Reaction of difluorocarbene with anions from amides and oximes: Synthesis of N-difluoromethyl substituted amides, O-difluoromethylimidates or O-difluoromethyl substituted oximes under phase-transfer catalysis conditions

Article abstract of DOI:10.1016/j.jfluchem.2006.04.006

N-Aryl substituted amides react with chlorodifluoromethane in the presence of concentrated aqueous sodium hydroxide and benzyltriethylammonium chloride (TEBAC) as a catalyst in benzene (phase-transfer catalysis, PTC), affording mixtures of N- and O-difluoromethyl substituted derivatives. Amide anions are involved in this process. The reaction carried out with oximes gives O-difluoromethyl oxime ethers.

Full text of DOI:10.1016/j.jfluchem.2006.04.006

Journal of Fluorine Chemistry 127 (2006) 943–947
www.elsevier.com/locate/fluor
Reaction of difluorocarbene with anions from amides and oximes: Synthesis
of N-difluoromethyl substituted amides, O-difluoromethylimidates or O-
difluoromethyl substituted oximes under phase-transfer catalysis conditions
*
´
Ewelina Nawrot, Andrzej Jonczyk
Warsaw University of Technology, Faculty of Chemistry, Koszykowa St. 75, 00-662 Warszawa, Poland
Received 1 February 2006; received in revised form 14 April 2006; accepted 18 April 2006
Available online 28 April 2006
Abstract
N-Aryl substituted amides react with chlorodifluoromethane in the presence of concentrated aqueous sodium hydroxide and benzyltriethy-
lammonium chloride (TEBAC) as a catalyst in benzene (phase-transfer catalysis, PTC), affording mixtures of N- and O-difluoromethyl substituted
derivatives. Amide anions are involved in this process. The reaction carried out with oximes gives O-difluoromethyl oxime ethers.
# 2006 Elsevier B.V. All rights reserved.
Keywords: Phase-transfer catalysis; Difluorocarbene; Amides; Oximes: N- and O-difluoromethylation
1. Introduction
case described, acetoxime gave an O-difluoromethyl derivative
in 3% yield when reacted with chlorodifluoromethane and
sodium methoxide in methanol–ethyl ether mixture [8].
We have reported [9] recently that some nitrogen hetero-
cycles are N-difluoromethylated with difluorocarbene gener-
ated from chlorodifluoromethane under conditions of phase-
transfer catalysis (PTC) [10–12], i.e. in the presence of
concentrated aqueous sodium hydroxide and benzyltriethy-
lammonium chloride (TEBAC) as a catalyst.
Amides treated with strong bases generate ambident anions
which react with alkylating agents giving N-substituted
products [1]. Primary and secondary amides form with reactive
alkylating agents (dialkylsulfates, trialkyloxonium salts, etc.)
alkoxymethyleneiminium salts which were converted into
O-substituted products (imidates) after deprotonation with a
weak base [2]. Difluorocarbene is an electrophilic species,
hence may react principally with amide or oxime anions at both
nucleophilic centers but such processes are not reported.
However, anions from some cyclic compounds exhibit
ambident nature toward difluorocarbene. Thus, chlorodifluor-
omethane, the precursor of this carbene, afforded with the
sodium salt of 2-hydroxyquinoline (carbostyril) [3] or with
quinoxalin-2-ones and potassium carbonate or caesium fluoride
[4], mixtures of O-difluoromethylated (main product) and N-
difluoromethylated derivatives, in addition to 2-fluoroquinox-
alines in the latter case.
2. Results and discussion
We studied the reaction of chlorodifluoromethane with
amides and oximes under PTC conditions. To select optimal
conditions, the amide 1a was allowed to react with chlorodi-
fluoromethane and sodium or potassium hydroxide in different
solvents, with or without a phase-transfer catalyst. These
preliminary experiments showed that difluorocarbene reacts
with ambident amide anion 1aÀ at both nucleophilic centers,
giving 2a and 3a. Of the basic systems studied, 50% aq. sodium
hydroxide/benzene/TEBAC as a catalyst gave the best results,
although total yields of difluoromethylated derivatives 2a and
3a did not exceed 54%. Potassium hydroxide in aqueous
acetone, the system used previously for difluoromethylation of
some nitrogen heterocycles with chlorodifluoromethane [13],
gave poor yields in this case as did solid–liquid variants of PTC.
