Vinyl tris(trimethylsilyl)silanes: substrates for Hiyama coupling

Article abstract of DOI:10.1016/j.tet.2008.03.024

The oxidative treatment of vinyl tris(trimethylsilyl)silanes with hydrogen peroxide in aqueous sodium hydroxide in tetrahydrofuran generates reactive silanol or siloxane species that undergo Pd-catalyzed cross-couplings with aryl, heterocyclic, and alkenyl halides in the presence of Pd(PPh₃)₄ and tetrabutylammonium fluoride. Hydrogen peroxide and base are necessary for the coupling to occur while activation of the silanes with fluoride is not required. The conjugated and unconjugated tris(trimethylsilyl)silanes serve as good cross-coupling substrates. The (E)-silanes undergo coupling with retention of stereochemistry while coupling of (Z)-silanes occurred with lower stereoselectivity to produce an E/Z mixture of products.

Full text of DOI:10.1016/j.tet.2008.03.024

Tetrahedron 64 (2008) 5322–5327
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Vinyl tris(trimethylsilyl)silanes: substrates for Hiyama coupling
Zhizhong Wang, Jean-Philippe Pitteloud, Lucresia Montes, Magdalena Rapp y,
*
Djenny Derane, Stanislaw F. Wnuk
Department of Chemistry and Biochemistry, Florida International University, Miami, FL 33199, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
The oxidative treatment of vinyl tris(trimethylsilyl)silanes with hydrogen peroxide in aqueous sodium
hydroxide in tetrahydrofuran generates reactive silanol or siloxane species that undergo Pd-catalyzed
cross-couplings with aryl, heterocyclic, and alkenyl halides in the presence of Pd(PPh3)4 and tetrabutyl-
ammonium fluoride. Hydrogen peroxide and base are necessary for the coupling to occur while activation
of the silanes with fluoride is not required. The conjugated and unconjugated tris(trimethylsilyl)silanes
serve as good cross-coupling substrates. The (E)-silanes undergo coupling with retention of stereo-
chemistry while coupling of (Z)-silanes occurred with lower stereoselectivity to produce an E/Z mixture of
products.
Received 18 January 2008
Received in revised form 16 February 2008
Accepted 7 March 2008
Available online 12 March 2008
Keywords:
Cross-couplings
Organosilanes
Pd-catalyzed reactions
Tris(trimethylsilyl)silanes
Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction
converted in situ into reactive alkenyl(propyl)silanol and disiloxane
species via the ring opening.7b When the alkenylsilanols were
Pd-catalyzed cross-coupling reactions are a powerful method
for the formation of carbon–carbon bonds under conditions that
are compatible with a broad range of functional groups.1 Among
these methods, couplings between organometallics derived from
group 14 metals and various electrophiles are well developed. Thus,
the Stille reaction has been widely applied in modern synthetic
organic chemistry.2 Furthermore, the Hiyama coupling has received
increased attention due to the lower toxicity of organosilicon
compounds.3 Procedures for coupling with organogermanes have
also been reported.4,5
Hiyama and Hatanaka reported on the positive effect that
fluoride ion imparts on the nucleophilicity of certain substituents
bonded to trialkylsilanes and attributed this enhancement to the
formation of pentacoordinated silicon species.6 This strategy has
opened up the door for the wide application of structurally diverse
silanes including: (i) heteroatom functionalized precursors such as
halosilanes, oxysilanes, silanols, and polysiloxanes and (ii) all-car-
bon substituted silicon species such as phenyl-, benzyl-, 2-thienyl-,
2-pyridyldimethylsilanes.3
synthesized independently, they were also found to undergo cou-
pling.8 Hiyama, Mori et al. independently found that silver(I) oxide
is an excellent activator for the coupling of alkenylsilanols with aryl
iodides.9 Generally, the organosilanols can couple by two mecha-
nistically distinct processes. One is operative under basic condi-
tions and requires only formation of the silanolates while the
second pathway involves the use of a fluoride activator that can
form an active fluorosiliconate.10
Recent developments in Si-cross-coupling include the applica-
tion of triallyl(aryl)silanes for the synthesis of biaryls,11a a palla-
dium-phosphinous acid-catalyzed and NaOH-promoted coupling
of arylsiloxanes in water,11b application of alkenyl- and aryl[2-
(hydroxymethyl)phenyl]dimethylsilanes as a reusable silicon based
precursor for the fluoride-free couplings via intramolecular acti-
vation,11c vinylation of aryl bromides using an inexpensive vinyl-
polysiloxane,11d application of (2-pyridyl)allyldimethylsilanes as
novel pyridyl transfer,11e and fluoride-free methodologies.11f,g
Herein, we report application of various tris(trimethylsilyl)silanes
as ‘masked’ substrates for the cross-coupling reactions with aryl
halides and alkenyl bromides, which occurred under oxidative
conditions in the presence of aqueous sodium hydroxide with or
without tetrabutylammonium fluoride.
Denmark et al. developed Pd-catalyzed coupling reactions
employing siletanes (alkenylsilacyclobutanes) in the presence of
TBAF and Pd(0) such as Pd2(dba)3.7a The siletanes were found to be
2. Results and discussion
* Corresponding author. Tel.: þ1 305 348 6195; fax: þ1 305 348 3772.
