Pleiadene dimerization and its application to the construction of novel carbon networks

Article abstract of DOI:10.1002/(SICI)1521-3765(19990104)5:1<289::AID-CHEM289>3.0.CO;2-1

The synthesis of the anti- and syn-pleiadene dimers 2 has been carried out in solution and in the solid state from (6b-10b- dihydrobenzo[j]cyclobut[a]acenaphthylene) (3). The structures of the anti- and syn-2 dimers were determined by X-ray crystallography and by NMR spectroscopy and mass spectrometry. An exhaustive thermal analysis of the two dimers has been carried out, establishing that the equilibrium for their interconversion occurs via pleiadene as an intermediate species. Vinyl-6b- 10b-dihydrobenzo[j]cyclobut[a]acenaphthylene (5) has been synthesized and used as a pre-monomer for the preparation of poly(pleiadene) dimer through either thermal or photochemical polymerization.

Full text of DOI:10.1002/(SICI)1521-3765(19990104)5:1<289::AID-CHEM289>3.0.CO;2-1

FULL PAPER
Pleiadene Dimerization and Its Application to the Construction of Novel
Carbon Networks
[a][b]
Gordana Srdanov,[a] and Fred Wudl*[a][b]
Â
Angela Sastre,
Abstract: The synthesis of the anti- and syn-pleiadene dimers 2 has been carried out
in solution and in the solid state from (6b-10b-dihydrobenzo[j]cyclobut[a]acenaph-
thylene) (3). The structures of the anti- and syn-2 dimers were determined by X-ray
crystallography and by NMR spectroscopy and mass spectrometry. An exhaustive
thermal analysis of the two dimers has been carried out, establishing that the
equilibrium for their interconversion occurs via pleiadene as an intermediate species.
Vinyl-6b-10b-dihydrobenzo[j]cyclobut[a]acenaphthylene (5) has been synthesized
and used as a pre-monomer for the preparation of poly(pleiadene) dimer through
either thermal or photochemical polymerization.
Keywords: biradicals ´ carbon net-
works ´ pleiadene ´ polymerizations
Introduction
Attempts in recent years to produce materials with properties
approaching those of diamond have increasingly focused on
the generation of cross-linked all-hydrocarbon polymers. The
exceptional physical properties of diamond (hardness, high
refractive index, and high thermal conductivity) can ultimate-
ly be ascribed to the compact, strong, directional carbon ±
carbon single covalent bonds that make up this three-dimen-
sional extended solid.[1] Many approaches have been utilized
to prepare high-strength organic materials by thermal cross-
linking[2] and hypercross-linking.[3±6]
lene)[9, 11] is the most efficient precursor for the thermal or
low-temperature photochemical generation of 1 as a short-
lived intermediate.[10, 12]
Though pleiadene dimerization has been briefly described
for the chain extension of macromolecules,[2] there are no
reports of its utilization as a polymerization functional group.
This pleiadene polymerization/cross-linking process possesses
the two distinct advantages usually associated with cyclo-
additive polymerizations such as the Diels ± Alder polymer-
ization: 1) it is achieved without the evolution of volatile by-
products, allowing easier processing of the polymer into the
final molded product in a single step and, 2) it does not
introduce any weak links into the polymer in the form of
heteroatoms or free radical-, anion-, or cation-stabilizing
groups. With this type of monomer we are able to fill a mold of
the desired shape and then heat it to obtain a finished product
directly without the properties of the polymer being degraded
by contamination with a catalyst or by-product fragment such
as CO2, H2O or N2.
The elusive hydrocarbon pleiadene 1[7] is of interest from a
theoretical[8±12] and, more recently, from a practical view-
point as a chain-extending end cap for flexible quinoline
oligomers.[2] Attempts to isolate it at room temperature have
always led to the formation of the anti dimer 2,[9, 11, 12, 13] as
1
characterized by H NMR spectroscopy. Though the pleia-
dene dimer can be generated from either 7,12-dibromo- or
7,12-dihydropleiadene-7,12-sulfone,[13] the strained benzocy-
clobutene 3 (6b-10b-dihydrobenzo[j]cyclobut[a]acenaphthy-
[a] Prof. Dr. F. Wudl, G. Srdanov, Dr. A. Sastre
Departments of Chemistry and Materials
Institute for Polymers and Organic Solids
University of California
Santa Barbara, CA 93106 ± 5090 (USA)
In their studies of the dimerization of 1 Kolc and Michl[10]
concluded that it is not possible to distinguish between a
stepwise biradical pathway (mechanism a, Scheme 1) or a
concerted, symmetry-controlled process (mechanism b,
Scheme 1). The principal argument against a concerted
thermal process is that a [p4s ‡ p4s] reaction is symmetry-
[b] Prof. Dr. F. Wudl, Dr. A. Sastre
Exotic Materials Institute
Department of Chemistry and Biochemistry
University of California, Los Angeles
CA 90095 ± 1569
Fax: (‡1)310-805-0767
E-mail: wudl@chem.ucla.edu
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F. Wudl et al.
