pahor, N. B.; Calligaris, M.; Randaccio, L. J . Chem. Soc., Perkin
Trans. 2 1978, 42-45.
(12) Ito, Y.; Miyata, S.; Nakatsuka, M.; Saegusa, T. J . Org. Chem.
1981, 46, 1043-1044.
(13) (a) Cassady, T. L. Ph.D. Dissertation, Iowa State University,
Ames, 1983. (b) Miller, D. L., M.S. Thesis, Iowa State University, Ames,
1984.
X Abstract published in Advance ACS Abstracts, J uly 15, 1997.
(1) (a) Szwarc, M. Nature 1947, 160, 403. (b) Szwarc, M. J . Chem.
Phys. 1948, 16, 128-136.
(14) Mu¨ngel, N.; Kesper, K.; Schweig, A.; Specht, H. Tetrahedron
Lett. 1988, 29, 6239.
(2) (a) Errede, L. A.; Landrum, B. F. J . Am. Chem. Soc. 1957, 79,
4952-4955. (b) Errede, L. A.; Hoyt, J . M. J . Am. Chem. Soc. 1960, 82,
436-439.
(3) (a) Williams, D. J .; Pearson, J . M.; Levy, M. J . Am. Chem. Soc.
1970, 92, 1436-1438. (b) Pearson, J . M.; Six, H. A.; Williams, D. J .;
Levy, M. J . Am. Chem. Soc. 1971, 93, 5034-5036.
(15) Chen, P.-S.; Chou, C.-H. J . Chin. Chem. Soc. (Taipei) 1992, 39,
251.
(16) Huang, C.-S.; Peng, C.-C.; Chou, C.-H. Tetrahedron Lett. 1994,
35, 4175.
(17) Shiea, J .; Wang, W.-S.; Wang, C.-H.; Chen, P.-S.; Chou, C.-H.
Anal. Chem. 1996, 68, 1062
S0022-3263(96)01528-9 CCC: $14.00 © 1997 American Chemical Society

Oligomerization of Thiopene-Based p-Quinodimethanes
J . Org. Chem., Vol. 62, No. 26, 1997 8981
Concerted or two-step diradical mechanisms could be
proposed for conversion of S-monomer 3 to its corre-
sponding SS-dimer 5. However, a thermal concerted [π6s
+ π6s] cycloaddition is not allowed by the Woodward-
Hoffmann rules18 and is thus unlikely. Moreover, forma-
tion of a large amount of SSS-trimer 7 is easily explained
by a diradical mechanism. We propose, therefore, the
mechanism presented in Scheme 1. In this mechanism
two molecules of S-monomer 3 combine to give diradical
11 which can close to give SS-dimer 5 or react with
another molecule of S-monomer 3 to give diradical 12,
the precursor of SSS-trimer 7. The production of a small
amount of SSSS-tetramer is consistent with diradical 12
reacting with a molecule of S-monomer 3 to give diradical
13 which can close to give an SSSS-tetramer. Each
diradical could either close to give an oligomer or react
with a molecule of S-monomer 3 to give the next higher
diradical. As the diradical becomes larger, closure to give
an oligomer should be less likely since it should be more
difficult for the two radical sites to encounter each other.
Apparently diradical 13 and the larger diradicals lead
primarily to polymer.
Attem p ted Tr a p p in g of Dir a d ica l In ter m ed ia te 11
or 12. Con ven tion a l Tr a p p in g Agen ts. In an at-
tempt to obtain evidence for a diradical mechanism for
dimerization and trimerization of S-monomer 3, a series
of trapping experiments was carried out. No evidence
for trapping of an intermediate diradical was obtained
with 1,4-cyclohexadiene, 9,10-dihydroanthracene, di-
methyl maleate, phenylsilane, or styrene. In these cases
the normal oligomerization products were obtained.
Several agents interfered with the oligomerization
reactions but still provided no evidence for an intermedi-
ate diradical. Thiophenol gave the 1:1 addition product
of thiophenol to S-monomer 3, and tri-n-butyltin hydride
gave several tin compounds. Neither thiophenol nor tri-
n-butyltin hydride, both good hydrogen atom donors, gave
14, the product expected from the addition of two
hydrogen atoms to diradical 11. Use of the stable
nitroxyl radical 2,2,6,6-tetramethylpiperidinyl-l-oxyl
(TEMPO)19 gave products which we were unable to
identify.
of the methyl derivative of 3, 2-ethylidene-5-methylene-
2,5-dihydrothiophene (10), prepared as a mixture of the
E and Z isomers by FVP of (5-ethyl-2-thiophene-yl)methyl
benzoate (9), was also studied. The results of this work
are reported herein.
Resu lts a n d Discu ssion
(5-Methyl-2-thiophene-yl)methyl benzoate (8) was pre-
pared in high yield by the reduction of 5-methyl-2-
thiophenecarboxaldehyde with lithium aluminum hy-
dride followed by the esterification of the resulting alcohol
with benzoyl chloride in the presence of triethylamine.
FVP of benzoate 8 at 650 °C and ca. 10-5 Torr produced
in ca. 75% yield S-monomer 3, which appeared as a
yellow band in the cold trap at -196 °C. Benzoic acid
and a small amount of a white polymer were also
produced. Carbon disulfide-chloroform was added to the
trap, and the mixture was then warmed to -78 °C. This
provided a solution of 3 which was relatively stable.
