Addition of dihalocarbenes to corannulene. A fullerene-type reaction

Article abstract of DOI:10.1016/S0040-4039(00)01734-2

Dihalocarbenes (:CCl₂, :CBr₂, and :CI₂) add preferentially to one of the radial double bonds of corannulene, rather than to the rim. These cyclopropanations strongly resemble the additions of dihalocarbenes to fullerenes, which likewise occur at 6:6-double bonds, destroy the cyclic conjugation in two adjacent benzene rings, and give 'closed' adducts. An explanation is offered for the abnormally high reactivity of the interior carbon atoms of corannulene. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)01734-2

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 9633–9637
Addition of dihalocarbenes to corannulene. A fullerene-type
†
reaction
Dorin V. Preda and Lawrence T. Scott*
Boston College, Merkert Chemistry Center, Chestnut Hill, MA 02467-3860, USA
Received 31 August 2000; accepted 29 September 2000
Abstract
Dihalocarbenes (:CCl , :CBr , and :CI ) add preferentially to one of the radial double bonds of
2
2
2
corannulene, rather than to the rim. These cyclopropanations strongly resemble the additions of
dihalocarbenes to fullerenes, which likewise occur at 6:6-double bonds, destroy the cyclic conjugation in
two adjacent benzene rings, and give ‘closed’ adducts. An explanation is offered for the abnormally high
reactivity of the interior carbon atoms of corannulene. © 2000 Elsevier Science Ltd. All rights reserved.
Keywords: carbenes; cyclopropanation; orbital effects; polycyclic aromatic compounds.
The pronounced bond length alternation around the six-membered rings of the bowl-shaped
1
corannulene molecule resembles that seen in the central ring of phenanthrene. It would be
natural to assume, therefore, that the cyclopropanation of corannulene with dichlorocarbene
2
should mimic that of phenanthrene to give adduct 1 (Scheme 1). We find, however, that this
mode of addition does not occur, despite the fact that formation of 1 would preserve the greatest
possible number of benzene rings and would introduce the least additional strain into the
corannulene skeleton.
Cl Cl
Cl Cl
:
CCl2
:CCl2
phenanthrene
corannulene
Scheme 1.
1
*
Corresponding author. Fax: 617-552-2705; e-mail: lawrence.scott@bc.edu
We dedicate this paper to Professor Harry Wasserman on the occasion of his 80th birthday.
†
0
040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)01734-2

9634
Instead of adding to one of the perimeter bonds of corannulene, dichlorocarbene adds
3,4
exclusively to one of the radial bonds, giving adduct 2 as the primary product. As the reaction
proceeds, the reactive olefinic bonds in 2 begin to compete with corannulene for the carbene,
5
and a 2:1 adduct (3) starts to accumulate as a secondary product (Eq. (1)). The reaction can be
conveniently monitored by NMR spectroscopy, but at no time do we ever see any evidence for
adduct 1.
Cl
Cl Cl
Cl Cl
Cl
CHCl3
(1)
aq. NaOH 50%
+
-
Bu N HSO
4
4
2
3
Because the 1:1 adduct (2) goes on to the 2:1 adduct (3) at a rate similar to that of its
formation, the concentration of 2 never reaches a high level. Nevertheless, by careful chromatog-
raphy we were able to obtain a relatively pure sample of 2, contaminated by only ca. 5% of
1
residual corannulene. The H NMR spectrum of 2 (Fig. 1) shows the expected singlet and two
doublets in the region for aromatic hydrogens and two doublets in the region for olefinic
hydrogens.
1
Figure 1. H NMR spectrum of the 1:1 adduct (2)
The large coupling constant observed for the two high field signals (J =9.6 Hz) is consistent
vic
with a ‘closed’ propellane-type structure for 2 that has an intact cyclopropane ring and two
6
localized double bonds. Density functional calculations (pBP/DN**//AM1) likewise favor the
closed structure over an ‘open’ structure (cf. 1,6-methano[10]annulene) as the global energy
minimum for 2. The dichlorocarbene adduct of corannulene thus resembles the dichlorocarbene
adduct of C , in which the carbene has added to a 6:6-double bond and the cyclopropane ring
6
0
7
remains closed.
Why does dichlorocarbene add to the radial bond of corannulene when that mode of addition
(
2) disrupts the cyclic conjugation in two benzene rings, rather than in only one, and clearly
introduces more strain than the perfectly reasonable alternative mode (1) would have? Like most
carbene additions, this one is almost certainly irreversible under the reaction conditions
employed, i.e. we are dealing with a kinetically controlled process (discussed further below).
8
Recent computational studies and kinetic isotope effect measurements indicate that the
transition state for dichlorocarbene addition to a double bond is very unsymmetrical, the

