Perfluorobarrelene

Article abstract of DOI:10.1016/j.tetlet.2008.10.017

The title fluorocarbon has been synthesized in three steps from cis-5,6-dichlorohexafluorocyclohexa-1,3-diene, a hexafluorobenzene synthon. Photolysis of perfluorobarrelene yields perfluorocyclooctatetraene. An extremely facile retro-Diels-Alder reaction is also described.

Full text of DOI:10.1016/j.tetlet.2008.10.017

Tetrahedron Letters 49 (2008) 7349–7351
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Perfluorobarrelene
*
Philip Ralli, Yin Zhang, David M. Lemal
Department of Chemistry, Dartmouth College, Hanover, NH 03755, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
The title fluorocarbon has been synthesized in three steps from cis-5,6-dichlorohexafluorocyclohexa-1,3-
diene, a hexafluorobenzene synthon. Photolysis of perfluorobarrelene yields perfluorocyclooctatetraene.
An extremely facile retro-Diels–Alder reaction is also described.
Received 12 July 2008
Accepted 3 October 2008
Available online 8 October 2008
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords:
Barrelene
Fluorocarbon synthesis
Fluorinolysis
retro-Diels–Alder
Photolysis
With its array of pi orbitals interacting through space in Möbius
fashion, bicyclo[2.2.2]octa-2,5,7-triene (barrelene, 1) is a particu-
larly interesting hydrocarbon.1–4 Sensitized photolysis of 1 yields
semibullvalene (2), another unusual member of the manifold of
(CH)8 hydrocarbons that undergoes degenerate rearrangement
with remarkable ease.5–8 Because hydrocarbons and their fluoro-
carbon counterparts typically contrast in their properties and
chemistry in ways that shed light on the effects of fluorine substi-
tution,9 we wished to synthesize perfluorobarrelene (3). We hoped,
however, that the contrast would not extend to the photoisomeriz-
aton reaction, as 3 might serve as a photoprecursor for the un-
known perfluorosemibullvalene.
F
SnR3
SnR3
Cl
F
F
F
+
Cl
F
F
5 R = Me
4
9 R = n-Bu
Cl
Cl
F
F
Cl
Cl
F
F
F
F
F
F
SnR3
SnR3
+
F
F
F
F
SnR3
SnR3
7 R = Me
F
6 R = Me
F
F
F
10 R = n-Bu
F
F
F2 / He, CH3CN
F
F
2
3
F
1
Cl
Cl
F
F
F
F
F
F
F
Zn
F
F
F
Synthesis of perfluorobarrelene,10 outlined in Scheme 1, was
greatly facilitated by the ready availability of 5,6-dichlorohexaflu-
orocyclohexa-1,3-diene (4), a synthon for perfluorobenzene that
had been prepared from that fluorocarbon in our laboratory in five
steps and in >50% overall yield.11 For addition to 4, a synthon for
the extremely labile difluoroacetylene12 was needed. In principle,
F
DMSO
F
F
F
F
3
8
Scheme 1.
chlorotrifluoroethylene could play that role, but it failed to react
with diene 4 even at 180 °C for 24 h, presumably because of its
electron-deficiency. Fortunately, the electron-rich bis(trimethyl-
stannyl)acetylene (5) reacted smoothly with 4 at 85 °C to give a
* Corresponding author. Tel.: +1 603 646 2989; fax: +1 603 646 3946.
E-mail address: david.m.lemal@dartmouth.edu (D. M. Lemal).
