LETTER
547
Tetrabutylammonium Butyldifluorodimethylsilicate and Difluorodimethyl-
phenylsilicate, New Nucleophilic Fluorinating Reagents
Jaroslav Kví ala,* Petr Mysík, Old ich Paleta
Department of Organic Chemistry, Institute of Chemical Technology Prague, Technická 5, 166 28 Prague 6, Czech Republic
Fax 420 2 2435 4288; E-mail kvicalaj@vscht.cz
Received 15 January 2001
Albanese and coworkers,4 who also reported an improved
synthesis of Pilcher reagent, TAMPS (1b) and other me-
thylphenylsilicates.
Abstract: Tetrabutylammonium butyldifluorodimethylsilicate
(TAMBS, 1a) and tetrabutylammonium difluorodimethylphenylsil-
icate (TAMPS, 1b) were conveniently prepared from the corre-
sponding substituted fluorodimethylsilanes and tetrabutyl-
ammonium fluoride. Both fluorosilicates 1a, 1b are powerful nu-
cleophilic fluorinating reagents which transform primary or second-
ary halides, tosylates, or mesylates to the corresponding fluorides in
moderate to good yields.
The aim of our work was to develop a convenient and safe
method for the synthesis of substituted difluorodimethyl-
silicates and to study the scope and limitations of their flu-
orinating abilities.
We started the syntheses from easily accessible substitut-
ed chlorodimethylsilanes, which are available commer-
cially but can be prepared in a more economical way from
cheap dichlorodimethylsilane and the corresponding
Grignard reagents. Small contamination of chlorodimeth-
ylphenylsilane by the corresponding bromide arising from
halogen exchange was not significant as both compounds
were transformed by fluorination to the identical product
(see Scheme). The key intermediates, substituted fluo-
rodimethylsilanes, can be prepared by numerous ways.
We preferred the fluorination of the starting chlorides by
alkali metal fluorides,5 which is a more safe and conve-
nient method than fluorination of the corresponding sil-
anols by hydrogen fluoride,3 and cheaper than fluorination
of silyl hydrides.4 The final step of the preparation con-
sisted of mixing fluorosilane with an acetonitrile solution
of TBAF. We employed a 70% aqueous solution of TBAF
from Aldrich, which after 3 h drying at 3 kPa afforded
crystalline trihydrate of sufficient purity. The presence of
water resulted in formation of small amounts of the corre-
sponding silanols, whose presence did not impede subse-
quent fluorination reactions. The formation of
Key words: nucleophilic fluorination, hypervalent silicon com-
pounds, butyldifluorodimethylsilicate, difluorodimethylphenylsili-
cate
Among three general fluorination methods, namely radi-
cal, electrophilic, and nucleophilic fluorinations, the latter
is still employed preferentially due to economic reasons
and easy application in the laboratory.1 From a practical
point of view nucleophilic fluorinating reagents can be di-
vided into several groups: a) cheap and easily available
fluorinating reagents, which either display low efficiency
(KF) or require special precautions (HF); b) fluorinating
reagents with acceptable price and moderate efficiency
(TBAF 3H2O, Py nHF, Et3N 3HF, Gingras reagent, KF/
18-crown-6 ether); c) comparably efficient reagents,
which are not commercially available and their prepara-
tion involve some level of risk, e.g. use of hydrogen fluo-
ride or insufficient stability (CuF Bipy, CHClF-CF2-
-
NEt2, R4P+HF2 ); d) efficient and commercially available,
but rather expensive reagents (Ishikawa reagent, DAST,
TASF).
1
difluorosilicates could be easily checked by both H and
Fluorinating reagents either require the presence of a good
leaving group (halogen, OSO3R) in the substrate, or create
it in the first step of fluorination reaction from the poor
leaving group (OH). While the latter reagents can be used
directly in combination with poor leaving group as hy-
droxyl, selectivity of fluorination by the former one can be
improved substantially in demanding cases by using effi-
cient leaving group (triflate).
19F NMR spectroscopy.6
The use of hypervalent silicon compound as the fluorinat-
ing reagent has been first reported by Doboszewski et al.,2
who employed TASF reagent for fluorination of protected
sugars. The main drawback of TASF is its high price due
to its synthesis from highly toxic SF4. Pilcher3 synthesized
tetrabutylammonium difluorotriphenylsilicate by a com-
parably more convenient way using aqueous hydrogen
fluoride, but efficient fluorination required high excess of
this reagent. Moreover, his results were not confirmed by
Scheme i Me2SiCl2, Et2O, reflux, R = Bu: X = Cl, 3 h, 42%,
R = Ph: X = Br, 2 h, 46%; ii KF, MeCN, rt, 16 h, R = Bu: 76%,
R = Ph: 47%; iii TBAF·3 H2O, MeCN, rt, 2 h, quant.
In contrast to tetrabutylammonium difluorotriphenylsili-
cate (TBAT) reported by Pilcher,3 we were not able to ob-
tain difluorosilicates TAMBS 1a and TAMPS 1b in the
Synlett 2001 No. 4, 547–549 ISSN 0936-5214 © Thieme Stuttgart · New York