E-mail address: wnuk@fiu.edu (S.F. Wnuk).
Present address: Faculty of Chemistry, Adam Mickiewicz University, Poznan,
y
The (E)-vinyl tris(trimethylsilyl)silanes (TTMS-silanes) 2a,c–f
Poland.
were prepared by stereoselective radical-mediated silyl-
0040-4020/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2008.03.024

Z. Wang et al. / Tetrahedron 64 (2008) 5322–5327
5323
desulfonylations5a of the corresponding (E)-vinyl sulfones 1 with
Table 1
Effect of reaction parameters on the cross-coupling of TTMS-silanes
(TMS)3SiH (Scheme 1). The cyclohexylidene silane 2g was similarly
prepared. The (Z)-vinyl silanes 4a–d were synthesized in 80–92%
yield by the radical hydrosilylation12 of the corresponding alkynes
3 with (TMS)3SiH. Attempted hydrosilylation of 1-alkynes 3a–c
with (TMS)3SiH in the presence of Rh(COD)2BF4/PPh3/NaI or
RhCl(PPh3)3/NaI catalyst system13 produced E isomers 2a–c in high
yields but complete control of stereochemistry was generally
difficult since Z isomers 4a–c were also formed (w5–20%). Thus,
hydrosilylation of 3b gave 2b/4b (E/Z, w9:1) mixture, which was
purified to afford 2b (82%). Isomerization (20 h, 60 ꢀC) of the (Z)-
silanes 4a and 4c with 0.5 equiv of (TMS)3SiH in the presence of Rh
catalyst13 also afforded the (E)-silanes 2a (78%) and 2c (89%). Pro-
longed heating (54 h) of 4b in the presence of (TMS)3SiH/
RhCl(PPh3)3/NaI effected quantitative conversion of 4b into 2b.
I
Si(SiMe3)3
Pd(PPh3)4 (10% mol)
THF/55 °C/10 h
4b
5b
Entry
Peroxide
Base
Fluoride
Yielda (%)
E/Zb
1
2
3
4
5
6
7
8
9
10
None
H2O2
H2O2
H2O2
H2O2
H2O2
None
H2O2
H2O2
t-BuOOH
None
TBAF
None
None
TBAF
TBAF
TBAF
TBAF
NaF
<5c
61d,e,f
60
5:95
15:85
25:75
3:97
2:98
67:33
n/a
40:60
60:40
75:25
NaOH
KOSiMe3
NaOH
KOSiMe3
None
KOSiMe3
NaOH
NaOH
KH
90
81
18c
<2c
60
g
CsF
12c
15c
Noneh
a
Isolated yields (combined for both isomers of 5b). Couplings were performed on
R1
R
Si(SiMe3)3
O
R1
R
S
a
0.1 mmol scale of silane (0.03 mM).
O
b
Determined by GC–MS [with internal standard of (E)- and (Z)-stilbenes] and/or
1H NMR of the crude reaction mixture.
2
c
1
Based on GC–MS.
Pd2(dba)3 also gave 5b (52%; E/Z, 25:75).
Attempted couplings with H2O2 (without NaOH) or with NaOH (without H2O2)
d
b
b
e
R1
R
failed to give 5b.
a
f
Coupling with bromobenzene instead of iodobenzene gave 5b (40%; E/Z, 20:80).
Reaction with NaOH instead of KOSiMe3 was also unsuccessful.
Coupling in the presence of TBAF gave 5b in w2% yield (GC–MS).
R
g
Si(SiMe3)3
h
3
4
Series: a R = Ph, R1 = H
b R = (4)CH3Ph, R1 = H
H2O (30%, 3 equiv) and NaOH (3 equiv)/H2O in THF followed by
addition of bromobenzene, Pd(PPh3)4, and TBAF gave stilbene 5a
(82%, Table 2, entry 1). Analogously, (Z)-4b coupled with iodo-
benzene and bromobenzene to give p-methylstilbene 5b in 90 and
86% yields, respectively (entries 3 and 4). Less reactive electrophiles
such as chlorobenzene and aryl triflate15 failed to give the desired
coupling products (entries 5 and 6). The substituent on the phenyl
ring in (Z)-silanes 4a–d (p-MeO, p-Me, H, p-CF3) effects coupling
reactions with bromobenzene, increasing both the yields (from 70
to 97%) and steroselectivity outcome (E/Z from 55:45 to 9:91) as the
substituent changed from an electron-withdrawing group to an
electron-donating group (entries 1, 4,12, and 13). TTMS-silanes also
coupled with both p-deficient (entry 2) and electron-rich hetero-
cyclic halides (entry 9) as well as with iodonaphthalene (entry 11).
Not only aryl halides but also vinyl halides can be coupled with
vinyl TTMS-silanes. Thus, oxidative treatment of (Z)-4b with b-
bromostyrene produced the diene 6b (72%, entry 7). The aliphatic
1-bromo-2-methyl-1-propene coupled with (Z)-4b yielding 7b in
high yield and high stereoselectivity (entry 8).
