Mild and efficient aryl-alkenyl coupling via Pd(II) catalysis in the presence of oxygen or Cu(II) oxidants

Article abstract of DOI:10.1021/jo020159p

We report herein a mild and efficient method for carbon-carbon bond formation between aryl stannanes and olefins via Pd(II) catalysis in the presence of oxygen or Cu(II) oxidants as a reoxidant. The process allows reactions between various olefins and aryl stannanes of varying electron density. Coupling methods under these oxidation conditions are comparatively described, and the benefits and limitations are also discussed.

Full text of DOI:10.1021/jo020159p

TABLE 1. Tr a n sition Meta l Oxid a n ts Scr een ed
Mild a n d Efficien t Ar yl-Alk en yl Cou p lin g
via P d (II) Ca ta lysis in th e P r esen ce of
Oxygen or Cu (II) Oxid a n ts
J ay P. Parrish, Young Chun J ung, Seung Il Shin, and
Kyung Woon J ung*
oxidant
oxidant
entry (3.0 equiv) yield (%)
entry (3.0 equiv) yield (%)
1
2
3
4
5
CuCl2
Cu(OAc)2
FeCl3
CuF2
Cu(acac)2
96
92
82
63
21
6
7
8
9
Cu(OH)2
CuCO3
AlCl3
15
14
14
10
Department of Chemistry, University of South Florida,
4
202 East Fowler Avenue, Tampa, Florida 33620-5250
MnO2
kjung@chuma.cas.usf.edu
Received March 8, 2002
6
bond-forming reactions at ambient temperatures. How-
ever, this modified procedure required a full equivalent
of a Pd(II) reagent unless used in conjunction with Cu(II)
salts as reoxidants. In addition, only limited examples
were provided in regards to aryl stannanes and olefins,
leaving this promising method far from general use.
To further develop the olefin/stannane coupling into
an efficient catalytic protocol, we embarked on develop-
ment of mild and versatile conditions by changing
oxidants, bases, and solvents. While conducting our
optimization studies, Mori reported a Pd(II)-catalyzed
Abstr a ct: We report herein a mild and efficient method
for carbon-carbon bond formation between aryl stannanes
and olefins via Pd(II) catalysis in the presence of oxygen or
Cu(II) oxidants as a reoxidant. The process allows reactions
between various olefins and aryl stannanes of varying
electron density. Coupling methods under these oxidation
conditions are comparatively described, and the benefits and
limitations are also discussed.
The Heck reaction has become a standard tool for
1
carbon-carbon bond formation and is frequently utilized
coupling of olefins and aryl stannanes using Cu(OAc)
2
to prepare valuable intermediates in organic syntheses.2
Typical conditions require high temperatures and long
reaction times as well as reactive substrates such as aryl
iodides or triflates.2,3 Due to recent advances addressing
these issues, room-temperature reactions are feasible,
even with unreactive aryl chlorides.4 Different ap-
proaches employing organometallic variations have also
been studied to avert the aforementioned shortcomings.5
For instance, Heck successfully introduced mercurial or
stannyl moieties as halide surrogates in carbon-carbon
,3
7
as a catalyst reoxidant. This methodology provided a
catalytic alternative to Heck’s conditions, allowing for the
coupling of nonallylic olefins with aryl stannanes. How-
ever, the reaction conditions remained harsh (100 °C, 24
h, DMF) and provided limited examples. In an effort to
mitigate these pitfalls, our studies have focused on
specific Pd(II) reoxidants including transition metals,
oxygen and air, and organic based oxidants.
Initially, we undertook reaction optimization using tert-
butyl acrylate 1 and commercially available tributylphen-
yltin as the coupling partner in the presence of various
transition metal reoxidants (Table 1). Of the oxidants
(1) For reviews, see: (a) Beletskaya, I. P.; Cheprakov, A. V. Chem.
Rev. 2000, 100, 3009. (b) Ikeda, M.; El Bialy, S. A. A.; Yakura, T.