FULL PAPER
column chromatography, which
could explain why the dimeri-
zation was previously thought
to be anti-specific. Special semi-
preparative HPLC (see Exper-
imental Section) was necessary
to separate the isomers. The EI-
MS analyses performed sepa-
rately on both anti-2 and syn-2
showed the parent ion at m/z
‡
456 (M ) as well as an addi-
tional peak at half the molec-
ular mass (m/z 228) corre-
sponding to the aromatic pleia-
dene. Proton NMR spectra of
the crude reaction mixture al-
lowed a clear distinction to be
Scheme 1. Mechanisms for the dimerization of 1.
made between the resonances
of the two isomers. Two sharp
singlets at d ˆ 5.07 and 5.36 are assigned to the four
benzhydryl protons of the anti isomer and the syn isomer
respectively. Decoupling experiments in CDCl3 for anti-2, and
in CD2Cl2 for syn-2, allowed the assignment of all the
aromatic protons, as shown in Figure 1. The 13C NMR spectra
of anti- and syn-2 both show 10 signals, corresponding to C2h
and C2v symmetry, respectively.
forbidden. Conversely, the primary argument in favor of
concertedness is the previously claimed selective anti-specific
product formation. In other words, the observation of anti-
specificity is a fundamental argument against a stepwise
process. The same authors have proposed that regardless of
whether mechanism a or mechanism b is operative, anti-
specific dimerization is to be expected on the basis of
secondary orbital interactions.[10]
UV/Vis spectroscopy showed a slight hypsochromic shift
and a broadening in the signals of the syn-pleiadene dimer
relative to the anti isomer. The major difference was in the
270 ± 330 nm region (Figure 2). The long-wavelength bath-
ochromic shift in the anti isomer may be due to a transannular
p ± p interaction between the phenylene and naphthylene
units, the same interaction which may account for the crystal
packing in the syn isomer (see below and Figure 4).
The anti dimer crystallizes (see Experimental Section)
together with one molecule of disordered o-dichlorobenzene
(ODCB). The two halves of the molecule either side of the
inversion center are crystallographically different. The struc-
ture has no unusual bond lengths or angles. The longest single
bond, the equatorial C(lA) ± C(5B) bond (1.606(10),
1.575(11) Š) is different by about 3s for the two independent
parts of the asymmetric unit and is shorter than the
corresponding bond (1.621(8) Š) in the syn conformer
(Figure 3). The average length of the phenylene bonds is
1.390(11) Š for the anti dimer, these being slightly shorter
(1.380(9) and 1.387(9) Š) for the syn isomer; the naphthylene
bond lengths are comparable with previously reported
data[14±16] and have average values of 1.393(11) and
1.401(11) Š (anti) and 1.393(11) and 1.396(11) Š (syn)
(Table 1). A comparison of the dihedral angles about the
two long bonds for the anti and syn isomers shows the isomers
to have different angle strains, with the two independent
halves of anti-2 also having slightly different molecular strain
(Table 1).
In this work, we have studied the pleiadene dimerization
reaction, and report herein the isolation, spectroscopic
properties, structural analysis, thermal analysis, and crystal
structures of the anti and the syn pleiadene dimers. We are
able to show unequivocally, and contrary to previous findings,
that the formation of 2 is not anti-specific and that the
observed ratio of syn to anti product is not a reflection of
relative thermodynamic stability. We also report the prepa-
ration of poly(pleiadene dimer) based on a self-initiated
thermal or photochemical polymerization of dihydrobenzo[j]-
cyclobut[a]acenaphthylene subunits.
Results and Discussion
Pleiadene dimerization: Compound 3 was synthesized ac-
cording to the experimental procedure described by Kolc and
Michl.[10] Heating the monomer 3 in a sealed tube at 2408C,
afforded a 4.9:1 mixture (83:17) of the anti- and syn-2 isomers,
respectively, in an overall yield of 84%. The same ratio was
also obtained as described previously[10] by refluxing for
15 min in a-chloronaphthalene under an argon atmosphere
(Scheme 2). The isomers differ in their physical and chemical
properties, but no separation could be achieved by normal
The packing of the two structures is significantly different
owing to the presence of a disordered solvent molecule in the
anti crystals. The syn molecules form stacks along the
crystallographic a axis in such a way that an o-phenylene ring
from one molecule and a naphthalene ring from a neighbor
Scheme 2. Reaction of 3 with achloronaphthalene to give anti-2 and syn-2.
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Pleiadene Dimerization
289±296
Figure 1. Top: 1H NMR spectrum of the
anti-2 isomer in CDCl3 solution; bottom:
1H NMR spectrum of the syn-2 isomer in
CD2Cl2 solution. Controls revealed no
change in chemical shift on changing from
CDCl3 to CD2Cl2. The latter was used for the
syn isomer because one of its resonances
overlapped with the CDCl3 signal.
facing one other with a normal van der Waals separation
(3.422 Š) (Figure 4).
