Difluoromethylation of some C-H acids with chlorodifluoromethane under conditions of phase transfer catalysis (PTC)

Article abstract of DOI:10.1016/j.jfluchem.2009.02.008

Selected C-H acids react with difluorocarbene generated from chlorodifluoromethane with concentrated aqueous solution of sodium hydroxide, and a catalyst, benzyltriethylammonium chloride (TEBAC) in benzene or THF affording C-difluoromethyl substituted der

Full text of DOI:10.1016/j.jfluchem.2009.02.008

Journal of Fluorine Chemistry 130 (2009) 466–469
Contents lists available at ScienceDirect
Journal of Fluorine Chemistry
journal homepage: www.elsevier.com/locate/fluor
Difluoromethylation of some C–H acids with chlorodifluoromethane under
conditions of phase transfer catalysis (PTC)
Ewelina Nawrot, Andrzej Jo n´ czyk *
Warsaw University of Technology, Faculty of Chemistry, Koszykowa Street 75, 00-662 Warszawa, Poland
A R T I C L E I N F O
A B S T R A C T
Article history:
Selected C–H acids react with difluorocarbene generated from chlorodifluoromethane with concentrated
aqueous solution of sodium hydroxide, and a catalyst, benzyltriethylammonium chloride (TEBAC) in
benzene or THF affording C-difluoromethyl substituted derivatives. This process is restricted to C–H
Received 29 November 2008
Received in revised form 8 February 2009
Accepted 9 February 2009
Available online 21 February 2009
acids of pK
a
ffi 16.3–19.1. The observed facts are rationalized.
ß 2009 Elsevier B.V. All rights reserved.
Keywords:
Phase transfer catalysis
Difluorocarbene
C-difluoromethylation
C–H acids
1
. Introduction
There are at least two reasons for which fluorine atoms are
sodium hexamethyldisilylamide (NaHMDS) [11,25] or (tris-
diethylamino)phosphine methylimide [26]. In the case of
nitriles, sodium t-butoxide [9,10] or sodium hydride was
applied. The C–H acids mentioned above possessed only one
acidic hydrogen atom. Depending on the structure of esters or
nitriles and reaction conditions, difluoromethylation occurred in
yield from 19% to high.
introduced to organic compounds. First, they are isosteric with
hydrogen ones, second – exhibit the highest electrophilicity from
all elements (4.0 in Pauling s scale). Hence, difluoromethyl and
trifluoromethyl groups significantly change physical and biological
properties of organic compounds [1,2]. Difluoromethyl group is
conveniently introduced by the reaction of organic anions with
chlorodifluoromethane (freon R-22). We have previously shown
that using freon R-22 in the presence of a concentrated aqueous
solution of sodium hydroxide and a quaternary ammonium salt as
a catalyst, in nonpolar aprotic solvent (phase transfer catalysis, PTC
0
For reaction of difluoromethylation we selected C–H acids listed
in Table 1.
Taking into account that in the case of diphenylacetonitrile (1a)
this process is fairly well described (it was carried out in the
presence of two different bases) [9,10,15], our preliminary
experiments were performed with 1a. Passing the stream of freon
R-22 into a vigorously stirred mixture of 1a, 50% aq. sodium
hydroxide, with benzyltriethylammonium chloride (TEBAC) as a
catalyst, in benzene led to formation of (2-difluoromethyl)diphe-
nylacetonitrile (2a) (Table 2, entry 1, Table 3).
[
3–6]), N-aryl amides are simultaneously N- and O-difluoromethy-
lated [7], oximes O-difluoromethylated [7], while some nitrogen
heterocycles, N-difluoromethylated [8].