The (E)-TTMS-silanes 2a–f underwent coupling with retention
of stereochemistry. Thus, coupling of conjugated silanes 2a–d with
aryl, heterocyclic, and aliphatic bromides or iodides (H2O2/NaOH/
Pd(0)/TBAF) provided products stereoselectively in good to excel-
lent yields (48–90%, Table 3, entries 1–15). The electron-deficient
aryl iodides gave to some extent higher yields than the electron-
rich aryl iodides in the reactions with silane 2a (entries 1, 3, and 4).
The nonconjugated (E)-vinyl silanes 2e–f also undergo coupling
with iodobenzene and bromobenzene under oxidative conditions
in the presence of aqueous NaOH and TBAF to give the desired E
alkenes 5e–f (70–85%; entries 16–19). The vinyl silane 2g, derived
from cyclohexanone, successfully coupled with aryl (entries 20 and
21), alkenyl (entry 22), and heterocyclic halides (entry 23) to
produce trisubstituted alkenes.
c R = (4)CH3OPh, R1 = H
d R = (4)CF3Ph, R1 = H
e R = PhCH2CH2, R1 = H
f R = C6H13, R1 = H
g R = R1 = -(CH2)5-
Scheme 1. Stereoselective synthesis of (E)- and (Z)-vinyl tris(trimethylsilyl)silanes (2
and 4). Reagents and conditions: (a) (Me3Si)3SiH/AIBN/toluene or benzene (85 ꢀC; oil
bath); (b) (Me3Si)3SiH/RhCl(PPh3)3 or Rh(COD)2BF4/PPh3/NaI/neat/D.
We started by examining different coupling conditions. Thus,
attempted coupling of (Z)-vinyl silane 4b with iodobenzene under
typical coupling conditions employed for organosilanes6 [e.g.,
TBAF/Pd(0)/THF] yielded the desired product 5b in less than 5%
(Table 1, entry 1). However, we found that fluoride-free treatment
of 4b with H2O2/NaOH5a in aqueous THF for 15 min followed by
addition of iodobenzene and Pd(PPh3)4 provided (E/Z)-5b in 61%
yield (entry 2). When NaOH was replaced with KOSiMe3 base14 the
yield did not improve, although the reaction mixture became ho-
mogeneous with the combination of KOSiMe3/H2O2 (entry 3). TBAF
was found to be an excellent activator under aqueous (H2O2/NaOH)
and ‘anhydrous’ (H2O2/KOSiMe3) oxidative conditions furnishing
coupling product 5b in 90 and 81% yields, respectively (entries 4
and 5). Both peroxide and base were necessary since only 18%
conversion to 5b was obtained without NaOH or KOSiMe3 (entry 6)
and almost no product was formed without peroxide (entry 7).
Other fluoride sources such as NaF and CsF did not show im-
provement over TBAF (entries 8 and 9). Anhydrous oxidative
conditions [t-BuOOH/KH/Pd(0)] with or without TBAF did not give
satisfactory results (entry 10), although such conditions worked
well for the coupling of analogous TTMS-germanes.5b These results
indicate that the peroxide and base are necessary for the coupling
of TTMS-silanes to occur while fluoride facilitates this conversion
(vide infra Tables 2 and 3 vs Table 4).
Although TBAF promotes couplings of TTMS-silanes in the
presence of H2O2/base, we found that fluoride activation of TTMS-
silanes was not necessary for the cross-coupling to occur. Thus,
oxidative treatment (H2O2/NaOH or KOSiMe3) of the conjugated
silane (E)-2a with bromo- and iodobenzene also produced
The protocols established above (Table 1, entries 2–5) have
proven to be general for the coupling of a range of alkenyl, aryl, and
heterocyclic iodides and bromides with an array of vinyl TTMS-si-
lanes. Thus, treatment of the conjugated silane (Z)-4a with H2O2/

5324
Z. Wang et al. / Tetrahedron 64 (2008) 5322–5327
Table 2
Table 3
The coupling of vinyl (Z)-TTMS-silanes
The coupling of vinyl (E)-TTMS-silanes
H2O2/NaOH (aq)
R2X, Pd(PPh3)4/TBAF
THF/55 °C/10 h
R1
H2O2/NaOH (aq)
R1
R1
Si(SiMe3)3
R1X, Pd(PPh3)4/TBAF
THF/55 °C/10 h
R2
Si(SiMe3)3
R
R
2
5 R2 = Ph
R
R
6 R2 = CH=CHPh
Series: a R = Ph, R1 = H
4
5 R1 = Ph
7 R2 = CH=C(CH3)2
b R = (4)CH3Ph, R1 = H
6 R1 = CH=CHPh
Series: a R = H
c R = (4)CH3OPh, R1 = H
d R = (4)CF3Ph, R1 = H
e R = PhCH2CH2, R1 = H
f R = C6H13, R1 = H
b R = CH3
c R = CH3O
d R = CF3
7 R1 = CH=C(CH3)2
8 R2
9 R2
=
=
S
Me
8 R1
9 R1
=
=
CH3
S
g R = R1 = -(CH2)5-
N
10 R2 = (4)BuC6H4
N
10 R1 = (4)BuC6H4
11 R2
=
11 R1
=
Entry
Silane
R2X
Producta (E)
Yieldb (%)
1
2
