3542
J . Org. Chem. 2000, 65, 3542-3543
Sch em e 1
On th e Syn th esis a n d NMR An a lysis of
Tetr a bu tyla m m on iu m
Tr ip h en yld iflu or osilica te
Christopher J . Handy, Yiu-Fai Lam, and
Philip DeShong*
Department of Chemistry and Biochemistry, The University
of Maryland, College Park, Maryland 20742
details. The previously reported synthesis of TBAT begins
with reaction of triphenylsilanol and aqueous hydrofluo-
ric acid in methanol. The resulting intermediate, triph-
enylsilyl fluoride, is then converted to 1 using a 1.0 M
solution of tetrabutylammonium fluoride (TBAF) in THF.
Experience has shown that impurities initially present
in the triphenylsilanol are carried through the synthesis.
Attempts to remove these impurities through repeated
recrystallization of the final product are tedious and often
unsuccessful. Therefore, it is recommended that the
starting silanol be recrystallized (hexanes/ethyl acetate)
prior to transformation to the fluoride.
Furthermore, the stoichiometry of the reagents in the
second step is of the utmost importance. Critical is the
need to avoid use of excess TBAF. As noted in the
previously published procedure, the stoichiometry is 1.0
equiv of Ph3SiF to 0.98 equiv of TBAF. In the previously
reported procedure, the amount of TBAF to be used is
based on the assumption that the crude fluoride contains
4% water. To ensure correct stoichiometry in the second
step, we recommend the use of anhydrous triphenylsilyl
fluoride, which may be obtained by recrystallization of
the crude fluoride from methanol/water followed by
drying under vacuum.
Received November 15, 1999
In tr od u ction
Since the initial report of the synthesis of tetrabutyl-
ammonium triphenyldifluorosilicate (TBAT, 1, Scheme
1),1 this and related hypervalent silicates have been
shown to be versatile reagents with numerous synthetic
applications.2-16 Our group has reported on the use of
TBAT in nucleophilic displacement reactions,1 as a
fluoride source for silicon-carbon bond cleavage,17 and
as a phenylation reagent in palladium-catalyzed cross
coupling reactions.18,19 Recently, the Denmark group at
the University of Illinois contacted us concerning dis-
crepancies in the initial characterization of TBAT. Ac-
cordingly, we have performed additional studies concern-
ing both the synthesis of TBAT and its behavior in
various NMR solvents and report the following clarifica-
tions regarding the original synthetic procedure. In the
intial publication, we were not aware of and did not stress
the importance of several of these factors.
A second matter of concern in this step is that com-
mercially available solutions of TBAF (1.0 M in THF)
contain varying amounts of bifluoride ion (FHF-), ob-
servable by 19F NMR spectroscopy.20 Since excess fluoride
and bifluoride have been implicated in the decomposition
of pentacoordinate fluorosilicates,21 it is essential that
these impurities not contaminate the final product.
Consequently, addition of excess or contaminated TBAF
is to be avoided.22
Following the azeotropic removal of water from the
reaction mixture and isolation of the crude product,
recrystallization affords analytically pure TBAT. We have
found two acceptable solvent systems for recrystalliza-
tion: ethyl acetate yields TBAT as fine needles, while
methylene chloride/hexanes gives TBAT as large blocks.23
NMR An a lysis of TBAT. Contrary to our previously
published results, 1H and 19F spectra should not be
acquired using deuteriochloroform (CDCl3) as the NMR
solvent. Solutions of TBAT in CDCl3 give one spectrum
immediately following preparation and an entirely dif-
ferent spectrum upon standing at room temperature for
several hours. This behavior is attributed to trace
amounts of acid in the CDCl3 that act to accelerate the
Resu lts a n d Discu ssion
Syn th esis of TBAT. Analytically pure TBAT (1) is
obtained by the previously published synthetic procedure,
provided careful attention is given to the following
* To whom correspondence should be addressed. Phone: 301-405-
1892. Fax: 301-314-9121. E-mail: pd10@umail.umd.edu.
(1) Pilcher, A. S.; Ammon, H. L.; DeShong, P. J . Am. Chem. Soc.
1995, 117, 5166-5167.
(2) Albanese, D.; Landini, D.; Penso, M. Tetrahedron Lett. 1995, 36,
8865-8866.
(3) Shibata, K.; Miyazawa, K.; Goto, Y. J . Chem. Soc., Chem.
Commun. 1997, 1309-1310.
(4) Scheidt, K. A.; Chen, H.; Follows, B. C.; Chemler, S. R.; Coffey,
D. S.; Roush, W. R. J . Org. Chem. 1998, 63, 6436-6437.
(5) Mowery, M. E.; DeShong, P. J . Org. Chem. 1999, 64, 1684-1688.
(6) Soli, E. D.; Manoso, A. S.; Patterson, M. C.; DeShong, P.; Favor,
D. A.; Hirschmann, R.; Smith, A. B., III. J . Org. Chem. 1999, 64, 3171-
3177.
(7) Soli, E. D.; DeShong, P. J . Org. Chem. 1999, 64, 9724-9726.
(8) Mowery, M. E.; DeShong, P. Org. Lett. 1999, 1, 2137-2140.
(9) Albanese, D.; Landini, D.; Penso, M.; Petricci, S. Synlett 1999,
199-200.
(10) Hojo, M.; Murakami, C.; Fujii, A.; Hosomi, A. Tetrahedron Lett.
1999, 40, 911-914.
(11) Gurvich, V.; Kim, H.-Y.; Hodge, R. P.; Harris, C. M.; Harris, T.
M. Nucleosides Nucleotides 1999, 18, 2327-2333.
(12) Denmark, S. E.; Choi, J . Y. J . Am. Chem. Soc. 1999, 121, 5821-
5822.
(13) Denmark, S. E.; Wu, Z. Org. Lett. 1999, 1, 1495-1498.
(14) Denmark, S. E.; Wehrli, D. Org. Lett. 2000, 2, 565-568.
(15) Chuit, C.; Corriu, R. J . P.; Reye, C. In Chemistry of Hypervalent
Compounds; Akiba, K., Ed.; Wiley-VCH: New York, 1999; pp 81-146.
(16) Kira, M.; Zhang, L. C. In Chemistry of Hypervalent Compounds;
Akiba, K., Ed.; Wiley-VCH: New York, 1999; pp 147-169.
(17) Pilcher, A. S.; DeShong, P. J . Org. Chem. 1996, 61, 6901-6905.
(18) Brescia, M.-R.; DeShong, P. J . Org. Chem. 1998, 63, 3156-
3157.
(20) Sharma, R. K.; Fry, J . L. J . Org. Chem. 1983, 48, 2112-2114.
(21) Marat, R. K.; J anzen, A. F. J . Chem. Soc., Chem. Commun.
1977, 671-672.
(22) We recommend 19F spectroscopy to check the purity of the 1.0
M solution of TBAF in THF prior to its use in the synthesis, as varying
amounts of bifluoride and other contaminants are often present in
commercially available solutions purchased from Acros and Aldrich.
Alternatively, the Denmark group recommends preparation of 1.0 M
TBAF in THF from crystalline TBAF purchased from Fluka. These
prepared solutions contain <1% bifluoride and are stable indefinitely.
(19) Mowery, M. E.; DeShong, P. J . Org. Chem. 1999, 64, 3266-
3270.
10.1021/jo991771s CCC: $19.00 © 2000 American Chemical Society
Published on Web 05/05/2000