The process requires rather extensive bubbling of freon R-22 to
remove air from the reaction flask, otherwise 1a is oxidized to
benzophenone. Oxidation of 2-arylalkanenitrles (including 1a) with
oxygen under PTC conditions to the corresponding phenones is
firmly established [27–29]. Reactions of 1a carried out either in the
presence of more diluted sodium hydroxide solutions or in different
solvents and temperatures gave usually lower content of 2a in the
reaction mixtures (Table 2, entries 2–6). The reaction done in an
aqueous–acetone solution of potassium hydroxide afforded parti-
cularly poor result (Table 2, entry 7). Previously, this combination of
base and solvents was successfully applied for difluoromethylation
of some nitrogen heterocycles [30]. Attempted difluoromethylation
2
. Results and discussion
Now, we wish to report the results of our research on
application of PTC to C-difluoromethylation of some C–H acids
with freon R-22 (Scheme 1). According to literature, difluor-
omethylation of esters with freon R-22 was carried out in
the presence of sodium or potassium t-butoxide [9,10–14],
sodium hydride (usually in THF) [15,16–21], LDA [22–24],
*
Corresponding author. Tel.: +48 22 621 42 30.
E-mail address: anjon@ch.pw.edu.pl (A. Jo n´ czyk).
0022-1139/$ – see front matter ß 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jfluchem.2009.02.008

E. Nawrot, A. Jo n´ czyk / Journal of Fluorine Chemistry 130 (2009) 466–469
467
Table 1
C–H acids which were reacted with freon R-22.
1,2
a
b
c
d
e
f
g
h
R
X
Ph
Ph
Ph
Ph
Ph
Ph
Et
PhS
Et
Ph
2–C10
H
7
4-O
2
6
NC H
4
NMe
2
CO
2
Et
Et
Y
CN
CN
CN
CN
CN
CN
CN
CO
2
1
,2
i
j
k
l
m
n
o
R
X
PhCH
2
Me
Et
Ph
Ph
Et
Ph
Bu
CO
2
Et
CO
2
Et
CO
2
Et
Et
CN
Y
CO
2
Et
CO
2
Et
CO
2
CO
2
Et
COPh
COPh
SMe
of other nitriles succeeded in the case of 1-naphtylphenylacetoni-
tryle (1b) only, giving 2b in a rather low yield (Table 3).
Pure 2a,b were isolated by column chromatography in a low
yield only, the majority of eluted fractions contained both 1a
and 2a or 1b and 2b. This problem was solved by PTC alkylation
of unreacted 1a,b with isopropylchloroacetate, then separation
of 2a,b from alkylated 1a,b by column chromatography
Scheme 1. Difluoromethylation of C–H acids.
(
Table 3).
Different reactivity of C–H acids with freon R-22 seems depend
on their acidities. Reported in literature pKa(DMSO) values of
searched compounds are collected in Table 4 [31].
2
-(4-Nitrophenyl)-(1c), (2-dimethylamino)phenylacetonitrile
(
1d), 2-phenyl- (1e) and (2-phenylsulfanyl)butyronitrile (1f) did
not react with freon R-22, at all. Similarly, 2-benzoyl-1-cyano-1,2-
dihydro-isoquinoline (1g, isoquinoline Reissert compound) proved
inert.
Slightly different reactivity was observed in the case of
substituted malonic esters 1h–k. Diethyl phenylmalonate (1h)
entered smooth reaction with freon R-22 (THF appeared a better
solvent than benzene) forming 2h, while alkyldiethylmalonates
Under PTC conditions freon R-22 formed products with C–H
acids of pKa(DMSO) ffi 16.3–19.1. Difluoromethylated C–H acids 2
resulted from reaction of difluorocarbene with carbanions and
protonation of the difluoromethylanions thus formed. It means
that both active particles, i.e., carbanions and difluorocarbene are
generated under PTC conditions. According to Hine et al. [32,33]
chlorodifluorocarbanion is not involved in alkaline hydrolysis of
freon R-22. We found that it hydrolyzed (via difluorocarbene)
when passed through a 50% aq. sodium hydroxide–benzene two-
phase system, more efficiently when a PT catalyst was present. It
seems that difluorocarbene formed difluoromethylanions with
carbanions when they are formed in sufficiently high concentra-
tion (but under PTC conditions not exceeding that of a catalyst [3–
6]) and exhibit high activity. Possibly, this is the case of C–H acids
1
i–k afforded the expected products 2i–k in a lower yield, under
harsher conditions (Table 3). On the other hand, neither
phenyl)ethylcyanoacetate (1l) nor 2-methylethylidenemalonate
(
reacted with freon R-22. Reactions of esters 1h–k were carried out
in large excess of THF, otherwise their hydrolysis was noticed.