A thermal analysis on the dimers was carried out to study
the possibility of interconversion between the two isomers.
Differential scanning calorimetry (DSC) of 3 between 40 and
1
4008C at 58Cmin under a nitrogen atmosphere showed a
melting endotherm at 1308C (m.p. 130 ± 1328C[10]) and a
maximum exotherm for the dimerization at 2168C. Integra-
tion of the area under the exotherm gave a value of
1
105.25 calg , which corresponds to an exothermicity of
1
31.58 kcalmol for the dimerization process. The melting
points of anti- and syn-2 were 377.45 and 338.928C, respec-
tively, and were followed very closely by broad exothermic
peaks centered at 380.218C and 340.698C. Pyrolysis in the
DSC apparatus of the less stable dimer, syn-2, with quenching
at 3408C before the exothermic reaction was complete,
yielded a sublimed mixture of anti- and syn-2 (60:40), which
1
equates with a very low difference in energy (0.5 kcalmol )
Figure 3. Labeling scheme for the anti-2 (top) and syn-2 (bottom)
molecular units. The actual asymmetric unit for anti consists of two
independent halves, denoted by the suffixes A and B, but drawn as a single
unit. See Table 1 for bond lengths.
Figure 2. UV/Vis spectrum of anti-2 (solid line) and syn-2 (dotted line) in
CHCl3 as solvent. The region below about 240 nm is an instrumental
artifact due to solvent absorption. A ˆ absorbance.
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F. Wudl et al.
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Table 1. Selected bond lengths [Š] and torsion angles [8].
anti-2
Molecule 1
Molecule 2
1.515(11)
C(1) ± C(2)
C(1) ± C(5)[a,b]
C(1) ± C(11)
C(5) ± C(4)
1.538(11)
1.606(10)
1.498(8)
1.521(11)
1.515(8)
27.90
162.0
106.89
27.20
1.575(11)
1.546(8)
1.524(11)
1.512(8)
26.26
162.75
109.47
27.02
C(5) ± C(16)
C(2)-C(1)-C(5)-C(16)
C(2)-C(1)-C(5)-C(4)
C(11)-C(1)-C(5)-C(16)
C(11)-C(1)-C(5)-C(4)
syn-2
C(03) ± C(02)
C(03) ± C(04)
C(03) ± C(41)
C(04) ± C(05)
C(04) ± C(31)
C(07)-C(08)-C(09)-C(10)
1.505(8) C(09) ± C(10)
1.621(8) C(09) ± C(46)
1.496(8) C(09) ± C(08)
1.504(9) C(08) ± C(07)
1.521(8) C(08) ± C(32)
1.525(8)
1.496(8)
1.614(8)
1.513(8)
1.494(8)
4.14
4.49
C(02)-C(03)-C(04)-C(05)
C(07)-C(08)-C(09)-C(46) 128.03
C(02)-C(03)-C(04)-C(31) 129.8
C(41)-C(03)-C(04)-C(05) 141.69
C(32)-C(8)-C(9)-C(10)
141.09
8.56
C(32)-C(08)-C(09)-C(46)
C(41)-C(03)-C(04)-C(31)
7.75
Symmetry transformations: [a] Molecule 1: x, y ‡ 2,
x ‡ 1, y ‡ 1, z.
z
1. [b] Molecule 2:
Figure 4. Stereodiagram of the packing for anti-2 (top; viewed along the b
axis) and syn-2 (bottom; viewed along the c axis).
This allows the course of the dimerization reaction to be
explained as represented in Scheme 3, in which the dimeriza-
tion is rendered reversible through the pleiadene monomer as
an intermediate. However, the pleiadene does not revert to
the starting material 3. The pleiadene monomer is a radicaloid
(singlet ground state, ESR silent),[10] which dimerizes in an
energetically downhill process. A further thermal study of the
mixture of the two isomers at > 4008C led to the isolation of a
black powder and dihydropleiadene (4, 20% yield). The
source of H ´ is not known at the present time.
between the two isomers. Moreover, at this temperature a
solid mixture of anti- and syn-2 (75:25) was left in the DSC
vessel, but no dihydropleiadene 4 was detected in either the
sublimate or the residue. Similar studies performed at the
lower temperatures of 280, 290, 300, 310, 320, and 3308C
afforded approximately the same mixture of sublimed prod-
uct, with isomer ratios of between 55:45 and 60:40, as
determined by HPLC. When the same experiment was
performed with pure anti-2 at 3808C, anti- and syn-2 were
observed in the sublimate in a ratio of 96:4, together with
dihydropleiadene, leaving a black powder as the DSC residue.