The structure of 3 was confirmed by its spectral
properties. The 1H and 13C NMR spectra agree with
those reported,16 and the IR and mass spectra, obtained
by GC/IR and GC/MS, are also consistent with structure
3.
When a 0.17 M solution of S-monomer 3 (0.323 mmol)
was allowed to warm to room temperature, SS-dimer 5
(0.0259 mmol, 14.7%), SSS-trimer 7 (0.0477 mmol,
44.3%), and polymer were produced. The structures of
products 5 and 7 are indicated by their spectral proper-
ties, which agree well with the available literature
data.5,12
Evidence for formation of an SSSS-tetramer with a
molecular weight of 440 was obtained by GC-MS. Quan-
titative GC showed 0.0042 mmol (0.68%, based on
amount of initial monomer) of the SSSS-tetramer.
A high-dilution experiment was carried out to explore
the possibility of obtaining a greater yield of SS-dimer
5. A preparative-scale pyrolysis (2.0 g) of benzoate 8 was
carried out with S-monomer 3 being dissolved in 1.025
L of carbon disulfide, which resulted in a 0.0066 M
solution of the S-monomer. After 5 days at room tem-
perature, the solution contained SS-dimer 5 and SSS-
trimer 7 in a mol ratio of 8.8:1.
When molecular oxygen, an excellent carbon radical
trap,20 was bubbled through solutions of S-monomer 3,
compound 15 was formed. This is a 2:2 O2:3 adduct
which has been characterized by C.-H. Chou16 and our
The formation of S-monomer 3 by FVP of the thiophene
benzoate 8 is explained by a mechanism proposed for the
furan analog, two [3,3] sigmatropic shifts followed by â
elimination of benzoic acid.9,13a
(18) Woodward, R. B.; Hoffmann, R. The Conservation of Orbital
Symmetry; Academic Press: New York, 1970.
(19) Ingold, K. U.; Walton, J . C. In Landolt-Bornstein: Reactical
Reaction Rates in Liquids; Fischer, M., Ed.; Springer-Verlag: New
York, 1994; Vol. 18c, pp 350-352.

8982 J . Org. Chem., Vol. 62, No. 26, 1997
Trahanovsky et al.
Sch em e 1
also result from OS-diradical 20, which could react with
S-monomer 3 to form either OSS-diradical 19 or 21, both
of which could close to give OSS-trimer 17. Because
O-monomer 2 is in higher concentration, one might
expect a comparable amount of OOS-trimer 18 to be
produced instead of just a trace. OS-Diradical 20 is
probably formed, but the rate of closure to give OS-dimer
16 must be faster than the rate of addition to another
monomer. This agrees with the results of the oligomer-
ization of O-monomer 2 alone which produces no or only
small amounts16 of OOO-trimer 6 in addition to OO-
dimer 4 and polymer. An explanation for the low yields
of OOO-trimer 6 is that once OO-diradical 22 is formed,
it rapidly closes to OO-dimer 4 rather than picking up a
third O-monomer 2 molecule. The polymer produced in
this reaction could be formed by an independent route
such as free radical polymerization. Another explanation
is that if OO-diradical 22 picks up a third O-monomer 2
1H NMR data match those reported. Separation of 15
was attempted, but it rearranged to other compounds
which we were unable to identify. Compound 15 comes
from the direct reaction of O2 and S-monomer 3 and gives
no evidence for the existence of diradical 11.
Tr a p p in g b y 2,5-Dim et h ylen e-2,5-d ih yd r ofu r a n
(2). Although the above radical trapping reactions failed,
trapping by O-monomer 2 was successful. O-Monomer
2 was selected as a potential trap of diradical intermedi-
ate 11 because it is similar both in size and reactivity to
S-monomer 3. O-Monomer 2 was prepared by FVP of
5-methylfurfuryl benzoate at 560 °C and ca. 10-5
Torr,9,13a,15 and the solution in CS2 was stored at -78 °C
overnight. The following day S-monomer 3 was prepared,
the solutions were mixed, and the relative concentrations
1
of the monomers in the mixture were determined by H
NMR spectroscopy. The mixture was allowed to react
at room temperature overnight until none of S-monomer
3 remained. However, due to the differences in reactiv-
ity, much of O-monomer 2 remained after all S-monomer
3 was gone. The excess O-monomer 2 was selectively
destroyed by the addition of acetic acid. The excess acid
was easily removed by basic workup without damage to
the oligomerization products. Four compounds contain-
ing the thiophene moiety were produced: OS-dimer 16,21
SS-dimer 5, OSS-trimer 17, and SSS-trimer 7. Relative
yields of each depended on the ratio of monomers used
and are summarized in Table 1. Some OO-dimer 4 was
also produced, but amounts varied due to the variation
in the length of time the solution of monomers was
allowed to stand before the acetic acid treatment to
destroy the excess O-monomer 2. It should be noted that
only a trace of OOS-trimer 18 was observed (by GC-MS)
and no OOO-trimer 616 was observed. The products were
separated by careful column chromatography on silica
gel with hexanes. OSS-Trimer 17 has not been previ-
ously reported, and its structure is indicated by its
spectral properties.
molecule, the resulting OOO-diradical 23 reacts with
another molecule of O-monomer 2 to form polymer faster
than closing to OOO-trimer 6.