Treating oximes with bases generated ambident anions
which, depending on the geometry of the substrate, kind of base
and type of alkylating agent, afforded O- or N-substituted
derivatives (nitrones) or their mixtures [5–7]. According to one
* Corresponding author. Tel.: +48 22 628 42 30; fax: +48 22 628 27 41.
´
E-mail address: anjon@ch.pw.edu.pl (A. Jonczyk).
0022-1139/$ – see front matter # 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jfluchem.2006.04.006

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E. Nawrot, A. Jonczyk / Journal of Fluorine Chemistry 127 (2006) 943–947
944
Scheme 1.
Next, a series of amides 1b–j was allowed to react with
chlorodifluoromethane under the above conditions, affording
mixtures of N- and O-difluoromethylated products, 2b–j and
3b–j, respectively, in moderate yields (Scheme 1; Table 1).
To check the stability of difluoromethylated products, a
mixture of 2d and 3d was stirred in benzene at room
temperature for 4–5 h with water, 20% or 50% aq. sodium
hydroxide or 5% hydrochloric acid. The products 2d and 3d
were stable to neutral or basic conditions; hydrochloric acid
caused cleavage of both isomers with the formation of 1d, but
3d reacted at a much higher rate. Evidently, the difluoromethyl
group in 2d was first converted into formyl, and the product thus
formed was easily deformylated giving 1d. Products 2 and 3
were stored in a refrigerator.
Scheme 2.
Among the products formed, we identified N-chlorodifluoro-
derivative 4, which resulted by chlorination of anion 2d with
carbon tetrachloride.
The experiments collected in Table 1 revealed that only
those amides which are N-substituted with an aryl group
(Table 1, Entries 1–7) were able to form the products 2 and 3 in
moderate yields, in other cases (Table 1, Entries 8–10), the
yields were negligible. This results are due to the increased
acidity of N-aryl substituted amides 1a–g which are able to
generate anions in sufficiently high concentration (but not
exceeding that of the catalyst) for reaction with difluorocar-
bene. Alternative mechanistic pathway, i.e. reaction of the
amide nitrogen electron pair with difluorocarbene is not
feasible since amides are too weak bases. To confirm the
presence of difluoromethyl carbanion, we carried out the
reaction of N-phenylbenzamide (1d) with chlorodifluoro-
methane, under PTC conditions in carbon tetrachloride
(Scheme 2). This reagent is known to chlorinate anions easily
via a halophilic process [14,15].
These investigations were completed by reaction of
chlorodifluoromethane with oximes 5a–e under PTC conditions
(Scheme 3, Table 2).
In this case we noticed formation of only O-difluoromethyl
substituted products 6a–e in a rather low yields. Ketoximes 5a
and b (Entries 1, 2) gave the products in higher yields than
aldoximes 5c–e, and oxime of methylethylketone did not react
at all. The products 6 are much more stable than 2 and
particularly 3.
Scheme 3.
Table 1
Reaction of amides 1a–j with chlorodifluoromethane under PTC conditions
Entry
Substrates 1, products 2, 3
R1
R2
Time (h)
Temperature (8C)
Yielda (%)
2
3
1
2
a
b
c
d
e
f
g
h
i
2-H3CC6H4
C6H5
CH3
CH3
CH3
C6H5
C6H5
C6H5
CH3
CH3
CH3
C6H5
2
20
40
20
20
20
20
20
20
20
20
12
20
13
26
28
13b
16
7c
42
6
3
3
4-H3COC6H4
C6H5
3
26
19
23
2b
4
3
5
4-H3CC6H4
4-H3COC6H4
4-NCC6H4
CH3
0.5
3
6
7
1
10
8
5
9
C2H5
6
6c
10
j
CH3
5
5c
a
Isolated pure (ꢀ97%) products.
b
c
The mixture of 2f and 3f was isolated in 41% yield.