9635
LUMO of the electrophilic carbene interacting most strongly with the largest HOMO coefficient
it can find in the p-system. In extreme cases, there is even experimental evidence that the process
9
may be stepwise. When dichlorocarbene approaches corannulene, therefore, it seeks out the site
where the highest occupied molecular orbital (HOMO) coefficient is the largest. MO calculations
reveal that the HOMO of corannulene is doubly degenerate and that both orbitals have their
largest coefficients at the interior carbon atoms (Fig. 2). Dichlorocarbene is thus drawn to the
five-membered ring of corannulene by a strong HOMO–LUMO interaction. If the addition were
stepwise, the zwitterionic intermediate would have structure 4. Regardless of whether the
reaction is concerted and asynchronous or completely stepwise, once bonding between the
carbene and an interior carbon atom of corannulene has begun, the formation of adduct 1
ceases to be an option.
Figure 2. Left: Superimposed maps of the two degenerate highest occupied molecular orbitals of corannulene
(
pBP/DN**//AM1). Right: The intermediate to be expected if the carbene addition were completely stepwise
One case in which a dichlorocarbene cyclopropanation has been observed to proceed
thermally in the reverse direction bears an intriguing structural resemblance to our adduct 2.
Vogel et al. found that propellane 6, though less stable than the ‘open’ 11,11-dichloro-1,6-meth-
ano[10]annulene 5, can be formed reversibly by heating 5 and that it slowly extrudes dichloro-
1
0
carbene at 50°C. When cyclohexene was present to trap the carbene, 7,7-dichloronorcarane (7)
and naphthalene were obtained as the major products (Eq. (2)).
Cl
Cl
Cl
Cl
∆
Cl
Cl
+
(2)
cyclohexene
5
6
7
Motivated by this example, we heated adduct 2 in cyclohexene and observed a slow increase
in the amount of corannulene (NMR with an internal standard). The absence of 7,7-
dichloronorcarane (7) in the reaction mixture (NMR and GC/MS), however, argues against
carbene extrusion as the pathway leading back to corannulene. What we did observe instead of
7
was a substantial amount of the dimer of cyclohexenyl radical (3,3%-dicyclohexenyl). These
11
results suggest a stepwise decomposition pathway in which 2 first breaks only one bond to give
a diradical analogous to 4; a hydrogen atom transfer from cyclohexene to the CCl radical
2
center would account for the formation of cyclohexenyl radical, and subsequent loss of a CHCl2
radical would generate corannulene. Thus, the cyclopropanation of corannulene with dichloro-
carbene does not appear to be thermally reversible, at least not as a concerted process.
Dibromocarbene, generated from bromoform by the same phase transfer method as in Eq.
(
1), behaves the same as dichlorocarbene toward corannulene, affording a 1:1 adduct of type 2

