Synthesis of a strained, air-sensitive, polycyclic aromatic hydrocarbon by means of a new 1,4-benzadiyne equivalent

Article abstract of DOI:10.1016/S0040-4020(02)01054-2

9,11,20,22-Tetraphenyltetrabenzo[a,c,l,n]pentacene (4) was prepared by the pyrolysis of 1,4-bis(phenyliodonio)benzene-2,5-dicarboxylate (9), a new 1,4-benzadiyne equivalent, in the presence of phencyclone. Although stable as a solid, solutions of 4 must be handled in the dark; otherwise, ambient light promotes its oxygenation to the 10,21-endoperoxide (10). The X-ray structures of both 4 and 10 were determined, as well as the stuctures of two dihydrophencyclones isolated as byproducts of the pyrolysis. Compound 4 adopts a conformation with approximate C_2h symmetry in the solid state, in contrast to the results of gas-phase calculations, which uniformly predict a twisted, D₂-symmetric ground state structure.

Full text of DOI:10.1016/S0040-4020(02)01054-2

Tetrahedron 58 (2002) 8875–8882
Synthesis of a strained, air-sensitive, polycyclic aromatic
hydrocarbon by means of a new 1,4-benzadiyne equivalent
*
Ingeborg I. Schuster, Liliana Craciun, Douglas M. Ho and Robert A. Pascal, Jr.
Department of Chemistry, Princeton University, Princeton, NJ 08544, USA
Received 19 June 2002; accepted 17 August 2002
Abstract—9,11,20,22-Tetraphenyltetrabenzo[a,c,l,n]pentacene (4) was prepared by the pyrolysis of 1,4-bis(phenyliodonio)benzene-2,5-
dicarboxylate (9), a new 1,4-benzadiyne equivalent, in the presence of phencyclone. Although stable as a solid, solutions of 4 must be
handled in the dark; otherwise, ambient light promotes its oxygenation to the 10,21-endoperoxide (10). The X-ray structures of both 4 and 10
were determined, as well as the stuctures of two dihydrophencyclones isolated as byproducts of the pyrolysis. Compound 4 adopts a
conformation with approximate C2h symmetry in the solid state, in contrast to the results of gas-phase calculations, which uniformly predict a
twisted, D2-symmetric ground state structure. q 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction
Polycyclic aromatic hydrocarbons (PAHs) of extraordinary
size or shape have received increased attention from
chemists, perhaps stimulated by the structures of the
fullerenes, during the last decade.1,2 Of the many new
compounds prepared, PAHs with helical conformations are
among the most aesthetically pleasing,3 none more so than
the longitudinally twisted derivatives of acenes. These
molecules are essentially twisted aromatic ribbons, but thus
far only one acene derivative has been reported with an end-
to-end twist greater than 708: 9,10,11,12,13,14,15,16-
octaphenyl-dibenzo[a,c]naphthacene4 (2, 1058 twist). This
molecule is formally derived from decaphenylanthracene5
(1, 638 twist) by joining two phenyl groups with a carbon–
carbon bond to yield the naphthacene core.
The greater twist in 2 results from the steric conflict of the
benzo hydrogens with the faces of the adjacent phenyl
groups. If two more phenyl groups on the opposite end of
the molecule were to be joined, the result would be the
predicting twists in excess of 908 (Table 1). The large
calculated twist of 4 and the existence of several reasonable
synthetic precursors made this hydrocarbon an attractive
target, and its synthesis and structure are the subject of this
paper.
pentacene derivative 3, which is calculated (AM16) to have
a 1438 twist. A less crowded, but perhaps more easily
accessible molecule is 9,11,20,22-tetraphenyltetra-
benzo[a,c,l,n]pentacene (4), in which the two central phenyl
groups have been removed. Semiempirical and ab initio
molecular orbital calculations of 4 at a wide variety of levels
yield helical geometries with end-to-end twists ranging
from 828 to 958, with the higher-level calculations all
2. Results and discussion
2.1. Synthesis of 4
Two short synthetic routes to compound 4 may be proposed.
The simplest is the reduction of the readily available
quinone 57 to its parent hydrocarbon 4. In a closely related
case, we recently prepared 1,2,3,4,5,6,7,8-octaphenyl-
Keywords: polycyclic aromatic hydrocarbon; endoperoxide; 1,4-
benzadiyne equivalent.
