Reductive defluorination of fluoroalkanes

Article abstract of DOI:10.1016/S0040-4020(03)00019-X

The reaction of an excess of lithium powder and a catalytic amount of DTBB with primary, secondary and tertiary fluoroalkanes in the presence of a substoichiometric amount of 1,2-bis(trimethylsilyl)benzene 1 afforded the corresponding alkanes resulting from a fluorine-hydrogen exchange. The method could be extended to non-geminal difluorides. The effect of the disilylated compound in the naphthalene-catalysed lithiation of fluorobenzene and benzyl fluoride was also studied.

Full text of DOI:10.1016/S0040-4020(03)00019-X

Tetrahedron 59 (2003) 1237–1244
Reductive defluorination of fluoroalkanes
*
´
David Guijarro, Pedro Martınez and Miguel Yus
´ ´
Departamento de Quımica Organica, Facultad de Ciencias, Universidad de Alicante, Apdo. 99, 03080 Alicante, Spain
Received 21 October 2002; revised 10 December 2002; accepted 20 December 2002
´
This paper is dedicated to the memory of Professor Antonio Gonzalez
Abstract—The reaction of an excess of lithium powder and a catalytic amount of DTBB with primary, secondary and tertiary fluoroalkanes
in the presence of a substoichiometric amount of 1,2-bis(trimethylsilyl)benzene 1 afforded the corresponding alkanes resulting from a
fluorine–hydrogen exchange. The method could be extended to non-geminal difluorides. The effect of the disilylated compound in the
naphthalene-catalysed lithiation of fluorobenzene and benzyl fluoride was also studied. q 2003 Elsevier Science Ltd. All rights reserved.
1. Introduction
the mentioned lithiation procedure which allows the
defluorination of aliphatic fluorides under very mild
reaction conditions.
The carbon–fluorine bond is the strongest one that a carbon
atom can form.1 Consequently, cleavage of this bond is not
easy at all, especially in aliphatic derivatives. Defluorination
procedures are important from an environmental point of
view, due to the problematic degradation of fluoroalkanes in
Nature.2 Apart from methods involving loss of fluorine or
hydrogen fluoride giving olefins,3 most of the defluorination
methodologies imply treatment with a metallic catalyst
under very drastic reaction conditions.4 In a few cases,
defluorination could be achieved electrochemically,5 using
alkali metals under special conditions,6 magnesium7 or
lithium aluminium hydride.8
2. Results and discussion
Continuing with our studies on the use of fluorocompounds
as starting materials in lithiation processes, 1-fluorononane
was allowed to react with an excess of lithium in the
presence of a catalytic amount of naphthalene, but no
reaction was observed after several hours at 208C. However,
we thought that our lithiation methodology could achieve
the carbon–fluorine bond cleavage if we could find a good
way to activate that bond. Recently, Maruoka et al.23
reported that different disilylated compounds were able to
form stable complexes with the fluoride anion of tetra-
butylammonium fluoride via coordination of fluoride ion to
both silicon atoms. This article prompted us to test some
silicon containing compounds as additives in our lithiation
reaction with the hope that they would activate the carbon–
fluoride bond and facilitate its reductive cleavage by
formation of a type I complex (Chart 1).
On the other hand, in the last few years we have been
developing an arene-catalysed lithiation reaction9–13 which
allows the preparation of different organolithium com-
pounds under very mild reaction conditions. Thus, using this
methodology, simple organolithium compounds can be
prepared from non-halogenated materials14 as well as
functionalised organolithium compounds15 by chlorine–
lithium exchange12 or by ring-opening of heterocycles.16 In
addition, this lithium activation has also been used for the
generation of polylithium synthons17 and for the activation
of other metals,18 especially nickel.19 Recently, we have
applied the arene-catalysed lithiation for the preparation of
aryllithiums20 or allylic and benzylic organolithium com-
pounds21 from the corresponding fluorides, working in the
second case under Barbier-type reaction conditions.22
However, when the same process was applied to aliphatic
fluorides, the reaction failed, recovering the starting
material unchanged. In this paper, we report a variant of
Chart 1.
We started testing 1,2-bis(trimethylsilyl)benzene 1
(Table 1), since a similar compound gave the best results
in Maruoka’s work. Thus, a mixture of 1-fluorononane
(1 mmol) and 1,2-bis(trimethylsilyl)benzene (0.15 mmol)
Keywords: lithium; lithiation; fluoroalkanes; alkyl fluorides; defluorination.
*
Corresponding author. Tel.: þ34-965903548; fax: þ34-965903549;
e-mail: yus@ua.es
0040–4020/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0040-4020(03)00019-X

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D. Guijarro et al. / Tetrahedron 59 (2003) 1237–1244
Table 1. Defluorination of 1-fluorononane in the presence of different additives 1–5
Entry
1
Additive (no.)
Additivea (mmol)
T (8C)
t (h)
Yield of nonane (%)b
Yield of fluorononane (%)b
0.15
0
3.5
64
–
2
3
CH2(SiMe3)2 (2)
(CH2SiMe3)2 (3)
0.15
0.15
20
20
14
18
2
22
84
58
4
0.30
0.30
20
20
19
19
41
21
39
30
5
a
Amounts of reagents used: fluorononane (1 mmol), additive (mmol indicated in the table), lithium (7 mmol), naphthalene (0.16 mmol), THF (7 mL).
