Palladium-catalyzed trimerization of strained cycloalkynes: Synthesis of decacyclene

Article abstract of DOI:10.1055/s-2002-20473

Palladium-catalyzed cyclotrimerization was applied to three strained cycloalkynes. Pd(PPh₃)₄ and Pt(PPh₃)₄-catalyzed cyclotrimerizations of cyclohexyne (2) afforded dodecahydrotriphenylene (3) in 64% and 62% yields, respectively, but subjecting cyclopentyne to the same conditions failed to afford isolable amounts of the cyclotrimer. Finally, decacyclene (15), a putative C₆₀-fullerene precursor, was obtained in 23% yield by Pd₂(dba)₃-catalyzed cyclotrimerization of acenaphthyne (14).

Full text of DOI:10.1055/s-2002-20473

486
LETTER
Palladium-catalyzed Trimerization of Strained Cycloalkynes: Synthesis of
Decacyclene
P
alladium-Cataly
e
ze
d
T
rimer
a
ization of
S
t
traine
d
r
Cycloa
l
i
kynes z Iglesias, Diego Peña, Dolores Pérez,* Enrique Guitián,* Luis Castedo
Departamento de Química Orgánica y Unidad Asociada al CSIC, Universidad de Santiago, 15782 Santiago de Compostela, Spain
E-mail: qoenrgui@usc.es
Received 26 November 2001
spectively.12 Copper-promoted oligomerization of
Abstract: Palladium-catalyzed cyclotrimerization was applied to
strained 1,2-diiodoperfluorocycloalkenes has also been
three strained cycloalkynes. Pd(PPh3)4 and Pt(PPh3)4-catalyzed cy-
clotrimerizations of cyclohexyne (2) afforded dodecahydrotriphe-
achieved in moderate yields.13
nylene (3) in 64% and 62% yields, respectively, but subjecting
Although the above compounds may formally be consid-
cyclopentyne to the same conditions failed to afford isolable
ered as oligomers of the corresponding cycloalkyne, this
amounts of the cyclotrimer. Finally, decacyclene (15), a putative
does not mean that free cycloalkynes are involved in their
C60-fullerene precursor, was obtained in 23% yield by Pd2(dba)3-
formation.14
catalyzed cyclotrimerization of acenaphthyne (14).
We decided to start our study with the trimerization of a
six-membered cycloalkyne, cyclohexyne (2). To keep the
reaction conditions as close as possible to those previous-
ly used for the trimerization of arynes we selected triflate
1 as the alkyne precursor.15 Cyclohexyne (2) was generat-
ed by addition of CsF to a solution of 1 and 10 mol% of
Pd(PPh3)4 in acetonitrile at 20 ºC. After 12 h at room tem-
perature a 64% yield of dodecahydrotriphenylene (3) was
isolated (Scheme 1).16
Key words: arynes, transition metals, palladium
The organometallic chemistry of arynes has been investi-
gated in recent years from several points of view. Studies
aimed at the synthesis of metal-aryne complexes have
been carried out by Bennett, Buchwald and Jones, among
others.1 To a lesser extent, applications of transition-metal
complexes of arynes to organic synthesis have also been
developed.2 However, the organometallic chemistry of
highly strained cycloalkynes3 is less explored due to the
inherent difficulty of their preparation and their low sta-
bility.1,4
In recent years we have reported both the first catalytic
[2+2+2] cycloaddition of arynes leading to triphe-
nylenes,5 and a modification of that procedure which al-
lows the cocyclization of arynes and alkynes.6 We have
now investigated whether strained cycloalkynes can un-
dergo similarly catalyzed [2+2+2] cycloadditions.
Scheme 1 a) Pd(PPh3)4 (10 mol%), CsF, CH3CN, 20 °C, 64%; b)
Pt(PPh3)4 (10 mol%), CsF, CH3CN, 20 °C, 62%
Similar results were obtained when cyclohexyne was gen-
erated as above in the presence of Pt(PPh3)4. In this case 3
was isolated in 62% yield.
Uncatalyzed cyclotrimerization of highly strained cy-
cloalkynes has often been reported, but yields are very
low.7,8 For some small cycloalkynes yield has been im-
proved by the use of transition-metal derivatives as cata-
lysts: Gassman and coworkers obtained the trimer of
bicyclo[2,2,1]heptyne in 9–11% without catalyst and in
37% yield in the presence of nickelocene,9 and Gassman's
method has also afforded the formal trimer of bicyc-
lo[2,2,2]octyne in 33% yield.10 Application of the same
procedure to more highly strained cycloalkynes, such as
bicyclo[2,1,1]hexyne has been unsuccessfull11 but the for-
mal trimer of this cycloalkyne was obtained in 43% yield
when 2,3-diiodobicyclo[2,1,1]hex-2-ene was sequentially
treated with n-BuLi, CuI and CuCl2. Interestingly, when
the reaction was carried out without CuCl2 the trimer and
the tetramer were obtained in 34% and 21% yields, re-
Although cyclotrimerization of arynes does not take place
to any appreciable extent in the absence of catalyst5a the
reports of uncatalyzed cycloalkyne trimerization7,8 led us
to try generating cyclohexyne in the absence of catalyst.
Under these conditions trimer 3 was isolated in 30% yield.
These results clearly show that a catalytic amount of pal-
ladium(0) or platinum(0) substantially improves the yield
of the trimerization.
After proving that cyclohexyne behaves similarly to
arynes in [2+2+2] cycloadditions we turned to the more
strained cyclopentyne (8),17 generation of which by the
same experimental procedure as above requires prepara-
tion of triflate 7. This was achieved as shown in
Scheme 2. Bromination of cyclopentenone (4) with bro-
mine and ketalization of the resulting bromoketone af-
forded 5. Replacement of bromine with TMS was
accomplished by metal-halogen exchange with n-BuLi
and trapping the anion with TMSCl. After deketalization,
Synlett 2002, No. 3, 04 03 2002. Article Identifier:
1437-2096,E;2002,0,03,0486,0488,ftx,en;D26001ST.pdf.
© Georg Thieme Verlag Stuttgart · New York
ISSN 0936-5214

