Synthesis of HOMSi reagents by Pd/Cu-catalyzed silylation of bromoarenes with disilanes

Article abstract of DOI:10.1246/cl.130919

Silylation of aryl bromides with disilanes of type {[2- (PGOCH2)C6H4]Me2Si}2 (PG: protecting group) successfully takes place in the presence of a Pd/Ruphos or Davephos/CuI catalytic system to afford HOMSi reagents containing various functional groups in good yields. BisHOMSi reagents were also prepared directly from the corresponding arylene dibromides.

Full text of DOI:10.1246/cl.130919

Received: October 2, 2013 | Accepted: October 18, 2013 | Web Released: October 26, 2013
CL-130919
Synthesis of HOMSi Reagents by Pd/Cu-Catalyzed Silylation of Bromoarenes with Disilanes
Yasunori Minami,*1,2 Kenta Shimizu,3 Chisato Tsuruoka,3 Takeshi Komiyama,3 and Tamejiro Hiyama*1,2
1Research and Development Initiative, Chuo University, Kasuga, Bunkyo-ku, Tokyo 112-8551
2JST, ACT-C, Kasuga, Bunkyo-ku, Tokyo 112-8551
3Faculty of Science and Engineering, Chuo University, Kasuga, Bunkyo-ku, Tokyo 112-8551
(E-mail: yminami@kc.chuo-u.ac.jp)
Silylation of aryl bromides with disilanes of type {[2-
(PGOCH2)C6H4]Me2Si}2 (PG: protecting group) successfully
takes place in the presence of a Pd/Ruphos or Davephos/CuI
catalytic system to afford HOMSi reagents containing various
functional groups in good yields. BisHOMSi reagents were also
prepared directly from the corresponding arylene dibromides.
2-(PGOCH2)C6H4Br
1) BuLi, -78 °C, 3 h
2) (SiMe2Cl)2
OPG
OH
OPG
electro-
phile
TsOH•H2O
MeOH
PG = THP
Si
Me2
Si
Me2
Si
Me2
2
2
2
1H, 90%
PG = THP: 1THP
83%
PG = Ac: 1Ac
91%
PG = TBDPS: 1TBDPS
Organosilicon compounds are selective, stable, and thus,
useful reagents for chemoselective organic synthesis. Of the
many functional tolerant transformations, metal-catalyzed cross-
coupling with organic halides are representative.1 Accordingly,
silicon-based cross-coupling reaction is gaining prominence
among various cross-coupling methodologies. In particular, the
tetraorganosilicon-type coupling reagents organo-[2-(hydroxy-
methyl)phenyl]dimethylsilanes (R-HOMSi) are unique for easy
handling and facile recovery and reuse of the Si moiety; these
reagents are now commercially available worldwide.2,3 Ar-
HOMSi reagents have high potential for the synthesis of
functionalized oligoarenes and polyarenes. A typical preparative
method for Ar-HOMSi reagents is the reaction of organometallic
reagents with a cyclic silyl ether, 1,2-(SiMe2OCH2)C6H4.
However, functionalized Ar-HOMSi reagents are less accessible
by this method because of the insufficient chemoselective
transformation. To overcome this disadvantage, transition-metal-
catalyzed silylation of organic halides with disilanes4-6 or
hydrosilanes7 appears to be highly promising for the synthesis of
functionalized organosilicon reagents. In this respect, prepara-
tion of Ar-HOMSi reagents by the silylation of bromoarenes
with hydrosilanes is attractive, as reported by Kondo et al.
However, yields remain only moderate and the scope is
limited.7k Because disilanes are also applicable to such
silylation, we focused on the use of disilanes for the silylation
of aryl halides and herein report a new synthesis of Ar-HOMSi
reagents by the palladium/copper-catalyzed silylation of organic
halides.
