Photostimulated reactions of vinyl phosphate esters with triorganostannides. Evidence for an SRN1 vinylic mechanism

Article abstract of DOI:10.1021/jo049762t

Ketones are converted into vinyl diethyl phosphate esters (VinDEP), which under photostimulation reacted with sodium trimethylstannide (1) or sodium triphenylstannide (2) in liquid ammonia affording vinylstannanes via a vinylic S_RN1 mechanism.

Full text of DOI:10.1021/jo049762t

P h otostim u la ted Rea ction s of Vin yl P h osp h a te Ester s w ith
Tr ior ga n osta n n id es. Evid en ce for a n SRN1 Vin ylic Mech a n ism
Alicia B. Chopa,*,† Viviana B. Dorn, Mercedes A. Badajoz, and Mar´ıa T. Lockhart
INIQO, Departamento de Qu´ımica, Universidad Nacional del Sur, Avenida Alem 1253,
8000 Bah´ıa Blanca, Argentina
abchopa@uns.edu.ar
Received February 10, 2004
Ketones are converted into vinyl diethyl phosphate esters (VinDEP), which under photostimulation
reacted with sodium trimethylstannide (1) or sodium triphenylstannide (2) in liquid ammonia
affording vinylstannanes via a vinylic SRN1 mechanism. Thus, (1-phenylvinyl)DEP (3), (3,4-dihydro-
1-naphthyl)DEP (7), (3,4-dihydro-2-naphthyl)DEP (9), (E)-(1,2-diphenylvinyl)DEP (12), (E/ Z)-(1-
methyl-2-phenylvinyl)DEP (14) and (E)-(1-phenyl-2-methylvinyl)DEP (16) react with 1 and 2, under
photostimulation, leading to the corresponding substitution products in good to excellent yields
(45-89%). On the other hand, there is no reaction between (1-cyclohexenyl)DEP (5) or (1-
benzylvinyl)DEP (18) with either 1 or 2, under similar conditions. These reactions appear to be
strongly dependent on structural features of the vinyl phosphate since only conjugated vinyl
phosphates afforded substitution products. These substitution reactions are completely regioselective
and stereoconvergent. It seems to be the first example of a vinylic SRN1 process involving organotin
anions as nucleophiles.
In tr od u ction
On the other hand, the reaction of R3SnM as nucleo-
philes with aryl halides has long been known.7 The
mechanisms involved in these reactions strongly depend
on the nature of the leaving group, the nucleophile, the
solvent, and the reaction conditions. For example, the
results obtained by Quintard in the reaction of p-chloro-
and p-fluorotoluenes with Bu3SnLi in THF suggest that
these reactions occur either through a radical or a
benzyne mechanism. The relationship of the cine or ipso
substitution products depends on whether the reactions
were carried out in the dark or under irradiation or in
the presence of added substances.8 Also, a few years ago,
Rossi reported that aryl halides react with Ph3Sn- and
Me3Sn- ions in liquid ammonia through an SRN1 and/or
an HME mechanism leading to the corresponding sub-
stitution and/or dehalogenation products, respectively.9
Results obtained in our laboratory showed that Ph3Sn-
ion also reacts with haloarenes in DMSO and acetonitrile
as solvents, through an SRN1 and/or an HME mechanism.
In general, the reactions carried out in DMSO give better
yields of substitution product than those carried out in
acetonitrile.10 Recently, we have also described that aryl
trimethylammonium salts as well as aryl diethyl phos-
phate esters react with R3SnNa in liquid ammonia via
an SRN1 mechanism leading to the substitution products
in good to excellent yields.11,12
Triorganostannyl anions (R3SnM) have been described
in the literature as “supernucleophiles”. The displace-
ment of halides and other groups from alkyl, aryl, and
vinyl substrates by organotin anions represents one of
the most important routes for the formation of new tin-
carbon bonds.1
Indeed, the reaction of R3SnM with alkyl derivatives
has been extensively studied by several groups.2-6 On the
basis of stereochemical studies, experiments in which
intermediates were trapped, and the effect of reaction
conditions, three mechanistic pathways have been pro-
posed: (i) direct SN2 at carbon; (ii) SN2 at halogen
(halogen-metal exchange, HME), and (iii) electron trans-
fer (ET) from R3SnM in a cage collapse or a chain
mechanism (SRN1).
