Preparation and Palladium-Catalyzed Cross-Coupling of Aryl Triethylammonium Bis(catechol) Silicates with Aryl Triflates

Article abstract of DOI:10.1021/jo035309q

Pentavalent aryl and heteroaryl bis(catechol) silicates undergo palladium-catalyzed cross-coupling with aryl and heteroaryl triflates in the presence of a fluoride source in excellent yields. These solid, air-stable bis(catechol) silicates are prepared from a high-yielding displacement reaction between catechol and an aryl siloxane in the presence of an amine base. The cross-coupling reaction is tolerant of a wide range of electron-donating and electron-withdrawing groups. Several examples of di-ortho-substituted triflates are successfully coupled with these reagents.

Full text of DOI:10.1021/jo035309q

P r ep a r a tion a n d P a lla d iu m -Ca ta lyzed Cr oss-Cou p lin g of Ar yl
Tr ieth yla m m on iu m Bis(ca tech ol) Silica tes w ith Ar yl Tr ifla tes
W. Michael Seganish and Philip DeShong*
Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742
deshong@umd.edu
Received September 5, 2003
Pentavalent aryl and heteroaryl bis(catechol) silicates undergo palladium-catalyzed cross-coupling
with aryl and heteroaryl triflates in the presence of a fluoride source in excellent yields. These
solid, air-stable bis(catechol) silicates are prepared from a high-yielding displacement reaction
between catechol and an aryl siloxane in the presence of an amine base. The cross-coupling reaction
is tolerant of a wide range of electron-donating and electron-withdrawing groups. Several examples
of di-ortho-substituted triflates are successfully coupled with these reagents.
SCHEME 1
In tr od u ction
Palladium-catalyzed cross-coupling reactions of aryl
halides or triflates to form unsymmetrical biaryls are one
of the most useful tools in synthetic organic chemistry.
Several of the most common methods for performing
these transformations are the Stille (organostannane),
Suzuki-Miyaura (organoboron), and Hiyama (organo-
silicon) reactions (Scheme 1).1-5 The more established
Suzuki-Miyaura and Stille reactions have benefited from
high yields and wide functional group tolerances. Despite
these advantages, organosilicon reagents have emerged
as a viable alternative for the synthesis of unsymmetrical
biaryls, due in part to the inherent drawbacks in the
Stille and Suzuki-Miyaura methodologies.6-23 For ex-
ample, Stille couplings require the use of toxic organotin
SCHEME 2
* To whom correspondence should be addressed. Tel: 301-405-1892.
Fax: 301-314-9121.
(1) Negishi, E. Handbook of Organopalladium Chemisty for Organic
Synthesis; J ohn Wiley and Sons: New York, 2002; Vol. 1.
(2) Fugami, K.; Kosugi, M. Top. Curr. Chem. 2002, 219, 87-130 and
references therein.
reagents, while the boronic acid derivatives utilized in
Suzuki couplings can be difficult to synthesize and purify.
Our laboratory has focused on the use of aryl trialkoxy-
siloxanes as alternatives to the organotin and organobo-
ron compounds. These siloxanes can be readily synthe-
sized via metalation (Li or Mg) of the corresponding aryl
halide followed by nucleophilic attack upon readily avail-
able tetraalkyl orthosilicates or through palladium-24,25
or rhodium-catalyzed26 silylation (Scheme 2). In the
presence of a fluoride activator, aryl siloxanes undergo
Pd(0)-catalyzed cross-coupling with aryl iodides,21 bro-
mides,21,22 and chlorides.22 In addition, Fu has recently
(3) Miyaura, N. Top. Curr Chem. 2002, 219, 11-59 and references
therein.
(4) Hassan, J .; Sevignon, M.; Gozzi, C.; Schulz, E.; Lemaire, M.
Chem. Rev. 2002, 102, 1359-1469 and references therein.
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9695.
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10.1021/jo035309q CCC: $27.50 © 2004 American Chemical Society
Published on Web 01/17/2004
J . Org. Chem. 2004, 69, 1137-1143
1137

Seganish and DeShong
SCHEME 3
SCHEME 4
extended this methodology to include couplings with alkyl
bromides and iodides.9
(60-84%) and in poor to moderate yields with aryl
bromides, triflates, and electron-rich aryl iodides (21-
57%) (Scheme 4).
In addition to low yields, these reactions suffered from
long reaction times (30-110 h). It is within this context
that we sought to develop a methodology for the efficient
cross-coupling of bis(catechol) silicates with aryl triflates.