In tr am olecu lar Dispr opor tion ation P r odu cts fr om
Oligom er iza tion of 2-Eth ylid en e-5-m eth ylen e-2,5-
d ih yd r oth iop h en e (10) As Evid en ce for a Dir a d ica l
In ter m ed ia te. In an attempt to provide direct evidence
for the diradical mechanism for the oligomerization of
Formation of OSS-trimer 17 provides strong support
for the diradical mechanism. Thus the following se-
quence involving the trapping of SS-diradical 11 accounts
for the formation of OSS-trimer 17. OSS-Trimer 17 could
(20) Scaiano, J . C.; J ohnston, L. J . Chem. Rev. 1989, 81, 521.
(21) Lee, A. W.; Keehn, P. M.; Ramos, S. M.; Rosenfeld, S. M.
Heterocycles 1977, 7, 81-104.

Oligomerization of Thiopene-Based p-Quinodimethanes
J . Org. Chem., Vol. 62, No. 26, 1997 8983
Ta ble 1. Su m m a r y of Yield s of P r od u cts in th e Co-oligom er iza tion of O-Mon om er 2 a n d S-Mon om er 3
run 3 6:1 ratioa
run 1 1:1 ratioa
rel yield, mol %b
run 2 3:1 ratioa
rel yield, mol %b
abs yield,
mgc
abs yield,
rel yield,
mol %b
d
product
%
OS-dimer 16
SS-dimer 5
OSS-trimer 17
SSS-trimer 7
34.2
24.1
10.4
31.3
56.6
12.6
12.6
18.2
44.1
25.6
7.2
18.1
19.5
3.8
58.3
31.3
6.2
5.1
3.8
4.2
a
b
Ratio of O-monomer 2 to S-monomer 3. Relative yields are moles per 100 moles of products based on only the OS, SS, OSS, and SSS
products and were determined by GC analysis (response factors proportional to weight were assumed). c Absolute yields in run 3 were
determined by GC using biphenyl as an internal standard. Absolute yield based on moles of S-monomer 3 available (this assumes a
d
75% yield from benzoate 8).
Ta ble 2. Su m m a r y of Oligom er iza tion P r od u cts of
2-Eth ylid en e-5-m eth ylen e-2,5-d ih yd r oth iop h en e (10)a
Sch em e 2
rel yield,b
product
%
SS-cyclic dimers 24a (cis, trans-A, trans-B)c
SS-cyclic dimers 24b (cis, trans-A, trans-B)c
SS-acyclic dimers 25a d
37
20
43
SS-acyclic dimers 25bd
SSS-cyclic trimers 26a (six possible stereoisomers)e
SSS-cyclic trimers 26b (two possible stereoisomers)e
a
A 0.05 M of 10 (a 3:1 mixture of E and Z or Z and E isomers
a 1:1 mixture of CS2:CDCl3 as solvent was oligomerized.
thiophene-based p-QDMs, we focused attention on the
trapping of diradical intermediate 11. Of the trapping
agents we used, only O-monomer 2 trapped the diradical.
The other trapping agents failed probably because (a) the
diradical intermediate is too reactive to be trapped by
the trapping agent or (b) some of the trapping agents,
such as oxygen and thiophenol, react directly with the
very reactive S-monomer 3. In an attempt to gain more
evidence for an intermediate diradical in the oligomer-
ization of thiophene-based p-QDMs, it was decided to
study 10. Other studies have shown that diradicals with
in
Isolated yields after flash chromatography. c The mixture of
b
isomers was isolated and identified by 1H NMR, 13C NMR, and
d
GC/MS. The mixture of isomers was isolated and identified by
1H NMR, COSY, 13C NMR, and GC/MS. The ratio of two acyclic
dimers, 25a and 25b, is ca. 2.2:1 on the basis of 1H NMR analysis.
e The mixture of isomers was isolated and identified by 1H NMR
and GC/MS.
R-methyl groups can give rise to intramolecular dispro-
portionation products.20,22 Observation of such a product
from 10 would be good evidence for the existence of a
diradical intermediate.
Compound 10 was prepared by FVP of (5-ethyl-2-
thiophene-yl)methyl benzoate (9) which was obtained in
high yield as shown in Scheme 2.
About 40% of 10 was obtained by FVP of 9 at 680 °C
and ca. 10-5. Product 10 appeared as a dark yellow band
in the liquid nitrogen trap at -196 °C. Benzoic acid and
white polymer were also produced.
1
The H NMR spectrum of compound 10 shows that it
NMR experiment23 using Cr(acac)3 in CDCl3 was per-
formed on the mixture. Analysis of these data showed
16 singlets in the thiophene carbon region (δ 159.8-
147.9). This is consistent with the conclusion that each
constitutional isomer exists in three stereoisomeric forms,
one cis and two trans isomers. The parent dimer 5 has
been shown to exist in the stepped anti conformation.6,11
The 1H NMR signal for the ethano bridge of 5 is an
AA′BB′ pattern. Analysis of the temperature dependence
exists as two isomers, the E and Z isomers, in a ratio of
ca. 3:1 (stereochemistry not assigned). When a ca. 0.05
M solution (1:1 CS2:CDCl3) of S-monomer 10 was allowed
to warm to room temperature, SS-cyclic dimers 24a and
24b, acyclic dimers 25a and 25b, and SSS-cyclic trimers
26a and 26b were produced. SSS-Acyclic trimers were
not observed. The results are summarized in Table 2.
