A new type of phase-transfer catalysis via continuous transfer of fluoride anions to the organic phase in the form of potassium difluorotriphenylstannate

Article abstract of DOI:10.1016/j.tetlet.2003.12.078

Fluorination of alkyl halides and sulfonates is efficiently executed in a liquid-solid phase-transfer catalysis system with Ph₃SnF as phase-transfer catalyst acting via continuous formation of lipophilic Ph ₃SnF₂ anions entering the organic phase in the form of the potassium salt.

Full text of DOI:10.1016/j.tetlet.2003.12.078

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 1385–1386
A new type of phase-transfer catalysis via continuous transfer of
fluoride anions to the organic phase in the form of potassium
difluorotriphenylstannate
*
Mieczysław Makosza and Robert Bujok
z
Institute of Organic Chemistry, Polish Academy of Sciences ul. Kasprzaka 44, 01-224 Warszawa 42, PO Box 58, Poland
Received 27 October 2003; revised 3 December 2003; accepted 11 December 2003
Abstract—Fluorination of alkyl halides and sulfonates is efficiently executed in a liquid–solid phase-transfer catalysis system with
Ph SnF as phase-transfer catalyst acting via continuous formation of lipophilic Ph SnF anions entering the organic phase in the
3 3 2
form of the potassium salt.
Ó 2003 Elsevier Ltd. All rights reserved.
Phase-transfer catalysis (PTC) is a general methodology
for catalysing reactions of inorganic and organic anions
with organic molecules. It operates via continuous
transfer of reacting anions into the nonpolar organic
phase in the form of ion-pairs with lipophilic cations,
most often tetraalkylammonium cations. Application of
this efficient methodology is hindered when the reacting
+
−
+ −
+ R SnX + K F
+
−
+
−
+ K X
2 org solid
(1)
(2)
Q
X
Q
R SnF
org
3
solid
3
R X + Q+ R SnF−
F + Q+X − + R SnF
R
org 3
3
2 org
Scheme 1.
In the course of these studies we have observed that in
sulfolane the fluorination process is promoted by R SnX
itself, and does not require the presence of Q X .
Apparently the potassium salt of the hypervalent anion
1
anion such as fluoride, exert low lipophilicity.
3
þ
À
On the other hand the introduction of fluorine into
organic molecules via nucleophilic substitution of other
À þ
R SnF K is sufficiently soluble in this solvent to enter
3
2
2
the solution where it reacts with R–X.
halogens or sulfonates is of great practical interest.
In our previous paper we reported that fluorination of
alkyl halides or sulfonates with KF can be efficiently
executed using a liquid–solid PTC methodology in the
presence of additional co-catalysts––triorganotin
On the basis of this observation we have formulated a
new concept of phase-transfer catalysis. In typical PTC,
the reacting anions are continuously transferred into the
organic phase in the form of lipophilic ion-pairs with
lipophilic cations, usually TAA cations. According to
the new concept, the reacting anions are converted into
lipophilic anions via complexation with appropriate
catalysts so that their potassium salts are sufficiently
lipophilic to enter the organic phase. The reaction of the
anion, delivered in the form of a complexed anion, takes
place in this phase whereas the liberated catalyst forms a
new complexed anion. The application of this process in
the fluorination reaction catalysed by triorganotin
fluoride is shown in Scheme 2.
3
halides. The co-catalytic process operates via continu-
ous formation of hypervalent difluorotriorganostannate
anions in the reaction of triorganotin fluorides with
solid KF and subsequent ion exchange with tetraalkyl-
þ
À
ammonium (TAA) salts Q X to produce lipophilic
ion-pairs of the complex anions with TAA cations.
These ion-pairs enter the organic phase where the anions
react with R–X to produce R–F and triorganotin fluo-
ride, which reacts again with KF as shown in Scheme 1.
The effectiveness of triphenyltin fluoride as a new phase-
transfer catalyst for the model reaction involving the
fluorination of benzyl bromide in comparison with
Keywords: Phase-transfer catalysis; Triaryltin halides; Fluoroalkanes.
Corresponding author. Tel.: +48-22-63-18788; fax: +48-22-63-26681;
e-mail: icho-s@icho.edu.pl
*
þ
À
Bu N HSO is shown in Scheme 3.
4
4
0
040-4039/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2003.12.078

