Tris(trimethylsilyl)sulfonium and Methylbis(trimethylsilyl)sulfonium Ions: Preparation, NMR Spectroscopy, and Theoretical Studies

Article abstract of DOI:10.1021/jo001248+

Tris(trimethylsilyl)sulfonium 1 and methylbis(trimethylsilyl)sulfonium 2 ions were prepared as long-lived species by reacting trimethylsilane and trityl tetrakis(pentafluorophenyl)borate (Ph₃C⁺ TPFPB) in the presence of precursor sulfides and characterized by ¹H, ¹³C, and ²⁹Si NMR spectroscopy at -78°C. Attempted preparation of dimethyl(trimethylsilyl)sulfonium ion under similar conditions failed as a result of the formation of the more stabilized trimethylsulfonium ion. Structures, and ¹³C and ²⁹Si NMR chemical shifts were calculated by density functional theory (DFT)IGLO methods. The calculated results agree well with the experimental data.

Full text of DOI:10.1021/jo001248+

7646
J . Org. Chem. 2000, 65, 7646-7649
Tr is(tr im eth ylsilyl)su lfon iu m a n d
Meth ylbis(tr im eth ylsilyl)su lfon iu m Ion s: P r ep a r a tion , NMR
Sp ectr oscop y, a n d Th eor etica l Stu d ies1
G. K. Surya Prakash,* Chulsung Bae, Qunjie Wang, Golam Rasul, and George A. Olah*
Donald P. and Katherine B. Loker Hydrocarbon Research Institute and Department of Chemistry,
University of Southern California, University Park, Los Angeles, California 90089-1661
olah@methyl.usc.edu
Received August 16, 2000
Tris(trimethylsilyl)sulfonium 1 and methylbis(trimethylsilyl)sulfonium 2 ions were prepared as
long-lived species by reacting trimethylsilane and trityl tetrakis(pentafluorophenyl)borate (Ph3C+
TPFPB) in the presence of precursor sulfides and characterized by 1H, 13C, and 29Si NMR
spectroscopy at -78 °C. Attempted preparation of dimethyl(trimethylsilyl)sulfonium ion under
similar conditions failed as a result of the formation of the more stabilized trimethylsulfonium ion.
Structures, and 13C and 29Si NMR chemical shifts were calculated by density functional theory
(DFT)/IGLO methods. The calculated results agree well with the experimental data.
In tr od u ction
lived trisilyloxonium ions.8c In our continuing studies of
silyl-substituted onium ions,8a-c we now wish to report
the preparation and NMR (1H, 13C, 29Si) spectroscopic
characterization of long-lived trisilylsulfonium and me-
thyldisilylsulfonium ions. We have also calculated the
structure of the sulfonium ions using density functional
theory (DFT) method. 13C and 29Si NMR chemical shifts
of the ions were also computed using the IGLO method,
and the results were compared with the experimental
data.
