A ligand free and room temperature protocol for Pd-catalyzed kumada-corriu couplings of unactivated alkenyl phosphates

Article abstract of DOI:10.1021/jo900098a

Kumada - Corriu cross-couplings of nonactivated cyclic and acyclic vinyl phosphates with aryl magnesium reagents afforded a series of 1,1-disubtituted alkenes in good yields for most cases when the reactions were performed at room temperature with the sim

Full text of DOI:10.1021/jo900098a

and a satisfactory reactivity in Pd-catalyzed coupling with the
use of appropriate ligands. Even if the oxidative addition step
to the catalyst is less favorable with these reagents, use of elec-
tron-rich tertiary phosphines can provide yields comparable to
those obtained with the usual electrophiles.4e,5f,8 The advantages
in the use of phosphates compared to tosylates are their stability
and their simple and inexpensive preparation from the reaction
of phosphoryl chloride with an in situ generated enolate. The
analogous vinyl tosylates require the use of the more expensive
tosyl anhydride for effective derivatization.5b
A Ligand Free and Room Temperature Protocol
for Pd-Catalyzed Kumada-Corriu Couplings of
Unactivated Alkenyl Phosphates
Delphine Gauthier, Stephan Beckendorf, Thomas M. Gøgsig,
Anders T. Lindhardt, and Troels Skrydstrup*
The Center for Insoluble Protein Structures (inSPIN),
Department of Chemistry and the Interdisciplinary
Nanoscience Center, Aarhus UniVersity,
Langelandsgade 140, 8000 Aarhus C, Denmark
In previous reports, we disclosed the application of unactivated
alkenyl phosphates for the direct synthesis of unsymmetrical diaryl
alkenes employing Ni-catalyzed Suzuki-Miyaura and the Pd-
catalyzed Negishi couplings.4a,j,k Hartwig and co-workers revealed
in 2005 the effective coupling of alkenyl tosylates with Grignard
reagents using Pd(dba)2 and JosiPhos ligands.5b Only few reports
on the couplings of unactivated vinyl phosphates with Grignard
reagents have previously been published.4c,9 For example, Kumada
et al. examined a Ni-catalyzed coupling of unactivated aryl phos-
phates with aryl Grignard reagents.9c,10 More recently, Miller
investigated a cross-coupling of an aryl Grignard reagent with in
situ-derived enol phosphates using PdCl2(PPh3)2 as catalyst.9e Iron
catalysis proved also to be particularly efficient for the coupling
of Grignard reagents with terminal dienol and trienol phosphates
as reported by Cahiez et al.11 Furthermore, previous reports have
described that NiBr2 can promote cross-coupling of enol phos-
phates with silylmethylmagnesium reagents without ligands.7c,d
From the point of view of experimental ease, the development
of a palladium ligandless system for promoting Kumada-Corriu
couplings would be of high interest. In this paper, we demon-
ts@chem.au.dk
ReceiVed January 19, 2009
Kumada-Corriu cross-couplings of nonactivated cyclic and
acyclic vinyl phosphates with aryl magnesium reagents
afforded a series of 1,1-disubtituted alkenes in good yields
for most cases when the reactions were performed at room
temperature with the simple palladium salt, PdCl2, without
the presence of phosphine ligands.
(5) (a) Ackermann, L.; Althammer, A. Org. Lett. 2006, 8, 3457. (b) Limmert,
M. E.; Roy, A. H.; Hartwig, J. F. J. Org. Chem. 2005, 70, 9364. (c) Roy, A. H.;
Hartwig, J. F. J. Am. Chem. Soc. 2003, 125, 8704. (d) Baker, W. R.; Pratt, J. K.
Tetrahedron 1993, 39, 8739. (e) Cahiez, G.; Avedissian, H. Synthesis 1998, 1199.
(f) Gelman, D.; Buchwald, S. L. Angew. Chem., Int. Ed. 2003, 42, 5993. (g) Lo
Galbo, F.; Occhiato, E. G.; Guarna, A.; Faggi, C. J. Org. Chem. 2003, 68, 6360.
