Fluorination of aromatic compounds with N-fluorobenzenesulfonimide under solvent-free conditions

Article abstract of DOI:10.1134/S107042800910008X

Reactions of N-fluorobenzenesulfonimide with methylbenzenes, phenols, and phenol ethers were studied under solvent-free conditions. The rate constant ratio for the reactions with mesitylene and durene indicates polar mechanism of the process. Solvent-free fluorination of aromatic compounds with N-fluorobenzenesulfonimide in some cases is more selective than reactions with other N-F reagents in a solvent.

Full text of DOI:10.1134/S107042800910008X

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2009, Vol. 45, No. 10, pp. 1468–1473. © Pleiades Publishing, Ltd., 2009.
Original Russian Text © R.V. Andreev, G.I. Borodkin, V.G. Shubin, 2009, published in Zhurnal Organicheskoi Khimii, 2009, Vol. 45, No. 10,
pp. 1483–1488.
Fluorination of Aromatic Compounds with N-Fluorobenzene-
sulfonimide under Solvent-Free Conditions
R. V. Andreev, G. I. Borodkin, and V. G. Shubin
Vorozhtsov Novosibirsk Institute of Organic Chemistry, Siberian Division, Russian Academy of Sciences,
pr. Akademika Lavrent’eva 9, Novosibirsk, 630090 Russia
e-mail: gibor@nioch.nsc.ru
Received December 9, 2008
Abstract—Reactions of N-fluorobenzenesulfonimide with methylbenzenes, phenols, and phenol ethers were
studied under solvent-free conditions. The rate constant ratio for the reactions with mesitylene and durene
indicates polar mechanism of the process. Solvent-free fluorination of aromatic compounds with N-fluoroben-
zenesulfonimide in some cases is more selective than reactions with other N–F reagents in a solvent.
DOI: 10.1134/S107042800910008X
used to effect fluorination of carbanions, enolates, and
enol ethers in the synthesis of fluorinated intermediate
products necessary for the manufacture of medicinal
agents [7–9]. Direct fluorination of aromatic com-
pounds with NFSI was performed less frequently than
with other N–F reagents, e.g., 1-chloromethyl-4-
fluoro-1,4-diazoniabicyclo[2.2.2]octane salts [1–24].
Fluorinated aromatic compounds are widely used as
pesticides, medicinal agents, and various functional
materials; therefore, development of ecologically
friendly and selective fluorination procedures seems to
be an important problem [1–3]. During the past two
decades, N–F reagents [1–17] have been extensively
used as source of fluorine in mild and selective fluori-
nation of organic compounds. In particular, such re-
agent is N-fluorobenzenesulfonimide (NFSI) [2, 4, 7–
9, 18–24]. It is nonhygroscopic, stable to storage, and
convenient to handle with. N-Fluorobenzenesulfon-
imide is quite reactive, but at the same time it exhibits
a moderate oxidative ability [2], which is important for
fluorination of organic compounds with low oxidation
potentials. N-Fluorobenzenesulfonimide is commonly
The goal of the present work was to study the selec-
tivity in fluorination of aromatic compounds, namely
methyl-substituted benzenes, phenols, and phenol
ethers, with NFSI in the absence of a solvent. The
selectivity problem in electrophilic fluorination under
these conditions remains poorly studied (cf. [22]).
We have found that the yield of monofluoro deriv-
atives in the fluorination of polymethylbenzenes
Scheme 1.
Me
Me
Me
F
F
F
NFSI
+
Me
Me
Me
Me
Me
Me
I
IV
V
F
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
F
F
NFSI
NFSI
+
Me
Me
Me
Me
IX
X
XI
R
R
II, III
VI–VIII
II, VI, R = H; III, VIII, R = Me; VII, R = F.
1468

