Acceleration of alkenyltrimethylsilane fluorination under mild conditions using ultrasound

Article abstract of DOI:10.1016/j.ultsonch.2010.02.007

Alkenyltrimethylsilanes are selectively fluorodesilated to alkenyl fluoride very readily by reaction with 1-chloromethyl-4-fluoro-1,4-diazoniabicyclo[2,2,2]octane bis-tetrafluoroborate (Selectfluor) and N-fluorobenzensulfonimids at room temperature under

Full text of DOI:10.1016/j.ultsonch.2010.02.007

Ultrasonics Sonochemistry 17 (2010) 768–769
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Ultrasonics Sonochemistry
journal homepage: www.elsevier.com/locate/ultsonch
Short Communication
Acceleration of alkenyltrimethylsilane fluorination under mild conditions
using ultrasound
*
Reza Ranjbar-Karimi
Department of Chemistry, Faculty of Science, Vali-e-Asr University, Rafsanjan 77176, Islamic Republic of Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Alkenyltrimethylsilanes are selectively fluorodesilated to alkenyl fluoride very readily by reaction with
1-chloromethyl-4-fluoro-1,4-diazoniabicyclo[2,2,2]octane bis-tetrafluoroborate (Selectfluor) and N-flu-
orobenzensulfonimids at room temperature under ultrasound. In the presence of ultrasound irradiation
in the case of one of the reactions, the yield was 85% after 25 min, but using the previously established
thermal method the yield was only 32% after 20 h.
Received 11 November 2009
Received in revised form 6 February 2010
Accepted 10 February 2010
Available online 16 February 2010
Crown Copyright Ó 2010 Published by Elsevier B.V. All rights reserved.
Keywords:
Fluorination
Selectfluor
N-Fluorobenzensulfonimid
Sonochemistry
Ultrasound
Trimethylsilanes
1. Introduction
trimethylsilanes compounds with electrophilic fluorinating agents
under ultrasonic irradiation.
The introduction of fluorine into organic compounds changes
dramatically the physical and biological properties and chemical
reactivity of organic materials. Selective fluorination under mild
reaction conditions using electrophilic reaction processes is one
of the most important strategies in organic synthesis. Various elec-
trophilic fluorinating reagents have been developed and evaluated,
and, in each case, optimum reagents have been selected and used
for each individual purpose. N–F reagent, in particular Selectfluor
reagent, is one of the best general-purpose user-friendly electro-
philic fluorinating agents [1–4].
The effect of ultrasound on different reactions has been widely
studied during the last two decades [5–7]. The application of ultra-
sound to chemical reactions can cause an increase in the yields of
reactions and in some cases the ratio of products formed. The most
important effect when ultrasound passes through a liquid medium
is the generation of many cavities. This leads to high temperatures
and high pressures within the cavities during their collapse.
Previously we reported the use of ultrasound irradiation for
electrophilic fluorination of CH and CH2 groups attached to
heterocyclic and nitro functionalities [8]. In this work we have
established new methodologies for the selective preparation of
some alkenyl fluorides by the reaction of corresponding alkenyl-
There is a recent report on the use of electrophilic fluorination
for the preparation of alkenyl fluoride from corresponding organo-
metallic compounds in low yield and long reaction times [9].
2. Experimental
2.1. Materials
1-Chloromethyl-4-fluoro-1,4-diazoniabicyclo[2,2,2]octane bis-
tetrafluoroborate (Selectfluor™, abbreviated Selectfluor) and
N-fluorobenzensulfonimids was purchased from Aldrich. Alkenyl-
trimethylsilanes have been prepared according to the reported
procedure [10].
2.2. Equipments
The ultrasonic device used was an UP 400 S instrument from Dr.
Hielscher GmbH. An S3 immersion horn emitting 24 kHz ultra-
sound at intensity levels tunable to maximum sonic power density
of 460 W cmÀ2 was used. Sonication was carried out at 100% (max-
imum amplitude 210 lm). A 3 mm long sonotrode (maximum im-
merse depth of 90 mm) was immersed directly into the reaction
mixture. NMR spectra were recorded on a Bruker DRX300 spec-
trometer operating at 475 MHz for 19F with trifluorochlorome-
thane as an internal standard. Thin layer chromatography (TLC)
was run on silica percolated aluminium plates (Merck Kieselgel
* Tel.: +98 391 3202162; fax: +98 391 3202427.
E-mail address: karimi_r110@yahoo.com
1350-4177/$ - see front matter Crown Copyright Ó 2010 Published by Elsevier B.V. All rights reserved.
doi:10.1016/j.ultsonch.2010.02.007