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.10.017

7350
P. Ralli et al. / Tetrahedron Letters 49 (2008) 7349–7351
rise to two bands in the near UV at 208.0 nm (log
239 nm (log
= 2.48) (ethanol).1
e = 3.05) and
F
Cl
Cl
F
F
e
F
F
F
F
F
Zn
Direct photolysis of 3 or sensitization with either acetone or
benzene resulted in isomerization to perfluorocyclooctatetraene
(14). None of the yet unknown perfluorosemibullvalene was ob-
tained, despite the fact that under similar conditions semibullva-
lene (2) is the principal product from the parent barrelene (1).5c
Sn(n-Bu)3
F
Sn(n-Bu)3
DMSO
F
F
F
Sn(n-Bu)3
10
Sn(n-Bu)3
11
F
Sn(n-Bu)3
F
F
F
F
F
F
+
F
F
hv
F
F
F
F
F
direct or
F
F
Sn(n-Bu)3
F
F
F
sensitized
9
12
F
F
F
3
14
F
F
Sn(n-Bu)3
Sn(n-Bu)3
F
F
F
1. Experimental
F
13
1.1. 7,8-Dichloro-2,3-bis(trimethylstannyl)perfluorobicyclo-
[2.2.2]octa-2,5-diene (6, 7)
Scheme 2.
In
a
25 mL round-bottomed flask were placed 10.39 g
of bis(trimethylstannyl) 7.65 g
acetylene,18
(29.6 mmol)
stereoisomeric mixture of Diels–Alder adducts 6 and 7 in the ratio
9:1 (87% yield). Fluorodestannylation of the mixture to afford chlo-
rofluorocarbon 8 was accomplished with a stream of fluorine in he-
lium (1:1) in acetonitrile at 0 °C (34% yield). To obtain
perfluorobarrelene, 8 was dechlorinated with zinc in DMSO as-
sisted by ultrasound13 and with chromous chloride in dimethyl-
formamide as well, albeit in quite low yield in each case.
Because bis(trimethylstannyl)acetylene (5) is a highly toxic
compound, the Diels–Alder reaction was also performed with its
tributyl analog 9.14 The cycloaddition with 9 required more vigor-
ous conditions (160 °C vs 85 °C), and gave a lower yield (51%15) of a
single adduct (10);16 the subsequent defluorostannylation reaction
to afford 8 proceeded in 35% yield.15 Bis(methylsilyl)acetylene re-
acted too in Diels–Alder fashion with diene 4 in 48 h at 150 °C
(50% yield15), but fluorinolysis of the adduct17 gave only polymeric
products.
(29.8 mmol) of cis-5,6-dichlorohexafluorocyclohexa-1,3-diene,11
and 2.5 g of calcium carbonate. Caution: Bis(trimethystannyl)acet-
ylene is a neurotoxin. The mixture was heated with stirring at
85 °C for 42 h (longer than necessary), then slurried in pentane,
and placed on a column of silica gel (45 g). Elution with pentane
gave the adduct as a white crystalline mass, 15.64 g (87% yield).
Mp 70–71.5 °C. 19F NMR (CDCl3) for 6 (90%): d À110.9 (2F, gem
to Cl), À153.5 (2F, vinyl), À175.4 (2F, bridgehead); for 7 (10%): d
À114.2 (2F, gem to Cl), À153.5 (2F, vinyl), À177.5 (2F, bridgehead).
IR (KBr): 2980, 2910, 1760, 1340, 1285, 1220 cmÀ1. MS m/e (rel.
int.): 593 (20%), 411 (56%), 361 (29%), 311 (22%), 257 (40%), 165
(100%). Anal. Calcd for C14H18Cl2F6Sn2: C, 27.63; H, 2.98; Cl,
11.65; F, 18.73. Found: C, 27.39; H, 3.01; Cl, 11.82; F, 18.48.
1.2. 7,8-Dichloroperfluorobicyclo[2.2.2]octa-2,5-diene (8)
In an attempt to reverse the order of the fluorinolysis and
dechlorination reactions, the tributylstannyl adduct 10 was sub-
jected to zinc in DMSO with ultrasound, but surprisingly perfluoro-
benzene (12) was the only fluorine-containing product (Scheme 2).