Heterocycles 1999, 51, 1957. (c) Shibasaki, M.; Boden, C. O. J .; Kojima,
A. Tetrahedron 1997, 53, 7371. (d) Gibson, S. E.; Middleton, R. J .
Contemp. Org. Synth. 1996, 3, 447. (e) Negishi, E.; Coperet, C.; Ma,
S.; Liou, S.-Y.; Liu, F. Chem. Rev. 1996, 96, 365. (f) de Meijere, A.;
Meyer, F. E. Angew. Chem., Int. Ed. Engl. 1994, 33, 2379. (g) Heck,
R. F. Org. React. 1982, 27, 345. (h) Heck, R. F. Acc. Chem. Res. 1979,
screened, CuCl
(entries 1 and 2) while FeCl
2
and Cu(OAc)
2
were the most efficient
gave lower yields,
3
and CuF
2
still providing some reoxidation (entries 3 and 4). How-
ever, other examined transition metals proved ineffective
for this transformation (entries 5-9). Although not shown
in the table, other components were also varied in the
1
2, 146.
2) (a) Brase, S.; de Meijere, A. In Metal-catalyzed Cross-coupling
(
presence of CuCl
conditions. It was found that Pd(OAc)
OCOCF were all effective catalysts for this transfor-
2
as a default oxidant to seek optimal
Reactions; Diederich, F., Stang, P. J ., Eds.; Wiley: New York, 1998;
pp 99-166. (b) Link, J . T.; Overman, L. E. In Metal-catalyzed Cross-
coupling Reactions; Diederich, F., Stang, P. J ., Eds.; Wiley: New York,
2
, PdCl , and Pd-
2
(
3 2
)
1
998; pp 231-269.
(
3) For examples related to total synthesis, see: (a) Muratake, H.;
mation, providing tert-butyl (E)-cinnamate 2 in greater
than 90% yield. Palladium catalysts containing electron-
donating ligands such as PdCl (PPh ) and Pd(PPh )
2 3 2 3 4
Abe, I.; Natsume, M. Tetrahedron Lett. 1994, 35, 2573. (b) Kojima, A.;
Takemoto, T.; Sodeoka, M.; Shibasaki, M. J . Org. Chem. 1996, 61, 4876.
(c) Overman, L. E.; Ricca, D. J .; Tran, V. D. J . Am. Chem. Soc. 1997,
1
1
19, 12031. (d) Tietze, L. F.; Schirok, H. Angew. Chem., Int. Ed. Engl.
997, 36, 1124.
were ineffective. In addition to THF as the solvent of
choice, polar solvents such as DMF and EtOH were
equally adequate as reaction solvents, whereas in ben-
zene, couplings remained incomplete after 24 h. More-
(4) For recent examples of low-temperature traditional Heck reac-
tions, see: (a) J effery, T. In Advances in Metal-Organic Chemistry;
Liebeskind, L. S., Ed.; J AI: London, UK, 1996; pp 153-260. For
examples of low-temperature Heck reactions with aryl chlorides, see:
(
b) Littke, A. F.; Fu, G. C. J . Am. Chem. Soc. 2001, 123, 6989.
(5) For some selected examples, see: (a) Cho, C. S.; Motofusa, S.-I.;
(6) For the initial report, see: Heck, R. F. J . Am. Chem. Soc. 1968,
90, 5518. For the mechanistic rationale, see: (a) Heck, R. F. J . Am.
Chem. Soc. 1969, 91, 6707. (b) Heck, R. F. J . Am. Chem. Soc. 1971,
93, 6896.
(7) (a) Hirabayashi, K.; Ando, J .-I.; Nishihara, Y.; Mori, A.; Hiyama,
T. Synlett 1999, 99. (b) Hirabayashi, K.; Ando, J .-I.; Kawashima, J .;
Nishihara, Y.; Mori, A.; Hiyama, T. Bull. Chem. Soc. J pn. 2000, 73,
1409.
Ohe, K.; Uemura, S. Bull. Chem. Soc. J pn. 1996, 69, 2341. (b) Oda,
H.; Morishita, M.; Fugami, K.; Sano, H.; Kosugi, M. Chem. Lett. 1996,
8
4
11. (c) Fugami, K.; Hagiwara, S.; Oda, H.; Kosugi, M. Synlett 1998,
77. (d) Oi, S.; Moro, M.; Ono, S.; Inoue, Y. Chem. Lett. 1998, 83. (e)
