Palladium-catalyzed silyl C(sp3)-H bond activation

Article abstract of DOI:10.1039/c2ob06941e

The first transition-metal-catalyzed activation of silyl C(sp ³)-H bond was realized and synthetically applied. A variety of organic skeletons substituted with SiMe₃ groups could undergo the Pd-catalyzed intramolecular coupling react

Full text of DOI:10.1039/c2ob06941e

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PAPER
Palladium-catalyzed silyl C(sp3)–H bond activation†
Yun Liang,a,b Weizhi Geng,a Junnian Wei,a Kunbing Ouyanga and Zhenfeng Xi*a,c
Received 18th October 2011, Accepted 1st December 2011
DOI: 10.1039/c2ob06941e
The first transition-metal-catalyzed activation of silyl C(sp3)–H bond was realized and synthetically
applied. Avariety of organic skeletons substituted with SiMe3 groups could undergo the Pd-catalyzed
intramolecular coupling reaction, resulting in an unprecedented synthetic method for yielding six-
membered silacycles. It was found that the adjacent Si atom played an essential role for the activation of
the C(sp3)–H bond of the SiMe3 group; no activation reaction of the C(sp3)–H bond of the CMe3 group
took place under the same reaction conditions.
bonds (Scheme 1). A wide variety of organic skeletons substi-
Introduction
tuted with a SiMe3 group could undergo a Pd-catalyzed cleavage
of the silyl C(sp3)–H bond, providing an unprecedented syn-
thetic method for yielding six-membered silacycles, which are
not readily accessible via known methods.12
Transition-metal-catalyzed activation and synthetic application
of C(sp3)–H bond is a great challenge and has significant syn-
thetic potential.1,2 Silyl C(sp3)–H bonds in trialkylsilyl groups
such as SiMe3 are among the most common C(sp3)–H bonds
because many compounds are substituted with trialkylsilyl
groups. Although remarkable achievements have been realized
on the activation and synthetic application of tertiary C(sp3)–H
bonds,3 the activation of C(sp3)–H bonds adjacent to a functional
group, such as oxime,4 oxazolne,5 pyridine,6 amide,7 carboxyl,8
and C(sp3)–H bonds adjacent to hetero-atoms, such as N,9 O,10
S,11 is unknown in the literature and remains a great challenge.
In this paper, we report the first example of transition-metal-cata-
lyzed activation and synthetic application of silyl C(sp3)–H
Results and discussion
Initially, we tested the compound 1a possessing a SiMe3 substi-
tuent as a model example (Table 1). After screening various reac-
tion conditions, an optimal reaction condition was realized:
Table 1 Optimization of reaction conditions (see ESI† for details)
Scheme 1 Silyl C(sp3)–H bond cleavage and intramolecular silyl
C(sp3)–C(sp2) coupling leading to six-membered silacycles.
Yield of
Entry [Pd]
Ligand Base
Solvent
2a (%)a
1
2
3
4
5
6
7
8
Pd(OAc)2
Pd2(dba)3
[PdCl(π-allyl)]2 Pt-Bu3
Pt-Bu3
Pt-Bu3
NaOt-Bu Dioxane 45
NaOt-Bu Dioxane 13
NaOt-Bu Dioxane 45
NaOt-Bu Dioxane 63
NaOt-Bu Dioxane 75 (64)
NaOt-Bu Dioxane 41
NaOt-Bu Dioxane 28
aBeijing National Laboratory for Molecular Sciences, MOE Key
Laboratory of Bioorganic Chemistry and Molecular Engineering, College
of Chemistry, Peking University, Beijing, 100871, China. E-mail:
zfxi@pku.edu.cn; Fax: +86-10-62751708; Tel: +86-10-62759728
bKey Laboratory of Resource Fine-Processing and Advanced Materials
of Hunan Province and MOE Key Laboratory of Chemical Biology and
Traditional Chinese Medicine Research, College of Chemistry and
Chemical Engineering, Hunan Normal University, Changsha, 410081,
China
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pt-Bu3
PCy3
PPh3
Pt-Bu3
Pt-Bu3
Pt-Bu3
Pt-Bu3
Pt-Bu3
Pt-Bu3
Pt-Bu3
Pt-Bu3
LiOt-Bu
KOt-Bu
K2CO3
Cs2CO3
CsF
Dioxane
9
9
Dioxane trace
Dioxane trace
Dioxane trace
Dioxane trace
Dioxane trace
10
11
12
13
14
15
cState Key Laboratory of Organometallic Chemistry, SIOC, CAS,
Shanghai, 200032, China
KOH
NaOt-Bu Toluene
NaOt-Bu Toluene
77 (70)
83 (74)b
†Electronic supplementary information available: Materials including
experimental procedures, NMR spectra of all new products and X-ray
data for 6e, 8a. CCDC reference numbers 823538 and 824123. For elec-
tronic supplementary information and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c2ob06941e
a GC yields. Isolated yields are given in parenthesis. b 20 mol% of
TBAB was added.