Formation of acyclic dimers 25a and 25b strongly
support the proposed diradical mechanism.20,22 Scheme
3 accounts for the formation of 24a , 24b, 25a and 25b,
and Scheme 4 accounts for the formation of 26a , 26b.
In an attempt to further understand the composition
of the mixture of dimers 24a and 25b, a quantitative 13C
1
of these H NMR spectra of 5 shows that the barrier to
interconversion of these forms is larger than 27 kcal/
mol.11 Thus, a mixture of 24a and 24b should consist of
the six isomers shown in Chart 1.
(22) (a) Bergman, R. G. In Free Radicals; Kochi, J ay K., Ed.; J ohn
Wiley and Sons: New York, 1973; Vol. I, p 231. (b) Leung, M.-k.;
Trahanovsky, W. S. J . Am. Chem. Soc. 1995, 117, 841.
(23) Wehrli, F. W.; Marchand, A. P.; Wehrli, S. Interpretation of
Carbon-13 NMR Spectra, 2nd ed.; J ohn Wiley & Sons: New York, 1988.

8984 J . Org. Chem., Vol. 62, No. 26, 1997
Trahanovsky et al.
Sch em e 3
Sch em e 4
Ch a r t 1
Laboratory, Eagle Harbor, MI, and Galbraith laboratories,
Knoxville, TN. Thiophene, ethyl bromine, n-butyllithium (n-
BuLi), N,N-dimethylformamide (DMF), lithium aluminum
hydride (LiAlH4), triethylamine, 5-methyl-2-thiophenecarbox-
aldehyde, and benzoyl chloride (BzCl) were purchased from
Aldrich Chemical Co.
(5-Meth yl-2-th iop h en e-yl)m eth yl Ben zoa te (8). To a
stirred slurry of 1.774 g (0.0467 mol) of LiAlH4 in 150 mL of
dry ether at 0 °C was slowly added a solution of 11.77 g (0.0933
mol) of 5-methyl-2-thiophenecarboxaldehyde in 50 mL of dry
ether. The mixture was stirred at room temperature for 30
min. A standard workup26 gave 10.86 g (0.0847 mol, 92%) of
5-methyl-2-thiophenemethanol: 1H NMR (CDCl3, 300 MHz)
δ 6.68 (d, J ) 3 Hz, 1 H), 6.50 (m, 1 H), 4.64 (s, 2 H), 2.46 (s,
3 H) [lit.27 1H NMR δ 6.65 (2 H), 4.52 (s, 2 H), 2.18 (s, 3 H)].
A solution of 12 mL (0.103 mol) of BzCl in 60 mL of dry
ether was added dropwise over a 50-min period to a stirred
solution of 10.86 g (0.086 mol) of 5-methyl-2-thiophenemetha-
nol and 12 mL of distilled triethylamine in 150 mL of dry ether.
After the solution was stirred at room temperature overnight
(ca. 20 h), 100 mL of distilled water was added and the layers
were separated. The aqueous layer was extracted with ether
(4 × 25 mL). The combined ether layers was washed succes-
sively with 1 M hydrochloric acid (HCl) (3 × 50 mL), saturated
sodium bicarbonate (NaHCO3) (3 × 50 mL), and saturated
sodium chloride (NaCl) (3 × 50 mL). After drying (MgSO4)
and concentrating, 18.7 g (0.081 mol) of 8 was recovered (95%)
(bp 124 °C, 0.45 mmHg): IR (thin film) 3080, 2960, 2930, 1725,
Con clu sion
It is concluded that the mechanism of the oligomer-
ization of thiophene-based p-quinodimethane 2,5-di-
methylene-2,5-dihydrothiophene (3) involves the initial
combination of two molecules of the monomer to give an
intermediate diradical (11) that can close to form dimer
5 or react with additional molecules of 3 to produce
higher oligomers. This mechanism is supported by
successful trapping of 11 by 2,5-dimethyl-2,5-dihydrofu-
ran (2) and the formation of intramolecular dispropor-
tionation products in the oligomerization of the methyl
derivative of 3, 2-ethylidene-5-methylene-2,5-dihydro-
thiophene (10).
(24) (a) Trahanovsky, W. S.; Ong, C. C.; Pataky, J . G.; Weitl, F. J .;
Mullen, P. W.; Clardy, J . C.; Hansen, R. S. J . Org. Chem. 1971, 36,
3575. Commercial apparatus is available from Kontes Scientific
Glassware, Vineland, NJ . (b) For review, see: Brown, R. C. F. Pyrolysis
Methods in Organic Chemistry; Academic: New York, 1980; Chapter
2.
(25) Chou, C.-H.; Trahanovsky, W. S. J . Org. Chem. 1986, 51, 4208.
(26) Fieser, L. F.; Fieser, M. Reagents for Organic Synthesis; J ohn
Wiley and Sons: New York, 1967; Vol. I, p 584.
Exp er im en ta l Section
Meth od s a n d Ma ter ia ls. The pyrolysis apparatus24 and
some general methods25 have been described previously.
