Application of silicon-based cross-coupling technology to triflates

Article abstract of DOI:10.1021/jo034809g

Aryl silatranes undergo fluoride-induced cross-coupling with aryl triflates to provide unsymmetrical biaryl derivatives in good to excellent yields. Silatranes also couple with aryl iodides and bromides, although the yields of adduct are lower than with the corresponding siloxane derivates. Aryl siloxanes (which had previously failed to couple with triflates) can be employed for triflate couplings using the Denmark modification, although the yields are lower than the corresponding silatrane reactions.

Full text of DOI:10.1021/jo034809g

Ap p lica tion of Silicon -Ba sed Cr oss-Cou p lin g Tech n ology to
Tr ifla tes
Shaundra Riggleman and Philip DeShong*
Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742
pd10@umail.umd.edu
Received J une 11, 2003
Aryl silatranes undergo fluoride-induced cross-coupling with aryl triflates to provide unsymmetrical
biaryl derivatives in good to excellent yields. Silatranes also couple with aryl iodides and bromides,
although the yields of adduct are lower than with the corresponding siloxane derivates. Aryl
siloxanes (which had previously failed to couple with triflates) can be employed for triflate couplings
using the Denmark modification, although the yields are lower than the corresponding silatrane
reactions.
SCHEME 1
In tr od u ction
Organosilicon reagents have played an increasingly
important role in Pd(0)-catalyzed cross-coupling with
organohalides.1-18 Our laboratory has developed the Pd-
(0)-catalyzed fluoride-promoted reaction of aryl halides
with aryl siloxanes. This methodology tolerates electron-
deficient, electron-rich, and ortho-substituted systems to
provide the unsymmetrical biaryl adduct in good to
excellent yields (Scheme 1).19,20
While siloxanes undergo cross-coupling with aryl ha-
lides in excellent yields, they failed to efficiently couple
* To whom correspondence should be addressed. Tel: 301-405-1892.
Fax: 301-314-9121.
(1) Hirabayashi, K.; Mori, A.; Kawashima, J .; Suguro, M.; Nishihara,
Y.; Hiyama, T. J . Org. Chem. 2000, 65, 5342-5349.
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(3) Hassan, J .; Sevignon, M.; Gozzi, C.; Schulz, E.; Lemaire, M.
Chem. Rev. 2002, 102, 1359-1469.
(4) Hiyama, T. In Metal-Catalyzed Cross-Coupling Reactions; Dieder-
ich, F., Stang, P. J ., Eds.; Wiley-VCH: New York, 1998; pp 421-453.
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1309-1310.
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Akiba, K.-Y., Ed.; Wiley-VCH: New York, 1999; pp 147-169.
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Chem. 2000, 65, 1554-1557.
(18) Hiyama, T.; Shirakawa, E. In Cross-Coupling Reactions; Miyaura,
N., Ed.; Springer-Verlag: Berlin, 2002; Vol. 219, pp 61-85.
(19) Mowery, M. E.; DeShong, P. J . Org. Chem. 1999, 64, 3266-
3270.
(20) Mowery, M. E.; DeShong, P. J . Org. Chem. 1999, 64, 1684-
1688.
with aryl triflates.21 When phenyltrimethoxysilane was
cross-coupled with an aryl triflate, the major product was
the result of hydrolysis of the aryl triflate. The goal of
this research was to explore the possibility of using
phenylsilatrane as a cross-coupling reagent with aryl
triflates, which are often more accessible than the halide
derivatives.
It was initially thought that phenylsilatrane (1) would
be adequately hypervalent to transfer its phenyl group
without the need for activation by fluoride because
silatrane and its derivatives constitute a class of formally
pentacoordinate silicon compounds by virtue of the
transannular Si-N bond.22-24 One of the benefits of the
silatrane derivatives is they have been found to be stable
under standard hydrolysis and alcoholysis conditions.25-28
Also, they are crystalline solids, do not polymerize, are
(21) Mowery, M. E. PhD, University of Maryland College Park, 2000.
(22) Frye, C. L.; Vogel, G. E.; Hall, J . A. J . Am. Chem. Soc. 1961,
83, 996-997.
(23) Frye, C. L.; Vincent, G. A.; Finzel, W. A. J . Am. Chem. Soc.
1971, 93, 6805-6811.
(24) Turley, J . W.; Boer, F. P. J . Am. Chem. Soc. 1968, 90, 4026-
4030.
(25) Verkade, J . G. Coord. Chem. Rev. 1994, 137, 233-295.
10.1021/jo034809g CCC: $25.00 © 2003 American Chemical Society
Published on Web 09/25/2003
8106
J . Org. Chem. 2003, 68, 8106-8109