† Member of CIC, Argentina.
(1) (a) Davies, A. G. Organotin Chemistry; VCH: Weinheim, 1997.
(b) Sato, T. Comprehensive Organometallic Chemistry II; Pergamon:
New York, 1995; Vol. 8.
(2) (a) Prezzavento, B. A.; Kuivila, H. G. J . Org. Chem. 1987, 52,
929. (b) Alnajjar, M. S.; Kuivila, H. G. J . Am. Chem. Soc. 1985, 107,
416. (c) Alnajjar, M. S.; Smith, G. F.; Kuivila, H. G. J . Org. Chem.
1984, 49, 1271. (d) Smith, G. F.; Kuivila, H. G.; Simon, R.; Sultan, L.
J . Am. Chem. Soc. 1981, 103, 833. (e) Alnajjar, M.; Kuivila, H. G. J .
Org. Chem. 1981, 46, 1053.
(3) (a) Ashby, E. C.; Gurumurthy, R.; Ridlehuber, R. W. J . Org.
Chem. 1993, 58, 5832. (b) Ashby, E. C. Acc. Chem. Res. 1988, 21, 414
and references therein.
(4) (a) Lee, K.-W.; San Filippo, J ., J r. Organometallics 1983, 2, 906.
(b) San Filippo, J ., J r.; Silbermann, J . J . Am. Chem. Soc. 1982, 104,
2831. (c) San Filippo, J ., J r.; Silbermann, J .; Fagan, P. J . J . Am. Chem.
Soc. 1978, 100, 4834.
(5) Adcock, W.; Clark, C. I.; Trout, N. A. J . Org. Chem. 2001, 66,
3362 and references cited therein.
(7) Pereyre, M.; Quintard, J . P.; Rahm, A. Tin in Organic Synthesis;
Butterworths: London, 1987.
(8) (a) Quintard, J . P.; Hauvette-Frey, S. J . Organomet. Chem. 1978,
159, 147. (b) 1976, 112, C11.
(9) Yammal, C. C.; Podesta´, J . P.; Rossi, R. A. J . Org. Chem. 1992,
57, 5720.
(6) Santiago, A. N.; Toledo, C. A.; Rossi, R. A. J . Org. Chem. 2002,
67, 2494 and references cited therein.
(10) Lockhart, M. T.; Chopa, A. B.; Rossi, R. A. J . Organomet.Chem.
1999, 582, 229.
10.1021/jo049762t CCC: $27.50 © 2004 American Chemical Society
Published on Web 04/30/2004
J . Org. Chem. 2004, 69, 3801-3805
3801

Chopa et al.
TABLE 1. Rea ction of Vin yl P h osp h a tes w ith Me3Sn Na
SCHEME 1
a
in Liqu id NH3
There are several antecedents in the literature about
the synthesis of vinylstannanes by the reaction of R3Sn-
ions with vinyl compounds supporting different leaving
groups. Thus, Piers has reported that the reaction of
â-iodo enones with Me3SnLi in THF resulted in the
formation of moderate amounts of the corresponding
substitution products although the results were not very
reproducible. Nevertheless, when these reactions were
carried out with lithium phenylthio(trimethylstannyl)-
cuprate instead of Me3SnLi, the yield of substitution
products was substantially increased.13 Stannyl cuprates
also react with â-X-R,â-unsaturated esters (X ) Cl, I,
OTos, OTf, SPh)14,15 as well as with vinyl iodides and
vinyl triflates16 to give vinylstannanes in good to excellent
yields. All of these reactions may be considered either
as nucleophilic substitutions or addition-elimination
reactions. It should be mentioned that Oshima16 has
reported that the reaction of vinyl phosphates with tin
anions (R3SnAlEt3) in the presence of Pd(PPh3)4 catalyst
gave no desired vinylstannane. We now report the results
obtained and the mechanistic aspects of the reaction of
Me3SnNa (1) and Ph3SnNa (2) with vinyl diethyl phos-
phate esters (VinDEP)17 in liquid ammonia. We have
found that an SRN1 reaction does indeed occur under
photostimulation.
Although rather scanty, there are antecedents in the
literature concerning vinyl substitutions by the SRN
1
mechanism, in which a carbon nucleophile reacts with a
halovinyl derivative.18 The proposed mechanism is a
chain process in which radicals and radical anions are
involved as intermediates, as is illustrated in Scheme 1.