Elemental analyses were carried out by Spang Microanalytical
(27) Lozanova, A. V.; Moiseenkov, A. M.; Semenovskii, A. V. Bull.
Acad. Sci. USSR, Div. Chem. Sci. 1981, 30, 619-623.

Oligomerization of Thiopene-Based p-Quinodimethanes
J . Org. Chem., Vol. 62, No. 26, 1997 8985
1
1610, 1455, 1270, 1100, 800, 710 cm-1; H NMR (CDCl3, 300
13,14-Dit h ia t r icyclo[8.2.1.13,6]t et r a d eca -4,6,10,12-t et -
r a en e ([2.2](2,5)th iop h en op h a n e, SS-d im er 5): mp and H
1
MHz) δ 8.08 (m, 2 H), 7.40 (m, 3 H), 6.90 (d, J ) 3.6 Hz, 1 H),
6.56 (m, 1 H), 5.37 (s, 2 H), 2.44 (s, 3 H); HRMS calcd for
C13H12O2S 232.05630, found 232.05581. Anal. Calcd: C,
67.22; H, 5.21; S, 13.80. Found: C, 67.22; H, 5.25; S, 13.68.
NMR spectrum are in accord with published data;12 13C NMR
(CDCl3, 75.45 MHz) δ 151.2, 126.8, 32.2; GC-MS m/e (relative
intensity) 222 (0.58), 221 (2), 220 (15), 112 (5), 111 (8), 110
(100.00), 109 (9), 84 (5), 77 (5), 66 (15), 58 (5) [lit.12 mass
spectrum m/e (relative intensity) 220 (24), 110 (100)]; HRMS
calcd for C12H12S2 220.0380, found 220.03818.
(5-Eth yl-2-th iop h en e-yl)m eth yl Ben zoa te (9). To a
stirred solution of 5.26 g (0.0625 mol) of thiophene in 9:1 THF
and HMPA (100 mL) at -78 °C was slowly added 1.98 M (35
mL) n-BuLi. After the solution was stirred for 2 h, 4.7 mL of
distilled ethyl bromide was added to the same flask. The
reaction mixture was stirred at room temperature overnight.
After the reaction was complete, HCl (0 °C) was added to the
reaction mixture until the solution turned yellow. The solution
was transferred to a separatory funnel, and the organic layer
was extracted with saturate NaHCO3 (2 × 50 mL) and
saturated NaCl (2 × 50 mL). After drying (MgSO4) and
concentrating, 5.66 g (0.051 mol) of 2-ethylthiophene was
recovered (81%). The bp28 and 1H NMR29 spectrum are in
accord with published data.
To 4.0 g (0.0357 mol) of distilled 2-ethylthiophene in 9:1 THF
and HMPA (100 mL) at -78 °C was slowly 1.9 M (23 mL) of
n-BuLi added. After the solution was stirred for about 45 min,
28 mL of DMF was added to the solution. The reaction
mixture was allowed to slowly warm to room temperature and
stirred overnight. A standard workup gave 4.0 g (0.0285 mol)
of 5-ethyl-2-thiophenecarboxaldhyde (80%). Pure aldehyde
19,20,21-Tr it h ia t et r a cyclo[14.2.1.13,6.19,12]h en eicosa -
4,6,10,12,16,18-h exa en e ([2.2.2](2,5)th iop h en op h a n e, SSS-
tr im er 7): mp and 1H NMR spectrum are in accord with
published data;12 13C NMR (CDCl3, 75.45 MHz) δ 140.3, 124.4,
31.2; GC-MS m/e (relative intensity) 332 (0.83), 331 (1), 330
(9), 222 (0.5), 221 (2), 220 (9), 112 (5), 111 (8), 110 (100), 84
(5), 77 (3), 66 (13) [lit.12 mass spectrum m/e (relative intensity)
330 (100), 220 (36), 110 (61)]; HRMS calcd for C18H18S3
330.05707, found 330.05712.
SSSS-Tetr a m er . A 0.781 g (0.00336 mol) quantity of 8 was
pyrolyzed at 680 °C in the normal manner. Upon completion,
10 mL of CS2 was distilled into the trap. After being warmed
to -78 °C, the solution was transferred to a flask containing
0.112 g (0.000724 mol) of biphenyl. A GC trace of the cold
solution was obtained, and multiple ion detection GC-MS
showed the expected SS-dimer 5 and SSS-trimer 7, as well as
1.88 mg (0.677 mol %, based on amount of initial monomer) of
a compound having a molecular weight of 440 and fragments
at m/e 330 and 220.
P r ep a r a tion of SS-Dim er 5 by High Dilu tion . A 2.10 g
(0.00902 mol) quantity of 8 was pyrolyzed at 625 °C in the
normal manner. Upon completion, 25 mL of CS2 was distilled
into the trap. After the solution was warmed to -78 °C, it
was added to 1 L of CS2, which resulted in a 0.00660 M solution
of S-monomer 3. After the solution was held at room temper-
ature for 5 days, the mol ratio of S-monomer 3 to SS-dimer 5
to SSS-trimer 7 was 1.00:32.89:5.58 as determined by GC.