FLUORINATION OF AROMATIC COMPOUNDS WITH N-FLUOROBENZENESULFONIMIDE
1469
Table 1. Fluorination of methyl-substituted benzenes, phenols, and phenol ethers with N-fluorobenzenesulfonimide under
solvent-free conditionsa
Compound no.
Temperature, °C
Time, h
Product (fraction, %)
IV (97), V (3)
Overall yield, %
I
105
105
105
105
105
105
105
105
105
105
090
1.67
1.67
1.67
1.67
22.000
0.50
3.00
3.00
0.50
0.33
3.00
65
13
10
06
52
24
30
07
66
75
61
II
VI (98), VII (2)
III
VIII
IX
X (59), XI (41)
IX
X (62), XI (38)
XX
XXIII
XXI
XXII
XXVI
XXX
XXXIII
XXIV
XXV
XXVII (92), XXVIII (4), XXIX (4)
XXXI
XXXIV (70), XXXV (30)
[mesitylene (I), durene (II), and pentamethylbenzene
(III)] with NFSI under solvent-free conditions de-
creases as the number of methyl groups in the aromatic
ring increases (Scheme 1, Table 1). A probable reason
is ipso-attack by the reagent, which promotes side
reactions (cf. [25]).
tion time almost did not change the ratio of products X
and XI, but the yield appreciably increased (Table 1).
The relative reactivity of durene and mesitylene is
commonly used to distinguish between the classical
polar mechanism involving formation of σ-complex
(SEAr) and single electron transfer (SET) mechanism
implying intermediate radical cation (Scheme 2)
[19, 27]. As applied to fluorination of aromatic com-
pounds with N–F reagents, the relative contributions of
the above mechanisms are likely to be determined by
the nature of the substrate and reagent and reaction
conditions. For example, high substrate selectivity was
revealed in the competitive fluorination of mesitylene
and durene with F-TEDA-BF4 in acetonitrile or ionic
liquid [emim][OTf] (kMes/kDur = 6 and 10, respectively
at 80°C), which indicated polar mechanism of the
process [19]. By contrast, the reverse reactivity ratio
(kMes/kDur = 0.24) was observed in the fluorination of
mesitylene and durene with the same reagent in aque-
ous acetonitrile; these data were interpreted in terms of
the SET mechanism [27]. We examined the kinetics of
competitive fluorination of mesitylene and durene with
The reactions of NFSI with mesitylene and durene
afforded mainly only one product, whereas fluorina-
tion of m-xylene (IX) under analogous conditions led
to the formation of a mixture of 1-fluoro-2,4-dimethyl-
and 2-fluoro-1,3-dimethylbenzenes X and XI at a ratio
of 1.4:1. A similar product ratio (1.8:1) was obtained
previously in the fluorination of m-xylene with
1-chloromethyl-4-fluoro-1,4-diazoniabicyclo[2.2.2]oc-
tane bis(tetrafluoroborate) (F-TEDA-BF4) in aceto-
nitrile MeCN [26]; in addition, difluoro-substituted
derivative, 1,5-difluoro-2,4-dimethylbenzene (XII),
was formed in 23% yield. The overall yield of fluori-
nated products obtained from m-xylene was consider-
ably lower than in the fluorination of mesitylene, other
conditions being equal (Table 1), which may be related
to lower reactivity of the former. Increase of the reac-
Scheme 2.
δ–
R2N
F
H
SEAr
–R2N–
δ+
F
F
H
X
R2NF
+
–H+
X
X
X
SET
·
[R2N– Ar+·]
–R2N–
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 45 No. 10 2009