Finally we found that alkyl derivatives of desoxybenzoine 1m,n
and 9-methylsulfanylfluorene (1o) did not enter the reaction with
freon R-22 under PTC conditions.
of pK
condition but probably not the latter one. On the other hand, C–
H acids of pK > 19 generate active carbanions but in concentration
insufficiently high for reaction with difluorocarbene. We did not
find measured pK for freon R-22, this value calculated from acidity
a a
= 16.3–19.1. C–H acids of pK < 16 fulfill the former
a
Table 2
Difluoromethylation of 1a under different conditions.
a
coefficients [34] is ca 22. The results published by Hine et al.
[32,33] mean that difluorocarbene from freon R-22 is generated at
the interphase of two-phase system, particularly in the presence of
Entry Base
Solvent
Temperature Time Content of 2a in
a
(
8C)
(h)
reaction mixture (%)
1
2
3
4
5
6
7
50% aq. NaOH Benzene
20
6
62
42
61
15
49
14
6
C–H acids of pK
interphase carbanions react with difluorocarbene. These consid-
erations are rather simplified since we used pK values of C–H acids
a
< 22. Therefore, it seems that also at the
50% aq. NaOH Benzene 35–40
6
40% aq. NaOH Benzene
30% aq. NaOH Benzene
50% aq. NaOH THF
20
20
7
5.5
6
a
20
measured in DMSO [31], the conditions which only roughly
resemble that of PTC. However, pK’s of investigated C–H acids,
measured in other solvents are scarcely available in literature.
We have demonstrated that PTC can be applied for difluor-
omethylation of C–H acids with freon R-22. The process is simple
50% aq. NaOH Dioxane
20
3
40% aq. KOH
Acetone
40–50
8
a
Determined by GC.
Table 3
Difluoromethylated C–H acids.
but restricted to C–H acids of pK
observed facts is given.
a
ffi 16.3–19.1. An explanation of
a
No.
Solvent
Temperature (8C)
Time (h)
Yield (%)
Table 4
a
pK of C–H acids.
2
2
2
2
2
2
a
b
h
i
C
C
6
H
H
6
20
20
6
6
43
12
64
40
36
6
6
No.
a
pK (DMSO)
THF
THF
THF
THF
20
1
50–60
20
11
5
1a
1h
1j
17.5
16.3
18.7
19.1
8.0
j
b
k
50–60
9
7
1
1
k
l
a
Isolated, pure (ꢀ98.5%) products.
b
1
Determinated by GC. Identified by H NMR.

4
68
E. Nawrot, A. Jo n´ czyk / Journal of Fluorine Chemistry 130 (2009) 466–469
3
. Experimental
of argon at temperature 10–20 8C for 0.5 h and at 60–70 8C for 0.5 h,
then cooled and diluted with water (5 ml). The reaction was worked
3.1. General
upasdescribedinSection3.3, theproduct2bwasisolatedbycolumn
chromatography and crystallized (yield 12%).
