Palladium-catalyzed cross-coupling reaction of aryl(trialkyl)silanes with aryl nitriles

Article abstract of DOI:10.1016/j.tetlet.2012.09.072

Using highly stable, readily accessible, and recyclable 2-(2-hydroxyprop-2-yl)cyclohexyl-substituted arylsilanes activated by a mild phosphate base, palladium-catalyzed silicon-based aryl-aryl cross-coupling reaction proceeds for the first time with aryl nitriles in a highly chemoselective manner.

Full text of DOI:10.1016/j.tetlet.2012.09.072

Tetrahedron Letters 53 (2012) 6743–6746
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Tetrahedron Letters
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Palladium-catalyzed cross-coupling reaction of aryl(trialkyl)silanes with
aryl nitriles
⇑
Shi Tang , Shu-Hua Li, Wen-bin Yan
College of Chemistry and Chemical Engineering, Jishou University, Jishou 416000, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 24 May 2012
Revised 17 September 2012
Accepted 18 September 2012
Available online 24 September 2012
Using highly stable, readily accessible, and recyclable 2-(2-hydroxyprop-2-yl)cyclohexyl-substituted
arylsilanes activated by a mild phosphate base, palladium-catalyzed silicon-based aryl–aryl cross-cou-
pling reaction proceeds for the first time with aryl nitriles in a highly chemoselective manner.
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Palladium
Cross-coupling
Recyclable
Silicon-based
Aryl nitrile
Introduction
the CN group acting as leaving group in cross coupling is, to date,
limited to the reaction with active organometallics (Mg, Zn, B)
Biaryls represent the mostly common structural motifs in a
broad range from naturally occurring, potentially useful therapeu-
tic agents to versatile building materials for light-emitting diodes
and liquid crystals. Metal-catalyzed cross-coupling reactions have
innovated a number of synthetic processes to access these struc-
via Ni catalyst. Increasing demands for chemoselectivity, availabil-
ity, stability, and non-toxicity have made arylsilanes an attractive
alternative to conventionally employed organomagnesium and
2
6–41
organoboron reagents in cross-coupling reaction.
Owing to
the weak nucleophilic feature of organosilanes, such protocols
have rarely met with success in silicon-based cross-coupling reac-
tion of aryl nitriles. On the other hand, we recently developed a
robust aryl(trialkyl)silane reagent that could participate in the
cross-coupling reaction of aryl chlorides and tosylates.42 The
attractive organosilane (HOMSi reagent) bearing a proximal hydro-
xyl group close to the silyl species allows fluoride-free intramolec-
ular activation, and formation of pentacoordinate silicates and/or
1
,2
tural motifs. While a number of protocols have been developed
using a wide range of conventional electrophiles (aryl halides, aryl
sulfonates, etc.), further extension of cross-coupling reaction
involving extremely challenged electrophilic partner recently has
attracted much attention to this field, such as cleavage of C–C
bond.3 Owing to its strength (>100 kcal mol ), the carbon–
–5
À1
carbon
r-bond of C–CN bond to be broken is kinetically inert,
and its activation is usually limited to systems in which relief of
metal alkoxides is likely responsible for the transmetallation under
6
43–45
strain or aromatization serves as the driving force. Nevertheless,
mild conditions with a carbonate salt as activator.
With the
7
–9
10,11
12,13
low-valent metal complexes containing Ni,
etc.,
Rh,
are able to cleave the carbon–carbon bond, and make
C–CN bond an attractive electrophile used in an array of transfor-
Pd,
background in mind, we examined a silicon-based aryl–aryl
cross-coupling reaction of aryl nitriles. We found that the newly
developed arylsilanes also achieve highly chemoselective C–C bond
forming reaction by the cleavage of C–CN bond. Herein, we report a
palladium-catalyzed aryl–aryl cross-coupling reaction of the
aryl(trialkyl)silanes with aryl nitriles.
1
4,15
16–18
19
20
mations, such as cross-coupling,
and carbocynation
silylation,
borylation,
2
1–24
of alkenes and alkynes. With respect to
arylation of C–CN bond to form biaryls, Miller et al., for example,
reported several cross-coupling reactions of aryl nitriles with aryl
1
6,17
Grignard reagents,
and Shi and co-workers also developed a
Results and discussion
25
boron-based cross-coupling reaction of aryl nitriles. However,
In the course of our research project aiming at metal-catalyzed
silicon-based arylation of C–CN bond, the cross-coupling reaction
of ethyl 4-cyanobenzonate 2a with phenyl[2-(2-hydroxyprop-2-yl)
⇑
Corresponding author. Tel./fax: +86 743 856 3911.
