Chiral Recognition between a Substituted Cyclooctatetraene Dianion and a Half Crown Ether Substituted with Ibuprofen

Article abstract of DOI:10.1021/jo000972p

(S)-Verbenol was substituted onto cyclooctatetraene (COT) via an ether linkage. In tetrahydrofuran (THF), Cs⁺ or Na⁺ counterions are tightly ion associated with the verbenoxy-COT dianion. A cosolvent, consisting of an ibuprofen unit connected to a half crown ether, was added to the verbenoxy-COT^2-,M⁺₂ solutions. The intimate interaction between the chiral cosolvent (ibuprofoxymethoxyethoxyethane) and the ion-associated counterion (either Na⁺ or Cs⁺) forces a chiral recognition between the verbenoxy moiety and the ibuprofoxy moiety. When a molar excess of the cosolvent is present in the dianion THF solution, separation of the cosolvent associated with the verbenoxy-COT^2-,M⁺₂ complex from the uncomplexed cosolvent allows partial resolution of the enantiomers of ibuprofoxymethoxyethoxyethane.

Full text of DOI:10.1021/jo000972p

7588
J . Org. Chem. 2000, 65, 7588-7594
Ch ir a l Recogn ition betw een a Su bstitu ted Cycloocta tetr a en e
Dia n ion a n d a Ha lf Cr ow n Eth er Su bstitu ted w ith Ibu p r ofen
Cheryl D. Stevenson* and Daniel K. Cashion
Department of Chemistry, Illinois State University, Normal, Illinois 61790-4160
Stevenson@xenon.che.ilstu.edu
Received J une 28, 2000
(S)-Verbenol was substituted onto cyclooctatetraene (COT) via an ether linkage. In tetrahydrofuran
(THF), Cs+ or Na+ counterions are tightly ion associated with the verbenoxy-COT dianion. A
cosolvent, consisting of an ibuprofen unit connected to a half crown ether, was added to the
verbenoxy-COT2-,M+ solutions. The intimate interaction between the chiral cosolvent (ibuprof-
2
oxymethoxyethoxyethane) and the ion-associated counterion (either Na+ or Cs+) forces a chiral
recognition between the verbenoxy moiety and the ibuprofoxy moiety. When a molar excess of the
cosolvent is present in the dianion THF solution, separation of the cosolvent associated with the
verbenoxy-COT2-,M+ complex from the uncomplexed cosolvent allows partial resolution of the
2
enantiomers of ibuprofoxymethoxyethoxyethane.
In tr od u ction
Shortly after the discovery of the amazing affinity the
crown ethers have for the alkali metal cation, attempts
were made to separate the cations from the anions in
tightly ion-associated anion radical complexes with agents
such as 18-crown-6. Indeed, EPR studies have shown that
the addition of 18-crown-6 to an organic anion radical-
alkali metal complex results in the loss of the metal
hyperfine structure. However, ion association still per-
sists, but the entire crown-metal cation is involved in
the ion associated complex, e.g., Structure I.1
of utilizing such chiral recognition to partially resolve
mixtures of enantiomers.
The cyclooctatetraene (COT) system is ideal to serve
as the anionic part of the proposed complex. Two elec-
trons can be added to COT, and the resulting dianion4
associates more strongly to alkali metal cations than do
the corresponding monoanions.5 If the COT were at-
tached to a chiral appendage, it seems reasonable that
it would recognize the chiral appendage on the diglyme
unit.