This indicates, as shown in Scheme 3, that the interconversion
between anti-2 and 1 at 3808C is
Pleiadene-based polymers: Armed with the above informa-
tion, we set out to establish the applicability of pleiadene
quite different from the inter-
conversion between syn-2 and 1
at 280 ± 3408C. On this basis it
would appear that under the
usual reaction conditions for
the conversion of 3 to 2 (neat
at 2408C or in refluxing a-
chloronaphthalene at 2598C),
interconversion does not occur.
The preference in this reac-
tion for the anti product could
be the consequence of slightly
higher p ± p electron repul-
sion in the syn isomer, with
an energy difference of about
1
2 kcalmol (calculated experi-
mentally by calorimetry). The
5:1 ratio observed in the
bulk reaction is therefore not
a reflection of the relative sta-
bilities of syn and anti dimers
of 2.
Scheme 3. Interconversion between syn-2 and 1 and between anti-2 and 1.
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Pleiadene Dimerization
289±296
dimerization in an actual polymerization reaction. To this end,
8-vinyl-6b-10b-dihydrobenzo[j]cyclobut[a]acenaphthylene
(5) was prepared from 8-iodo-6b-10b-dihydrobenzo-[j]cyclo-
but[a]acenaphthylene (6) by a Negishi coupling.[17] The [2‡2]
cycloaddition of iodobenzyne, generated from 4-iodobenze-
nediazonium-2-carboxylate (7), to acenaphthylene (8) gave a
20% yield of the iodo derivative 6. Reaction of a vinylzinc
with 6 in the presence of a Pd0 catalyst proceeded readily to
give the vinyl pleiadene derivative in 98% yield (Scheme 4).
Substitution of the bromo pleiadene derivative[2] for the iodo
Figure 5. DSC of vinylbenzocyclobutane derivative 5.
Scheme 4. Synthesis of 5.
compound, reduced the yield substantially (10 ± 30%). DSC
analysis of the iodo derivative showed a melting endotherm at
1628C, a dimerization exotherm peak at 2188C, and the ring-
opening exotherm at 3068C.
The DSC analysis of vinylbenzocyclobutane 5 (Figure 5)
exhibited a melting endotherm at 1028C and an exotherm
peak at 2148C that could in principle, by analogy with the data
for the parent compound 3 and the iodo derivative 6, be
attributable to the dimerization process. However, opening of
the dimer, as evidenced by an exotherm peak in the DSC
between 300 and 4008C, was not observed, in contrast to the
other examples already mentioned. Pyrolysis of 5 at 2308C for
15 min in the DSC apparatus afforded an insoluble, thick
monolith.
A photochemical polymerization with a 450 W mercury
lamp, produced material with similar properties. These
observations prompted to us to consider the possibility of a
spontaneous polymerization having occurred during the
pleiadene dimerization process. Thermal gravimetric analysis
(TGA) of 5 demonstrated the high thermal stability of the
compound generated after heating at 2148C (Figure 6). In the
first step of the curve, beyond the melting point, a 29% loss of
mass can be attributed to sublimation. The sublimation was
fortunately incomplete, and the
Figure 6. TGA of vinylbenzocyclobutane derivative 5.
dimer 9 can be easily prepared from the iodo benzocyclobu-
tene 6. Heating compound 6 for 15 min in refluxing a-
chloronaphthalene gave the diiodopleiadene dimer 10 in 79%
yield. In this case, a 98% yield of anti-iodo pleiadene dimer 10
was obtained as a an inseparable mixture of the three possible
stereoisomers depicted in Scheme 5, as was established by
HPLC analysis.[18] The divinyl pleiadene dimer 9 was then
obtained in 70% yield, likewise as a mixture of stereoisomers,
under the Negishi conditions described above (Scheme 5).
The DSC analysis of the dimer 9 showed only an
exothermic peak at 2988C, corresponding to the polymer-
ization process. Heating 9 for 15 min at 3008C in the DSC
apparatus afforded an insoluble monolith with the same
residue polymerized to a poly-
mer of exceptional thermal sta-
bility with an onset of decom-
position of 4808C (Figure 6).
Upon quenching the poly-
merization process by sudden
cooling, the only isolated solu-
ble product was the divinyl
pleiadene dimer 9. No evidence
of a Diels ± Alder adduct was
Scheme 5. Synthesis of 9.
found. The divinyl pleiadene
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properties as the material generated from the vinyl monomer
5 described above. TGA analysis of compound 9 showed no
loss of mass until 4808C, at which point decomposition
occurred.
Figure 7 shows the FTIR spectra of the divinyl pleiadene
dimer 9 and the cross-linked polymer generated by pyrolysis
ˆ
of 5 or 9. In the latter spectrum the -CH CH2 stretching
vibration of dimer 9 at 989 cm 1 has clearly disappeared. The
spectra of both the vinyl dimer 9 and the cross-linked polymer
are otherwise similar, indicating that the anti-pleiadene
skeleton is still present in the cross-linked polymer.
Scheme 6. Chain polymerization of the vinyl groups by I.
post-processing cross-linking, and the examination of the
mechanical properties of the materials obtained.