In a few systems, this chain process is initiated by
spontaneous ET from the nucleophile to the substrate
forming the vinylic radical anion (eq 1). Stimulation by
UV light or by Fe2+ has been recently applied in vinylic
SRN1 reactions.18 It should be noted that the coupling with
a
b
Substrate/Me3SnNa, 1/1.2. Substrate/Me3SnNa, 1/5. No sub-
stitution products were detected without irradiation, unless where
it is stated. c Together with 11, ca. 2%. Together with 11, ca. 15%.
d
e 20% p-DNB added. f E/ Z, 39/61. E/ Z, 76/24.
g
the nucleophile (eq 3) is not the only reaction that vinyl
radicals can undergo: hydrogen atom transfer from the
solvent is one of the most important side reactions (eq
5). This competitive reaction is reduced by using liquid
ammonia as solvent, which is a poor hydrogen atom
donor.19
As far as we know, our results are the first example of
SRN1 vinylic substitution with organotin anions as nu-
cleophiles.
(11) Chopa, A. B.; Lockhart, M. T.; Silbestri, G. Organometallics
2001, 20, 3358.
(12) (a) Chopa, A. B.; Lockhart, M. T.; Silbestri, G. Organometallics
2002, 21, 5874. (b) Chopa, A. B.; Lockhart, M. T.; Dorn, V. B.
Organometallics 2002, 21, 1425. (c) Chopa, A. B.; Lockhart, M. T.;
Silbestri, G. Organometallics 2000, 19, 2249.
(13) Piers, E.; Morton, H. E. J . Chem. Soc., Chem. Commun. 1978,
1033.
(14) Piers, E.; Tse, L. A. Tetrahedron Lett. 1984, 25, 3155.
(15) Seitz, D. E.; Lee, S.-H. Tetrahedron Lett. 1981, 22, 4909.
(16) Matsubara, S.; Hibino, J .-I; Morizawa, Y.; Oshima, K.; Nozaki,
H. J . Organomet. Chem. 1985, 285, 163.
Resu lts a n d Discu ssion
Rea ction s w ith Me3Sn Na (1). The results obtained
are summarized in Table 1.
(17) Vinyl phosphates were prepared from the corresponding car-
bonyl compounds by trapping the enolate with diethyl chlorophosphate
according to: Ireland, R. E.; Pfister, G. Tetrahedron Lett. 1969, 2145.
The IR spectra of the esters (as films) present characteristic absorption
We have found that under irradiation (1-phenylvinyl)-
DEP (3)20 reacts with 1 to give the corresponding
at 1030, 1155-1164, 1183-1214, and 1265-1274 cm-1
.
(18) (a) Annunziata, A.; Galli, C.; Gentili, P.; Guarnieri, A.; Beit-
Yannai, M.; Rappoport, Z. Eur. J . Org. Chem. 2002, 2136 and
references cited therein. (b) Santiago, A. N.; Lassaga, G.; Rappoport,
Z.; Rossi, R. A. J . Org. Chem. 1996, 61, 1125.
(19) Rossi, R. A.; Pierini, A. B.; Santiago, A.N. Aromatic Substitution
by the SRN1 Reaction; Organic Reactions; Paquette, L.A., Bittman, R.,
Eds.; J ohn Wiley & Sons: New York, 1999; Vol. 54, p 1.
3802 J . Org. Chem., Vol. 69, No. 11, 2004

Reactions of Vinyl Phosphate Esters with Triorganostannides
substitution product, 1-phenyl-1-(trimethylstannyl)ethene
(4) in 82% yield (2 h)21 (eq 6). There is no reaction without
photostimulation (in the dark) and the substrate 3 was
recovered unchanged (Table 1, entry 1).
the nucleophile to the substrate occurs which is stimu-
lated by irradiation.