Yield determined by quantitative GLC, using commercially available 1-fluorononane, nonane and n-dodecane (internal standard) in the determination of
response factors.
b
was treated with an excess of lithium (1:7 molar ratio) and a
defluorination was conducted in the presence of trimethyl-
silylbenzene 4 (0.30 equiv., in order to have the same
concentration of silicon atoms), 41% yield of nonane and
39% of unchanged fluorononane were obtained after 19 h at
208C (Table 1, entry 4). No improvement was observed with
additive 5, which possesses a more electrophilic silicon
atom (Table 1, entry 5).
substoichiometric amount of naphthalene (1:0.16 molar
ratio, 8 mol%) in THF at 08C. After 3.5 h stirring at this
temperature, the starting material was consumed and, after
hydrolysis with water, nonane was formed in 64% yield
(Table 1, entry 1). Some other silicon containing com-
pounds were tested as additives and the results are collected
in Table 1. Commercially available bis(trimethylsilyl)-
methane 2 was not effective to promote the defluorination
reaction. After 14 h at 208C, only 2% of nonane was
obtained together with 84% of unchanged starting material
(Table 1, entry 2). When compound 3 was used as additive,
only 3% of nonane was formed after 3 h at 08C. Additional
stirring for 18 h at 208C led to 22% of nonane together with
58% of unreacted fluorononane. According to these results,
it seems that the presence of an aromatic ring in the additive
is crucial in order to get good results. It is also very
important to have two silicon atoms in the additive in order
to get dicoordination to the fluorine atom. When the
After having established that compound 1 was the best
additive, we performed several experiments in order to
optimise the reaction conditions (Table 2). First, we studied
the effect of the amount of additive used, which was varied
from 0.25 to 0.05 equiv. related to fluorononane. Increasing
the amount of additive to 0.25 equiv. led to a reduction in
the reaction time and a slight increase in yield (Table 2,
entry 1), but the amount of by-products formed was higher
than in the reaction with 0.15 equiv. of the additive. With
0.10 equiv. of 1, a very small amount of the starting material
did not react and a 48% yield of nonane was obtained after
Table 2. Optimisation of the reaction conditions in the defluorination of 1-fluorononane
Entry
1a (mmol)
Arene (%)
T (8C)
t (h)
Yield of nonane (%)b
Yield of fluorononane (%)b
1
2
3
4
5
6
7
8
0.25
0.15
0.10
0.05
0.15
0.15
0.15
0.15
C10H8 (8)
C10H8 (8)
C10H8 (8)
C10H8 (8)
C10H8 (8)
DTBB (5)
DTBB (5)
–
0
0
0
0
1
70
64
48
39
76
78
77
33
–
–
1
21
2
–
3.5
3.5
7
230 to 0
3.5c
2.5
3.5c
18
0
230 to 0
20
–
45
a
Amounts of reagents used: fluorononane (1 mmol), additive 1 (mmol indicated in the table), lithium (7 mmol), arene (0.16 mmol of C10H8 or 0.10 mmol of
DTBB), THF (7 mL).
Yield determined by quantitative GLC, using commercially available 1-fluorononane, nonane and n-dodecane (internal standard) in the determination of
response factors.
The reaction was stirred for 1 h at 2308C and for 2.5 additional hours allowing the temperature to rise from 230 to 08C.
b
c

D. Guijarro et al. / Tetrahedron 59 (2003) 1237–1244
1239
3.5 h at 08C (Table 2, entry 3). The amount of unreacted
fluorononane increased when 0.05 equiv. of 1 were used,
even with an extended reaction time (Table 2, entry 4).
Since full conversion, a good yield and few by-products
were obtained when employing 0.15 equiv. of 1, we decided
to use that amount of additive in future experiments.
the results collected in Table 2, we chose the reaction
conditions in entry 6 as the optimal ones.
After having determined the optimal reaction conditions, we
studied the scope of the defluorination reaction by testing
different fluoroalkanes. Primary, secondary and tertiary
alkyl fluorides were defluorinated in moderate to good
yields when treated with an excess of lithium (1:7 molar
ratio) and a catalytic amount of DTBB (1:0.10 molar ratio,
5 mol%) in the presence of additive 1 (Table 3, entries 1–3).
In the case of 1-fluorononane, the reaction was complete
after 2.5 h at 08C. However, in reactions with secondary and
tertiary alkyl fluorides, 6b and 6c, respectively, there was
some unreacted starting material after 2.5 h and the
defluorination did not proceed further when the reaction
was stirred for a longer time. Increasing the amount of
additive 1 to 0.25 equiv. led to full conversion of the starting
fluoroalkane after 2.5 h at 08C (Table 3, entries 2 and 3).
Next, the effect of temperature and the electron carrier were
studied. When the mixture of lithium, naphthalene, 1-
fluorononane and 1 (0.15 equiv.) in THF was stirred at
2308C for 1 h and additional 2.5 h allowing the temperature
to rise to 08C, the yield increased to 76% (Table 2, entry 5).