9636
1
2
and a 2:1 adduct of type 3. To allay concerns that the 1:1 adduct might be arising from
nucleophilic addition of the initially formed trihalomethyl anion, rather than from a true
carbene cycloaddition, we generated dibromocarbene independently from Seyferth’s reagent
1
3
(
PhHgCBr ) in the presence of corannulene under neutral conditions in refluxing benzene. The
3
same 1:1 adduct of type 2 was obtained from this reaction; no evidence could be seen, however,
for any 2:1 adduct. The exclusive formation of a 1:1 adduct of type 2 in this reaction confirms
that free dihalocarbenes exhibit a kinetic preference for cyclopropanation of the radial bonds of
corannulene. The absence of any 2:1 adduct in this case suggests that the concentration of 1:1
adduct never built up to a high enough level to compete with corannulene for the carbene.
Control experiments confirm that the 1:1 dibromocarbene adduct of type 2 decomposes readily
in refluxing benzene.
Diiodocarbene generated from iodoform by the same phase transfer method as in Eq. (1)
14
likewise gives a 1:1 adduct of type 2 in low yield. No 2:1 adduct of type 3 was seen in this case,
perhaps owing to the steric demands of the iodine atoms, although this might also be just a
consequence of the low concentration of 1:1 adduct.
In conclusion, the addition of dihalocarbenes to corannulene provides a striking example of
the reactivity of interior carbon atoms of open geodesic polyarenes. The only prior examples of
dihalocarbene additions to polycyclic p-systems that destroy simultaneously the cyclic conjuga-
7
tion in two benzene rings are the additions of dihalocarbenes to fullerenes and to diindenochry-
sene, a bowl-shaped alkene-centered C H fullerene fragment that also exhibits fullerene-like
2
6
12
1
5
reactivity.
Acknowledgements
Financial support from the National Science Foundation is gratefully acknowledged.
References
1
2
3
. (a) Hanson, J. C.; Nordman, C. E. Acta Crystallogr., Sect. B 1976, 32, 1147–1153. (b) Hedberg, L.; Hedberg, K.;
Cheng, P.-C.; Scott, L. T. J. Phys. Chem. 2000, 104, 7689–7694.
. (a) Joshi, G. C.; Singh, N.; Pande, L. M. Synthesis 1972, 317. (b) Blume, G.; Neumann, T.; Weyerstahl, P. Justus
Liebigs Ann. Chem. 1975, 201–208.
. To a solution of corannulene (100 mg, 0.4 mmol) in 50 mL of chloroform were added 10 mL of a 50% aqueous
solution of NaOH and tetrabutylammonium hydrogen sulfate (20 mg) as a phase transfer catalyst. The mixture
was stirred at room temperature for 20 h. The resulting brown solution was poured into 200 mL of water and
extracted with chloroform (3×15 mL). The combined organic solutions were dried over anhydrous magnesium
sulfate, and the solvent was removed under reduced pressure to give a dark brown solid. The crude product (ca.
1
1
05 mg) contains ca. 90% of corannulene, 8% of 2 and 2% of 3 (as determined by H NMR spectroscopy).
Column chromatography on neutral alumina with 6:1 pentane:ether as eluant followed by preparative TLC (on
silica gel with 6:1 pentane:ether as eluant) afforded relatively pure samples of 2 and 3.
1
4
. Selected data for compound 2: H NMR (400 MHz, CDCl ) l (ppm): 7.78 (s, 2H), 7.66 (d, J=8.8 Hz, 2H), 7.16
3
13
(
d, J=8.8 Hz, 2H), 6.80 (d, J=9.6 Hz, 2H), 5.88 (d, J=9.6 Hz, 2H); C NMR (100 MHz, CD Cl ) l (ppm):
2 2
1
41.0, 138.3, 133.1, 129.3, 128.5, 128.3, 127.8, 126.0, 118.3, 93.0, 86.7, 50.1; HRMS: calcd. for C H Cl :
21 10 2
3
34.0130. Found: 334.0127.
1
5
. Selected data for compound 3: H NMR (300 MHz, CDCl ) l (ppm): 7.69 (d, J=9.0 Hz, 1H), 7.63 (d, J=7.8
3
Hz, 1H), 7.62 (d, J=8.4 Hz, 1H), 7.60 (d, J=8.7 Hz, 1H), 7.16 (d, J=8.4 Hz, 1H), 7.14 (d, J=8.4 Hz, 1H), 6.87

