Phenols as starting materials for the synthesis of arylstannanes via SRN11

Article abstract of DOI:10.1021/om010878e

Phenols are converted into aryl diethyl phosphate esters (ArDEP), which on reaction with sodium trimethylstannide (1) or sodium triphenylstannide (2) in liquid ammonia afford arylstannanes by the S_RN1 mechanism. Thus, the photostimulated reaction of phenylDEP (3), (4-methoxyphenyl)DEP (4), (4-biphenyl)DEP (5), (1-naphthyl)DEP (6), (2-naphthyl)DEP (7), and 2- (34), 3- (32), and (4-pyridyl)DEP (35) with 1 leads to monostannylated product in fair to excellent yields (20-98%). Also, substrates containing two or three leaving groups react with 1 under irradiation, affording the corresponding di- or tristannylated aryl compounds. With tetraethyl m-phenylene bisphosphate (15), tetraethyl p-phenylene bisphosphate (21), (4-chlorophenyl)DEP (22), and 1,3,5-tris(diethylphospho)benzene (30), the di- or trisubstitution products 1,3-bis(trimethylstannyl)benzene (19) (79%), 1,4-bis(trimethylstannyl)benzene (23) (95 and 97%), and 1,3,5-tris(trimethylstannyl)benzene (31) (57%) are obtained, respectively. Also, the reaction of 6 and 7 with 2 leads to substitution products in quantitative yields, and the reaction of 21, 22, and (4-bromophenyl)DEP (24) with 2 affords 1,4-bis(triphenylstannyl)benzene (38) in high yields (70-100%). On the other hand, the results obtained in the photostimulated reaction of 24 and (4-iodophenyl)DEP (25) with 1, as well as in the reaction of 25 with 2, clearly indicate a fast HME reaction.

Full text of DOI:10.1021/om010878e

Organometallics 2002, 21, 1425-1429
1425
P h en ols a s Sta r tin g Ma ter ia ls for th e Syn th esis of
Ar ylsta n n a n es via SRN11
Alicia B. Chopa,* Mar´ıa T. Lockhart, and Viviana B. Dorn
INIQO, Departamento de Quı´mica e Ingenierı´a Quı´mica, Universidad Nacional del Sur,
Avenida Alem 1253, 8000 Bah´ıa Blanca, Argentina
Received October 5, 2001
Phenols are converted into aryl diethyl phosphate esters (ArDEP), which on reaction with
sodium trimethylstannide (1) or sodium triphenylstannide (2) in liquid ammonia afford
arylstannanes by the SRN1 mechanism. Thus, the photostimulated reaction of phenylDEP
(3), (4-methoxyphenyl)DEP (4), (4-biphenyl)DEP (5), (1-naphthyl)DEP (6), (2-naphthyl)DEP
(7), and 2- (34), 3- (32), and (4-pyridyl)DEP (35) with 1 leads to monostannylated product in
fair to excellent yields (20-98%). Also, substrates containing two or three leaving groups
react with 1 under irradiation, affording the corresponding di- or tristannylated aryl
compounds. With tetraethyl m-phenylene bisphosphate (15), tetraethyl p-phenylene bis-
phosphate (21), (4-chlorophenyl)DEP (22), and 1,3,5-tris(diethylphospho)benzene (30), the
di- or trisubstitution products 1,3-bis(trimethylstannyl)benzene (19) (79%), 1,4-bis(trimethyl-
stannyl)benzene (23) (95 and 97%), and 1,3,5-tris(trimethylstannyl)benzene (31) (57%) are
obtained, respectively. Also, the reaction of 6 and 7 with 2 leads to substitution products in
quantitative yields, and the reaction of 21, 22, and (4-bromophenyl)DEP (24) with 2 affords
1,4-bis(triphenylstannyl)benzene (38) in high yields (70-100%). On the other hand, the
results obtained in the photostimulated reaction of 24 and (4-iodophenyl)DEP (25) with 1,
as well as in the reaction of 25 with 2, clearly indicate a fast HME reaction.
Sch em e 1
In tr od u ction
The preparation of organotin compounds and its
application as intermediates has become a very impor-
tant aspect in organic chemistry.2 For example, pal-
ladium-catalyzed reactions involving aryltin compounds
have found wide application in the synthesis of aromatic
and heterocyclic compounds.3 In connection with the
synthetic importance of these reactions, we are inter-
ested in searching new routes to arylstannanes.