*
Corresponding author. Tel.: þ1-609-258-5417; fax: þ1-609-258-6746;
e-mail: snake@chemvax.princeton.edu
0040–4020/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020(02)01054-2

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I. I. Schuster et al. / Tetrahedron 58 (2002) 8875–8882
Table 1. Computational data for the D2 and C2h conformations of compound 4
Level
E (D2, aua)
Twistb (deg.)
E (C2h, aua)
Half-twistb (deg.)
DEc (kcal/mol)
AM1
HF/STO-3G
HF/3-21G
HF/6-31G(d)
B3PW91/6-31G(d)
B3PW91/cc-pVDZ
B3LYP/6-31G(d)
B3LYP/6-311G(d,p)
0.493344
22341.204399
22356.797109
22370.029798
22384.647171
22384.803559
22385.549854
22386.079867
84.5
82.4
86.6
94.7
92.8
91.6
92.2
92.4
0.496425
22341.200673
22356.792761
22370.024263
22384.641431
22384.797820
22385.545221
22386.074395
38.3
36.9
39.0
42.4
39.9
39.3
40.2
40.4
þ1.9
þ2.3
þ2.7
þ3.5
þ3.6
þ3.6
þ2.9
þ3.4
a
1 au¼627.503 kcal/mol.
b
Using the crystallographic numbering scheme (Fig. 2), for the D2 conformation the ‘twist’ is the torsion angle C(10)–C(11)–C(11A)–C(10A); for the C2h
conformation, the twist is 08 by symmetry; the half-twist is the torsion angle C(10)–C(11)–C(1A)–C(1). The experimental value for the latter is 36.78.
DE¼E(C2h)2E(D2).
c
co-workers have developed a variety of 1,4-benzadiyne
equivalents for the synthesis of anthracene derivatives. The
simplest precursors are the 1,2,4,5-tetrahalobenzenes,11 but
these are unsuitable for our purpose because the alkyl-
lithium reagents used in such reactions are incompatible
with phencyclone. A more elegant 1,4-benzadiyne equival-
ent is Hart and Ok’s 1,5-diamino-1,5-dihydrobenzo[1,2-
d:4,5-d0]bistriazole,12 but its synthesis had frustrated us in
the past. A neutral, thermally activated 1,4-benzadiyne
equivalent might be the best precursor of 4, and after
examining several possible candidates we decided to
prepare 1,4-bis(phenyliodonio)benzene-2,5-dicarboxylate
(9, Scheme 2). 2-(Phenyliodonio)benzoate is a well-known
thermal benzyne precursor.13 At temperatures above 1608C,
benzyne, iodobenzene, and carbon dioxide are the major
products of its decomposition,13b and the bisiodonium
dicarboxylate 9 should undergo similar reactions. Com-
pound 9 was prepared from 2,5-diiodoterephthalic acid (8)
by using a standard method.14 The doubly zwitterionic 9 is
anthracene by reducing octaphenylanthraquinone with HI in
extremely insoluble; this property facilitates its purification
(impurities can be washed away with hot solvents) but
hinders its characterization and further reaction. Never-
theless, a clean, if noisy, proton NMR spectrum and a
satisfactory FAB mass spectrum were obtained for 9, and
we turned to the synthesis of 4.
refluxing acetic acid.8 Unfortunately, this very useful
method for quinone reduction9 is somewhat compromised
by steric hindrance: anthraquinone is completely reduced in
15 h,9 but octaphenylanthraquinone requires two weeks’
reduction for a 30% yield of the anthracene.8 Compound 5
proved to be even more resistant; no reduction was observed
at all. Reduction of 5 with hydride reagents has been shown
to give partially reduced and isomerized products,10 so we
turned to the second approach to 4 (Scheme 1).
Compounds 6 and 9 and a small amount of g-butyrolactone
(the preferred solvent for such reactions13b) were mixed in a
screw-capped tube and heated to 2708C for 1.5 h. After
cooling, the reaction mixture was dissolved in CH2Cl2 and
fractionated by silica gel preparative TLC, ultimately
yielding a small amount of a pale yellow material with
Reaction of two molecules of phencyclone (6) with a 1,4-
benzadiyne equivalent (7) should give 4 directly. Hart and
Scheme 1.