Content in the reaction mixture.

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E. Nawrot, A. Jonczyk / Journal of Fluorine Chemistry 127 (2006) 943–947
945
Table 2
Reaction of oximes 5a–e with chlorodifluoromethane under PTC conditions
Entry
5 and 6
R1
R2
Time (h)
Content of 6 in the
reaction mixture
Yielda (%)
determined by GC (%)
1
2
3
4
5
a
b
cb
db
e
C6H5
C6H5
CH3
H
1.5
3
48
45
27
24
13
22
13
7
C6H5
4-CH3OC6H4
C6H5
5
H
3
6
–(CH2)5–
5
–
a
Purity ꢀ 99% (by GC).
b
syn-Oximes 5c and d were used.
We have shown that difluorocarbene (generated from
50% aq. NaOH (1.6 ml, 2.40 g, 30 mmol), TEBAC (0.11 g,
0.5 mmol) and benzene (20 ml) were placed. The content of the
flask was stirred for ca. 2 min, then chlorodifluoromethane was
bubbled through the mixture. The progress of the reaction was
monitored by GC, the reaction was diluted with benzene
(50 ml) when content of 2 and 3 in the reaction mixture did not
increase. The organic phase was decanted from semi-solid
material and dried over MgSO4, the solvent was evaporated and
the residue was purified by column chromatography on
aluminum oxide.
2a, solid, mp 37–40 8C, 1H NMR d 1.80 (s, 3H, CH3), 2.27
(s, 3H, CH3), 7.20–7.38 (m, 4H, aromatic C), 7.57 (t, J = 61 Hz,
1H, CHF2). 13C NMR d 17.9 (CH3), 22.6 (CH3), 108.5 (t,
J = 241 Hz, CHF2), 127.3, 130.0, 130.9, 131.6, 133.4, 138.2,
171.8 (C O). 19F NMR d À94.1, À102.5 (part AB of ABX, JF–
F = 230 Hz, JH–F = 61 Hz, 2F). HRMS calcd. for C10H11NOF2:
199.0809. Found: 199.0815.
chlorodifluoromethane) react with ambident amide anions
under typical PTC conditions giving mixtures of N- 2 and O-
difluoromethylated 3 products, in low to moderate yields. A
similar process applicable to oximes 5 led to formation of only
O-difluoromethylated derivatives 6, in a rather low yield.
Taking into account availability of difluorocarbene precursor
and amides or oximes as well as the simplicity of PTC
procedures, these processes may be applied for synthesis of
difluoromethyl substituted derivatives 2, 3 and 6. However,
similar retention times of 2 and 3 create some problems during
their separation by column chromatography.
3. Experimental
3.1. General
Melting points were measured on a capillary melting point
apparatus and are uncorrected. 1H NMR spectra were recorded
on a Varian Mercury (at 400 MHz) or Varian Gemini (at
200 MHz), 13C NMR spectra on a Varian Mercury (at
100 MHz) spectrometers in CDCl3 with tetramethylsilane as
internal reference. 19F NMR spectra were recorded on a Varian
Mercury (at 376 MHz) in CDCl3 with trifluoroacetic acid as
external reference (d = À77.00 ppm). All chemical shifts are
reported in ppm. The abbreviations used are as follows: s,
singlet; d, doublet; t, triplet; q, quartet; m, multiplet. Gas
chromatography (GC) analyses were carried out on Agilent
6850 Series GC System equipped with HP-50+ (30 m) column.
Microanalyses were obtained using a CHN/S Perkin-Elmer
2400 element analyzer. Column chromatography was per-
formed using Merck basic aluminum oxide 90 (70–230 mesh)
or silica gel with hexane and ethyl acetate mixtures (gradient)
as eluents. Chlorodifluoromethane and amides 1b, 1h–j were
commercial materials while amides 1a [16], 1c [17], 1d [18],
1e, 1f [19], 1g [20] and oximes 5a [21], 5b [22], 5c [23], 5d
[24], 5e [25] were prepared by literature procedures.
