Novel one-pot synthesis of aryltris(trimethylsilyl)silanes

Article abstract of DOI:10.1021/jo0268660

The simple combination of tris(trimethylsilyl)-potassium, ArMgBr, and ArBr provides a novel one-pot synthesis of aryl(tristrimethylsilyl)silanes. A mechanistic rationale for this conversion is proposed.

Full text of DOI:10.1021/jo0268660

silyl)silane 1 and, in this paper, report a simple one-pot
procedure for the preparation of this and related aryltris-
(trimethylsilyl)silanes from readily available tetrakis-
(trimethylsilyl)silane.
Novel On e-P ot Syn th esis of
Ar yltr is(tr im eth ylsilyl)sila n es
Mahesh J . Sanganee,† Patrick G. Steel,*,‡ and
Daniel K. Whelligan‡
Department of Chemistry, University of Durham,
Science Laboratories, South Road Durham DH1 3LE, U.K.,
and Synthetic Chemistry, GlaxoSmithKline,
Gunnels Wood Road, Stevenage, Herts SG1 2NY, U.K.
Recent reports by Goossen and Ferwanah have shown
that trimethylsilylarenes can be conveniently prepared
by a palladium-catalyzed silylation of aryl bromides using
hexamethyldisilane, eq 1.17
p.g.steel@durham.ac.uk
Received December 18, 2002
Abstr a ct: The simple combination of tris(trimethylsilyl)-
potassium, ArMgBr, and ArBr provides a novel “one-pot”
synthesis of aryl(tristrimethylsilyl)silanes. A mechanistic
rationale for this conversion is proposed.
In connection with a project investigating the use of
silenes as reagents for organic synthesis,1 we required
multigram quantities of organotris(trimethylsilyl)silanes.
Furthermore, to facilitate subsequent “Fleming-Tamao”-
type oxidations, aryl derivatives were of particular inter-
est. In addition to functioning as a bulky steric (“hyper-
silyl”) substituent in synthetic applications,2,3 aryl(tris-
trimethylsilyl)silanes have found widespread use in
assorted structural, spectroscopic,4-7 and photochemical
studies.8-12 Reflecting these applications, a number of
preparative methods have been reported.8,10,13-16 How-
ever, although 1 can be obtained, these involve either
multiple steps, the use of hazardous reagents or inter-
mediates or require extensive and difficult purification
giving only modest overall yields. Consequently, we have
sought an alternative synthesis of phenyltris(trimethyl-
Our initial approach was to attempt this procedure
replacing the disilane with commercially available tet-
rakis(trimethylsilyl)silane 2.18 However, all attempts to
achieve this conversion, employing a variety of catalysts
and conditions, afforded an intractable mixture of sily-
lated aromatics. We speculate that the product is equally
vulnerable to Si-Si bond activation by the palladium
catalysts producing the observed product mixtures. This
result suggested that if the reactivity of the original tris-
(trimethylsilyl)silane unit could be enhanced it would be
possible to find conditions that allowed the efficient
isolation of the desired silane 1. Consequently, we
explored the development of a silicon modification of the
Hayashi procedure for the cross-coupling of Grignard
reagents with aryl bromides.19,20 Treatment of tetrakis-
(trimethylsilyl)silane with MeLi, according to the proce-
dure of Gilman,21 generated tris(trimethylsilyl)silyllith-
ium which could be transmetalated with anhydrous
MgBr2 to produce the corresponding silyl-Grignard re-
agent, [(Me3Si)3SiMgBr]. Subsequently, we found that
this procedure could be substantially accelerated and
simplified by replacing MeLi with KOtBu.2 Following the
Hayashi protocol, treatment of this Grignard reagent
with bromobenzene in the presence of (dppf)PdCl2 af-
forded the desired phenyltris(trimethylsilyl)silane 1 in
20% yield, eq 2.
† GlaxoSmithKline.
‡ Durham University.
(1) Batsanov, A. S.; Clarkson, I. M.; Howard, J . A. K.; Steel, P. G.
Tetrahedron Lett. 1996, 37, 2491-2494.
(2) Marschner, C. Eur. J . Inorg. Chem. 1998, 221-226.
(3) Bochmann, M.; Green, M. L. H.; Powell, A. K.; Sassmann-
shausen, J .; Triller, M. U.; Wocadlo, S. J . Chem. Soc., Dalton Trans. 1
1999, 43-49.
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1970, 23, 85.
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118.
(6) Sipe, H. J .; West, R. J . J . Organomet. Chem. 1974, 70, 353.
(7) Shizuka, H.; Obuchi, H.; Ishikawa, M.; Kumada, M. J . Chem.
Soc., Faraday Trans. 1 1984, 80, 383-401.
