Palladium complexes of 1,3,5,7-tetramethyl-2,4,8-trioxa-6-phenyl-6- phosphaadamantane: Synthesis, crystal structure and use in the Suzuki and Sonogashira reactions and the α-arylation of ketones

Article abstract of DOI:10.1021/jo049474x

Palladium complexes of 1,3,5,7-tetramethyl-2,4,8-trioxa-6-phenyl-6- phosphaadamantane were prepared and characterized with Pd[1,3,5,7-tetramethyl-2, 4,8-trioxa-6-phenyl-6-phosphaadamantane]₂·dba shown to be an effective catalyst for use in the

Full text of DOI:10.1021/jo049474x

P a lla d iu m Com p lexes of
1,3,5,7-Tetr a m eth yl-2,4,8-tr ioxa -6-p h en yl-6-p h osp h a a d a m a n ta n e:
Syn th esis, Cr ysta l Str u ctu r e a n d Use in th e Su zu k i a n d
Son oga sh ir a Rea ction s a n d th e r-Ar yla tion of Keton es
George Adjabeng,† Tim Brenstrum,† Christopher S. Frampton,‡ Al J . Robertson,§
J ohn Hillhouse,§ J ames McNulty,† and Alfredo Capretta*,†
Department of Chemistry, McMaster University, Hamilton, Ontario, Canada L8S 4M1,
Bruker Nonius B.V., Oostsingel 209, 2612 HL Delft, The Netherlands, and Cytec Canada, Inc.,
P.O. Box 240, Niagara Falls, Ontario, Canada L2E 6T4
capretta@mcmaster.ca
Received March 31, 2004
Palladium complexes of 1,3,5,7-tetramethyl-2,4,8-trioxa-6-phenyl-6-phosphaadamantane were
prepared and characterized with Pd[1,3,5,7-tetramethyl-2,4,8-trioxa-6-phenyl-6-phospha-
adamantane]2·dba shown to be an effective catalyst for use in the Suzuki and Sonogashira reactions
and the R-arylation of ketones. Couplings using this versatile complex proceeded in excellent yields
under mild conditions.
The efficacy and scope of applicability of palladium-
catalyzed cross-coupling chemistry1 has been bolstered
in recent years by the implementation of catalysts
incorporating bulky, electron-rich phosphine ligands.
These systems are particularly mild and extremely
versatile,2 permitting reactions such as the Suzuki,3
Heck,4 and Stille5 couplings of even the least reactive
coupling partners. Work in our laboratories has allowed
for the development of a new class of sterically hindered
tertiary phosphine based on a phosphaadamantane frame-
work.6 The use of a catalytic system incorporating Pd2-
(dba)3‚CHCl3 and 1,3,5,7-tetramethyl-2,4,8-trioxa-6-phenyl-
6-phosphaadamantane (PA-Ph, 1) was shown to promote
the Suzuki cross-coupling of aryl iodides, bromides, and
chlorides with a variety of aryl boronic acids under mild
conditions in high yields. The present paper builds on
these initial results and describes the preparation and
characterization of palladium complexes of 1 and reveals
them to be effective catalysts for use in the Suzuki and
Sonogashira reactions and the R-arylation of ketones.
Palladium complexes of 1,3,5,7-tetramethyl-2,4,8-tri-
oxa-6-phenyl-6-phosphaadamantane were readily pre-
pared by dissolving (1) and Pd2(dba)3‚CHCl3 in toluene
and stirring the resultant solution at room temperature
for 2 h before diluting with hexane (a 10-fold volume)
(Figure 1). Two crystalline products were obtained:
brown crystals of Pd(PA-Ph)2‚dba (2) and small amounts
of green Pd(PA-Ph)2‚O2 (3). The major product 2 was
physically separated from 3, and both were characterized
via MS and NMR. Integration of 1H NMR spectrum of 2
clearly reveals the presence of both phosphaadamantanyl
and dibenzylideneacetone (dba) moieties in a 2:1 ratio.
Interestingly, the chemical shifts of the olefin protons of
dba were clearly visible at 7.76 and 7.11 ppm and indicate
that the dba is not coordinated with the Pd in solution.