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1185. (i) Huang, X.; Anderson, K. W.; Zim, D.; Jiang, L.; Klapars, A.; Buchwald,
S. L. J. Am. Chem. Soc. 2003, 125, 6653.
Palladium-catalyzed coupling reactions have emerged as
powerful tools in synthetic organic chemistry for the construction
of carbon-carbon bonds.1 Along side the traditional electro-
philes,1a,2 alkenyl phosphates have proved to be attractive and
convenient coupling partners in such transformations.3-7 Com-
pared to the corresponding and more widely used triflates or
nonaflates, phosphates and tosylates display a higher stability
(6) (a) Hansen, A. L.; Skrydstrup, T. Org. Lett. 2005, 7, 5585. (b) Fu, X.;
Zhang, S.; Yin, J.; McAllister, T. L.; Jiang, S. A.; Chou-Hong, T.; Thiruven-
gadam, K.; Zhang, F. Tetrahedron Lett. 2002, 43, 573.
(1) (a) Tsuji, J. Palladium Reagents and Catalysts, New PerspectiVes For
the 21st Century; John Wiley & Sons: Chichester, U.K., 2004. (b) de Meijere,
A.; Diederich, F. Metal-catalysed Cross-coupling Reactions, 2nd ed.; Wiley-
VCH: Weinheim, Germany, 2004. (c) Handbook of Organopalladium Chemistry
for Organic Synthesis; Negishi, E., Ed.; John Wiley & Sons: New York, 2002.
(d) Tsuji, J. Transition Metal Reagents and Catalysts: InnoVations in Organic
Synthesis; John Wiley & Sons: Chichester, U.K., 2004.
(2) (a) Littke, A. F.; Fu, G., F. Angew. Chem., Int. Ed. 2002, 41, 4176. (b)
Beletskaya, I. P.; Cheprakov, A. V. Chem. ReV. 2000, 100, 3009.
(3) (a) Protti, S.; Fagnoni, M. Chem. Com. 2008, 3611. (b) Steinhuebel, D.;
Baxter, J. M.; Palucki, M.; Davies, I. W. J. Org. Chem. 2005, 70, 10124. (c)
Nicolaou, K. C.; Shi, G.-Q.; Gunzner, J. L.; Ga¨rtner, P.; Yang, Z. J. Am. Chem.
Soc. 1997, 119, 5467. (d) Zhou, J.; Fu, G. C. J. Am. Chem. Soc. 2003, 125,
12527. (e) Wu, J.; Yang, Z. J. Org. Chem. 2001, 66, 7875.
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2006, 47, 2427. (c) Larsen, U. S.; Martiny, L.; Begtrup, M. Tetrahedron Lett.
2005, 46, 4261. (d) Tang, Z.-Y.; Hu, Q.-S. J. Am. Chem. Soc. 2004, 126, 3058.
(e) Nguyen, H. N.; Huang, X.; Buchwald, S. L. J. Am. Chem. Soc. 2003, 125,
11818. (f) Zim, D.; Lando, V. R.; Dupont, J.; Monteiro, A. L. Org. Lett. 2001,
3, 3049. (g) Lepifre, F.; Clavier, S.; Bouyssou, P.; Coudert, G. Tetrahedron
2001, 57, 6969. (h) Nan, Y.; Yang, Z. Tetrahedron Lett. 1999, 40, 3321. (i)
Percec, V.; Bae, J.-Y.; Hill, D. H. J. Org. Chem. 1995, 60, 1060. (j) Hansen,
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(k) Lindhardt, A. T.; Gøgsig, T. M.; Skrydstrup, T. J. Org. Chem. 2009, 74,
135.
(7) Representative references for Ni- or Pd-catalyzed cross-coupling of enol
phosphates with organometallic reagents: (a) Pd/R3Al: Takai, K.; Sato, M.;
Oshima, K.; Nozaki, H. Bull. Chem. Soc. Jpn. 1984, 57, 108. (b) Ni/R3Al:
Fukumiya, N.; Oki, M.; Aratani, T. Chem. Ind. (London) 1981, 606. (c) Ni or
Pd/Me3SiCH2MgCl: Hayashi, T.; Fujiwa, T.; Okamoto, Y.; Katsuro, Y.; Kumada,
M. Synthesis 1981, 1001. (d) Ni or Pd/(i-PrO)2MeSiCH2MgCl: Tamao, K.; Ishida,
N.; Kumada, M. J. Org. Chem. 1983, 48, 2122. (e) Ni/RMgX: Sofia, A.;
Karlstrom, E.; Itami, K.; Backvall, J. J. Org. Chem. 1999, 64, 1745. (f) Ni/
RZnBr: Wu, J.; Yang, Z. J. Org. Chem. 2001, 66, 7875. (g) Pd/RnMn: Fugami,
K.; Oshima, K.; Utimoto, K. Chem. Lett. 1987, 2203.