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1980, 190, 117; ibid 1979, 178, 105.
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Chem. Soc., Faraday Trans. 1 1984, 80, 341-357.
(10) Oka, K.; Nakao, R. J . Organomet. Chem. 1990, 390, 7-18.
(11) Kusukawa, T.; Ando, W. J . Organomet. Chem. 1998, 559, 11-
22.
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223.
(13) Baines, K. M.; Brook, A. G.; Ford, R. R.; Lickiss, P. D.; Saxena,
A. K.; Chatterton, W. J .; Sawyer, J . F.; Behnam, B. A. Organometallics
1989, 8, 693-709.
(14) Gilman, H.; Harrell, R. L.; Smith, C. L.; Shiina, K. J . Orga-
nomet. Chem. 1966, 387.
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299.
(17) Goossen, L. J .; Ferwanah, A. R. S. Synlett 2000, 1801-1803.
(18) Gilman, H.; Smith, C. L. J . Am. Chem. Soc. 1964, 86, 1454; J .
Organomet. Chem. 1967, 8, 245.
(19) Hayashi, T.; Konishi, M.; Fukushima, M.; Mise, T.; Kagotani,
M.; Tajika, M.; Kumada, M. J . Am. Chem. Soc. 1982, 104, 180-186.
(20) Hayashi, T.; Konishi, M.; Kobori, Y.; Kumada, M.; Higuchi, T.;
Hirotsu, K. J . Am. Chem. Soc. 1984, 106, 158-163.
(21) Gilman, H.; Smith, C. L. J . Organomet. Chem. 1968, 14, 91-
101.
(16) Baumeister, U.; Schenzel, K.; Zink, R.; Hassler, K. J . Orga-
nomet. Chem. 1997, 543, 117-124.
10.1021/jo0268660 CCC: $25.00 © 2003 American Chemical Society
Published on Web 03/21/2003
J . Org. Chem. 2003, 68, 3337-3339
3337

TABLE 1. Yield s of Ar yltr is(tr im eth ylsilyl)sila n e,
Ar Si(SiMe3)3
SCHEME 1
Ar
yield (%)
Ph (1)
Tol (9)
4-(MeO)C6H4 (10)
4-FC6H4 (11)
70
48
55
64
31
4-(F3C)C6H4 (12)
magnesium 8, Scheme 2.23 This reacts rapidly, at low
temperatures, with the aryl bromide to produce the silyl
bromide 4, as detected by GCMS, and relatively unreac-
tive diarylmagnesium. Final substitution occurs on ad-
dition of the second equivalent of the Grignard reagent.
Attempts to intercept the intermediate silyl bromide
using a different Grignard reagent, tolylmagnesium
bromide, afforded a 3:7 mixture of phenyl- and tolyl-
substituted products, respectively. This can be attributed
to the Schlenk equilibria between Ph2Mg and PhMgBr
being accelerated by the magnesium bromide liberated
in the final substitution step, eqs 3 and 4.24,25
SCHEME 2
In attempts to optimize this process, a range of
conditions, including alternative silylmetallics (organo-
lithium, potassium, or magnesium reagent), catalysts,
modes of addition, and temperatures were investigated.
However, little benefit accrued. During these studies, it
was observed that an initial reaction occurred on mixing
the silyl organometallic and aryl bromide even in the
absence of the catalyst. This suggested that 1 could be
formed via a metal-halogen exchange between the
initially formed silyl potassium 3 and bromobenzene to
form silyl bromide 4 and phenylpotassium which then
combine with elimination of KBr, Scheme 1. From GCMS
analysis of the crude reaction mixture the alternative
silanes 522 and 6 were identified as the major side
products in this process. These could arise from reaction
of silylpotassium 3 with the bromide 4 and trimethylsi-
lane abstraction by phenylpotassium, respectively. While
use of the corresponding magnesium reagents minimized
the latter problem, conversion of the silyl bromide was
poor as ascertained by isolation of the corresponding
silanol 7.
Having established an optimal procedure for the
preparation of 1, we then explored its application to other
substrates. Both electron-rich (4-MeO) and electron-poor
(4-CF3) aryl bromides prove to be viable substrates for
this conversion, Table 1.
In conclusion, this process provides a convenient one-
pot access to aryl(tristrimethylsilyl)silanes via a novel
sequence of reactions. Efforts to further enhance this
preparation and fully elucidate the mechanism of this
process are in progress. Results of this and the studies
of these reagents as silene precursors will be reported in
due course.