PG = MOM: 1MOM
90%
99%
Scheme 1. Preparation of disilanes 1.
duced (See Supporting Information).8 Thus, we next examined
many other activation protocols for 1 as the silylation reagent
and found the catalyst system recorded by Hosomi9a to be
informative: in situ generation of silylcopper reagents by the
reaction of disilanes with Cu(OTf).9 On the basis of this report,
we planned the generation of protected HOMSi copper reagent
by treatment of disilyl reagents 1 with Cu(I) for the silylation of
organic halides.10 Of the many catalysts and additives examined,
we found that the reaction system consisting of [Pd(allyl)Cl]2,
Ruphos (2-dicyclohexylphosphino-2¤,6¤-diisopropoxybiphenyl),
CuI, K2CO3, and THF/DMF (3:1) as a solvent showed catalytic
activity to afford 3a in 34% (NMR) yield together with siloxane
4 in 92% yield (Table 1, Run 1). Other copper salts such as
CuBr, CuBr¢SMe2, and CuCl were less effective. K2CO3 was
proved to be the best; other bases such as Na2CO3, Cs2CO3, and
K3PO4 resulted in lower yields of 3a. In the absence of CuI,
palladium, or K2CO3, formation of 3a was not observed, though
4 was generated, maybe via an attack of the carboxylate anion
or iodonium ion from CuI to 1THP (Runs 2-4). These results
indicate that the silyl copper reagent is formed as an active silyl
nucleophile, possibly by the reaction of 1THP with CuI in the
presence of K2CO3. To enhance the efficiency, various solvents
were further examined and the reaction in dioxane/NMP (4:1) at
100 °C afforded 3a in 80% yield (Run 5). In this solvent without
CuI, 3a was obtained in 52% yield (Run 6). These results
suggest that the highly polar solvents definitely increase the
silylation efficiency, as is evidenced by the couse of H2O that
further increased the yield and shortened the reaction time
(Run 7). Finally, the yield was improved to 87% in the presence
H2O (Run 8).
Protected 2-bromophenylmethanol was lithiated with butyl-
lithium; the resulting aryllithium was allowed to react with
1,2-dichlorotetramethyldisilane to afford protected disilanes
(Scheme 1). Thus, THP-protected disilane 1THP was isolated
in 83% yield. In a similar way, MOM-protected disilane 1MOM
was prepared in 90% yield. Other protected disilanes having
acetyl (1Ac) and TBDPS (1TBDPS) groups were prepared by the
deprotection of 1THP, followed by the protection of the resulting
1H by acetylation or silylation to afford 1Ac and 1TBDPS in 91%
and quantitative yields, respectively.
Deprotection of 3a by TsOH in methanol readily afforded
the cross-coupling active p-tolyl-HOMSi reagent 5.
The scope and limitations of the synthetic method under
the optimized conditions are summarized in Table 2. Protected
disilanes 1MOM, 1Ac, and 1TBDPS reacted with 2a in a similar
manner to afford products 6a, 7a, and 8a in moderate to high
We first tested the reported conditions4f-4h,4j for the
silylation of p-bromotoluene (2a) using 1THP in the presence
of a base without any success: no trace or only small amounts of
the desired product, protected p-tolyl-HOMSi (3a), was pro-
Chem. Lett. 2014, 43, 201–203 | doi:10.1246/cl.130919
