Br?nsted acid-promoted formation of stabilized silylium ions for catalytic friedel-crafts C-H silylation

Article abstract of DOI:10.1021/jacs.6b04878

A counterintuitive approach to electrophilic aromatic substitution with silicon electrophiles is disclosed. A strong Br?nsted acid that would usually promote the reverse reaction, i.e., protodesilylation, was found to initiate the C-H silylation of electron-rich (hetero)arenes with hydrosilanes. Protonation of the hydrosilane followed by liberation of dihydrogen is key to success, fulfilling two purposes: to generate the stabilized silylium ion and to remove the proton released from the Wheland intermediate.

Full text of DOI:10.1021/jacs.6b04878

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Communication
Brønsted Acid-Promoted Formation of Stabilized
Silylium Ions for Catalytic Friedel-Crafts C-H Silylation
Qing-An Chen, Hendrik F. T. Klare, and Martin Oestreich
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b04878 • Publication Date (Web): 15 Jun 2016
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Brønsted Acid-Promoted Formation of Stabilized Silylium Ions
for Catalytic Friedel–Crafts C–H Silylation
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Qing-An Chen, Hendrik F. T. Klare, and Martin Oestreich*
Institut für Chemie, Technische Universität Berlin, Strasse des 17. Juni 115, 10623 Berlin, Germany
Supporting Information Placeholder
A) Proton removal using excess amine base (Simchen, 1984)
ABSTRACT: A counterintuitive approach to electrophilic aro-
SiMe3
matic substitution with silicon electrophiles is disclosed. A strong
Brønsted acid that would usually promote the reverse reaction,
i.e., protodesilylation, was found to initiate the C–H silylation of
electron-rich (hetero)arenes with hydrosilanes. Protonation of the
hydrosilane followed by liberation of dihydrogen is key to suc-
cess, fulfilling two purposes: to generate the stabilized silylium
ion and to remove the proton released from the Wheland interme-
diate.
Me3SiOTf (1.0 equiv)
Et3N·HOTf
+
Et3N (6.5 equiv)
5 °C to rt
N
Me
N
Me
81%
(C3:C2 > 99:1)
B) Protolysis of hydrosilanes for trialkylsilyl triflate synthesis (Corey, 1981)
iPr3SiH
+
HOTf
iPr3SiOTf + H2
0 °C to rt
(1.1 equiv)
97%
C) Proton-catalyzed C–H silylation of electron-rich (het)arenes (this work)
Electrophilic aromatic substitution (SEAr) is a valuable method
for the C–H functionalization of arenes. By exploiting the elec-
trophilicity of Me3SiOTf, Frick and Simchen accomplished a
highly regioselective C–H silylation of indoles and pyrroles three
decades ago (Scheme 1A).1 To overcome competing protodesi-
lylation, i.e., the reverse reaction, excess base had to be added to
absorb the released protons. According to a straightforward pro-
cedure reported by Corey and co-workers, such Alkyl3SiOTf are
accessible from the reaction between Alkyl3SiH and TfOH
(Scheme 1B).2,3 It is notable that the hydride and proton are re-
moved from the reaction in form of dihydrogen. Inspired by Co-
rey’s work, we imagined that Brønsted acids with weakly coordi-
nating counteranions [X]‒ could promote the catalytic formation
of stabilized silicon cations from hydrosilanes.4,5 The
thus-generated silicon electrophiles could then participate in-situ
in the Friedel–Crafts C–H silylation of electron-rich (hetero)arenes
(Scheme 1C).1,6–8 Owing to the proton removal as dihydrogen, we
expected this catalytic system to suppress protodesilylation.
H+[X]– (catalytic)
SiR3
R3SiH (stoichiometric)
H2
+
N
involving
[R3Si]+[X]–
Me
N
Me
To test our hypothesis, we investigated the stoichiometric for-
mation of the silicon electrophile using Brønsted acid. Due to
facile cleavage of the phenyl group (= protodesilylation) rather
than loss of the hydride, the reaction of Me2PhSiH (1a) with
TfOH leads to HMe2SiOTf but not to Me2PhSiOTf (Equation 1).9
We therefore envisioned using a substantially weaker but still
strong acid to avoid dephenylation. Accordingly, Brookhart’s acid
[H(OEt2)2]+[BArF ]– (2)10 was employed to generate the corre-
4
sponding ether-stabilized silicon cation.11 D–H gas immediately
evolved from the reaction after treatment of deuterium-labeled
Me2PhSiD (1a-d1) with 2 (Equation 2), indicating smooth proton
transfer with gas evolution and coordination of Et2O as driving
forces. The formation of D–H was verified by a triplet at δ 4.44
ppm with a diagnostic coupling constant of 1J = 42.6 Hz in the 1H
NMR spectrum. No cleavage of the phenyl group was observed.
