Photoinduced catalytic defluorination of perfluoroalkanes to give perfluoroalkenes

Full text of DOI:10.1021/ja9539599

J. Am. Chem. Soc. 1996, 118, 2525-2526
2525
We have adopted decamethylferrocene (FeCp*2) as a pho-
tosensitizer, because it is nonnucleophilic and the oxidized form
is easily reduced by Zn, making catalysis feasible. A Li salt,
previously used by Richmond and co-workers,3g is required for
reaction and acts as an F- ion acceptor from the substrate.
Finally, the polar solvent THF favors the photoinduced charge
transfer chemistry.
Photoinduced Catalytic Defluorination of
Perfluoroalkanes To Give Perfluoroalkenes
Juan Burdeniuc and Robert H. Crabtree*
Department of Chemistry, Yale UniVersity
225 Prospect Street, New HaVen, Connecticut 06511
In a stoichiometric version of the reaction, a solution of
purified7 perfluoromethylcyclohexane (C7F14, 4.00 g, 11.43
mmol), FeCp*2 (100 mg, 0.300 mmol), and Li[O3SCF3] (1.30
g, 8.33 mmol) in dry THF (10 mL) was irradiated (17 h, medium
pressure 200 W Hg lamp, Pyrex vessel) under N2. After 30
min, the color changed from the yellow color of FeCp*2 to the
green color of the ferricinium ion. After 17 h, the mixture
contained unreacted FeCp*2 (40 mg, 0.120 mmol) and
[FeCp*2][O3SCF3] (90 mg, 0.180 mmol). The condensed
volatiles consisted of a THF solution of the C7F14 and 1-per-
fluoromethylcyclohexene, as shown by the appearance of a band
at 262 nm (ꢀ ) 17 000) in the UV spectrum and of the
characteristic8 19F NMR resonances. Isolation of the alkene
proved difficult because of the large excess of C7F14, but we
were able to use the product mixture for the synthesis and
isolation of perfluoroalkene derivatives. For example, reaction
with NH3 (1 atm, 60 min) gave 1 (26.0 mg, 90.0 µmol),
previously synthesized9a from the alkene. Purification of the
solids either by chromatography (TLC on alumina, CH2Cl2
eluent Rf ) 0.6) or by recrystallization from CCl4 yielded pure
ReceiVed NoVember 27, 1995
Fluoroorganic compounds are of current interest in connection
with their important industrial applications1 and the ozone
depletion potential of CFCs.2 Saturated perfluorocarbons are
extremely inert,3a and few reactions are known.3b-k Reactions
with strong reducing agents such as alkali or alkali earth metals
are difficult to control and lead to complete mineralization.3b,c
Controlled defluorinations have been observed with d-3f,g and
f-block3e metal complexes, in one case3g giving a perfluoroarene.
A radical anion has also been shown to aromatize cyclic
perfluorocarbons.3h Very recently, we proposed4 that Hg-
photosensitized reduction of perfluoroalkanes by NH3 gives the
perfluoroalkene as an intermediate, which rapidly reacts with
NH3 to give perfluoroimines and perfluoronitriles. Perfluoro-
alkenes are also plausible intermediates in some prior reduction
reactions,3g,h and are formed3j,k by photoinduced electron transfer
from amines, but only where the substrate has two vicinal
tertiary C-F bonds.
In many prior cases, complete reduction to the aromatic
perfluorocarbon is observed. Stopping at the earlier perfluo-
roalkene stage is a serious challenge since the alkenes are known
to be even more reactive toward electron transfer reactions than
are their aromatic counterparts.5 Perfluoroalkenes are valuable
intermediates,6 but there is still no general method to obtain
them from the readily available saturated perfluoroalkanes. We
now report such a reaction.
1
1. The physical data proved to be identical (MS, UV, IR, H
NMR, 19F NMR, mp) with those previously reported for 1.9c
We find that 2 mol of ferricinium salt are formed per mole of
perfluoroalkene, so the equation is
(1) (a) Anderson, R. F.; Punderson, J. O. In Organofluorine Chemicals
and their Industrial Applications; Banks, R. E., Ed.; Ellis Horwood:
Chichester, 1979; pp 235-247. (b) Slinn, D. S. L.; Green, S. W. In
Prepararion, Properties and Industrial Applications of Organofluorine
Compounds; Banks, R. E., Ed.; Ellis Horwood: Chichester, 1982; pp 45-
82. (c) Chambers, R. D. Fluorine in Organic Chemistry; Wiley: New York,
1973; pp 3-4. (d) Banks, R. E., Smart, B. E., Tatlow, J. C., Eds.