2b, solid, mp 74–77 8C, 1H NMR d 1.88 (s, 3H, CH3), 7.2–
7.5 (m, 5H, aromatic H), 7.55 (t, J = 61 Hz, 1H, CHF2). 13C
NMR d 22.6 (CH3), 107.9 (t, J = 241.15 Hz, CHF2), 129.3,
129.4, 130.0, 134.0, 171.2 (C O). 19F NMR d À97.5 (d,
J = 61 Hz, 2F). HRMS calcd. for C9H9NOF2: 185.0652. Found:
185. 0655.
2c, solid, mp 58–60 8C, 1H NMR d 1.86 (s, 3H, CH3), 3.83,
(s, 3H, CH3–O), 6.93–7.20 (m, 4H, aromatic H), 7.52 (t,
J = 62 Hz, 1H, CHF2). 13C NMR d 22.9 (–CH3), 55.4 (–O–
CH3), 107.9 (t, J = 240.4 Hz), 114.7, 126.6, 131.2, 160.3,
171.9. 19F NMR d À97.4 (d, J = 62 Hz, 2F). HRMS calcd. for
C10H11NO2F2: 215.0758. Found: 215.0749.
1
2d, solid, mp 92–95 8C, H NMR d 7.20–7.41 (m, 10H,
aromatic H), 7.62 (t, J = 60 Hz, 1H, CHF2). 13C NMR d 109.4
(t, J = 243 Hz, 1C, CHF2), 128.0, 128.7, 128.8, 129.1, 130.0,
131.1, 133.5, 134.9, 170.6 (C O). 19F NMR d À94.9 (d,
J = 53 Hz, 2F). HRMS calcd. for C14H11NOF2: 247.0809.
Found: 247.0803.
2e, solid, mp 37–39 8C, 1H NMR d 2.31 (s, 3H, CH3), 7.00–
7.42 (m, 9H, aromatic H), 7.59 (t, J = 61 Hz, 1H, CHF2). 13C
NMR d 21.2 (CH3), 109.6 (t, J = 242.4 Hz, CHF2), 125.1,
128.2, 128.9, 129.0, 131.2, 132.1, 134.0, 138.9, 170.9 (C O).
19F NMR d À96.1 (br s, 2F) HRMS calcd. for C15H13NOF2:
261.0965. Found: 261.0969.
3.2. General procedure for difluoromethylation of amides
1a–j
1
Into a three-necked, round bottomed flask equipped with a
reflux condenser, mechanical stirrer and glass pipe for
introducing chlorodifluoromethane, amide 1a–j (10 mmol),
2f, solid, mp 68–71 8C, H NMR d 3.81 (s, 3H, CH3–O),
6.85–7.49 (m, 9H, aromatic H), 7.66 (t, J = 61 Hz, CHF2). 13
NMR d 55.4 (CH3), 109.5 (t, J = 243.1 Hz), 114.4, 127.4, 128.1,
C

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E. Nawrot, A. Jonczyk / Journal of Fluorine Chemistry 127 (2006) 943–947
946
128.9, 131.2, 131.4, 133.8, 159.7, 171.0 (C O). 19F NMR d
À96.5 (br s, 2F). HRMS calcd. for C15H13NO2F2: 277.0914.
Found: 277.0901.
the flask was stirred for ca. 2 min, then chlorodifluoromethane
was bubbled through the reaction mixture for 3 h. The progress
of the reaction was monitored by GC, the reaction was diluted
with CCl4, when content of 4 in the reaction mixture did not
increase. The organic phase was decanted (GC indicated 14%
of 4, 15% of 2d, 17% of 3d and unreacted 1d), the product 4
was isolated with 2% yield by column chromatography on
silica gel.