Additionally, we have extended the scope of the coupling
to electron-rich aryl iodides.68
One limitation of the siloxane-based methodology is
that aryl siloxanes do not couple efficiently to aryl
triflates due to competing hydrolysis of the triflate under
the basic reaction conditions. However, the use of aryl
triflates is advantageous because they can easily be
derived from the corresponding phenol and are often
more accessible than the analogous aryl halide.27 Recent
studies in our laboratory have demonstrated that silox-
ane derivatives can be employed for the coupling of aryl
triflates under strictly controlled conditions.28 Nonethe-
less, we sought to develop new silicon-based reagents that
would give better yields and show greater functional
group tolerance and synthetic utility than the siloxane
based approach. In this paper, we describe the prepara-
tion and coupling reactions of aryl bis(catechol) silicate
anions with aryl triflates.
Resu lts a n d Discu ssion
Aryl bis(catechol) silicates are readily prepared in
excellent yield by the reaction of their siloxane counter-
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thesized by Frye in 1962.29 They are readily prepared in
excellent yields from alcoholic solutions of catechol, the
respective siloxane, and triethylamine (Scheme 3).29
The structure30-52 and reactivity53-63 of these com-
plexes have been studied extensively. However, their use
as carbon-carbon bond-forming reagents remains rela-
tively unexplored.64-67 In one of the few examples of their
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of a suitable palladium catalyst in refluxing dioxane will
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Org. Chem. 1990, 55, 2415-2420.
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3050-3053.
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1138 J . Org. Chem., Vol. 69, No. 4, 2004

Aryl Triethylammonium Bis(catechol) Silicates
TABLE 1. Syn th esis of Tr ieth lya m m on iu m Ar yl
Bis(ca tech ol) Silica tes fr om Ar yl Siloxa n esa
polarity of the reaction solvent has an adverse effect upon
the reaction. This result can be attributed to the ability
of the solvent to coordinate to the palladium and disrupt
the catalytic cycle.
Two notable features of this reaction were discovered
during the course of the optimization studies. It was
found that catechol could be quantitatively recovered
from the crude cross-coupling reaction mixture by extrac-
tion with aqueous base (1 M NaOH), followed by acidi-
fication and re-extraction of the aqueous layer. Also,
approximately 50% of the phosphine 2 could be recovered
during chromatographic separation of the reaction prod-
ucts.
entry
R
yield (%)
1
2
3
4
5
6
7
8
9
H
88
82
94
86
83
90
88
82
89
2-OCH3
3-OCH3
4-OCH3
2-CH3
3-CH3
4-CH3
Efficient cross-coupling reactions with aryl triflates is
very gratifying because previously we had reported that
siloxanes failed to undergo cross-coupling with triflates
due to facile hydrolysis of the triflate under the coupling
conditions.21,22 Subsequently, we had reported a revised
protocol using silatrane analogues that allows for cross
coupling with electron-rich aryl triflates in fair to good
overall yields.28 The silicate methodology discussed above,
however, is more general and consistently gives higher
yields of product than the silatrane method.
3,4-methylenedioxy
4-CO2Et
a
Reactions were perfomed at room temperature overnight.
parts with 2 equiv of catechol in the presence of triethy-
lamine (see Table 1). The silicates are white, air stable,
crystalline solids that can be recrystallized from metha-
nol-ether (when necessary). The salts are freely soluble
in warm ethereal solvents (THF, dioxane) and DMF.
Employing the protocol established in our laboratory
for siloxane couplings, the bis(catechol) silicates did not
cross-couple well with aryl bromides (Table 2, entries
1-4) or chlorides (data not shown). With bromides, the
yields of coupled product were <40% and chlorides gave
no product at all. For comparison, the analogous reaction
with the aryl trialkoxysiloxane afforded 90% of the
respective unsymmetrical biaryl.21 The bis(catechol) sili-
cates did couple successfully with aryl iodides (Table 2,
entries 5 and 6). However, the best coupling substrates
for the bis(catechol) silicates were aryl triflates (Table
2, entries 7-16). As summarized in Table 2 (entries 9,
14-16), the best conditions for the cross-coupling with
anisoyl triflate were 5 mol % palladium catalyst and 5
mol % dicyclohexylphosphinobiphenyl (Buchwald phos-
phine) 2 in refluxing dioxane, although satisfactory yields
can be obtained at lower temperatures in refluxing THF.