Contrast these chemical shifts with those reported by
Stahl and co-workers wherein their (bathocupronine)Pd-
(η2-dba) showed the coordinated olefinic protons at δ 4.47
and 4.24 ppm.7 It would appear, therefore, that the dba
is readily dissociated from the complex in solution to open
two vacant coordination sites on the palladium (presum-
ably occupied by solvent). Mass spectrometry of 2 reveals
a base peak corresponding to the mass of a Pd-bisphos-
phine complex at m/z ) 690. While crystals of the 2 failed
to diffract, an X-ray structure of the bis(phospha-ada-
mantanyl)peroxopalladium(II) complex (3) was obtained
and is shown in Figure 2.8,9 It should be noted that both
2 and 3 are air stable. Curiously, attempts at preparing
solely Pd(PA-Ph)2‚dba under the strict exclusion of
oxygen failed to result in the crystallization of either
complex.
† McMaster University.
‡ Bruker-Nonius.
(7) Stahl, S.; Thorman, J . L.; Nelson, R. C.; Kozee, M. A. J . Am.
Chem. Soc. 2001, 123, 7188-7189.
§ Cytec Canada, Inc.
(1) Metal-Catalyzed Cross Coupling Reactions; Diederich, F., Stang,
P., Eds.; Wiley-VCH: New York, 1998.
(2) Littke, A. F.; Fu, G. C. Angew. Chem., Int. Ed. 2002, 41, 4176.
(3) Littke, A. F.; Dai, C.; Fu, G. C. J . Am. Chem Soc. 2000, 122,
4020; Yin, J .; Rainka, M. P.; Zhang, X.; Buchwald, S. L. J . Am. Chem.
Soc. 2002, 124, 1162.
(4) Littke, A. F.; Fu, G. C. J . Org. Chem. 1999, 64, 10.
(5) Littke, A. F.; Fu, G. C. Angew. Chem., Int. Ed. 1999, 38, 2411.
(6) Adjabeng, G.; Brenstrum, T.; Wilson, J .; Frampton, C.; Robert-
son, A.; Hillhouse, J .; McNulty, J .; Capretta, A. Org. Lett. 2003, 5, 953.
(8) Details regarding the crystal structure of Pd[1,3,5,7-tetramethyl-
2,4,8-trioxa-6-phenyl-6-phosphaadamantane]2‚O2 may be obtained from
the Cambridge Crystallographic Data Centre (deposition no. 233089).
It should be noted that crystals of Pd[1,3,5,7-tetramethyl-2,4,8-trioxa-
6-(o-tolyl)-6-phosphaadamantane]2‚O2 were also characterized (deposi-
tion no. 233088).
(9) Other η2-peroxopalladium species have been reported in the
literature including ref 7: Valentine, J . S. Chem. Rev. 1973, 73, 235.
Yoshida, T.; Tatsumi, K.; Matsumoto, M.; Nakatsu, K.; Nakamura, A.;
Fueno, T.; Otsuka, S. Nouv. J . Chim. 1979, 3, 761-774.
10.1021/jo049474x CCC: $27.50 © 2004 American Chemical Society
Published on Web 06/29/2004
5082
J . Org. Chem. 2004, 69, 5082-5086

Pd Complexes of 6-Phosphaadamantane Compounds
TABLE 2. Son oga sh ir a Rea ction of Ar yl Iod id es
F IGURE 1. Synthesis of palladium/PA-Ph complexes.
a
b
Isolated yields. 0.75% catalyst. c Also successful with 1.2
equiv of Cs2CO3.
than previously described utilizing the PA-Ph/Pd2(dba)3
system.6 Finally, the reaction of sterically hindered
2-bromomesitylene with 1-naphthaleneboronic acid (Table
1, entry 4) was catalyzed by 2 to afford an 85% conversion
at room temperature. The η2-peroxopalladium species 3
proved to be a less effective catalyst, requiring heat to
promote the same reactions. This is not surprising since
the Pd(PA-Ph)2‚O2 complex needs to undergo a reductive
elimination of O2 before the active palladium species is
liberated and can undergo its first oxidative addition with
the aryl halide.
F IGURE 2. Structure of Pd(PA-Ph)2‚O2 (3).