3
4
5
6
7
8
2a
2a
2a
2a
2a
2a
2a
2b
2b
2b
2b
2b
2c
2c
2d
2e
2e
2f
PhI
PhBr
5a
5a
5c
83
67
79
90
85
58
70
72
66
59
48
70
63
60
86
85
72
70
78
84
65
50
83
Entry
Silane
R1X
Product
Yielda (%)
E/Zb
1
2
3
4
5
6
7
8
4a
4a
4b
4b
4b
4b
4b
4b
4b
4b
4b
4c
4d
PhBr
2-Iodopyridine
PhI
PhBr
PhCl
5a
9a
82
60
90
86
<5c
<5c
72
87
75
61
73
97
70
40:60
2:98
3:97
30:70
n/a
(4)CH3OPhI
(4)CF3PhI
(CH3)2C]CHBr
Iodothiophenec
2-Iodopyridine
PhI
5d
7a
8a
9a
5b
8b
10b
11b
11b
5c
5b
5b
5b
5b
6b
7b
8b
10b
11b
5c
PhOTf
n/a
PhCH]CHBrd
(CH3)2C]CHBr
Iodothiophenef
(4)BuPhI
1-Iodonaphthaleneg
PhBr
44:56e
4:96
9
Iodothiophenec
(4)BuPhI
1-Bromonaphthalene
1-Iodonaphthalene
PhBr
10
11
12
13
14
15
16
17
18
19
20
21
22
23
9
15:85
24:76
15:85
9:91
10
11
12
13
PhCH]CHBrd
PhBr
PhI
PhBr
PhI
PhBr
PhI
PhBr
6ce
5d
5e
5e
5f
PhBr
5d
55:45
a
Isolated yields (combined for the E/Z isomers). Couplings were performed on
0.1–1.0 mmol scale of silanes (0.03 mM); Pd(PPh3)4 (10 mol %).
b
Determined by GC–MS and/or 1H NMR of the crude reaction mixture.
GC–MS.
E/Z, 88:12.
1E,3E/1Z,3E.
2-Iodo-5-methylthiophene.
Coupling with 1-bromonaphthalene gave 11b (51%, E/Z, 27:73).
c
2f
5f
d
2g
2g
2g
2g
5g
5g
7g
8g
e
f
(CH3)2C]CHBr
Iodothiophenec
g
Only E isomers were detected (1H NMR, GC–MS) except for the g series where
stereochemistry is not relevant. Couplings were performed on 0.1–0.5 mmol scale of
silanes (0.03 mM); Pd(PPh3)4 (10 mol %).
a
(E)-stilbene (entries 1 and 2, Table 4). Other conjugated and non-
conjugated (E)- and (Z)-silanes coupled with aryl, heterocyclic, and
aliphatic alkenyl halides (entries 3–12). Again coupling of (Z)-
silanes occurred with lower stereoselectivity to produce E/Z mix-
ture (entries 9–12). It is noteworthy that TBAF containing reactions
are, however, generally higher yielding and more stereoselective
than the fluoride-free reactions.
b
Isolated yields.
2-Iodo-5-methylthiophene.
E/Z, 88:12.
c
d
e
1E/3E.
corresponding aryl bromides (see also Table 2, entry 3 vs 4; Table 4,
entry 10 vs 11). Longer stirring of the silanes 2 and 4 with H2O2/
NaOH (45 vs 15 min) prior to the addition of the aryl halide and the
catalyst resulted in no improvement of yield and stereoselectivity.
Denmark and Tymonko have recently utilized substrates bear-
ing two distinct silyl subunits [RSiMe2OH vs RSiMe2Bn], which
required complementary activations (TMSOK vs TBAF), for the
construction of unsymmetrical disubstituted 1,4-butadienes.17
TTMS-silanes can also serve as alternative organosilane substrates
in Pd-catalyzed couplings. For example, TTMS-silane 2a remained
intact under typical conditions employed in the coupling of dime-
thylsilanols14,17 [TMSOK(2 equiv)/Pd2(dba)3/dioxane/rt/4 h] with
more than 95% of 2a being recovered after 4 h and w85% after 24 h.
This experiment demonstrated that TTMS-silanes could act as
masked silanols, which require hydrogen peroxide for initiation.
It is noteworthy that under the oxidative conditions required for
coupling of TTMS-silanes, the reductive self-coupling of the halides
has not been observed for the fluoride promoted reactions (Tables 2
and 3) and was only sporadically observed for the fluoride-free
reactions (Table 4, entries 2, 5, and 9; 1–3%, GC–MS). Moreover,
The lack of stereoselectivity for the coupling of (Z)-silanes
probably results from the isomerization of the vinyl siloxane in-
termediates derived from (Z)-TTMS-silanes under the coupling
conditions. Isomerization16 of the products was excluded based on
the following experiments: (i) no isomerization of the (Z)-stilbene
5a was observed when (Z)-5a was refluxed in THF in the presence of
H2O2/NaOH or TMSOK with or without Pd(0) and/or TBAF, (ii)
coupling of the (Z)-silane 4b under typical conditions [H2O2/NaOH/
Pd(0)THF/with or without TBAF] with phenyl iodide or bromide in
the presence of 0.25 or 1.0 equiv of the (Z)-stilbene 5a produced
product 5b as the E/Z mixture [see Table 2 (entries 3 and 4) and
Table 4 (entries 10 and 11)], while isomerization of the (Z)-stilbene
5a into E isomer was not observed (GC–MS).