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Table 2 Formation of benzo- and dibenzo-fused six-membered
silacycles
Table 3 Formation of heteroarene-fused silacycles 6 and 8 from their
corresponding Me3Si-substituted compounds 5 and 7
Pd(PPh3)4 (5 mol%), Pt-Bu3 (10 mol%), TBAB (20 mol%),
NaOt-Bu (3 equiv), in toluene, 120 °C, 12 h. Notably, in
addition to the remarkable effect of different catalysts and
ligands, the base was found to be very sensitive for this trans-
formation. NaOt-Bu was found to be the most effective base,
while most other bases such as LiOt-Bu, KOt-Bu, K2CO3,
Cs2CO3, CsF, KOH afforded very low yields of products, or no
products at all (see ESI† for details). With the optimized reaction
condition, the dibenzo-fused six-membered silacycle 2a was
obtained in 74% isolated yield (entry 15, Table 1).
corresponding siloles formed, via cleavage of the Si–Me
bond,13,14 and de-brominated products were detected by
GC-MS.
As further demonstration of the usefulness of the above-men-
tioned reaction, we subjected Me3Si-substituted-heteroarenes 5
and 7 to the reaction conditions, expecting the formation of
complex structures containing six-membered silacyclic skele-
tons. As shown in Table 3, a variety of silyl[2,3-b]indoles 6
could be obtained in good isolated yields from their correspond-
ing 3-(2-bromoaryl)-2-(trialkylsilyl)-1H-indoles 5 under the
optimal reaction condition via the silyl C(sp3)–H bond cleavage
and consequent intramolecular silyl C(sp3)–C(aryl sp2) coupling.
The substituent of the aryl moiety could be H (6a), Me (6b), F
(6c) and Cl (6d) groups, and the yield of their corresponding
products was 91%, 86%, 80%, 61%, respectively. The N-substi-
tuent of the indole moiety could be aryl (6a–h), benzyl (6i, 6j),
and an alkyl (6k) groups. The N-aryl could be substituted with
both electron-donating groups such as OMe, Me (6e–g) and
electron-withdrawing group such as CN (6h). The structure of
product 6e was determined by single-crystal X-ray structural
analysis (Fig. 1, on the left).
Similarly, as given in Table 2, the substituted derivatives 1b
and 1c could afford their corresponding dibenzo-fused silacycles
2b and 2c in moderate yields.
Investigation into the effect of substituents on the silicon
center disclosed, from which, as demonstrated in the cases of 1d
(SiMe2Et), 1e (SiMe2i-Pr) and 1f (SiMe2Ph), the methyl C(sp3)–
H bond was selectively cleaved to afford their corresponding
silacycles 2d, 2e, and 2f in good isolated yields (Table 2). The
reaction of 1g (SiEt3) is of particular interest, affording the
methylated six-membered silacycle 2g in 41% isolated yield. In
this case, the methylene C(sp3)–H bond was selectively cleaved;
The methyl C(sp3)–H bond was not involved.
The above silyl C(sp3)–H bond cleavage and consequent intra-
molecular silyl C(sp3)–C(aryl sp2) coupling was also success-
fully applied to the vinylic bromide derivatives 3a and 3b
(Table 2). Their corresponding benzo-fused silacycles 4a and 4b
were obtained in 46% and 76% isolated yields, respectively.
It should be mentioned that, as given in Table 2, in most cases
no Si–Me bond cleavage was observed.13,14 For the cases of 1g
and the vinylic bromide derivatives 3, small amounts of their
Under the optimal reaction condition, the benzo[e][1,3]azasi-
lanes 8a, 8b, and 8c (Table 3) could be also efficiently obtained
in 57%, 84%, 68% isolated yields, respectively, from their corre-
sponding indole derivatives, 7. The structure of product 8a was
determined by single-crystal X-ray structural analysis and is
given in Fig. 1.
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Fig. 1 ORTEP drawings of 6e (left) and 8a (right) with 30% thermal
ellipsoids. Selected bond lengths [Å] for 6e: Si(1)–C(3) 1.851(4), Si(1)–
C(11) 1.847(4), C(3)–C(4) 1.500(5), C(10)–C(11) 1.376(5), C(11)–N(1)
1.396(4); for 8a: Si(1)–C(8) 1.869(2), Si(1)–C(15) 1.872(2), C(7)–C(8)
1.380(3), C(8)–N(1) 1.415(2), C(14)–C(15) 1.496(3).
Scheme 3 A proposed reaction mechanism for the formation of the
six-membered silacycles, 2, from 1.