1
Melting points were measured on a capillary melting point
apparatus and are uncorrected. H and C NMR spectra were
recorded on a Varian Mercury spectrometer (at 400 and 100 MHz,
2b, solid, mp 134–135 8C, H NMR (400 MHz, CDCl
3
):
d
6.63 (t,
1
13
13
J = 54 Hz, 1H, CHF
CDCl ): 54.41, 114.19 (t, J = 252 Hz, CHF
125.36, 126.13, 126.76, 127.98, 128.77, 129.11, 129.17, 129.32,
129.89, 130.90, 133.54, 134.88. Anal. Calcd. for C19 N: C, 77.80,
2
), 7.31–7.98 (m, 12H, ArH), C NMR (100 MHz,
3
d
2
), 117.13, 124.59, 125.03,
respectively) in CDCl
ence, chemical shifts are reported in ppm (
3
with tetramethylsilane as external refer-
= 0.00). The following
d
13 2
H F
abbreviations are used: s, singlet; d, doublet; t, triplet; q, quartet;
m, multiplet. Gas chromatography (GC) analyses were carried out
on gas chromatograph Agilent 6850 Series GC System equipped
with HP-50+ (30 m) column. Microanalyses were obtained using a
CHN/S PerkinElmer 2400 element analyzer. Column chromato-
graphy was performed on Merck Kieselgel 60 (230–400 mesh)
with hexane and ethyl acetate mixtures (gradient) as eluents.
Chlorodifluoromethane, nitrile 1a, malonates 1h–k and 9-methyl-
sulfanylfluorene (1o) were commercial, while nitriles 1b [35], 1c
H, 4.47, N, 4.78. Found: C, 77.76, H, 4.46, N, 4.78.
3.4. General procedure for preparation of 2h–k
Into the three-necked, round bottomed flask equipped with
reflux condenser, mechanical stirrer, thermometer and glass pipe
for introducing chlorodifluoromethane, malonate 1h–k (4 mmol),
50% aq. NaOH (0.96 g, 0.64 ml, 12 mmol), TEBAC (0.05 g,
0.20 mmol) and THF (35 ml) were placed. The content of the flask
was vigorously stirred and chlorodifluoromethane was bubbled
through the mixture at the temperature and the time indicated in
Table 3. After every 1.5 h, further portions of 50% aq. NaOH (0.96 g,
[
(
36], 1d [37], 1e [38], 1f [39], 1g [40], (phenyl)ethylcyanoacetate
1l) [41] and desoxybenzoines 1m,n [42] were prepared by
literature procedures.
0.64 ml) were added. The progress of the reaction was monitored
3.2. Procedure for preparation of 2a
by GC. The mixture was diluted with CH Cl (35 ml), the organic
2
2
phase was decanted from the semisolid inorganic one which stuck
to the wall of the flask, filtered through filter, and dried over
Into three-necked round bottomed flask equipped with reflux
condenser, mechanical stirrer, thermometer and glass pipe for
4
MgSO . The solvent was evaporated, and the products 2h–j were
introducing chlorodifluoromethane nitrile 1a (0.97 g, 5 mmol),
isolated by column chromatography in yields 36–64% (Table 3).
Reactions of nitriles 1c–g with chlorodifluoromethane were
carried out using Section 3.3 while C–H acids 1l–o using Section
50% aq. NaOH (1.20 g, 0.80 ml, 15 mmol), TEBAC (0.06 g,
.25 mmol) and benzene (10 ml) were placed. The content of
0
the flask was vigorously stirred and chlorodifluoromethane was
bubbled through the mixture for 6 h (the progress of the reaction
was monitored by GC). Then the mixture was diluted with water
3.4.
1
2h, colorless oil, H NMR (400 MHz, CDCl
3
d 1.31 (t, J = 7.2 Hz,
):
), 6.56 (t, J = 55 Hz, 1H,
), 7.35–7.41 (m, 5H, ArH). C NMR (100 MHz, CDCl ): 13.80,
62.57, 67.26 (t, J = 21.3, C-CHF ), 114.03 (t, J = 247 Hz, CHF ),
128.12, 128.58, 129.34, 130.24, 165.99, F NMR (376 MHz, CDCl ):
6H, 2 Â CH
CHF
3
), 4.30–4.41 (m, 4H, 2 Â CH
2
1
3
(
10 ml), the water phase was extracted with benzene (3 Â 5 ml),
2
3
d
the organic extracts were dried over MgSO and the solvent was
4
2
2
1
9
evaporated. The residue (1.16 g) was dissolved in benzene (1 ml),
3
cooled to 15 8C, 50% aq. NaOH (0.72 g, 0.48 ml, 9 mmol), TEBAC
d
À128.28 (d, J = 55 Hz, 2F).