LETTER
Palladium-catalyzed Trimerization of Strained Cycloalkynes
487
Scheme 2 a) Br2, Py, CH2Cl2, 96%; b) 1,2-ethanediol, p-TsOH,
benzene, 72%; c) n-BuLi, –78°C, then TMSCl, 91%; d) L-Selectride,
then PhNTf2, 49%
1,4-addition of hydride to cyclopentenone 6 and reaction
of the resulting enolate with PhNTf2 afforded triflate 7.
Attempts to trimerize cyclopentyne as above by treatment
of 7 with CsF in the presence of Pd(PPh3)4 failed
(Scheme 3). Nor did changes in the reaction conditions
(temperature, source of fluoride, addition of crown ether,
etc) ever allow isolation of 9, though its presence was de-
tected by mass spectrometry. We attribute this failure to
scant generation of such a highly strained molecule as cy-
clopentyne, and to its very short life.
Scheme 4 a) NBS, DMSO, 67%; b) i-Pr2NEt, Tf2O, 93%; c) TMSCl
then n-BuLi, –100°C, 58%
was brominated and oxidized in a single operation by
treatment with NBS and DMSO; the resulting bromoke-
tone 11 was enolized; the enolate was trapped with Tf2O,
giving triflate 12 in 93% yield; and metal-halogen ex-
change and reaction of the organolithium intermediate
with TMSCl afforded 13.
Attempts at generation and trimerization of 14 with CsF
and Pd(PPh3)4 in acetonitrile afforded only very low
yields. After some experimentation we found that the use
of TBAF as fluoride source and Pd2(dba)3 as catalyst gave
better results, allowing isolation of decacyclene (15) in
23% yield (Scheme 5).25
Scheme 3 a) Pd(PPh3)4, CsF, CH3CN, 20 °C
It occurred to us that a more aryne-like cyclopentyne, such
as acenaphthyne (14), might be more easily trimerized.
Although recent calculations have shown that 14 is very
highly strained,18 it is one of the few arynes to have been
characterized spectroscopically, Chapman and coworkers
having recorded its infrared and UV spectra by irradiation
of 1,3-bis(diazo)phenalen-2-one with UV light (>274 nm)
in an argon matrix.19 Its trimer decacyclene (15) is very in-
teresting because it has been transformed by FVP into the
hydrocarbon C36H12, the largest subunit of fullerene C60 so
far synthesized.20
The idea of preparing decacyclene by trimerization of
acenaphthyne is not new. In fact, decacyclene was ob-
tained this way in low yield almost 100 years ago follow-
ing generation of acenaphthyne by oxidation of
acenaphthene with sulfur at 200–300 °C.21 There have
been several attempts to improve the yield by using other
procedures to generate acenaphthyne, such as base-in-
duced decomposition of a bis(tosylhydrazone),22 Ram-
berg-Bäcklund rearrangement of a dibromosulfone23 or
deoxygenation of acenaphthoquinone,24 but none has en-
joyed more than limited success.
Scheme 5 a) Pd2(dba)3 (5mol%), TBAF, CH3CN, molecular sieves,
60°C, 23%
This palladium-catalyzed cyclotrimerization is the mild-
est and most efficient procedure hitherto reported for syn-
thesis of decacyclene, a precursor of the largest subunit of
fullerene C60 so far synthesized (the hydrocarbon C36H12).
To sum up, like arynes, highly strained cycloalkynes such
as cyclohexyne and acenaphthyne can undergo palladi-
um-catalyzed cyclotrimerization.
Further work is in progress to extend the scope of this pal-
ladium-catalyzed cyclotrimerization to the synthesis of
functionalized decacyclenes and to other highly strained
hydrocarbons.
With arynes, palladium-catalyzed trimerization takes
place under mild conditions in moderate to good yields.
To apply this procedure to acenaphthyne, precursor 13
was prepared as shown in Scheme 4. Acenaphthylene (10)
Synlett 2002, No. 3, 486–488 ISSN 0936-5214 © Thieme Stuttgart · New York