Aromatic nucleophilic substitution via the SRN1 mech-
anism enables the substitution of appropriate nucleo-
fuges on unactivated aromatic systems with suitable
nucleophiles.4 The proposed mechanism is a chain
process (Scheme 1). In the initiation step there is an
electron transfer (ET) from the nucleophile to the
starting substrate (eq 1). If this ET is not spontaneous,
it could be induced by UV light.4
nucleophile to give ArNu•- (eq 3), which in turn reduces
the starting aromatic substrate. The radical anion ArX•-
is regenerated according to reaction 4. It should be noted
that the coupling with the nucleophile is not the only
reaction that aryl radicals can undergo: hydrogen atom
transfer from the solvent is one of the most important
side reactions (eq 5). This competitive reaction is
prevented by using liquid ammonia as solvent, which
is a poor hydrogen atom donor.4
The key species of the process, Ar•, obtained by the
reductive cleavage of ArX•- (eq 2), combines with the
Triorganostannyl anions have proved to be excellent
nucleophiles in SRN1 reactions.5 Application of these
reactions to the synthesis of arylstannanes is currently
in progress in our laboratory. We have recently de-
scribed the photostimulated reactions of a number of
haloarenes and haloheteroarenes with triphenylstannyl
anions in dimethyl sulfoxide (DMSO) and acetonitrile
(ACN) as solvents.6 The photostimulated reactions of
1-chloronaphthalene, 2- and 3-chloropyridines, and 1,4-
* Member of CIC. To whom correspondence should be addressed.
E-mail: abchopa@uns.edu.ar. Fax: 54 291 4595187.
(1) A portion of this work has previously appeared; see: Chopa, A.
B.; Lockhart, M. T.; Silbestri G. Organometallics 2000, 19, 2249.
(2) (a) Davies, A. G. Organotin Chemistry; VCH: Weinheim, 1997.
(b) Sato, S. Comprehensive Organometallic Chemistry II; Pergamon:
New York, 1995; Vol. 8.
(3) For a review see: Farina V.; Krishnamurthy V.; Scott W. J . The
Stille Reaction. In Organic Reactions; Paquette, L. A., Ed.; J ohn Wiley
& Sons: New York, 1997; Vol. 50.
(4) For reviews see: (a) Bowman, W. R. Chem. Soc. Rev. 1988, 17,
283. (b) Norris, R. K. Comprehensive Organic Synthesis; Trost, B. M.,
Ed.; Pergamon: New York, 1991; Vol. 4, p 451. (c) Rossi, R. A.; Pierini,
A. B.; Santiago, A.N. Aromatic Substitution by the SRN1 Reaction. In
Organic Reactions; Paquette, L. A., Bittman, R., Eds.; J ohn Wiley &
Sons: New York, 1999; Vol. 54, p 1.
(5) Co´rsico, E. F.; Al Postigo; Rossi, R. A. Molecules 2000, 5, 1076
(6) Lockhart, M. T.; Chopa, A. B.; Rossi, R. A. J . Organomet. Chem.
1999, 582, 229
10.1021/om010878e CCC: $22.00 © 2002 American Chemical Society
Publication on Web 03/03/2002

1426 Organometallics, Vol. 21, No. 7, 2002
Chopa et al.
Ta ble 1. Rea ction of Ar yl- a n d Heter oa r yl Dieth yl
P h osp h a tes w ith Me3Sn Na in Liqu id Am m on ia a
dichlorobenzene with potassium triphenylstannide
(Ph3SnK) lead to high yields (70-90%) of the corre-
sponding mono- or distannylated benzenes. The reac-
tions of arylbromides (ArBr) and aryliodides (ArI) in the
dark or under irradiation lead only to dehalogenation
products through a halogen metal exchange (HME)
mechanism. In general the reactions in DMSO give
better yields than in ACN, probably because the hydro-
gen abstraction reaction by aryl radicals from ACN
competes with the coupling reaction with Ph3Sn- ions.
We have also studied the reactions of several aryl-
trimethylammonium salts with trimethylstannyl anions
in liquid ammonia.7 Arylamines are converted into
aryltrimethylammonium salts, which on reaction with
sodium trimethylstannide (Me3SnNa) in liquid ammonia
afford aryltrimethylstannanes by the SRN1 mechanism
in good to excellent yields (45-100%).