On the other hand, (1-cyclohexenyl)DEP (5)20 is unre-
active toward 1 even after 5 h under irradiation in the
presence of an excess of 1. No substitution product, i.e.,
1-(trimethylstannyl) cyclohexene (6), was detected, and
the starting substrate was almost completely recovered
(Table 1, entries 2 and 3). Otherwise, the photostimulated
reaction of both (3,4-dihydro-1-naphthyl)DEP (7)22 and
(3,4-dihydro-2-naphthyl)DEP (9)22 with 1 led to the
corresponding substitution products, i.e., 3,4-dihydro-1-
(trimethylstannyl)naphthalene (8)23 and 3,4-dihydro-2-
(trimethylstannyl)naphthalene (10),23 although in very
low yields (10% and 4%, respectively). Considerable
amounts of unreacted precursors were present in each
case. It should be noted that an increase in the 1/7 and
1/9 ratio (5/1) produced a substantial increment on the
yield of 8 (47%) and 10 (45%) (eq 7) (Table 1, entries 4-7).
In experiments 6 and 7 (Table 1), together with the
substitution product 10, the reduction product 3,4-
dihydronaphthalene (11) was detected in ca. 2% and 15%
yields, respectively. When these reactions were carried
out in the dark neither substitution nor reduction
products were detected. The presence of 11 is probably
due to the hydrogen abstraction reaction by the radical
intermediate from ammonia (eq 5).
Also, the photostimulated reactions of 1 with (E/ Z)-
(1-methyl-2-phenylvinyl)DEP (14)22 and (E)-(1-phenyl-2-
methylvinyl)DEP (16)20 yield the corresponding substi-
tution products 1-phenyl-2-(trimethylstannyl)propene (15)
in 73% yield (E/ Z, 92/8; 2 h)24 and 1-phenyl-1-(trimeth-
ylstannyl)propene (17) in 85% yield (E/ Z, 42/58; 2 h),23
respectively (eq 8). It should also be noted that both
reactions are partially inhibited by the addition of p-DNB
(20%) and that no substitution product is formed in the
dark (Table 1, entries 11-15). To analyze the stereo-
chemistry of these reactions, we carried out similar
photostimulated reactions starting from different E/ Z
(39/61; 76/24) mixtures of compound 14. In all the reac-
tions, similar E/Z-mixtures (within 2%) of product 15
were obtained independently of the starting substrate
configuration; i.e., the reaction is stereoconvergent (en-
tries 11 and 13). It should be noted that in both experi-
ments 11 and 13 the unreacted substrate 14 was recov-
ered with unchanged configuration, showing that E and
Z isomers are stable to isomerization under the reaction
conditions. Additional checks confirmed that product 15
undergoes no isomerization under the reaction conditions.
Thus, after irradiation (2 h), but in the absence of the
organotin anion, different (E,Z)-15 mixtures were recov-
ered unchanged. We conclude that the loss of configura-
tion must take place during the substitution process. The
stereochemistry of the products probably depends on the
structure of the vinyl radical intermediate.25 It seems
that the final E/ Z mixtures depend on the nature of the
â-substituents. Thus, compounds 12 and 14, supporting
a Ph group on the â-carbon, led to higher yields of (E)
isomers, whereas compound 16, supporting a â-Me group,
led to higher yields of (Z) isomer.
We have also found that there was a slow reaction
between (E)-(1,2-diphenylvinyl)DEP (12)20 and 1 in the
dark, giving the substitution product 1,2-diphenyl-1-
(trimethylstannyl)ethene (13, E/ Z, 89/11) in 6% yield (3
h).21 This reaction was totally inhibited by the addition
of p-dinitrobenzene (p-DNB), a well-known inhibitor of
SRN1 reactions.12 On the other hand, when the reaction
was carried out under irradiation, the yield of 13 (as a
mixture of isomers E/ Z, 86/14) was substantially in-
creased to 70% in the same time period (eq 8) (Table 1,
entries 8-10). The slow dark reaction inhibited by the
addition of p-DNB indicates that a spontaneous ET from
When we carried out the reaction of (1-benzylvinyl)-
DEP (18)26 with 1, no substitution product was detected
even after 5 h under irradiation, and the starting sub-
strate was almost completely recovered (Table 1, entry
16).
From the above-mentioned results it is evident that,
in the systems studied, the substitution products were
formed through an SRN1 mechanism: (i) the reactions did
not take place in the dark and (ii) the rate of the reactions
were significantly reduced when p-DNB was added. The
occurrence of the SRN1 mechanism appears to be depend-
ent on some structural features of the starting substrate.
(20) Spectroscopic data were in agreement with those reported by:
Borowitz, I. J .; Firstenberg, S.; Casper, E. W. R.; Crouch, R. K. J . Org.