TABLE 1. Su zu k i Cou p lin g Rea ction s
The Sonogashira reaction has proven to be an effective
method for the synthesis of conjugated alkene/alkyne
functionalities.10 Application of the Pd(PA-Ph)2‚dba (2)
complex to this class of organopalladium cross-coupling
chemistry was highly successful. In the reactions involv-
ing aryl iodides, preliminary screening revealed that
coupling could be effected in less than 1 h, in high yields,
by using a combination of 1.5% Pd(PA-Ph)2‚dba and 2%
CuI. Optimization of the reaction parameters revealed
that acetonitrile was the best solvent and both diisopro-
pylethylamine and Cs2CO3 were effective bases. These
conditions were then applied to an array of aryl iodides
and terminal alkynes (Table 2). The general protocol
effected Sonogashira coupling of activated (entry 2),
deactivated (entries 3, 5, and 6), and sterically demanding
(entry 4) systems at room temperature, in approximately
1 h, in excellent yields. It should be noted that for
reactions involving aryl iodides, omission of the copper
cocatalyst necessitated longer reaction times (at room
temperature) or heating to achieve comparable results.
Sonogashira coupling of aryl bromides was also effected
using the Pd(PA-Ph)2‚dba complex. In this series, how-
a
b
Isolated yield. 3% catalyst, 60 °C, 20 h. c 3% catalyst,
40 h.
The catalytic activity of the complexes was then
determined. Satisfyingly, Suzuki cross-coupling of 4′-
iodoacetophenone and 4′-bromoacetophenone with 1-naph-
thaleneboronic acid (Table 1, entries 1 and 2, respec-
tively) using the (PA-Ph)2Pd‚dba complex (2) revealed
extraordinary activity. In both cases, the reaction to form
4′-(1-naphthyl)acetophenone was completed within 10
min at room temperature with catalyst loadings as low
as 0.5%. The Pd(PA-Ph)2‚dba complex was also used to
effect the Suzuki coupling of 2-chlorotoluene with phe-
nylboronic acid to yield 2-phenyltoluene (Table 1, entry
3) in 88% yield at a slightly milder temperature (60 °C)
(10) Tykwinski, R. R. Angew. Chem., Int. Ed. 2003, 42, 1566.;
Sonogashira, K. In Comprehensive Organic Synthesis; Trost, B. M.,
Ed.; Pergamon: New York, 1991; Vol. 3, Chapter 2.4. Hundertmark,
T.; Littke, A. F.; Buchwald, S. L.; Fu, G. C. Org. Lett. 2000, 2, 1729.
Bohm, V. P. W.; Herrmann, W. A. Eur. J . Org. Chem. 2000, 22, 3679.
J . Org. Chem, Vol. 69, No. 15, 2004 5083

Adjabeng et al.
TABLE 3. Cop p er -F r ee Son oga sh ir a Rea ction of Ar yl
Br om id es
TABLE 4. r-Ar yla tion of Keton es
a
b
Isolated yields. 3.7% catalyst, 1.2 equiv of base. c 60
°C.
ever, addition of CuI was deleterious to the reaction. It
was found that copper-free reactions of aryl bromides at
50 or 60 °C occurred efficiently, in high yield, and were
complete within 6-20 h (entries 1-8, Table 3). The
analogous reactions, using CuI as cocatalyst, were in-
complete even after 24 h. It is unclear why the copper
cocatalyst promotes the Sonogashira coupling of aryl
iodides while hampering that of aryl bromides, although
a plausible explanation could involve oxidation of the
phosphine ligand by small amounts of Cu(II) present in
the CuI. Since the oxidative addition of aryl bromides is
slower than that of aryl iodides, the kinetics, in the
former case, may be such that oxidation of the phosphine
by Cu(II) now competes with the rate of oxidative
addition and ultimately results in the poor yields ob-
tained. Further mechanistic studies to explain this
surprising result are currently underway.
The palladium-catalyzed R-arylation of ketones is
another useful, general synthetic method11 to which, it
was envisaged, the Pd(PA-Ph)2‚dba (2) complex might be
applied. Using the coupling of 4-bromoanisole with pro-
piophenone for the initial screening allowed for rapid
determination of the optimum reaction parameters such
as solvent, base, temperature, and catalyst loading. While
the reaction proceeded using various bases or solvents,
the best results were obtained using NaOtBu as the base,
toluene as the solvent, and a catalyst loading of 1.5%.