Kang, S.-K.; Choi, S.-C.; Ryu, H.-C.; Yamaguchi, T. J . Org. Chem. 1998,
3, 5748. (f) Matoba, K.; Motofusa, S.-I.; Cho, C. S.; Ohe, K.; Uemura,
S. J . Organomet. Chem. 1999, 574, 3.
6
1
0.1021/jo020159p CCC: $22.00 © 2002 American Chemical Society
Published on Web 09/05/2002
J . Org. Chem. 2002, 67, 7127-7130
7127

TABLE 2. Effect of Ba se Ch oice on Cou p lin g
TABLE 3. Effect of Oxid a n t Ch oice on Rea ction of
ter t-Bu tyl Acr yla te a n d P h Sn Bu 3
entry
base (2.0 equiv)
time (h)
yield (%)
1
2
3
4
5
NaOAc
LiOAc
CsOAc
Et3N
1
1
1
24
5
96
95
89
70
67
entry
oxidant
time (h)
yield (%)
1
2
3
nonea
air
O2
48
48
12
15
37
74
No base
a
Reaction was run under a N2 atmosphere.
over, reaction conditions were tolerant to moisture, and
an inert atmosphere was not necessary.
reduces cost and cleanup as well as avoids side reactions
caused by metal salts and their byproducts. Salient
14
As shown in Table 2, base selection played an impor-
tant role in the reaction. NaOAc and LiOAc were the
bases of choice, giving tert-butyl (E)-cinnamate 2 in 96
and 95%, respectively (entries 1 and 2), and CsOAc was
also effective (entry 3). Although tertiary amine bases
are known to reduce Pd(II) catalysts in situ to Pd(0),8
we observed a decrease in reaction efficiency when
triethylamine was used, providing the coupled product
in 70% yield after 24 h. Interestingly, a feature of this
reaction was the ability to use base-free conditions in
certain cases, therefore eliminating another undesirable
reaction component. In the absence of a base, the reaction
still proceeded in moderate yields, delivering tert-butyl
examples of oxygen-promoted metal-catalyzed carbon-
carbon bond formations include aryl-aryl and aryl-
15,16
alkenyl couplings.
Moreover, oxygen can be utilized
17
as the sole source of catalyst reoxidant presumably
through coordination of oxygen to the metal. Under
18
similar conditions, carbon-carbon bond formation has
been demonstrated between aryl stannanes and olefins,
prompting us to adopt molecular oxygen as a reoxidizing
19
agent in our Pd(II)-catalyzed coupling protocol.
In comparison, the coupling reactions were run under
nitrogen, air, or oxygen (Table 3). These gases were
delivered either by bubbling or via oxygen balloon. Very
low yields were found when the reaction was run under
a nitrogen atmosphere, indicating the need for an oxygen
source (entry 1). When air was used, only 37% yield of
the desired product was obtained (entry 2). Likewise with
oxygen, a 74% yield of 2 was acquired in a shorter
reaction time (entry 3). From these results, we inferred
that oxygen was the source of reoxidation and best
applied in a pure form. We then established the optimal
reaction conditions using NaOAc as the base. Other bases
such as K CO were also effective; however, organic bases
(
E)-cinnamate 2 in 67% after 5 h (entry 5). Despite these
benefits, we found several limitations in the Cu(II)-
promoted reaction. The use of CuCl for allylic systems
2
9
,10
led to undesired chloroalkylation
although Cu(OAc)
2
avoided this side reaction.11 This Cu(II)-promoted reac-
tion was limited in substrates (vide infra), which was
enough to invoke our attention to discover more efficient
oxidants.
Since the discovery of the Wacker process as an
2
3
1
2
industrially viable route to acetaldehyde, molecular
oxygen together with metal salts has been used to
promote numerous palladium-catalyzed reactions.13 As
an environmentally benign source, oxygen drastically
including Et N were not. Solvent selection was crucial
3
as only DMF or NMP worked well in all cases, while
solvents such as THF or benzene did not. Using these
optimized conditions, we investigated substrate limita-
tions and compared their results with the Cu(II)-
promoted reactions.
(
8) Negishi, E.; Coperet, C.; Ma, S.; Liou, S.-Y.; Liu, F. Chem. Rev.
996, 96, 365.
2
9) Use of CuCl effected chloroalkylation to produce iii in 73% yield.
1
(
(14) (a) Tsuji, J .; Shimizi, I.; Kobayashi, Y. Isr. J . Chem. 1984, 24,
1
1
1
53. (b) Tsuji, J .; Shimizi, I.; Yamamoto, K. Tetrahedron Lett. 1976,
7, 2975. (c) McOuillin, F. J .; Parker, D. G. J . Chem. Soc., Perkin Trans.
1974, 809.
(
15) For recent examples, see: (a) Wong, M. S.; Zhang, X. L.