However, 2% of unreacted fluorononane was recovered in
this case. Using 4,40-di-tert-butylbiphenyl (DTBB, 5 mol%)
instead of naphthalene under the same reaction conditions,
full conversion and 77% yield of the defluorination product
was obtained (Table 2, entry 7). Finally, when the reaction
was run at 08C for 2.5 h using DTBB as an electron carrier,
78% yield of nonane was obtained and the starting material
was not detected (Table 1, entry 6). The need of the use of
the electron carrier was also established. No reaction was
observed when a mixture of fluorononane, additive 1 and
lithium were stirred for 3 h at 08C. A small amount of
defluorination product was formed when the latter reaction
was stirred for 18 h at 208C (Table 2, entry 8). According to
The process could be extended to some functionalised
fluoroalkanes. Compounds 6d and 6e gave the correspond-
ing defluorinated ethers under the optimal reaction con-
ditions in moderate to good yields (Table 3, entries 4 and 5).
The reductive defluorination of difluorides 6f and 6g was
also investigated in order to check if mono- or direduction
Table 3. Reductive defluorination of fluoroalkanes 6 using additive 1
Entry
1
Fluoroalkane (no.)
1a (mmol)
Product (no.)b
Yield (%)c
32 (78)d
0.15
2
3
0.25
0.25
40d
79
4
5
6
0.15
0.15
0.30
46
86
28d (24)d,e
F
f
f
f
(6g)
7
0.30
2.00
–
–
–
F
6
f
8
6g
–
a
Amounts of reagents used: fluoroalkane 6 (1 mmol), additive 1 (mmol indicated in the table), lithium (7 mmol), DTBB (0.10 mmol), THF (7 mL). All
reactions were set up as described in Section 4.
b
c
d
e
All isolated compounds 7 were .95% pure (300 MHz 1H NMR and/or GLC).
Isolated yield after flash chromatography (silica gel, hexane) based on the starting fluoroalkane 6.
Yield determined by quantitative GLC, using commercially available alkane and n-dodecane (internal standard) in the determination of response factors.
In brackets, yield of 1-fluorononane determined by quantitative GLC, using commercially available 1-fluorononane and n-dodecane (internal standard) in the
determination of response factors.
f
No reaction occurred.

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D. Guijarro et al. / Tetrahedron 59 (2003) 1237–1244
took place. When 1,9-difluorononane 6f reacted for 2.5 h
with an excess of lithium and a catalytic amount of DTBB in
the presence of 0.30 equiv. of additive 1, a mixture of
nonane (28%) and 1-fluorononane (24%) was obtained
(Table 3, entry 6). The defluorination did not proceed
further with a prolonged reaction time. Concerning geminal
difluoride 6g, no reaction was observed under the same
reaction conditions as for 6f, the starting material being
quantitatively recovered after work-up (Table 3, entry 7). As
it was demonstrated at the beginning of this article, it is
necessary to have coordination of fluorine to both silicon
atoms of the additive in order for the complex to undergo
reductive cleavage of the carbon–fluorine bond. We assume
that the lack of reactivity in this case is due to the formation
of chelate II (Chart 1), in which each fluorine atom is
coordinated to only one silicon moiety. We even tried to
accomplish the defluorination of 6g by using 2 equiv. of
additive 1, but no reaction took place after stirring for 4.5 h
at 08C (Table 3, entry 8).
(1:7 molar ratio) and DTBB (1:0.10 molar ratio, 5 mol%) in
THF at 08C, in a Barbier-type process.22 After 1.5 h stirring
at that temperature, the starting fluoride had disappeared
completely and the major reaction product was nonane. The
addition product to the electrophile was not detected (GC–
MS, 1H NMR). According to these results, it seems that the
generated organolithium compound prefers to abstract a
proton either from the carbon atom in a position to the
carbonyl group of the electrophile or from the reaction
medium25 rather than add to the electrophile. In these tests,
we used 1 equiv. of additive 1 in order to get a fast
defluorination process, which would diminish the formation
of pinacol-type coupling products resulting from the
reaction of the electrophile with lithium.
In previous papers, we have been able to prepare
organolithium compounds from fluoroarenes20 and from
allylic and benzylic fluorides21 using our arene-catalysed
lithiation procedure. After having demonstrated that addi-
tive 1 was able to facilitate the reductive cleavage of the
carbon-fluorine bond in fluoroalkanes, we decided to
investigate if it could also improve the results we had
obtained in the lithiation of aromatic, allylic and benzylic
fluorides. Thus, a mixture of fluorobenzene 8 (1 mmol) and
additive 1 (0.15 mmol) was treated with an excess of lithium
(1:7 molar ratio) and a catalytic amount of naphthalene
(1:0.16 molar ratio, 8 mol%) in THF at 2308C. After
30 min, cyclohexanone was added and the reaction was
stirred for 3 h allowing the temperature to rise to 08C. After
hydrolysis with water and usual work-up, the expected
addition product 9 was isolated in 72% yield (Eq. (1)). A
parallel experiment was set up without additive 1, and the
yield was 71%.