Monitoring the reaction by 19F NMR revealed the intermediacy of
triene 11 (d À157.5, 4 vinyl Fs; À194.5, 2 bridgehead Fs), which
fragmented at rt. This extraordinarily facile retro-Diels–Alder reac-
tion probably occurred via zwitterion 13 (or a transition state with
the character of 13), the negative charge being stabilized by conju-
gation and the positive charge by the b-C–Sn bond. Under the same
conditions, the trimethylsilyl adduct behaved identically, presum-
ably with assistance from the b-C–Si bond in the fragmentation
step. C–Sn and C–Si bonds alpha to a positive charge also provide
some stabilization by induction.
Perfluorobarrelene (3) is a highly volatile white solid, mp 68.5–
70 °C (mp 15–16 °C for the parent hydrocarbon). Its 19F NMR spec-
trum (CDCl3) comprises signals at d À150.1 and À219.0 ppm (3:1),
representing the vinyl and bridgehead fluorines, respectively. The
fluorocarbon displays double bond stretching absorption at
1746 cmÀ1 in its infrared spectrum, corresponding to a band at
1580 cmÀ1 for the parent. Its mass spectrum features a major
molecular ion peak, the base peak corresponding to loss of CF3. Per-
A solution of the adduct mixture 6 and 7 (17.7 g, 29.0 mmol)
in 175 mL of acetonitrile contained in
a cylindrical reactor
(4.5 cm diam.) was cooled to 0 °C. A 1:1 mixture of fluorine in he-
lium was bubbled into the bottom of the magnetically stirred
solution at the rate of 15 mL/min for 16 h. Caution: Fluorine at
high concentrations in helium is a very aggressive reagent. Reac-
tion progress was monitored by 19F NMR, and the gas inlet jet
was occasionally unclogged with a copper wire. Sodium fluoride
(8 g) was added late in the reaction to scavenge any HF that
was formed, and could have been introduced at the outset. After
trimethyltin fluoride had been removed by vacuum filtration, the
reaction mixture was poured into cold water (250 mL). The yel-
low oil that separated was collected, then the aqueous phase
was extracted with isopentane (3 Â 20 mL), and the extracts were
combined with the yellow oil. This was followed by washing with
water (100 mL) and 10% aqueous sodium bicarbonate (50 mL).
The aqueous washings were back-washed with isopentane
(2 Â 20 mL), and the combined organic extracts were dried
(MgSO4) and concentrated under reduced pressure. Short-path
distillation of the resulting yellow oil gave the diene as colorless,
low-melting prisms, 3.13 g (34%). Bp 40–45 °C (11 Torr). 19F NMR
(CDCl3: d À116.0 (2F, gem to Cl), À150.4 (4F, vinyl), À209.1 (2F,
bridgehead). IR (neat): 1780, 1755, 1350, 1280, 1245, 1150,
fluorobarrelene has a maximum at 218 nm (log
e = 2.9) (cyclohex-
ane) in its ultraviolet spectrum, whereas the hydrocarbon gives
1053, 1000, 955, 795 cmÀ1
.

P. Ralli et al. / Tetrahedron Letters 49 (2008) 7349–7351
7351
1.3. Perfluorobicyclo[2.2.2]octatriene (3)
References and notes
1. Zimmerman, H. E.; Paufler, R. M. J. Am. Chem. Soc. 1960, 82, 1514; Zimmerman,
H. E.; Grunewald, G. L.; Paufler, R. M.; Sherwin, M. A. J. Am. Chem. Soc. 1969, 91,
2330.
2. Zimmerman, H. E. J. Am. Chem. Soc. 1966, 88, 1564; Zimmerman, H. E. Science
1966, 153, 837.
3. Hoffmann, R.; Heilbronner, E.; Gleiter, R. J. Am. Chem. Soc. 1970, 92, 706; Bodor,
N.; Dewar, M. J. S.; Worley, S. D. J. Am. Chem. Soc. 1970, 92, 19.
4. Palmer, M. H. J. Mol. Struct. 1987, 161, 333.
5. (a) Zimmerman, H. E.; Grunewald, G. L. J. Am. Chem. Soc. 1966, 88, 183; (b)
Zimmerman, H. E.; Binkley, R. W.; Givens, R. S.; Sherwin, M. A. J. Am. Chem. Soc.