It is likely that this electron-rich phosphine facilitates
rapid oxidative addition into the aryl triflate, while at
the same time its steric bulk allows it to readily disas-
sociate and allow transmetalation with the hypervalent
silicate species. Employing a similar ligand to 2, di-tert-
butylphosphinobiphenyl (3)69 provided only a 23% yield
of the cross-coupled product (Table 3, entry 12), providing
further evidence that phosphine 2 combines the optimal
combination of steric and electronic effects to promote
the reaction.
Having established that iodides and triflates are ef-
fective coupling substrates with aryl bis(catechol) sili-
cates, a systematic investigation of the scope and limi-
tations of the reaction was undertaken. The results
summarized in Table 3 indicate that ortho-, meta-, and
para-substituted electron-rich aryl iodides (entries 1-3)
couple with phenyl bis(catechol) silicate. In addition,
meta- and para-substituted bis(catechol) silicates were
successfully coupled with iodobenzene (Table 3, entries
5 and 6). However, the ortho-substituted silicate coupled
in poor yield (entry 4). This is most likely due to steric
crowding around the silicon atom, which hinders trans-
metalation of the aryl ring to the palladium.
The analogous study was performed with aryl triflates
to evaluate the functional group and steric tolerances of
the reaction. The results are summarized in Table 4. The
data indicates that this new methodology is applicable
to a wide range of aryl triflates. Attempted coupling of
very strongly electron-deficient triflates (Table 4, entry
12) proceeded in only moderate yield, with the remainder
of the mass balance being 4-nitrophenol (hydrolysis
product). Coupling of unprotected 4-aniline triflate was
not successful (Table 4, entry 9); however, when the
amine was acylated, cross-coupling proceeded in excellent
yield (Table 4, entry 7). The inability of the cross-coupling
reaction to proceed in the presence of a free amino group
is attributed to coordination of the amine to the pal-
ladium and poisoning of the catalyst.
Entries 16 and 17 are of particular interest. These di-
ortho-substituted aryl triflates coupled in only moderate
yield (Table 4, entry 17), or not at all (entry 16) when
the reaction was run in THF. However, when the reaction
was run at a higher temperature (refluxing dioxane),
these substrates coupled in excellent yields. This result
indicates that it is possible to couple more stericly
demanding substrates by switching the solvent to dioxane
and performing the reaction at a higher temperature.70
It is thought that the higher reaction temperature
achieved in refluxing dioxane facilitates oxidative addi-
The poor yields of coupled product obtained in DMF
versus THF or dioxane indicate that increasing the
(69) Wolfe, J . P.; Singer, R. A.; Yang, B. H.; Buchwald, S. L. J . Am.
Chem. Soc. 1999, 121, 9550-9561.
J . Org. Chem, Vol. 69, No. 4, 2004 1139

Seganish and DeShong
TABLE 2. Op tim iza tion of th e P a lla d iu m -Ca ta lyzed Cr oss-Cou p lin g Rea ction of Tr ieth yla m m on iu m Bis(ca tech ol)
P h en yl Silica te w ith Ar yl Tr ifla tes
entry
R
silicate complex (equiv)a
PdLn (mol %)
phosphine (mol %)
solvent (T, °C)
yield (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Br
Br
Br
Br
I
I
OTf
OTf
OTf
OTf
OTf
OTf
OTf
OTf
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
1.0
1.5
Pd(OAc)2 (10)
Pd(OAc)2 (10)
Pd(dba)2 (10)
Pd(dba)2 (10)
Pd(OAc)2 (10)
Pd(dba)2 (10)
Pd(OAc)2 (10)
Pd(OAc)2 (10)
Pd(dba)2 (10)
Pd(dba)2 (10)
Pd(dba)2 (10)
Pd(dba)2 (10)
Pd(dba)2 (10)
Pd(dba)2 (10)
PPh3 (20)
PPh3 (20)
2 (10)
2 (10)
PPh3 (20)
2 (10)
PPh3 (20)
PPh3 (20)
2 (10)
none
PPh3 (10)
3 (10)
DMF (90)
DMF (150)
DMF (90)
THF (65)
DMF (90)
THF (65)
DMF (90)
THF (65)
THF (65)
THF (65)
THF (65)
THF (65)
THF (65)
THF (65)
40
40
38
7
87
9
75
0
96
11
0
23
49
95
2 (10)
2 (10)
15
16
OTf
OTf
1.5
1.5
Pd(dba)2 (5)
Pd(dba)2 (5)
2 (5)
2 (5)
THF (65)
dioxane (101)
93
97
a
For each equivalent of the catechol complex, 1.0 equivalent of TBAF was added.