ANDREEV et al.
1470
Scheme 3.
OR
OR
OR
OH
F
+
F
NFSI
+
F
XV, XIX
F
XVI
XIII, XVII
XIV, XVIII
XIII–XV, R = H; XVII–XIX, R = Me.
NFSI in the absence of a solvent. The rate constant
ratio kMes/kDur was estimated at 3.4 (105°C), indicating
that the reaction follows polar mechanism.
was almost the same (1.4) and similar to the isomer
ratio observed in the fluorination of anisole with other
N–F reagents in various solvents: 1-fluoro-1-hydroxy-
1,4-diazoniabicyclo[2.2.2]octane bis(tetrafluoroborate)
in [emim][CF3SO3] (1.3) [19]; N,N′-difluoro-2,2′-bi-
pyridinium bis(tetrafluoroborate) in acetonitrile (1.4)
[29]; N-fluoroquinuclidinium trifluoromethanesulfo-
nate in acetonitrile (1.5) [31]; F-TEDA-BF4 in aceto-
nitrile or trifluoroacetic acid (1.5 and 1.4, respectively)
[32, 33]; 1-fluoro-4-hydroxy-1,4-diazoniabicyclo-
[2.2.2]octane bis(tetrafluoroborate) in acetonitrile (1.5)
[32]; N-fluorobenzene-1,2-disulfonimide in CDCl3
(1.5) [34]. These data suggest weak dependence of the
regioselectivity on the nature of N–F reagent and
polarity of the medium provided that specific effects
are absent. The reverse ortho/para-isomer ratios were
reported, e.g., for the fluorination with F-TEDA-BF4 in
strongly acidic medium (CF3SO3H, 0.8) [35] and with
2,4,6-trichloro-1-fluoro-1,3,5-triazinium tetrafluoro-
borate in nitromethane (0.5) [36]. Presumably, in the
first case the reactive species is protonated trifluoro-
methanesulfonyl hypofluorite SF3SO2OHF+ rather than
F-TEDA-BF4 [35], while in the second steric interac-
tion between the methoxy group in the substrate and
chlorine atoms in the reagent could hinder attack by
the latter on the ortho position.
Phenols and phenol ethers turned out to be fairly
active in the fluorination with NFSI under solvent-free
conditions. Fluorination of phenol (XIII) at 105°C (re-
action time 3 h; substrate–reagent ratio 5:1) afforded
mainly monofluoro derivatives XIV and XV at a ratio
(ortho/para) of 1.2:1 (Scheme 3, Table 2). The regio-
selectivity in this reactions almost did not change as
the temperature decreased. Reversal of the substrate–
reagent ratio to 1:5 resulted in increased fractions of
2,4-difluorophenol (up to 8%) and 2-fluorophenol
(ortho/para = 1.8:1).
The ortho/para ratios obtained in the fluorination of
phenol with NFSI were similar (or even equal) to those
observed in the fluorination of the same substrate with
other N–F reagents in various solvents, e.g., with bis-
(4-fluoro-1,4-diazoniabicyclo[2.2.2]oct-1-yl)propane
tetrakis(trifluoromethanesulfonate) in methanol (~1.5)
[28], N,N′-difluoro-2,2′-bipyridinium bis(tetrafluoro-
borate) in acetonitrile (1.2) [29], and 1-fluoro-4-
methyl-1,4-diazoniabicyclo[2.2.2]octane bis(trifluoro-
methanesulfonate) in methanol (~1.5) [30].
The overall yield of the fluorination products ob-
tained from anisole (XVII) and NFSI at 105°C (no sol-
vent) was considerably higher than in the fluorination
of phenol (Table 2), but the ortho/para isomer ratio
The reactions of NFSI with p-methylphenol (XX),
4-methylanisole (XXI), and 4-chloroanisole (XXII)
Table 2. Fluorination of phenol and anisole with N-fluorobenzenesulfonimide under solvent-free conditions
Fractions of fluorination products, %
PhX–NFSI
molar ratio
X in PhX
Temperature, °C Time, h
Overall yield, %
ortho
para
2,4-F2
OH
5:1
5:1
5:1
1:5
5:1
6:1
105
090
080
080
105
080
3
5
3
3
3
3
55
54
56
59
59
59
44
45
44
33
41
41
1
1
–
8
–
–
56
44
47
24
71
33
OH
OH
OH
OMe
OMe
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 45 No. 10 2009

FLUORINATION OF AROMATIC COMPOUNDS WITH N-FLUOROBENZENESULFONIMIDE
1471
F-TEDA-BF4 in acetonitrile [23], ~33% in the reaction
with 1-fluoro-4-methyl-1,4-diazoniabicyclo[2.2.2]oc-
tane bis(trifluoromethanesulfonate) in methanol [30],
~33% in the reaction with N-fluoroquinuclidinium
salts in acetonitrile [31], 18% in the fluorination with
N,N′-difluoro-2,2′-bipyridinium bis(tetrafluoroborate)
in formic acid [29], and 11% in the reaction with
3,5-dichloro-1-fluoropyridinium trifluoromethanesul-
fonate in methylene chloride [38] (Scheme 6). The
fluorination of 2-naphthol with NFSI under solvent-
free conditions yields 1-fluoronaphthalen-2-ol (XXXI)
as the major product, and almost no ketone XXXII is
formed (Table 1). This reaction is likely to begin in the
solid phase, and the mixture turns liquid in 5 min.
selectively afforded products of hydrogen replacement
in the ortho position with respect to stronger electron-
donating group (Scheme 4, Table 1).
Scheme 4.
X
X
F
NFSI
Y
Y
XX–XXII
XXIII–XXV
XX, XXIII, X = OH, Y = Me; XXI, XXIV, X = OMe,
Y = Me; XXII, XXV, X = OMe, Y = Cl.
Scheme 6.
The reaction of p-cresol (XX) with F-TEDA-BF4 as
more active N–F reagent in acetonitrile was accom-
panied by formation of an appreciable amount of
4-fluoro-4-methylcyclohexa-2,5-dien-1-one (in addi-
tion to phenol XXIII) [37]. Likewise, 4-fluoroben-
zene-1,3-diol (XXVII) was obtained with high selec-
tivity by fluorination of resorcinol (XXVI) with NFSI;
also, small amounts of 2-fluoro- and 4,6-difluoroben-
zene-1,3-diols XXVIII and XXIX were formed
(Scheme 5, Table 1).
F
OH
OH
F+
XXX
XXXI
F
F
O
+
XXXII
Scheme 5.
OH
OH
Likewise, no 2,2-difluoronaphthalen-1(2H)-one
was detected among products of the reaction of
1-naphthol (XXXIII) with NFSI (cf. [23, 38]), and the
main products were 2- and 4-fluoronaphthalen-1-ols
XXXIV and XXXV at a ratio of 7 : 3 (Scheme 7,
Table 1).
NFSI
OH
OH
OH
F
XXVII
XXVI
OH
Scheme 7.
F
F
OH
OH
+
+
F
NFSI
OH
OH
F
XXVIII
XXIX
XXXIII
XXXIV
OH
Analogous results were obtained previously using
N,N′-difluoro-2,2′-bipyridinium bis(trifluoromethane-
sulfonate) in acetonitrile; in this case, 10% of 4,6-di-
fluorobenzene-1,3-diol was also formed [29].
+
F
XXXV
Fluorination of 2-naphthol (XXX) with N–F re-
agents in organic solvents is usually accompanied by
formation of 1,1-difluoronaphthalen-2(1H)-one
(XXXII) together with the corresponding monofluoro
derivative, 1-fluoronaphthalen-2-ol (XXXI). The yield
of ketone XXXII was 27% in the reaction with
Thus the fluorination of aromatic compounds with
N-fluorobenzenesulfonimide under solvent-free condi-
tions in some cases ensures higher selectivity than the
use of other N–F reagents in solution.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 45 No. 10 2009