Sulfonium salts containing a tricoordinated sulfur
atom attached to three carbon atoms have been well-
known since the early 1900s. Trialkylsulfonium ions are
more stable than the corresponding trialkyloxonium ions
and can be easily prepared by the alkylation of saturated
dialkyl sulfides under mild conditions because of suf-
ficient nucleophilicity of the sulfur atom.2 Sulfonium salts
and the ylides derived from them have become increas-
ingly useful in organic synthesis,3 and more recently
sulfonium ions have found specific industrial applications
as cationic initiators for polymerization.4 Although there
has been increasing interest in the chemistry of hetero-
sulfonium salts in which one or more of the ligand is a
heteroatom (halogen, sulfur, oxygen, or nitrogen), silyl-
substituted sulfonium ions are rare and their structures
have not been confirmed.5
Resu lts a n d Discu ssion
Syn t h esis of Tr isilylsu lfon iu m Ion a n d NMR
St u d ies. In contrast to studies on trisilyloxonium ion
reported by Olah and Prakash,8c when the hydride
abstraction from trimethylsilane by trityl TPFPB was
carried out in the presence of 3-5 equiv of hexamethyl-
disilathiane 3 at -78 °C, trisilylsulfonium was not
formed. In a separate experiment, it was shown that
hexamethyldisilathiane undergoes fast exchange with
Hydride transfer of organohydrosilane to trityl cations,
known as Corey hydride transfer,6 has been well adopted
for the production of silyl cation7 and synthesis of
silylated onium ions.8 We previously reported the prepa-
ration and NMR spectroscopic characterization of long-
1
trityl TPFPB in CD2Cl2 even at -78 °C, as shown by H,
13C, and 29Si NMR. To avoid this exchange, hexameth-
yldisilathiane and trityl TPFPB were used in equimolar
amount, so that all hexamethyldisilathiane could react
with the in-situ-generated trimethylsilyl cation. Thus
when trityl TPFPB in CD2Cl2 solution was added to a
mixture of trimethylsilane and 1 equiv of hexamethyl-
disilathiane at -78 °C under argon in a NMR tube,
successful formation of tris(trimethylsilyl)sulfonium ion
1 was observed (eq 1).
(1) Considered Onium Ions. Part 53. For Part 52, see: Olah, G. A.;
Prakash, G. K. S.; Rasul, G. Proc. Natl. Acad. Sci., U.S.A. 1999, 96,
3494.
(2) Olah, G. A.; Laali, K. K.; Wang, Q.; Prakash, G. K. S. Onium
Ions; J ohn Wiley & Sons: New York, 1998.
(3) Stirling, C. J . M., Ed. The Chemistry of Sulphonium Group. In
The Chemistry of Functional Groups; Patai, S., Series Ed.; Wiley: New
York, 1981; Parts 1 and 2.
(4) (a) Sundell, P.-E.; J o¨nsson, S.; J ult, A. J . Polym. Sci. Polym.
Chem. 1991, 29, 1535. (b) Hamazu, F.; Akashi, S.; Koizumi, T.; Takata,
T.; Endo, T. J . Polym. Sci. Polym. Chem. 1991, 29, 1845. (c) Decker
C.; Le Xuam, H.; Nguyen Thi Viet, T. J . Polym. Sci. Polym. Chem.
1995, 33, 2759. (d) Crivello, J . V. Adv. Polym. Sci. 1984, 62, 1.
(5) Although there have been two reports on the preparation of
silylsulfonium ions, the claims are not true; see refs 10 and 11.
(6) (a) Corey, J . Y. J . Am. Chem. Soc. 1975, 92, 3237. (b) Corey, J .
Y.; West R. J . Am. Chem. Soc. 1963, 85, 2430.
(7) Lambert, J . B.; Kania, L.; Zhang, S. Chem. Rev. 1995, 95, 1191.
(8) (a) Olah, G. A.; Rasul, G.; Prakash, G. K. S. J . Organomet. Chem.
1996, 521, 271. (b) Prakash, G. K. S.; Wang, Q.; Rasul, G.; Olah, G. A.
J . Organomet. Chem. 1998, 550, 119. (c) Olah, G. A.; Li, X.-Y.; Wang,
Q.; Rasul, G.; Prakash, G. K. S. J . Am. Chem. Soc. 1995, 117, 8962.
(d) Kira, M.; Hino, T.; Sakurai, H. J . Am. Chem. Soc. 1992, 114, 6697.
1H NMR spectrum recorded at -78 °C shows a peak
at δ 0.75 corresponding to the methyl groups of trisilyl-
10.1021/jo001248+ CCC: $19.00 © 2000 American Chemical Society
Published on Web 09/30/2000

Silyl-Substituted Sulfonium Ions
J . Org. Chem., Vol. 65, No. 22, 2000 7647
Ta ble 1. NMR Ch em ica l Sh ifts of Som e Silylsu lfon iu m
Ion s a n d Th eir P r ecu r sor sa
is interesting to note that this exchange process is
thermally reversible, and a sharp singlet resonance at δ
38.8 was obtained in the 29Si NMR spectrum upon cooling
the solution back to -78 °C.