Fluoride 6a was commercially available and it was used
without further purification. The rest of the fluorides were
prepared from the corresponding alcohols (for 6b–6f) or
aldehyde (for 6g) by reaction with either DAST24a or bis(2-
methoxyethyl)amino sulfur trifluoride24b following litera-
ture procedures. Secondary (6b) and tertiary (6c) fluorides
showed to be relatively unstable. Hydrogen fluoride
elimination took place to some extent when they were
stored as pure liquids at room temperature. However, they
could be stored for a longer time without apparent
decomposition at 48C in solution in hexane. The best results
in defluorination reactions with 6b and 6c were obtained
with freshly prepared fluorides.
ð1Þ
Additive 1 could not be recovered after the defluorination
reactions. As the reaction proceeds, it is gradually decom-
posing into several by-products that could contain tri-
methylsilyl groups (according to GC–MS), which could not
be identified. However, the separation of the desired
defluorination product from these by-products could easily
be performed by flash chromatography (silica gel, hexane).
On the other hand, naphthalene-catalysed lithiation of
a mixture of benzyl fluoride 10 (1 mmol) and additive
1 (0.15 mmol) in the presence of cyclohexanone
(1.2 mmol) (Barbier-type reaction conditions) gave
the expected product 11 in 43% yield (Eq. (2)). A
parallel reaction under the same conditions but in the
absence of additive 1 gave compound 11 in 45% yield.
According to these results, it seems that the use of the
disilylated compound 1 does not cause any effect on the
lithiation of both aromatic and benzylic fluorides,
probably due to the high rate with which our lithiation
procedure is able to cleave the carbon–fluoride bond in
those cases.
We also tried to trap the supposed intermediate organo-
lithium reagent with a carbonyl compound as electrophile.
Thus, a mixture of 1-fluorononane (1 mmol), additive 1
(1 mmol) and the electrophile (isobutyraldehyde or 3-
pentanone, 1.2 mmol) was reacted with an excess of lithium
ð2Þ

D. Guijarro et al. / Tetrahedron 59 (2003) 1237–1244
1241
Concerning the starting fluorides, fluorobenzene 8 was
commercially available and benzyl fluoride 10 was prepared
as previously described by us.21
solution of 6-undecanone (2.0 mL, 9.5 mmol) in anhydrous
THF (10 mL), under Ar, at 08C. The mixture was stirred for
2 h allowing the temperature to rise to 208C. Then, water
(10 mL) was added and the mixture was extracted with ethyl
acetate (3£10 mL). The combined organic phases were
washed with water (10 mL) and brine (10 mL) and then
dried over magnesium sulfate. After evaporation of the
solvents (15 Torr), the resulting residue was purified by
column chromatography (silica gel, hexane/ethyl acetate),
giving 84% of the titled compound. Physical, spectroscopic
and analytical data follow: colourless oil; Rf 0.29 (hexane/
ethyl acetate: 9/1); tr 9.7 [Tcolumn¼608C (3 min) and 60–
2708C (15 8C min21)]; n (film) 3456 (OH), 1145 cm21 (CO);
dH 0.71–0.99 (9H, m, 3£Me), 1.08–1.62 (23H, m, 11£CH2
and OH); dC 14.0 (3C, 3£Me), 22.6 (2C), 23.1 (2C), 23.3,
25.65, 32.45 (2C), 38.9, 39.15 (2C) (11£CH2), 74.3 (CO); m/z
210 (Mþ2H2O, 1%), 158 (10), 157 (100), 97 (17), 83 (43), 71
(13), 69 (53), 58 (22), 57 (39), 56 (17), 55 (99); HRMS:
Mþ2H2O, found 210.2322. C15H30 requires 210.2347.
3. Conclusions
From the results described in this paper, we conclude that
disilylated compound 1 is a very efficient additive to
promote the reductive cleavage of the carbon–fluorine bond
of fluoroalkanes in combination with an excess of lithium
and a catalytic amount of an arene. It has been demonstrated
that primary, secondary and tertiary monofluorides and non-
geminal difluorides can easily be converted into the
corresponding defluorinated products in moderate to good
yield under mild reaction conditions. Our procedure has
shown to be a good alternative for the defluorination
methods that have already been reported.
4.2.3. 9-Methoxy-1-nonanol.30 n-BuLi (6.4 mL 1.7 M
solution in hexane, 11.1 mmol) was dropwise added to a
stirred solution of 1,9-nonanediol (1.815 g, 11.1 mmol) in
THF (20 mL), under Ar, at 08C. A white solid appeared,
which could not be dissolved even after addition of 10
more mL of THF. Methyl iodide (0.7 mL, 12.2 mmol) was
added and the reaction was stirred at 208C for 60 h. Then,
water (10 mL) was added and the mixture was extracted
with ethyl acetate (3£15 mL). The combined organic phases
were washed with water (10 mL) and brine (10 mL) and then
dried over magnesium sulfate. Evaporation of the solvents
gave a mixture of starting diol, 9-methoxy-1-nonanol and 1,9-
dimethoxynonane. Column chromatography of the crude
mixture gave the titled compound in 16% yield. Physical and
spectroscopic data follow: colourless oil; Rf 0.36 (hexane/
ethylacetate:1/1);tr 9.9;n(film)3400(OH), 1117, 1057 cm21
(CO); dH 1.04–1.44, 1.47–1.69 [10H and 4H, respectively,
2m, (CH2)7CO], 1.77 (1H, s, OH), 3.33 (3H, s, Me), 3.37 (2H,
t, J¼6.6 Hz, CH2OMe), 3.63 (2H, t, J¼6.6 Hz, CH2OH); dC
25.7, 26.1, 29.3, 29.4, 29.5, 29.6, 32.75 [(CH2)7CO], 58.45
(Me), 62.9 (CH2OH), 72.9 (CH2OMe); m/z 125 (Mþ2H2O–
MeOH, ,1%), 99 (11), 96 (50), 95 (53), 85 (19), 83 (26), 82
(14), 81 (11), 71 (47), 70 (13), 69 (42), 67 (100), 58 (14), 57
(28), 56 (25), 55 (95), 54 (60), 53 (11).