Organofluorine Chemistry. Principles and Commercial Applications;
Plenum Press: New York, 1994.
An excess of C7F14 was necessary for the desired perfluoro-
alkene to be formed. Reduction to the perfluoroarene was not
our goal since we recently reported10 very efficient aromatization
of saturated cyclic perfluorocarbons.
(2) (a) Ravishankara, A. R.; Solomon, S.; Turnipseed, A. A.; Warren,
R. F. Science 1993, 259, 194. (b) Molina, M. J.; Rowland, F. S.; Nature
1974, 249, 810.
(3) (a) Smart, B. E. In The Chemistry of Halides, Pseudo-Halides and
Azides; Patai, S., Rappoport, Z., Eds.; Wiley: New York, 1983; Suppl. D,
pp 604-606. (b) Sheppard, W. A.; Sharts, C. M. Organic Fluorine
Chemistry; W. A. Benjamin: New York, 1969; p 450. (c) Hudlicky, M.
Chemistry of Organic Fluorine Compounds; Ellis Horwood: Chichester,
1976; pp 563-566. Perry, R. Fluorine- The First Hundred Years; Banks,
R. E., Sharp, D. W. A., Tatlow, J. C., Eds.; Elsevier: New York, 1986; pp
293-295. (d) MacNicol, D. D.; Robertson, C. D. Nature 1988, 332, 59.
(e) Weydert, M.; Andersen, R. A.; Bergman, R. G. J. Am. Chem. Soc. 1993,
115, 8837. (f) Harrison, R. G.; Richmond, T. G. J. Am. Chem. Soc. 1993,
115, 5303. (g) Bennett, B. K.; Harrison, R. G.; Richmond, T. G. J. Am.
Chem. Soc. 1994, 116, 11165. (h) Marsella, J. A.; Gilicinski, A. G.;
Coughlin, A. M.; Pez, G. P. J. Org. Chem. 1992, 57, 2856. (i) For a review,
see: Kiplinger, J. L.; Richmond, T. G.; Osterberg, C. E. Chem. ReV. 1994,
94, 341. (j) Allmer, K.; Feiring, A. E. Macromolecules 1991, 24, 5487.
(k) Kaprinidis, N. A.; Turro, N. J. Personal communication, 1995.
(4) Burdeniuc, J.; Chupka, W.; Crabtree, R. H. J. Am. Chem. Soc. 1995,
117, 10119.
(5) (a) Compare the electron affinities of hexafluorobenzene (0.51 eV)
and perfluorocyclohexene (>1.4 eV). (b) Chowdhury, S.; Grimsrud, E.
P.; Heinis, T.; Kebarle, P. J. Am. Chem. Soc. 1986, 108, 3630. (c) Drzaic,
P. S.; Marks, J.; Brauman, J. I. In Gas Phase ion Chemistry; Bowers, M.
T., Ed.; Academic Press: Orlando, FL, 1984; Vol. 3, p 167.
(6) (a) Chambers, R. D. Fluorine in Organic Chemistry; Wiley: New
York, 1973; Chapter 5. (b) Fokin, A. V.; Kolomiets, A. F.; Vasil’ev, N.
V. Russ. Chem. ReV. 1984, 53, 238. (c) Bryce, M. R.; Chambers, R. D.;
Kirk, J. R. J. Chem. Soc., Perkin Trans. 1 1984, 1391. (d) Kolenko, I. P.;
Filyakova, T. I.; Zapelov, A. Y.; Lur’e, E. P. IzV. Akad. Nauk SSSR, Ser.
Khim. 1979, 2509. (e) A general review: Silvester, M. J. Aldrichimica
Acta 1995, 28 (2), 45-54.