1H
13C
29Si
(SiCH3) ppm (SiCH3) ppm
ppm
We treated trimethylsilyl trifluoromethanesulfonate
(TMSOTf) with hexamethyldisilathiane in CD2Cl2 to
study the effect of other low nucleophilicity counteran-
ions. However, we observed only the starting materials
by 1H, 13C, and 29Si NMR with no evidence for the
formation of trisilylsulfonium ion. This result can be
explained by the strength of the stronger Si-O bond in
TMSOTf compared to the Si-S bond in 1.
compounds
Me3SiH
exp
exp
calcd
exp
calcd
0.11
0.33
0.75
0.30
0.68
-2.6 -2.2 -16.3 -16.3
(Me3Si)2S (3)
4.1
2.9
0.3
3.7
3.0
0.3
12.9
38.8
15.9
40.8
15.3
42.9
16.0
46.8
(Me3Si)3S+ (1)
Me3SiSCH3 (4)
(Me3Si)2S+CH3 (2)
-0.2 -0.3
a
Experimental and calculated chemical shifts are referenced
to TMS.
Syn th esis of Meth yldisilylsu lfon iu m Ion an d NMR
Stu d ies. Similarly, methylthiotrimethylsilane 4 and
trimethylsilane in the presence of 1 equiv of trityl TPFPB
formed the corresponding methylbis(trimethylsilyl)sul-
fonium ion 2, whose NMR data are also shown in Table
1. A solution of ion 2 was found to be more stable than
1
that of ion 1, and H, 13C, and 29Si NMR chemical shifts
of 2 did not show any temperature dependence up to room
1
temperature. The H NMR chemical shifts of 2 at δ 0.68
(Si-CH3) and δ 2.37 (S-CH3) were found deshielded
compared to the progenitor, methylthiotrimethylsilane
at δ 0.30 (Si-CH3) and δ 2.02 (S-CH3). The 13C NMR
resonances at -0.2 (Si-CH3) and 10.8 ppm (S-CH3) also
confirm the formation of 2. 29Si NMR chemical shift of 2
was found be at 40.8 ppm.
Attempted preparation of 2 either by reacting meth-
ylthiotrimethylsilane and TMSOTf or by reacting hexa-
methyldisilathiane and methyl iodide did not proceed;
only starting materials were observed in both cases.
Independent reaction of hexamethyldisilathiane and
methyl triflouromethanesulfonate (MeOTF) in either
CD2Cl2 or CD3CN resulted in trimethylsulfonium triflate
and TMSOTF quantitatively (eq 2). The products were
1
identified by H, 13C, and 29Si NMR spectroscopy.
Attem p ted Syn th esis of Dim eth ylsilylsu lfon iu m
Ion . When dimethyl sulfide was allowed to react with
trimethylsilane in the presence of 1 equiv of trityl TPFPB
in CD2Cl2 at -78 °C, we observed a white precipitate that
was characterized as trimethylsulfonium ion and tri-
phenylmethane by NMR spectroscopy. Even under a
variety of conditions no evidence for the formation of
dimethylsilylsulfonium ion was found. In CD3CN solvent,
in the above experiment at -40 °C, the NMR data
showed only the formation of trimethylsulfonium ion (1H
δ 2.79, 13C δ 27.4 ppm) and triphenylmethane as well as
few unidentified silyl compounds.