I. I. Schuster et al. / Tetrahedron 58 (2002) 8875–8882
8877
Scheme 2.
proton NMR and EI mass spectra seemingly consistent with
compound 4. Slow evaporation of the NMR sample gave
single crystals, and X-ray analysis revealed this material to
be the endoperoxide 10 (Fig. 1). Undoubtedly compound 4
had been formed, but it had subsequently reacted with
oxygen. It was then recalled that during chromatography an
initially red band of product had turned yellow. No doubt
this event was the oxygenation in progress! The endo-
peroxide is quite stable (crystals of 10 were unchanged after
six months), but it gave no molecular ion in either EI or FAB
mass spectra; however, its MALDI-TOF mass spectrum
clearly showed an MþH ion (m/z 815).
The pyrolysis of 6 and 9 was then repeated with greater
attention paid to the extraction and fractionation of the
products. Once again, the endoperoxide 10 was the only
high molecular weight product observed, but the cis- and
trans-dihydrophencyclones 11 and 12 were also isolated. It
was noted that the dark red organic extracts faded to yellow
in a matter of hours in ambient light, but no bleaching was
observed when they were kept in the dark. Thus 4 probably
acts as a sensitizer for its own photooxygenation. The
pyrolysis was again performed, but this time precautions
were taken to exclude light during the isolation and
purification of the products. The initial fractions from
Figure 1. Molecular structure of endoperoxide 10. Thermal ellipsoids have been drawn at the 50% probability level.

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I. I. Schuster et al. / Tetrahedron 58 (2002) 8875–8882
Figure 2. Molecular structure of hydrocarbon 4. Thermal ellipsoids have been drawn at the 50% probability level.
column chromatography (silica gel, 1:1 hexanes–benzene)
were red, and upon evaporation of the solvent and addition
of chloroform they deposited a red precipitate of 4, which
was collected and dried in the dark. The proton NMR
spectra of 4 and 10 are easily distinguished by the location
of the singlet due to the central ring protons; in 4 this falls at
d 8.46, in 10 at d 6.79.
molecules, compound 4 lacks phenyl groups on the central
aromatic ring to shield its carbon atoms from contact with
oxygen.
2.2. Structure of 4
The X-ray structure of compound 4 is illustrated in Fig. 2.
To our surprise (and, in truth, disappointment), this
molecule is not a continuously twisted acene ribbon such
as 1 and 2, but rather it possesses crystallographic Ci
symmetry and approximate C2h symmetry. The overall twist
for the molecule is 08 (by symmetry), although the twist
from one end of the molecule to the center [C(10)–C(11)–
C(1A)–C(1)] is 36.78. The observed C2h conformation is in
contradistinction to the calculated D2-symmetric, twisted,
gas-phase conformation of 4, which is favored by 2–4 kcal/
mol by a wide variety of computational methods (Table 1).
In the crystal, packing forces must provide the energy
needed to populate the less stable C2h conformation.
However, the higher-level calculations favor the D2
conformation by as much as 3.6 kcal/mol, which is, in our
experience, a large energy difference to overcome for the
appearance of a less stable conformer in a hydrocarbon
The low yields in the syntheses of 4 and/or 10 from 9
(usually only 1–2% of the purified product) persuaded us to
‘bite the bullet’ and carry out the seven-ste0p synthesis of
1,5-diamino-1,5-dihydrobenzo[1,2-d:4,5-d ]bistriazole12
which we had previously avoided. Unfortunately, the yields
of 4/10 obtained when using this 1,4-benzadiyne precursor
were no better than those from 9 (data not shown).
Recrystallization of 4 from toluene in the dark gave bright
red-orange, single crystals, which proved to be stable
indefinitely to air and light. However, dilute solutions of 4 in
chloroform, initially red-orange, faded to pale yellow in
ambient light within one hour. None of twisted acenes that
we have previously prepared, even the exceptionally twisted
compound 2, exhibit such air sensitivity, but unlike those
Figure 3. Stereo view of the unit cell and crystal packing of hydrocarbon 4.

I. I. Schuster et al. / Tetrahedron 58 (2002) 8875–8882
8879
crystal structure. The crystals contain two molecules of
toluene per molecule of 4, which fit nicely into small clefts
between the tilted phenyl groups of the larger hydrocarbon
(Fig. 3). These interactions and a favorable stacking of the
toluenes may provide the needed stabilization of the C2h
conformation.