Instead, we obtained a biphasic system that usually indicates
clathrate formation of the solvent and the newly generated silicon
cation.12 Identification of that cation by 29Si NMR spectroscopy
was however hampered by dynamic exchange between reversibly
bound Et2O11a and the benzene solvent, apparent from significant
line broadening in the 1H NMR spectrum. By replacing C6D6 with
Scheme 1. Merger of Electrophilic C‒H Silylation and
Brønsted Acid-Promoted Formation of Silicon Cations
1,2-Cl2C6D4 as solvent, we were then able to detect
[Me2PhSi(OEt2)]+[BArF ]– (3a) by H/29Si HMQC measurements
1
4
and clearly establish the formation of the desired silyloxonium ion
(Equation 3 and Figure 1). The 29Si NMR spectrum showed a
characteristic signal at δ 53.2 ppm. In turn, the combination of
TfOH and Et2O in 1:2 ratio did not evolve any gas on addition to
Me2PhSiH but led to slow dephenylation.
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H
1
2
3
4
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Me2PhSiH +
HOTf
HMe2SiOTf +
86%
(1)
(2)
5 min at rt
(Ref 9)
1a
(1.0 equiv)
1H NMR
4.44 ppm
[H(OEt2)2]+
C6D6
Me2PhSiD +
D–H
[BArF
]
–
4
10 min at rt
(t, 1J = 42.6 Hz)
1a-d1
2 (1.0 equiv)
as above
(biphasic
system)
entry
2 (mol%)
nbe (equiv)
yield (%)b
9
[H(OEt2)2]+
[Me2PhSi(OEt2)]+ 29Si NMR
Me2PhSiH +
(3)
[BArF
]
–
–
[BArF
]
4
53.2 ppm
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4
then
1
2
3
4
5
6
7
8
1.0
—
—
70
1a
2 (1.0 equiv)
3a
1,2-Cl2C6D4
—
no reaction
c
—
—
no reaction
2.0
4.0
2.0
2.0
1.0
—
77
53
85
95
97
—
0.50
1.0
1.0
aAll reactions were performed on a 0.20 mmol scale (based on the hy-
drosilane) using double the amount of the indole (0.40 mmol, 2.0 equiv) as
well as the indicated amount of catalyst 2 and norbornene in toluene (0.10
mL) at 80 °C for 18 h. bBased on hydrosilane and determined by 1H NMR
spectroscopy or GLC analysis using 1,3,5-trimethoxybenzene as internal
standard. cNaBArF4 (1.0 mol%) added instead of [H(OEt2)2]+[BArF ]– (2).
4
Figure 1. H/29Si HMQC spectrum of [Me2PhSi(OEt2)]+[BArF ]–
(3a) recorded in 1,2-Cl2C6D4 at room temperature.
1
4
Next, we examined the hydrosilane scope (Table 2). With
Me2PhSiH (1a) the isolated yield was essentially quantitative, and
the reaction proceeded smoothly with MePh2SiH (1b) even at
room temperature (entries 1 and 2). Probably due to steric hin-
drance, low conversion was observed for Ph3SiH (1c) and, like-
wise, for Et3SiH (1d) (entries 3 and 4). The protocol was not
compatible with (EtO)2MeSiH (1e) as a result of silylated oxoni-
um ion formation11b (entry 5). Dihydrosilanes 1f‒1h also served
as efficient coupling partners (entries 6–8). Monosubstitution was
observed exclusively with Ph2SiH2 (1f) at room temperature
(entry 6) but, using a three-fold excess of the indole, MePhSiH2
(1g) underwent two-fold C‒H silylation to afford the
bis(indol-3-yl)-substituted silane 7ag (entry 7). Selective mono-
substitution was achieved with Et2SiH2 (1h) when using the in-
dole as the limiting reagent (entry 8). Again, bis(indol-3-yl)-
substituted silane 7ai formed from trihydrosilane PhSiH3 (1i)
(entry 9); the molecular structure of 7ai was confirmed by X-ray
diffraction (see the Supporting Information for details).