Tr a p p in g Exp er im en ts. To a 0.5 mL of S-monomer 3
solution (0.1 M) were added 0.3615 g of purified 9,10-dihy-
droanthracene and 4.5 mL of CCl4. This results in a 1:40
monomer:9,10-dihydroanthracene ratio. To a 0.5 mL of S-
monomer 3 solution (0.1 M) was added 0.3606 g of anthracene
in 4.5 mL of CCl4. Both reactions were allowed to stand at
room temperature overnight. GC, GCMS, and NMR were used
to analyze the final products. The same procedure was used
for the rest of the conventional trapping agents.
Oxygen Used a s Tr a p p in g Agen t. To each of six NMR
tubes was added 0.5 mL of an S-monomer 3 (0.1 M) solution.
Oxygen then was bubbled through the monomer solution at
different time periods: 5, 10, 20, 50, 60, and 180 s. To each
of six NMR tubes was added 0.5 mL of an S-monomer 3 (0.1
M) solution. Nitrogen was bubbled to each tube at different
times: 5, 10, 20, 50, 60, and 180 s. 1H NMR was used to follow
both reactions. After standing at room temperature for 2 days,
the solution was analyzed by GC.
Co-Oligom er iza tion of O-Mon om er 2 a n d S-Mon om er
3. P r ep a r a tion of O-m on om er 2.9,13a A 1.77 g (0.00819 mol)
quantity of 5-methyl-2-furfuryl benzoate was pyrolyzed at 560
°C in the normal manner. Upon completion, 17 mL of CS2
was distilled into the trap. After warming to -78 °C, the
solution containing O-monomer 2 was stored at -78 °C.
A 0.370 g (0.00159 mol) quantity of 8 was pyrolyzed at 680
°C in the normal manner. Upon completion, 10 mL of CS2
was distilled into the trap. After warming to -78 °C, the
solution containing S-monomer 3 was combined with the
O-monomer 2 solution and allowed to stand at room temper-
ature overnight. Integration of the 1H NMR obtained just
before warming showed the ratio of O-monomer 2 to S-
monomer 3 to be 5.6:1.
1
was obtained by vacuum distillation; bp and H NMR spectrum
match literature30 values. IR (thin film) cm-1: 2966, 1663,
1464, 1452, 1227, 812.
As described above for the preparation of 5-methyl-2-
thiophenemethanol, 2.78 g (0.0198 mol) of 5-ethyl-2-thiophen-
ecarboxaldehyde was reduced with 0.3760 g (0.0099 mol) of
LiAIH4 to 2.512 g (0.0177 mol) of 5-ethyl-2-thiophenemethanol
(89%): 1H NMR (CDCl3, 300 MHz) δ 6.818 (d, J ) 3.3 Hz, 1H),
6.648 (d, J ) 3.6 Hz,, 1H), 4.746 (s, 2H), 2.861 (q, J ) 7.2 Hz,
J ) 7.8 Hz, 2H), 1.648 (broad, 1H); IR (thin film) cm-1 3333,
2966, 2930, 1456, 1207, 1005, 804.
As described above for the preparation of 8, 2.512 g (0.0177
mol) of 5-ethyl-2-thiophenemethanol was converted to 9 (82%).
Pure 9 was obtained by column chromatography (silica gel,
1:100 ethyl acetate to hexane as eluent) (bp 145 °C, 1.2
mmHg): 1H NMR (CDCl3, 300 MHz) δ 8.072 (m, 2H), 7.451
(m, 3H), 6.909 (d, J ) 3.3 Hz, 1H), 6.683 (d, J ) 3.6 Hz, 1H),
5.435 (s, 2H), 2.848 (q, J ) 7.5 Hz, 2H), 1.307 (t, J ) 7.5 Hz,
3H); 13C NMR (CDCl3, 75.5 MHz) δ 166.10, 149.22, 135.05,
132.89, 129.87, 129.57, 128.18, 128.05, 122.88, 61.29, 23.44,
15.76; IR (thin film) cm-1 3063, 2964, 1718, 1267, 1096, 712;
HRMS calcd for C14H14O25 246.0715, found 246.0720. Anal.
Calcd: C, 68.3; H, 5.73. Found: C, 68.4; H, 6.06.
P yr olysis of (5-Met h yl-2-t h iop h en e-yl)m et h yl Ben -
zoa te (8). A 0.3360 g (0.0136 mol) of 8 was pyrolyzed at 640
°C in the normal manner.24 Upon completion, 5 mL of a 1:1
mixture of CS2 and CDCl3 was distilled into the trap, which
resulted in a 0.10 M solution of 2,5-dimethylene-2,5-dihy-
drothiophene (3). A quantitative low-temperature 1H NMR
analysis of the pyrolysate using dibromoethane as a standard
showed the presence of 3 in 74.4% yield. After the pyrolysate
was allowed to stand at room temperature overnight, sub-
stantial amounts of [2.2](2,5)thiophenophane (SS-dimer 5, 14.7
mol %) and [2.2.2](2,5)thiophenophane (SSS-trimer 7, 44.3 mol
%) were formed. The products were separated by column
chromatography on silica gel (1:100 ethyl acetate to hexanes).