Application of the optimum conditions determined
allowed for smooth couplings of aryl bromides and
a
b
d
Isolated yields. 0.75% catalyst. c 70 °C. 2.2% catalyst.
chlorides with either propiophenone or isobutyrophenone
within a few hours in high yields. As Table 4 illustrates,
the Pd(PA-Ph)2‚dba complex is not only an effective
catalyst for the R-arylation of ketones but is also one of
the most active systems reported to date. For example,
the Pd(PA-Ph)2‚dba-catalyzed reaction of the unactivated
4-bromoanisole with propiophenone at 40 °C afforded the
coupled product in 96% yield (Table 4, entry 4) in 24 h
using NaOtBu as the base. A survey of the chemical
literature has revealed that other leading ligands such
as P(t-Bu)3 effect the same reaction 20 °C higher at best.9
The Pd(PA-Ph)2‚dba system provides excellent yields
with an array of electron-rich and hindered aryl bro-
mides. For instance, the coupling of 2,4-dimethoxybro-
mobenzene and propiophenone at 40 °C affords the
coupled product in 80% isolated yield (Table 4, entry 7)
while the reaction of sterically hindered 2-bromomesity-
lene with propiophenone at 40 °C affords an 87% yield
(11) Culkin, D. A.; Hartwig. J . F. Acc. Chem. Res. 2003, 36, 234.
5084 J . Org. Chem., Vol. 69, No. 15, 2004

Pd Complexes of 6-Phosphaadamantane Compounds
the reaction, the reaction mixture was loaded onto small plug
of silica gel and washed with copious amounts of Et2O or
EtOAc. The washings are concentrated and purified by column
chromatography on silica gel. 4′-(1-Na p h th yl)a cetop h en on e
(Ta ble 1, En tr ies 1 a n d 2). Pd(PA-Ph)2‚dba (4.6 mg, 0.0050
mmol), K3PO4 (509 mg, 2.4 mmol), 1-naphthaleneboronic acid
(142 mg, 1.20 mmol), and either 4′-bromoacetophenone (199
mg, 1.00 mmol) or 4′-iodoacetophenone (246 mg, 1.00 mmol)
in toluene (3 mL) were used. Both reactions were complete
after 10 min and after workup and column chromatography
(20% EtOAc in hexane) yielded 245 mg (99% in both cases) of
the title compound as a white solid. Compound showed: 1H
NMR (CDCl3, 200 MHz) δ 8.10 (d, J ) 8.1 Hz, 2H), 8.00-7.80
(m, 3H), 7.61 (d, J ) 8.3 Hz, 2H), 7.58-7.40 (m, 4H), 2.69 (s,
3H); 13C NMR (CDCl3, 50 MHz) δ 198.0, 145.9, 139.1, 136.0,
133.9, 2 × 130.4, 5 × 128.4, 127.0, 126.5, 126.1, 125.6, 125.4,
26.8; MS[EI+] m/z (RI) 246 (100), 231 (72), 202 (68); HRMS
for C18H14O calcd 246.1045, obsd 246.1051.
of the diortho-substituted R-arylated ketone. Despite the
additional steric crowding about the R-position in isobu-
tyrophenone, reaction with electron-rich N,N-dimethyl-
4-bromoaniline afforded a 93% yield of the coupled
product (Table 4, entry 6). Finally, the Pd(PA-Ph)2‚dba-
catalyzed R-arylation of ketones using aryl chlorides was
also achieved. Entries 9 and 10 gave satisfactory results
(comparable to those already described in the literature)
at 70 °C.
Overall, the Pd(PA-Ph)2‚dba complex has been dem-
onstrated to be an effective catalyst for the Suzuki
reaction, the R-arylation of ketones, and the Sonogashira
reaction and offers a number of advantages to other
catalytic systems. The complex is simple to prepare, air
stable, and provides an “all-in-one” system delivering
both the metal and ligand required for reaction catalysis.
Furthermore, the activity of the 2 is comparable to and,
in some instances better than, other systems described
in the chemical literature. Finally, the ability to alter the
aryl moiety of the phosphaadamantane ligand affords the
opportunity to sterically and electronically fine-tune the
phosphine and hence generate palladium complexes with
differing catalytic potential. Applications involving 2 in
other palladium-promoted cross-coupling reactions are
currently under investigation.