In the course of our investigations we have found that
the photochemically induced reaction of some aryl
diethyl phosphate esters (ArDEP) with trimethyl- and
triphenylstannyl anions in liquid ammonia1 proceeds
cleanly via an SRN1 mechanism. All these substrates,
under the reaction conditions studied, gave substitution
products in excellent yields (70-100%). The main
advantage of these reactions is that they enable the
direct synthesis of organostannanes with different aryl
ligands,4 avoiding the use of organomagnesium or
organolithium reagents. It is known, for example, that
the reaction of 2-halopyridines with n-butyllithium
followed by the reaction with a triorganostannyl chloride
led to the corresponding organostannane in low yield
due to the formation of complex mixtures of intermedi-
ates during the lithiation process.8
conditions
time (h)
entry
aryl moiety
C6H5
4-MeOC6H4
4-biphenyl
1-C10H7
Ar-SnMe3, yield %b
1
2
3
hν, 2
C6H5-SnMe3, 85
4-MeO-C6H4-SnMe3, 81
4-biphenyl-SnMe3, 80
1-C10H7-SnMe3, 93
2-C10H7-SnMe3, 98
0c
hν, 1
hν, 1
4
hν, 1
5
2-C10H7
hν, 1
6
7
8
9
10
11
12
13
14
15
16
17
18
4-MeCOC6H4
4MeOOCC6H4
hν, 2
hν, 2
0c
d
1,3-C6H4
1,4-C6H4
4-ClC6H4
4-BrC6H4
hν, 4
1,3-(Me3Sn)2-C6H4, 79e
1,4-(Me3Sn)2-C6H4, 95e
1,4-(Me3Sn)2-C6H4, 97e
C6H5-SnMe3, 37g
0h
d
hν, 1
d
hν, 1
d
hν, 1.5
dark, 1.5
hν, 1.5
dark, 1.5
hν, 6
d
4-BrC6H4
d
4-IC6H4
4-IC6H4
C6H5-SnMe3, 26g
0i
d
f
1,3,5-C6H3
1,3,5-(Me3Sn)3C6H3, 57j
2-Me3Sn pyridine, 35
3-Me3Sn pyridine, 74
4-Me3Sn pyridine, 20
2-pyridyl
3-pyridyl
4-pyridyl
hν, 4
hν, 3
hν, 5
a
Substrate/Me3SnNa, 1:1.2. There is no reaction in the dark,
except were it is stated. Determined by GC. c Complex mixtures.
b
Substrate/Me3SnNa, 1:2.2. e Isolated yield. f Substrate/Me3SnNa,
d
1:3.3. g Together with 3 and 29. 3, 79%. 3, 83%. j Together with
h
i
traces of disubstitution product.
Similar results were obtained in the reactions of (4-
methoxyphenyl)DEP (4), (4-biphenyl)DEP (5), (1-naph-
thyl)DEP (6), and (2-naphthyl)DEP (7) with 1. Thus,
under irradiation, the corresponding substitution prod-
ucts (4-methoxyphenyl)trimethylstannane (9), (4-bi-
phenyl)trimethylstannane (10), (1-naphthyl)trimethyl-
stannane (11), and (2-naphthyl)trimethylstannane (12)
were formed in excellent yields (entries 2-5), according
to eqs 6 and 7. The NMR spectra of the reaction
mixtures showed no signals corresponding to the other
possible isomers, indicating the complete regioselec-
tivity of the substitution. When these reactions were
carried out in the dark, the corresponding starting
materials were almost completly recovered. These re-
Further studies on the reaction of aryl diethyl phos-
phate esters with triphenyl- and trimethylstannyl-
sodium have revealed that this would be a convenient
method for the synthesis of mono-, di-, and tristannyl-
ated aryl- and heteroaryl compounds.
In this paper, we report the results obtained and the
mechanistic aspects of the reactions of several mono-,
di-, and trisubstituted ArDEP with trimethyl- (1) and
triphenylstannyl (2) anions in liquid ammonia.
sults suggest that these reactions occur through an SRN
mechanism.