4. Experimental
4.1. General
For general information, see Ref. 26. tr values are given in
min under the conditions described in Ref. 26, unless
otherwise stated. Flash chromatography was performed
using silica gel 60 of 0.040–0.063 mm. Additives 1 and 2,
fluorides 6a and 8, naphthalene, DTBB, cyclohexanone and
the starting materials needed for the preparation of fluorides
6b–6g were commercially available (Acros, Aldrich) and
were used without further purification. Additives 327 and 528
were prepared according to literature procedures. Additive
4,20 benzyl fluoride 1021 and lithium powder26 were
prepared as previously reported by us. Commercially
available anhydrous THF (99.9%, water content#0.006%,
Acros) was used as solvent in all the lithiation reactions.
CH2Cl2 was dried by refluxing it with phosphorous
pentoxide under Ar and distilled before use.
4.2. Preparation of the required alcohols (precursors of
fluorides 6b–6e)
4.2.1. 6-Undecanol. 6-Undecanone (2.0 mL, 9.5 mmol) was
dropwise added to a stirred suspension of lithium aluminium
hydride (454 mg, 13.0 mmol) in anhydrous THF (10 mL),
under Ar, at 08C. The mixture was stirred for 2 h allowing
the temperature to rise to 208C. Then, the reaction was
quenched following a literature procedure.29 After filtration
of the solids, solvent was evaporated (15 Torr) and pure (1H
NMR) 6-undecanol was obtained in 90% yield. Physical,
spectroscopic and analytical data follow: colourless oil; Rf
0.34 (hexane/ethyl acetate: 4/1); tr 7.0 [Tcolumn¼608C
(3 min) and 60–2708C (158C min21)]; n (film) 3353 (OH),
1128 cm21 (CO); dH 0.91 (6H, t, J¼6.7 Hz, 2£Me), 1.19–
1.51 (16H, m, 8£CH2), 1.60 (1H, s, OH), 3.53–3.68 (1H, m,
CHO); dC 14.01 (2C, 2£Me), 22.6 (2C), 25.3 (2C), 31.9
(2C), 35.3 (2C) (8£CH2), 72.0 (CO); m/z 172 (Mþ, ,1%),
101 (43), 83 (100), 57 (15), 56 (11), 55 (86); HRMS: Mþ,
found 172.1789. C11H24O requires 172.1827.
4.2.4. 9-(Tetrahydro-2-pyranyloxy)-1-nonanol.31 The
title compound was prepared according to a literature
procedure.31 Physical and spectroscopic data follow: green-
ish oil; Rf 0.47 (hexane/ethyl acetate: 1/1); tr 13.8; n (film)
3410 (OH), 1128, 1071, 1030 cm21 (CO); dH 1.00–1.37,
1.40–1.89 [10H each, 2m, (CH2)7CO and (CH2)3CH], 2.10
(1H, s, OH), 3.25–3.38, 3.40–3.50, 3.62–3.74, 3.76–3.94
(1H each, 4m, 2£CH2OCH), 3.56 (2H, t, J¼6.7 Hz,
CH2OH), 4.48–4.60 (1H, m, CH); dC 19.5, 25.35, 25.6,
26.05, 29.2, 29.25, 29.4, 29.6, 30.65, 32.65 [(CH2)7CO and
(CH2)3CH], 62.15, 62.7, 67.55 (3£CH2O), 98.7 (CH); m/z
244 (Mþ, ,1%), 101 (20), 85 (100), 84 (11), 83 (17), 69
(29), 67 (12), 57 (14), 56 (20), 55 (34).
4.3. Synthesis of fluorides 6b–6e using DAST as
fluorinating agent: general procedure24a
4.2.2. 6-Butyl-6-Undecanol. n-BuLi (6.5 mL 1.6 M sol-
ution in hexane, 10.4 mmol) was dropwise added to a stirred
To a solution of DAST (0.41 mL, 3.1 mmol) in dry CH2Cl2

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D. Guijarro et al. / Tetrahedron 59 (2003) 1237–1244
(3 mL), under Ar, at 2788C, the solution of the correspond-
ing alcohol (3.1 mmol) was dropwise added (ca. 10 min).