9637
13
(
d, J=9.6 Hz, 1H), 6.07 (d, J=9.6 Hz, 1H), 3.07 (d, J=10.2 Hz, 1H), 2.95 (d, J=10.2 Hz, 1H); C NMR (75
MHz, CDCl ) l (ppm): 145.3, 142.5, 138.9, 137.6, 136.7, 132.7, 130.9, 129.5, 120.9, 128.2, 127.5, 126.8, 125.4,
3
1
20.5, 63.6, 62.5, 46.7, 43.3, 36.5, 35.7 (20 signals seen out of 22 expected); HRMS: calcd. for C H Cl :
22 10 4
4
14.9615. Found: 414.9615.
6. Minimum energy molecular geometries were optimized at the AM1 level of theory, and single point energies were
calculated at the pBP/DN** level of theory using the computational chemistry programs implemented in Spartan
5
.0 (Wavefunction, Inc., Irvine, CA 92612).
7. Tsuda, M.; Ishida, T.; Nogami, T.; Kurono, S.; Ohashi, M. Tetrahedron Lett. 1993, 34, 6911–6912.
8. Keating, A. E.; Merrigan, S. R.; Singleton, D. A.; Houk, K. N. J. Am. Chem. Soc. 1999, 121, 3933–3938.
9. (a) Murray, R. W. Tetrahedron Lett. 1960, 7, 27–29. (b) Weber, J.; Brinker, U. H. Angew. Chem., Int. Ed. Engl.
1
997, 36, 1623–1626.
10. Rautenstrauch, V.; Scholl, H. J.; Vogel, E. Angew. Chem., Int. Ed. Engl. 1968, 7, 288–289.
1
1. (a) Hoffmann, R. W. Angew. Chem., Int. Ed. Engl. 1971, 10, 529–537. (b) Hoffmann, R. W. Acc. Chem. Res.
1
985, 18, 248–253.
1
1
2. Selected data for dibromocarbene adducts: 1:1 adduct: H NMR (400 MHz, CDCl ) l (ppm): 7.77 (s, 2H), 7.66
3
1
(
d, J=8.4 Hz, 2H), 7.15 (d, J=8.4 Hz, 2H), 6.84 (d, J=9.6 Hz, 2H), 5.88 (d, J=9.6 Hz, 2H); 2:1 adduct: H
NMR (400 MHz, CDCl ) l (ppm): 7.68 (d, J=9.2 Hz, 1H), 7.64 (d, J=8.8 Hz, 1H), 7.62 (d, J=8.8 Hz, 1H),
3
7
.59 (d, J=9.2 Hz, 1H), 7.17 (d, J=8.4 Hz, 1H), 7.13 (d, J=8.4 Hz, 1H), 6.92 (d, J=9.6 Hz, 1H), 6.11 (d, J=9.6
Hz, 1H), 3.09 (d, J=10.0 Hz, 1H), 3.04 (d, J=10.0 Hz, 1H).
1
1
3. Seyferth, D. Accounts Chem. Res. 1972, 5, 65–74 and references cited therein.
4. Selected data for the diiodocarbene adduct: 1:1 adduct: H NMR (400 MHz, CDCl ) l (ppm): 7.74 (s, 2H), 7.65
1
3
(
d, J=8.8 Hz, 2H), 7.13 (d, J=8.8 Hz, 2H), 6.94 (d, J=9.6 Hz, 2H), 5.82 (d, J=9.6 Hz, 2H).
5. Scott, L. T.; Bronstein, H. E.; Preda, D. V.; Ansems, R. B. M.; Bratcher, M. S.; Hagen, S. Pure Appl. Chem.
999, 71, 209–219.
1
1
.