© 2014 The Chemical Society of Japan | 201

a
Table 1. Reaction of 2a with 1THP
Table 2. Silylation of organobromides 2 using disilane 1a
[Pd(allyl)Cl]2
(2.5 mol%)
Ruphos (10 mol%)
[Pd(allyl)Cl]2 (2.5 mol%)
ligand (10 mol%)
CuI (10 mol%)
OPG
OPG
Si
Si
1THP
(1.2 equiv)
+
+
Ar Br +
O
K2CO3 (2.2 equiv)
Ar
K2CO3 (2.2 equiv)
H2O (x equiv)
dioxane/NMP (4:1), 100 °C
Br
Si
Si
Me2
Si
Me2
2
2a
4
3a
2
Si = {2-(THPOCH2)C6H4}Me2Si
3
1
Yield/%b
Temp Time
H2O
Time Product
Run Additive
Solvent
Run
1
2
Ligand
3a
4c
(x equiv) /h /%b
/°C
/h
1
2
CuI (10 mol %) THF/DMFd
®
80
80
80
80
24
24
24
24
24
24
2
34
0
0
92
71
72
13
85
76
11
1
2
3
1MOM 2a
1Ac 2a
1TBDPS 2a
L1
L1
L1
4
4
4
24 6a, 80%
24 7a, 60%
24 8a, 54%
THF/DMFd
3e CuI (10 mol %) THF/DMFd
4f CuI (10 mol %) THF/DMFd
0
R
5
6
7
CuI (10 mol %) dioxane/NMPg 100
80
52
83
®
dioxane/NMPg 100
Br
CuI (10 mol %) dioxane/NMPg 100
H2O (4 equiv)
4
5
6
7
8
1THP
1THP
1THP
1THP
1THP
2b: R = OMe
2c: R = NH(Boc)
2d: R = NPh2
2e: R = 3,5-xylyl
2f: R = H
L1
L1
L1
L1
L2
L2
L2
L2
L2
L2
L2
L2
4
4
4
0
4
0
0
0
0
1
1
0
24 3b, 75%
13 3c, 74%
16 3d, 67%
24 3e, 63%
24 3f, 64%
24 3g, 60%
16 3h, 63%
24 3i, 70%
48 6j, 35%
31 3k, 26%
23 3l, 63%
48 3m, 27%
8
CuI (10 mol %) dioxane/NMPg 100
H2O (4 equiv)
24
87 (73)h 77
aUnless otherwise noted, 2, 1THP (1.2 equiv), [Pd(allyl)Cl]2
(2.5 mol %), ligand (10 mol %), CuI (10 mol %), K2CO3
(2.2 equiv), and solvent (0.36 M). NMR yield. Yields based
on 1THP. 3:1. Without [Pd(allyl)Cl]2. Without K2CO3. 4:1.
hIsolated yield.
9c 1THP
10c 1THP
11 1THP
2g: R = F
2h: R = Cl
2i: R = Ph
b
c
d
e
f
g
12 1MOM 2j: R = Ac
13d 1THP
14d 1THP
15 1THP
2k: R = CN
2l: R = CF3
2m: R = CHO
yields (Runs 1-3). Various organic bromides were next
subjected to the reaction with 1THP and 1MOM. Electron-donating
groups such as p-MeO, p-NHBoc, and p-Ph2N did not hamper
the reaction in the presence of Ruphos, and the corresponding
protected HOMSi reagents 3b-3d were isolated in moderate to
good yields (Runs 4-6). In the case of 3,5-xylyl bromide (2e),
H2O did not enhance the reactivity: 3e was isolated in 63% yield
in the absence of H2O (Run 7). In contrast, electron-neutral or
electron-deficient aryl bromides afforded the corresponding
protected Ar-HOMSi in low yields. As pointed out in many
reports on the silylation of haloarenes with hydrosilanes or
disilanes,4f,4j,7d,7e we further screened ligands for silylation using
these bromides and found that Davephos, 2-dicyclohexylphos-
phino-2¤-(N,N-dimethylamino)biphenyl, led to the improvement
of the catalytic activity. Thus, with Davephos, bromobenzene
(2f) was converted to protected Ph-HOMSi 3f in 64% yield
(Run 8). Electron-deficient aryl bromides preferred the reaction
in the absence or 1 equiv amount of H2O (Runs 9-15). The
reaction of 1-bromo-4-fluorobenzene (2g) afforded protected
4-F-C6H4-HOMSi reagent 3g in 60% yield (Run 9). Other
electron-deficient groups such as chloro, phenyl, acetyl, cyano,
trifluoromethyl, and formyl tolerated well the silylation with
1THP and 1MOM to afford the corresponding protected HOMSi
reagents 3h-3i, 6j, and 3k-3m in 26-70% yields (Runs 10-15).