1
(0.005 g, 0.02 mmol) and isopropylchloroacetate (0.41 g, 3 mmol)
2i, colorless oil, H NMR (400 MHz, CDCl
3
):
d
1.25 (t, J = 7 Hz,
), 6.05 (t,
), 7.18–7.28 (m, 5H, ArH). C NMR (100 MHz,
13.82, 35.91, 62.14, 62.15, 114.37 (t, J = 247 Hz, CHF ),
):
: C, 59.99, H,
were added. In atmosphere of argon, the mixture was vigorously
stirred at temperature 10–20 8C for 0.5 h, then at 60–70 8C for
6H, 2 Â CH
J = 54 Hz, 1H, CHF
CDCl ):
127.40, 128.42, 130.12, 134.22, 166.38. F NMR (376 MHz, CDCl
3
), 3.44 (s, 2H, CH
2
), 4.17–4.30 (m, 4H, 2 Â CH
2
1
3
2
0
.5 h, cooled and diluted with water (5 ml). The organic phase was
3
d
2
1
9
separated, the water phase was extracted with benzene (3 Â 5 ml),
3
the organic extracts were washed with water (10 ml), dried over
d
À131.76 (d, J = 54 Hz, 2F). Anal. Calcd. for C15
18 2 4
H F O
MgSO
isolated by column chromatography and crystallized (yield 43%).
4
and the solvent was evaporated. The product 2a was
6.04. Found: C, 59.88, H, 5.91.
1
2j, colorless oil, H NMR (400 MHz, CDCl
3
):
d
1.26 (t, J = 7.2 Hz,
), 6.34
12.03, 13.84,
), 62.25, 114.46 (t, J = 245 Hz, CHF ),
À132.97 (d, J = 55 Hz, 2F).
1
2
CDCl
a, Solid, mp 84–85 8C, lit. [9] mp 85 8C, H NMR (200 MHz,
6H, 2 Â CH
(t, J = 55 Hz, 1H, CHF
58.13 (t, J = 22.8, C-CHF
3
), 1.52 (s, 3H, CH
3
), 4.23 (q, J = 7.2 Hz, 4H, 2 Â CH
2
1
3
13
3
):
d
6.43 (t, J = 54 Hz, 1H, CHF
): 56.44 (t, J = 20.5 Hz, C-CHF
), 117.70, 128.11, 128.27, 128.79, 128.92, 129.27,
30.16, 134.12, 140.30.
2
), 7.19–7.45 (m, 10H, ArH),
C
2
). C NMR (100 MHz, CDCl ):
3
d
NMR (50 MHz, CDCl
J = 252 Hz, CHF
3
d
2
), 114.25 (t,
2
2
1
9
2
3
166.91. F NMR (376 MHz, CDCl ): d
1
Acknowledgments
3.3. Procedure for preparation of 2b
The financial support of this work by the Ministry of Science and
Higher Education, Warsaw, Poland (grant no. N204 038 31/09630)
is gratefully acknowledged.
Into three-necked, round bottomed flask equipped with reflux
condenser, mechanical stirrer, thermometer and glass pipe for
introducing chlorodifluoromethane, nitrile 1b (0.97 g, 4 mmol), 50%
aq. NaOH (0.96 g, 0.64 ml, 12 mmol), TEBAC (0.05 g, 0.2 mmol) and
benzene(10 ml)wereplaced. The contentofthe flaskwasvigorously
stirred and chlorodifluoromethane was bubbled through the
mixture for 6 h at room conditions (the progress of the reaction
was monitored by GC). The mixture was diluted with water (10 ml),
the water phase was extracted with benzene (3 Â 5 ml), the organic
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