Experimental Section
NMR spectra were recorded with a Varian Inova 400 spectrometer
operating at 400 MHz for 1H and 125 MHz for 13C, and were referenced to
tetramethylsilane. UV/Vis spectra were obtained with a Hewlett Packard
8452A diode array spectrometer. TGA and DSC were carried out using a
Perkin Elmer 7 Series thermogravimetric analyzer and differential scan-
ning calorimeter, respectively. HPLC measurements were performed by
using a Nacalai tesque, Cosmosil Buckyprep (4.6 Â 250 mm) column with
UV detection at 318 nm, 80:20 hexanes:toluene as eluent, and a flow rate of
1 mLmin 1. Retention times: 5.79 min (anti-2), 8.01 min (syn-2).
Dimerization of 6b-10b-dihydrobenzo[j]cyclobut[a]acenaphthylene (3): In
solution: A solution of 3 (190 mg, 0.84 mmol) in a-chloronaphthalene
(5 mL) was refluxed under an argon atmosphere for 15 min. After cooling,
MeOH (25 mL) was added, precipitating a white solid. The solid was
separated and washed several times with MeOH to obtain 160 mg (84%) of
a mixture of anti- (83%) and syn- (17%) pleiadene dimers (determined by
analytical HPLC). The isomers were separated by semipreparative HPLC
(Nacalai tesque, Semi-preparative Cosmosil Buckyprep Waters instrument
with
a
10 Â 250 mm column), with UV detection at 318 nm, 80:20
hexanes:toluene as eluent, and a flow rate of 5 mLmin 1. Retention times:
6.02 min (anti-2), 9.4 min (syn-2). Recrystallization from toluene resulted in
the exclusive precipitation of the anti isomer.
Neat reaction:[10] Compound 3 (50 mg, 0.22 mmol) was sealed in an
ampoule under high vacuum (10 6 mm Hg) after several repeated freeze-
thaw cycles, and was then heated in a tube furnace at 230 ± 2408C. The
crude reaction mixture consisted of a mixture of the anti- (84%) and syn-
pleiadene dimers (16%) (determined by analytical HPLC). Recrystalliza-
tion from toluene gave 26 mg of the anti-2 isomer (52%).
Figure 7. a) FTIR spectrum of divinyl pleiadene dimer 9. b) FTIR
spectrum of the cross-linked polymer generated by pyrolysis or photolysis
of vinylbenzocyclobutene derivative 5 or divinyl pleiadene dimer 9.
Taking into consideration all the experimental data shown
above, we can postulate a spontaneous cross-linking polymer-
ization of 8-vinyl-6b-10b-dihydrobenzo[j]cyclobut[a]acenaph-
thylene (5), via the divinyl pleiadene dimer as an intermedi-
ate. An intriguing possibility here is that the pleiadene
biradical (I) initiates the chain-polymerization of the vinyl
groups (Scheme 6).
In conclusion, contrary to previous findings, we have
proved that both the syn and anti-2 dimers are produced in
the pleiadene dimerization reaction and that the dimerization
process is reversible through pleiadene as an intermediate. We
have also established the thermolysis or photolysis of 8-vinyl-
6b-10b-dihydrobenzo[j]cyclobut[a]acenaphthylene (5) as a
promising new approach to the generation of carbon net-
works. Further work will be directed at the preparation of
polymers incorporating these units, the investigation of their
anti-2: 1H NMR (400 MHz, CDCl3, 258C): d ˆ 7.53 (dd, 4H, J ˆ 8.2, 1.3 Hz,
H3), 7.49 (dd, J ˆ 7.2, 1.3 Hz, H1, 4H), 7.30 (dd, 4H, J ˆ 8.2, 7.2 Hz, H2), 6.64
(8H, AA'BB' system, H7-10), 5.07 (s, 4H, CH); 13C NMR (125 MHz,
CDCl3): d ˆ 140.3, 137.4, 135.8, 131.6, 130.7, 128,7, 128.4, 126.4, 125.2, 63.4;
UV/Vis (CHCl3): lmax (log e) ˆ 246 (4.17), 294 (sh), 306 (4.19), 318
‡
‡
(4.15) nm; MS (70 eV): m/z (%): 456 (3) [M ], 228 (100), [C18H12].
syn-2: 1H NMR (400 MHz, CD2Cl2, 258C): d ˆ 7.26 (dd, 4H, J ˆ 8.2, 1.3 Hz,
H3), 7.18 (dd, J ˆ 7.0, 1.3 Hz, H1, 4H), 7.00 (dd, 4H, J ˆ 8.2, 7.0 Hz, H1), 7.07
(8H, AA'BB' system, H7-10), 5.36 (s, 4H, CH); 13C NMR (125 MHz,
CDCl3): d ˆ 140.8, 136.7, 134.8, 131.0, 130.2, 130.1, 128.2, 127.1, 124.5, 60.5;
UV/Vis (CHCl3): lmax (log e) ˆ 246 (4.15), 280 (sh), 298 (4.11), 312 (sh) nm;
‡
‡
MS (70 eV): m/z (%): 456 (3) [M ], 228 (100), [C18H12].