Attempts to prepare dimethylsilylsulfonium ion by
reacting thiomethyltrimethylsilane 4 and MeOTF in
either CD2Cl2 or CD3CN also resulted in TMSOTf and
trimethylsulfonium ion. The formation of trimethylsul-
fonium ion instead of dimethylsilylsulfonium ion is
presumably due to displacement of the Me3Si group in
the initially formed dimethylsilylsulfonium ion to produce
the more stable trimethylsulfonium ion.9 Since the sul-
fonium ions are known to disproportionate, the dimeth-
F igu r e 1. 1H NMR spectra of (Me3Si)3S+ at variable temper-
atures.
sulfonium ion. 13C and 29Si NMR resonances for the
trisilylsulfonium ion appeared at 2.9 and 38.8 ppm,
respectively (see Table 1). The 29Si NMR shift of 1 is
deshielded by 26 ppm compared to that of hexamethyl-
disilathiane (29Si δ 13). Even though the chemical shifts
cannot be directly related to positive charge density, the
results qualitatively indicate that the positive charge of
the trisilylsulfonium ion is delocalized on both S and Si
atoms. To examine the stability of 1 in the solution, we
further recorded the NMR spectra of the trisilylsulfonium
ion at different temperatures. The NMR spectra of the
trisilylsulfonium ion reaction mixture did not change
below -60 °C. However, the signals of 1 and a small
amount of remaining hexamethyldisilathiane coalesced
on raising the temperature above -60 °C (Figure 1). This
exchange was clearly indicated by the observed averaged
1
peaks in H, 13C, and 29Si NMR at room temperature; the
resonances were 0.68, 3.4, and 8.7 ppm, respectively. It
(9) Ray, F. E.; Levine, I. J . Org. Chem. 1937, 2, 267.

7648 J . Org. Chem., Vol. 65, No. 22, 2000
Prakash et al.
ylsilylsulfonium ion appears to desilylate and react with
another molecule of MeOTf, giving trimethylsulfonium
ion in all cases.
Abel et al. have claimed that a mixture of n-butylthio-
trimethylsilane and methyl iodide upon standing in the
dark for several days resulted in n-butylmethyl(trimeth-
ylsilyl)sulfonium iodide as a fine white precipitate.10 The
evidence for the formation of silylsulfonium ion was based
on poor elemental analysis data. We have repeated the
work of Abel et al. by reacting methylthiotrimethylsilane
and methyl iodide in either CD2Cl2 or CD3CN. The
precipitation did occur in both solvents after 2 days;
however, no evidence for the formation of dimethylsilyl-
sulfonium ion was obtained by 13C and 29Si NMR spec-
troscopy. The white precipitate was found to be only the
trimethylsulfonium ion (1H δ 2.73, 13C δ 27.4). Recently
Franek and El-Sayed reported the first preparation of
dialkylsilylsulfonium ions by reacting ethyl- or propyl-
thiotrimethylsilane and Meerwein’s trimethyloxonium
salt. The claim for the purported silylsulfonium ions was
1
based on the H NMR data.11 Repetition of this work by
treating ethylthiotrimethylsilane and trimethyloxonium
tetrafluoroborate in CD3NO2 at -10 °C gave almost
1
identical reported H NMR resonances (δ 0.22, 1.50, 2.91,
1
3.34) with a product yield of 50%.12 However, H NMR
integration and 13C NMR data (δobs 8.9, 24.7, 38.9)
suggest that the observed species is actually ethyldi-
methylsulfonium tetrafluoroborate (eq 3). The presence
of trimethylfluorosilane13 was also confirmed by 29Si and
19F NMR spectra (lit.14 29Si, δ 32.9 in acetone-d6 relative
to internal TMS, doublet, J ) 274 Hz; 19F, δ -156 relative
to internal CFCl3). Trimethylfluorosilane is produced by
the reaction of initially formed ethylmethyl(trimethyl-
silyl)sulfonium ion with the F- ions of BF4- moiety. Since
ethylthiotrimethylsilane requires 2 equiv of (CH3)3O+BF4
-
to form the ethyldimethylsulfonium ion (eq 3), when both
reagents were mixed in 1:1 ratio, only half of the sulfide
reacted. This result is consistent with the Franek and
El-Sayed’s work wherein roughly 56% yield of the product
was observed.
