Reductive transformations of organofluorine compounds: III. Hydrodefluorination of perfluoroalkylbenzenes effected by Zn(Cu). The unusual behavior of compounds containing perfluorinated tert-butyl group

Article abstract of DOI:10.1023/A:1012429802321

Perfluoroalkylbenzenes under a treatment of a system Zn(Cu) - DMF - H₂O - electrolyte give rise in high yield to products of fluorine substitution by hydrogen in position 4. In perfluoro-tert-butylbenzene are substituted by hydrogen the fluorin

Full text of DOI:10.1023/A:1012429802321

Russian Journal of Organic Chemistry, Vol. 37, No. 4, 2001, pp. 517 522. Translated from Zhurnal Organicheskoi Khimii, Vol. 37, No. 4, 2001,
pp. 552 557.
Original Russian Text Copyright
2001 by Krasnov, Platonov.
Reductive Transformations of Organofluorine Compounds:
III.* Hydrodefluorination of Perfluoroalkylbenzenes
Effected by Zn(Cu). The Unusual Behavior of Compounds
Containing Perfluorinated tert-Butyl Group**
B. I. Krasnov and V. E. Platonov
Vorozhtsov Novosibirsk Institute of Organic Chemistry, Siberian Division, Russian Academy of Sciences,
Novosibirsk, 630090 Russia
Received November 10, 1999
Abstract Perfluoroalkylbenzenes under a treatment of a system Zn(Cu) DMF H2O electrolyte give rise
in high yield to products of fluorine substitution by hydrogen in position 4. In perfluoro-tert-butylbenzene
are substituted by hydrogen the fluorine atoms in positions 2 and 4; here the DMF water ratio affects the
regioselectivity of the process. In the perfluoro-4-tert-butyltoluene is mainly substituted the fluorine in the
ortho-position to the C(CF3)3 group.
The synthetic value of hydrogen introduction into a
perfluoroalkylarene consists in the possibility to
replace it by functional group with the use of a
Grignard reagent [2]. It is presumable that hydroper-
flyoroalkylarenes will turn out to be convenient
precursors for preparation of perfluoroalkylaryllith-
ium derivatives as has been already done e.g., with
pentafluorobenzene and tetrafluorobenzenes [3, 4].
The preparation methods for hydro compounds
involve nucleophilic substitution of fluorine atom in
the para-position with respect to perfluoroalkyl group
effected by LiAlH4 or NH2NH2 followed in the latter
case by treatment of the intermediate polyfluoroaryl-
hydrazine with Fehling reagent [5, 6]. The yields of
hydro derivatives here are moderate. Recently the
hydrodefluorination of perfluorotoluene was carried
out in high yield with the use of P(NEt2)3 in aqueous
dioxane [7] and with Zn in aqueous ammonia [8]. We
reported lately on substitution of fluorine in the para-
position of perfluoroalkylbenzenes by hydrogen
effected by Zn(Cu) in aqueous DMF [9].
the hydrodefluorination, and also of copper(II)
chloride instead of copper powder.
The reaction of octafluorotoluene (I) with the
system Zn(Cu) DMF H2O as we stated earlier
resulted in a single hydrodefluorination product,
, , ,2,3,5,6-heptafluorotoluene (II). Here is
important the completeness of conversion of the
original compound. We found that addition of electro-
lytes favors the hydrodefluorination effected by
Zn(Cu) [10]. The favorable effect of salts additives
may be understood, e.g., as increase in the electro-
conductivity of the solution if the role of Zn(Cu) is
that of a short-circuited voltaic couple.
Perfluorotoluene (I) in the presence of NH4Cl
within 20 h at 70 C is fully consumed and furnishes
a mixture containing mainly toluene II and a little of
N,N-dimethyl-4-trifluoromethyl-2,3,5,6-tetrafluoro-
aniline (III) (run no. 1, Table 1).
In the present study the preparation method for
hydro derivatives of perfluorinated monoalkyl-
benzenes was modified. The modification consists in
the application of electrolytes in order to accelerate
*
For communication II, see [1].
Under the same conditions but without NH4Cl
additive 21% of the initial compound remains un-
reacted (run no. 2, Table 1). The reaction is limited
to hydrogenolysis of a single C F bond. Hepta-
**
The study was carried out under financial support of the
Russian
Foundation
for
Basic
Research
(grant
no. 98-03-332966a).
1070-4280/01/3704-0517$25.00 2001 MAIK Nauka/Interperiodica

518
KRASNOV, PLATONOV
Table 1. Reactions of perfluoroalkylarenes with the systems Zn(CuCl2) DMF H2O electrolyte and
Zn(Cu) DMF H2O electrolyte
Run
no.