The reaction was stirred for ca. 5 h allowing the temperature
to rise to 208C. After hydrolysis with water (5 mL) at 08C,
the organic phase was separated, diluted with CH2Cl2
(10 mL), washed with water (3£5 mL) and dried over
magnesium sulfate. Solvent was evaporated (15 Torr) and
the resulting residue was purified by column chromato-
graphy (silica gel, hexane), affording the expected fluoride
in the following yields: 6b (50%), 6c (42%), 6d (44%) and
6e (40%). Physical, spectroscopic and analytical data
follow.
(10), 85 (100), 84 (10), 69 (29), 57 (20), 56 (29), 55 (23);
HRMS: Mþ, found 246.1976. C14H27FO2 requires
246.1995.
4.4. Preparation of fluorides 6f and 6g using bis(2-
methoxyethyl)amino sulfur trifluoride as fluorinating
agent24b
4.4.1. 1,9-Difluorononane (6f).32 A solution of bis(2-
methoxyethyl)amino sulfur trifluoride (1.7 mL, 9.2 mmol)
in CH2Cl2 (2 mL) was dropwise added to a solution of 1,9-
nonanediol (1.512 g, 9.3 mmol) in dry CH2Cl2 (6 mL),
under Ar, at 2788C (on cooling the latter solution to 2788C,
it became solid). The reaction mixture was stirred for ca. 5 h
allowing the temperature to rise to 208C. Then, a saturated
aqueous solution of NaHCO3 was slowly added until the gas
evolution ceased. Layers were separated and the aqueous
phase was extracted with CH2Cl2 (3£15 mL). The com-
bined organic phases were dried over magnesium sulfate.
After evaporation of the solvent (15 Torr), the resulting
residue was purified by column chromatography (silica gel,
hexane), giving difluoride 6f in 10% yield. Physical and
spectroscopic data follow: colourless oil; tr 4.4 [Tcolumn¼608
4.3.1. 6-Fluoroundecane (6b). Colourless oil; tr 5.4
[Tcolumn¼608C (3 min) and 60–2708C (158C min21)]; n
(film) 1015 cm21 (CF); dH 0.92 (6H, t, J¼6.4 Hz, 2£Me),
1.23–1.77 (16H, m, 8£CH2), 4.35–4.49, 4.51–4.69 (1H, m,
CHF); dC 13.95 (2C, 2£Me), 22.55 (2C), 31.7 (2C)
[2£Me(CH2)2], 24.8 (2C, d, J¼4.4 Hz, 2£CH2CH2CF),
35.1 (2C, d, J¼20.8 Hz, 2£CH2CF), 94.6 (d, J¼165.8 Hz,
CF); m/z 154 (Mþ2HF, 2%), 126 (11), 111 (23), 98 (22), 97
(43), 84 (36), 83 (45), 82 (12), 71 (18), 70 (82), 69 (71), 61
(13), 59 (10), 57 (57), 56 (98), 55 (100); HRMS: Mþ2HF,
found 154.1752. C11H22 requires 154.1721.
C
(3 min) and 60–2708C (158C min21)];
n (film)
1048 cm21 (CF); dH 1.17–1.51 [10H, m, (CH2)5CH2CF],
1.59–1.81 (4H, m, 2£CH2CF), 4.42 (4H, dt, J¼47.5,
6.2 Hz, 2£CH2F); dC 25.1 (2C, d, J¼5.5 Hz, 2£CH2CH2-
CF), 29.1 (2C, 2£CH2CH2CH2CF), 29.35 [CH2(CH2)3CF],
30.35 (2C, d, J¼18.7 Hz, 2£CH2CF), 84.1 (2C, d,
J¼164.7 Hz, 2£CF); m/z 144 (Mþ2HF, ,1%), 88 (14),
83 (28), 74 (17), 70 (22), 69 (58), 67 (12), 61 (31), 59 (13),
57 (20), 56 (38), 55 (100).
4.3.2. 6-Butyl-6-fluoroundecane (6c). Colourless oil; tr 8.8
[Tcolumn¼608C (3 min) and 60–2708C (158C min21)]; n
(film) 1056 cm21 (CF); dH 0.79–1.04 (9H, m, 3£Me),
1.22–1.44 [16H, m, 2£Me(CH2)3CH2CF and Me(CH2)2-
CH2CF], 1.50–1.71 (6H, m, 3£CH2CF); dC 14.0 (3C,
3£Me), 22.6 (3C, 3£MeCH2), 23.0 (d, J¼5.5 Hz, MeCH2-
CH2CH2CF), 25.55 (2C, d, J¼5.5 Hz, 2£MeCH2CH2CH2-
CH2CF), 32.3 (2C, 2£MeCH2CH2CH2CH2CF), 36.75 (d,
J¼23.1 Hz, MeCH2CH2CH2CF), 37.0 (2C, d, J¼23.0 Hz,
2£MeCH2CH2CH2CH2CF), 99.3 (d, J¼169.1 Hz, CF); m/z
210 (Mþ2HF, 2%), 158 (13), 111 (11), 97 (70), 83 (100), 71
(12), 69 (52), 57 (33), 56 (14), 55 (54); Mþ2HF, found
210.2355. C15H30 requires 210.2347.
4.4.2. 1,1-Difluorononane (6g). To a solution of nonanal
(5.4 mL, 30.0 mmol) in dry CH2Cl2 (10 mL), under Ar, at
208C was successively added a solution of bis(2-methoxy-
ethyl)amino sulfur trifluoride (9.4 mL, 51.0 mmol) in dry
CH2Cl2 (6 mL) and absolute ethanol (0.35 mL, 6.0 mmol).