(8) (a) Ebran, J.-P.; Hansen, A. L.; Gøgsig, T. M.; Skrydstrup, T. J. Am.
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Norrby, P.-O.; Skrydstrup, T. Angew. Chem., Int. Ed. 2006, 45, 3349.
(9) Csp2-Csp2(aryl Grignard) linkages: (a) Cossy, J.; Belotti, D. Tetrahedron
1999, 55, 5145. (b) Sahlberg, C.; Quader, A.; Claesson, A. Tetrahedron Lett.
1987, 24, 5137. (c) Hayashi, T.; Katsuro, Y.; Okamoto, Y.; Kumada, M.
Tetrahedron Lett. 1981, 22, 4449. (d) Schweir, T.; Sromek, A. W.; Yap, D. M. L.;
Chernak, D.; Gevorgyan, V. J. Am. Chem. Soc. 2007, 129, 9868. (e) Miller,
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William, A. D.; Kobayashi, Y. J. Org. Chem. 2002, 67, 8771.
(10) For other examples of ligand free Kumada-Curriu couplings with Ni,
see ref 9g: Lipshutz, B. H.; Tasler, S.; Chrisman, W.; Spliethoff, B.; Tesche, B.
J. Org. Chem. 2003, 68, 1190.
(11) Cahiez, G.; Habiak, V.; Gager, O. Org. Lett. 2008, 10, 2389.
3536 J. Org. Chem. 2009, 74, 3536–3539
10.1021/jo900098a CCC: $40.75 2009 American Chemical Society
Published on Web 04/03/2009

TABLE 1. Optimization Studies in the Kumada-Corriu Coupling
of the O-Cyclohexenyl Phosphate 1a
TABLE 2. Kumada-Corriu Couplings of Cycloalkenyl
Phosphatesa
entry
[Pd] (mol %)
[1] (M)
time (h)
2 (%)b
3 (%)b
c
1
2
Pd(OAc)2 (5)
Pd(dba)2 (5)
PdCl2(COD) (5)
PdCl2(COD) (5)
PdCl2 (5)
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.3
0.3
0.3
24
3
3
3
3
4
7
7
24
6
41
80
91
84
74
83
56
46
51
82
85
82
-
4
3
2
2
3
4d
5
c
6d
7
PdCl2 (5)
-
c
PdCl2(COD) (2)
PdCl2(COD) (1)
PdCl2(COD) (2)
PdCl2(COD) (2)
PdCl2 (2)
-
8
2
2
2
9e
10e f
,
c
11f
3
24
-
c
12f,g
PdCl2 (2)
-
a Reaction conditions: 1 (0.50 mmol), p-tolyl magnesium bromide 1
M in THF (1.5 equiv, 0.75 mmol) added slowly over 2 h, catalyst
(1.0-5.0 mol %), THF (Vtot ) 2.75 mL), in sealed sample vials.
b Isolated yield after column chromatrography. All reactions went to
completion as determined by 1H NMR. c 3 was not isolated (<1%).
d The phosphate was diluted in toluene so as to obtain a mixture of
THF/toluene (0.75:1, 1.75 mL). e The reaction was conducted at 40 °C.
f The reaction was conducted with Vtot ) 1.75 mL of THF. g The
reaction was carried out with the direct addition of o-tolyl magnesium
chloride.
strate the use of PdCl2 as a simple catalyst precursor without
the requirement of phosphine ligands for the room temperature
Kumada-Corriu cross-coupling of unactivated alkenyl phos-
phates. These conditions provide straightforward access to
numerous 1,1-disubstituted alkenes in good yields.
