Palladium-catalyzed cross-coupling reaction of 1-aryltriazenes with aryl- and alkenyltrifluorosilanes

Article abstract of DOI:10.1002/adsc.200404212

The palladium-catalyzed cross-coupling reaction of 1-aryltriazenes with aryl- and alkenyltrifluorosilanes occurs readily at room temperature to yield the corresponding biaryl and stilbene products in moderate to good yields. In contrast to the previous re

Full text of DOI:10.1002/adsc.200404212

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Palladium-Catalyzed Cross-Coupling Reaction of 1-Aryltriazenes
with Aryl- and Alkenyltrifluorosilanes
Tomoyuki Saeki,* Tadafumi Matsunaga, Eun-Cheol Son, Kohei Tamao*
International Research Center for Elements Science, Institute for Chemical Research, Kyoto University, Uji, Kyoto
611-0011, Japan
Fax: (þ81)-774-38-3186, e-mail: tamao@scl.kyoto-u.ac.jp
Received: July 5, 2004; Accepted: November 1, 2004
Abstract: The palladium-catalyzed cross-coupling re-
action of 1-aryltriazenes with aryl- and alkenyltri-
fluorosilanes occurs readily at room temperature to
yield the corresponding biaryl and stilbene products
in moderate to good yields. In contrast to the previ-
ous results for the reaction with areneboronic acids,
in which an additional Lewis acid such as boron tri-
fluoride is essential for the activation of the 1-aryl-
Scheme 1. General scheme for cross-coupling reaction of are-
neboronic acids with 1-aryltriazenes.
triazenes, the Lewis acidity of organotrifluorosilanes
seems to be strong enough to directly activate the tri-
azene moiety to enter into the palladium-catalyzed
cross-coupling reaction without an extra Lewis acid.
Keywords: aryltriazene; biaryls; cross-coupling; pal-
ladium; stilbene; trifluorosilane
Figure 1. Schematic representations of
a concerted-like
mechanism for the formation of the diaryl-palladium inter-
mediate.
The transition metal-catalyzed cross-coupling reaction
has become a promising synthetic tool for the construc- organometallic compounds, instead of the boronic acids,
tion of carbon–carbon and carbon–heteroatom bonds to construct a synthetically versatile reaction. Herein,
in the field of organic synthesis, material science, and we report the cross-coupling reaction of the 1-aryltri-
pharmaceuticals.[1]
azenes with aryl- and alkenyltrifluorosilanes in the pres-
We have recently reported that the 1-aryltriazenes can ence of a palladium catalyst and a phosphine ligand.
be employed as an electrophile in the palladium-cata- At the beginning of the investigation, we attempted
lyzed cross-coupling reaction with areneboronic acids,[2] the cross-coupling reaction of p-anisyltriazene 2 with
in which the addition of Lewis acids such as boron tri- various organosilicon compounds such as p-Tol-SiMe3,
fluoride is essential to enhance the reactivity of the tri- -Si(OMe)3, -SiCl3, -SiMe2F, -SiMeF2, and -SiF3 (1) under
azene compounds toward the palladium catalyst a catalyst system similar to that utilized for the arenebor-
(Scheme 1). The 1-aryltriazenes can readily be prepared onic acids [Pd2(dba)3 (2 mol %), P(t-Bu)3 (8 mol %), and
from the corresponding arylamines,[3] and the reaction is BF3 ·OEt2 (1 equiv.)][2] to discover that only the trifluoro-
generally completed within 10 min at room tempera- silane 1 having the highest Lewis acidity afforded the
ture, thus providing a quick and organic halide-free cou- corresponding biaryl product 3 in 17% yield after 24 h
pling system under mild conditions.[4] Although the ac- stirring at room temperature; the other silicon com-
tual reaction mechanism remains unclear, the boron tri- pounds were totally ineffective.[5] In sharp contrast to
fluoride might play two crucial roles during the catalysis the previous results for the areneboronic acids, the addi-
as depicted in Figure 1a: (i) as a Lewis acid toward the tional Lewis acid turned out not to be necessary for the
triazene to enhance the cleavage of the sp2-carbon–ni- cross-coupling reaction of the trifluorosilanes. Thus, fi-
trogen bond and (ii) as a fluoride source to promote a nally, the biaryl product 3 was obtained in 76% yield un-
transmetalation of the aryl substituent on the resulting der the optimized catalyst system without boron trifluor-
borate moiety.
