DOI:
10.1039/A908852K
(Communication)
Chem. Commun., 2000, 19-20
Bucky-bowls. A general approach to benzocorannulenes:
synthesis of mono-, di- and tri-benzocorannulenes
Received
8th November 1999
, Accepted 16th November 1999
First published on 24th December 1999
Abstract
We outline a conceptually simple and general route to
bowl-shaped benzocorannulenes based on readily assembled PAHs which on
flash vacuum pyrolysis result in the sequential formation of a five- and
six-membered ring; following this approach, syntheses of mono-, di- and
tri-benzocorannulenes have been achieved.
As a part of our continuing interest in the synthesis of
C60 fullerene (bucky-ball) and its fragments
(bucky-bowls),1,2 we became interested
in developing a synthetic approach to pentabenzocorannulene 1en route (see transannular bridging indicated in 1) to
the ‘deep-bowl’
2,
C40H10.3 Bowl-shaped
1 and 2 represent 2/3 of the carbon content of
C60 with eleven and sixteen rings, respectively, constituting a
dominant cross-section on the fullerene surface. Both 1 and
2 evoke considerable synthetic interest and are formidable
objectives. As a prelude to efforts towards 1 and 2, we
have developed a new and general synthetic route to benzoannulated
corannulenes in which an appropriately constructed aromatic array upon
flash vacuum pyrolysis (FVP) undergoes two-fold C–C bond formation
involving cyclodehydrogenation to generate a five-membered ring, followed
by insertion of vinylidene carbene or equivalent species to form a
six-membered ring. Herein, we report the synthesis of mono-, di- and
tri-benzocorannulenes 3–5.
Our
approach to benzocorannulene 3 emanated from 13-
methylbenzo[g]chrysene 7, readily available from
9-methyl- phenanthrene 6 through a tactical modification of the
reported procedure.4 The methyl group in
7 was oxidised to the required aldehyde 9 in two steps
via the intermediate bromide 8 (Scheme 1). The aldehyde functionality in
9 was then elaborated to 10–12 having
active functionalities, which on thermal activation under FVP conditions
were expected to result in the projected two-fold cyclization. Indeed, FVP
of 10–12 furnished 3, albeit in low yields
characteristic of such reactions (Scheme
2).2 Benzocorannulene 3
was readily identified through its spectral characteristics (UV, 2D NMR,
mass).5,6
![Reagents and conditions: i, NBS, AIBN, CCl4, 73%;
ii, (Bu4N)2Cr2O7,
CHCl3, 76%.](/image/article/2000/CC/a908852k/a908852k-s1.gif) |
| Scheme 1
Reagents and conditions: i, NBS, AIBN, CCl4, 73%;
ii, (Bu4N)2Cr2O7,
CHCl3, 76%.
| |
![Reagents and conditions: i, CBr4, PPh3,
Zn, CH2Cl2, 85%; ii,
ClCH2PPh3Cl, ButOK, 0.5 h, 68%; iii,
ClCH2PPh3Cl, ButOK, 2 h, 60%; iv, FVP,
1150 °C, 5–7%; v, FVP, 1150 °C, 2–3%; vi, FVP, 1150
°C, 1–2%.](/image/article/2000/CC/a908852k/a908852k-s2.gif) |
| Scheme 2
Reagents and conditions: i, CBr4, PPh3,
Zn, CH2Cl2, 85%; ii,
ClCH2PPh3Cl, ButOK, 0.5 h, 68%; iii,
ClCH2PPh3Cl, ButOK, 2 h, 60%; iv, FVP,
1150 °C, 5–7%; v, FVP, 1150 °C, 2–3%; vi, FVP, 1150
°C, 1–2%.
| |
Our approach to dibenzocorannulene 4 originated from
5-methylbenzo[c]phenanthrene 13, in turn readily
accessible from commercial 2-methylnaphthalene.7 Naphthoannulation on 13 through the
intermediacy of the Wittig salt 14 and photocyclization of the
resulting stilbene derivative 15 led to
13-methyldibenzo[c,p]chrysene 16 (Scheme 3). The methyl group in 16 was
oxidised to the aldehyde 17 and in the light of the relatively
more efficient conversion 10→3 was further
transformed to the hexacyclic gem-dibromoalkene 18, the
desired FVP precursor. On thermal activation 18 underwent the
expected double cyclization to furnish the new dibenzocorannulene
4 and was fully characterised on the basis of incisive spectral
analyses5 (Scheme
3).
![Reagents and conditions: i, NBS 99%; ii, PPh3,
C6H6, 79%; iii,
p-MeC6H4CHO, Cs2CO3,
PriOH, 80%; iv, hv, I2,
C6H6, propylene oxide, 65%; v, NBS, CCl4,
44%; vi, (Bu4N)2Cr2O7,
CHCl3, 77%; vii, CBr4, PPh3, Zn,
CH2Cl2, 85%; viii, FVP, 1150 °C, 5–7%.](/image/article/2000/CC/a908852k/a908852k-s3.gif) |
| Scheme 3
Reagents and conditions: i, NBS 99%; ii, PPh3,
C6H6, 79%; iii,
p-MeC6H4CHO, Cs2CO3,
PriOH, 80%; iv, hv, I2,
C6H6, propylene oxide, 65%; v, NBS, CCl4,
44%; vi, (Bu4N)2Cr2O7,
CHCl3, 77%; vii, CBr4, PPh3, Zn,
CH2Cl2, 85%; viii, FVP, 1150 °C, 5–7%.