An initial study was carried out on the cross-coupling reaction
of the unactivated vinyl phosphate 1 with p-tolyl magnesium
bromide testing a variety of reaction conditions (Table 1). The
use of PdCl2(COD) at room temperature in THF provided a
satisfactory 91% isolated yield of the coupling product 2 (entry
3) and only 3% of the diene 3, the formation of which was
previously reported by Lipschutz.12 To avoid the generation of
the homocoupling byproduct 3, the THF solution of the or-
ganomagnesium reagent was slowly added over a period of 2 h
with a syringe pump. The efficiency of PdCl2(COD) as a simple
catalyst precursor is surprising considering the requirement of
electron-rich diphosphines in the Pd-catalyzed Kumada-
Corriu coupling of vinyl tosylates with arylmagnesium halides
promoted by JosiPhos ligands.5b Both Pd(dba)2 (entry 2) and
even the less expensive catalyst PdCl2 (entry 5) proved effective
in this coupling leading to an 80% and 74% yield of 2,
respectively, whereas Pd(OAc)2 (entry 1) proved less efficient.
Adding toluene to the media revealed only a slight improvement
in the yield with PdCl2 as catalyst, whereas the opposite effect
was noted for PdCl2(COD) (entries 4 and 6).13 However, by
increasing the reagent concentrations from 0.2 to 0.3 M and
lowering the catalyst loading from 5 to 2 mol %, PdCl2 remained
an effective catalyst for this reaction in contrast to PdCl2(COD),
furnishing 2 in an 85% yield (entry 11) with only traces of the
dimer byproduct 3 (<1%).
a Reaction conditions: phosphate (0.50 mmol), Grignard reagent 1 M
in THF (1.5 equiv, 0.75 mmol), PdCl2 (2.0 mol %), THF (Vtot ) 1.75
mL), rt. Isolated yield after column chromatrography. b The p-tolyl
Grignard reagent was added over a period of 2 h with a syringe pump,
whereas the o-tolyl reagent was added directly.
satisfactory 82% yield of the Kumada-Corriu product could
also be secured with only 2 mol % of PdCl2 (entry 12). Notably,
o-tolyl magnesium chloride could be rapidly added without
formation of the dimer product 3.
With this catalytic system in hand, we then examined the
scope of the Kumada-Corriu coupling using a variety of alkenyl
phosphates. The alkenyl phosphates were obtained by proton
abstraction of the corresponding ketones with LiHMDS in THF
at -78 °C followed by the addition of o,o-diphenyl phosphoryl
chloride.5a,8b
As depicted in Table 2, several cycloalkenyl phosphates
underwent successful coupling in moderate to good yields in
the presence of the aryl Grignard reagents. The coupling with
the more hindered o-tolyl magnesium chloride generally required
longer reaction times (entries 2, 4, and 8). In all reactions, no
vinyl phosphate homocoupling product was detected.
These ligandless conditions proved also applicable to the
Kumada-Corriu coupling of alkyl- and arylvinyl phosphates.
As illustrated in Table 3, vinyl phosphates possessing a secon-
dary or tertiary C1-alkyl substituent were sufficiently reactive
In the case of the coupling of 1 with the sterically more
encumbered Grignard reagent, o-tolyl magnesium chloride, a
(12) Tasler, S.; Lipshutz, B. H. J. Org. Chem. 2003, 68, 1190.
(13) Banno, T.; Hayakawa, Y.; Umeno, M. J. Organomet. Chem. 2002, 653,
288.
J. Org. Chem. Vol. 74, No. 9, 2009 3537

TABLE 3. Kumada-Corriu Couplings of Alkylvinyl Phosphatesa
TABLE 4. Kumada-Corriu Couplings of Arylvinyl Phosphatesa
a Reaction conditions: phosphate (0.50 mmol), Grignard reagent 1 M
in THF (1.5 equiv, 0.75 mmol), PdCl2 (2.0 mol %), THF (Vtot ) 1.75
mL), rt. Isolated yield after column chromatrography. b The p-tolyl
Grignard reagent was added over 2 h with a syringe pump, whereas the
o-tolyl reagent was added directly. c Yield determined by 1H NMR of a
mixture of the desired product and the Grignard homocoupling bypro-
duct (1:0.25). d Yield determined by 1H NMR of mixture of the desired
product, the vinyl phosphate homocoupling byproduct and the Grignard
homocoupling byproduct (8.7:1:0.3).
a Reaction conditions: phosphate (0.50 mmol), Grignard reagent 1 M
in THF (1.5 equiv, 0.75 mmol), PdCl2 (2.0 mol %), THF (Vtot ) 1.75
mL), rt. Isolated yield after column chromatrography. b The p-tolyl
Grignard reagent was added over 2 h with a syringe pump, whereas the
o-tolyl reagent was added directly.