1
2g, solid, mp 115–118 8C, H NMR d 1.89 (s, 3H, CH3),
7.42–7.79 (m, 4H, aromatic H), 7.49 (t, J = 61 Hz, 1H, CHF2).
13C NMR d 22.9 (CH3), 108.0 (t, J = 242.6 Hz, 1C, CHF2),
113.8, 117.5, 131.1, 133.5, 138.2, 169.9. 19F NMR d À106.1 (br
s, 2F) HRMS calcd. for C10H8N2OF2: 210.0605. Found:
210.0599.
3a, liquid, 1H NMR d 1.90 (s, 3H, CH3), 2.11 (s, 3H, CH3),
6.65–7.21 (m, 4H, aromatic H), 7.42 (t, J = 73 Hz, 1H, CHF2).
13C NMR d 15.8 (CH3), 17.8 (CH3), 113.2 (t, J = 254 Hz, 1C,
CHF2), 119.8, 124.3, 126.7, 128.8, 130.7, 145.1, 156.8. 19F
NMR d À86.9 (d, J = 73 Hz, 2F). HRMS calcd. for
C10H11NOF2: 199.0809. Found: 199.0814.
Stirring of 2d with carbon tetrachloride under the same
conditions did not afford 4.
4, solid, mp 40–43 8C, 1H NMR d 7.19–7.33 (m, 8H,
aromatic H), 7.48–7.51 (m, 2H, aromatic H). 13C NMR d 123.2
(t, J = 285.1, 1C, CHF2), 128.0, 129.2, 129.3, 130.3, 131.6,
133.6, 136.7, 170.1. 19F NMR d À39.8 (s, 2F). Anal. calcd. for
C14H10NOClF2: C, 59.7, H, 3.6, N, 5.0, Cl, 12.6. Found: C,
59.5, H, 3.4, N, 5.2, Cl, 12.3.
3b, liquid, 1H NMR d 2.03 (s, 3H, CH3), 6.86–7.46 (m, 5H,
aromatic H), 7.46 (t, J = 72 Hz, 1H, CHF2). 13C NMR d 15.6
(CH3), 112.9 (t, J = 253.7 Hz, CHF2), 120.5, 124.1, 129.2,
146.3, 157.2. 19F NMR d À86.7 (d, J = 72 Hz, 2F). HRMS
calcd. for C9H9NOF2: 185.0652. Found: 185. 0655.
3c, liquid, 1H NMR d 1.96 (s, 3H, CH3), 3.80 (s, 3H, CH3–
O), 6.72–6.88 (m, 4H, aromatic H), 7.37 (t, J = 72 Hz, 1H,
CHF2). 13C NMR d 19.4 (–CH3), 55.5 (–O–CH3), 113.1 (t,
J = 253, 6 Hz, CHF2), 114.0, 114.5, 121.7, 124.6, 130.5, 139.6,
156.5. 19F NMR d À86.7 (d, J = 72 Hz, 2F). HRMS calcd. for
C10H11NO2F2: 215.0758. Found: 215.0757.
3.4. General procedure for difluoromethylation of
oximes 5a–e
The reactions were carried out as described in Section 3.2,
but products were isolated by column chromatography on silica
gel.
6a, liquid, 1H NMR d 6.76 (t, J = 73 Hz, 1H, CHF2), 7.36–
7.51 (m, 10H, aromatic H). 13C NMR d 118.8 (t, J = 259 Hz,
1C, CHF2), 128.2, 128.4, 128.6, 129.1, 129.7, 130.6, 131.9,
134.8, 162.2. 19F NMR d À91.6 (d, J = 73 Hz, 2F). Anal. calcd.
for C14H11NOF2: C, 68.0, H, 4.5, N, 5.7. Found: C, 67.9, H, 4.5,
N, 5.8.