Application of Silicon-Based Cross-Coupling Technology to Triflates
TABLE 1. Op tim iza tion of th e Cou p lin g of
4-Meth oxyp h en yl Tr ifla te w ith P h en ylsila tr a n e
SCHEME 2
entry
na
phosphine
time (h)
yield (%)
stable at room temperature, and are easily prepared from
their siloxane precursors. The most common method for
synthesis of aryl silatranes is the alcoholysis of the
corresponding trialkoxysilane by triethanolamine, which
leads to the exclusive formation of crystalline monomeric
product in excellent yield.22,23 In turn, the siloxane
precursors for the synthesis of silatrane derivatives can
be easily prepared using either a palladium(0) or rhod-
ium(I) catalyst.29-31
1
2
3
4
5
6
7
3
10
20
30
20
20
20
2
2
2
2
12
12
12
24
24
48
48
65
83
90
93
3
35
PPh3
P(o-tol)3
<5b
<5c
a
Hydrated levels of TBAF were made by premixing appropriate
b
amounts of water with the TBAF‚3 H2O. 85% starting material
recovered. c 80% starting material recovered.
The initial studies involving phenylsilatrane as a
phenyl-transfer agent revealed that the dative bond in
this pentacoordinate silicon system was not sufficient to
promote transfer of phenyl. However, with TBAF activa-
tion, phenylsilatrane was found to couple with aryl
triflates (Scheme 2).
Recently, Denmark reported that the addition of water
to his silanol coupling reactions would slow the rate of
triflate hydrolysis, enabling a higher yield of coupled
product.32 The original reaction conditions for couplings
utilized TBAF as a 1 M solution with a 3:1 mole ratio of
H2O to TBAF (Table 1, entry 1). In an attempt to increase
the yield of coupled product, the amount of water in the
TBAF solution was increased. Table 1 describes the
results of varying first the amount of water added and
then changing the phosphine ligand.
TABLE 2. Cou p lin g of P h en ylsila tr a n e w ith Ar yl
Tr ifla tesa ,b
entry
R
time (h)
yield (%)
Upon addition of 20 equiv of water to 1 equiv of TBAF
to the coupling medium, no hydrolyzed product was
detected (Table 1, entry 3) and an excellent yield of the
coupled adduct was obtained. Entries 1 and 2 showed less
than 10% hydrolyzed product, and addition of water did
not affect reaction time until after 30 equiv of water had
been added as seen in entry 4.
1
2
3
4
5
6
7
8
9
4-OMe
2-OMe
2,3-dimethoxy
2,6-dimethoxy
4-H
4-Me
4-NO2
4-Ac
4-CO2Me
12
12
12
48
12
12
1
90
90
73
0c,d
75
87
50d
56d
21d
1
1