1386
M. Makosza, R. Bujok / Tetrahedron Letters 45 (2004) 1385–1386
z
of alkyl halides or sulfonates, like base induced b-elim-
ination, is substantially suppressed.
−
+
Ar SnF2 K
3 org
Ar SnF + KFsolid
3
org
−
+
R − Xorg+ Ar SnF2
K
R − F + Ar SnF + KXorg
3 solid
3
org
Some examples of fluorination in this new phase-trans-
3
fer catalysis system with Ph SnF serving as the phase-
Scheme 2.
transfer catalyst are collected in Table 1.
o
The continuous formation of potassium difluorotriph-
enylstannate acting as an intermediate in the catalytic
fluorination process was proved by the isolation and
6
0 C, 24 h
PhCH Br
+
KF
PhCH2F
2
solvent, catalyst (5 mol %)
characterisation of this salt. Stirring of Ph
3
SnF and KF
+
−
Solvent,
Catalyst: none,
Bu N HSO ,
Ph SnF
4
4
3
in DMF and removal of the solid gave a solution from
À
2
þ
which Ph SnF K was isolated by evaporation and
3
sulfolane
DMF
0
0
0
5%
9%
6%
54%
54%
3%
precipitation with diethyl ether. These simple operations
4
gave the salt high purity. Although tetraalkylammo-
nium salts of such hypervalent anions have found wide
CH CN
3
5
Scheme 3.
application in synthesis, to the best of our knowledge
the use of the corresponding potassium salts has not
been reported previously.
Table 1
conditions
Ph SnF, 10 mol %
R – X + KF
R – F + KX
Time Yield (%)
3
Acknowledgements
a
R–X
Solvent
Temper-
ature (°C)
(h)
of R–F
This work was generously supported by Bayer Pharma,
Wuppertal, Germany. We thank the Foundation for
Polish Science for Professorial Subsidy.
b
PhCH Br
PhCOCH
2
Mix
95
85
24
4
100
92
70
90
2
Br
Sulf
Mix
95
4
c
n-C
2-C
8
8
H
17OMs
17OTs
Sulf
Sulf
105
85
24
48
H
80
References and notes
4
-NO PhCHCH
2
3
Mix
95
8
85
OMs
1
. (a) Makosza, M.; Fedory nꢀ ski, M. In Phase Transfer
z
a
b
c
Yield determined by GLC.
Mix-sulfolane + acetonitrile, 1:2.
Sulfolane.
Catalysis in Interfacial Catalysis; Volkov, A. G., Ed.;
Marcel Dekker: New York, 2003; pp 159–201; (b)
Makosza, M.; Fedory nꢀ ski, M. Adv. Cat. 1987, 35, 375–
z
4
22; (c) Dehmlov, E. V.; Dehmlov, S. S. Phase Transfer
Catalysis, 3rd ed.; Chemie: Weinheim, 1993; (d) Starks, C.
M.; Liotta, C. L.; Halpern, M. In Phase-Transfer Catalysis:
Fundamentals, Applications and Industrial Perspectives;
Chapman & Hall: New York, 1994.
This new type of phase-transfer catalysed process for the
fluorination of RX with solid KF proceeds well in sul-
folane and dimethylformamide, but not in acetonitrile.
2
. Subramanian, L. R.; Siegmund, G. In Houben-Weyl, 4th
ed.; 1999; Vol. E10a, pp 548.
3. Makosza, M.; Bujok, R. Tetrahedron Lett. 2002, 12, 1285.
À þ
Obviously solubility of the ion-pair Ph SnF K in the
3
2
latter solvent is insufficient. However, the solubility of
this salt in a mixture of sulfolane–acetonitrile is suffi-
cient, thus for some processes a 1:2 mixture of these
z
1
4. Yield 80%, mp>250 °C; H NMR (DMSO-d
6
, 400 MHz):
Sn– H] ¼ 64 Hz;
1
17; 119
1
7.25 (br s, 9H); 7.94 (Sn satellites: J [
H); F NMR (DMSO-d , 376 MHz): )164.3 (Sn satellites:
1
9
6
6
solvents can be used. The rate constant of the reaction of
À
1
19
J [Sn–F] ¼ 2021 Hz). Sn NMR (DMSO-d ): )350.0 (t, J
6
the Ph SnF anion with alkyl halides and sulfonates is
3
2
1
19
19
[
Ph
Sn– F] ¼ 1938 Hz);
(LSIMS(-)NBA):
SnF : 389. Found 389; Anal. Calcd for
calcd
for
not very high, because the reaction does not proceed via
dissociation of the complex anion in the organic phase
1
20
À
3
2
C
18
H
15
F
2
KSn: C, 50.62; H, 3.54. Found C, 50.25; H, 3.83.
. (a) Gingras, M. Tetrahedron Lett. 1991, 32, 7381;
b) Mart ꢀı nez, A. G.; Barcina, J. O.; Rys, A. Z.; Subrama-
À
and reaction of R–X with liberated F anions, but via
À
5
direct transfer of F to R–X from the complex anion.
À
(
Due to this feature the basic character of F is sub-
À
nian, L. R. Tetrahedron Lett. 1992, 33, 7787; (c) Mart ꢀı nez,
A. G.; Barcina, J. O.; del Rosario Colorado Heras, M.; de
Fresno Cereso, A. Org. Lett. 2000, 2, 1377.
stantially diminished, thus typical side reactions of F
anions, which are usually observed during fluorination