Selective reduction of halopolyfluorocarbons by organosilicon hydrides

Article abstract of DOI:10.1021/jo9807363

It is demonstrated that silicon hydrides can be used for reduction of polyfluorinated halocarbons. For example, the reaction between CF₃CCl₂F and excess triethylsilane, catalyzed by benzoyl peroxide, leads to the formation of a mixture containing CF₃CHClF (major), CF₃CH₂F, and ClSi(C₂H₅)₃. On the other hand, the reaction of chlorofluoroalkanes, containing an internal -CCl₂- group readily proceeds with reduction of both chlorines, leading to compounds such as (CF₃)₂CH₂ and CF₃CH₂C₂F₅. In contrast to chlorofluoroalkanes, bromofluoroalkanes are much more reactive and reaction with hydrosilane rapidly proceeds without the catalyst at elevated temperature.

Full text of DOI:10.1021/jo9807363

7294
J . Org. Chem. 1998, 63, 7294-7297
Selective Red u ction of Ha lop olyflu or oca r bon s by Or ga n osilicon
Hyd r id es
Viacheslav A. Petrov
DuPont Central Research and Development,† Experimental Station,
P.O. Box 80328, Wilmington, Delaware 19880-0328
Received April 20, 1998
It is demonstrated that silicon hydrides can be used for reduction of polyfluorinated halocar-
bons. For example, the reaction between CF3CCl2F and excess triethylsilane, catalyzed by ben-
zoyl peroxide, leads to the formation of a mixture containing CF3CHClF (major), CF3CH2F, and
ClSi(C2H5)3. On the other hand, the reaction of chlorofluoroalkanes, containing an internal
-CCl2- group readily proceeds with reduction of both chlorines, leading to compounds such as
(CF3)2CH2 and CF3CH2C2F5. In contrast to chlorofluoroalkanes, bromofluoroalkanes are much
more reactive and reaction with hydrosilane rapidly proceeds without the catalyst at elevated
temperature.
In tr od u ction
only reported example of such reductions of fluorinated
substrates.
Conversion of chloro- and bromopolyfluorocarbons into
the corresponding hydropolyfluorocarbons can be carried
out by several methods. Reduction by tributyltin hy-
dride1-3 and catalytic4 or high temperature5 hydrodeha-
logenation by hydrogen are probably the most widely
employed methods. A common disadvantage of thermal
reduction is low selectivity, and the method based on tin
hydrides, although rather powerful, involves the use and
handling of toxic organotin compounds.
The reaction of halofluoroalkanes with hydrosilanes in
the presence of radical initiators has been investigated
as a general method for the preparation of hydrofluoro-
alkanes.
Resu lts
Chloropolyfluorocarbons were found to be much more
active in this reaction than alkyl chlorides. In general,
the reaction of chlorofluorocarbons with silanes proceeds
rapidly at elevated temperature in the presence of
peroxide initiators.
For instance, CF3CCl2F (1) reacts with an excess of
triethylsilane (2) in the presence of 5 mol % of benzoyl
peroxide (BP) to give a mixture of CF3CHFCl (3), CF3-
CH2F (4), and ClSi(C2H5)3 (Table 1, entry 1).
Silicon hydrides are a relatively cheap class of com-
pounds of low toxicity, and the reduction of alkyl ha-
lides by silicon hydrides in the presence of radical
initiators has been known for a long time.6 In contrast
to the use of organotin hydrides, reductions with these
reagents do not find wide application in organic synthe-
sis, due to a significantly lower activity of silicon hydrides
compared to tin hydrides. On the other hand, silicon
hydrides can be used successfully for the selective reduc-
tion of polychloroalkanes.7,8 For example, reduction of
CF2Cl2 by silicon hydrides under radical conditions
(copyrolysis or mercury photosensitization of the Si-H
8
bond)9 and selective reduction of ClCF2CFCl2 by C6H5-
SiH(CH3)2 in the presence of benzoyl peroxide afford
monohydroproducts in high yield. This seems to be the
The isomeric 1,2-dichlorotetrafluoroethane (5) is less
active in this process, as the reaction between a mixture
of 1 and 5 and excess reducing agent leads to a mixture
of products 3, 4, 6, and 7, with a higher conversion of
the gem-dichloride 1 (96% vs 71%; Table 1, entries 1 and
2).
† Publication No. 7753.
(1) Krespan, C. G. In Chemistry of Organic Fluorine Compounds
II; Hudlicky, M., Pavlath, A. E., Eds.; ACS Monograph 187; American
Chemical Society: Washington, DC, 1995; p. 297.