8-Iodo-6b-10b-dihydrobenzo[j]cyclobut[a]acenaphthylene (6): The prepa-
ration was carried out according to a slight modification of the synthesis of
8-bromo-6b-10b-dihydrobenzo[j]cyclobut[a]acenaphthylene. A solution of
iodoanthranilic acid (25 g, 95 mmol) and trichloroacetic acid (260 mg,
1.6 mmol) in dry THF (350 mL) was cooled to 08C. Isoamyl nitrite (16 mL,
120 mmol) was then added dropwise over a period of 30 min, keeping the
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Pleiadene Dimerization
289±296
temperature at 08C for 30 min at room temperature for a further 2 h to
yield the diazonium salt as a tan solid. The precipitate was cooled at 08C,
filtered off (Caution! The filter paper must not be allowed to dry out,
because when dry benzenediazonium-2-carboxylate can detonate violently
on being scraped or heated, and it is strongly recommended to keep it wet
with solvent at all times), and washed with cold THF until the filtrate was
colorless. This material was used directly and was assumed to have been
formed in quantitatively yield. This solid was added to a solution of
acenaphthylene (30.4 g, 200 mmol) in 1,2-dichloroethane (400 mL) and the
yellow mixture was stirred overnight at room temperature. It was then
heated for 2 d at 458C and finally overnight to 608C. The solvent was
removed under vacuum to obtain a brownish paste which was triturated
with hot pentane. The yellow precipitate obtained on evaporation of the
solvent was washed several times with methanol to obtain 2.76 g of the pure
iodobenzocyclobutene derivative. An additional 3.2 g of pure iodo com-
pound was obtained by silica column purification of the mother liquor
together with the brownish paste, using petroleum ether as eluent to give an
4.95 (m, 4H); 13C NMR (125 MHz, CDCl3): d ˆ 142.5, 142.34, 139.9, 139.8,
139.1, 136.2, 135.8, 135.4, 132.5, 131.8, 131.2, 129.4, 129.3, 128.8, 128.7, 128.5,
‡
128.4, 125.4, 125.3, 62.7, 62.6, 62.5; MS (FAB): m/z (%): 708 ([M ], 10), 354
(100); IR (KBr): nÄ ˆ 3052, 3035, 2920, 1601, 1582, 1507, 838, 772 cm 1; UV/
Vis (toluene): lmax ˆ 236 (sh), 244 (sh), 250 (sh), 254 (sh), 306, 318 nm;
elemental analysis calcd for C36H22I2 (Mw ˆ 708.38): C 61.04, H 3.13, I
35.83; found C 60.82, H 3.20, I 35.68.
anti-Divinyl pleiadene dimer (9): Into a three-necked round-bottom flask
was added at
788C vinyllithium (0.3 mL of a 2m THF solution,
0.6 mmol,), ZnCl2 (0.6 mL of a 0.5m THF solution, 0.3 mmol), and dry
THF (2 mL), and the mixture was warmed up to 08C during 30 min. A
solution of the anti-diiodopleiadene dimer 10 (200 mg, 0.28 mmol) and
palladium tetrakis(triphenylphosphane) (600 mg, 0.02 mmol) in freshly
distilled THF was then added by cannula, and the mixture was heated to
458C overnight. Water was added and the mixture was extracted with
diethyl ether (3 Â 200 mL). The combined organic layers were washed with
brine, dried with MgSO4, filtered through a plug of silica, and evaporated
under reduced pressure to obtain a yellow product. After purification by
column chromatography (silica gel, petroleum ether), 100 mg (70%) of a
1
overall yield of 6 g (18%); m.p. 1628C; H NMR (400 MHz, CDCl3): d ˆ
7.63 (m, 2H, Har), 7.52( s, 1H, Har), 7.47 (m, 5H, Har), 6.94 (d, 1H, J ˆ 7.2 Hz,
Har), 5.33 (d, 1H, J ˆ 3.6 Hz), 5.28 (d, 1H, J ˆ 3.6 Hz); 13C NMR (125 MHz,
CDCl3): d ˆ 149.0, 146.4, 142.3, 142.2, 136.8, 131.8, 128.1, 124.8, 124.2, 124.1,
1
white solid was obtained; m.p. > 3008C; H NMR (400 MHz, CDCl3): d ˆ
4.95 (d, 2H, J ˆ 10.8 Hz), 5.06 (m, 4H, CH-Ar), 5.38 (d, 2H, J ˆ 17.6 Hz),
6.30 (dd, 2H, J ˆ 10.8, 17.6 Hz), 6.63 (m, 4H, Har), 6.72 (m, 2H, Har), 7.28
(m, 4H, Har), 7.48 (m, 4H, Har); 13C NMR (125 MHz, CDCl3): d ˆ 140.4,
140.3, 140.0, 139.9, 137.3, 137.2 (2C), 137.1, 136.5, 135.8 (2C), 131.5, 130.9
(2C), 130.0, 128.9, 128.8, 128.7 (2C), 128.3, 128.2, 125.2, 125.1, 124.2, 112.9,
54.2, and 53.9 ppm; MS (FAB): m/z (%): 354 (100); IR (KBr): nÄ ˆ 3051,
1
3039, 3028, 2951, 1605, 1497, 1446, 1400, 1371, 1245, 881, 804, 784, 770 cm
;
UV/Vis (toluene): lmax ˆ 280 (sh), 287 (sh), 295, 305 (sh), 308 (sh), 318 (sh),
322 (sh) nm; elemental analysis calcd for C18H11I: C 61.04, H 3.13, I 35.83;
found C 60.97, H 3.17, I 35.74.