Assorted phosphines were surveyed as additives in the
coupling reaction (Table 1, entries 5-7). On the basis of
the results summarized in Table 1, the optimal reaction
conditions for coupling of 4-methoxyphenyl triflate were
20:1 H2O/TBAF and the use of the Buchwald ligand (2)
(Table 1, entry 3).33-35
a
Hydrated levels of TBAF were made by premixing appropriate
b
amounts of water with the TBAF‚3H2O. Conditions for entries
1-5 utilized 20 equiv of H2O and 2-(dicyclohexylphosphino)biphen-
yl; entries 6-8 utilized 10 equiv of H2O and PPh3. c 56% starting
d
material recovered. Mass balance was hydrolyzed triflate.
(26) Garant, R. J .; Daniels, L. M.; Das, S. K.; J anakiraman, M. N.;
J acobson, R. A.; Verkade, J . G. J . Am. Chem. Soc. 1991, 113, 5728-
5735.
After optimization of the coupling of phenylsilatrane
with 4-methoxyphenyl triflate, other triflate substrates
were investigated (Table 2). Interestingly, an electron-
donating ortho substituent on the aryl triflate ring did
not affect the coupling reaction (Table 2, entries 2 and
3). Unfortunately, the sterically hindered 2,6-dimeth-
oxyphenyl triflate does not couple with phenylsilatrane.
Presumably, it is unable to undergo oxidative addition
with the Pd(0) catalyst. Aryl substrates bearing strong
electron-donating groups gave the best yields, although
weaker donating groups also gave good yields of adduct
(Table 2, entries 5 and 6). We also saw efficient coupling
with 1-naphthalene triflate (68%). With electron-deficient
aryl triflates, (Table 2, entries 7-9), the yields were lower
(27) Sidorkin, S. F.; Pestunovich, V. A.; Voronkov, M. G. Russ. Chem.
Rev. Engl. Transl. 1980, 49, 414-427.
(28) Yoshikawa, A.; Gordon, M. S.; Sidorkin, V. F.; Pestunovich, V.
A. Organometallics 2001, 20, 927-931.
(29) Murata, M.; Suzuki, K.; Watanabe, S.; Masuda, Y. J . Org.
Chem. 1997, 62, 8569-8571.
(30) Manoso, A. S.; DeShong, P. J . Org. Chem. 2001, 66, 7449-7455.
(31) Murata, M.; Shimazaki, R.; Watanabe, S.; Masuda, Y. Synthesis
2001, 2231-2233.
(32) Denmark, S. E.; Sweis, R. F. Org. Lett. 2002, 4, 3771-3774.
(33) Torraca, K. E.; Kuwabe, S. I.; Buchwald, S. L. J . Am. Chem.
Soc. 2000, 122, 12907-12908.
(34) Harris, M. C.; Huang, X.; Buchwald, S. L. Org. Lett. 2002, 4,
2885-2888.
(35) Aranyos, A.; Old, D. W.; Kiyomori, A.; Wolfe, J . P.; Sadighi, J .
P.; Buchwald, S. L. J . Am. Chem. Soc. 1999, 121, 4369-4378.
J . Org. Chem, Vol. 68, No. 21, 2003 8107