Tetrahedron Lett. 2001, 42, 4087. (b) Smith, K. A.; Campi, E. M.;
J ackson, W. R.; Marcuccio, S.; Naeslund, C. G. M.; Deacon, G. B. Synlett
1
997, 131. (c) Alcaraz, L.; Taylor, R. J . K. Synlett 1997, 791. (d)
Shirakawa, E.; Murota, Y.; Nakao, Y.; Hiyama, T. Synlett 1997, 1143.
(e) Moreno-Manas, M.; Perez, M.; Pleixats, R. J . Org. Chem. 1996, 61,
2346.
(16) For recent examples, see: (a) Weissman, H.; Song, X.; Milstein,
D. J . Am. Chem. Soc. 2001, 123, 337. (b) Matoba, K.; Motofusa, S.-I.;
Cho, C. S.; Ohe, K.; Uemura, S. J . Organomet. Chem. 1999, 574, 3. (c)
Miura, M.; Tsuda, T.; Satoh, T.; Pivsa-Art, S.; Nomura, M. J . Org.
Chem. 1998, 63, 5211.
(17) (a) van Benthem, R. A. T. M.; Hiemstra, H.; Michels, J . J .;
Speckamp, W. N. J . Chem. Soc., Chem. Commun. 1994, 357. (b) Larock,
R. C.; Hightower, T. R. J . Org. Chem. 1993, 58, 5298.
(18) (a) Stahl, S. S.; Thorman, J . L.; Nelson, R. C.; Kozee, M. A. J .
Am. Chem. Soc. 2001, 123, 7188. (b) Thiel, W. R. Angew. Chem., Int.
Ed. Engl. 1999, 38, 3157.
(19) For the initial report, see: (a) Heck, R. F. J . Am. Chem. Soc.
1968, 90, 5518. For recent examples, see: (b) Hirabayashi, K.; Ando,
J .-I.; Kawashima, J .; Nishihara, Y.; Mori, A.; Hiyama, T. Bull. Chem.
Soc. J pn. 2000, 73, 1409. (c) Hirabayashi, K.; Ando, J .-I.; Nishihara,
Y.; Mori, A.; Hiyama, T. Synlett 1999, 99.
We think that the desired product ii does form, but reacts quickly by
recoordinating with Pd(II). After metal-olefin coordination, chloride
attacks the most activated, electron poor position, which is usually
benzylic in our examples.
(10) For examples of chloroalkylation, see: (a) Heck, R. F. J . Am.
Chem. Soc. 1968, 90, 5538. (b) Backvell, J .-E.; Nordberg, R. E. J . Am.
Chem. Soc. 1980, 102, 393. (c) Tamaru, Y.; Hojo, M.; Higashimura,
H.; Yoshida, Z. Angew. Chem., Int. Ed. Engl. 1986, 25, 735.
(11) The oxidation of Pd(0) is aided by the presence of chloride ions,
which helps to stabilize Pd(II) and Cu(I). For a reference, see: J ira,
R.; Freiesleben, W. Organomet. React. 1972, 3, 5.
(12) (a) Smidt, J . Chem. Ind. 1962, 54. (b) Smidt, J .; Hafner, W.;
J ira, R.; Sieber, R.; Sedlmeier, J .; Sabel, J . Angew. Chem., Int. Ed.
Engl. 1962, 1, 80. (c) Smidt, J .; Hafner, W.; J ira, R.; Sedlmeier, J .;
Sieber, R.; Ruttinger, R.; Kojer, H. Angew. Chem. 1959, 71, 176.
(
13) For a review, see: Tsuji, J . Palladium Reagents and Catalysis,
Innovations in Organic Synthesis; Wiley: New York, 1996; pp 19-
24.
1
7
128 J . Org. Chem., Vol. 67, No. 20, 2002

TABLE 4. Com p a r ison of Reoxid a n ts
TABLE 5. Cou p lin g w ith Non a llylic Olefin s
a
Cu(II) ) CuCl2; solvent ) THF. Solvent ) DMF. 55 °C. d E/
b
c
Z ) 2.3/1. Cu(II) ) Cu(OAc)2. f E/Z ) 1/11. E/Z ) 1/10.
e
g
a
Cu(II) ) CuCl ; solvent ) THF. b Solvent ) DMF. c 55 °C. d E/
As illustrated in Table 4, this newly developed oxygen
protocol was generally more efficient than the Cu(II)
method. In particular, some alkenes were not effected
under Cu(II) conditions, however compatible with the
oxygen conditions. Unactivated aliphatic alkenes such as
2
Z ) 2.6/1.