488
B. Iglesias et al.
LETTER
(10) Komatsu, K.; Akamatsu, H.; Jinbu, Y.; Okamoto, K. J. Am.
Chem. Soc. 1988, 110, 633.
(11) Frank, N. L.; Baldridge, K. K.; Siegel, J. S. J. Am. Chem.
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(12) Matsuura, A.; Komatsu, K. J. Am. Chem. Soc. 2001, 123,
1768.
Acknowledgement
Financial support from the DGES under projects PB96-0967 and
BQU2000-0464 is gratefully acknowledged. We also thank the
MEC and the Xunta de Galicia for the award of fellowships to D.
Peña and B. Iglesias, respectively.
(13) (a) Camaggi, G. J. Chem. Soc.(C) 1971, 2382. (b) Soulen,
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(25) Decacyclene (15): A solution of 13 (160 mg, 0.175 mmol)
in dry CH3CN (1.5 mL) was added to a suspension of
Pd2(dba)3 CHCl3 (9 mg, 0.0087 mmol) in dry CH3CN (1
mL) in a Schlenk flask containing molecular sieves. Then n-
Bu4NF solution (175 mL, 0.175 mmol) was added dropwise
at 60 ºC. At the end of the addition the heating was
suppressed and the mixture was stirred until cooled to room
temperature. The solvent was evaporated in vacuo and the
residue was solved in CH2Cl2 and washed with H2O. The
organic phase was dried over anhyd Na2SO4 and evaporated
in vacuo. The residue was chromatographed (SiO2, hexane)
to yield decacyclene (15, 10 mg, 23%).
dibromobicyclo[2,2,1]hexene.
Synlett 2002, No. 3, 486–488 ISSN 0936-5214 © Thieme Stuttgart · New York