The reaction of 2-bromonaphthalene (2n) afforded 3n in 70%
yield (Run 16). 3-Bromothiophene (2o) was silylated with 1THP
in the presence of Ruphos and 4 equiv of H2O to afford protected
3-thienyl-HOMSi reagent 3o in 42% yield (Run 17).
16 1THP
L2
L1
0
4
24 3n, 70%
24 3o, 42%
Br
2n
S
17 1THP
Br
2o
aUnless otherwise noted, 2 (0.5 mmol), 1 (0.6 mmol), [Pd-
(allyl)Cl]2 (0.0125 mmol), ligand (0.05 mmol), CuI (0.05
mmol), K2CO3 (1.1 mmol), and 1,4-dioxane/NMP (4:1, 1.4
mL) were heated at 100 °C. Isolated yield. 120 °C. 80 °C.
L1 = Ruphos. L2 = Davephos.
b
c
d
[Pd(allyl)Cl]2 (5 mol%)
Ruphos (20 mol%)
CuI (20 mol%)
Br Ar Br + 1THP
Si Ar Si
K2CO3 (4.4 eq), H2O (8 eq)
dioxane/NMP (4:1),100 °C, 48 h
9
10
2.2 eq
Oct
Oct
Si
Si
Si
Si
10a, 64%
10b, 65%
Si = {2-(THPOCH2)C6H4}Me2Si
Scheme 2.
This silylation can be applied to the synthesis of protected
bis-HOMSi reagents 10 by the reaction of dibromoarenes 9
(Scheme 2). For example, 4,4¤-dibromobiphenyl (9a) was
bissilylated by 2.2 equiv of 1THP under the optimized conditions
using Ruphos to afford protected 4,4¤-biphenylylene-bisHOMSi
reagent 10a in 64% yield. Similarly, 2,7-dibromo-9,9¤-dioctyl-
9H-fluorene (9b) was double silylated to form the corresponding
products 10b in 65% yield.
We propose the reaction mechanism illustrated in Figure 1.
Oxidative addition of aryl bromide 2 to palladium(0) complex
forms an Ar-Pd-Br complex, which transmetalates to the silyl
copper(I) reagent derived from the reaction of disilane 1 with
202 | Chem. Lett. 2014, 43, 201–203 | doi:10.1246/cl.130919
© 2014 The Chemical Society of Japan

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Ar
Br
Si
Si
LPd
LPd
Cu Si
Si OY
O
4
Pd(0)L
K2CO3
Ar
Cu
X
Si Si
1
3
Ar Si
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Si = Si{2-(PGOCH2)C6H4}Me2
X = Br, I Y = K, CO2K
Figure 1. Proposed reaction mechanism.
Cu-X and K2CO3 to afford Ar-Pd-Si complex with the
generation of an oxysilane (Si-OY) and Cu-X. Finally,
reductive elimination of Ar-Pd-Si complex produces protected
HOMSi reagents 3 and regenerates palladium(0) to complete the
catalytic cycle. The coproduced oxysilane appears to be readily
converted to siloxane 4.11 The additive H2O may be attributed to
enhancement in the solubility of K2CO3 and thus promote the
reactivity of 1 toward Cu(I) and K2CO3.
In conclusion, we have demonstrated that the silylation
of aryl bromides with disilanes is a powerful tool for the
preparation of aryl-HOMSi reagents. This method allows us to
synthesize variously functionalized aryl-HOMSi reagents useful
for the C-C bond-forming cross-coupling with aryl bromides
after deprotection. Furthermore, bisHOMSi reagents also are
readily prepared and conveniently used for polyarylene synthe-
sis.
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Chem. Lett. 2014, 43, 201–203 | doi:10.1246/cl.130919
© 2014 The Chemical Society of Japan | 203