In a closely related example, we have reported 12 crystal
structures of simple derivatives of 9,14-diphenylbenzo[b]-
triphenylene (14) which differ only in the identity of the
X-substituents.15 Compound 14 has two low-energy con-
formations of C2 and Cs symmetry, and the twisted C2
conformation is calculated to be 2–3 kcal/mol more stable
than the Cs by a variety of methods. One might expect the Cs
conformation to occur frequently in crystal structures of this
class of molecules. However, although several packing
arrangements were observed for 14, in only one of 12
structures was the Cs conformation observed.15 It is notable
that the average twist of the derivatives of 14 possessing the
C2 conformation is 38.38,15 almost the same as the 36.78
‘half-twist’ for compound 4. In the case of 4, additional
crystal structures would be required in order to determine
whether the high energy, C2h conformation observed here is
an anomaly or a structure generally favored in the solid
state.
Figure 4. Molecular structures of the cis-dihydrophencyclone 11 (top) and
the trans-dihydrophencyclone 12 (bottom). Thermal ellipsoids have been
drawn at the 50% probability level.
several previous occasions,16–18 but the stereochemistry of
the phenyl groups has not been specified. This compound
Evidence for the D2 conformation of 4 in solution might be
obtained by means of NMR in the presence of a chiral
solvent or chiral shift reagent, but the success of such an
experiment requires that racemization of 4 be slow on the
NMR time scale. This is extremely unlikely. A single
transition state for the conversion of the D2 to the C2h
conformation was located at the AM1 level, but the barrier
for this process is only 4.5 kcal/mol. Since racemization
merely requires the reaction sequence (þ)-D2!C2h!(2)-
D2, the detection of the D2 conformation would be
impossible under normal conditions.
¨
has been described in the literature as ‘schone weisse
Krystalnadeln’16 and ‘pearly colorless flakes’;17 thus we
suspect that this material was the cis isomer. However, the
reported melting points of these samples were invariably
sharp (314–3158C,16 320–3238C,17 and 3138C18) with no
reference made to any unusual behavior or decomposition.
This is inconsistent with our own experience; both 11 and 12
exhibit broad melting point ranges, color changes upon
heating, and slow decomposition accompanied by gas
evolution. Upon heating to temperatures near 3008C,
compound 11 becomes quite yellow, and NMR analysis of
material recovered from a melt, before decomposition is too
far advanced, shows it to be a roughly 1:1 mixture of 11 and
12. Compounds 11 and 12 are stable in chloroform solution
at room temperature, but in DMSO solution each also
isomerizes into a mixture of the two. The interconversion is
easily monitored by NMR; the separate resonances for each
isomer change in intensity over a period of several hours,
although the rate is variable and may reflect the presence of
some trace catalyst. No sample of either 11 or 12, including
the crystals used for X-ray analysis, was found with a
melting point range of less than 108C.
2.3. Concerning dihydrophencyclone
Two dihydrophencyclones (11 and 12) are by-products of
the synthesis of 4. Their mechanism of formation is
unclear, but surely some transhydrogenation can occur
under the high temperatures and concentrations of this
reaction. The structures of 11 and 12 were unambiguously
determined by X-ray analysis (Fig. 4); the colorless
compound 11 is the cis isomer, and the bright yellow 12
is the trans isomer.
Interestingly, ‘dihydrophencyclone’ has been reported on

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I. I. Schuster et al. / Tetrahedron 58 (2002) 8875–8882
For comparison, we prepared an authentic sample of
dihydrophencyclone by the method of Sonntag et al.17
Treatment of tetraphenylcyclopentadienone (13) with AlCl3
smoothly gave the colorless cis-dihydrophencyclone 11.
This material was judged to be very clean by NMR, and its
properties proved to be identical in all respects to those of
our own samples, including a broad melting point range and
a propensity for isomerization to 12.
(t, J¼8 Hz, 4H), 7.83 (t, J¼8 Hz, 2H), 8.19 (d, J¼8 Hz, 4H);
MS (FAB), m/z 571 (MþH, 8), 495 (MþH–C6H4, 75), 465
(M2CO–C6H5, 100).