ide (Scheme 2). The direct acid-base interaction be-
In the course of our research, we next examined the tween the silicon atom of the trifluorosilane and the ni-
cross-coupling reaction of the 1-aryltriazenes with other trogen atom at the 3-position of the triazene compound
Adv. Synth. Catal. 2004, 346, 1689–1692
DOI: 10.1002/adsc.200404212
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Tomoyuki Saeki et al.
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Table 2. Cross-coupling of aryltriazenes with aryltrifluorosi-
lanes.[a]
Entry
X
Y
Yield[b]
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Me
Me
H
OMe
F
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
o-Me
m-Me
p-Me
p-Me
p-Me
2,4,6-Me3
p-OMe
p-NEt2
p-F
p-Cl
p-Br
71%
74%
74%
60%
82%
63%
76%
30%
38%
42%
trace
0%
Scheme 2. Cross-coupling reaction of p-tolyltrifluorosilane
(1) with 1-(p-anisyl)triazene 2.
Table 1. Screening of catalyst system.[a]
[b]
Entry
Catalyst
Ligand
Yield
1
2
3
4
5
6
7
8
9
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd(dba)2
Pd(PPh3)4
none
PPh3
P(o-Tol)3
PCy3
P(t-Bu)3
dppb
dppf
P(t-Bu)3
none
37%
26%
46%
28%
76%
9%
41%
62%
26%
p-I
p-OTf
p-C(O)Me
p-CO2Me
36%
55%
22%
[a]
Reaction conditions: p-tolyltrifluorosilane (0.20 mmol),
aryltriazene (0.24 mmol), Pd2(dba)3 ·CHCl3 (2 mol %),
and P(t-Bu)3 (8 mol %), DME (2.0 mL), room tempera-
ture.
Yields are determined by GC analysis with eicosane as an
internal standard.
[a]
Reaction conditions: trifluorosilane 1 (0.20 mmol), aryl-
triazene 2 (0.24 mmol), catalyst (2 mol %), and ligand
(8 mol %), DME (2.0 mL), room temperature.
Yields are determined by GC analysis with eicosane as an
internal standard.
[b]
[b]
creased the yields (entries 9–15); in the case of bro-
might be a crucial step in the catalysis as shown in Figure mides and iodides, no product was obtained at all and
1b, accounting for the effectiveness of the highly Lewis most of the starting triazenes were recovered un-
acidic trifluorosilane.[6]
changed.[8] In contrast, electron-withdrawing substitu-
The screening of catalysts as summarized in Table 1 ents on the aryltrifluorosilanes increased the yield to
showed that the combination of Pd2(dba)3 with some extent, as observed for p-F-C6H4SiF3 in compari-
P(t-Bu)3 was most effective (entry 5), whereas the triar- son with p-OMe- and non-substituted analogues (en-
ylphosphines (entries 2 and 3) and bidentate ligands tries 3–5). These results are consistent with our hypo-
(entries 6 and 7) resulted in low yields. It is notable thetical mechanism shown in Figure 1b, in that the elec-
that, in the screening of solvents, the addition of metha- tron-withdrawing substituents on the aryltrifluorosi-
nol as a co-solvent was found to accelerate the reaction lanes enhance the Lewis acidity of the silicon center
of the model case in Scheme 2 to give a slightly increas- making the silicon-nitrogen interaction more favored,
ing yield of 3 (4 h, 90% yield). This acceleration effect and the electron-donating substituents on the triazene
was most remarkable in rather sterically hindered 1,2- moiety then stabilize the cationic character of the aro-
disubstituted alkenyltrifluorosilanes, as will be men- matic group developed in the reaction intermediate
tioned below.[7]
(or transition state).[9]
We next examined the scope of the cross-coupling re-
The reaction system was extended to the cross-cou-
action with respect to the substituents on the 1-aryltri- pling reaction of some alkenyltrifluorosilanes (4a – d)
azenes and aryltrifluorosilanes (Table 2). On the whole, (Scheme 3).[10,11] Among the alkenylsilanes examined,