The reaction mixture was stirred for 4.5 h at 208C and then it
was quenched and worked-up as described above for 1,9-
difluorononane. Purification of the crude residue by column
chromatography (silica gel, hexane) afforded difluoride 6g
in 38% yield. Physical, spectroscopic and analytical data
follow: colourless oil; tr 2.6 [Tcolumn¼608C (3 min) and 60–
2708C (158C min21)]; n (film) 1048 cm21 (CF); dH 0.89
(3H, t, J¼6.6 Hz, Me), 1.05–1.57 [12H, m, Me(CH2)6],
1.69–1.92 (2H, m, CH2CF), 5.78 (1H, tt, J¼57.0, 4.5 Hz,
CHF); dC 14.05 (Me), 22.65, 29.35, 31.6, 31.8 [Me(CH2)4],
22.15 (t, J¼5.5 Hz, CH2CH2CF), 28.95–29.2 (m, CH2CH2-
CH2CF), 34.15 (t, J¼20.3 Hz, CH2CF), 117.5 (t,
J¼238.2 Hz, CF2); m/z 164 (Mþ, 2%), 98 (15), 95 (23),
87 (12), 85 (26), 83 (12), 82 (28), 81 (20), 73 (32), 71 (65),
70 (38), 69 (28), 68 (13), 67 (13), 61 (16), 59 (22), 57 (100),
56 (51), 55 (79), 53 (13), 51 (20). HRMS: Mþ, found
164.1375. C9H18F2 requires 164.1377.
4.3.3. 9-Fluorononyl methyl ether (6d). Colourless oil; Rf
0.45 (hexane/ethyl acetate: 4/1); tr 7.7; n (film) 1127,
1043 cm21 (CO and CF); dH 1.22–1.49, 1.50–1.85 [10H
and 4H, respectively, 2m, (CH2)7CO], 3.31 (3H, s, Me),
3.35 (2H, t, J¼6.6 Hz, CH2O), 4.41 (2H, dt, J¼47.3, 6.2 Hz,
CH2F); dC 26.05, 29.1, 29.35, 29.4, 29.6 [(CH2)5CO], 25.05
(d, J¼5.5 Hz, CH2CH2CF), 30.35 (d, J¼19.8 Hz, CH2CF),
58.45 (Me), 72.9 (CH2O), 84.1 (d, J¼163.6 Hz, CF); m/z
144 (Mþ2MeOH, 18%), 116 (53), 102 (13), 97 (11), 96
(14), 95 (19), 88 (46), 83 (42), 82 (45), 81 (37), 74 (21), 71
(11), 70 (50), 69 (67), 68 (58), 67 (28), 59 (12), 57 (18), 56
(57), 55 (100), 54 (31), 53 (12); HRMS: Mþ2MeOH, found
144.1333. C9H17F requires 144.1314.
4.3.4. 2-(9-Fluorononyloxy)tetrahydropyran (6e). Green-
ish oil; Rf 0.49 (hexane/ethyl acetate: 4/1); tr 12.3; n (film)
1129, 1073, 1031 cm21 (CO and CF); dH 1.04–1.95 [20H,
m, (CH2)7CF and (CH2)3CH], 3.32–3.45, 3.46–3.58, 3.68–
3.81, 3.83–3.95 (1H each, 4m, 2£CH2O), 4.42 (2H, dt,
J¼47.3, 6.2 Hz, CH2F), 4.55–4.66 (1H, m, CH); dC 19.65,
25.45, 26.15, 29.15, 29.35, 29.4, 29.7, 30.75 [(CH2)5CO and
(CH2)3CH], 25.1 (d, J¼5.5 Hz, CH2CH2CF), 30.35 (d,
J¼19.8 Hz, CH2CF), 62.3, 67.6 (2£CH2O), 84.2 (d,
J¼163.6 Hz, CF), 98.7 (CH); m/z 246 (Mþ, ,1%), 101
4.5. DTBB catalysed defluorination of fluorides 6 in the
presence of additive 1. Isolation of compounds 7: general
procedure
To a green suspension of lithium powder (50 mg, 7.2 mmol)
and DTBB (27 mg, 0.10 mmol), under Ar, at 08C, was
dropwise added a solution of the corresponding fluoride 6

D. Guijarro et al. / Tetrahedron 59 (2003) 1237–1244
1243
(1.0 mmol) and additive 1 [0.15 mmol (for 6a, 6d and 6e),
0.25 mmol (for 6b and 6c) or 0.30 mmol (for 6f and 6g)] in
THF (2 mL). After 2.5 h stirring at 08C, the reaction was
hydrolysed with water (5 mL) at the same temperature and
acidified with 2 M HCl (5 mL). The mixture was extracted
with ethyl acetate (3£15 mL) and the combined organic
phases were washed with saturated aqueous NaHCO3
(5 mL), water (5 mL) and brine (5 mL), being then dried
over magnesium sulfate. After evaporation of the solvents
(15 Torr), the resulting residue was purified by flash
chromatography [silica gel, hexane (for 7a–7c) or hexane/
ethyl acetate (for 7d, 7e)], affording the expected products
in the yields indicated in Table 3. Compounds 7a and 7b
were characterized by comparison with authentic samples
(commercially available). For compounds 7c–7e, physical,
spectroscopic and analytical (for 7c) data follow.