Based on all the above preliminary experimental results, a pro-
posed catalytic cycle for the formation of the six-membered sila-
cycles 2 from 1 is given in Scheme 3. A seven-membered
palladacycle intermediate, 12, might be formed via the NaOt-
Bu-mediated proton abstraction of 11. This process might be
facilitated by the α-carboanion stabilizing effect of the Si atom15
or by the hypervalent Si enter formation.16 The palladacycle, 12,
would then give rise to the six-membered silacycle derivative, 2,
via reductive elimination.
Conclusions
In summary, the first Pd-catalyzed selective cleavage of the silyl
C(sp3)–H bond in a SiMe3 group and consequent intramolecular
silyl C(sp3)–C(sp2) bond forming process was developed. It was
found that the silicon atom played a key role in the activation of
the silyl C(sp3)–H bond, making the SiMe3 group remarkably
different from the CMe3 group. This finding would open a new
way for the selective cleavage of the silyl C(sp3)–H bond in a
trialkylsilyl group and for the synthesis of silacycles.
Scheme 2 Selective activation of the silyl C(sp3)–H bond vs selective
activation of the C(sp3)–Si bond of a SiMe3 group. No reaction took
place in the case of the Me3C-substituted compound.10.
The reaction mechanism for the selective cleavage of the silyl
C(sp3)–H bond is intriguing. It is interesting to compare the
selective activation of the silyl C(sp3)–H bond described in this
work with the selective activation of the Si–Me bond of the
SiMe3 group reported previously by us13 and by others.14 As
shown in Scheme 2, the major differences of these two reactions
are the Pd catalysts and the bases used.13 Although it is fre-
quently reported in the literature that NaOt-Bu and KOt-Bu
demonstrate remarkably different effects from those of LiOt-Bu,
it is not clear yet what the major reason for this is. In our case,
no remarkable change was observed when 1a was treated in
toluene with 3 equivalents of LiOt-Bu or NaOt-Bu, even at
120 °C, as followed by in situ NMR spectra. Furthermore, the
addition of an aldehyde did not show remarkable effect on the
selective activation of the silyl C(sp3)–H bond.13
Experimental
General information
Unless otherwise noted, all starting materials are commercially
available and were used without further purification. Solvents
were purified using an M-Braun SPS-800 Solvent Purification
System. n-BuLi and PhLi were obtained from Acros. All reac-
tions were carried out under a dry and oxygen-free nitrogen
atmosphere in slight positive pressure by using Schlenk tech-
niques. 1H and 13C NMR spectra were recorded on a JEOL
1
JNM-AL300 spectrometer (FT, 300 MHz for H; 75 MHz for
13C), or a Bruker ARX400 spectrometer (FT, 400 MHz for
1H; 100 MHz for 13C) at room temperature. High-resolution
mass spectra (HRMS) were recorded on a Bruker Apex IV
FTMS mass spectrometer using electrospray ionization (ESI).
There are many examples of the activation of tertiary C(sp3)–
H bonds in the literature.3 In order to know if the silicon atom
plays a key role in the activation of the C(sp3)–H bond of the
SiMe3 group, the analogue compound 10, with a CMe3 substitu-
ent instead of the SiMe3 group, was synthesized and evaluated
(Scheme 2). Experimental results showed that no reaction took
place when the compound 10 was subjected to the reaction con-
ditions described in Scheme 2. Most of the starting compound,
10, was recovered. These results indicate that the silyl group is
not innocent and not equivalent to simple alkyl groups.
Typical procedure for the preparation and characterization of
2, 4, 6, 8. Under the protection of nitrogen, Pd(PPh3)4 (5 mol%),
Pt-Bu3 (10 mol%) and NaOt-Bu (0.9 mmol), were added to
2 mL of toluene. After this, reaction mixture was stirred at room
temperature for 15 min, 1, 3, 5, or 7 (0.3 mmol), TBAB (20 mol
%) were added and this reaction mixture was stirred at 120 °C
for 12 h. The reaction mixture was quenched with water and
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extracted with Et2O. The extraction was washed with brine and
dried over MgSO4. The solvent was then evaporated in vacuo
and the residue was purified by using silicon gel column with
petroleum ether and ethyl acetate as eluent, to afford the final
products 2, 4, 6, or 8.
(d, J = 7.8 Hz, 3H), 0.81–0.65 (m, 4H); 13C NMR (75 MHz,
CDCl3) δ: 0.91, 2.21, 7.35, 7.42, 14.65, 21.82, 126.05, 126.08,
126.46, 127.74, 128.30, 128.67, 130.13, 132.80, 133.62, 137.46,
141.95, 144.73; HRMS (ESI, m/z) calcd for [C18H22Si]H+:
267.1564; found 267.1558.