triazenes with electron-donating substituents such as 2-substituted (E)-alkenyltrifluorosilanes 4a and 4b
methyl and methoxy groups afforded the corresponding were more reactive, giving higher yields of coupling
biaryl products in good yields, ranging from 63% to 76% products 6 than 1,2-disubstituted alkenylsilanes 4c and
(entries 1, 2, 6 and 7); it is noted that the bulky mesityl 4d. The latter lower yields are probably due to the steric
group can also be coupled. The diethylamino group, hindrance around the silicon center, preventing the in-
one of the most electron-donating substituents, excep- teraction with triazene molecules. The addition of meth-
tionally decreased the yield probably due to the unde- anol was effective again to accelerate the reaction, espe-
sired interaction between the nitrogen atom and the cially for the sterically hindered 1,2-disubstituted alke-
Lewis acidic silicon center to retard the coupling reac- nyltrifluorosilanes as shown in the parentheses in
tion (entry 8). Electron-withdrawing substituents de- Scheme 3.[7] The cross-coupling reactions of 4a with var-
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Pd-Catalyzed Cross-Coupling Reaction of 1-Aryltriazenes
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Scheme 3. Cross-coupling reaction of alkenylsilanes 4a – d with 1-(p-tolyl)triazene 5.
added P(t-Bu)3 (10 wt % hexane solution, 32 mg,
0.016 mmol) and p-tolyltrifluorosilane 1 (35 mg, 0.2 mmol) at
room temperature. The reaction wasmonitored bygas chroma-
tography using eicosane (28 mg, 0.10 mmol) as an internal
standard to confirm the formation of the desired biaryl product
3 in 76% yield after 18 h stirring. The reaction mixture was fil-
tered through a pad of silica gel and the filtrate was evaporated
under reduced pressure to give a viscous oil. The crude product
was purified by HPLC using hexane/AcOEt (10:1) as an eluent
to afford 4-methyl-4’-methoxybiphenyl (3); yield: 29 mg
(0.15 mmol; 75%).
Table 3. Cross-coupling of (E)-styryltrifluorosilane (4a) with
aryltriazenes.[a]
Entry
X
Time
Yield[b]
1
2
3
4
5
6
7
Me
OMe
OTf
F
Cl
Br
9 h
9 h
92%
95%
55%
82%
85%
52%
trace
[c]
36 h
18 h
18 h
48 h
48 h
I
Acknowledgements
[a]
Reaction conditions: (E)-styryltrifluorosilane (4a)
(0.20 mmol), aryltriazene (0.24 mmol), Pd2(dba)3 ·CHCl3
(2 mol %), and P(t-Bu)3 (8 mol %), DME (2.0 mL),
room temperature.
Yields are determined by GC analysis with eicosane as an
internal standard.
We thank the Ministry of Education, Culture, Sports, Science
and Technology, Japan, for the Grant-in-Aid for COE Research
on Elements Science, No. 12CE2005, and Hokko Chemical In-
dustry Co., Ltd., for their gift of phosphine ligands.
[b]
[c]
Isolated yield.
References and Notes
ious triazenes are summarized in Table 3. In contrast to
the results obtained for the aryltrifluorosilanes, not only
the electron-donating groups but also the electron-with-
drawing groups including halogen atoms were tolerated.
Thus, the coupling reaction of 4a with 4-bromophenyl-
triazene readily proceeded to give the bromine-substi-
tuted stilbene in 52% yield with good chemoselectivity
(entry 6).
In conclusion, the palladium-catalyzed cross-coupling
reaction of 1-aryltriazenes with aryl- and alkenyltri-
fluorosilanes has been achieved under mild conditions
to give the corresponding biaryl and stilbene products
in good yields. The Lewis acidity and the steric effect
of the silicon compounds are responsible for the yields,
thereby supporting our working hypothesis involving
the direct acid-base interaction between the trifluorosi-
lanes and the triazenes.
[1] a) Metal-Catalyzed Cross-Coupling Reactions, (Eds.: F.