TABLE 6. Cou p lin g w ith Allylic Olefin s
1
-heptene (3) gave 82% yield of (E)-hept-1-enylbenzene
in only 2 h whereas the Cu(II) protocol gave no desired
product (entry 1). Aromatic nonallylic olefins including
styrene (4) were comparable in both cases (entry 2).
4
-Methyl-5-vinylthiazole (5), a heterocyclic nonallylic
compound, gave 51% yield of the desired product under
oxygen conditions, albeit in low yield and at 55 °C;
however, Cu(II) conditions were not effective (entry 3).
We attributed this moderate yield to the steric effects of
the ortho methyl group. Allyl phenyl sulfone (6), which
is an electron-poor allylic system, was converted smoothly
to the (E)-(3-phenylsulfonylpropenyl)benzene in 73%
yield with a 2.3/1 E/ Z ratio after 20 h while resulting in
no reaction under Cu(II) conditions (entry 4). Both allyl
benzyl ether (7), an electron-rich allylic system, and
3
-methylene-2-norbnanone (8), an activated vicinal di-
substituted olefin, offered higher yields of the desired
products in the presence of oxygen than Cu(II) oxidants
(entries 5 and 6). These representative examples implied
that both reoxidants were efficient and oxygen conditions
were more versatile.
Next, we screened various nonallylic olefins to probe
general applicability of both methodologies (Table 5).
Electron- poor alkenes were sluggish in the presence of
oxygen, and Cu(II) conditions were more reliable (entries
a
Cu(II) ) Cu(OAC)2; solvent ) THF. b Solvent ) DMF. c E/Z
)
2.3/1. d E/Z ) 1.6/1.
1
and 2). With 4-chloro- (11) and 4-bromostyrene (12),
also furnished the corresponding coupling products with-
out much difficulty (entries 6 and 7, respectively).
As shown in Table 6, these improved methods were
effective in the formation of disubstituted olefins uniquely
from allylic systems. Under both conditions, allylbenzene
(16) was converted to (E)-1,3-diphenylpropene smoothly
(entry 1), and electron-rich eugenol derivative (17) gave
the coupled product in good yields (entry 2). However,
electron-poor pentafluorobenzene (18) was successfully
oxygen conditions gave no Stille products, producing only
desired products in good yields (entries 3 and 4). Under
the oxygen conditions, 5-bromo-1-pentene (13) gave 80%
of the coupling product without any side products result-
ing from elimination or transmetalation, whereas the
Cu(II) protocol failed to deliver the desired product (entry
5
). Electron-rich 2,3,4-trimethoxystyrene (14) and gem-
disubstituted olefins such as tert-butyl methacrylate (15)
J . Org. Chem, Vol. 67, No. 20, 2002 7129

reacted only under oxygen conditions (entry 3). From
these examples, it was inferred that electron-rich sub-
strates offered higher yields than electron-poor congeners
as observed in the previous nonallylic systems. Subse-
quently, olefins with sensitive functional groups were
subjected to both coupling reactions, delivering the
desired olefins in moderate to good yields. Allyl phenyl-
acetate (19) produced only 67% yield of the desired
product along with 30% of ester hydrolysis product under
the oxygen conditions; however, the Cu(II) method de-
livered 81% of desired olefin with no hydrolysis product
TABLE 7. Va r iou s Ar yl Sta n n a n es Scr een ed w ith
Va r yin g Electr on Den sitiesa
(entry 4). Using both methods, allyl glycidyl ether (20)
provided the coupled product in good yields (entry 5), and
no epoxide ring-opening was detected. N-Allyl pyrrole-
carboxaldehyde (21) was also compatible with both
methodologies without any allylic migration (entry 6).