No. of
initial
compound compound,
mmol
Amount
of initial
Metal
amount,
mmola
Salt
amount,
mmol
Ratio
DMF
waterb
Temperature
of process, C in the reaction mixture, %
(duration, h)
Reaction products, content
(yield, %)
1
2
3
4
5
6
7
8
I
I
I
I
I
I
I
I
50
50
610, 155
610, 155
224c
31, 47 70 (20)
31, 47 70 (20)
31, 42 70 (20)
31, 42 70 (20)
31, 41 70 (18)
26, 41 70 (19)
0, 23 boil. (20)
0, 117 boil. (30)
30, 45 70 (19), Ar V, 94 (83)
30, 45 70 (19), Ar V, 97 (82)
30, 45 70 (19.5), Ar V, 96 (86)
30, 45 70 (19), Ar IV, 64 (47); V, 26 (21)
28, 45 70 (19), Ar II, 98 (72)
40, 35 70 (18), Ar IX, 80 (71), X, 13 (12), XI,
5 (4); XII 1 (1)
II, 95 (70); III, 1 (1)
I, 21 (14); II, 75 (55); III, 4 (2)
II, 92 (77)
II, 90 (77)
II, 93 (79)
II, 88 (77)
I, 85 (72); II, 15 (14)
I, 51 (39); II, 49 (40)
250
250
508
508
47.5
47.5
69.9
35
3050, 775
1525, 625
3048, 762
3048, 762
60.8, 80.8
460.8, 80.8
838.8, 209.7
420e, 3.5f
838.8e, 7.0f
105e, 3.5f
60.0e, 0.50f
50.4e, 0.42f
935c
467c
935c
935c
467c
428d
210d
104d
210d
104d
14.9d
12.7d
9
V
IV
IV
IV
VI
VIII
10
11
12
13
14
69.9
35
5
4.2
15
16
17
18
19
20
21
VIII
VIII
VIII
VIII
VIII
VIII
VIII
1
1
1
1
1
1
1
30.0, 8.0
30.0, 8.0
31.5g, 3.2
31.5g, 3.2
31.5g, 3.2
31.5g, 3.2
24.4, 7.8
89, 17 70 (10), Ar VIII, 19 (9), IX, 45 (22), X,
16 (8), XI, 16 (7)
89, 167 70 (10), Ar VIII, 28 (11), IX, 41 (17), X,
3 (1), XI, 26 (9)
100, 20 70 (10), Ar VIII, 2 (1), IX, 53 (40), X,
10 (7), XII, 6 (4), XI, 28 (19)
80, 40 70 (10), Ar VIII, 1 (<1), IX, 42 (28), X,
27 (18), XII, 9 (6), XI, 22 (15)h
60, 69 70 (10), Ar VIII, 4 (3), IX, 55 (34), X,
20 (12), XII, < 1 , XI, 20 (12)h
20, 100
70 (10), Ar VIII, 57 (18), IX, 34 (11), X,
< 1 , XII, 0, XI, 9 (3)h
78, 67 70 (10), Ar VIII, 0 (0), IX, 29 (15), X, 46
(24), XII, 5 (3), XI, 18 (9)
22
23
II
XIII
25
12.6
305, 77.5
151.2, 37.8
86d
43d
31, 42 70 (20)
56, 111 70 (9)
II, 99 (79)
XIV, 92 (87), XV, 4 (4)
a
b
c
d
e
f
Zn, Cu.
1
mmol mmol of the initial compound for DMF and H2O respectively.
NH4Cl additive.
NaCl additive.
Zn amount given.
CuCl2 2H2O additive.
Activated zinc was prepared by washing with 1.7% hydrochloric acid, then with water followed by decantation and drying in a
vacuum in an argon flow at 30 35 C.
The composition (mol %) was estimated from 19F and 1H NMR spectra.
g
h
fluorotoluene (II) treated with the system Zn(Cu)
NaCl-DMF H2O returned intact (run no. 22, Table1).
The results of run no.1 are well reproduced at in-
creased charge (run nos. 3 5, Table 1), and there-
with the amount of DMF and metals may be reduced
(run nos. 4, 5, Table 1). Note that conversion of
compound I into reaction product II occurs also on
boiling with Zn(Cu) in the water solutions of NH4Cl
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REDUCTIVE TRANSFORMATIONS OF ORGANOFLUORINE COMPOUNDS: III.
519
Table 2. 1H and 19F NMR spectra of compounds IX XII, XIV
19F NMR spectrum, F, ppm
1H NMR
spectrum,
, ppm
Compd.
no.