4.1.2. 9,11,20,22-Tetraphenyltetrabenzo[a,c,l,n]penta-
cene 10,21-endoperoxide (10). Compound 9 (0.26 g,
0.46 mmol), phencyclone16 (6, 0.45 g, 1.18 mmol), and
g-butyro-lactone (1.5 mL) were heated in a screw-capped
Pyrex tube at 2508C for 3 h. After cooling, methanol was
added to precipitate a brown solid which was collected by
filtration. This material (0.25 g) was subjected to column
chromatography (silica gel, 1:1 hexanes–benzene). Several
early fractions (Rf 0.6–0.8, alumina TLC, 1:1 hexanes–
benzene) contained the peroxide 10, and these were
combined and further fractionated by preparative TLC
(silica gel GF, 1:1 hexanes–benzene) to give the pure
endoperoxide 10 (4.1 mg, 0.0050 mmol, 1.1%). Further
elution of the silica gel column gave a white solid (Rf 0.2,
alumina TLC, toluene) which proved to be cis-dihydro-
phencyclone 11 (15.1 mg, 0.039 mmol, 3%). The original
methanol filtrate gradually deposited a yellow solid upon
standing, the trans-dihydrophencyclone 12 (36.4 mg,
0.095 mmol, 8%). A second preparation gave a 1.6% yield
of 10.
3. Conclusion
9,11,20,22-Tetraphenyltetrabenzo[a,c,l,n]pentacene (4) has
two surprising properties: it is air-sensitive, and it adopts the
‘wrong’ conformation, at least in the solid state. Given the
stability of our previously prepared polyphenylacenes and
related hydrocarbons to air, light, heat, acids, and bases,4,5,19
the sensitivity of 4 to air was unexpected. This reactivity can
be rationalized in terms of a lack of steric protection at the
central ring, but 1,2,3,4,5,6,7,8-octaphenylanthracene,
which is similarly unencumbered, does not seem to be air-
sensitive.8 A somewhat analogous situation exists for the
polymethylanthracenes. Meador and Hart20 studied the
photochemistry of many of these molecules and observed
that ‘several somewhat less methylated analogues… had to
be prepared and isolated in the dark since even laboratory
fluorescent lights were sufficient to bring about their
[photooxidation].’ The unexpected preference for the C2h
conformation of 4 in the solid state is probably due to a
crystal packing effect. However, in two other studies
where we have compared the crystal conformations of a
series of related polycyclic aromatics with their com-
putational geometries, the higher energy structure was
very seldom found in the crystalline state.15,21 In order to
establish the intrinsic conformational preference of 4, one
would need to devise an experimental method which
could elucidate the geometry of 4 in solution or in the gas
phase.
For 10, mp 2208C; 1H NMR (CDCl3) d 6.79 (s, 2H), 6.94–
7.00 (m, 8H), 7.14 (t, J¼8 Hz, 4H), 7.35–7.41 (m, 20H),
8.39 (d, J¼8 Hz, 4H); 13C NMR (CDCl3) d 74.6, 123.1,
125.4, 126.6, 127.5, 129.2, 129.5, 129.8, 130.3, 130.5,
131.38, 131.44, 131.50, 134.0, 134.6, 139.6 (16 of 16
expected resonances under conditions of slow phenyl
rotation); MS (MALDI-TOF), m/z 815 (MþH); MS
(FAB), m/z 782 (M2O2); MS (EI), m/z 782 (M2O2, 9),
503 (5), 429 (15), 369 (38), 355 (38), 295 (55), 281 (48), 221
(100), 207 (63); exact mass 782.2936, calcd for C62H38
(M2O2) 782.2975. Single crystals of 10 were obtained by
the slow evaporation of a CDCl3 solution.
For 11, mp behavior: ca. 2958C becomes distinctly yellow
and shrinkage occurs; 309–3228C melts with slow gas
evolution [lit. (see text) 314–3158C,16 320–3238C,17
4. Experimental
1
3138C18]; H NMR (CDCl3) d 5.20 (s, 2H), 7.19–7.28 (m,
10H), 7.49 (ddd, J¼8, 7, 1 Hz, 2H), 7.58 (dd, J¼8, 1 Hz,
2H), 7.68 (ddd, J¼8, 7, 1 Hz, 2H), 8.82 (d, J¼8 Hz, 2H);
MS (EI), m/z 384 (Mþ, 100), 382 (M2H2, 15), 356
(M2CO, 19), 354 (M2H2–CO, 18), 279 (M2CO–C6H5,
87). Single crystals of 11 were obtained from CHCl3–
acetone.