2,5-Dim eth ylen e-2,5-d ih yd r oth iop h en e (3): H and 13C
1
NMR data are in good accord with published values;16 GC-IR
3101, 3001, 1589, 1192, 837 cm-1 [lit.14 IR (Ar 15 K) 1586.6,
1187.4, 840.8, 831.2, 801.6 cm-1]; GC-MS m/e (relative inten-
sity) 112 (4.94), 111 (8.51), 110 (100.00), 109 (40.59), 95 (5.4),
84 (23.36), 77 (14.31), 74 (4.26), 71 (12.90), 69 (14.92), 66
(54.29), 65 (13.86), 63 (6.49), 58 (25.78), 55 (10.33), 51 (30.51),
50 (24.22), 47 (2.09).
Oligom er iza tion P r od u cts. After standing at room tem-
perature overnight, a number of dimers and trimers derived
from S-monomer 3 were formed, as well as OO-dimer 4
(identified by GC/MS) and polymer. Yields were determined
by GC analysis using biphenyl as an internal standard. The
compounds and yields are summarized in Table 1.
(28) King, W. J .; Nord, F. F. J . Org. Chem. 1949, 14, 638-42.
(29) The Aldrich Library of NMR spectra, Vol. 2, p 466; Pouchert,
Ed. II, Aldrich Chemical Co., Milwaukee, 1983.
It should be noted that using a 1:1 ratio of O-monomer 2 to
S-monomer 3 gave the highest relative yield of trimers but
(30) Lantz, R.; Ho¨rnfeldt, A.-B. Chem. Scr. 1972, 2, 9-15.

8986 J . Org. Chem., Vol. 62, No. 26, 1997
Trahanovsky et al.
that using a 3:1 ratio gave a greater OSS-trimer 17 to SSS-
trimer 7 ratio.
th r ee isom er s; cis, tr a n s-A, a n d tr a n s-B): 1H NMR (CDCl3,
300 MHz) δ 6.8-6.6 (m, 8H), 3.7-2.3 (m, 12H), 1.6-1.2
(several d, J ) 6.9 Hz, 12H); GC/MS: m/e (relative intensity)
Because excess O-monomer 2 remained for several days
after all of the S-monomer 3 had reacted, the excess O-
monomer 2 was destroyed by addition of an equal volume of
glacial acetic acid to the reaction mixture. This mixture was
allowed to stand for 30-90 min. Excess acetic acid was
removed by first adding ethyl ether and then extracting with
water (2 × 50 mL) followed by saturated NaHCO3 until the
ether solution was basic. After drying (MgSO4) and concen-
trating, the products were dissolved in a minimum amount of
hexanes. Separation by careful column chromatography on
silica gel (hexanes) yielded pure samples of all five products.
13-Oxa -14-th ia tr icyclo[8.2.1.13,6]tetr a d eca -4,6,10,12-tet-
248 (M+, 4), 124 (100), 97.1 (6.02), 65.0 (2.82), 39.1 (5.17); 13
C
NMR (CDCl3, 75.5 MHz, 7 mg of Cr(acac)3 was added) δ 159.78,
158.07, 157.26, 157.16, 156.73, 156.19, 155.94, 154.12, 150.54,
150.46, 150.23, 149.59, 149.55, 148.80, 148.39, 147.93 (16
quaternary carbons), 126.94,126.26, 125.87, 125.82, 125.32,
125.26, 124.57, 124.38, 122.82, 122.48, 122.26 (11 signals),
47.02, 44.96, 43.64, 41.69, 41.47, 39.77, 39.57 (7 signals), 36.98,
32.37, 32.31, 31.68, 31.11 (5 signals), 19.49, 19.30, 18.56, 17.19,
16.17, 12.46 (6 signals).
5-Eth yl-5′-vin yl-2,2′-eth ylen edith ioph en e (acyclic dim er
25a ) a n d 5-m eth yl-5′-vin yl-2,2′-(r-m eth yleth ylen e)d ith io-
p h en e (a cyclic d im er 25b): 1H NMR (CDCl3, 300 MHz) for
25a δ 6.7 (m, 2H), 6.5 (m, 2H), 6.7 (q, J ) 17.1 Hz, J ) 10.8
Hz, 1H), 5.48 (d, J ) 17.1 Hz, 1H), 5.08 (d, J ) 10.8 Hz, 1H),
3.1 (s, 4H), 2.8 (q, J ) 7.5 Hz, 2H), 1.3 (t, J ) 7.2 Hz, 3H); for
25b δ 6.7 (m, 2H), 6.5 (m, 2H), 6.7 (q, J ) 17.4 Hz, 1H), 5.48
(d, J ) 17.4 Hz, 1H), 5.08 (d, J ) 10.8 Hz, 1H), 3.25-2.9 (m,
3H), 2.4 (s, 3H), 1.3 (d, J ) 6.9 Hz, 3H); 13C NMR (CDCl3, 75.5
MHz) for both 25a and 25b δ 145.6, 143.3, 141.2, 140.96, 130.2,
125.8, 124.8, 124.2, 122.8, 112.2, 40.1, 37.6, 32.5, 32.2, 29.8,
23.5, 22.2, 16.0; GC/MS m/e (relative intensity) 248.2 (8), 125.1
(100), 123 (35).