J, Hz
C(CF3 )3
2
3
4
5
6
IXa
Xa
100.7 t 34.5 m 26.9 m
99.6 d 36.4 m 11.1 d 13.6 d. 26.0 d
26.9 m 34.5 m 7.36 t.t
25 (F F2(6)), 8.5 (H4F3(5)), 7.0 (H4F2(6)
)
7.47 d.d.d.d 22 (F F2), 22(F4F5), 20(F3F4), 18(F2F3),
12.5 (H6F5), 11 (F2F5), 10.5 (F2F4), 7.5
d.d.d
d.d.d
d.d.d
(H6F4), 6.5 (H6F2), 3 (F3F5), 2.5 (H6F3)
XIa
100.1 t 31.6 m 11.0 m
99.3 d 32.0 m 18.9 m
11.0 m 31.6 m 3.10 t
25 (F F2(6)), 2.5 (H4F3(5)
)
XIIa
37.3 m
49.0
7.15 m (H6), 21.5 (F F2), 2.5 (H4F3(5)
3.04 t (NCH3)
7.48 d.d.d
)
XIV 99.9 d 34.8 m 33.0 m 104.2 t
21.5 (F F2), 20.5 (F3F4 and F4F5), 12
(H6F5), 5.5 (H6F2), 2.5 (H6F3)
(CF3)
a
The signals were assigned from analysis of 19F and 1H NMR spectra of reaction mixtures.
and NaCl with no DMF but the rate of the reaction
is considerably slower (run nos. 7, 8, Table 1).
The treatment with the system Zn(Cu) NaCl-
DMF H2O of perfluoroethylbenzene afforded
1-pentafluoroethyl-2,3,5,6-tetrafluorobenzene
(V)
(run no. 9, Table 1). We found that in the presence
of salt additive the amount of the copper used can be
reduced to 0. 1 mole per 1 mole of substrate when it
is introduced into the system as CuCl2 (runs
nos. 10 12, Table 1). Therewith the efficiency of the
dehydrofluorination remains on the same level. The
complete conversion of perfluoroisopropylbenzene
(VI) into heptafluoroisopropyl-2,3,5,6-tetrafluoro-
benzene (VII) was carried out under similar condi-
tions (run no. 13, Table 1).
At complete conversion of compound VIII (run
no. 14, Table 1) in the reaction mixture were present
1-nonafluoro-tert-butyl-2,3,5,6-tetrafluorobenzene
(IX) and 1-nonafluoro-tert-butyl-2,3,4,5-tetrafluoro-
benzene (X) that were separated by distillation from
small impurity containing N,N-dimethyl-4-nona-
fluoro-tert-butyl-2,3,5,6-tetrafluoroaniline (XI) and
N,N-dimethyl-4-nonafluoro-tert-butyl-2,3,6-trifluoro-
aniline (XII) (1H and 19F NMR spectra see in
Table 2). Dimethylanilines XI and XII were not
isolated, and their formation in the presence of the
system Zn(Cu) DMF H2O was concluded from
RF
= CF2CF3 (IV, V), CF(CF3)2 (VI, VII).
The treating of perfluoro-tert-butylbenzene (VIII)
with Zn(Cu) in aqueous DMF unexpectedly provided
derivatives with hydrogen atoms both in para and
ortho-positions with respect to the perfluoroalkyl
group unlike the behavior of perfluorobenzenes I, IV,
VI and of perfluoropropylbenzene [9] containing less
bulky substituents. In the nucleophilic reactions of
arene VIII also was not observed substitution in the
ortho-position [11, 12].
1
identity of their H and 19F NMR spectra to those of
compounds obtained by treating perfluoroarene VIII
or the mixture of compounds IX and X with di-
methylamine solution in DMF. The distilled off
mixture of compounds IX and X (run no. 14,
Table 1) according to GC-MS data contained as
impurity 6% of compound with molecular weight
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 37 No. 4 2001

520
KRASNOV, PLATONOV
corresponding to a product with two fluorine atoms
of arene VIII replaced by hydrogen.
The influence of the DMF H2O ratio on the
physical characteristics of this binary system and rate
constants of some reactions therein is well known
[13]. We observed the change in the regioselectivity
of the hydrodefluorination of arene VIII effected by
Zn(Cu) at variation in the amounts of DMF and water
(preliminary communication see [14]). For instance,
ten-fold increase in water amount (the content in
the binary mixture DMF water grew from 16 to
65 mol %) at the same ratio of the other reagents and
the same reaction conditions resulted in 4 times lower
contribution of the ortho-dehydrofluorination (run
nos. 15, 16, Table 1).