Chem. 1971, 36, 3282. It was also characterized by 13C NMR and MS
spectra.
(24) NMR spectroscopic data are in agreement with those reported
by: Mitchell, T. N.; Amamria, A. J . Organomet. Chem. 1983, 252, 47.
It was also characterized by MS spectra.
(25) Galli, C.; Gentili, P.; Guarnieri, A.; Rappoport, Z. J . Org. Chem.
1996, 61, 8878.
(21) NMR spectroscopic data were in agreement with those reported
by: Cochran, J . C.; Phillips, H. K.; Tom, S.; Hurd, A. R.; Bronk, B. S.
Organometallics 1994, 13, 947.
(26) 1H NMR spectroscopic data were in agreement with those
reported by: Kosugi, M.; Miyajima, Y.; Nakanishi, H.; Sano, H.; Migita,
T. Bull. Chem. Soc. J apan 1989, 62, 3383. It was also characterized
by 13C, 31P NMR and MS spectra.
(22) Characterized by its IR, 1H, 13C, 31P NMR, and MS spectra.
(23) Characterized by its 1H, 13C, 119Sn NMR, and MS spectra.
J . Org. Chem, Vol. 69, No. 11, 2004 3803

Chopa et al.
TABLE 2. Rea ction of Vin yl P h osp h a tes w ith P h 3Sn Na
Again, the stereochemical results show that these
reactions are stereoconvergent. Once more, â-Ph vinyl
phosphates 12 and 14 afforded mixtures of substitution
products showing a high excess of (E) over (Z) isomer
whereas â-Me vinyl phosphate 16 led to a mixture
enriched on (Z) isomer.
a
in Liqu id NH3
On the other hand, results similar to those obtained
in the reaction between compound 18 and 1 were
obtained in the reaction of 18 with 2. Thus, there is no
reaction and 18 was recovered unchanged under these
reaction conditions.
In all cases, the NMR spectra of the reaction mixtures
showed no signals corresponding to the other possible
isomers, thus indicating the complete regioselectivity of
these substitution reactions. The stereochemistry of
1
vinyltins was defined on the basis of 13C and H chemical
shifts and vicinal 1H-119Sn and13C-119Sn couplings,
which show a sensitive dependence on dihedral angle.28,29
Con clu sion s
Recently, we have demonstrated that the diethyl
phosphate group can act as a nucleofuge in the reaction
of aryl phosphate esters with trimethyl- and triphenyltin
anions in liquid ammonia under irradiation and that
these reactions take place through an SRN1 mechanism.12
Our present results confirm that vinyl phosphate esters
also react with these tin anions in liquid ammonia by a
comparable substitution mechanism. We have found that
these vinylic SRN1 reactions strongly depend on structural
features of the vinyl phosphate. Thus, only conjugated
vinyl phosphates afforded the substitution product by
SRN1. It should be mentioned that similar results have
been found by Galli and Rappoport in the reaction of vinyl
halides with carbon nucleophiles.18a Nevertheless, they
have also found that those reactions are only limited to
conjugated vinyl halides lacking either a vinylic or allylic
hydrogen. Thus, vinyl halides having â-vinylic hydrogen
undergo substitution by an elimination-addition route,
whereas vinylic halides supporting allylic or vinylic
hydrogen(s) give substituted allene. In our reactions we
have not found these limitations.
a
b
Substrate/Ph3SnNa, 1/1.2. E/ Z, 76/24.
Thus, both compounds 5 and 18, which do not react with
1 under photostimulation, are unconjugated vinyl phos-
phates whereas all the conjugated vinyl systems studied
afford the substitution products in good to excellent
yields. Probably, this different reactivity could be due to
the fact that a conjugated vinyl phosphate is easier to
reduce than an unconjugated one, favoring the ET from
the nucleophile 1 to the substrate (eq 1, Scheme 1).
Rea ction s w ith P h 3Sn Na (2). To analyze the effect
of the triorganostannyl anion on the reactivity with vinyl
phosphates, we studied the reactions of 2 with different
substrates in liquid ammonia.
The reactions informed here would be of interest not
only from a mechanistic point of view but also as an
alternative route14,16,30 for the synthesis of vinylstannanes
starting from unsymmetrical ketones. The method in-
volves the regioselective conversion of the starting ketone
into the corresponding kinetic or thermodynamic phos-
phate ester and treatment of the latter with tin anions
in liquid ammonia.