4.1. Data for compounds
4.1.1. 1,4-Bis(phenyliodonio)benzene-2,5-dicarboxylate
(9). Potassium persulfate (3.00 g, 11.1 mmol) was added
in small portions to a solution of 2,5-diiodoterephthalic
acid22 (8, 0.96 g, 2.3 mmol) in concentrated sulfuric acid
(10 mL) at 2108C. The resulting mixture was stirred for
5 min at 2108C, and then it was allowed to warm to room
temperature. After 20 min stirring, the mixture was again
cooled to 2108C, benzene (2.5 mL) was added, and the
mixture was stirred for 1 h below 108C. The reaction
mixture was then cooled to 2158C, and ice water (22 mL)
was added slowly so that the temperature did not exceed
108C. The pH was adjusted to 9 by the dropwise addition of
concentrated ammonia (ca. 27 mL). The resulting precipi-
tate was collected by filtration, washed with water, and dried
overnight under vacuum. The yield of the gray product 9
was 0.89 g (1.6 mmol, 68%). Further purification of this
material was achieved by leaching the solid with hot
methanol followed by hot acetone; mp .4008C (but
For 12, mp behavior: ca. 2408C partly melts to give a
resinous semisolid; ca. 2708C darkens; 300–3108C melts
1
completely with slow gas evolution; H NMR (CDCl3) d
5.24 (s, 2H), 7.14–7.16 (m, 4H), 7.25–7.29 (m, 6H), 7.48
(ddd, J¼8, 7, 1 Hz, 2H), 7.58 (dd, J¼8, 1 Hz, 2H), 7.67
(ddd, J¼8, 7, 1 Hz, 2H), 8.80 (d, J¼8 Hz, 2H); MS (EI), m/z
384 (Mþ, 100), 382 (M2H2, 25), 370 (M2CH2, 50), 356
(M2CO, 21), 354 (M2H2–CO, 26), 279 (M2CO–C6H5,
97). Single crystals of 12 were obtained from CH2Cl2–
acetone.
4.1.3. 9,11,20,22-Tetraphenyltetrabenzo[a,c,l,n]penta-
cene (4). Compound 9 (0.259 g, 0.455 mmol), phencyclone
(0.451 g, 1.18 mmol), and g-butyrolactone (1.5 mL) were
1
becomes dark). H NMR (DMSO-d6) d 7.52 (s, 2H), 7.65

I. I. Schuster et al. / Tetrahedron 58 (2002) 8875–8882
Table 2. Crystallographic data for compounds 4, 10–12
8881
10
11
12
4
Chemical formula
Formula weight
Crystal size (mm)
Space group
C
62H38O2·CHCl3
C29H20O·C3H6O
442.53
C29H20
384.45
O
C62H38·2C7H8
967.19
934.29
0.22£0.17£0.11
Pbcn (No. 60)
13.7466(5)
17.9472(6)
18.6378(7)
90
90
90
4598.2(3)
4
1.350
0.21£0.14£0.10
0.32£0.25£0.12
0.15£0.14£0.02
P21/c (No. 14)
13.7224(7)
14.5041(7)
14.3718(7)
90
117.077(2)
90
2546.9(2)
2
1.261
ꢀ
P 1 (No. 2)
ꢀ
P 1 (No. 2)
˚
a (A)
10.1687(3)
10.3813(3)
12.6836(4)
110.809(1)
101.383(1)
101.647(1)
1171.68(6)
2
1.254
0.077
200(2)
27.5
10.2497(3)
10.5972(4)
11.2543(4)
109.198(1)
105.151(2)
110.492(2)
978.57(6)
2
1.305
0.077
200(2)
27.5
˚
b (A0
˚
c (A)
a (deg.)
b (deg.)
g (deg.)
3
˚
V (A )
Z
rcalcd (g/cm3)
m (mm21
T (K)
)
0.248
200(2)
25.0
0.071
200(2)
22.5
umax (deg.)