6b, liquid, 1H NMR d 2.36 (s, 3H, CH3), 6.78 (t, J = 73 Hz,
1H, CHF2), 7.41–7.46 (m, 3H, aromatic H), 7.66–7.71 (m, 2H,
aromatic H). 13C NMR d 13.9, 119.2 (t, J = 256.4 Hz, 1C,
CHF2), 126.7, 128.7, 130.5, 134.9, 160.5. 19F NMR d À91.4 (d,
J = 73 Hz, 2F). Anal. calcd. for C9H9NOF2: C, 58.4, H, 4.9, N,
7.6. Found: C, 58.6, H, 4.7, N, 7.6.
6c, solid, mp 26–28 8C, 1H NMR d 3.84 (s, 3H, CH3O), 6.69
(t, J = 73 Hz, 1H, CHF2), 6.91–6.94 (m, 2H, aromatic H), 7.56–
7.59 (m, 2H, aromatic H), 8.17 (s, 1H). 13C NMR d 55.3, 114.3,
118.5 (t, J = 257.1 Hz, 1C, CHF2), 122.7, 129.5, 153.5, 162.1.
19F NMR d À91.8 (d, J = 73 Hz, 2F). Anal. calcd. for
C9H9NO2F2: C, 53.7, H, 4.5, N, 6.9. Found: C, 53.8, H, 4.5,
N, 6.8.
1
3d, solid, mp 158–160 8C, H NMR d 6.82–7.45 (m, 10H,
aromatic H), 7.53 (t, J = 72 Hz, 1H, CHF2). 13C NMR d 113.8
(t, J = 254.6 Hz, 1C, CHF2), 121.0, 124.1, 128.4, 129.4, 129.7,
131.4, 146.2. 19F NMR d À85.4 (d, J = 72 Hz, 2F). HRMS
calcd. for C14H11NOF2: 247.0809. Found: 247.0812.
3e, liquid, 1H NMR d 2.32 (s, 3H, CH3), 6.70–7.42, (m, 9H,
aromatic H), 7.52 (t, J = 72 Hz, 1H, CHF2). 13C NMR d 20.9
(CH3), 113.8 (t, J = 254,4 Hz), 120.9, 128.4, 129.6, 129.9,
131.2, 133.6, 143.5. 19F NMR d À86.3 (d, J = 72 Hz,
2F). HRMS calcd. for C15H13NOF2: 261.0965. Found:
261.0974.
3f, liquid, 1H NMR d 3.77 (s, 3H, CH3O), 6.73–7.38 (m, 9H,
aromatic H), 7.48 (t, J = 76 Hz, C, 1H, CHF2). 13C NMR d 55.5
(CH3–O), 113.8 (t, J = 261.3 Hz, CHF2), 114.6, 122.3, 128.4,
129.6, 131.2, 139.0, 156.5. 19F NMR d À86.2 (d, J = 73 Hz,
2F). HRMS calcd. for C15H13NO2F2: 277.0914. Found:
277.0906.
3g, solid, mp 32–35 8C, 1H NMR d 1.96 (s, 3H, CH3), 6.86–
7.64 (m, 4H, aromatic H), 7.30 (t, J = 71 Hz, 1H, CHF2). 13C
NMR d 15.9, 107.8, 112.9 (t, J = 255.1 Hz, 1C, CHF2), 118.8
(CN), 121.4, 133.4, 150.4, 157.8. 19F NMR d À96.0 (d,
J = 71 Hz, 2F). HRMS calcd. for C10H8N2OF2: 210.0605.
Found: 210.0609.
6d, liquid, 1H NMR d 6.72 (t, J = 72 Hz, 1H, CHF2), 7.40–
7.47 (m, 3H, aromatic H), 7.63–7.66 (m, 2H, aromatic H), 8.24
(s, 1H). 13C NMR d 118.4, 118.5 (t, J = 256.6 Hz, 1C, CHF2),
124.7, 127.8, 128.9, 130.2, 131.3, 153.9. 19F NMR d À91.9 Hz
(d, J = 72 Hz, 2F). Anal. calcd. for C8H7NOF2: C, 56.1, H, 4.1,
N, 8.2. Found: C, 56.2, H, 4.3, N, 8.2.
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¨
¨
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