1
Resu lts a n d Discu ssion
Aryl and heteroaryl diethyl phosphate esters were
prepared in high yields from the corresponding phenols.9
Triorganostannyl anions were generated from the ap-
propriate triorganostannyl chloride according to the
reaction conditions established in previous work.7 Reac-
tions were conducted without irradiation (dark) and
under irradiation. The experiments were run under
nitrogen or argon, and the product compositions were
monitored by gas chromatography.
Rea ction s w ith Tr im eth yltin Sod iu m (1). The
results obtained are summarized in Table 1. Photo-
stimulated reaction of phenylDEP (3) with 1 proceeded
smoothly (2 h) to afford phenyltrimethylstannane (8) in
85% yield (entry 1).
On the other hand, the photostimulated reaction of
(4-acetylphenyl)DEP (13) with 1 gave (2 h) a complex
reaction mixture, from which could not be detected
neither the expected stannylated derivative nor the
starting material. Similar results were obtained in the
reaction of (4-carbomethoxyphenyl)DEP (14) with 1
(entries 6 and 7).
(7) Chopa, A. B.; Lockhart, M. T.; Silbestri, G. Organometallics 2001,
20, 3358.
(8) (a) Wakefield, B. J . Organolithium Methods; Academic Press:
New York, 1988. (b) Hughes, D. L.; Verhoeven, T. R. Tetrahedron Lett.
1996, 37, 2537.
We have also found that there is no reaction between
tetraethyl m-phenylene bisphosphate (15) and 1 in the
(9) Kenner, G. W.; Williams, N. R. J . Chem. Soc. 1955, 522.

Synthesis of Arylstannanes via SRN
1
Organometallics, Vol. 21, No. 7, 2002 1427
Sch em e 2
Sch em e 3
As reported in our preliminary communication, simi-
lar results have been obtained in the reactions carried
out with tetraethyl p-phenylenebisphosphate (21) and
(4-chlorophenyl)DEP (22) (entries 9 and 10).
In contrast, (4-bromophenyl)DEP (24) and (4-iodo-
phenyl)DEP (25) react with 1, under irradiation, to give
a mixture of 3 (38% and 53%, respectively) and 8 (37%
and 26%, respectively). Neither monosubstitution (20b)
nor disubstitution (23) products were detected (entries
11 and 13). The reactions of 24 and 25 with 1 in the
dark lead only to dehalogenation product 3 (entries 12
and 14). Under both reaction conditions hexamethyl-
distannane (29) was also identified as a significant
reaction product. These results suggest a fast HME
reaction followed by the protonation of the intermediate
anion 26 by the ammonia to give 3. Under irradiation
3 reacts with 1, leading to 8 through an SRN1 mecha-
nism. The fact that no monosubstitution product 20b
is found indicates that 26 does not react with Me3SnBr
(27) or Me3SnI (28) in liquid ammonia (Scheme 3). The
formation of 29 is probably due to the reaction between
27 or 28 with 1 (eq 9).
We have also found that a substrate containing three
leaving groups reacts with 1 under irradiation to afford
the corresponding trisubstituted product. Thus, 1,3,5-
tris(diethylphospho)benzene (30) led after 6 h to 1,3,5-
tris(trimethylstannyl)benzene (31, 57%) (eq 10). It is to
be noted that traces of disubstitution product were
detected (ca. 1% by GC) and that no reaction occurred
in the dark (entry 15). Recently, Rossi has reported the
synthesis of 31 by the reaction of 1,3,5-trichlorobenzene
with an excess of 1 in liquid ammonia, under irradia-
tion.10
dark (4 h). However, under irradiation in the same time
period, the disubstitution product 1,3-bis(trimethyl-
stannyl)benzene (19) was obtained in 79% yield (entry
8). These results clearly indicate that this disubstituted
substrate also reacts with 1 by the SRN1 mechanism,
as shown in Scheme 2.
It should be noted that no monosubstitution prod-
uct was detected. The absence of monosubstituted
product 20a could be attributed to preferential expul-
sion of the nucleofuge from the intermediate radical
anion 17a to form radical 18a rather than ET to 15
(eq 8).
We have studied the reaction of some pyridyl deriva-
tives with 1, to investigate the possible utility of these
reactions in the synthesis of (trimethylstannyl)pyri-
dines. It can be observed in Table 1 that the best results
were obtained in the photostimulated reaction of (3-
pyridyl)DEP (32) with 1, which led to (3-pyridyl)tri-
(10) Co´rsico, E. F.; Rossi, R. A. Synlett 2000, 227-229.