Diederich, P. J. Stang), Wiley-VCH, Weinheim, 1998;
b) Cross-Coupling Reactions: A Practical Guide (Ed.:
N. Miyaura), Topics in Current Chemistry, Vol. 219,
Springer-Verlag, Heidelberg, 2002; c) 30 Years of Cross-
Coupling Reaction (Eds.: K. Tamao, E.-i. Negishi, T.
Hiyama), J. Organomet. Chem. (Special Issue) 2002, 653.
[2] T. Saeki, E.-C. Son, K. Tamao, Org. Lett. 2004, 6, 617.
[3] D. B. Kimball, M. M. Haley, Angew. Chem. Int. Ed. 2002,
41, 3338 and references cited therein. Caution: the tria-
zene compounds are carcinogenic; direct hand contact
should be avoided.
[4] Cross-coupling reactions of arenediazonium salts, one of
the attractive candidates for halogen-free electrophiles,
have been extensively studied: a) K. Kikukawa, K.
Kono, F. Wada, T. Matsuda, J. Org. Chem. 1983, 48,
1333; b) S. Sengupta, S. Bhattacharya, J. Org. Chem.
1997, 62, 3405; c) M. B. Andrus, C. Song, Org. Lett.
2001, 3, 3761.
[5] This result is in sharp contrast to the fluoride ion-pro-
moted Hiyama coupling in which aryldifluorosilanes
are more reactive than aryltrifluorosilanes: Y. Hatanaka,
K. Goda, Y. Okahara, T. Hiyama, Tetrahedron 1994, 50,
8301.
[6] An alternative reaction mechanism based on the gener-
ally accepted three-step mechanism is also possible: the
Lewis acid-base interaction between the silicon atom
Experimental Section
Typical Procedure for the Cross-Coupling Reaction of
Aryltriazenes with Trifluorosilanes
To a solution of p-anisyltriazene 2 (49 mg, 0.24 mmol) and
Pd2(dba)3 ·CHCl3 (4 mg, 0.004 mmol) in DME (2.0 mL) was
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Tomoyuki Saeki et al.
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and the nitrogen atom enhances the oxidative addition of
the sp2-carbon–nitrogen bond to the zero-valent palladi-
um complex to form a cationic aryl-palladium intermedi-
ate. Then, transmetalation of the aryl substituent on the
resulting pentacoordinate silicon atom and subsequent
reductive elimination afford the biaryl product.
[8] A competitive reaction between p-anisyltriazene 2 and
p-bromoanisole, in the reaction with p-tolyltrifluorosi-
lane (1), afforded the corresponding biaryl 3 in satisfac-
tory yield and the aryl bromide was recovered un-
changed. Chemoselective differential reactions of 4-bro-
mobenzene diazonium salts have been reported: See re-
fs.[4b,10a, 11a]
[9] The electron-donating substituents can also stabilize the
cationic aryl-palladium species generated as an initial in-
termediate in the three-step mechanism.[6]
[10] Synthesis of symmetrical stilbene compounds via a dou-
ble Heck reaction of arenediazonium salts, generated in
situ from 1-aryltriazenes, with vinyltriethoxysilane has
been reported: a) S. Sengupta, S. K. Sadhukhan, Org.
Synth. 2002, 79, 52; b) S. Sengupta, S. Bhattacharyya,
S. K. Sadhukhan, J. Chem. Soc. Perkin Trans. 1 1998, 275.
[11] It is widely accepted that the palladium-catalyzed cou-
pling reaction of arenediazonium salts with vinylsilanes
proceeds by a Mizoroki–Heck-type addition-elimination
mechanism: a) K. Kikukawa, K. Ikenaga, F. Wada, T.
Matsuda, Chem. Lett. 1983, 1337; b) K. Ikenaga, S. Mat-
sumoto, K. Kikukawa, T. Matsuda, Chem. Lett. 1990,
185.
[7] The role of the methanol has not been clarified at the
present time, but it can be assumed that the oxygen
atom of methanol coordinates to the silicon center to
form a pentacoordinate silicon species, and the resulting
acidic hydrogen atom on the methanol interacts with the
nitrogen atom of the triazene to enhance the oxidative
addition to palladium complex.
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Adv. Synth. Catal. 2004, 346, 1689–1692