Our group recently introduced catalytic electrophilic C‒H si-
lylations6 of electron-rich arenes such as indoles and anilines
based on cooperative Si‒H bond activation7b and Lewis-acid
catalysis,7d respectively. With the present work, we now aim at
the development of a complementary process catalyzed by
Brønsted acid 213 (Table 1). Good yield and excellent regioselec-
tivity were obtained using 1.0 mol % of Brookhart’s acid 2 in the
reaction between 1-methylindole (4a) and hydrosilane 1a (4a →
5aa, entry 1).14,15 No reaction was seen in the absence of
[H(OEt2)2]+[BArF ]– (2), and Na+[BArF ]‒ alone did not promote
this transformation (entries 2 and 3). A gradual increase of the
catalyst loading from 1.0 to 4.0 mol % led to diminished yields
(entries 4 and 5). This unusual trend is understood as the result of
protodesilylation prevalent at higher proton concentrations.1 It
also emphasizes that proton release and removal, i.e., dihydrogen
release, must be well balanced to overcome this intrinsic problem.
To our delight, the addition of norbornene (nbe) as a proton scav-
enger16 dramatically improved the yield to near-quantitative (en-
tries 6‒8). We note here that 1-methylindoline (6a) always formed
as byproduct which is why these reactions were performed with
the hydrosilane as the limiting reagent. Importantly, the silylated
indole 5aa (major) and the indoline 6a (minor) did not form in
equimolar ratio (for an explanation, see Scheme 4).
4
4
Table 2. Screening of Hydrosilanes in the Indole Silylationa
Table 1. Optimization of the C3 Silylation of Indolea
entry
hydrosilane
T (°C)
yield (%)b
1
2
3
Me2PhSiH (1a)
MePh2SiH (1b)
Ph3SiH (1c)
80
rt
96 (5aa)
93 (5ab)
8c (5ac)
80
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Et3SiH (1d)
80
80
rt
7c (5ad)
0d (5ae)
96 (5af)
73e (7ag)
61c,f (5ah)
74e (7ai)
1
2
3
4
5
6
7
5
(EtO)2MeSiH (1e)
Ph2SiH2 (1f)
MePhSiH2 (1g)
Et2SiH2 (1h)
PhSiH3 (1i)
6
7
rt
8
9
rt
rt
8
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aAll reactions were performed on a 0.20 mmol scale (based on 1) using
double the amount of the indole (0.40 mmol, 2.0 equiv), [H(OEt2)2]+
[BArF ]– (2, 1.0 mol %), and norbornene (1.0 equiv) in toluene (0.10 mL)
4
b
at the indicated temperature for 18 h. Isolated yield after flash chroma-
tography on silica gel. cDetermined by 1H NMR spectroscopy using
CH2Br2 as internal standard. d(EtO)3SiMe and [(EtO)2MeSi]2O detected by
e
b
1
aC3:C2 = 92:8. Determined by H NMR spectroscopy using
GLC–MS analysis. Three-fold excess of the indole (0.60 mmol) used to
obtain the bis(indol-3-yl)-substituted silane exclusively. The indole used
f
CH2Br2 as internal standard. cC3:C2 = 87:13.
as the limiting reagent (0.20 mmol) together with hydrosilane 1h (0.40
mmol); trace amounts of the corresponding bis(indol-3-yl)-substituted
silane observed by GLC‒MS analysis.
We then turned toward aniline derivatives as promising elec-
tron-rich arenes in the Brønsted acid-promoted silylation with
hydrosilanes (Scheme 3).7d,7e As aniline reduction was not ob-
served with this setup, we returned to using the more conventional
substrate-to-reagent ratio; a two-fold excess of the hydrosilane
was required to reach high yields. The addition of nbe was also
crucial.16 Indeed, N,N-dimethylaniline and N-phenylpyrrolidine
reacted highly regioselectively in good yields at room temperature
and 80 °C, respectively (8a → 9af and 8b → 9bf). The success of
this silylation relies heavily on the electronic property of substit-
uents on the aniline as ortho-fluoro-substituted congeners did not
react (not shown). Alkylation in the ortho position to the elec-
tron-donating amino group as in 1-methyl- indoline and
1-methyl-1,2,3,4-tetrahydroquinoline was tolerated (8c → 9cf and
8d → 9df). The meta-substituted derivative also participated in
similar yield, maintaining excellent para selectivity (8e → 9ef),
whereas the para-substituted isomer was unreactive (8f not to 9ff).
No silylation occurred in the reaction with anisole (8g).