Gen er a l P r oced u r e for th e Son oga sh ir a Cr oss-Cou -
plin g Reaction s. All liquid reagents (phenylacetylene, 2-meth-
yl-3-butyn-2-ol, diisopropylethylamine, aryl iodides, and aryl
bromides) and solvents were degassed under argon prior to
use. The palladium source, Cs2CO3, and CuI (if required) were
placed in an oven-dried reaction tube. The reaction tube was
sealed with a rubber septum, evacuated, and refilled with
argon. Next, the aryl halide (if a liquid; if a solid, then the
aryl halide was added prior to the evacuation-refill cycle), the
alkyne, and acetonitrile were added. The reaction was stirred
under argon at the indicated temperature for the indicated
amount of time. At the conclusion of the reaction, the reaction
mixture was diluted with Et2O or EtOAc, filtered through a
pad of silica gel with copious washings, concentrated, and
purified by column chromatography on silica gel. 4-(P h en yl-
eth yn yl)tolu en e (Ta ble 2, En tr y 1). Following the general
procedure, using Pd(PA-Ph)2‚dba (7 mg, 0.0075 mmol), 4-io-
dotoluene (218 mg, 1.00 mmol), phenylacetylene (0.165 mL,
1.50 mmol), CuI (4 mg, 0.02 mmol), (i-Pr)2NEt (0.210 mL, 1.20
mmol), and acetonitrile (1 mL). After 1 h at room temperature,
workup and column chromatography (hexane) yielded 188 mg
(96%) of the title compound as a white solid. The compound
showed: 1H NMR (CDCl3, 200 MHz) δ 7.55-7.46 (m, 2H), 7.43
(d, J ) 8.1 Hz, 2H), 7.40-7.26 (m, 3H), 7.16 (d, J ) 8.1 Hz,
2H), 2.37 (s, 3H); 13C NMR (CDCl3, 75 MHz) δ 138.4, 4 × 131.6,
2 × 129.2, 2 × 128.4, 128.1, 123.5, 120.2, 89.6, 88.8, 21.5; MS-
[EI+] m/z (RI) 192 (100), 191 (42); HRMS for C15H12 calcd
192.0939, obsd 192.0940.
Exp er im en ta l Section
Syn th esis of th e P a lla d iu m Com p lexes of 1,3,5,7-
Tet r a m et h yl-2,4,8-t r ioxa -6-p h en yl-6-p h osp h a a d a m a n -
ta n e: P d (P A-P h )2‚d ba (2) a n d P d (P A-P h )2‚O2 (3). Pd2-
(dba)3‚CHCl3 (1.040 g, 1.0 mmol), PA-Ph (1) (2.340 g, 8.0
mmol), and toluene (70 mL) were placed in a round-bottom
flask, and the dark-purple mixture was stirred under argon
for 2 h. At that time, the original dark-purple color of the
mixture became yellow. The reaction mixture was diluted with
hexane (700 mL) and allowed to stand overnight. Two crystal-
line compounds were formed: brown needles [Pd(PA-Ph)2‚dba]
and green microcrystals [Pd(PA-Ph)2·O2], the former being the
major product. The two products were obtained in roughly 90%
overall yield and separated manually. Pd(PA-Ph)2‚dba (2)
showed: 1H NMR (CDCl3, 300 MHz) δ 7.76 (d, J ) 16 Hz, 2H),
7.74-7.63 (m, 8H), 7.45-7.37 (m, 6H), 7.22-7.17(m, 6H), 7.11
(d, J ) 16 Hz, 2H), 2.76 (dd, J ) 13.5 & 4.5 Hz, 1H,), 1.84-
1.78 (m, 4H), 1.58-1.55 (m, 2H), 1.52 (s, 6H), 1.47 (d, J ) 14.5
Hz, 6H), 1.33 (s, 6H), 1.25 (d, J ) 13.3 Hz, 6H); 13C NMR
(CDCl3, 75 MHz) δ 189.0, 143.4, 135.2, 135.1, 133.3, 130.6,
129.1, 129.0, 128.7, 128.3, 125.5, 97.7, 97.0, 96.4, 96.3, 74.6,
74.2, 73.9, 72.9, 43.3, 40.3, 40.3, 39.2, 27.5, 21.3; MS[FAB] m/z
(RI) 692 (79), 691 (35), 690 (100), 689 (74), 688 (33). Pd(PA-
Ph)2‚O2 (3) showed: 1H NMR (CDCl3, 300 MHz) δ 7.92-7.86
(m, 4H), 7.32-7.23 (m, 6H), 2.50 (dd, J ) 13.6, 4.4 Hz, 2H),
1.71-1.56 (m, 6H), 1.48 (s, 6H), 1.47 (d, J ) 14.7 Hz, 6H),
1.33 (s, 6H), 1.04 (d, J ) 13.3 Hz, 6H); 13C NMR (CDCl3, 75
MHz) δ 2 × 134.6, 2 × 133.3, 2 × 131.1, 2 × 128.7, 96.7, 96.6,
96.2, 96.1, 74.5, 74.4, 74.0, 73.7, 43.8, 38.2, 27.7, 26.2; MS-
[FAB] m/z (RI) 693 (29), 692 (79), 691 (35), 690 (100), 689 (74),
688 (33).