High level ab initio calculation also predicts that when
lithium perchlorate is dissolved in diglyme, the lithium
cation is tightly ion associated to the perchlorate anion
and simultaneously ligated to the oxygens of the di-
glyme.2
The presence of the anion lowers the Li+-diglyme
binding energy. However, the dissociation energy for
[diglyme-LiClO4 f LiClO4 + diglyme] is still predicted
to be over 45 kcal/mol, and the distance between the
diglyme oxygens and the Li+ is relatively short (about
2.0 Å, see Structure II).2,3 It follows that if a chiral
hydrocarbon replaces one of the methyl protons of the
diglyme system, this chiral moiety could be forced into
close proximity to the anion. Further, if the anionic ligand
were also chiral, then chiral recognition should take
place. We were motivated to investigate the possibility
The possibility of chiral recognition between the sub-
stituted diglyme and the substituted COT dianion is
7
further suggested by recent NMR studies.5 The Li NMR
signals for the R and S enantiomers of the dilithium salts
of the sec-butoxycyclooctatetraene dianion in a chiral
solvent {S,S-(+)-2,3-dimethoxy-1,4-bis(dimethylamino)-
butane} were shown to be considerably different indicat-
ing solvent-ion pair chiral recognition. The equilibrium
controlling the ion pair dissociation of the double tight
ion associated species to form the monosolvent separated
(1) (a) Eastman, M. P.; Ramirez, D. A.; J ager, C. D.; Watts, M. T. J .
Phys. Chem. 1976, 80, 182. (b) Watts, M. T.; Lu, M. L.; Chen. R. C.;
Eastman, M. P. J . Phys. Chem. 1973, 77, 2959.
(2) Baboul, A. G.; Redfern, P. C.; Sutjianto, A.; Curtiss, L. A. J . Am.
Chem. Soc. 1999, 114, 968.
(4) (a) Strauss, H. L.; Katz, T. J .; Fraenkel, G. K. J . Am. Chem. Soc.
1963, 85, 2360. (b) Stevenson, C. D., Concepcion, J . G. J . Phys. Chem.
1972, 85, 2176.
(5) (a) Stevenson, C. D.; Fico, J r., R. M.; Brown, E. C. J . Phys. Chem.
B 1998, 102, 2841. (b) Cox, R. H.; Harrison, L. W.; Austin, W. K., J r.
J . Phys. Chem. 1973, 77, 200.
(3) Sutjianto, A.; Curtiss, L. A. J . Phys. Chem. A 1998, 102, 968.
10.1021/jo000972p CCC: $19.00 © 2000 American Chemical Society
Published on Web 10/10/2000

Chiral Recognition
J . Org. Chem., Vol. 65, No. 22, 2000 7589
species is shifted to the right in the case of the S dianion.
The equilibrium (reaction 1, double wavy line indicates
a molecule of solvent) can be shifted either to the left or
the right by varying the chirality of the solvent or of the
dianion.5
by one enantiomer of the cosolvent. Consequently, sepa-
ration of the ion pair complexed cosolvent (V) from the
free cosolvent may result in a partial chiral resolution of
the enantiomers of IV. The two cosolvent ion pairs are
diasteriomeric possibly rendering the thermodynamic
parameters controlling reactions 2 and 3 different. It
seems possible that the two chiral moieties in the
solvent-ion pair complex (V) could avoid a steric interac-
This necessarily means that the association of the
solvent with the COT2-,Li+2 complex is dependent upon
the enantiomer of the solvent, and it raises the possibility
of using an enantiomerically substituted COT dianion to
resolve the enantiomers of a cosolvent (a coordinating
solvent added to the primary solvent). The cosolvent, such
as a substituted diglyme, has a much higher affinity for
the ion-associated cations than does a simple ethereal
solvent (e.g., tetrahydrofuran). This allows analogous
chiral recognition in the presence of a bulk nonchiral
solvent system. The chiral recognition might be enhanced
by utilizing a relatively large, rigid (bicyclic) substituent
on the COT dianion. The verbenoxy unit has all of the
desired properties.
tion by being on opposite sides of the complex. Could this
minimize the chiral recognition and result in a chiral
perturbation upon reactions 2 and 3 is unobservable? We
were motivated to answer this question.