1428 Organometallics, Vol. 21, No. 7, 2002
Chopa et al.
under irradiation.11 Reaction products were tetra-
phenylstannane (39, 40%) and 3 (52%) (entry 7). This
result indicates that the reaction goes through steps
similar to those proposed for the reaction of 25 with 1
(Scheme 3). That is, in a first step the HME reaction
operates with 25, leading to 3. Under irradiation 3
reacts with 2, leading to 39 through an SRN1 mecha-
nism. The fact that no monosubstitution product, i.e.,
4-(triphenylstannyl)phenyl DEP (40), is found indicates
that the intermediate anion and Ph3SnI do not react in
liquid ammonia.
Ta ble 2. Rea ction of Ar yld ieth yl P h osp h a tes w ith
P h 3Sn Na in Liqu id Am m on ia a
aryl
moiety
conditions,
time (h)
entry
Ar-SnPh3, yield %
1
2
3
4
5
6
7
1-C10H7
2-C10H7
1,4-C6H4
hν, 4
1-Ph3Sn-C10H7, 100b
2-Ph3Sn-C10H7, 100b
1,4-(Ph3Sn)2C6H4, 70d
1,4-(Ph3Sn)2C6H4, 100d
1,4-(Ph3Sn)2C6H4, 100d
0e
hν, 6
c
hν, 2
c
4-ClC6H4
hν, 1.5
hν, 1.5
dark, 0.5
hν, 0.5
c
4-BrC6H4
c
4-IC6H4
4-IC6H4
c
C6H5-SnPh3, 40b,f
a
Substrate/Ph3SnNa, 1:1.2; no substitution products were
detected without irradiation. Determined by GC. c Substrate/
b
Ph3SnNa, 1:2.2. Isolated yield. e A mixture of 3 and (Ph3Sn)2O.
d
Con clu sion s
f Together with 3, 52%.
The present results demonstrate that the diethyl
phosphate group can act as a nucleofuge in the reaction
of arylDEP esters with trimethyl- and triphenyltin
anions in liquid ammonia under irradiation and that
these reactions take place through an SRN1 mechanism.
The attractive feature is that a triorganostannyl group
(Me3Sn- and Ph3Sn-) can, with interposition of one
additional step, be introduced in place of a phenolic
hydroxy group. In the first step phenols are converted
to the corresponding arylDEP esters with high yields
(85-90%). In the second step, in most of the cases we
have examined, the substitution reactions led to the
corresponding mono-, di-, or tristannylated substrates
in 50-100% yield. The synthesis of arylstannanes
following this method has the advantage that both
operational steps can be performed at moderate tem-
perature, so it should be feasible for use with thermally
labile molecules.12
methylstannane (35) in 74% yield after 3 h. The photo-
stimulated reaction of (2-pyridyl)DEP (33) and (4-
pyridyl)DEP (34) with 1 led to (2-pyridyl)trimethyl-
stannane (36) (4 h) and (4-pyridyl)trimethylstannane
(37) (5 h) in lower yields (entries 16-18) (eq 11). When
these reactions were carried out in the dark, no substi-
tution product was detected. From the above mentioned
results it is evident that these reactions take place
through an SRN1 mechanism.
The experiments carried out indicate, qualitatively,
that 1 is more reactive than 2 in SRN1 reactions. For
example, compare experiments 4, 5, 9, and 10 in Table
1 with experiments 1-4 in Table 2. These results are
in agreement with those obtained by Rossi in the
reaction between haloarenes and organotin anions.13
Rea ction s w ith Tr ip h en yltin Sod iu m (2). In Table
2 it can be seen that the photostimulated reactions of
6, 7, 21, 22, and 24 with 2 gave the expected stannylated
derivatives in excellent yields. When these reactions
were carried out in the dark, no substitution product
was detected.
The reactions of compounds 21, 22, and 24, i.e.,
substrates containing two leaving groups, led to the
disubstitution product 1,4-bis(triphenylstannyl)benzene
(38), in 70-100% yield, according to eq 12. No mono-
substitution product has been formed. All of these
observations strongly suggest that these reactions also
take place through an SRN1 mechanism.