A ‘side-by-side’ comparison of the coupling of (Z)-4b with 1-
iodonaphthalene [1 h (30%, E/Z, 0:100); 3 h (58%, E/Z, 3:97)] and
1-bromonaphthalene [1 h (22%, E/Z, 5:95); 3 h (35%, E/Z, 13:87)]
showed that product 11b is formed at different pace (Table 2, entry
11). It appears that coupling with the aryl iodides is faster and
occurs with a higher degree of stereoretention than with the

Z. Wang et al. / Tetrahedron 64 (2008) 5322–5327
5325
Table 4
compound (Z)-12 [(4)CH3C6H4CH]CHSi(OSiMe3)3] was isolated in
7% yield in addition to product 11b (12%). The structure of 12 was
assigned based on the HRMS and NMR spectra. Subjection of 12 to
TBAFpromotedcoupling with1-iodonaphthalene producing product
11b but in low yield (8%; GC–MS).
Fluoride-free coupling of the vinyl TTMS-silanes
H2O2/NaOH (aq)
R1
R1
R2X, Pd(PPh3)4
Si(SiMe3)3
R2
R
R
THF/55 °C/10 h
5 R2 = Ph
In summary, we demonstrated that the conjugated and un-
conjugated vinyl tris(trimethylsilyl)silanes undergo Pd-catalyzed
cross-coupling with aryl, heterocyclic, and alkenyl iodides and bro-
midesunderaqueous oxidative conditions in the presence of sodium
hydroxide with or without fluoride activation. Contrary to (E)-si-
lanes, which undergo coupling with retention of stereochemistry,
coupling of (Z)-silanes occurred with lower stereoselectivity giving
an E/Z mixture of products. The best stereoselectivity was achieved
when either aryl iodides or electron-rich TTMS-silanes were used.
Under the oxidative coupling conditions neither reductive self-
coupling of the halides nor oxidative homocoupling of the vinyl
TTMS-silanes were observed. The tris(trimethylsilyl)silanes
remained intact under typical basic conditions employed in the
coupling of dimethylsilanols, thus making stable and readily acces-
sible vinyl TTMS-silanes alternative substrates (‘masked’ silanols)
for the Hiyama coupling. Hydrogen peroxide presumably chemo-
selectively cleaves Si–Si bond(s) generating active silanol/siloxane
species that undergo coupling in the presence of base.
2(E) or 4(Z)
Series: a R = Ph, R1 = H
6 R2 = CH=CHPh
7 R2 = CH=C(CH3)2
b R = (4)CH3Ph, R1 = H
8 R2
=
c R = (4)CH3OPh, R1 = H
e R = PhCH2CH2, R1 = H
g R = R1 = -(CH2)5-
Me
S
11 R2
=
Entry
Silane
R1X
Product
Yielda (%)
E/Zb
1
2
3
4
5
6
7
8
2a
2a
2b
2b
2c
2e
2g
2g
4a
4b
4b
4b
PhI
5a
5a
8b
11b
6c
75c
50
52
46
45
56
65
42
55
61g
40h
30
100:0
100:0
100:0
100:0
100:0f
100:0
n/a
PhBr
Iodothiophened
1-Iodonaphthalene
PhCH]CHBre
PhI
PhI
5e
5g
7g
5a
5b
5b
11b
(CH3)2C]CHBr
PhBr
PhI
PhBr
n/a
9
25:75
15:85
20:80
17:83
10
11
12
1-Bromonaphthalene
3. Experimental section
3.1. General
a
Isolated yields. Couplings were performed on 0.1 mmol scale of silanes
(0.03 mM); Pd(PPh3)4 (10 mol %).
b
Determined by 1H NMR and/or GC–MS of the crude reaction mixture.
c
With KOSiMe3 instead of NaOH yield was 60% (E/Z, 100:0).
2-Iodo-5-methylthiophene.
E/Z, 88:12.
Only E,E isomer was observed.
With KOSiMe3 instead of NaOH yield was 60% (E/Z, 25:75).
With KOSiMe3 instead of NaOH yield was 48% (E/Z, 10:90).
d
1H NMR (Me4Si) spectra at 400 or 600 MHz, 13C NMR (Me4Si) at
100.6 MHz, 19F NMR (CCl3F) at 376.4 MHz and 29Si NMR (Me4Si) at
119.2 MHz were determined with solutions in CDCl3 unless other-
wise noted. Mass spectra (MS) were obtained by electron ionization
(EI) technique and HRMS were acquired by AP-ESI technique. Re-
agent grade chemicals were used and solvents were dried by reflux
over CaH2 and distillation from CaH2 under an argon atmosphere.
TLC was performed on Merck Kieselgel 60-F254 and products were
detected with 254 nm light or by development of color with I2.