(2) Dolbier, W. R. Chem. Rev. 1996, 96, 1557.
(3) Van Der Puy, M.; Belter, R. K.; Borowski, R. J .; Ellis, L. A. S.;
Pershichini, P. J ., III; Poss, A. J .; Rygas T. P.; Tung, H. S. J . Fluorine
Chem. 1995, 71, 59.
(4) Lunin, V. V.; Lokteva, E. S. Izv. Akad. Nauk, Ser. Khim. 1996,
1609.
(5) Rao, V. N. M. In Organofluorine Chemistry. Principles and
Commercial Applications; Banks, R. E., Smart, B. E., Tatlow, J . C.,
Eds.; Plenum Press: New York, 1994; p 159.
(6) Weber, W. P. Silicon Reagents for Organic Chemistry; Springer-
Verlag: New York, 1983; pp 323-324.
(7) Nagai, Y.; Yamazaki, K.; Shiojima, I.; Kobori, N.; Nagashi, M.
J . Organomet. Chem. 1967, 9, P21.
(8) Nagai, Y.; Yamazaki, K.; Shiojima, I. J . Organomet. Chem. 1967,
9, P25.
(9) Clark, M. P.; Conqueror, M.; Morgan, G. H.; Davidson, I. M. T.
J . Organomet. Chem. 1996, 521, 395.
Reduction of 1,1,2-trichlorotrifluoroethane (8) by 2
(ratio 1:1), although not highly selective, produces mono-
hydrogenated ethane 9 as a major product, along with
small amounts of dihydrocompounds 11 and 12.
S0022-3263(98)00736-1 CCC: $15.00 © 1998 American Chemical Society
Published on Web 09/23/1998

Selective Reduction of Halopolyfluorocarbons
J . Org. Chem., Vol. 63, No. 21, 1998 7295
Ta ble 1. Red u ction of Ha lop olyflu or oa lk a n es by Or ga n osila n es
molar
ratio
initiator
(mol %)
temp (°C)
(time, h)
entry no.
compd
silane
conver (%)
100
5 (96), 1 (71)
82
67
85
96
100
products (wt % in mixture)
1
2
3
4
5
6
7
1
2
2
1:2
1:2
1:2
1:2
1:2
1:2
1:2
BPa (5)
80 (10)
100 (4)
80 (10)
80 (10)
80 (10)
100 (12)
100 (12)
3 (88), 4 (12)
5 + 1b
BP (8.6)
BP (4)
BP (4)
BP (5)
BP
3 (19.5), 4 (5.5), 6 (44), 7 (31)
9 (90), 10 (2), 11 (1.2), 12 (6.8)
9 (97), 10 (2), 12 (1)
9 (97), 10 (3)
14 (64), 15 (26.5), 16 (9.5)
18 (28), 18a (5), 19 (49), 19a (5),
20 (10.5), unknown (2.5)
22 (94), 23 (6)
8
8
8
13
17
2
PSc
C6H5Si(CH3)2H
2
2
BP (8)
8
9
10
11
12
13
21
24
21
21
21
13
2
2
1:2
1:2
1:1
1:2
1:2
1:2
BP (4)
100 (12)
120 (13)
120 (12)
120 (12)
120 (12)
120 (12)
100
100
93
6
99.5
83
BP (3)
25 (75), 25a (3), 26 (21), 26a (1)
22 (7.5), 23 (92.5)
(CH3)2SiH2
Cl3SiH
TBPd (4)
TBP (6)
23
2
2
H2PtCl6‚6H2O (1)
22 (32.5), 23 (67.5)
14 (62), 15 (19), 16 (11),
CFHdCFCF3 (8)
Pd/Ce
14
15
16
17
18
19
5 + 27f
2
1:2.5 BP (4)
100 (12)
120 (13)
120 (13)
120 (12)
100 (12)
80 (5)
27 (100), 5 (83) 28 (40), 6 (48), 7 (12)
100
75
90
50
29
29
29
32
34
2
1:2
1:2
1:1
1:2
1:2
BP (5)
BP (4)
BP (4)
26 (95), 27 (5)
30 (70), 31 (30)
30 (85), 31 (15)
33
PSc
TMSg
2
2
100
35 (95)i
Benzoyl peroxide. Ratio 88:12. c Poly(methylsiloxane), [-(H)Si(CH3)O-]n, trimethylsilyl-terminated, equivalent weight 70. tert-
a
b
d
Butyl peroxide. e 0.5 wt % of Pd on carbon, 0.5 g. f Ratio 65:35. Tetramethylsiloxane. 35 was isolated in 86% yield.
g
i
1,2,3-Trichloropentafluoropropane (17) with excess of
silane 2 (ratio 1:2) behaves similarly, producing a mixture
of alkanes 18-20 along with small amounts (2.5%) of
unidentified product (Table 1, entry 7).