After screening various classes of olefins, we investi-
gated the scope and limitation of aryl stannane coupling
partners. In general, electron-rich aryl stannanes were
effective as demonstrated with 3,4-dimethoxyphenyl
stannane (22), giving almost quantitative yields under
both reoxidant conditions (entry 1). On the other hand,
aryl stannanes with electron-withdrawing groups includ-
ing 4-trifluoromethylphenylstannane (23) and (4-tribu-
tylstannyl)benzaldehyde (24) were relatively sluggish
under the oxygen conditions compared to the Cu(II)
protocol (entries 2 and 3). Sterically congested 2-meth-
oxyphenylstannane (25) was also compatible with both
conditions (entry 4). These stannanes reacted with dif-
ferent olefins, exhibiting similar trends (entries 5-7).In
conclusion, we applied oxygen and Cu(II) as catalyst
reoxidants in the palladium-catalyzed coupling of olefins
and aryl stannanes. Our methodologies allowed for the
synthesis of disubstituted olefins from nonallylic and
allylic systems in favor of E-isomers. The use of aryl
stannanes with various electron densities and sterics was
also feasible with this reaction. Oxygen conditions offered
an improvement over transition metal promoted cou-
plings due to elimination of metal salts and their byprod-
ucts that might cause side reactions. Thus, these oxygen
conditions successfully facilitated aryl-alkenyl bond
formation with several olefins incompatible with Cu(II)
conditions. The synthetic applications of this method will
be discussed in due course.
a
A: oxidant ) O2, solvent ) DMF. B: oxidant ) CuCl2, solvent
)
THF.
trated in vacuo and purified by flash chromatography (30 g of
SiO ). Elution with petroleum ether (100 mL), then 9:1 hexanes/
EtOAc afforded (E)-tert-butyl cinnamate (2; 98 mg, 96%) as a
clear oil. Data for 2: R
250 MHz, CDCl ) δ 1.56 (s, 9 H), 6.40 (d, J ) 16.0 Hz, 1 H),
7.39 (m, 2 H), 7.53 (m, 3 H), 7.62 (d, J ) 16.0 Hz, 1 H); C NMR
(62.5 MHz, CDCl ) δ 30.9, 55.4, 71.2, 114.4, 122.4, 128.4, 131.9,
40.3, 144.2, 158.7, 166.6.
Rep r esen ta tive Exp er im en ta l P r oced u r e (Oxygen con -
d ition ): tert-Butyl acrylate (1; 64 mg, 0.5 mmol, 1 equiv) was
dissolved in DMF (2.5 mL, 0.2 M solution) and stirred at room
temperature. To this clear solution was added tributylphenyltin
(184 mg, 0.5 mmol, 1 equiv) followed by a single addition of
2
1
f
) 0.60 (9:1 hexanes/EtOAc); H NMR
(
3
1
3
3
1
NaOAc‚3H
2 2
O (136 mg, 1.0 mmol, 2 equiv) and Pd(OAc) (11 mg,
0
.05 mmol, 0.1 equiv). The reaction flask was suspended over a
balloon of oxygen, and the solution was stirred at room temper-
ature for 12 h. The mixture was then diluted with diethyl ether
10 mL), filtered through a plug of neutral alumina, and washed
with diethyl ether (3 × 10 mL). The filtrate was concentrated
in vacuo, and subjected to flash chromatography (30 g of SiO ).
(
2
Exp er im en ta l Section
Elution with petroleum ether (100 mL), then 9:1 hexanes/EtOAc
afforded (E)-tert-butyl cinnamate (75 mg, 74%).
Rep r esen ta tive Exp er im en ta l P r oced u r e (Cu (II) con d i-
tion ): tert-Butyl acrylate (1; 64 mg, 0.5 mmol, 1 equiv) was
dissolved in tetrahydrofuran (2.5 mL, 0.2 M solution) and stirred
at room temperature. To this clear solution was added tribu-
tylphenyltin (184 mg, 0.5 mmol, 1 equiv) followed by a single
Ack n ow led gm en t. We acknowledge generous fi-
nancial support in part from the National Institutes of
Health (RO1 GM 62767).
addition of CuCl
mg, 1.0 mmol, 2 equiv), and Pd(OAc)
2
(201 mg, 1.5 mmol, 3 equiv), NaOAc‚3H
2
O (136
(11 mg, 0.05 mmol, 0.1
2
Su p p or tin g In for m a tion Ava ila ble: Spectral data for all
the compounds in the schemes and tables. This material is
available free of charge via the Internet at http://pubs.acs.org.
equiv). The suspension was stirred at room temperature for 1
h, after which the mixture was diluted with diethyl ether (10
mL), filtered through a plug of neutral alumina, and washed
with diethyl ether (3 × 10 mL). The filtrate was then concen-
J O020159P
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130 J . Org. Chem., Vol. 67, No. 20, 2002