E-mail address: Shitang@126.com (S. Tang).
0
040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.09.072

6
744
S. Tang et al. / Tetrahedron Letters 53 (2012) 6743–6746
Table 1
Table 2
Optimization of conditions for Pd-catalyzed cross-coupling of 1a with 2aa
Pd-catalyzed cross-coupling reactions of aryl nitriles and aryl(trialkyl)silanesa
metal
ligand
[Pd(allyl)Cl]2 (5 mol%)
PMe (20 mol%)
HO
Ph
3
Ar-Ar1
HO
Ar
1
1
Ar1 Ph
Ar -CN
+
4a
+
Ar -CN
+
O
+
K PO (4 eq)
3
4
Si
K PO , toluene
Si
Si
(> 95 % )
3
4
Me
2
toluene, 110 °C
3
2
Me
a
Me
2
2
1
10 °C, 24 h
1a
2a
3a
4
1
Ar1 = EtO C-C H
2
6
4
Ar = 4-Me-C H4 (1b), 2-Me-C H4 (1c), 4-CF -C H (1d), 4-F-C H (1e)
6 6 3 6 4 6 4
Entry
Metal
Ligand
Yield of 3ab (%)
Yield of 4ac (%)
Yield of 3b (%)
Entry
1
2
Time (h)
1
2
3
4
5
6
7
8
9
Ni(cod)
Ni(cod)
2
2
PMe
PEt
PMe
PEt
P(i-Pr)
PCyP
P(n-Bu)
PCy
PPhMe
3
46
<10
85
<10
Trace
Trace
Trace
Trace
Trace
58
34
61
<10
67
27
>95
>95
>95
>95
>95
>95
>95
>95
>95
>95
>95
>95
>95
>95
52
1
2
3
4
5
6
7
8
9
1a
1a
1a
1a
1a
1a
1a
1a
1a
4-EtO
2
CC
6
H
4
CN (2a)
CN (2b)
CN (2c)
CN (2d)
CN (2e)
CN (2f)
CN (2g)
CN (2h)
4-(TBSOCH )C CN
2i)
24
24
36
36
36
24
24
24
24
78 (3a)
65 (3b)
51 (3c)
62 (3d)
35 (3e)
71 (3f)
69 (3g)
60 (3h)
51 (3i)
3
4-MeC
4-MeOC
3,5-(MeO)
2-MeC
4-FC
4-AcC
4-CF
6 4
H
[Pd(allyl)Cl]
[Pd(allyl)Cl]
[Pd(allyl)Cl]
[Pd(allyl)Cl]
[Pd(allyl)Cl]
[Pd(allyl)Cl]
[Pd(allyl)Cl]
2
2
2
2
2
2
2
3
6 4
H
3
2 6 4
C H
3
6 4
H
3
6 4
H
3
H
4
6
3
3
C
6
H
4
2
6 4
H
2
1
1
1
1
1
1
1
0
1
2
3
4
5
6
Pd(OAc)
Pd(dba)
Pd(CH CN)
PdCl
2
PMe
PMe
PMe
PMe
PMe
PMe
PMe
3
3
3
3
3
3
3
(
2
10
11
12
13
14
15
1a
1a
1a
1b
1c
1d
1e
3-Pyridyl nitrile (2j)
Styryl nitrile (2k)
2-Naphthyl nitrile (2l)
2a
2a
2a
36
24
36
24
24
24
24
42 (3j)
<5 (3k)
67 (3l)
73 (3m)
63 (3n)
68 (3o)
62 (3f)
3
2
Cl
2
2
d
[Pd(allyl)Cl]
[Pd(allyl)Cl]
[Pd(allyl)Cl]
2
2
2
e
f
<10
20
a
16
6 5
C H CN (2m)
Reaction conditions: 1a (0.4 mmol), 2a (0.2 mmol), metal source (10 mol % with
respect to metal), K PO (0.8 mmol), ligand (0.04 mmol), toluene (1 mL), 110 °C,
4 h.
a
3
4
Reaction conditions: 1 (1 mmol), 2 (0.5 mmol), [Pd(allyl)Cl]
equiv), PMe (0.1 mmol), toluene (2 mL), 110 °C.