Poly(methylhydrosiloxane), [-(H)Si(CH3)O-]n, was also
shown to be an efficient reducing agent. The reaction of
this material with 8 was slower (67% conversion) but
slightly more selective for 9 (97% vs 90%) compare to
reaction of 8 and 2, (Table 1, entries 3 and 4).
The ratios of alkanes 14/15 (eq 6), 18/18a , and 19/19a
in the crude reaction mixtures are indicative of a higher
rate of reduction of internal -CFCl- groups than of
terminal -CF2Cl group in this process, as befits a free
radical process.
An internal dichloromethylene group -CCl2- of poly-
chlorofluorocarbons was found to be even more active in
the reduction, which is also in accord with a free radical
mechanism for this process. Compound 21, for example,
was reduced to (CF3)2CH2 by reaction with 2 mol of 2.
Only a small amount of monohydropropane 23 was found
in this reaction mixture.
The reaction of 8 with dimethylphenylsilane was
reported to proceed with exclusive reduction of one
chlorine in the -CCl2F group.8 The present work con-
firms that the interaction of 8 with dimethylphenylsilane
under conditions similar to those used in ref 8 is indeed
highly selective and leads to monohydrogenated products
in high yield. However, this process is not entirely
regiospecific and results in the formation of a small
amount of 10 along with compound 9.
Reaction of 1,2-dichlorohexafluoropropane (13) with a
2-fold excess of silane 2 leads to a mixture of two isomeric
monohydropropanes 14 and 15 as major products, along
with small amount of dihydropropane 16.
The reduction of compound 24 by a 2-fold excess of 2
results in the formation of a mixture of compounds 25
and 26 as major products, along with small amounts of
25a and 26a .
The preferential formation of compounds 25 and 26
may be interpreted as evidence for the higher reactivity
of the -CHCl- group compared to the -CF2Cl group.
Reduction of a mixture of 5 and butane 27 with an
excess of 2 is another example. Under conditions similar

7296 J . Org. Chem., Vol. 63, No. 21, 1998
Petrov
into hydrobromodifluoromethane (33) simply by heating
a mixture of reagents.
to reduction of 21 a complete conversion of 27 into
dihydrobutane 28 was observed in contrast to 83%
conversion of 5 (Table 1, entry 14).
The corresponding dihydropentane 35 was isolated in
high yield from a noncatalyzed reaction of 2,2-dibromooc-
tafluoropentane (34) with a 2-fold excess of HSi(C2H5)3
(Table 1, entry 19).
Discu ssion
Reduction of halopolyfluoroalkanes by hydrosilanes
such as triethylsilane probably occurs by a radical chain
mechanism. A radical derived from the decomposition
of the initiator starts the radical chain by abstraction of
a hydrogen atom from the silane, leading to the formation
of a silyl radical 36 (Scheme 1).
The reactivity of the silane has a significant effect on
the reduction process. For example, in the reaction of
21 catalyzed by tert-butyl peroxide (TBP), the replace-
ment of 2 by diethylsilane (ratio 1:1) changes the
selectivity of the reaction, and the monohydropropane 23
becomes the major product.
Sch em e 1
Trichlorosilane has the lowest activity among silanes
used in this study. The reaction of HSiCl3 and 21 is slow
and leads to the formation of 23 in low yield (Table 1,
entry 11).
The propagation steps involve abstraction of a chlorine
atom from chloropolyfluoroalkane by radical 36 to form
triethylchlorosilane and fluoroalkyl radical 37. Abstrac-
tion of H• from silane by 37 leads to the formation of
hydrochloropolyfluoroalkane 38 and regeneration of the
silyl radical 36. Alkane 38 can be either an intermediate
or final product of the reaction. The order of decreasing
activity of different groups in the reduction (-CCl2- >
-CClF- g -CClH- > -CF2Cl) can be correlated to the
stability of corresponding radical intermediates and is
in good agreement with a radical mechanism for the
process. The fact that reduction of chloropolyfluoroal-
kanes is catalyzed by peroxides is, of course, also indica-
tive of a radical chain mechanism, which is very efficient,
based on the amount of peroxide (5-6 mol %) necessary
to complete the reduction.