TABLE 3. P a lla d iu m -Ca ta lyzed Cr oss-Cou p lin gs of
Tr ieth yla m m on iu m Bis(ca tech ol) P h en yl Silica te w ith
Ar yl Iod id es
TABLE 4. P a lla d iu m -Ca ta lyzed Cr oss-Cou p lin gs of
Tr ieth yla m m on iu m Bis-ca tech ol P h en yl Silica te w ith
Ar yl Tr ifla tesa
entry
R
yieldb (%)
bromidec
88
1
2
3
4-OCH3
2-OCH3
4-CHO
96
95
98
4
5
6
7
4-COCH3
2-COCH3
4-CO2CH3
4-NHAc
96
93
94
92
86
93
77
8
9
4-tert-butyl
4-NH2
95
0
90
24
10
11
12
13
14
15
16
17
4-CN
2-CN
4-NO2
1-naphthyl
2,6-dimethoxy
2-CH3
2,6-dimethyl
2-methoxy-6-formyl
91
89
46d
86
58
0e (0)f
92
96e (0)f
86e (64)f
a
b
Reactions were perfomed for 6 h. Reaction was performed
in refluxing THF as solvent unless otherwise noted. c Yields are
obtained using the corresponding aryl bromide in place of the
tion of the palladium into these hindered systems. This
assertion is supported by the fact that at lower temper-
triflate, coupling to phenyltrimethoxysilane.21,22 Remainder of
d
mass balance was hydrolyzed triflate. e Yield in refluxing dioxane.
f Yield in refluxing THF.
(70) It was noted by a reviewer that the effect seen by switching
the solvent from THF to dioxane may not be entirely attributable to
the increased temperature of the reaction. Rather, the difference in
dielectric constant (2.21 for dioxane, 7.58 for THF) may play a greater
role. However, two experimental results indicate that temperature,
not polarity may, be the significant factor. When the coupling reactions
were performed in dioxane at the reflux temperature of THF, no
improvement in the yield of product was observed. In addition, when
toluene was used as the solvent (dielectric constant of 2.15 at reflux),
no cross-coupling was observed.
atures (THF), the triflate is recovered quantitatively
(Table 4, entries 14 and 16). However, 2,6-dimethoxyphe-
nyl trifluoromethanesulfonate (Table 4, entry 14) would
not couple with the silicate, even in refluxing dioxane.
The triflate was recovered.
1140 J . Org. Chem., Vol. 69, No. 4, 2004

Aryl Triethylammonium Bis(catechol) Silicates
TABLE 5. P a lla d iu m -Ca ta lyzed Cr oss-Cou p lin gs of
Tr ieth yla m m on iu m Bis-ca tech ol P h en yl Silica te w ith
Heter oa r yl Tr ifla tesa
TABLE 6. P a lla d iu m -Ca ta lyzed Cr oss-Cou p lin g of
Tr ieth yla m m on iu m Bis-ca tech ol P h en yl Silica te w ith
High ly F u n ction a lized Tr ifla tes
a
Reactions were run using 1.5 equiv of triethylammonium
bis(catechol) phenyl silicate, 1.5 equiv of TBAF, 5 mol % of
b
Pd(dba)2, and 5 mol % of 2 in refluxing dioxane for 6 h. Value in
parentheses is the yield obtained in refluxing THF. c Bromide yield
refers to the coupling of phenyl trimethoxysilane with the corre-
a
b
Solvent was THF. Solvent was dioxane. c Triflate was hy-
d
sponding heteroaryl bromide.22,23 Reaction run in refluxing THF.
drolyzed under all conditions investigated.
The coupling of bis(catechol) silicates with heteroaryl
triflates was also investigated. Palladium-catalyzed cross-
coupling reactions of heteroaryl halides with aryl silox-
anes proceed in only moderate yields and we were
therefore interested to see if we could improve on the
yield by employing a coupling between heteroaryl tri-
flates and aryl bis(catechol) silicates.18,71 The results are
presented in Table 5.