2a: Colorless oil, isolated yield 74% (50 mg); 1H NMR
(400 MHz, CDCl3) δ: 7.68 (d, J = 8.0 Hz, 1H), 7.61 (d, J = 7.6
Hz, 1H), 7.54 (d, J = 8.4 Hz, 1H), 7.47 (t, J = 7.6 Hz, 1H), 7.30
(t, J = 7.2 Hz, 1H), 7.26–7.16 (m, 3H), 2.17 (s, 2H), 0.21 (s,
6H); 13C NMR (75 MHz, CDCl3) δ: −4.35, 21.25, 126.11 (2C),
126.56, 127.45, 127.93, 130.19, 131.21, 132.52, 135.48, 136.17,
137.55, 144.44; HRMS (ESI, m/z) calcd for [C15H16Si]H+:
225.1094; found 225.1093.
4a: Colourless oil, isolated yield 46% (36 mg); 1H NMR
(400 MHz, CDCl3) δ: 7.43 (d, J = 7.2 Hz, 1H), 7.37–7.35 (m,
2H), 7.20–7.17 (m, 1H), 2.23 (s, 3H), 1.52 (s, 2H), 0.23 (s, 9H),
0.21 (s, 6H); 13C NMR (75 MHz, CDCl3) δ: −4.46, 0.66, 17.07,
23.01, 124.84, 126.02, 129.53, 131.73, 133.07, 135.61, 141.17,
145.91; HRMS (ESI, m/z) calcd for [C15H24Si2]H+: 259.1333;
found 259.1330.
4b: Colourless oil, isolated yield 76% (69 mg); 1H NMR
(400 MHz, CDCl3) δ: 7.44 (d, J = 6.8 Hz, 1H), 7.37–7.35 (m,
2H), 7.20–7.16 (m, 1H), 2.66 (t, J = 7.4 Hz, 2H), 1.48 (s, 2H),
1.32–1.26 (m, 4H), 0.86 (t, J = 6.8 Hz, 3H), 0.23 (s, 9H), 0.21
(s, 6H); 13C NMR (75 MHz, CDCl3) δ: −4.61, 0.88, 14.08,
16.99, 22.91, 32.49, 35.26, 124.84, 125.90, 129.47, 131.91,
134.81, 136.76, 144.15, 146.87; HRMS (ESI, m/z) calcd for
[C18H30Si2]H+: 303.1959; found 303.1967.
2b: Colourless solid, isolated yield 58% (49 mg); mp:
94.2–94.8 °C; 1H NMR (400 MHz, CDCl3) δ: 7.48 (s, 1H), 7.37
(s, 1H), 7.27 (s, 1H), 6.94 (s, 1H), 2.32 (s, 3H), 2.29 (s, 3H),
2.27 (s, 3H), 2.23 (s, 3H), 2.06 (s, 2H), 0.19 (s, 6H); 13C NMR
(75 MHz, CDCl3) δ: −4.09, 19.26, 19.31, 19.45, 20.12, 20.60,
127.12, 128.80, 132.29, 132.70, 133.07, 133.78, 133.88, 134.52,
134.96, 135.32, 138.48, 142.32; HRMS (ESI, m/z) calcd for
[C19H24Si]H+: 281.1720; found 281.1716.
6a: Pale yellow solid, isolated yield 91% (93 mg); mp:
1
2c: Colourless solid, isolated yield 42% (43 mg); mp:
110.5–111.6 °C; H NMR (400 MHz, CDCl3) δ: 8.18–8.15 (m,
1
155.2–155.8 °C; H NMR (400 MHz, CDCl3) δ: 7.16 (s, 1H),
1H), 7.99 (d, J = 7.6 Hz, 1H), 7.50–7.46 (m, 2H), 7.42–7.38 (m,
3H), 7.34–7.32 (m, 1H), 7.29–7.20 (m, 4H), 7.08 (t, J = 7.4 Hz,
1H), 2.20 (s, 2H), −0.04 (s, 6H); 13C NMR (75 MHz, CDCl3) δ:
−3.55, 22.64, 110.63, 120.42, 120.54, 122.79, 125.01, 125.53,
125.70, 126.15, 126.29, 127.21, 127.72, 129.34, 131.07, 134.49,
134.60, 138.05, 140.26, 140.34; HRMS (ESI, m/z) calcd for
[C23H21NSi]H+: 340.1516; found 340.1520.