Merck Kieselgel 60 (230–400 mesh) was used for column chro-
matography. Purity and identity of the products (crude and/or
purified) were also established using a GC–MS (EI) system with
a HP 5973 mass selective detector [capillary column HP-5MS
(30 mꢂ0.25 mmꢂ25 mm)]. The vinyl sulfones 1a,5a 1c,22 1e,5a 1f,5a
1g5a and silanes 2e–g,5a 4a12a were prepared as reported.
e
f
g
h
byproducts resulting from the oxidative homocoupling18 of the
vinyl silanes 2 and 4 have not been observed. Also, although the
oxidative conditions employed for generation of the active orga-
nosilane species are similar to the ones used in Tamao–Kumada and
Fleming oxidation of silanes to alcohols (including vinyl silanes to
aldehydes and ketones), which involve cleavage of the C–Si bond,19
we did not observe conversion of the vinyl silanes 2 and 4 to the
corresponding aldehydes. Apparently, Si–Si bond cleavage takes
place chemoselectively with the C–Si bond tolerating the relatively
mild oxidative conditions required for coupling.20
We have not yet had the opportunity to systematically in-
vestigate the mechanism(s) of the TTMS-silanes Pd-catalyzed
coupling but it appears that hydrogen peroxide chemoselectively
cleaves20b the Si–Si bond(s) to generate silanol species
RSi(OH)n(SiMe3)3ꢁn (n¼1, 2, or 3). Subsequently, the silanol(s) are
converted by the base to a silanolate anion, which might follow the
coupling mechanism suggested by Denmark et al. for the organo-
silanols.10 Alternatively, TTMS-silanes can be converted by hydro-
gen peroxide to siloxane species RSi(OSiMe3)n(SiMe3)3ꢁn (n¼1, 2, or
3), that can be further transformed to the reactive pentacoordinate
species (hypervalent silicate anion)3b,e,10 by fluoride or base.
In order to obtain additional mechanistic insights, we examined
the coupling reaction of 2a with iodobenzene by 29Si NMR. Thus,
treatment of 2a with hydrogen peroxide (THF-d8/NaOH/H2O) resul-
ted in the appearance ofnew peaks at9.82, 7.20, and5.59 ppm, which
are characteristic for the species having oxygen attached to sili-
con,7b,10a,20a,b,21 with concurrent disappearance of the two distinctive
peaks at ꢁ85.31 ppm (Si atom attached to sp2 carbon) and
ꢁ14.37 ppm (SiMe3) for the silicon atoms present in substrate 2a.
Addition of Pd catalyst and phenyl iodide to the resulting mixture
resulted in the formationof stilbene (E)-5a. Moreover, when coupling
of 4b with 1-iodonaphthalene under fluoride-free conditions was
quenched after 2 h, the corresponding tris(trimethylsiloxy)silyl
3.2. (E)-2-(4-Methoxyphenyl)-1-[tris(trimethylsilyl)-
silyl]ethene (2c)
3.2.1. Procedure A
Argon was bubbled through a solution of 1c22 (E; 150 mg,
0.55 mmol) in anhydrous benzene (10 mL) for 15 min at ambient
temperature. (Me3Si)3SiH (0.51 mL, 410 mg, 1.65 mmol) and AIBN
(24 mg, 0.14 mmol) were added, and deoxygenation was continued
for another 10 min. The resulting solution was refluxed at 85 ꢀC for
4 h [additional AIBN (92 mg, 0.55 mmol) in degassed benzene
(2 mL) was injected through a septum via an automatic syringe over
the 4 h period or dropwise manually]. The volatiles were evapo-
rated, and the residue was chromatographed in hexane to give 2c
(134 mg, 64%) as a colorless oil. 1H NMR d 0.25 (s, 27H), 3.84 (s, 3H),
6.29 (d, J¼18.8 Hz, 1H), 6.87 (d, J¼18.8 Hz, 1H), 6.89 (d, J¼8.8 Hz,
2H), 7.35 (d, J¼8.8 Hz, 2H); 13C NMR d 1.0, 55.4, 114.0, 119.6, 127.2,
132.4, 145.0, 159.3; MS (EI) m/z 380 (15, Mþ), 174 (100). AP-ESI-
HRMS calcd for C18H36ONaSi4 (MNaþ): 403.1735. Found: 403.1739.
Treatment of 4c (381 mg, 1 mmol) with (Me3Si)3SiH (0.15 mL,
124 mg, 0.5 mmol) in the presence of Rh catalyst, as described for
2a [GC–MS of the crude reaction mixture: 2c/4c (E/Z, 92:8; tR
22.02 min, Z; tR 22.34 min, E)], gave 2c (338 mg, 89%).