In Table 2 it can be seen that the photostimulated
reactions of 3, 12, 14, and 16 with 2 gave the expected
stannylated derivatives. Thus, 1-phenyl-1-(triphenylstan-
nyl)ethene (19),27 1,2-diphenyl-1-(triphenylstannyl)ethene
(20, E/ Z, 81/19),23 1-phenyl-2-(triphenylstannyl)propene
(21, E/ Z, 100/0),23 and 1-phenyl-1-(triphenylstannyl)-
propene (22, E/Z, 34/66)23 were obtained in 82, 86, 78,
and 89% yields, respectively (eq 9). The substitution
products were easily detected by GC-MS analysis.
When these reactions were carried out in the dark no
substitution product was detected. All of these observa-
tions strongly suggest that these reactions also take place
through an SRN1 mechanism.
Work is in progress to determine the scope of these
reactions.
Exp er im en ta l Section
Gen er a l Meth od s. Irradiation was conducted in a reactor
made of Pyrex, equipped with four 250 W UV lamps emitting
(27) NMR spectroscopic data were in agreement with those reported
by: Nonaka, T.; Okuda, Y.; Matsubara, S.; Oshima, K.; Utimoto, K.;
Nozaki, H. J . Org. Chem. 1986, 51, 4716. It was also characterized by
MS spectra.
(28) Betzer, J .-F.; Le Me´nez, P.; Prunet, J .; Brion, J .-D.; Ardisson,
J .; Pancrazi, A. Synlett 2002, 1.
(29) Mitchell T. N.; Amamria, A.; Killing, H.; Rutschow, D. J .
Organomet. Chem. 1986, 304, 257.
(30) Wulff, W. D.; Peterson, G. A.; Bauta, W. E.; Clan, K.-S.; Faron,
K. L.; Gilbertson, S. R.; Kaesler, R. W.; Yang, D. C.; Murray, C. K. J .
Org. Chem. 1986, 51, 277.
3804 J . Org. Chem., Vol. 69, No. 11, 2004

Reactions of Vinyl Phosphate Esters with Triorganostannides
maximally at 350 nm, water-cooled. To carry out the reactions
in the dark, the reaction flask was wrapped with aluminum
foil.
All the products were purified by column chromatography
on silica gel treated with triethylamine in order to avoid
destannylation of vinylstannanes.
1H, 13C, 119Sn, and 31P NMR spectra were recorded in CDCl3
on a 300 MHz spectrometer and are referenced to TMS (1H
and 13C) or Me4Sn (119Sn) or (31P). Mass spectra were recorded
by GC/MS.
Ack n ow led gm en t. We acknowledge financial sup-
port from the Consejo Nacional de Investigaciones
Cient´ıficas y Te´cnicas (CONICET), Comisio´n de Inves-
tigaciones Cient´ıficas (CIC), Fundacio´n Antorchas and
the Universidad Nacional del Sur, Argentina. CONICET
is also thanked for a research fellowship to V.B.D.
Gen er a l P r oced u r e for Ir r a d ia ted Rea ction s. A 125 mL
portion of sodium-dried ammonia was condensed into a three-
necked, round-bottomed Pyrex flask equipped with a coldfinger
condenser, a nitrogen inlet, and a magnetic stirrer. Me3SnCl
(1.1 mmol) and Na metal (2.60 mg atom) were added. When
the blue color disappeared, 0.90 mmol of vinyl phosphate was
added and then the mixture irradiated with stirring as
indicated in Tables 1 and 2. The reaction was quenched by
addition of ammonium chloride, and ammonia was allowed to
evaporate. The residue was treated with water and then
extracted with ether. The vinyl trimethyltin derivatives were
quantified by GC using the external standard method. On the
other hand, vinyltriphenyltin products were quantified by
isolation and weighing.
Su p p or tin g In for m a tion Ava ila ble: Tables containing
1H, 13C, and 31P NMR data and MS data of new vinyl diethyl
phosphates; 1H, 13C, and 119Sn NMR data and MS data of new
organotin derivatives. 13C NMR spectra of vinyl diethyl
phosphates and vinyltin derivatives. This material is available
free of charge via the Internet at http://pubs.acs.org.
J O049762T
J . Org. Chem, Vol. 69, No. 11, 2004 3805