At the same time in the reaction of compound XIII
with LiAlH4 the replacement of fluorine by hydrogen
occurred to a greater degree in the ortho position with
respect to the less bulky perfluorinated substituent.
The ratio of these two routes was estimated as 1: 2.3
by the integral intensities of CF3 and C(CF3)3
groups of different multiplicity in the 19F NMR
spectrum. The fragment CF=CRFCF= in the
19F NMR spectrum gives a signal of the perfluoro-
alkyl grout RF as a triplet with the coupling constant
no less than 20 Hz. In contrast the signals of the
perfluoroalkyl groups in the fragment CH=CRFCF=
are doublets.
When the water amount reached 83% occurred
prevailing para-dehydrofluorination (run no. 20,
Table 1). The possible cause thereof are difficulty in
solvation with water of the fluoride ion cleaved from
the ortho-position with respect to perfluoroalkyl. The
highest content of the ortho-dehydrofluorination
products IX and X is observed not at the maximum
content of DMF in the mixture but at intermediate
amounts. At 46% of water (run no. 21, Table 1) we
achieved the highest fraction of the ortho-isomer in
the product: 1.5 times more than para-isomer.
The formation of 1-nonafluoro-tert-butyl-4-tri-
fluoromethyl-2,3,6-trifluorobenzene (XVI) is sup-
ported by the value of m/z measured with GC-MS
method in the mass spectrum, and by analysis of 19F
and 1H NMR spectra of the reaction products mixture.
In the 19F NMR spectrum after assignment of the
signals belonging to compounds XIII and XIV stands
In the run no. 14 (Table 1) the ratio DMF water
was about the same, as in the run no. 19 (Table 1)
but the former was carried out with NaCl additive,
and the contribution from the ortho-dehydrofluorina-
tion was only 15%. The latter fact suggests that the
presence of salts can influence the regioselectivity of
the reaction. Here we wish to state that the de-
pendence of the regioselectivity on the DMF water
ratio we treat on a qualitative level.
out a group of intense signals ( , ppm, J, Hz): 100.
F
9 t (25.5), 99. 3 d (12), 64. 1 m, 45. 4 m, 23. 8 m
1
(9: 3: 1: 1: 1). In the H NMR spectrum is observed
a signal of the proton in aromatic ring ( , ppm,
J, Hz): 7. 40 d. d. d (13, 5. 5, 2. 5).
We observed formerly the hydrodefluorination in
the ortho-position to the perfluorobutyl group as the
prevailing path in the reaction of perfluoro-4-tert-
butyltoluene (XIII) with the system Zn(Cu) DMF
H2O [15]. When this system was used with NaCl
additive perfluoroarene XIII within 9 h at 70 C (run
no. 23, Table 1) virtually completely was converted
into 1-nonafluoro-tert-butyl-4-trifluoromethyl-2,3,5-
trifluorobenzene (XIV) (87%) (NMR spectra are
given in Table 2). Product XIV was separated by
distillation from a small impurity, hydrodefluorina-
tion product at the benzyl position nanafluoro-tert-
butyl-4-methyl-2,3,5,6-tetrafluorobenzene (XV) (4%).
In this reaction did not occur replacement of fluorine
by hydrogen in the ortho-position with respect to
trifluoromethyl group.
The revealed difference in the main products
obtained by hydrodefluorination of compound XIII
effected by LiAlH4 and Zn(Cu) may be caused by
dissimilar mechanisms of the processes. In the first
case the reaction probably includes an electron
transfer and goes via decomposition of the inter-
mediate anion-radical [17] whereas in the second case
apparently occurs a nucleophilic attack on the sterical-
ly the least hindered position in the aromatic ring.
The effect of the size of the perfluoroalkyl substituent
on the direction of the nucleophilic attack was
formerly observed with perfluoro-p-cymene [18].
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 37 No. 4 2001

REDUCTIVE TRANSFORMATIONS OF ORGANOFLUORINE COMPOUNDS: III.
521
Compound XIV may arise both from nucleophilic
substitution and electron transfer. In some studies the
mechanism of halogen replacement by hydrogen
effected by LiAlH4 was regarded as electron transfer
process [19, 20]. The direction of hydrodefluorina-
tion is changed with reaction mechanism also with
perfluoro-p-xylene: The latter when treated with the
system Zn(Cu) DMF H2O yields 4-methylhepta-
fluorotoluene [17] whereas with LiAlH4 occurs
fluorine substitution in the aromatic ring [21].
of dimethylamine, and 1.6% of water was maintained
at room temperature for 4 days. Then was added
CCl4, and the mixture was diluted with 3.5% HCl.