7.09 (s, 1H), 6.99 (s, 1H), 6.71 (s, 1H), 3.97 (s, 3H), 3.94 (s,
3H), 3.93 (s, 3H), 3.91 (s, 3H), 2.06 (s, 2H), 0.21 (s, 6H); 13C
NMR (75 MHz, CDCl3) δ: −4.16, 20.70, 55.78, 55.89, 55.95,
56.19, 109.63, 111.31, 114.43, 114.82, 126.46, 128.35, 129.77,
137.95, 147.05, 147.51, 147.71, 150.51; HRMS (ESI, m/z) calcd
for [C19H24O4Si]H+: 345.1517; found 345.1522.
2d: Colourless oil, isolated yield 69% (49 mg); 1H NMR
(400 MHz, CDCl3) δ: 7.68 (d, J = 7.6 Hz, 1H), 7.58 (d, J = 8.0
Hz, 1H), 7.51 (d, J = 7.2 Hz, 1H), 7.45 (t, J = 7.8 Hz, 1H), 7.28
(t, J = 7.2 Hz, 1H), 7.24–7.13 (m, 3H), 2.20–2.12 (m, 2H), 0.89
(t, J = 7.8 Hz, 3H), 0.70–0.64 (m, 2H), 0.21 (s, 3H); 13C NMR
(75 MHz, CDCl3) δ: −6.46, 4.37, 7.33, 19.50, 126.09, 126.14,
126.47, 127.43, 127.94, 130.17, 131.16, 132.81, 134.66, 136.18,
137.68, 144.70; HRMS (ESI, m/z) calcd for [C16H18Si]H+:
239.1251; found 239.1247.
6b: Colourless solid, isolated yield 86% (91 mg); mp:
133.8–134.9 °C; H NMR (400 MHz, CDCl3) δ: 8.16–8.13 (m,
1
1H), 7.88 (d, J = 7.6 Hz, 1H), 7.48 (t, J = 7.4 Hz, 2H),
7.42–7.40 (m, 3H), 7.33–7.31 (m, 1H), 7.21–7.19 (m, 2H), 7.09
(d, J = 7.6 Hz, 2H), 2.33 (s, 3H), 2.16 (s, 2H), −0.04 (s, 6H);
13C NMR (75 MHz, CDCl3) δ: −3.48, 21.06, 22.56, 110.58,
120.41, 120.45, 122.71, 125.58, 126.17, 126.33, 127.20, 127.64,
128.36, 129.32, 131.71, 132.00, 134.38, 134.45, 137.57, 140.29,
140.32; HRMS (ESI, m/z) calcd for [C24H24NSi]H+: 354.1673;
found 354.1676.
6c: Colourless solid, isolated yield 80% (86 mg); mp:
155.2–156.0 °C; 1H NMR (400 MHz, CDCl3) δ: 8.09 (d, J = 9.2
Hz, 1H), 7.92 (d, J = 8.4 Hz, 1H), 7.51 (t, J = 7.2 Hz, 2H),
7.45–7.41 (m, 3H), 7.35–7.31 (m, 1H), 7.23–7.21 (m, 2H), 6.98
(d, J = 8.8 Hz, 2H), 2.18 (s, 2H), −0.03 (s, 6H); 13C NMR
(75 MHz, CDCl3) δ: −3.63, 22.98, 110.72, 112.14 (d, J = 20.4
Hz), 117.75 (d, J = 20.4 Hz), 120.10, 120.63, 122.93, 124.80,
125.96, 127.18, 127.32 (d, J = 32.1 Hz), 127.8, 129.41, 130.67
(d, J = 3.1 Hz), 137.21 (d, J = 7.4 Hz), 137.44, 140.19, 140.26,
160.24 (d, J = 242.3 Hz); HRMS (ESI, m/z) calcd for
[C23H21FNSi]H+: 358.1422; found 358.1429.
2e: Colourless oil, isolated yield 79% (60 mg); 1H NMR
(400 MHz, CDCl3) δ: 7.69 (d, J = 8.0 Hz, 1H), 7.57 (d, J = 8.0
Hz, 1H), 7.52 (d, J = 7.2 Hz, 1H), 7.46 (t, J = 7.8 Hz, 1H), 7.28
(t, J = 7.4 Hz, 1H), 7.23–7.14 (m, 3H), 2.22–2.14 (m, 2H),
0.90–0.87 (m, 7H), 0.23 (s, 3H); 13C NMR (75 MHz, CDCl3) δ:
−8.21, 11.37, 17.39, 17.54, 18.64, 126.09, 126.16, 126.42,
127.46, 127.96, 130.14, 131.08, 133.22, 134.21, 136.22, 137.87,
144.90; HRMS (ESI, m/z) calcd for [C17H20Si]H+: 253.1407;
found 253.1407.