Given the potential for further derivatization, Ph2SiH2 (1f) was
used to study the scope of the regioselective C‒H silylation of
heteroarenes (Scheme 2). The isolated yield for 1-methylindole
was 96% (4a → 5af), and slightly higher temperature was re-
quired to obtain 93% yield for 1,2-dimethylindole (4b → 5bf).
Conversely, 1,3-dimethylindole did not react (4c not to 5cf),
furnishing proof of an SEAr mechanism with the more nucleo-
philic indole C3 position blocked by
a methyl group.
1,5-Dimethylindole underwent the C3-selective SEAr at room
temperature in high yield (4d → 5df) as did the 5-halogenated
1-methylindoles (4e‒4g → 5ef‒5gf); no dehalogenation was
detected. While these reactions were highly regioselective
(C3:C2 > 95:5), minor C‒H silylation was observed at the C2
position in the case of 6-fluoro-substituted 1-methylindole (4h →
5hf; C3:C2 = 92:8). Dehydrogenative Si‒N coupling occurred
with unprotected indole (4i → 5if),17 and no further reaction at C3
was found. Moderate yield (58%) and regioselectivity (C3:C2 =
88:12) was achieved with the more challenging pyrrole substrate
(4j → 5jf). This catalytic system was not able to facilitate the C‒
H silylation of benzofuran (4k) and benzothiophene (4l).—To
demonstrate the practicability of this protocol, a gram-scale syn-
thesis of a C3-silylated indole using MePh2SiH (1b) was per-
formed (4a → 5ab, cf. Table 2, entry 2). With just 0.5 mol %
Scheme 3. Regioselective Silylation of Aniline Derivatives
and Attempted Silylation of Anisole (EDG
tron-Donating Group)
= Elec-
loading of [H(OEt2)2]+[BArF ]– (2), the reaction on a 5.0 mmol
4
scale furnished 1.6 gram of silylated indole 5ab in 95% isolated
yield.
Scheme 2. Regioselective C‒H Silylation of Heteroarenes
On the basis of the literature precedence2,18,20 and our own ob-
servations, we propose the following dominating catalytic cycle16
for the Brønsted acid-promoted SEAr with an in-situ-generated
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Page 4 of 6
silicon electrophile (Scheme 4). Brookhart’s acid 2 is sufficiently
Experimental procedures, spectral data for all new compounds,
1
2
3
4
5
6
7
8
strong to protonate the hydrosilane to form a pentacoordinate
siliconium ion (1 → 9).3,19 That transient intermediate will release
dihydrogen18 to afford the donor-stabilized silylium ion
and crystallographic data. This material is available free of charge
via the Internet at http://pubs.acs.org.
[R3Si(donor)]+ [BArF ]– (10
→ 3). Et2O introduced with
AUTHOR INFORMATION
4
[H(OEt2)2]+[BArF ]– (2) is likely to act as the stabilizing donor (cf.
4
Equation 3 and Figure 1) but the toluene solvent12 will assume
this role18b if ether cleavage occurs in the course of the reaction.
The cationic silicon electrophile 3 is then attacked by the nucleo-
philic indole (4a → 11a). The resulting Wheland complex is a
Corresponding Author
martin.oestreich@tu-berlin.de
Notes
9
strong Brønsted acid with the weakly coordinating [BArF ]–
The authors declare no competing financial interest.
4
counteranion, and direct protonation of another hydrosilane mol-
ecule closes the catalytic cycle (1 → 10) concomitant with for-
mation of the C3-silylated indole (11a → 5a).20,21
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ACKNOWLEDGMENT
Q.-A.C. gratefully acknowledges the Alexander von Humboldt
Foundation for a postdoctoral fellowship (2015–2017). M.O. is
indebted to the Einstein Foundation (Berlin) for an endowed
professorship. We thank Dr. Elisabeth Irran (TU Berlin) for the
X-ray analysis.
Scheme 4. Proposed Catalytic Cycle of the Brønsted Ac-
id-Promoted Electrophilic Indole Silylation
REFERENCES
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(2) (a) Corey, E. J.; Cho, H.; Rücker, C.; Hua, D. H. Tetrahedron Lett.
1981, 22, 3455–3458 (iPr3SiOTf). (b) Aizpurua, J. M.; Palomo, C.
Tetrahedron Lett. 1985, 26, 6113–6114 (tBuMe2SiOTf).