When a similar silylonium ion, trimethylsilylnitrilium
tetrafluoroborate (Me3SiNCCH3+ BF4-) was reported by
Wenkert et al.,15 Bassindale refuted their claim,14 con-
cluding that the reported preparation of silylnitrilium
tetrafluoroborate actually gave trimethylfluorosilane and
boron trifluoride coordinated acetonitrile complex.
F igu r e 2. DFT B3LYP/6-3 1 G* optimized structures of 1,
2, and 5.
Den sity F u n ction a l Th eor y (DF T)/IGLO Stu d ies.
To substantiate the observed experimental results we
have calculated the structures and 13C and 29Si NMR
chemical shifts of tris(trimethylsily)sulfonium 1 and
methylbis(trimethylsilyl)sulfonium 2, as well as dimeth-
yl(trimethylsilyl)sulfonium 5 ions. The structures were
fully optimized at the density functional theory (DFT)
B3LYP/6-31 G* level. The minimum energy structure
of tris(trimethylsilyl)sulfonium ion was found to be of C3
symmetry, as shown in Figure 2, with Si-S bond distance
of 2.292 Å. The sulfur atom in 1 is modestly pyramidal.
(10) Abel, E. W.; Armitage, D. A.; Bush, R. P. J . Chem. Soc. 1964,
2455.
(11) Franek, W.; El-Sayed, I. Sulfur Lett. 1996, 19, 181.
(12) Reported 1H NMR resonances (CDCl3) are δ 0.18, 1.44, 2.85,
3.30. Yield 56%.
(13) Observed Me3SiF has 1H δ 0.22 (d, J ) 7.8 Hz), 13C δ -0.1 (J
) 15.1 Hz), 29Si δ 34.3 (J ) 273 Hz), 19F δ -156 ppm.
(14) Bassindale, A. R.; Stout, T. Tetrahedron Lett. 1984, 25, 1631.
(15) Caputo, R.; Ferreri, C.; Palumbo, G.; Wenkert, E. Tetraheron
Lett. 1984, 25, 577.

Silyl-Substituted Sulfonium Ions
J . Org. Chem., Vol. 65, No. 22, 2000 7649
The pyramidalization level is about 16°, expressed as the
out-of-plane bending angle of the central sulfur atom
relative to the plane defined by its three bonding part-
ners. Optimized structures of methylbis(trimethylsilyl)-
sulfonium and dimethyl(trimethylsilyl)sulfonium ions
were found to be of Cl and Cs symmetry, respectively,
with pyramidalization levels of 19° and 21° (Figure 2).
The relative stability of trisilylsulfonium and trisilyl-
oxonium ions were compared by using the following
isodesmic reaction (eq 4). Trimethylsilyl group transfer
synthesis of the dimethylsilylsulfonium ion. The struc-
tural parameters and chemical shifts of the ions were also
computed by DFT/IGLO methods. The calculated results
agree reasonably well with the experimental data.
Exp er im en ta l Section
Ap p a r a tu s a n d Ma ter ia ls. Trimethylsilane (Gelest), hex-
amethyldisilathiane, methylthiotrimethylsilane, and CD2Cl2
(Aldrich) are commercially available and were used as re-
ceived. Trityl tetrakis(pentafluorophenyl)borate (Ph3C+ TPF-
PB) was prepared according to a literature method.16 All NMR
spectra were obtained on a Varian 300 spectrometer equipped
with a variable temperature probe, and chemical shifts (1H,
13C, and 29Si) were referenced to tetramethylsilane.
Tr is(tr im eth ylsilyl)su lfon iu m Ion . Trimethylsilane (0.1
mL, ca. 0.8 mmol) was condensed in a 5 mm J . Young NMR
tube and cooled to -78 °C in a dry ice/acetone bath. Hexa-
methyldisilathiane (0.24 mmol) and trityl TPFPB (0.23 mmol)
in CD2Cl2 (0.75 mL) were added subsequently to the NMR
tube. The NMR tube sealed off at -78 °C, and the NMR
spectra of the sample were recorded at -78 °C.