1967, 89, 3932; (c) Zimmerman, H. E.; Binkley, R. W.; Givens, R. S.; Grunewald,
G. L.; Sherwin, M. A. J. Am. Chem. Soc. 1969, 91, 3316; (d) Zimmerman, H. E.;
Sulzbach, H. M.; Tollefson, M. B. J. Am. Chem. Soc. 1993, 115, 6548.
6. Frutos, L. M.; Sancho, U.; Castano, O. Org. Lett. 2004, 6, 1229.
7. Cheng, A. K.; Anet, F. A. L.; Mioduski, J.; Meinwald, J. J. Am. Chem. Soc. 1974, 96,
2887.
8. Hrovat, D. A.; Brown, E. C.; Vaughan, R.; Quast, H.; Borden, W. T. J. Org. Chem.
2005, 70, 2627.
9. See, for example, Lemal, D. M. J. Org. Chem. 2004, 69, 1.
10. Noted earlier in Lemal, D. M. Acc. Chem. Res. 2001, 34, 662.
11. Dailey, W. P.; Correa, R. A.; Harrison, E., III.; Lemal, D. M. J. Org. Chem. 1989, 54,
5511.
A 25 mL round-bottomed flask with a septum-capped sidearm
and stir bar was clamped to a straight tube, which was clamped
in turn to an oscillating motor. The tube was connected via vacuum
tubing to a Firestone valve through which the system could be
evacuated or filled with nitrogen. The flask was charged with
freshly activated zinc dust (4.0 g, 0.06 mol) and dimethyl sulfoxide
(DMSO, 7.0 mL, distilled from calcium hydride). After the system
had been purged with nitrogen, dichloride 8 (0.918 g, 2.88 mmol)
was introduced by syringe as a solution in DMSO, and the flask
was rocked in an ultrasonic bath for 7 min at 21–23 °C. Following
degassing by two freeze-pump-thaw cycles using liquid nitrogen,
volatile product was vacuum transferred statically into the bottom
of a small sublimer, while the reaction mixture was magnetically
stirred. Sublimation at 11 Torr and 32 °C gave perfluorobarrelene
as colorless prisms (0.125 g, 17.5%). Mp 58–60 °C. 19F NMR
(C6D6): d À149.9 (6F, vinyl), À219.0 (2F, bridgehead). IR (vapor):
1746, 1332, 1260, 1145, 940, 895 cmÀ1. MS m/e (rel. int.): 248
(63%), 229 (30%), 210 (6%), 198 (46%), 186 (50%), 179 (100%).
Through a combination of low temperature recrystallization from
isopentane and sublimation, a sample melting at 68.5–70 °C was
obtained.
12. Brahms, J. C.; Dailey, W. P. J. Am. Chem. Soc. 1989, 111, 8940.
13. Waldron, R. F.; Barefoot, A. C., III.; Lemal, D. M. J. Am. Chem. Soc. 1984, 106,
8301.
14. For a comparison of the toxicity of trialkyltin compounds, see Snoeij, N. J.; Van
Iersel, A. A. J.; Penninks, A.; Seinen, W. Toxicol. Appl. Pharmacol. 1985, 81, 274.
15. NMR yields; all others are isolated yields.
16. 19F NMR (CDCl3): d À110.6 (2F, gem to Cl), À154.3 (2F, vinyl), À174.8 (2F,
bridgehead). 1H NMR (CDCl3): bands in the d 0.8–1.7 range.
17. 19F NMR (CDCl3): d À111.4 (2F, gem to Cl), À153.5 (2F, vinyl), À183.7 (2F,
bridgehead). 1H NMR (CDCl3): d 0.34 (s, 18H).
Acknowledgment
The authors thank the National Science Foundation for the sup-
port of this research.
18. Seyferth, D.; Sarafidis, C.; Evnin, A. B. Organomet. Chem. 1964, 2, 417.