Riggleman and DeShong
TABLE 3. P h en ylsila tr a n e Cou p lin g w ith Ar yl Ha lid es
SCHEME 3
entry
X
R
yield (%) (siloxane yieldsa (%))
1
2
3
4
5
I
I
Br
Br
Br
4-acetyl
4-methoxy
4-acetyl
4-methoxy
2,6-dimethyl
80 (77b)
78 (91b)
71 (86a)
73 (74a)
65 (85a)
a
Results of the reaction with phenyltrimethoxysilane as re-
ported by Mowery and DeShong: Mowery, M. E.; DeShong, P. Org.
b
Lett. 1999, 1, 2137-2140. Results of the reaction with phenyl-
SCHEME 4
trimethoxysilane using the method of Mowery and DeShong:
Mowery, M. E.; DeShong, P. Org. Lett. 1999, 1, 2137-2140.
TABLE 4. Cou p lin g 4-Meth oxyp h en yl Tr ifla te w ith
P h en yltr im eth oxysila n e
entry
na
0b
3
10
20
30
time (h)
yield (%)
1
2
3
4
5
12
12
12
12
24
<10c
36c
53c
79
72
due to competing hydrolysis of aryl triflate. It is note-
worthy that the rate of hydrolysis of the triflates in these
instances is faster than the rate of the cross-coupling
reaction.
a
Hydrated levels of TBAF were made by premixing appropriate
b
amounts of water with the TBAF‚3H2O. Using “anhydrous”
TBAF solutions purchased from Acros (contains <5% water).
d
Results from Mowery and DeShong, ref 21. Mass balance was
There are two main strategies in dealing with the
hydrolysis of the aryl triflate: (1) the use of silatrane
derivates to decrease hydrolyzed triflate and (2) the use
of water to control the rate of hydrolysis. We propose that
hydrolysis of the aryl triflate occurs via the mechanism
described in Scheme 3. First, the siloxane 4 undergoes
hypervalent silicate formation with fluoride to give
silicate 5. Presumably, silicate 5 is in equilibrium with
silyl fluoride 6 and methoxide, and it is the alkoxide that
is responsible for hydrolysis of the aryl triflate. If the rate
of hydrolysis of the triflate is faster than either aryl
transfer of silicate 5 to aryl palladium intermediate 7 or
the rate of oxidative addition of Pd(0) into the triflate,
then hydrolysis of the triflate will occur faster than
oxidative addition and cross-coupling. The goal was to
inhibit hydrolysis either by decreasing the effective
concentration of methoxide or by decreasing the rate of
triflate hydrolysis.
One effective tactic to accomplish this goal was to
employ silatrane 8 rather than siloxane reagents. The
rate of hydrolysis of the triflate substrate was dramati-
cally reduced when using phenylsilatrane. Fluoride ad-
dition to silatrane 8 gave silicate 9 which can undergo
alkoxide loss (10 in Scheme 4). Unlike the situation with
siloxane reagents, however, the effective concentration
of compound 10 is low because of the intramolecular
(unimolecular) return to silicate. The bimolecular hy-
hydrolyzed triflate.
drolysis reaction becomes less competitive. The end result
is that the rate of coupling exceeds the rate of hydrolysis.
Having demonstrated that aryl triflates underwent
coupling with phenylsilatrane, aryl halides were also
coupled to phenylsilatrane (Table 3). As with phenyltri-
methoxysilane, phenylsilatrane gives higher yields with
aryl iodides (Table 3, entries 1 and 2) than with aryl
bromides (Table 3, entries 3-5). Electron-withdrawing
and -donating groups did not greatly effect the reactivity
of iodides or bromides. Entry 5 is particularly noteworthy,
demonstrating that phenylsilatrane will couple with
ortho-ortho hindered systems. Phenylsilatrane does not
couple well with aryl chlorides (0-30%).
Previously, we had reported that aryl triflates were not
viable substrates for coupling with siloxanes due to rapid
hydrolysis of the triflate under the coupling conditions.21
However, by adding excess water to the reaction mixture
as noted by Denmark,32 hydrolysis of the triflate was
dramatically suppressed and the coupling of the siloxane
derivatives proceeded in good yields as summarized in
Table 4. In entry 4, Table 4, no hydrolysis of starting
material was observed with 20 equiv of water in the
8108 J . Org. Chem., Vol. 68, No. 21, 2003

Application of Silicon-Based Cross-Coupling Technology to Triflates
TBAF solution, while there was significant hydrolysis of
the triflate with lesser amounts of water in the reaction
media.
enabled the parent siloxane derivative to couple with aryl
triflates in good yield.
In conclusion, phenylsilatrane has been shown to
undergo palladium-catalyzed cross-coupling with aryl
triflates and halides to give unsymmetrical biaryls in
good yields. Phenylsilatrane has been found to be stable
to standard hydrolysis and alcoholysis conditions.25-28
Also, it is a crystalline solid, does not polymerize, is stable
at room temperature, and is easily prepared from the
commercially available phenyltrimethoxysilane precur-
sor. Addition of water to the reaction medium was
observed to increase the yields of the coupled adduct and
Ack n ow led gm en t. We thank Dr. Yiu-Fai Lam for
his assistance in obtaining NMR spectra. The generous
financial support of the National Institutes of Health
(CA 82169-01) is acknowledged.
Su p p or tin g In for m a tion Ava ila ble: Experimental pro-
1
cedures and H and 13C NMR spectra for all compounds. This
material is available free of charge via the Internet at
http://pubs.acs.org.
J O034809G
J . Org. Chem, Vol. 68, No. 21, 2003 8109