Gen er a l P r oced u r e for th e P d (P A-P h )2‚d ba -Ca ta lyzed
Keton e Ar yla tion Rea ction s. (PA-Ph)2Pd‚dba (2, 0.015
mmol), NaOtBu (1.50 mmol), and the aryl halide (if a solid)
were placed in a reaction vessel containing a magnetic stir
bar. The reaction vessel was evacuated and then refilled with
argon. Toluene (1.5 mL) was added followed by the aryl halide
(if a liquid), the ketone, and the remaining toluene (1.5 mL)
[or THF (0.5 mL)]. The reaction was stirred at the indicated
temperature for the indicated amount of time and was
1
monitored by H NMR. At the conclusion of the reaction, the
mixture was diluted with CH2Cl2 or EtOAc, filtered through
a pad of silica gel with copious washings, concentrated, and
purified by column chromatography on silica gel. 1,2-Dip h e-
n ylp r op a n -1-on e (Ta ble 4, En tr y 1). (PA-Ph)2Pd‚dba (7 mg,
0.0075 mmol), NaOtBu (144 mg, 1.50 mmol), bromobenzene
(0.105 mL, 1.00 mmol), propiophenone (0.146 mL, 1.10 mmol),
and toluene/THF (1.5 mL: 0.5 mL) were used. After 20 h at
40 °C, workup and column chromatography (5% EtOAc in
hexane) gave 195 mg (93%) of the title compound as a pale
yellow liquid. The compound showed: 1H NMR (CDCl3, 300
MHz) δ 8.00 (d, J ) 7.2 Hz, 2H), 7.43-7.22 (m, 8H), 4.73 (q,
J ) 6.9 Hz, 1H), 1.59 (d, J ) 6.8 Hz, 3H); 13C NMR (CDCl3, 75
MHz) δ 200.2, 141.4, 136.4, 132.8, 2 × 128.9, 2 × 128.7, 2 ×
128.4, 2 × 127.7, 126.8, 47.8, 19.5; MS[EI+] m/z (RI) 210 (4),
105 (100); HRMS for C15H14O calcd 210.1045, obsd 210.1046.
For each of the coupling protocols described, a general
procedure is provided as well as one specific example. All other
experimental procedures are available in the Supporting
Information.
Gen er a l P r oced u r e for th e P d (P A-P h )2‚d ba -Ca ta lyzed
Su zu k i Cou p lin g Rea ction s. The aryl halide (1 mmol),
arylboronic acid (1.2 mmol), K3PO4 (2.4 mmol), Pd(PA-Ph)2‚
dba (4.6 mg, 0.0050 mmol), and toluene (3 mL) were added to
the reaction vessel, and the mixture was stirred at room
temperature (or heated as required) under argon and the
course of the reaction monitored by TLC. At the conclusion of
J . Org. Chem, Vol. 69, No. 15, 2004 5085

Adjabeng et al.
2, entries 2-6, Table 3, entries 1-8, and Table 4, entries 2-10;
IR spectra for compounds shown Table 3, entries 5-7; and H
NMR spectra for all compounds described. This material is
available free of charge via the Internet at http://pubs.acs.org.
Ack n ow led gm en t. We thank Cytec Canada, Inc.,
and the Natural Sciences and Engineering Research
Council of Canada for financial support.
1
Su p p or tin g In for m a tion Ava ila ble: Experimental pro-
cedures for compounds shown in Table 1, entries 3 and 4, Table
J O049474X
5086 J . Org. Chem., Vol. 69, No. 15, 2004