Primary alkyl radicals are known to be much less
reactive in hydrogen abstraction than polyfluorinated
radicals,2,4 and this is why the reduction of alkyl halides
usually requires much more powerful reagents than
silane 2, examples being tributyltin hydride,13 tris-
(trimethylsilyl)silane,14 and pentamethyldisilane.15
The relative reactivity of silanes in reduction of chlo-
ropolyfluoroalkanes is also an important factor; the
significant difference in activity between Cl3SiH and 2
The reaction of chloropolyfluoroalkanes with silanes
requires a radical initiator. No interaction between 21
and 2 was observed without peroxide initiator even at
elevated temperature (100 °C, 12 h). However, the
reduction also proceeds rapidly in the presence of “clas-
sical” hydrogenation catalysts such as hexachloroplatinic
acid or Pd/C. Reaction of 21 and 2 catalyzed by hexachlo-
roplatinic acid produced a mixture of 22 and 23, while
reaction of 13 and 2 catalyzed by Pd/C formed a sub-
stantial amount of CFHdCFCF3 along with a mixture
of compounds 14-16 (Table 1, entries 12 and 13).
1,2-Dichlorohexafluorocyclobutane (29) was also re-
acted with an excess of triethylsilane, producing 1,2-
dihydrohexafluorocyclobutane (30) along with a small
amount of monohydrocyclobutane 31.
(10) Paleta, O.; Dadak, V.; Dedek, V. J . Fluorine Chem. 1988, 39,
397.
(11) Dedek, V.; Chvatal, Z. J . Fluorine Chem. 1986, 31, 363.
(12) Ramig, K.; Quiroz, F.; Vernice, G. G.; Rozov, L. A. J . Fluorine
Chem. 1980, 80, 101.
Reactions of 29 with poly(methylsiloxane) and tetra-
methylsiloxane proceed similarly, but they are less
selective (see Table 1, entries 16 and 17). Bromopoly-
fluorocarbons are more active than chloropolyfluoroal-
kanes, and reaction with 2 usually proceeds without
catalyst. Dibromodifluoromethane (32) was converted
(13) Kuivila, H. G. Synthesis 1970, 499.
(14) Ballestri, M.; Chatgilialogu, C.; Clark, K. B.; Griller, D.; Giese,
B.; Kopping, B. J . Org. Chem. 1991, 56, 678.
(15) Lusztyk, J .; Maillard, B.; Ingold, K. U. J . Org. Chem. 1986, 51,
2457.

Selective Reduction of Halopolyfluorocarbons
J . Org. Chem., Vol. 63, No. 21, 1998 7297
peroxides, and compounds 1, 5, 13, 29, and 32 were commercial
and used without further purification. Compounds 21, 27,
34,16 and 2417 were prepared using known procedures. Prod-
ucts 3, 4, 6, 7, 9-12, 19a ,18 20,3 and 2819 were identified by
comparison of 1H and 19F data with reported values. Materials
15, 16, 22, 23, 30, and 33 were identified by comparison with
authentic samples.
Ca u tion ! We did not experience any problems in this work;
however, use a peroxide initiator and general consideration
of radical chain mechanism of reduction polyfluoroalkanes by
silanes suggest that the reaction under certain conditions can
be hard to control. Reductions should be carried behind a
shield or in a barricade.
Red u ction of P olyh a loflu or oa lk a n es by Or a n osila n es
Hyd r id es. (Gen er a l P r oced u r e). A mixture of polyhalo-
fluoroalkane (0.05-0.2 mol) and silane (0.05-0.4 mol) was
placed in Hastelloy reactor, and initiator/catalyst was added.
The reactor was closed, purged with N2, and kept 1 h at 25 °C
and 6-13 h at 80-120 °C. The reactor was cooled to -78 °C
and unloaded. The crude reaction mixture was analyzed by
GC and 1H and 19F NMR spectroscopy. Reaction conditions,
ratio of reagents, and products are given in Table 1.
is probably the result of a much slower hydrogen transfer
from trichlorosilane to the polyfluoroalkyl radical.