Exp er im en ta l Section
Speculating that an additional aliquot of the aryl
Grignard reagent would accelerate trapping of the in-
termediate bromide and thus reduce the amount of
octasilane 5 formed, a mixture of silylpotassium 3 and 2
equiv of PhMgBr were combined to produce PhMgSi-
(SiMe3)3 and PhMgBr. Addition of aryl bromide then
afforded the desired phenyltris(trimethylsilyl)silane 1 in
65% yield (GC) with minimal byproducts. Further refine-
ments to this process involved portionwise addition of the
aryl Grignard reagent and a final reflux to ensure
complete displacement of the bromide. With these condi-
tions a reproducible yield of 60-69% of the desired silane
could be obtained on scales up to 20 g.
All reactions were carried out under an argon atmosphere in
glassware dried under high vacuum by a heat-gun. THF was
distilled from sodium benzophenone ketyl under nitrogen. All
other reagents were used directly as supplied. Flash column
chromatography was carried out using silica gel 40-60 mesh.
High-resolution mass spectra were performed at the EPSRC
service at the University of Swansea.
Gen er a l P r oced u r e As Ap p lied to P h en yltr is(tr im eth -
ylsilyl)sila n e 1. Dry tetrakis(trimethylsilyl)silane26 2 (18.66 g,
58.31 mmol) and potassium tert-butoxide (6.87 g, 61.23 mmol)
were combined under argon. Dry THF (280 mL) was added and
(23) Hayami, H.; Sato, M.; Kanemoto, S.; Morizawa, Y.; Oshima,
K.; Nozaki, H. J . Am. Chem. Soc. 1983, 105, 4491-4492.
(24) Markies, P. R.; Altink, R. M.; Villena, A.; Akkerman, O. S.;
Bickelhaupt, F.; Smeets, W. J . J .; Spek, A. L. J . Organomet. Chem.
1991, 402, 289-312.
(25) Holtkamp, H. C.; Schat, G.; Blomberg, C.; Bickelhaupt, F. J .
Organomet. Chem. 1982, 240, 1-8..
(26) This reagent is widely used as an NMR standard and is
commercially available but can be readily prepared on a multigram
scale. See reference 18.
We hypothesise that the reaction proceeds via initial
reaction of the silyl potassium 3 with PhMgBr to pre-
cipitate KBr and generate tristrimethylsilyl(phenyl)-
(22) Bock, H.; Meuret, J .; Ruppert, K. J . Organomet. Chem. 1993,
445, 19-28.
3338 J . Org. Chem., Vol. 68, No. 8, 2003

the solution stirred for 2 h after which time it was dark red.
Phenylmagnesium bromide (1.0 M, 64.1 mL, 64.1 mmol) was
added upon which a white precipitate formed. The mixture was
stirred for 1 h and then cooled to -78 °C. Bromobenzene (9.2
mL, 87.5 mmol) was added quickly with rapid stirring causing
a significant exotherm. After being stirred for a further 1 h at
-78 °C, the mixture was warmed to rt for 30 min, and more
phenylmagnesium bromide (1.0 M, 93.3 mL, 93.3 mmol) was
added. The mixture was refluxed until GC showed the reaction
complete. Saturated ammonium chloride solution (280 mL) was
then added. The aqueous layer was separated and extracted with
ether (3 × 280 mL). The combined organic layers were dried
over MgSO4, filtered, concentrated, and dried in vacuo. Flash
column chromatography on silica gel (petroleum ether) followed
by Kugelrohr distillation afforded the title compound 1 as an
amorphous semiclear solid (13.03 g, 69%): Rf (n-hexane) 0.74;
bp 95 °C/0.1 mmHg (lit.10 bp 132 °C/1.5 mmHg); mp 82.0-83.0
°C (lit.8 mp 84-85 °C); νmax (thin film) 3066 (CH, aromatic), 2954,
2896, 1430, 1250 (SiC) cm-1; δH (300 MHz; CDCl3) 7.46-7.42 (2
H, m), 7.26-7.23 (3 H, m), 0.22 (27 H, s); δC (63 MHz; CDCl3)
136.56, 135.53, 127.68, 127.31, 1.17; δSi (99 MHz; CDCl3) -12.79,
) 8.5 Hz), 6.84 (2 H, d, J ) 8.5 Hz), 3.79 (3 H, s), 0.22 (27 H, s);
δC (63 MHz; CDCl3) 159.31, 137.70, 125.23, 113.68, 54.88, 1.15;
δ
Si (99 MHz; CDCl3) -12.94, -77.76; m/z (CI, NH4+) 355 (MH+,
100), 339 (M+ - Me, 2), 297 (3), 281 (M+ - SiMe3, 2), 264 (13);
HRMS (EI) found M+, 354.1684, C16H34OSi4 requires M 354.1687.