The first report of diasteroselective solvation involves
an organolithium compound [lithium 2-methoxy-((R)-1-
phenylethyl)((S)-1-phenylethyl)amine] in (R)- and (S)-2-
methyl-THF.7 In this case, the two diasteriomers proved
to be analytically different with respect to 13C NMR
chemical shifts. In this elegant study the authors used
their NMR results to show that the two enantiomers of
the organolithium compound have different affinities for
the (R) and (S) enantiomers of the 2-methyl-THF.7
Verbenol is a bark beetle pheromone and is readily
available as a pure enantiomer. Since it is an alcohol, it
can be readily attached to COT via an ether linkage.6 In
a THF solution of the dianion of verbenoxy-COT (III),
Resu lts a n d Discu ssion
containing a molar excess of the racemic cosolvent, one
of the two enantiomers of the cosolvent should be
preferentially coordinated to the counterion. This coun-
terion is, in turn, tightly associated with the enantio-
merically pure verbenoxy-COT2-. The unassociated enan-
tiomer of the cosolvent might then be washed from the
mixture with hexane. This should result in some separa-
tion of the isomers of the cosolvent.
The cosolvent of choice should contain a large chiral
hydrocarbon unit, which is, in turn, attached to a diglyme
moiety. The experiment would be of further interest if
the cosolvent were bonded to a molecule of pragmatic
interest. Ibuprofoxymethoxyethoxyethane (IV) has all of
the desirable traits. If the equilibrium constants for the
solvation competition reactions (reactions 2 and 3) differ
from unity, then the two-electron reduction of III in the
presence of a mixture of enantiomers of IV will predomi-
nantly yield a dianion, which is preferentially solvated
The reduction of verbenoxy-COT with a molar deficient
amount of lithium metal in HMPA (hexamethylphos-
phoramide) leads to a solution that yields a strong well
resolved EPR signal corresponding to an anion radical
that is free of ion association,8 and EPR analysis exhibits
a quartet (aH(odd) ) 5.64 G, 3 Hs) of pentets (aH(even) ) 0.68
G, 4 Hs), which is consistent with the published quantum
mechanical model for monosubstituted cyclooctatetraene
anion radicals (Figure 1).9 When the reduction is carried
out in THF, where both the anion radicals and dianion
are more strongly ion associated,10 only a very weak EPR
(7) Hilmersson, G.; Ahlberg, P.; Davidsson, O. J . Am. Chem. Soc.
1996, 118, 3539.
(8) (a) Stevenson, C. D.; Echegoyen L.; Lizardi, L. R. J . Phys. Chem.
1972, 76, 1439. (b) Levin, G.; J agur-Grodzinski, J .; Szwarc, M. J . Am.
Chem. Soc. 1970, 92, 2268. (c) Szwarc, M. In Ions and Ion Pairs in
Organic Reactions; Szwarc, M., Ed.; J ohn Wiley and Sons: New York,
1974; Vol. 2, Chapter 1.
(9) (a) Stevenson C. D.; Davis, J . P.; Reiter, R. C. J . Phys. Chem. A
1999, 103, 5343. (b) Hammons, J . H.; Hrovat, D. A.; Borden, W. T. J .
Am. Chem. Soc. 1991, 113, 4500.
(6) (a) Krebs, A. Angew. Chem. 1965, 77, 1966. (b) Stevenson C. D.;
Davis, J . P.; Reiter, R. C., J . Phys. Chem. A 1999, 103, 5343.

7590 J . Org. Chem., Vol. 65, No. 22, 2000
Stevenson and Cashion
F igu r e 1. X-band EPR spectrum (y-axis in Gauss) of the anion radical of verbenoxycyclooctatetraene in HMPA at ambient
temperature. The modulation amplitude was 0.05 G.
pure (R)-Ibuprofen is not available, racemic ibuprofen
was similarly reduced with lithium aluminum deuteride.