‡
‡
63.4, 63.3, 63.1, 63.0; MS (FAB): m/z (%): 509 ([MH] , 3), 508 ([M] , 3),
254 (100); IR (KBr): nÄ ˆ 1623, 1600, 1580, 1505, 1493, 997, 900, 841, 817, 775,
650, 550, 535 cm 1; UV/Vis (toluene): lmax ˆ 286, 307, 319 nm; elemental
analysis calcd for C40H28 (Mw ˆ 508.66): C 94.5, H 5.55; found C 94.2, H
5.71.
Differential scanning calorimetry was carried out under nitrogen on a
sample of pure 6, heating from 50 to 4008C at a rate of 108Cmin 1. The
melting endotherm was observed at 1628C, the dimerization exotherm at
2188C, and the ring-opening exotherm at 3068C. Integration of the area
under the first exotherm gave a value of 63.91 calg 1, which corresponds to
1
22.62 kcalmol 1, and of the second exotherm gave a value of 53.03 calg
,
Differential scanning calorimetry was carried out under nitrogen on a
sample of pure 9, heating from 50 to 5008C at a rate of 108C min 1. The
polymerization reaction exotherm occurred at 2988C. The sample is stable
up to about 4008C.
1
which corresponds to an exothermicity of 18.77 kcalmol for the process.
8-Vinyl-6b-10b-dihydrobenzo[j]cyclobut[a]acenaphthylene (5): Into
a
three-necked round-bottom flask was added at 788C vinyllithium
(5 mL of a 2m THF solution, 10 mmol,), ZnCl2 (10 mL of a 0.5m THF
solution, 5 mmol), and dry THF (10 mL), and the mixture was warmed up
to 08C during 1 h. A solution of the iodo derivative 6 (3.2 g, 9 mmol) and
palladium tetrakis(triphenylphosphane) (600 mg, 0.5 mmol) in freshly
distilled THF was then added by cannula and the mixture was heated to
458C overnight. Water was added and the mixture was extracted with
diethyl ether (3 Â 200 mL). The combined organic layers were washed with
brine, dried with MgSO4, filtered through a plug of silica, and evaporated
under reduced pressure to obtain a yellow product, which upon purification
by column chromatography (silica gel, petroleum ether as eluent) afforded
1.9 g (83%) of a white solid; m.p. 1028C; 1H NMR (400 MHz, CDCl3): d ˆ
5.13 (d, 1H, J ˆ 10.8 Hz), 5.37 (s, 2H, CH-Ar), 5.62 (d, 1H, J ˆ 17.6 Hz),
6.62 (dd, 1H, J ˆ 10.8, 17.6 Hz), 7.16 (m, 2H, Har), 7.27 (s, 1H, Har), 7.48 (m,
4H, Har), 7.63(d, 2H, J ˆ 7.7 Hz, Har); 13C NMR (125 MHz, CDCl3): d ˆ
140.3, 137.4, 135.8, 131.6, 130.7, 128,7, 128.4, 126.4, 125.2, 63.4; MS (FAB):
m/z (%): 254 (100); IR (KBr): nÄ ˆ 1623, 1600, 1492, 1469, 1419, 1364, 1167,
1154, 1116, 1104, 997, 903, 889, 869, 829, 807, 781, 771, 741, 666 cm 1; UV/Vis
(toluene): lmax ˆ 274 (sh), 286 (sh), 296, 304 (sh), 315 (sh), 320 (sh) nm;
elemental analysis calcd for C20H14 (254.33): C 94.45, H 5.55; found C 94.20,
H 5.28.