5326
Z. Wang et al. / Tetrahedron 64 (2008) 5322–5327
3.3. (Z)-2-(4-Methylphenyl)-1-[tris(trimethylsilyl)-
3.6. (E)-1-(4-Methylphenyl)-2-phenylethene (5b)
silyl]ethene (4b)
Treatment of 2b (36.5 mg, 0.10 mmol) with iodobenzene (17 mL,
31 mg, 0.15 mmol) by procedure C gave (E)-5b (14 mg, 72%) with
data as reported.23 1H NMR d 2.22 (s, 3H), 6.92 (d, J¼18.1 Hz, 1H),
6.98 (d, J¼18.1 Hz, 1H), 7.03 (d, J¼7.9 Hz, 2H), 7.13 (t, J¼7.3 Hz, 1H),
7.22 (t, J¼7.4 Hz, 2H), 7.30 (d, J¼8.1 Hz, 2H), 7.39 (d, J¼7.9 Hz, 2H);
GC–MS (tR 19.6 min) m/z 194 (100, Mþ).
3.3.1. Procedure B
(Me3Si)3SiH (0.31 mL, 248 mg,1 mmol) was added in one portion
via a syringe to a degassed solution of 3b (0.13 mL, 116 mg, 1 mmol)
in dry benzene (3 mL) at ambient temperature under nitrogen at-
mosphere. AIBN (83.8 mg, 0.50 mmol) was then added and the
resulting solution was heated (oil bath, 85 ꢀC) for 3 h or until the
alkyne was consumed (GC). The volatiles were evaporated in vacuo
and the oily residue was flash chromatographed (hexane) on silica
gel to give 4b (336 mg, 92%) as a colorless oil. 1H NMR d 0.16 (s, 27H),
2.37 (s, 3H), 5.82 (d, J¼14.5 Hz, 1H), 7.16 (d, J¼7.8 Hz, 2H), 7.22 (d,
J¼7.8 Hz, 2H), 7.40 (d, J¼14.5 Hz,1H); 13C NMR d 1.4, 21.3,123.1,128.1,
129.0,137.1,137.8,146.6; 29Si NMR d ꢁ88.33 [s, Si(SiMe3)3], ꢁ11.67 [s,
Si(SiMe3)3]; GC–MS: (tR 22.12 min) m/z 364 (6, Mþ),174 (100). HRMS
calcd for C18H36Si4 (Mþ): 364.1894. Found: 364.1896.
3.7. (E/Z)-1-(4-Methylphenyl)-2-phenylethene (5b)
Treatment of 4b (364 mg, 1.0 mmol) with iodobenzene (0.17 mL,
306 mg, 1.5 mmol) by procedure C gave 5b23b (E/Z, 3:97; 175 mg,
90%); GC–MS (tR 16.8 min, Z; tR 19.6 min, E) m/z 194 (100, Mþ).
HRMS calcd for C15H15 (MHþ): 195.1174. Found: 195.1179. (Z)-5b
had: 1H NMR d 2.20 (s, 3H), 6.43 (s, 2H), 6.92 (d, J¼7.9 Hz, 2H), 7.03
(d, J¼7.9 Hz, 2H), 7.11–7.20 (m, 5H).
Treatment of 4b (36.5 mg, 0.10 mmol) with bromobenzene
(16 mL, 23.6 mg, 0.15 mmol) by procedure C gave 5b (E/Z, 30:70;
17 mg, 86%).
3.4. (E)-1,2-Diphenylethene (5a)
3.4.1. Procedure C
Analogous treatment of 4b (36.5 mg, 0.10 mmol) with iodo-
benzene (17 mL, 31 mg, 0.15 mmol) by procedure C (without TBAF)
gave 5b (E/Z, 15:85; 12 mg, 61%). Identical coupling with bromo-
benzene (0.15 mmol) gave 5b (E/Z, 20:80; 8 mg, 40%).
Treatment of 4b (36.5 mg, 0.10 mmol) with iodobenzene (17 mL,
31 mg, 0.15 mmol) by procedure D gave 5b (E/Z, 25:75; 11.6 mg,
60%). Identical coupling with bromobenzene (0.15 mmol) gave 5b
(E/Z, 10:90; 9 mg, 48%).
Analogous treatment of 4b (36.5 mg, 0.10 mmol) with iodo-
benzene (17 mL, 31 mg, 0.15 mmol) by procedure D [with addition
of TBAF (0.3 mmol) as described in procedure C] gave 5b (E/Z, 2:98;
15.7 mg, 81%).
Analogous treatment of 4b (36.5 mg, 0.10 mmol) with iodo-
benzene by procedure C [using aqueous NaF (12.6 mg, 0.3 mmol)
instead of TBAF] gave 5b (E/Z, 40:60; 11.6 mg, 60%).
Analogous treatment of 4b (36.5 mg, 0.10 mmol) with iodo-
benzene by procedure C [using Pd2(dba)3 (9.2 mg, 0.01 mmol) in-
stead of Pd(PPh3)4 and without addition of TBAF] gave 5b (E/Z,
25:75; 10 mg, 52%).
Solution of NaOH (60 mg, 1.5 mmol) and H2O2 (30% solution,
0.15 mL, 1.5 mmol) in deionized H2O (1.5 mL) was added to a stirred
solution of 2a (175 mg, 0.5 mmol) in THF (15 mL) at ambient tem-
perature. After 15 min, iodobenzene (84 mL, 153 mg, 0.75 mmol),
Pd(PPh3)4 (58 mg, 0.05 mmol), and tetrabutylammonium fluoride
(1 M/THF, 1.5 mL,1.5 mmol) were added and the resulting brownish
mixture was heated at 55 ꢀC (oil bath) for 10 h. The volatiles were
evaporated and the residue was partitioned (H2O/CHCl3). The ali-
quot of the organic layer was subjected to GC–MS and/or 1H NMR
analysis in order to establish the overall stereochemistry. The or-
ganic layer was dried (MgSO4), evaporated, and column chroma-
tographed (hexane) to give (E)-5a (74 mg, 83%) with data identical
to commercial sample; GC–MS (tR 17.9 min, E) m/z 180 (100, Mþ).