(3) Conversely, treatment of R3SiD (R = Me, Et, and iPr) with HI in the
1
presence of AlI3 leads to H/2H exchange: Olah, G. A.; Heiliger, L.;
Aniszfeld, R.; Prakash, G. K. S. New J. Chem. 1990, 14, 877–881.
(4) Recent reviews of silylium ions: (a) Müller, T. In Structure and
Bonding; Scheschkewitz, D., Ed.; Springer: Berlin, 2014; Vol. 155,
pp 107–162. (b) Müller, T. In Science of Synthesis: Knowledge
Updates 2013/3; Oestreich, M., Ed.; Thieme: Stuttgart, 2013; pp 1–
42. (c) Klare, H. F. T.; Oestreich, M. Dalton Trans. 2010, 39, 9176–
9184.
(5) Known strategies to generate silylium ions: (a) Corey, J. Y. J. Am.
Chem. Soc. 1975, 97, 3237–3238 (silicon-to-carbon hydride transfer).
(b) Lambert, J. B.; Zhao, Y.; Wu, H.; Tse, W. C.; Kuhlmann, B. J.
Am. Chem. Soc. 1999, 121, 5001–5008 (allyl-leaving-group
approach). (c) MacLachlan, M. J.; Bourke, S. C.; Lough, A. J.;
Manners, I. J. Am. Chem. Soc. 2000, 122, 2126–2127 (ring-opening
protonolysis). (d) Schäfer, A.; Reißmann, M.; Schäfer, A.; Saak, W.;
Haase, D.; Müller, T. Angew. Chem., Int. Ed. 2011, 50, 12636–
12638 (substituent exchange). (e) Schäfer, A.; Reißmann, M.;
Schäfer, A.; Schmidtmann, M.; Müller, T. Chem. - Eur. J. 2014, 20,
9381–9386 (silylene protonation). (f) Simonneau, A.; Biberger, T.;
Formation of the indoline byproduct 6a is rationalized by com-
peting silylium-ion catalysis. Proton transfer from intermediate
11a to the indole substrate 4a used in excess not only liberates the
C3-silylated indole 5a but also arrives at another Wheland com-
plex 12a. This step was NMR spectroscopically corroborated by
the reaction of 4a with an independently prepared sample of 11a.
Iminium ion 12a then accepts a hydride from hydrosilane 1 to
yield indoline 6a as well as donor-stabilized silylium ion 3; quan-
titative deuterium incorportation at C2 of 6a was seen when using
Me2PhSiD (1a-d1). This reduction pathway will not occur with the
aniline substrates (not shown).
Oestreich,
M.
Organometallics
2015,
34,
3927–3929
(cyclohexadienyl-leaving-group approach).
(6) Recent reviews of catalytic C–H silylation: (a) Cheng, C.; Hartwig, J.
F. Chem. Rev. 2015, 115, 8946–8975. (b) Yang, Y.; Wang, C. Sci.
China Chem. 2015, 58, 1266–1279. (c) Xu, Z.; Huang, W.-S.; Zhang,
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R.; Kumar, I.; Singh, B.; Sharma, U. Synthesis 2015, 47, 2347–2366.
(7) Intermolecular Friedel–Crafts-type C–H silylations: (a) Furukawa, S.;
Kobayashi, J.; Kawashima, T. Dalton Trans. 2010, 39, 9329–9336.
(b) Klare, H. F. T.; Oestreich, M.; Ito, J.-i.; Nishiyama, H.; Ohki, Y.;
Tatsumi, K. J. Am. Chem. Soc. 2011, 133, 3312–3315. (c) Curless, L.
D.; Clark, E. R.; Dunsford, J. J.; Ingleson, M. J. Chem. Commun.
2014, 50, 5270–5272. (d) Yin, Q.; Klare, H. F. T.; Oestreich, M.
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Zhang, L.; Hou, Z. J. Am. Chem. Soc. 2016, 138, 3663–3666.
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Kobayashi, J.; Kawashima, T. J. Am. Chem. Soc. 2009, 131, 14192–
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To recap, we disclosed here a counterintuitive C–H silylation of
electron-rich (hetero)arenes passing through an SEAr mechanism.
The transformation is initiated by Brønsted acid-mediated genera-
tion of a highly electrophilic silicon cation from hydrosilanes.