Meth ylbis(tr im eth ylsilyl)su lfon iu m Ion . By using a
similar procedure as described above, the reaction of trimeth-
ylsilane (0.1 mL) and methylthiotrimethylsilane (0.17 mmol)
in the presence of 1 equiv of trityl TPFPB (0.17 mmol) in CD2-
Cl2 at -78 °C gave a solution of methylbis(trimethylsilyl)-
sulfonium TPFPB with triphenylmethane.
Ca lcu la tion Meth od s, Ba sis Set, a n d Geom etr y. Cal-
culations were carried out with the Gaussian 98 program
system.17 The geometry optimizations were performed using
the DFT18 method at the B3LYP19/6-31G* levels.20 Vibrational
frequencies at the B3LYP/6-31G*//B3LYP/6-31G* level were
used to characterize stationary points as minima and to
evaluate zero point vibrational energies (ZPE), which were
scaled by a factor of 0.96. Isodesmic energies were calculated
at the B3LYP/6-31G*//B3LYP/6-31G* + ZPE level. 13C and
29Si NMR calculations were performed according to the
reported method using IGLO programs21 at the IGLO II′ levels
using B3LYP/6-31G* geometries. Huzinaga22 Gaussian lobes
were used as follows. Basis II′′: Si, 11s 7p 2d contracted to
[5111111, 211111, 11], d exponent ) 1.4 and 0.35; C, O: 9s
5p 1d contracted to [51111, 2111, 1], d exponent: 1.0; H: 3s
contracted to [21]. The 13C and 29Si NMR chemical shifts were
referenced to TMS (calculated absolute shift, i.e., δ(Si) ) 380.6
and δ(C) ) 196.4).
from silylated sulfonium ion (Me3Si)3S+ 1 to (Me3Si)2O
giving silylated oxonium ion (Me3Si)3O+ and (Me3Si)2S
was computed to be endothermic by 11.8 kcal/mol. This
again indicates that trialkylsulfonium ions are more
stable than the corresponding trialkyloxonium ions. In
comparison, the methyl group transfer from methylated
sulfonium ion (CH3)3S+ to (CH3)2O giving methylated
oxonium (CH3)3O+ and (CH3)2S was also calculated to be
endothermic by 16.4 kcal/mol (eq 5).
We have also reproduced the 13C and 29Si NMR
chemical shifts of 1 and 2 at the IGLO II//B3LYP/6-31G*
level with reasonable degree of accuracy (Table 1). The
calculated 29Si δ of 1, 42.9 ppm, agrees well with the
experimental value of 38.8 ppm. The calculated 13C δ of
1 is 3.0 ppm, also very close to the experimental value of
2.9 ppm. The calculated average 29Si δ of 2 is 46.8 ppm,
6.0 ppm more deshielded than the experimental value
of 40.8 ppm. On the other hand, the calculated average
13C δ of Si-CH3 in 2 is -0.3 ppm, close to the experi-
mental value of -0.2 ppm. The 29Si δ of 5 was computed
to be 54.4 ppm.
Con clu sion s
The first silylsulfonium ions, tris(trimethylsilyl)sulfo-
nium and methylbis(trimethylsilyl)sulfonium ions, were
prepared as long-lived species and unequivocally char-
acterized by 1H, 13C, and 29Si NMR spectroscopy. The
strong tendency for formation of the more stable tri-
methylsulfonium ion in solution has prevented successful
Ack n ow led gm en t. Partial support of our work by
the National Science Foundation is gratefully acknowl-
edged.
Su p p or tin g In for m a tion Ava ila ble: 1H, 13C, and 29Si
NMR spectra of tris(trimethylsilyl)sulfonium and methylbis-
(trimethylsilyl)sulfonium ions at -78 °C and room tempera-
ture. This material is available free of charge via the Internet
at http://pubs.acs.org.
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