Treatment of 2a (35 mg, 0.10 mmol) with bromobenzene (16 mL,
23.6 mg, 0.15 mmol) by procedure C gave (E)-5a (12 mg, 67%).
Analogous treatment of 2a (35 mg, 0.10 mmol) with iodo-
benzene (17 mL, 31 mg, 0.15 mmol) by procedure C (without TBAF)
gave (E)-5a (13.5 mg, 75%).
Analogous treatment of 2a (35 mg, 0.10 mmol) with bromo-
benzene (16 mL, 23.6 mg, 0.15 mmol) by procedure C (without TBAF)
gave (E)-5a (9 mg, 50%). Also, biphenyl (1%; e.g., 2% consumption
of bromobenzene) was detected; GC–MS (tR 11.3 min) m/z
154 (100, Mþ).
3.8. (Z)-2-(4-Methylphenyl)-1-[tris(trimethylsiloxy)-
silyl]ethene (12)
3.4.2. Procedure D
Treatment of 4b (50 mg, 0.14 mmol) with 1-iodonaphthalene
(22 mL, 35 mg, 0.14 mmol) by procedure C [without TBAF, 2 h, NaOH
(5 equiv)] and column chromatography (hexane) gave 12 (4 mg, 7%)
and 11b (E/Z, 7:93; 4 mg, 12%). Compound 12 had: 1H NMR d 0.07
(br s, 27H), 2.36 (s, 3H), 5.50 (d, J¼15.5 Hz, 1H), 7.12 (d, J¼7.9 Hz,
2H), 7.21 (d, J¼15.5 Hz, 1H) 7.44 (d, J¼8.0 Hz, 2H); GC–MS (tR
17.60 min) m/z 412 (6, Mþ), 175 (100). HRMS calcd for C18H37O3Si4
(MHþ): 413.1814. Found: 413.1823.
KOSiMe3 (38.5 mg, 0.3 mmol) and H2O2 (30% solution, 31 mL,
0.30 mmol) were added to a stirred solution of 2a (35 mg,
0.10 mmol) in THF (3 mL) at ambient temperature. After 20 min,
iodobenzene (17 mL, 31 mg, 0.15 mmol) and Pd(PPh3)4 (11 mg,
0.01 mmol) were added and the resulting mixture was heated at
55 ꢀC (oil bath) for 10 h. Aqueous work-up and purification as
described in procedure C gave (E)-5a (11 mg, 60%).
3.5. (E/Z)-1,2-Diphenylethene (5a)
Acknowledgements
Treatment of 4a12a (35 mg, 0.10 mmol) with bromobenzene
(16 mL, 23.6 mg, 0.15 mmol) by procedure C gave 5a (E/Z, 40:60;
15 mg, 82%) with data identical to commercial sample; GC–MS (tR
15.1 min, Z; tR 17.9 min, E) m/z 180 (100, Mþ). HRMS calcd for C14H13
(MHþ): 181.1073. Found: 181.1079.
Analogous treatment of 4a (35 mg, 0.10 mmol) with bromo-
benzene (16 mL, 23.6 mg, 0.15 mmol) by procedure C (without
TBAF) gave 5a (E/Z, 25:75; 10 mg, 55%). Also, biphenyl (3%; e.g., 6%
consumption of bromobenzene) was detected (GC–MS).
We thank NIH/NIGMS program (S06GM08205) for supporting
this research. L.M. and D.D. were sponsored by the MBRS RISE pro-
gram (NIH/NIGMS;R25 GM61347). ThesupportofUSARO (W911NF-
04-1-0022) for the purchase of 600 MHz NMR spectrometer is
deeply acknowledged. We thank Dr. Donald V. Eldred (Dow Corning
Co., Midland, MI) and Dr. Vered Marks (FIU) for assistance with 29Si
NMR and Myron Georgiadis from the Advanced Mass Spectrometry
Facility at FIU for his assistance with GC–MS.

Z. Wang et al. / Tetrahedron 64 (2008) 5322–5327
5327
10. (a) Denmark, S. E.; Sweis, R. F.; Wehrli, D. J. Am. Chem. Soc. 2004, 126, 4865–
4875; (b) Denmark, S. E.; Sweis, R. F. J. Am. Chem. Soc. 2004, 126, 4876–4882; (c)
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Supplementary data
Experimental procedures and characterization data for com-
pounds 1d, 2a, 2b, 2d, 4c, 4d, 5c–g, 6b, 6c, 7a, 7b, 7g, 8a, 8b, 8g, 9a,
10b, and 11b. Supplementary data associated with this article can
be found in the online version, at doi:10.1016/j.tet.2008.03.024.
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