SCHEME 1. The Kumada-Corriu Coupling of the
Arylphosphate 4 with p-Tolyl Magnesium Bromide
with the o-tolyl or p-tolyl magnesium reagents under these
catalytic conditions producing the Kumada-Corriu product in
moderate to good yields.
The results for the Kumada-Corriu coupling of C1-aryl-
substituted vinyl phosphates under ligandless conditions are
depicted in Table 4. Again, this simple coupling procedure
proved to be useful with the four alkenyl phosphates tested
providing coupling yields in the range of 44-74% with the
o-tolyl and p-tolyl Grignard reagents. With the exception of
entry 3, no vinyl phosphate homocoupling product was detected
for these examples. We also briefly studied the Kumada-Corriu
coupling of a (Z)-alkenyl phosphate bearing the reactive function
in a terminal position to study the possible stereoselectivity of
this protocol. The coupling of (Z)-butadienyl diethyl phosphate
with p-tolyl magnesium bromide was not very productive, and
from the 1H NMR spectrum of the crude reaction mixture, both
the cis- and trans-diene products could be detected. Hence,
further work is required to optimize these reaction conditions.
Compared to the reactivity of the alkenyl tosylates, which
were reported to require an electron-rich diphosphine ligand for
promoting the Pd-catalyzed coupling with Grignard reagents,
it is surprising that these ligandless conditions are effective for
performing similar transformations with the vinyl phosphates.5b
In an earlier report, Sekiya and Ishikawa suggested that PdII-
salts in the presence of aryl magnesium halides form reactive
nanopalladium particles.14 Whether or not such nanopalladium
particles are the reactive species under these conditions is cur-
rently under investigation.
Finally, in order to explore the scope of this catalytic protocol,
we examined two aryl phosphates in their coupling with p-tolyl
magnesium bromide. Whereas the more electron-rich diethyl
4-methoxyphenyl phosphate proved unreactive, diphenyl 4-(tri-
fluoromethyl)phenyl phosphate 4 displayed higher reactivity and
could be coupled furnishing the biaryl system 5 in a good 57%
isolated yield after a reaction time of 24 h at room temperature
(Scheme 1).
In conclusion, we have successfully developed a ligandless room
temperature protocol for the Pd-catalyzed Kumada-Corriu cou-
pling of unactivated C1-alkenyl phosphates with aryl magnesium
(14) Sekiya, A.; Ishikawa, N. J. Organomet. Chem. 1977, 125, 281.
3538 J. Org. Chem. Vol. 74, No. 9, 2009

quenched with methanol (0.5 mL), concentrated in vacuo, and
purified by flash chromatography on silica gel with 10% CH2Cl2
in pentane as the eluant. This afforded 94.2 mg of the title
reagents. These reactions were catalyzed by the simplest palladium
salt, PdCl2, at room temperature without the necessity of phosphine
ligands. By avoiding expensive ligands and thermal energy, this
work contributes to the challenge of developing economical pro-
tocols for the creation of carbon-carbon bonds. Further inves-
tigations are ongoing in an attempt to improve these coupling
conditions with other aromatic phosphates.
1
compound (82% yield) as a colorless oil. H NMR (400 MHz,
CDCl3) δ (ppm) 7.20-7.08 (m, 4H), 5.61-5.58 (m, 1H), 2.40-2.19
(m, 3H), 2.32 (s, 3H), 2.03-1.94 (m, 2H), 1.47-1.31 (m, 2H),
0.96 (s, 9H). 13C NMR (100 MHz, CDCl3) δ (ppm) 144.5, 138.8,
135.2, 130.1, 128.4, 126.5, 126.0, 125.6, 44.1, 32.4, 31.8, 27.4 (3C),
27.3, 24.7, 20.0. GCMS C17H24 [M] calcd 228, found 228.
Spectroscopic data were in accordance with those reported in the
literature.