Reflections
Total
Unique
25263
4033
2688
0.082
0.218
1.13
0.120
0.246
1.03
13699
5344
3819
0.050
0.124
1.04
0.077
0.145
1.02
11893
4470
3143
0.048
0.120
1.04
0.082
0.151
1.02
11281
3315
1861
0.063
0.133
1.121
0.1315
0.1648
0.999
Observed [I.2s(I)]
R(F) (obs. data)a
wR(F 2) (obs. data)a
S (obs. data)a
R(F) (all data)a
wR(F 2) (all data)a
S (all data)a
P
P
P
P
P
R(F)¼ llFol2lFcll/ lFol; wR(F 2)¼[ w(Fo22F2c)2/ w(Fo2)2]1/2; S¼goodness-of-fit on F 2¼[ w(F2o2F2c)2/(n2p)]1/2, where n is the number of reflections
and p is the number of parameters refined.
a
heated in a screw-capped Pyrex tube at 2658C for 2.25 h.
KappaCCD diffractometer. The diffraction data were
processed by using the program DENZO.23 All structures
were solved by direct methods using Siemens SHELXTL,24
and all were refined by full-matrix least-squares on F 2 using
SHELXTL. All nonhydrogen atoms were refined aniso-
tropically, and hydrogens were included with a riding
model. Specific crystal, reflection, and refinement data are
contained in Table 2. Crystallographic data (excluding
structure factors) for the structures in this paper have
been deposited with the Cambridge Crystallographic Data
Centre as supplementary publication numbers CCDC
187845-187848. Copies of the data can be obtained, free
of charge, on application to CCDC, 12 Union Road,
Cambridge, CB2 1EZ, UK [fax: þ44-1223-336033 or
e-mail: deposit@ccdc.cam.ac.uk].
After cooling, methanol was added to precipitate 0.184 g of
a brown solid which was collected by filtration, and the
filtrate deposited an additional 0.023 g upon standing. The
combined solids (0.207 g) were air dried while protected
from light. This material was then chromatographed on a
silica gel column (1:1 hexanes–benzene), with both the
column and receiving flasks wrapped in aluminum foil to
protect them from light. The initial fractions from the
column were red in color, and after concentration and then
addition of chloroform, they deposited a red precipitate.
This material was collected and dried in the dark to yield
pure compound 4 (4.3 mg, 0.0055 mmol, 1.2%), mp
.3008C. Concentration of the remaining solution to dryness
left a mixture of 4 and 10 (41.2 mg, ,0.0515 mmol, 11.4%),
which indicates that the yield of the double aryne addition
was at least 12%. 1H NMR (CDCl3) d 6.87 (t, J¼8 Hz, 4H),
7.22–7.40 (m, 24H), 7.46 (d, J¼8 Hz, 4H), 8.12 (d, J¼8 Hz,
4H), 8.46 (s, 2H); 13C NMR (CDCl3) d 123.3, 125.3, 125.9,
127.0, 127.3, 128.3, 128.7, 128.8, 130.5, 130.9, 131.7,
132.3, 135.5, 141.5 (14 of 14 expected resonances); MS
(EI), m/z 782 (Mþ, 100), 391 (M2þ, 10); exact mass
782.2943, calcd for C62H38 782.2975.
4.3. Computational studies
All semiempirical (AM16), ab initio [HF/STO-3G, HF/3-
21G, HF/6-31G(d)25], and hybrid density functional
[B3PW91/6-31G(d), B3PW91/cc-pVDZ, B3LYP/6-
31G(d), B3LYP/6-311G(d,p)26–30] calculations were per-
formed by using GAUSSIAN 98.31 The built-in default
thresholds for wave function and gradient convergence were
employed. Transition states for the conformational inter-
conversions were located by using the QST3 function in
GAUSSIAN 98.
Compound 4 is an air- and light-stable solid, but in
solution in the presence of air under ambient room light it
is rapidly oxygenated to give 10. Single crystals of 4
were obtained by the evaporation of a toluene solution in the
dark.
4.2. General X-ray crystallographic procedures
Acknowledgements
X-Ray data were collected by using graphite mono-
˚
chromated Mo Ka radiation (0.71073 A) on a Nonius
This work was supported by National Science Foundation
Grant CHE-0077990, which is gratefully acknowledged.

8882
I. I. Schuster et al. / Tetrahedron 58 (2002) 8875–8882
19. Pascal, Jr. R. A.; McMillan, W. D.; Van Engen, D.; Eason,
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