The coupling of pyridyl triflates with triethylammo-
nium bis(catechol) phenyl silicate in refluxing dioxane
gave the desired cross-coupled products in yields that
were comparable to the equivalent heteroaryl bromide/
siloxane reactions (Table 5, entries 1 and 2).22 However,
the coupling of unprotected 5-indole trifluoromethane-
sulfonate (Table 5, entry 3) proceeded in a significantly
higher yield than that observed in the corresponding
siloxane reaction.18 In addition, the coupling of 2-quino-
line trifluoromethanesulfonate (Table 5, entry 4) pro-
ceeded in excellent yield.
The scope and limitations of triflate coupling were
extended to the highly functionalized triflates shown in
Table 6. the first coupling was 2-tropolone trifluo-
romethanesulfonate (4) with bis(catechol) phenyl silicate
(Table 6, entry 1). The tropolone ring system is present
in the colchicine family of alkaloids, and these compounds
have shown promise as anti-cancer agents by binding to
cellular tubulin in a Taxol-like mechanism.72 Thus, there
is interest in developing novel analogues. Tropolone 4
cross coupled in excellent yield with triethylammonium
bis(catechol) phenyl silicate to produce the functionalized
tropone 5, thus demonstrating the ability to add aryl
functionality to the tropolone system.
Similarly, the triflate of R-tocopherol (6) (vitamin E)
was chosen to evaluate the ability of the bis(catechol)
silicates to cross couple with highly hindered systems
(Table 6, entry 2). When triflate 6 was allowed to couple
with bis(catechol) phenyl silicate in refluxing dioxane, the
desired cross-coupled product 7 was obtained in excellent
yield.
Attempts to couple 5-coumarin trifluoromethane-
sulfonate (8) yielded none of the desired cross-coupled
product 9 (Table 6, entry 3). Instead, the triflate was
quantitatively hydrolyzed. This example clearly demon-
strates that the reaction conditions that are appropriate
for the coupling of aryl triflates are not suitable for the
coupling of vinyl triflates.
Having demonstrated the scope and limitations of the
reaction with various triflate substrates, attention was
turned to a systematic investigation of substituent effects
on the aryl bis(catechol) silicates. These results are
presented in Table 7. In summary, the presence of
electron-rich (Table 7, entries 5 and 6) and electron-
deficient (entry 7) functionalities is well tolerated. The
presence of functional groups in the meta and para
positions of the aryl ring do not adversely affect the cross-
coupling yields. However, when a substituent is present
in the ortho position, the yield drops dramatically (Table
7, entries 1 and 4). The yield can be increased slightly
by performing the reaction at a higher temperature
(refluxing dioxane). This outcome is not entirely unex-
(71) Handy, C. J . Preparation and NMR Studies of Hypervalent
Silicates and Their Use in Palladium-Catalyzed Cross Coupling
Reactions; PhD Dissertation, University of Maryland, College Park,
2002.
J . Org. Chem, Vol. 69, No. 4, 2004 1141

Seganish and DeShong
TABLE 7. P a lla d iu m -Ca ta lyzed Cr oss-Cou p lin gs of Ar yl Tr ieth yla m m on iu m Bis-ca tech ol Silica tes w ith Ar yl Tr ifla tes
a
b
Yield obtained in refluxing dioxane. Yield obtained in refluxing THF.
pected as a similar result was obtained in the coupling
of the aryl iodides (Table 3, entry 4).
tempted (Scheme 5). We were pleased to find that this
coupling proceeded smoothly to produce the quinolopy-
ridine 15 in 97% yield. It is anticipated that an analogous
coupling reaction will be applicable to the total synthesis
of streptonigrin and related substances.
Another interesting system that would further define
the scope of the bis(catechol) silicate coupling technology
was the coupling of a heteroaromatic triflate with a
heteroaromatic bis(catechol) silicate. Quinoline-pyridyl
moieties are found in multiple natural products such as
streptonigrin73 and camptothecin.74 We, therefore, de-
signed a template that would probe the feasibility of
applying the bis(catechol) silicate coupling technology to
these systems. Pyridyl derivative 11 is readily available
in excellent yield from the respective pyridyl siloxane 10
(Scheme 5).25 Having obtained the desired heteroaromatic
bis(catechol) silicate 11, a simple triflate was chosen to
probe the feasibility of the coupling reaction. Accordingly,
heteroaromatic bis(catechol) silicate 11 was coupled with
4-methoxyphenyl trifluromethanesulfonate 12 to yield
the heteroaromatic biaryl 13 in 96% yield (Scheme 5).