Experimental Section
1-(4-tert-Butylcyclohex-1-enyl)-4-methylbenzene (2):5b Gen-
eral Procedure for the Kumada-Corriu Coupling of Vinyl
Phosphates with p-Tolyl Magnesium Bromide. 4-tert-Butylcy-
clohex-1-enyl diphenyl phosphate (193 mg, 0.50 mmol) and PdCl2
(1.8 mg, 0.01 mmol) were dissolved in THF (1 mL). The p-tolyl
magnesium bromide (0.75 mL of a 1.0 M solution in THF, 1.5
equiv) was added slowly over 2 h at room temperature. The sample
vial was then fitted with a Teflon sealed screwcap and removed
from the glovebox. After an additional 3 h of stirring, the reaction
mixture was quenched with methanol (0.5 mL), concentrated in
vacuo, and purified by flash chromatography on silica gel with
pentane as the eluant. This afforded 97.1 mg of the title compound
(85% yield) as a colorless oil. 1H NMR (400 MHz, CDCl3) δ (ppm)
7.30 (d, J ) 8.0 Hz, 2H), 7.13 (d, J ) 8.0 Hz, 2H), 6.12-6.10 (m,
1H), 2.56-2.50 (m, 2H), 2.47-2.37 (m, 1H), 2.35 (s, 3H),
2.30-2.22 (m, 2H), 1.43-1.27 (m, 2H), 0.94 (s, 9H). 13C NMR
(100 MHz, CDCl3) δ (ppm) 139.5, 136.3, 136.2, 129.6 (2C), 124.9
(2C), 124.2, 44.0, 32.4, 29.0, 27.6, 27.4 (3C), 24.6, 21.2. GCMS
C17H24 [M] calcd 228, found 228. Spectroscopic data were in
accordance with those reported in the literature.
1-(4-tert-Butylcyclohex-1-enyl)-2-methylbenzene (Table 1,
entry 12):15 General Procedure for the Kumada-Corriu Coup-
ling of Vinyl Phosphates with o-Tolyl Magnesium Chloride.
4-tert-Butylcyclohex-1-enyl diphenyl phosphate (193 mg, 0.50
mmol) and PdCl2 (1.8 mg, 0.01 mmol) were dissolved in THF (1
mL). o-Tolyl magnesium chloride (0.75 mL of a 1.0 M solution in
THF, 1.5 equiv) was added at room temperature. The sample vial
was then fitted with a Teflon sealed screwcap and removed from
the glovebox. After 24 h of stirring, the reaction mixture was
4-Methyl-4′-(trifluoromethyl)biphenyl (5).5a Diphenyl 4-(tri-
fluoromethyl)phenyl phosphate (197 mg, 0.50 mmol) and PdCl2
(1.8 mg, 0.01 mmol) were dissolved in THF (1 mL). p-Tolyl
magnesium bromide (0.75 mL of a 1.0 M solution in THF, 1.5
equiv) was added slowly over 2 h at room temperature. The sample
vial was then fitted with a Teflon sealed screwcap and removed
from the glovebox. After an additional 22 h of stirring, the reaction
mixture was quenched with methanol (0.5 mL), concentrated in
vacuo, and purified by flash chromatography on silica gel with
pentane as the eluant. This afforded 67.6 mg of the title compound
1
(57% yield) as a colorless solid. H NMR (400 MHz, CDCl3) δ
(ppm) 7.68 (m, 4H), 7.51 (d, J ) 8.0 Hz, 2H), 7.27 (d, J ) 8.0 Hz,
2H), 2.43 (s, 3H). 13C NMR (100 MHz, CDCl3) δ (ppm) 144.8,
138.3, 137.0, 129.8 (2C), 129.2 (q, J ) 32.3 Hz), 127.3 (2C), 127.2
(2C), 125.8 (q, J ) 3.8 Hz, 2C), 124.5 (q, J ) 271.7 Hz), 21.3. 19
F
NMR (376 MHz, CDCl3) δ (ppm) -62.75. GCMS C14H11F3 [M]
calcd 236, found 236. Mp 121-122 °C. Spectroscopic data were
in accordance with those reported in the literature.
Acknowledgment. We are deeply appreciative of generous
financial support from the Danish National Research Foundation,
The Danish Research Council for Technology and Production
Sciences, the Carlsberg Foundation, the COST D40 Action, the
OChem and iNANO Graduate Schools, and Aarhus University.
Supporting Information Available: Experimental details for
all compounds including copies of 1H NMR and 13C NMR spectra.
This material is available free of charge via the Internet at
http://pubs.acs.org.
(15) Littke, A. F.; Dai, C.; Fu, G. C. J. Am. Chem. Soc. 2000, 122, 4020.
JO900098A
J. Org. Chem. Vol. 74, No. 9, 2009 3539