In the 19F NMR spectrum of the reaction products
were observed the signals from compounds IX, X,
XII (Table 20 in 14: 1: 2 ratio.
Compounds III, XV were identified in the reac-
1
tion mixtures by means of GLC, H and 19F NMR
spectroscopy by comparison with authentic samples
prepared by procedures [16] and [1] respectively.
EXPERIMENTAL
Heptafluoroisopropyl-2, 3, 5, 6-tetrafluoro-
benzene (VII). IR spectrum (3% solution ˆ CCl4, ,
1
NMR spectra of reaction mixtures and of solu-
tions in CCl4 of individual compounds (c <
11 mol%) were registered on spectrometer Bruker
WP-200SY at operating frequencies 188. 3 MHz for
cm ): 3080, 1612w, 1505s, 1400 w, 1301 s, 1290 s,
1264 s, 1240 s, 1185, 1178, 1145 w, 1132, 1064 w,
987 s, 932 s, 960, 923, 712. 19F NMR spectrum
(CCl4), F, ppm: 27.8 (F arom), 26. 7 (F arom), 25.3
(2F arom), 16.9 [CF(CF3)2], 86.2 [CF(CF3)2].
1H NMR spectrum (CCl4), , ppm: 7. 32 t.t. Found,
%: C 33.71, 33.94; H 0.48, 0.39. M+ 317.98978.
C9HF11. Calculated, %: C 33.99; H 0.32. M+
317.99026.
1
19F and 200 MHz for H), internal references C6F6
and HMDS ( 0.04 ppm). IR spectra were recorded
on spectrophotometer Specord M-80. Elemental
composition of molecular ions was determined from
high resolution mass spectra measured on Finnigan
MAT 8200 instrument. GC-MS measurements were
carried out on Hewlett-Packard G1800A instrument
equipped with electron-ionization detector. GLC
analyses were performed on LKhM-72 device (50
270 C, column 400 4 mm, stationary phases 15 and
25% SKTFT-50 or 15% E-301 on Chromosorb W,
1-Nonafluoro-tert-butyl-4-trifluoromethyl-2,3,5-
trifluorobenzene (XIV). IR spectrum (3% solution
1
in CCl4, , cm ): 3175 w, 3140 w, 1655, 1600,
1505, 1490, 1467 s, 1350, 1313 s, 1285 s, 1270 s,
1255 s, 1205, 1185, 1160 s, 1058, 1055, 975, 962,
910, 857. Found, %: C 31.14, 31.44; H 0.15, 0.19;
F 67.90, 67.67. M+ 417.98390. C11HF15. Calculated,
%: C 31.60; H 0.24; F 68.16. M+ 417.983874.
1
carrier gas He, flow rate 60 ml min . In reactions
was used zinc powder produced in Russia according
to state standard GOST 12601-76.
Reaction of perfluoro-4-tert-butyltoluene (XIII)
with LiAlH4. To a solution of 1.03 g of compound
XIII in 2 ml of anhydrous ethyl ether under argon
Reaction of perfluoroalkylbenzenes I, IV, VI,
VIII and polyfluorotoluene II with the systems
Zn(CuCl2) DMF H2O electrolyte and Zn(Cu)
DMF H2O electrolyte (Table 1). A mixture of DMF, atmosphere at 5...0 C within 5 min was added 0.1 g
water and metals was prepared. Copper(II) chloride
and the other salts were added in water solution.
Copper metal was used as freshly prepared water
suspension. Polyfluoroarenes were added to the reac-
tion mixture while stirring. The reaction products
were separated by steam distillation. The following
individual compounds were isolated: II (bp 113.5 C,
publ. 111 112 C [5], 110. 5 111. 5 C [8]), (V)
(bp 120 122 C, publ. 124 C [6]), (VII) (bp 139
140 C), (XIV) [bp 81 82 C (65 mm Hg)].
of lithium aluminum hydride in 4 ml of anhydrous
ethyl ether. The mixture was stirred for 1h, then by
portions was added 10 ml of 20% sulfuric acid. The
ether layer was separated and dried with CaCl2. After
the ethyl ether was distilled off we obtained 0.81 g of
mixture containing according to the data of 19F NMR
spectrum and GC-MS method 60% of compound
XIII, 11% of compound XIV, and 24% of compound
with molecular ion m/z 418.
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according to 19F NMR and GC-MS data compounds
IX (74%), X (16%), and 6% of compound with
molecular ion m/z 350. This mixture (0.1 g) and
0.14 g of solution containing 97.6% of DMF, 0.8%
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