Isolated yield.
3 4
(5 mol%), K PO (4
2
2
3
b
The yield was determined by GC.
The yield was, with respect to 1a, determined by GC.
Using a 2.5 mol % of [Pd(allyl)Cl] .
2
K
K
b
c
d
e
f
3
PO
PO
4
4
was replaced with K
was replaced with Na
2
CO
3
.
could also afford desired heteroarene 3j, the yield, but, decreased
to some extent. While styryl nitrile 2k did not undergo arylation,
the reaction of 2-naththyl nitrile 2l gave a 67% yield of 3l (entries
11 and 12). Sequentially, we attempted to probe the efficiency of
other aryl(trialkyl)silanes prepared similarly from the correspond-
ing aryl Grignard reagents. We were pleased to find that 4-tolyl
silane 1b and 2-tolyl silane 1c underwent the cross-coupling reac-
tion in good yield (entries 13 and 14). Additionally, 4-trifluorome-
thyphenyl silane 1d and 4-fluorophenyl silane 1e also proceeds the
reaction to furnish corresponding fluoro-containing biaryls, which
would provide a new alternative to other type of organometallics
(Mg, B, etc.) in the synthesis of pharmaceutically important fluoro-
arenes (entries 15 and 16).
3
2
CO
3
.
cyclohexyl]dimethylsilane 1a was firstly conducted to screen the
optimal reaction conditions (Table 1). In an initial experiment, we
delightedly observed the formation of desired coupling product 3a
in a 46% yield when the reaction was catalyzed by Ni(cod)
5 mol %) in the presence of PMe (entry 1). Further investigation re-
vealed the yield was dramatically increased to 85% when the metal
Ni was switched to Pd (entry 3). Moreover, the reusable silyl ether
a was also formed in almost quantitative yield by GC. Only a trace
amount of product 3a was detected when other phosphine ligands
were screened. However, we also observed a quantitative recovery
of the cyclic silyl ether 4a by GC regardless of efficiency of cross-
coupling, which could be in part due to the protiodesilylation of aryl
silane to give 4a (entries 4–9).46 Further screening of Pd sources re-
2
(
3
4
A working mechanism was also proposed as outlined in Figure 1
for the palladium-catalyzed silicon-based cross-coupling reaction
on the basis of previously reported mechanism.
42–45
Firstly,
vealed [Pd(allyl)Cl]
tion of Pd loading retarded the reaction, and gave an unparallel
yield (entry 14). Na CO and K CO resulted in the part recovery of
a, and thus afforded 3a in a poor yield (entries 15 and 16).
With the optimization conditions in hand, we next investigated
the substrate scope of the reaction. We were pleased to find
that phenyl[2-(2-hydroxyprop-2-yl)cyclohexyl]dimethylsilane 1a
could couple efficiently with a wide array of aryl nitriles to yield
various biaryls in moderate to good yield irrespective of steric
and electronic character of the substituent groups. For example,
p-methylphenyl, p-methoxylphenyl, 3-dimethoxylpheny nitriles
2
as the best choice (entries 10–13). The reduc-
in situ reduction of Pd precursor forms LnPd(0), followed by oxida-
tive addition of aryl nitrile to generate LnPd(II)(Ar)(CN), and then
intermediate LnPd(II)(Ar)(CN) is transmetallated by the pentacoor-
dinated silicate intermediate A originating from intermolecular
2
3
2
3
1
3 4
activation of aryl(trialkyl)silane using K PO as an activator, and
1
the resulted LnPd(II)(Ar )(Ar) finally proceeds the reductive elimi-
1
nation to give desired biaryls (Ar –Ar), as well as Pd(0) to involve
next catalytic cycles. It is worth to mention that the by-product,
a cyclic silyl ether 4a, is reusable by treating with ArMgBr.
In summary, we have demonstrated that a robust aryl(trial-
kyl)silane proceeded fluoride-free cross-coupling reaction with ex-
tremely inert but synthetically interesting C–CN bond for the first
time. It is worth to mention merits including the recycle of orga-
nosilane and fluoride-free manner would enhance the widespread
acceptability of silicon-based cross-coupling protocol. Additionally,
it represents a rare example of silicon-based C–C bond activation,
and provides a novel pathway to redirect the synthetic strategy
for the construction of C–C bond. Further efforts to expand utiliza-
tion scope of the robust aryl(trialkyl)silanes in organic synthesis
are current issues in our laboratory.