dried over sodium sulfate. After evaporation of the solvents
(15 Torr), the resulting residue was purified by column
chromatography (silica gel, hexane/ethyl acetate) giving
compound 9 in 72% yield. Another reaction that was set up
in parallel in the absence of additive 1 gave 71% yield of the
expected product. Compound 9, previously prepared in our
laboratory,35 was characterised by comparison of its
physical and spectroscopic data with an authentic sample.
4.7. Naphthalene-catalysed lithiation of benzyl fluoride
10 in the presence of additive 1 and cyclohexanone
To a green suspension of lithium powder (70 mg,
10.0 mmol) and naphthalene (20 mg, 0.16 mmol) in THF
(5 mL), under Ar, at 2308C, was dropwise added (ca.
20 min) a solution of fluoride 10 (1.0 mmol), additive 1
(38 mL, 0.15 mmol) and cyclohexanone (0.13 mL,
1.2 mmol) in THF (2 mL). The mixture was stirred for ca.
3 h allowing the temperature to rise to 08C. The reaction was
then hydrolysed with water (10 mL), acidified with 2 M HCl
and extracted with ethyl acetate (3£20 mL). The combined
organic layers were successively washed with a saturated
solution of NaHCO3 (5 mL), water (5 mL) and brine
(5 mL), being then dried over sodium sulfate. After
evaporation of the solvents (15 Torr), the resulting residue
was purified by column chromatography (silica gel,
hexane/ethyl acetate), affording compound 11 in 43%
yield. Another reaction that was set up in parallel in the
absence of additive 1 gave 45% yield of the expected
product. Compound 11, previously prepared in our labora-
tory,36 was characterised by comparison of its physical and
spectroscopic data with an authentic sample.
4.5.1. 6-Butylundecane (7c). Colourless oil; tr 8.2
[Tcolumn¼608C (3 min) and 60–2708C (158C min21)]; n
(film) 2957, 2926, 2858, 1466, 1378 cm21 (CH); dH 0.89
(9H, t, J¼6.9 Hz, 3£Me), 1.09–1.49 (23H, m, 11£CH2 and
CH); dC 14.15 (3C, 3£Me), 22.75 (2C), 23.2, 26.4 (2C),
29.0, 32.45 (2C), 33.4, 33.7 (2C) (11£CH2), 37.4 (CH); m/z
212 (Mþ, ,1%), 154 (13), 140 (29), 99 (15), 85 (59), 71
(57), 57 (100), 56 (14), 55 (25); Mþ, found 212.2486.
C15H32 requires 212.2504.
4.5.2. Methyl nonyl ether (7d).33 Colourless oil; Rf 0.59
(hexane/ethyl acetate: 4/1); tr 6.5; n (film) 1120, 1031 cm21
(CO); dH 0.87 (3H, t, J¼6.7 Hz, MeCH2), 1.15–1.46, 1.49–
1.68 [10H and 4H, respectively, 2m, (CH2)7Me], 3.32 (3H,
s, MeO), 3.36 (2H, t, J¼6.6 Hz, CH2O); dC 14.05 (MeCH2),
22.65, 26.15, 29.25, 29.5, 29.55, 29.65, 31.85, [(CH2)7Me],
58.45 (MeO), 72.95 (CH2O); m/z 158 (Mþ, ,1%), 126 (37),
98 (43), 97 (37), 84 (29), 83 (45), 82 (20), 71 (15), 70 (76),
69 (60), 68 (28), 57 (30), 56 (100), 55 (70).
Acknowledgements
This work was financially supported by the DGES from the
´
4.5.3. 2-Nonyloxytetrahydropyran (7e).34 Greenish oil; Rf
0.60 (hexane/ethyl acetate: 4/1); tr 11.5; n (film) 1128, 1076,
1032 cm21 (CO); dH 0.86 (3H, t, J¼6.5 Hz, Me), 1.01–1.95
[20H, m, Me(CH2)7 and (CH2)3CH], 3.32–3.46, 3.47–3.59,
3.69–3.81, 3.83–3.96 (1H each, 4m, 2£CH2O), 4.55–4.67
(1H, m, CH); dC 14.05 (Me), 19.65, 22.6, 25.5, 26.2, 29.25,
29.45, 29.5, 29.7, 30.75, 31.85 [Me(CH2)7 and (CH2)3CH],
62.25, 67.65 (2£CH2O), 98.7 (CH); m/z 228 (Mþ, ,1%), 85
(100), 84 (11), 71 (11), 57 (19), 56 (27), 55 (18).
Spanish Ministerio de Educacion y Cultura (MEC) (project
no. PB97-0133 and BQU2001-0538). P. M. thanks the
University of Alicante for financial support.
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To a green suspension of lithium powder (70 mg,
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