X-ray structure analysis: Data were collected at 298 K with a Siemens
Smart diffractometer with CCD detector, using MoKa radiation, l ˆ
0.71073 Š and operating at 50 kV and 40 mA. The structure was solved
and refined by using the Siemens SHELXTL program package. Data
collection: w scan; 2q range(8): 3.3 ± 40/2.8 ± 40; h,k,l range: 12 < h < 12,
13 < k < 12, 24 < l < 24/ 11 < h < 10, 13 < k < 13, 19 < l < 19; a
total of 7338/5751 and 2787/2169 independent reflections for anti and syn,
respectively, were collected. Full matrix least square refinement was
performed with 2787/2169 unique reflections with I > 2s(I). The final
agreement factors were: R(F) ˆ 0.096/0.089 (where R(F) ˆS j jFo j j Fc j j/
j Fo j with Fo > 2.0s(F)), Rw(F 2) ˆ 0.231/0.202 (where Rw(F 2) ˆ S(Fo2
Fc2)2]/S[w(Fo2)2]]1/2 with Fo > 2.0s(F); w ˆ 1/[s2(Fo2) ‡ (0.0612P)2] where
P ˆ (Fo2 ‡ 2Fc2)/3), and S(F 2) ˆ 1.328/1.163 for 366/326 parameters. In the
final stages of the refinement a secondary extinction correction was
included (0.007(4)/0.001(2)). The final parameters shifts were smaller than
0.001s; in the final difference map the largest peak and a hole were 0.238/
3
0.260 and 0.290/0.233 eŠ . The benzo groups for the anti isomer were
refined as idealized hexagons with individual anisotropic thermal param-
eters. All other non-hydrogen atoms in the two structures were refined
anisotropically with the hydrogens in the riding mode.
Differential scanning calorimetry was carried out under nitrogen on a
sample of pure 5, heating from 50 to 5008C at a rate of 108Cmin 1. The
melting endotherm was observed at 1028C and the dimerization ± poly-
merization exotherm at 2148C. Integration of the area under the exotherm
gave a value of 109.45 calg 1, which corresponds to an exothermicity of
anti-2: C42H26Cl2, rectangular colorless transparent plate from ODCB
(0.28  0.22  0.04 mm), MW ˆ 601.53; triclinic, space group P1, 1calcd
ˆ
3
1.32 gcm , m(MoKa) ˆ 0.246 mm 1, Z ˆ 2, a ˆ 9.499(3), b ˆ 9.8995(3), c ˆ
18.22470(10) Š, a ˆ 78.190(2), b ˆ 77.092(2), g ˆ 66.059(2)8, Vˆ
1514.33(7) Š3.
1
27.81 kcalmol for the process. The sample appears to be stable up to
about 4008C.
syn-2: C39H22, irregular colorless opaque crystal (0.2 Â 0.12 Â 0.08 mm)
3
anti-Diiodopleiadene dimer (10): A solution of 8-iodo-6b-10b-dihydro-
benzo[j]cyclobut[a]acenaphthylene (500 mg, 1.4 mmol) in a-chloronaph-
thalene (5 mL) was refluxed under an argon atmosphere for 15 min. After
cooling to room temperature, MeOH (50 mL) was added, affording a
yellowish-white precipitate which was centrifuged and washed several
times with methanol to yield 10 (390 mg, 79%: 96% anti isomer, as
determined by HPLC): m.p. >3008C; 1H NMR (400 MHz, CDCl3): d ˆ 7.59
(d, 4H, J ˆ 8.24 Hz, Har), 7.45 (m, 4H, Har), 7.35 (m, 4H, Har), 7.04 (dd, 2H,
J ˆ 2.4, 1.3 Hz), 6.91 (d, 2H, J ˆ 7.9, 1.3 Hz), 6.38 (dd, 2H, J ˆ 7.9, 2.4 Hz),
from CHCl3, MW ˆ 454.54; triclinic, space group P1, 1calcd ˆ 1.283 gcm
,
1
m(MoKa) ˆ 0.073 mm
,
Z ˆ 2, a ˆ 8.2711(7), b ˆ 10.0185(8), c ˆ
14.8885(12) Š,
a ˆ 83.038(2),
b ˆ 75.245(2),
g ˆ 82.079(2)8,
Vˆ
1176.9(2) Š3. Crystallographic data (excluding structure factors) for the
structures reported in this paper have been deposited with the Cambridge
Crystallographic Data Center as supplementary publication no. CCDC-
108046 (anti-2) and 108047 (syn-2). Copies of the data can be obtained free
of charge on application to CCDC, 12 Union Road, Cambridge CB21EZ,
UK (fax: (‡44)1223-336-033; e-mail: deposit@ccdc.cam.ac.uk).
Chem. Eur. J. 1999, 5, No. 1
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1999
0947-6539/99/0501-0295 $ 17.50+.50/0
295

F. Wudl et al.
FULL PAPER
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Acknowledgements
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This work was supported through NSF grant No. DMR-9704143, the
Materials Research Laboratory at UCSB, and by the Exotic Materials
Â
Institute (EMI) at UCLA. Angela Sastre thanks the Ministerio de
Â
Educacion y Cultura (Spain) for a postdoctoral fellowship.
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296
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0947-6539/99/0501-0296 $ 17.50+.50/0
Chem. Eur. J. 1999, 5, No. 1