2
b–2d proceeded the cross-coupling reaction in moderate yield
(Table 2, entries 2–4). Notably, substituents ortho to cyano group,
even simple methyl group, would retard the coupling reaction
somewhat (entry 5). In addition, various functional groups, such
as carboxylic, trifluoromethyl, and fluoro groups, could tolerate
the conditions (entries 6–8). Next, we are very pleased to find that
silicon-based protecting groups successfully tolerated the coupling
condition in contrast to the conventional fluoride-activation which
results in desilylation (entry 9). The reaction of 3-cyanopyridine 2j

S. Tang et al. / Tetrahedron Letters 53 (2012) 6743–6746
6745
Pd Precursor + L
1
Ar -CN
LnPd(0)
Ar1
Ar
K PO4
3
Ar1
K
OH
O
LnPd
Si Ar
CN
Si Ar
Me
KHPO4
Me
Me
2
A
1
O K
Ar
A
Me Me
Ar1
Ar
LnPd
O
Si
ArMgBr
Me
O
OH
4a
2
Si
Si Ar
Me
2
Me
2
4a
1
Figure 1. A possible mechanistic pathway.
Experimental section
28.33, 27.40, 26.11, 25.70, 24.15, À0.86, À1.33. Anal. Calculated
for C17 27FOSi: C, 69.34; H, 9.24; Found: C, 69.09; H, 9.24.
H
All manipulations of oxygen- and moisture-sensitive materials
were conducted with a Schlenk technique or in a dry box under
a nitrogen or argon atmosphere. Flash column chromatography
was performed using EM Silica gel 60 (300–400 mesh). Visualiza-
tion was accomplished with UV light (254 nm) and/or an aqueous
General procedure for cross-coupling reaction of
aryl(trialkyl)silanes with aryl nitriles
All reactions and manipulations were run under nitrogen atmo-
sphere. Under nitrogen atmosphere, a reaction tube was charged
with Aryl nitrile 2 (0.5 mmol), aryl(trialkyl)silane 1 (1.0 mmol),
alkaline KMnO
4
solution followed by heating. Proton and carbon
1
13
nuclear magnetic resonance spectra ( H and C NMR) were re-
1
13
corded on a Varian Mercury 400 ( H NMR, 400 MHz; C NMR
01 MHz) spectrometer with solvent resonance as the internal
[
Pd(allyl)Cl]
2 3 3 4
(9.2 mg, 13 lmol), PMe (7.6 mg, 0.1 mmol), K PO
1
(
0.42 g, 2.0 mmol), and toluene (2 mL). The mixture was stirred
1
13
standard ( H NMR, CHCl
3
at 7.26 ppm;
3
C NMR, CDCl at
at 110 °C until a GC analysis showed complete consumption of
the starting materials (usually 24–36 h), the solvent was evapo-
rated under reduced pressure and the resulted mixture was filtered
7
7.0 ppm). Unless otherwise noted, reagents were commercially
available and were used without further purification. Anhydrous
toluene was distilled from sodium/benzophenone prior to use.
Preparation of aryl(trialkyl)silanes (1a–1d) and 4a are described
in our previous publications.42
through a Florisil pad, diluted with Et
2
O, and washed with water
and then brine. The organic layer was dried over anhydrous MgSO
and concentrated in vacuo. The residue was purified by flash chro-
matography on silica gel to give the corresponding biaryl.
4
Preparation of 4-fluorophenyl[2-(hydroxypro-2-
yl)cyclohexyl]dimethylsilane (1e)
Acknowledgments
To a suspension of Mg (0.26 g, 11 mmol) in THF (2 mL) was
The research was financially supported from Jishou University
and JSPS (Japan Society for the Promotion of Science) postdoctoral
fellowship.
added dropwise
a solution of 4-bromofluorobenzene (1.7 g,
1
0 mmol) in THF (10 mL) over 15 min at rt, and the resulting mix-
ture was stirred at rt for 0.5 h. To the solution of the aryl Grignard
reagent thus obtained was added 4a (0.99 g, 5.0 mmol) at 0 °C. After
being stirred at rt overnight, the reaction mixture was quenched
References and notes
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4
Cl aqueous solution at 0 °C. The aqueous layer
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protiodesilyation
HO
Ar
K
Ar-H + O
O
Si
Si
Ar Si
Me Me
Me
2
Me
K HPO
2 4
2
4
a
1
20.