1
r a en e (OS-d im er 16): H NMR (CDCl3, 300 MHz) δ 6.93 (s,
2 H), 5.96 (s, 2 H), 2.90 (m, 8 H) [lit.13a 1H NMR (CDCl3) δ
6.92 (s, 2 H), 5.94 (s, 2 H), 2.88 (m, 8 H)]; 13C NMR (CDCl3,
75.45 MHz) δ 154.9, 151.6, 128.3, 108.5, 33.1, 31.0; GC-MS
m/e (relative intensity) 206 (1.28), 205 (3.17), 204 (24.40), 112
(4.69), 111 (7.48), 110 (100.00), 94 (29.94), 84 (3.70), 77 (7.36),
66 (22.20), 51 (17.82) [lit.21 mass spectrum m/e 110, 94].
19-Oxa -20,21-d ith ia tetr a cyclo[14.2.1.13,6.19,12]h en eicosa -
4,6,10,12,-16,18-h exa en e (OSS-tr im er 17): mp 79.5-84 °C;
1H NMR (CDCl3, 300 MHz) δ 6.58 (q, AB, 2 H), 5.98 (s, 4 H),
2.96 (m, 12 H); 13C NMR (CDCl3, 75.5 MHz) δ 152.4, 141.4,
140.5, 124.2, 123.9, 106.7, 32.0, 29.7, 29.2; HRMS calcd for
C18H18OS2 314.07991, found 314.07872.
2,3,14-Trimethyl-19,20,21-trithiatetracyclo[14.2.1.13,6.19,12]-
h en eicosa -4,6,10,12,16,18-h exa en e (SSS-tr im er 26a , p os-
sibly six ster eoisom er s) a n d (3,9,15-tr im eth yl)-19,20,21-
tr ith ia tetr a cyclo[14.2.1.13,6.19,12]h en eicosa -4,6,10,12,16,18-
h exa en e (SSS-tr im er 26b, p ossibly tw o ster eoisom er s):
GC/MS m/e (relative intensity) 372.5 (M+, 0.3), 234.1 (11),
123.0 (54), 111 (100); 1H NMR (CDCl3, 300 MHz) δ 6.63-6.55
(m, 12H), 3.47 (s, 4H), 3.0-3.2 (m, 14H), 1.37-1.27 (m, 18H).
P yr olysis of (5-Eth yl-2-th iop h en e-yl)m eth yl Ben zoa te
(9). A 0.1439 g (0.000 584 5 mol) of quantity of 9 was
pyrolyzed at 680 °C in the normal manner. Upon completion,
4.4 mL of 1:1 mixture of CS2 and CDCl3 was distilled into the
trap, which resulted in a 0.05 M solution of 10.
A quantitative low-temperature 1H NMR analysis of the
pyrolysate using diphenylmethane as a standard showed the
presence of 10a in 34% yield and 10b in 9% yield. After the
pyrolysate was allowed to stand at room temperature under
argon for ca. 3 days, substantial amounts of SS-dimers 24,
acyclic dimers 25, and SSS-trimers 26 were formed. The
products were separated by flash column chromatography
(silica gel; ethyl acetate and hexane (1:200) was used as
eluent).
Ack n ow led gm en t. This work was supported by the
U.S. Department of Energy, Office of Basic Energy
Sciences, Division of Chemical Sciences, under Contract
W-7405-ENG-82. We thank Arthur B. Ferruzzi, Steven
P. Lorimor, and J ames L. Malandra for help in the
preparation of the manuscript.
2-Eth ylid en e-5-m eth ylen e-2,5-d ih yd r oth iop h en e (m ix-
tu r e of tw o isom er s) (m a jor isom er 10a ): 1H NMR (CDCl3,
300 MHz) δ 6.435 (d, J ) 6 Hz, 1H), 6.399 (d, J ) 5.7 Hz, 1H),
5.635 (q, J ) 7.2 Hz, 1H), 5.147 (s, 1H), 4.979 (s, 1H), 1.803
(d, J ) 7.2 Hz, 3H).
Su p p or tin g In for m a tion Ava ila ble: 1H NMR spectra for
compounds 3, 5, 7-10, 15-17, 24a /b, 25a /b, 26a /b, and
several synthetic intermediates; 1H NMR COSY for compunds
25a /b; 13C NMR spectra for compounds 3, 17, 24a /b, and 25a /
b; IR spectra for compounds 3, 9, and several intermediates;
MS for compounds 3, 24a /b, 25a /b, and 26a /b (31 pages). This
material is contained in libraries on microfiche, immediately
follows this article in the microfilm version of the journal, and
can be ordered from the ACS; see any current masthead page
for ordering information.
2-Eth ylid en e-5-m eth ylen e-2,5-d ih yd r oth iop en e (m in or
1
isom er 10b): H NMR (CDCl3, 300 MHz) δ 6.49-6.4 (d, J )
6.6 Hz, 2H), 5.4 (q, J ) 6.9 Hz, 1H), 5.1 (s, 1H), 4.9 (s, 1H), 1.9
(d, J ) 7.5 Hz, 3H).
2,3-Dim eth yl-13,14-d ith ia tr icyclo[8.2.1.13,6]tetr a d eca -
4,6,10,12-tetr a en e (SS-d im er 24a , th r ee isom er s; cis,
tr a n s-A, a n d tr a n s-B) a n d 2,8-d im eth yl-13,14-d ith ia tr i-
cyclo[8.2.1.13,6]tetr adeca-4,6,10,12-tetr aen e (SS-dim er 24b,
J O961528I