2f: Colourless oil, isolated yield 53% (46 mg); 1H NMR
(400 MHz, CDCl3) δ: 7.74 (d, J = 7.6 Hz, 1H), 7.63 (d, J = 8.0
Hz, 1H), 7.50–7.43 (m, 4H), 7.35–7.14 (m, 7H), 2.54–2.32 (m,
2H), 0.51 (s, 3H); 13C NMR (75 MHz, CDCl3) δ: −5.90, 20.55,
126.17, 126.34, 126.62, 127.61, 127.82, 127.98, 129.55, 130.51,
131.45, 133.63, 133.77, 134.44, 135.22, 135.49, 137.67, 144.90;
HRMS (ESI, m/z) calcd for [C20H18Si]H+: 287.1251; found
287.1251.
6d: Pale yellow solid, isolated yield 61% (68 mg); mp:
1
156.7–158.3 °C; H NMR (400 MHz, CDCl3) δ: 8.10–8.08 (m,
1H), 7.89 (d, J = 9.2 Hz, 1H), 7.53 (t, J = 7.4 Hz, 2H),
7.48–7.42 (m, 3H), 7.35–7.33 (m, 1H), 7.26–7.22 (m, 4H), 2.18
(s, 2H), −0.02 (s, 6H); 13C NMR (75 MHz, CDCl3) δ: −3.60,
22.70, 110.79, 120.15, 120.77, 123.02, 124.59, 125.68, 125.95,
127.27 (2C), 127.93, 129.45, 130.00, 130.77, 133.19, 136.67,
138.11, 140.15, 140.40; HRMS (ESI, m/z) calcd for
[C23H21ClNSi]H+: 374.1126; found 374.1121.
2g: Colourless oil, isolated yield 41% (33 mg); 1H NMR
(400 MHz, CDCl3) δ: 7.67 (d, J = 7.6 Hz, 1H), 7.56 (d, J = 9.6
Hz, 1H), 7.50–7.43 (m, 2H), 7.29–7.21 (m, 4H), 2.34–2.28 (m,
1H), 1.21 (d, J = 7.2 Hz, 3H), 0.97 (t, J = 7.8 Hz, 3H), 0.85
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6e: Colourless solid, isolated yield 88% (93 mg); mp:
165.8–167.0 °C; 1H NMR (400 MHz, CDCl3) δ: 8.18 (d, J = 8.8
Hz, 1H), 8.01 (d, J = 8.4 Hz, 1H), 7.34–7.27 (m, 7H), 7.23 (t,
J = 5.4 Hz, 2H), 7.11 (t, J = 7.4 Hz, 1H), 2.46 (s, 3H), 2.23 (s,
2H), 0.03 (s, 6H); 13C NMR (75 MHz, CDCl3) δ: −3.47, 21.19,
22.67, 110.71, 120.39, 120.43, 122.68, 124.93, 125.24, 125.68,
126.06, 126.26, 127.04, 129.92, 131.08, 134.50, 134.71, 137.63,
138.19, 140.49; HRMS (ESI, m/z) calcd for [C24H23NSi]H+:
354.1673; found 354.1675.
6f: Pale yellow solid, isolated yield 85% (94 mg); mp:
118.2–119.1 °C; 1H NMR (400 MHz, CDCl3) δ: 8.16 (d, J = 6.8
Hz, 1H), 8.02 (d, J = 7.6 Hz, 1H), 7.40 (t, J = 8.0 Hz, 1H),
7.30–7.22 (m, 3H), 7.20–7.17 (m, 2H), 7.08–7.01 (m, 4H), 3.66
(s, 3H), 2.25–2.16 (m, 2H), −0.04 (s, 3H), −0.13 (s, 3H); 13C
NMR (75 MHz, CDCl3) δ: −4.56, −3.56, 22.66, 55.42, 110.98,
111.89, 120.22, 120.33, 120.60, 122.45, 124.74, 124.91, 125.61,
125.79, 126.15, 128.54, 129.64, 129.94, 131.09, 134.54, 134.88,
138.77, 140.83, 155.80; HRMS (ESI, m/z) calcd for
[C24H23NOSi]H+: 370.1622; found 370.1623.
Hz, 1H), 7.37 (d, J = 8.4 Hz, 1H), 7.25–7.21 (m, 3H), 7.16 (t,
J = 7.6 Hz, 1H), 7.06 (t, J = 7.4 Hz, 1H), 4.01 (d, J = 7.6 Hz,
2H), 2.28–2.17 (m, 3H), 0.93 (d, J = 6.4 Hz, 6H), 0.36 (s, 6H);
13C NMR (75 MHz, CDCl3) δ: −2.75, 20.41, 22.64, 30.30,
53.97, 110.23, 119.76, 120.56, 122.15, 124.14, 124.67, 125.67,
125.98, 126.25, 130.88, 134.12, 134.85, 137.14, 139.90; HRMS
(ESI, m/z) calcd for [C21H26NSi]H+: 320.1829; found 320.1831.