| |
Interestingly, 5-methylbenzo[c]phenanthrene 13 and the
Wittig salt 14 derived from it also served as the precursor for
the synthesis of tribenzocorannulene 5. Wittig coupling between
14 and 4-methylnaphthaldehyde gave 19 which on
photocyclization led to the naphtho[1,2-f]picene derivative
20 (Scheme 4). The methyl group
in 20 was again elaborated to the aldehyde 21 and further
to the FVP precursor 22. As planned, FVP on 22 furnished
the desired tribenzocorannulene 5, which was spectroscopically
characterised (Scheme 4).5,6
![Reagents and conditions: i, 4-methylnaphthaldehyde,
Cs2CO3, PriOH; ii, hv,
I2, C6H6, propylene oxide, 50% (2 steps);
iii, NBS, CCl4, 45%; iv,
(Bu4N)2Cr2O7, CHCl3,
70%; v, CBr4, PPh3, Zn, CH2Cl2,
55%; vi, FVP, 1150 °C, 1–2%.](/image/article/2000/CC/a908852k/a908852k-s4.gif) |
| Scheme 4
Reagents and conditions: i, 4-methylnaphthaldehyde,
Cs2CO3, PriOH; ii, hv,
I2, C6H6, propylene oxide, 50% (2 steps);
iii, NBS, CCl4, 45%; iv,
(Bu4N)2Cr2O7, CHCl3,
70%; v, CBr4, PPh3, Zn, CH2Cl2,
55%; vi, FVP, 1150 °C, 1–2%.
| |
In short, we have accomplished the syntheses of bowl-shaped
benzocorannulenes 3–5 from appropriate polycyclic
aromatics employing FVP as the key step, in which a five- and a
six-membered rings are sequentially formed. The precursor polycyclic
platforms were assembled from simple aromatic starting materials through an
iterative sequence involving Wittig olefination and photocyclization steps.
Notwithstanding the low yields in the final FVP step, which is not uncommon
for such cyclizations,1,2 this work
demonstrates the generality of our approach and sets the stage for the
synthesis of 1 and 2.
Acknowledgements
We thank JNCASR for financial support and the SIF facility at I.I.Sc for
high field NMR data. One of us (P. V. V. S. S.) thanks CSIR for a research
fellowship. We thank Professor L. T. Scott for generously providing copies
of spectra for comparison purposes.
References
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- All new compounds reported here were fully characterised on the
basis of their spectral (UV, IR, 2D 1H and 13C NMR,
MS) and analytical data. Selected data for 3: mp 253
°C; λmax(MeOH)/nm 305, 275, 260 and 240;
δH(300 MHz; CDCl3), 8.68 (2H, dd, J 6
and 3.3), 8.26 (2H, d, J 8.7), 7.95 (2H, d, J 8.7), 7.84
(4H, ABq, J 8.7), 7.76 (2H, dd, J 5.7 and 3.3);
δC(75 MHz; CDCl3) 137.6 (qC), 135.4 (qC), 134.6
(qC), 133.1 (qC), 130.8 (qC), 130.5 (qC), 128.9 (qC), 127.5 (CH), 127.3
(CH), 127.1 (CH), 126.9 (CH), 125.1 (CH) and 124.3 (CH); m/z 300
(M+). For 4: mp >250 °C (decomp.);
λmax(MeOH)/nm 319, 272, 257 (sh), 242 (sh);
δH(300 MHz; CDCl3) 9.41 (2H, d, J
8.4), 8.83 (2H, d, J 7.5), 8.35 (2H, d J 8.7), 8.01 (2H,
d, J 8.4), 7.91 (2H, s), 7.88–7.77 (4H, m);
δC(75 MHz; CDCl3) 136.7 (qC), 134.2 (qC), 134.0
(qC), 133.9 (qC), 133.7 (qC), 130.2 (qC), 128.5 (qC), 127.8 (CH), 127.5
(CH), 127.1 (CH), 127.0 (CH), 126.5 (CH), 125.5 (CH), 124.5 (qC), 123.9
(CH); m/z 350 (M+). For 5:
λmax(MeOH)/nm 347, 334, 279, 252; δH(400
MHz; CDCl3) 9.41 (2H, d, J 8), 8.86 (2H, d, J
7.2) 8.73 (2H, dd, J 6.4 and 3.6), 8.45 (4H, ABq, J 8.4),
7.87 (2H, d, J 8), 7.83 (2H, d, J 9.2), 7.79 (2H, dd,
J 6 and 3.2); m/z 400 (M+)..
- Mono- 3 and tri-benzocorannulene 5 reported here
have been prepared previously by Scott et al. [ref. 1(b),
(e)] following entirely different routes. See also: B. McMahon,
B.S. Thesis, Boston College, 1997; C. C. McComas, B.S. Thesis, Boston
College, 1996. Since the details of this work are not published, we have
provided here the spectral data and also compared the spectra of our
synthetic compounds with theirs. Dibenzocorannulene 4 has been
prepared for the first time..
- D. L. Nagel, R. Kupper, K. Antonson and L. Wallcave, J. Org. Chem., 1977, 42, 1977
.
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