p-Flu or oph en yltr is(tr im eth ylsilyl)silan e 11 (1.23g,64%): Rf
(petroleum ether) 0.85; bp 72-75 °C/0.07 mmHg; νmax (thin film)
2949, 2893, 1586, 1493, 1397, 1245, 1231, 1161, 1083, 865, 834,
746, 687, 624, 513 cm-1; δH (200 MHz; CDCl3) 7.43-7.36 (2 H,
m), 7.01-6.92 (2 H, m), 0.21 (27 H, s); δC (126 MHz; CDCl3)
162.87 (d, J ) 247.5 Hz), 137.95 (d, J ) 7.3 Hz), 130.52 (d, J )
3.8 Hz), 114.95 (d, J ) 19.7 Hz), 1.08; δF (376 MHz; CDCl3)
-114.98 (m); δSi (99 MHz; CDCl3) -12.85, -76.84; m/z (GCEI)
342 (M+, 76), 327 (M+ - Me, 36), 269 (M+ - SiMe3, 8), 254 (32),
253 (30), 250 (M+ - SiMe3 - F, 48), 239 (45), 235 (48), 211 (33),
209 (42), 208 (39), 195 (61), 192 (100), 174 (100), 159 (79), 153
(79); HRMS (EI) found M+, 342.1490, C15H31FSi4 requires M
342.1487.
p-Tr iflu or om eth ylph en yltr is(tr im eth ylsilyl)silan e 12 (0.68
g, 31%): νmax (thin film) 2951, 2894, 1604, 1389, 1325, 1260,
1247, 1165, 1127, 1101, 1057, 1017, 835, 694 cm-1; δH (500 MHz;
CDCl3) 7.57 (2 H, d, J ) 8.0 Hz), 7.50 (2 H, d, J ) 8.0 Hz), 0.24
(27 H, s); δC (126 MHz; CDCl3) 141.90, 136.52, 129.36 (q, J )
32.3 Hz), 124.42 (q, J ) 272.5 Hz), 124.08 (q, J ) 3.3 Hz), 1.10;
δF (282 MHz; CDCl3) -63.20 (s); δSi (99 MHz; CDCl3) -12.60,
-75.79; m/z (GCEI) 392 (M+ + H, 5), 304 (5), 300 (5), 259 (4),
245 (17), 230 (17), 203 (25), 193 (64), 174 (46), 159 (26), 145 (40),
131 (40); HRMS (EI) found M+, 392.1456, C16H31F3Si4 requires
M 392.1455).
-76.82; m/z (EI) 324 (M+, 30), 309 (M+ - Me, 12), 251 (M+
-
SiMe3, 17), 236 (M+ - Me - SiMe3, 22), 191 (25), 174 (M+ - Ph
- SiMe3, 70), 159 (M+ - Ph - SiMe3 - Me, 26), 135 (24), 73
(Me3Si+, 100); all data agree with the literature values.13
p-Tolyltr is(tr im eth ylsilyl)sila n e 9 (1.03 g, 48%): Rf (n-
hexane) 0.79; bp 65-70 °C/0.04 mmHg; νmax (thin film) 3064,
3029, 3013, 2949, 2893, 1495, 1440, 1394, 1245, 1085, 866, 834,
796, 746, 687 cm-1; δH (500 MHz; CDCl3) 7.35 (2 H, d, J ) 8.0
Hz), 7.09 (2 H, d, J ) 8.0 Hz), 2.32 (3 H, s), 0.22 (27 H, s); δC
(126 MHz; CDCl3) 136.99, 136.54, 131.22, 128.63, 21.33, 1.16;
Ack n ow led gm en t. We thank the EPSRC and Glax-
oSmithKline for financial support of this work (CASE
award to D.K.W.); The EPSRC Mass Spectrometry
Service for accurate mass determinations; Dr. A. M.
Kenwright for assistance with NMR experiments; and
Dr. M. J ones for mass spectra.
δ
Si (99 MHz; CDCl3) -12.89, -77.46; m/z (GCEI) 338 (MH+, 31),
323 (M+ - Me, 12), 265 (M+ - SiMe3, 27), 250 (12), 249 (12),
234 (9), 205 (26), 191 (51), 174 (100), 159 (38), 149 (72); HRMS
(EI) found M+, 338.1738, C16H34Si4 requires M 338.1738.
p-Meth oxyp h en yltr is(tr im eth ylsilyl)sila n e 10 (1.50 g,
55%): Rf (petroleum ether) 0.29 νmax (thin film) 2949, 2894, 2834,
1592, 1562, 1496, 1461, 1441, 1395, 1273, 1244, 1180, 1089,
1034, 866, 833, 687 cm-1; δH (250 MHz; CDCl3) 7.36 (2 H, d, J
J O0268660
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