R. Ranjbar-Karimi / Ultrasonics Sonochemistry 17 (2010) 768–769
769
Table 1
Fluorodesilylation of alkenyltrimethylsilanes.
R1
R2
SiMe3
R3
i or ii
R1
R2
F
R3
1a-i
2a-i
Entry
Products
R1
R2
R3
% Yield
% Yield
% Yield
(time/min)a,e
(time/min)b,e
(time/h)c,e
1
2
3
4
5
6
7
8
9
2a
2b
2c
2d
2e
2f
2j
2h
2i
H
H
H
H
Br
C6H5
Et
H
C6H5
H
H
H
C6H5
H
H
H
H
H
85 (25)
87 (25)
88 (25)
85 (30)
85 (30)
90 (30)
80 (30)
75 (30)
80 (35)
80 (30)
85 (25)
80 (25)
82 (30)
76 (30)
84 (35)
80 (40)
70 (35)
83 (40)
32 (20)d
34 (21)
30 (20)
35 (23)
48 (22)
32 (20)
57 (21)d
45 (22)d
29 (23)
p-MeC6H4
p-ClC6H4
C6H5
C6H5
H
C6H5
n-C6H13
n-C4H9
H
a
b
c
Conditions i: Selectfluor, CH3CN.
Conditions ii: N-fluorobenzensulfonimids, CH3CN.
Selectfluor, CH3CN, stirring at room temperature (without sonication).
According to Ref. [9].
d
e
All compounds were obtained as Z/E mixtures and identified by comparison of their physical and spectral data with those of authentic samples.
F254). Melting points were determined on a Kofler hot-stage
apparatus.
H
SiMe3
F
H
H
F
Cl
H
SiMe3
H
N
Ph
H
Ph
2BF4
N
F
Ph
2.3. General procedure for fluorodesilylation reactions
Conditions i: To a solution of the appropriate alkenyltrimethylsi-
lane compound (2.9 mmol) in dry CH3CN (10 mL) was added
selectfluor (2.9 mmol 1.02 g). The reaction mixture was irradiated
with ultrasound for 25–35 min. The mixture was filtered and
poured into saturated aqueous sodium hydrogen carbonate
(20 ml) and extracted with diethyl ether (2 Â 20). The organic layer
was separated, washed with brine (50 mL), dried over MgSO4, fil-
tered and the solvent was removed. The product was purified on
a silica column eluted with a 1:1 mixture of dichloromethane
and hexane.
Scheme 1. The reaction of 1 to form 2 mechanism.
at room temperature after 20 h [9], whereas under sonication 2a
was obtained in 85% at room temperature within 25 min. In terms
of the mechanism, the reaction of 1 to form 2 might involve an
addition–elimination pathway via a carbocationic intermediate
(Scheme 1).
4. Conclusion
Condition ii: To a solution of the appropriate alkenyltrimethylsi-
lane compound (2.9 mmol) in dry CH3CN (10 mL) was added N-flu-
orobenzensulfonimids (2.9 mmol, 0.68 g). The reaction mixture
was irradiated with ultrasound for 25–40 min. The mixture was fil-
tered and poured into saturated aqueous sodium hydrogen carbon-
ate (20 ml) and extracted with diethyl ether (2 Â 20). The organic
layer was separated, washed with brine (50 mL), dried over MgSO4,
filtered and the solvent was removed. The product was purified on
a silica column eluted with a 1:1 mixture of dichloromethane and
hexane.
In conclusion, the use of ultrasound enabled the easy prepara-
tion of alkenyl fluorides by reaction of corresponding alkenyltrim-
ethylsilanes compounds with electrophilic fluorinating agents. The
advantages of ultrasound in fluorination are shorter reaction times
and higher yields.
Acknowledgement
The author is thankful from Rafsanjan Vali-e-Asr University
(Rafsanjan, Iran) for the partial support of this work.
The authenticity of the products was established by comparing
their melting points with the data in the literature and 1H NMR
spectra.
References
3. Results and discussion
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fluoro-1,4-diazoniabicyclo[2,2,2]octane bis-tetrafluoroborate (Sele-
ctfluor™, abbreviated Selectfluor) and N-fluorobenzensulfonimids
as fluorinating agents. Table 1 summarizes the yield of the reactions
using these two new methods. The expected fluoroalkenes 2a–i
were obtained as Z/E mixtures in good yields. It is clear that reac-
tions of 1 with these electrophilic fluorinating reagents under ultra-
sonic irradiation are much more selective and have shorter reaction
times. For example, compound 2a (entry 1) was previously pre-
pared in 32% yield in the presence of 1 eq selectfluor in acetonitrile
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