8a: Yellow solid, isolated yield 57% (58 mg); mp:
129.1–130.2 °C; 1H NMR (400 MHz, CDCl3) δ: 7.88 (d, J = 8.4
Hz, 1H), 7.78 (d, J = 7.6 Hz, 1H), 7.75 (d, J = 8.0 Hz, 1H), 7.54
(d, J = 6.8 Hz, 2H), 7.44 (t, J = 7.6 Hz, 2H), 7.35 (t, J = 7.4 Hz,
1H), 7.30–7.26 (m, 3H), 7.17 (t, J = 7.0 Hz, 1H), 7.12 (t, J = 7.4
Hz, 1H), 2.13 (s, 2H), 0.04 (s, 6H); 13C NMR (75 MHz, CDCl3)
δ: −3.37, 20.36, 111.76, 119.84, 120.60, 121.52, 123.16,
124.83, 125.98, 126.81, 128.34, 128.53, 128.67, 129.76, 130.44,
131.56, 134.95, 136.01, 137.67, 137.83; HRMS (ESI, m/z) calcd
for [C23H22NSi]H+: 340.1516; found 340.1519.
8b: Pale yellow oil, isolated yield 84% (89 mg); 1H NMR
(400 MHz, CDCl3) δ: 7.86 (d, J = 8.4 Hz, 1H), 7.78 (d, J = 7.6
Hz, 1H), 7.63 (d, J = 8.0 Hz, 1H), 7.54 (d, J = 7.6 Hz, 2H), 7.44
(t, J = 7.6 Hz, 2H), 7.36 (d, J = 7.6 Hz, 1H), 7.28 (t, J = 7.0 Hz,
1H), 7.16 (t, J = 7.4 Hz, 1H), 7.11 (s, 1H), 7.08 (d, J = 8.0 Hz,
1H), 2.36 (s, 3H), 2.09 (s, 2H), 0.04 (s, 6H); 13C NMR
(75 MHz, CDCl3) δ: −3.33, 20.26, 20.89, 111.73, 119.79,
120.42, 121.35, 123.05, 126.52, 126.75, 128.25, 128.32, 128.51,
129.78, 130.17, 132.21, 134.35, 135.01, 135.39, 136.11, 137.65;
HRMS (ESI, m/z) calcd for [C24H24NSi]H+: 354.1673; found
354.1675.
6g: Yellow oil, isolated yield 87% (104 mg); 1H NMR
(400 MHz, CDCl3) δ: 8.15 (d, J = 8.8 Hz, 1H), 7.97 (d, J = 7.6
Hz, 1H), 7.45–7.42 (m, 1H), 7.29–7.20 (m, 4H), 7.08 (d, J = 7.4
Hz, 1H), 6.61 (s, 2H), 6.54 (s, 1H), 3.79 (s, 6H), 2.21 (s, 2H),
0.06 (s, 6H); 13C NMR (75 MHz, CDCl3) δ: −3.49, 22.75,
55.53, 99.91, 105.44 (2C), 110.86, 120.43, 120.59, 122.85,
125.07, 125.58, 125.70, 126.21, 126.32, 131.07, 134.56, 137.80,
140.08, 141.92, 161.21; HRMS (ESI, m/z) calcd for
[C25H25NO2Si]H+: 400.1727; found 400.1728.
6h: Colourless solid, isolated yield 71% (78 mg); mp:
238.9–239.6 °C; H NMR (400 MHz, CDCl3) δ: 8.17–8.14 (m,
8c: Yellow solid, isolated yield 68% (73 mg); mp:
135.6–136.2 °C; H NMR (400 MHz, CDCl3) δ: 7.82–7.77 (m,
1
1
1H), 7.94 (d, J = 7.6 Hz, 1H), 7.83 (d, J = 8.8 Hz, 2H), 7.57 (d,
J = 8.4 Hz, 2H), 7.39–7.35 (m, 1H), 7.32–7.25 (m, 4H), 7.14 (t,
J = 7.4 Hz, 1H), 2.21 (s, 2H), 0.02 (s, 6H); 13C NMR (75 MHz,
CDCl3) δ: −3.36, 22.42, 110.13, 110.92, 118.17, 120.71,
121.32, 123.51, 125.59, 125.83, 126.48, 126.75, 127.38, 127.44,
131.08, 133.43, 133.91, 134.41, 137.14, 139.65, 144.33; HRMS
(ESI, m/z) calcd for [C24H20N2Si]H+: 365.1469; found
365.1469.