In reductions of chloropolyfluoroalkanes, triethylsilane
is less reactive than tributyltin hydride since the latter
effectively replaces several chlorine atoms in chloropoly-
fluorocarbons under mild conditions.3 This difference in
hydrodechlorination reactivity is not surprising, since the
latter was shown to be more than 200 times more active
as a hydrogen transfer agent in reactions involving
polyfluorinated radicals.2 An advantage of the less
reactive silanes lies in the greater selectivities available
with them in the reduction of chloropolyfluoroalkanes.
The radical chain mechanism proposed for the reduc-
tion of halopolyfluoroalkanes by organosilanes is similar
to that offered for the reduction of polychloroalkanes by
silanes7,8 and for polychloro- and chloropolyfluoroalkanes
or polyfluorinated ethers by 2-propanol10-12 under UV-
irradiation. All of these processes were shown to have a
radical chain mechanism involving the formation of
electrophilic polyhalogenated radicals6,7,12 as intermedi-
ates.
Since compounds 18, 19, and 19a were previously reported
in patent literature only, they were characterized by 19F NMR.
On the other hand, the mechanism for the reduction
of chloropolyfluoroalkanes catalyzed by H2PtCl6 and Pd/C
may be much more complex. There are several possibili-
ties including formation of active metal clusters as a
result of reduction by H2PtCl6, followed by oxidative
addition of the C-Cl bond of chloropolyfluoroalkane to
an electron-rich metal center and further formation of a
polyfluoroalkyl σ-complex of Pt (or Pd). A polyfluoroalkyl
derivative of the metal may be converted to final product
either through the sequence of formation of metal hydride
complex and reductive elimination of polyfluoroalkane or
as a result of the homolysis of a carbon-metal bond with
generation of fluoroalkyl radical. Unfortunately, experi-
mental data are not sufficient to choose between these
mechanisms.
The experimental results obtained in this work are in
excellent agreement with a recently published opinion
based on an analysis of the reactivity of polyfluorinated
radicals that triethylsilane may be “a very useful agent
for relatively slow chain processes involving fluorinated
radicals”.2 Much lower reactivity of silicon hydrides vs
tin hydrides in radical reductions can be compensated
for by much higher reactivity of polyfluorinated radicals
vs the corresponding hydrocarbon radicals, since the
reactivities of fluorinated radicals approach that of the
highly electronegative tert-butoxy radical.2
18: -63.91 (2F, dm, 176 Hz), -64.82 (2F, dm, 176 Hz),
-193.66 (1F, dm, 56 Hz).
19: -61.94 (1F, dm, 187 Hz), -65.38 (1F, dm, 187 Hz),
-129.10 (1F, dm, 326 Hz), -130.75 (1F, dm, 326 Hz), -206.71
(1F, dm, 57 Hz).
19a : -132.55 (4F, dm, 54 Hz), -147.13 (1F, m).
Compounds 25 and 26 were not isolated but were character-
ized in mixture by 19F NMR spectroscopy.
25: -49.77 (2F, m; 11 Hz), -86.82 (3F, s), -118.15 (2F, m).
26: -54.18 (1F, dm; 168 Hz), -57.00 (1F, dm; 168 Hz),
-78.68 (3F, m), -116.08 (1F, dm; 272 Hz), -121.50 (1F, dm;
272 Hz).
Red u ction of 34 by Tr ieth ylsila n e. 10 mL of HSi(C2H5)3
was added dropwise to 10 g of 34 with stirring at 80 °C over
10 min. The reaction mixture was kept at this temperature
for 5 h, collecting the crude product which passed a short reflux
water condenser in a cold trap (-78 °C). Collected product
(based on GC and NMR data 35 was 95% purity) was
redistilled to give 4.8 g (86%) of 35.
Ack n ow led gm en t. The author thanks Dr. V. V.
Grushin and Dr. C. G. Krespan for helpful discussion,
reviewer A for a number useful comments, and Dr. D.
D. Khasnis for technical assistance.
J O9807363
(16) Petrov, V. A.; Krespan, C. G.; Smart, B. E. J . Fluorine Chem.
1998, 89, 125.
(17) Sievert, A. C.; Nappa, M. J . PCT WO 95/16656, 1995 (to
Dupont); Chem. Abstr. 1995, 123, 339128.
(18) Weigert F. J . J . Fluorine Chem. 1990, 46, 375.
(19) Burdon, J .; Ezmirly, S. T.; Huckerby, T. N. J . Fluorine Chem.
1988, 40, 283.
Exp er im en ta l Section
19F and 1H NMR spectra were recorded using CFCl3 as
internal standard and either chloroform-d or acetone-d as a
lock solvent. Silanes, poly(methylhydrosiloxane) (Gelest),