The (S)-ibuprofenol and racemic dideuterioibuprofenol
were attached to methoxyethoxyethane via ether link-
ages. The differing masses of the (S)-IV and the racemic
IV-d2 allow for unambiguous GC-mass spectral analysis.
Mixtures of the (S)-IV and racemic IV-d2 were used as
the cosolvent during the two electron reductions of
verbenoxy-COT in THF with either sodium or cesium
metal. Since the cosolvents were present in molar excess
(more than two moles of cosolvent per mole of verbenoxy-
COT dianion), only a fraction of the cosolvent can be
associated with the dianion. Hence the equilibria de-
scribed in reactions 2 and 3 were established. After
removal of the THF under reduced pressure, the uncom-
plexed cosolvent mixture was separated from the dian-
ion-cosolvent-metal complexes via dissolution in hex-
ane. This solution was subsequently labeled Phase I.
The dianion-cosolvent-metal complexes proved to be,
as expected, insoluble in hexane. A solution of I2 in
signal is observed. This is a consequence of tight ion
(10) Ion association strongly promotes disproportionation and ren-
association forcing the disproportionation equilibrium
ders the concentration of anion radical small; see: (a) Stevenson, C.
D.; Catlett, D. L.; Sedwick, J . B.; Wiedrich, R. J . Phys. Chem. 1983,
87, 578. (b) Smentowski, F. J .; Stevenson, C. D. J . Phys. Chem. 1969,
73, 340. (c) Smentowski, F. J .; Stevenson, C. D. J . Am. Chem. Soc.
1967, 89, 5120.
(reaction 5) far to the right.4a,10
(S)-Ibuprofen was reduced to the corresponding alcohol
with lithium aluminum hydride. Since enantiomerically

Chiral Recognition
J . Org. Chem., Vol. 65, No. 22, 2000 7591
Ta ble 1. Meta l Used for th e Red u ction , Ra tio of th e
Tota l Am ou n t of Deu ter a ted Cosolven t over th e Tota l
Am ou n t of P r otia ted Cosolven t in th e Or igin a l Mixtu r e,
Q, a n d th e Sep a r a tion F a ctor (r)
factors deviate from unity, although it is physically
possible for the two chiral moieties to remain on opposite
sides of the cyclooctatetraenyl ring system as dia-
grammed in Structure V.
metal total D/total H
Apparently the molecular dynamics in the cosolvent-
ion pair complex does result in some physical interaction
between the two chiral moieties. This chiral recognition
is of sufficient magnitude to render the free energy
changes of reactions 2 and/or 3 finite in value. At this
point it is impossible to delineate which equilibria
(reactions 2-4) is the largest contributor to R, but most
likely all three have nonzero values for ∆G°. The net
value for ∆∆G°, which represents the difference in the
overall abilities of the (R) and (S) cosolvents to associate
with the C10H14-O-COT2-,2M+ complex, is simply de-
fined as ∆∆G° ) -RT ln R.12 With this in mind ∆∆G° )
-48 and 77 cal/mol for the Cs and Na systems, respec-
tively.
The separations described above establish the existence
of chiral recognition between the ion pair and the
cosolvent. More insight into the nature of this interaction
may be possible through nuclear Ovehauser experiments.
The chiral recognition between the cosolvent and the ion-
associated dianion could possibly be greatly augmented
by placing a second verbenoxy group on the COT ring
system, perhaps in the number four position. There are
some synthetic challenges involved in this proposed
experiment, but we are currently addressing the problem.