Exp er im en ta l Section
Gen er a l P r oced u r es. Irradiation was conducted in a
reactor made of Pyrex, equipped with four 250 W UV lamps
emitting maximally at 350 nm, water-cooled. Proton NMR
spectra were recorded on a Bruker AC 200 spectrometer, using
CDCl3 as solvent. Mass spectra were obtained with a GC/MS
HP 6890. Infrared spectra were recorded on a Nicolet-Nexus
FTIR. Most of the reagents were commercially available. Aryl
diethyl phosphate esters were prepared by the method of
Kenner9 and characterized by IR and proton NMR spectros-
copy.14 All the products obtained were characterized by
comparison of their MS and proton NMR spectroscopic char-
acteristics with those of an authentic sample prepared by
known procedures, i.e., 8,7 9,13 10,15 11,7 12,16 19,10 31,10 35,17
(11) The best conditions for the SRN1 mechanism.
(12) Further work is in progress.
(13) Yammal, C. C.; Podesta´, J . C.; Rossi, R. A. J . Org. Chem. 1992,
57, 5720.
(14) The IR spectra of the esters (as films) present characteristic
On the other hand, the reaction of 25 with 2 in the
dark led to a mixture of phenylDEP (3) with triphenyltin
hydroxide and bis(triphenyltin)oxide (entry 6). It should
be noted that without adding water during the workup
it was possible to isolate triphenyltin iodide as product.
These results clearly suggest a fast HME reaction in
the dark. To know if the ET process could compete with
the HME mechanism, we added 25 to a solution of 2
absorption at 1030, 1155-1164, 1183-1214, and 1265-1274 cm-1. The
1H NMR spectra present a double triplet at 1.35-1.51 ppm (3J H,H
)
4
7.0-7.2 Hz, J H,P ) 1.1 Hz) and a double quartet at 4.25-4.45 ppm
(3J H,H ) 7.0-7.2 Hz, 3J H,P ) 8.1-8.4 Hz) as well as the absorption due
to the aryl groups.
(15) Seyferth, D.; Sarafidis, C.; Evnin, A. B. J . Organomet. Chem.
1964, 2, 417.
(16) Buchman, O.; Grosjean, M.; Nasielski, J . Bull. Soc. Chim. Belges
1962, 71, 467.
(17) Yamamoto, Y.; Yanagi, A. Chem. Pharm. Bull. 1982, 30, 1731.

Synthesis of Arylstannanes via SRN
1
Organometallics, Vol. 21, No. 7, 2002 1429
36,7 37,17 and 38.6 In most cases the yields were determined
by GC using the external standard method, but in a few they
were by isolation and weighing. To carry out the reactions in
the dark, the reaction flask was wrapped with aluminum foil.
P h otostim u la ted Rea ction of (4-Bip h en yl) Dieth yl
P h osp h a te (5) w ith Me3Sn Na (1). The reactions were
performed following the same procedure in all cases.
Sodium-dried ammonia (150 mL) was condensed into a 250
mL two-necked, round-bottomed Pyrex flask equipped with a
coldfinger condenser, a nitrogen inlet, and a magnetic stirrer.
Me3SnCl (0.219 g, 1.1 mmol) was dissolved in the ammonia,
and Na metal (0.058 g, 2.53 mg atom) was added until a blue
color persisted for at least 5 min. When the blue color
disappeared, 5 (0.293 g, 1.00 mmol) was added and then the
mixture irradiated with stirring, for 5 h. The reaction was
quenched by adding MeI in excess, and ammonia was allowed
to evaporate. The residue was treated with water and then
extracted with ether. Ether extracts were washed with an
aqueous saturated solution of NaCl and dried over MgSO4. The
ether was removed and the residue was recristallized in
ethanol, yielding 0.250 g (0.79 mmol, 79%) of (4-biphenyl)-
trimethylstannane (10), mp 47-49 °C (lit.15 mp 46-47 °C).
Ack n ow led gm en t. This work was partially sup-
ported by the Consejo Nacional de Investigaciones
Cient´ıficas y Te´cnicas (CONICET) and the Universidad
Nacional del Sur, Argentina. Special thanks go to Dr.
A. E. Zu´n˜iga for obtaining the mass spectra.
Su p p or tin g In for m a tion Ava ila ble: NMR and MS data
of trimethylstannyl derivatives. This material is available free
of charge via the Internet at http://pubs.acs.org.
OM010878E