2H), 7.70–7.67 (m, 1H), 7.53 (d, J = 8.0 Hz, 2H), 7.45 (t, J =
7.6 Hz, 2H), 7.37 (d, J = 7.6 Hz, 1H), 7.29 (t, J = 7.6 Hz, 1H),
7.22–7.16 (m, 1H), 7.03–6.95 (m, 2H), 2.12 (s, 2H), 0.05 (s,
6H); 13C NMR (75 MHz, CDCl3) δ: −3.45, 20.76, 111.35,
112.44 (d, J = 21.6 Hz), 117.87 (d, J = 21.6 Hz), 119.94,
120.66, 122.67 (d, J = 8.7 Hz), 123.32, 126.90, 128.38, 128.49,
128.67, 129.71, 133.12 (d, J = 8.0 Hz), 133.92 (d, J = 2.5 Hz),
134.60, 135.85, 137.66, 159.46 (d, J = 242.3 Hz); HRMS (ESI,
m/z) calcd for [C24H24FNSi]H+: 358.1422; found 358.1427.
6i: Pale yellow solid, isolated yield 64% (68 mg); mp:
1
109.4–110.6 °C; H NMR (400 MHz, CDCl3) δ: 8.19–8.16 (m,
1H), 8.02 (d, J = 7.6 Hz, 1H), 7.27–7.16 (m, 8H), 7.05 (t, J =
7.4 Hz, 1H), 6.92 (d, J = 6.4 Hz, 1H), 5.41 (s, 2H), 2.23 (s, 2H),
0.11 (s, 6H); 13C NMR (75 MHz, CDCl3) δ: −3.25, 22.45,
49.74, 109.96, 120.13, 120.77, 122.59, 124.66, 124.77, 125.71,
125.87, 126.09, 126.13, 127.33, 128.67, 131.08, 134.03, 134.73,
137.65, 138.01, 140.10; HRMS (ESI, m/z) calcd for
[C24H24NSi]H+: 354.1673; found 354.1671.
X-ray crystallographic studies for 6e and 8a
The single crystals of 6e and 8a suitable for X-ray analysis were
grown in mixed solvent of hexane, diethyl ether and ethyl
acetate. Data collections were performed at 20 °C on a Rigaku
RAXIS RAPID IP diffractometer, using graphite-monochro-
mated Mo Kα radiation (λ = 0.71073 Å). The determination of
crystal class and unit cell parameters was carried out by the
Rapid-AUTO (Rigaku 2000) program package. The raw frame
data were processed using Crystal Structure (Rigaku/MSC 2000)
to yield the reflection data file. The structure was solved by use
of the SHELXTL program.17 Refinement was performed on F2
anisotropically for all the non-hydrogen atoms by the full-matrix
least-squares method. The hydrogen atoms were placed at the
calculated positions and were included in the structure calcu-
lation without further refinement of the parameters. Crystal data,
data collection and processing parameters for compounds 6e and
8a were given only in the ESI†. Crystallographic data (excluding
structure factors) have been deposited with the Cambridge
6j: Pale yellow solid, isolated yield 94% (121 mg); mp:
129.7–130.1 °C; 1H NMR (400 MHz, CDCl3) δ: 8.14 (d, J = 8.0
Hz, 1H), 8.00 (d, J = 7.2 Hz, 1H), 7.31–7.23 (m, 8H), 7.17 (t,
J = 3.8 Hz, 4H), 7.10–7.04 (m, 2H), 7.00 (s, 1H), 6.90 (t, J =
7.6 Hz, 1H), 6.72 (d, J = 8.8 Hz, 1H), 2.27 (s, 2H), 0.17 (s, 6H);
13C NMR (75 MHz, CDCl3) δ: −2.81, 22.58, 66.57, 113.26,
119.83, 120.72, 122.08, 124.82, 124.88, 125.65, 126.32, 127.41,
127.73, 128.14, 128.56, 130.99, 134.01, 134.65, 139.17, 139.21,
139.51; HRMS (ESI, m/z) calcd for [C30H28NSi]H+: 430.1986;
found 430.1988.
6k: Pale yellow oil, isolated yield 51% (49 mg); 1H NMR
(400 MHz, CDCl3) δ: 8.12 (d, J = 8.0 Hz, 1H), 7.95 (d, J = 8.0
This journal is © The Royal Society of Chemistry 2012
Org. Biomol. Chem., 2012, 10, 1537–1542 | 1541

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Crystallographic Data Centre as supplementary publication nos.
CCDC-823538 (6e), and CCDC-824123 (8a). Copies of these
data can be obtained free of charge from the Cambridge Crystal-
lographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Acknowledgements
This work was supported by the Natural Science Foundation of
China and the Major State Basic Research Development
Program (2012CB821603). Y. L. is thankful for financial support
from the Natural Science Foundation of China (21072054). We
thank Prof. Zhangjie Shi for valuable discussion.
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1542 | Org. Biomol. Chem., 2012, 10, 1537–1542
This journal is © The Royal Society of Chemistry 2012