ANDREEV et al.
necessary at a required temperature, ~0.5 ml of CDCl3
1472
EXPERIMENTAL
or CDCl3– DMSO-d6 (3:1) and PhCF3 (internal refer-
ence) were added, and 1H and 19F NMR spectra were
recorded. The yields and product ratios were calculated
from signal intensities in the 19F NMR spectra.
The H and 19F NMR spectra were recorded from
solutions in CDCl3 on a Bruker AV-300 spectrometer at
300 and 282.4 MHz, respectively. The chemical shifts
were measured relative to the residual solvent signal
(CHCl3, δ 7.24 ppm) or PhCF3 (internal reference,
δF –63.73 ppm relative to CFCl3). The mass spectra
were obtained on an Hewlett–Packard G1800A GC–
MS system consisting of an HP 5971 mass-selective
detector and an HP 5890 Series II gas chromatograph.
The structure of the obtained compounds was con-
firmed by the 1H and 19F NMR and mass spectra which
were consistent with published data for 2-fluoro-1,3,5-
trimethylbenzene, 2,4-difluoro-1,3,5-trimethylbenzene
[39], 3-fluoro-1,2,4,5-tetramethylbenzene [39],
1-fluoro-2,3,4,5,6-pentamethylbenzene [40], 1-fluoro-
2,4-dimethylbenzene, 2-fluoro-1,3-dimethylbenzene*
[41], 2- and 4-fluorophenols [41–43], 2,4-difluoro-
phenol [42], 2-fluoro-4-methylphenol [41], 2-fluoro-1-
methoxy-4-methylbenzene, 4-chloro-2-fluoro-1-me-
thoxybenzene [19], 2-fluorobenzene-1,3-diol, 4-fluoro-
benzene-1,3-diol, 4,6-difluorobenzene-1,3-diol [29, 44],
1-fluoronaphthalen-2-ol [31], 2- and 4-fluoronaphtha-
len-1-ols [23], and 2- and 4-fluoro-1-methoxybenzenes
[19, 41].
1
Competitive fluorination of mesitylene and
durene with N-fluorobenzenesulfonimide. The
reagent (NFSI) and substrates (mesitylene and durene)
were mixed at a molar ratio of 1:5:5) at room tem-
perature in a Teflon ampule, the mixture was heated
for 35 min at 105°C and cooled, and CDCl3–DMSO-d6
mixture (3:1) was added. The ratio of the fluorinated
products was determined from the intensities of the
corresponding signals in the 19F NMR spectrum.
This study was performed under financial support
by the Russian Foundation for Basic Research (project
no. 07-03-00475-a) and by the Chemistry and Mate-
rials Science Division, Russian Academy of Sciences
(program no. 5.1.9).
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