We verified that the ferricinium salt is readily and quantita-
tively reduced by granular Zn in THF. This allowed us to effect
catalytic reduction of the perfluoroalkane, by irradiating a
mixture of C7F14 (6 g, 17.14 mmol), FeCp*2 (2.0 mg, 6.1 µmol),
and Li[O3SCF3] (1.30 g, 8.33 mmol) in 30 mL dry THF in the
(7) Traces of unsaturated perfluorocarbons in commercial C7F14 (Aldrich)
were eliminated by reaction with n-BuNH2. After filtration of the
fluoroimines and n-BuNH3F, the material was washed with 1.0 M HCl and
then with water. C7F14 was then dried over MgSO4 and distilled (59-61
°C). The UV spectra of all starting materials showed that unsaturation was
completely absent.
(8) (a) Observed: -136.3 (CF2, 2F), -136.2 (CF2, 2F), -122.1 (CF2,
2F), -112.3 (C-F, 1F), -112.0 (CF2, 2F), -61.1 (CF3, 3F). Literature:
-136.4 (CF2), -135.9 (CF2), -121.3 (CF2), -115.4 (C-F), -110.9 (CF2),
-61.3 (CF3). (b) Dungan, C. H.; Van Wazer, J. R. Compilation of Reported
19F NMR Chemical Shifts; Wiley-Interscience: New York, 1970. (c)
Campbell, S. F.; Hudson, A. G.; Mooney, E. F.; Pedler, A. E.; Stevens, R.;
Wood, K. N. Spectrochim. Acta 1967, 23A, 2119.
(9) (a) Derek, J.; Burdon, J.; Carter, P. A.; Patrick, C. R.; Tatlow, J. C.
J. Fluorine Chem. 1982, 21 (3), 305. (b) Doyle, A. M.; Pedler, A. E. J.
Chem. Soc. C 1971, 282. (c) MS (70 eV): 286 (M+), 267 (M+ - F), 186
(M+ - C2F4). 1H NMR (CD2Cl2): 5.75 (2H), 10.5 (1H). 19F NMR (CD2-
Cl2): -57.8 (3F), -117.9 (2F), -123.0 (2F), -136.6 (2F). IR (NaCl
film): 3505 (NH), 3345 (NH), 1676 (CdC), 1611 (NH), 1268 (CF), 1096
(CF). UV (EtOH): 274 (21800). Mp ) 99-100 °C. Crystals of this
compound can also be obtained by slow evaporation of a CH2Cl2 solution.
(10) Burdeniuc, J.; Crabtree, R. H. Science 1996, 271, 340.
0002-7863/96/1518-2525$12.00/0 © 1996 American Chemical Society

2526 J. Am. Chem. Soc., Vol. 118, No. 10, 1996
Communications to the Editor
t.o.) to the corresponding trisubstituted perfluoroalkene,14 (eq
4) suggesting initial attack at the tertiary CF bond.
FeCp*2
(CF3)2CF-CF2CF2CF3 + Zn + 2LiX
8
(CF3)2CdCFCF2CF3 + ZnX2 + 2LiF (4)
Unlike in prior systems,3k there is no need that both CF bonds
be tertiary. Perfluoro-2-methyl-2-pentene (2) was identified in
the THF solution from the UV and 19F NMR spectra,12a and
the derivative 3 was isolated13 after reaction with NH3.
Figure 1.
presence of Zn (0.50 g, 7.6 mmol).11 After 40 h, 1 was isolated
(190 mg, 0.67 mmol) after treatment with NH3 (110 turnovers
(t.o.), 3.9% conversion, 96% yield). Figure 1 shows the catalytic
cycle.
(CF3)2CdCFCF2CF3 + 10NH3 f
(CN)2CdC(NH2)CF2CF3 + 7NH4F (5)
Perfluoro-1,3-dimethylcyclohexane was also catalytically
reduced (60 t.o.),15 and reaction with NH3 gives a mixture of 4
and 4′, identified from their physical data.12b
Conversion can be maximized (11.9%) by using C7F14 (4.00 g,
11.43 mol), FeCp*2 (100 mg, 0.30 mmol), and Li[O3SCF3] (1.30
g, 8.33 mmol) in dry THF (10 mL) and irradiating for 19 h. As
before, compound 1 (0.39 g, 1.36 mmol) was isolated after
reaction with NH3. The catalyst can be recovered in 90% yield.