Protonation of the hydrosilane leads to loss of dihydrogen and
release of the stabilized silylium ions. The Wheland intermediate
then largely maintains the catalytic cycle as the proton source. No
protodesilylation is observed when the amount of acid is well
balanced. This protocol is a practical and straightforward way for
the installation of silicon groups on arenes, thereby complement-
ing existing transition-metal and Lewis-acid catalyses.6
(9) (a) Bassindale, A. R.; Stout, T. J. Organomet. Chem. 1984, 271, C1–
C3. (b) Uhlig, W.; Tzschach, A. J. Organomet. Chem. 1989, 378,
C1–C5.
(10) Brookhart, M.; Grant, B.; Volpe, Jr., A. F. Organometallics 1992, 11,
3920–3922.
ASSOCIATED CONTENT
Supporting Information
(11) For studies on silyloxonium ions, see: (a) Kira, M.; Hino, T.; Sakurai,
H. J. Am. Chem. Soc. 1992, 114, 6697–6700. (b) Olah, G. A.; Li,
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X.-Y.; Wang, Q.; Rasul, G.; Prakash, G. K. S. J. Am. Chem. Soc.
1995, 117, 8962–8966. (c) Olah, G. A.; Rasul, G.; Prakash, G. K. S.
J. Organomet. Chem. 1996, 521, 271–277.
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(12) (a) Lambert, J. B.; Zhang, S.; Stern, C. L.; Huffman, J. C. Science
1993, 260, 1917–1918. (b) Lambert, J. B.; Zhang, S.; Ciro, S. M.
Organometallics 1994, 13, 2430–2443.
(13) We were aware of the fact that [H(OEt2)2]+[BArF ]– (2) is not stable
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in CH2Cl2, forming HArF and BArF .10 The electron-deficient borane
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BArF could act as the Lewis acid catalyst (cf. Refs 7d and 7e).
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While we verified the instability of 2 in toluene, we also found that 2
shows enhanced stability in the presence of the hydrosilane and is
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perfectly stable in the presence of the indole. The [BArF ]–
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counteranion is recovered after the reaction.
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(14) Catalytic C2 silylation of indoles: (a) Lu, B.; Falck, J. R. Angew.
Chem., Int. Ed. 2008, 47, 7508–7510. (b) Minami, Y.; Komiyama, T.;
Hiyama, T. Chem. Lett. 2015, 44, 1065–1067. (c) Toutov, A. A.; Liu,
W.-B.; Betz, K. N.; Fedorov, A.; Stoltz, B. M.; Grubbs, R. H. Nature
2015, 518, 80–84. (d) Devaraj, K.; Sollert, C.; Juds, C.; Gates, P. J.;
Pilarski, L. T. Chem. Commun. 2016, 52, 5868–5871.
(15) Catalytic C3 silylation of indoles: (a) Ishiyama, T.; Sato, K.; Nishio,
Y.; Saiki, T.; Miyaura, N. Chem. Commun. 2005, 5065–5067. (b)
Ref 7b. (c) Ref 7c. (d) Sunada, Y.; Soejima, H.; Nagashima, H.
Organometallics 2014, 33, 5936–5939. (e) Ref 8c. (f) Cheng, C.;
Hartwig, J. F. J. Am. Chem. Soc. 2015, 137, 592–595. (g) Ito, J.-i.;
Hosokawa, S.; Khalid, H. B.; Nishiyama, H. Organometallics 2015,
34, 1377–1383.
(16) The fate of nbe is likely its cationic polymerization as we detected
neither norbornane nor its silylated congener by GLC–MS analysis.
(17) Königs, C. D. F.; Müller, M. F.; Aiguabella, N.; Klare, H. F. T.;
Oestreich, M. Chem. Commun. 2013, 49, 1506–1508.
(18) (a) Description of gas evolution: Nava, M.; Reed, C. A.
Organometallics 2011, 30, 4798–4800. (b) Detection of dihydrogen
gas and an explanation of its origin: Connelly, S. J.; Kaminsky, W.;
Heinekey, D. M. Organometallics 2013, 32, 7478–7481.
(19) This assumption is corroborated by theoretical and gas-phase studies
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on SiH5 : (a) Sefcik, M. D.; Henis, J. M. S.; Gaspar, P. P. J. Chem.
Phys. 1974, 61, 4329–4334. (b) Hu, C.-H.; Shen, M.; Schaefer III, H.
F. Chem. Phys. Lett. 1992, 190, 543–550.
(20) Heinekey and co-workers also noted in their work catalytic
hydrosilane consumption by the benzene-stabilized silicon cation in
benzene solvent (cf. Ref 18b).
(21) Douvris, C.; Ozerov, O. V. Science 2008, 321, 1188–1190.
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