Encouraged by the high yield obtained in this reaction,
the coupling of 2-quinoline trifluoromethanesulfonate 14
with heteroaromatic bis(catechol) silicate 11 was at-
A final feature of the cross-coupling reaction that was
explored was the fluoride activation source. The typical
fluoride source was tetrabutylammonium fluoride (TBAF)
in THF solution (Acros) containing 5% water. It was
observed that this reagent gave extensive hydrolysis of
triflates derived from phenols bearing strong electron-
withdrawing groups (Table 4, entry 12). A variety of
alternative fluorides was investigated and the results are
summarized in Table 8.
In the absence of a fluoride source, the cross-coupling
reaction does not occur (entry 1). Alkali fluoride salts
were ineffective as fluoride sources for the coupling
reactions (entries 2-6). TBAF trihydrate, available as a
crystalline solid, was as effective as the 1.0 M solution
of TBAF in THF in promoting the cross-coupling reaction
(entries 7 and 8). Tetramethylammonium fluoride trihy-
drate (TMAF) was also found to have similar activity
(entry 9). It is interesting to note that the use of
anhydrous TMAF actually lowered the yield of the cross-
coupling reaction, with the remainder of the triflate
(72) Nicolaou, K. C.; Dai, W. M.; Guy, R. K. Angew. Chem., Int. Ed.
Engl. 1994, 106, 15-44.
(73) Godard, A.; Marsais, F.; Ple, N.; Trecourt, F.; Turck, A.;
Queguiner, G. Heterocycles 1995, 40, 1055-1091.
(74) Hutchinson, C. R. Tetrahedron 1981, 37, 1047-1065.
1142 J . Org. Chem., Vol. 69, No. 4, 2004

Aryl Triethylammonium Bis(catechol) Silicates
SCHEME 5
anols.75 However, this reagent was not effective when
extended to the coupling of bis(catechol) silicates with
aryl triflates (Table 8, entry 11).
TABLE 8. Effect of Va r yin g th e F lu or id e Sou r ce on
Cr oss-Cou p lin g Efficien cy of Tr ieth yla m m on iu m
Bis-ca tech ol P h en yl Silica te w ith 4-Meth oxyp h en yl
Tr iflu or om eth a n esu lfon a te
Con clu sion
Aryl siloxanes efficiently couple to aryl iodides, bro-
mides, and chlorides to provide unsymmetrical biaryl
derivatives. However, the cross-coupling of aryl triflates
to aryl siloxanes has been largely unsuccessful due to
hydrolysis of the triflate under the reaction conditions.
The studies reported in this paper demonstrate that
pentacoordinate aryl bis(catechol) silicates serve as si-
loxane surrogates and couple to a wide range of aryl and
heteroaryl triflates in excellent yields. the application of
this methodology to the synthesis of heteroaromatic
natural products is underway and will be reported in due
course.
entry
“F- source”
yield (%)
1
2
3
4
5
6
7
8
9
none
KF
NaF
CsF
LiF
0a
29a
0a
3a
0a
NaBF4
TBAF‚3H2O
30a
94
TBAF (1.0 M in THF)
TMAF‚3H2O
95
96
10
11
TMAF (anhydrous)b
NaOSiMe3
74c
33a
Ack n ow led gm en t. We thank Dr. Yiu-Fai Lam for
his assistance in obtaining NMR spectra and Noel
Whittaker for help in preparing mass spectral data. The
generous financial support of the National Institutes of
Health (CA 82169) is acknowledged.
Remainder of triflate was unreacted. b Commerically available
a
“anhydrous” TMAF contains less than 3% water. c Remainder of
triflate was hydrolyzed.
Su p p or tin g In for m a tion Ava ila ble: Experimental pro-
undergoing hydrolysis (entry 10). It is plausible that the
water present in the hydrated TMAF is serving to
moderate the bacicity of the fluoride ion in solution and
slow the rate of hydrolysis of the triflate. A similar effect
was noticed in the coupling of aryl triflates and non-
aflates with silanols.7
cedures for the preparation and characterization of all com-
1
pounds is provided. H NMR spectra are available for all new
compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
J O035309Q
Denmark had previously reported that silanolates were
effective promoters in the cross-coupling of organosil-
(75) Denmark, S. E.; Sweis, R. F. J . Am. Chem. Soc. 2001, 123,
6439-6440.
J . Org. Chem, Vol. 69, No. 4, 2004 1143