Attempts to increase the conversion gave byproducts that are
still under study; no aromatics were found, however.
We find that perfluoroalkenes are obtained from perfluoro-
alkanes by photoinduced electron transfer from FeCp*2. In the
presence of Zn, the reaction becomes catalytic because the
ferricinium product of the photolysis is reduced to the starting
FeCp*2.
Acknowledgment. J.B. thanks 3M Corporation, and R.H.C. thanks
the U.S. Department of Energy for funding. We also thank Kaprinidis
and Turro for sending us a preprint of their article.
Perfluoro-2-methylpentane is also catalytically reduced (40
JA9539599
(11) We avoided powdered Zn because it tends to absorb light.
(12) (a) 19F NMR analysis of the THF/C6H6-d6 solution showed new
resonances at -59.41 (s, br, 3F), -61.9 (m, 3F), -85.1 (s, 3F), -118.5
(dd, J ) 17.1, 1F), -98.8 (m, 2F) corresponding to perfluoro-2-methyl-2-
pentene, as verified by comparison with a commercial sample (Aldrich).
UV (THF) ) 226 (110). (b) MS (70 eV): 336 (M+), 317 (M+ - F), 290
(M+ - F - CNH), 186 (M+ - C3F6), 166 (M+ - C3F6 - HF). IR (NaCl
film): 3570 (NH), 3448 (NH), 3350 (NH), 3180 (NH), 1652 (CdC), 1614
(NH), 1310 (CF), 1266 (CF), 1218 (CF), 1133 (CF). 19F NMR (CCl2D2):
-57.0 (CF3, 3F), -57.5 (CF3, 3F), -70.4 (CF3, 3F), -74.2 (CF3, 3F),
-106.1 (d, JFFg ) 290 Hz, 1F, CF2), -107.8 (d, JFFg ) 290 Hz, 1F, CF2),
-115.6 (d, JFFg ) 297 Hz, 1F, CF2), -115.8 (d, JFFg ) 293 Hz, 1F, CF2),
-119.8 (d, JFFg ) 290 Hz, 1F, CF2), -122.6 (d, JFFg ) 270 Hz, 1F, CF2),
-123.0 (d, JFFg ) 290 Hz, 1F, CF2), -130.7 (d, JFFg ) 290 Hz, 1F, CF2),
-184.6 (m, br, CF, 1F), -184.9 (m, br, CF, 1F). 1H NMR (CCl2D2): 5.7
(br, 2H), 10.6 (br, 1H). UV (EtOH): 274 (16 400).
(14) Experimental procedure: perfluoro-2-methylpentane (6.00 g, 17.7
mmol), FeCp*2 (2.0 mg, 6.1 µmol), and Li[O3SCF3] (1.30 g, 8.33 mmol)
are dissolved in dry THF (30 mL) and irradiated for 40 h. Perfluoro-2-
methyl-2-pentene (2, 72.0 mg, 0.240 mmol) was produced (75% yield,
1.36% conversion, 40 turnovers) which was converted into the isolable
derivative (CN)2CdC(NH2)CF2CF3 (3, 50 mg, 0.24 mmol) by reaction with
NH3.
(15) Perfluoro-1,3-dimethylcyclohexane (6.00 g, 15.0 mmol), FeCp*2 (2.0
mg, 6.1 µmol), and Li[O3SCF3] (1.30 g, 8.33 mmol) are dissolved in dry
THF (30 mL) and irradiated for 48 h. A mixture of isomers 4 and 4′ was
obtained after reaction with NH3 in a 60% yield (2.7% conversion from
perfluoroalkane). Conversion was maximized (10%) by using FeCp*2 (100
mg, 0.300 mmol) in 10 mL of dry THF together with Li[O3SCF3] (1.30 g,
8.33 mmol) and perfluoro-1,3 dimethylcyclohexane (4.00 g, 10.0 mmol)
and irradiating for 17 h. FeCp*2 was recovered (95% yield), and reaction
with NH3 gave 4 and 4′ (310 mg, 0.93 mmol, 60%).
(13) Josey, A. D. J. Org. Chem. 1964, 29, 707.