The contrasting values for R in the sodium and cesium
systems came as a surprise. The subtle interactions
between two chiral moieties, the counterion, and the THF
are very complex. Further, the large number of atoms
involved rules out the use of sophisticated quantum
mechanical treatments. Nevertheless cosolvent-ion pair
chiral recognition has been achieved, and it is of sufficient
magnitude to allow the partial physical separation of the
enantiomers of ibuprofen. Further, the results of the
partial separations allow useful insight into the nature
of the solvent-ion pair interaction.
used
in original mix
XI
XII
Q
R
Na
1.004
(0.004
0.976
1.079
0.990
0.502
0.846
(0.015 (0.005 (0.004 (0.030
0.999 1.053 0.488 1.11
(0.005 (0.006 (0.002 (0.02
Cs
(0.003
hexane was then added to the complexed dianion salt,
whereby it was reoxidized to the neutral state (C10H14
-
O-COT2-,2M+:2IV + I2 f C10H14-O-COT + 2MI +
2IV). This reoxidation freed the cosolvent, which was
dissolved in hexane and labeled Phase II.
When Na served as the reducing agent (M+ ) Na+ in
reactions 2 and 3), GC-mass spectral analysis revealed
that Phase I is enriched in IV-d2 and depleted in the
perprotio-IV, while the reverse is true in Phase II, Table
1. This means the R isomer is enriched in Phase I and
depleted in Phase II relative to the S isomer. The
interaction between S enantiomer of the cosolvent and
the disodium salt of the (S)-verbenoxy-COT dianion must
be more favorable than the analogous interaction with
the R enantiomer of the cosolvent.
Let ShI, ShII, RhI, and RhII represent the relative
amounts of perprotiated (S)- and (R)-cosolvents in Phases
I and II, respectively. Likewise, let SdI, SdII, RdI, RdII
represent the relative amounts of dideuterated (S)- and
(R)-cosolvents in Phases I and II. The mass spectral data
yields the ratio of dideuterated to perprotiated material
in each phase [X1 ) (SdI + RdI)/ShI and X2 ) (SdII + RdII)/
ShII]. Keep in mind that there is no perprotiated R isomer;
hence, RhI and RhII ) 0. The separation factor (R) is
defined by the ratio of S enantiomer to R enantiomer in
Phase I divided by that ratio in Phase II12 and is given
by eq 6.
(ShI + SdI)(RdI)
[(S)-IV]/[(R)-IV] in Phase I
R )
)
The use of the verbenoxy-COT dianion or diverbenoxy-
COT dianion as a stationary phase in a chromatographic
column represents another experimental challenge that
we are currently addressing. Indeed according to Linder,12
a ∆∆G° value of 29 cal/mol require 15000 theoretical
plates to generate a complete separation of a pair of
enantiomers. Our ∆∆G° values are larger in magnitude
than this.
[(S)-IV]/[(R)-IV] in Phase II (ShII + SdII)(RdII
)
(6)
The ratio of dideuterated to perprotiated (S) cosolvent
is the same in both phases (SdI/ShI ) SdII/ShII ) Q), which
is the same as that in the original mixture of racemic
dideuterated and S perprotiated cosolvents. This value,
Q, is easily obtained from the composition of the original
mixture. Substituting the expressions for XI, XII, and Q
in eq 6 leads to an expression which can be directly
evaluated from the mass spectral data, eq 7.
Exp er im en ta l Section
Ver ben oxycyclooctatetr aen e. The synthesis of verbenoxy-
COT was based on Kreb’s preparation of tert-butoxycyclo-
octatetraene.6 Verbenol (5.0 g, 33 mmol), from Aldrich Chemi-
cal Co., was refluxed in 20 mL of tetrahydrofuran (THF) with
a molar excess of potassium metal to form the alkoxide. The
potassium alkoxide salt in THF was then added to 4.3 g of
bromocyclooctatetraene (23 mmol) in 20 mL of THF at -78
°C. The reaction mixture (Scheme 1) was maintained at -78
°C for 24 h. After 24 h, the reaction mixture was allowed to
warm to room temperature, and the THF was distilled from
the reaction vessel. Verbenxoy-COT, a viscous yellow oil, was
purified by vacuum (10-3 Torr) distillation in 14% yield and
collected from 140 to 150 °C. The structure was verified by
NMR, mass spectral, and C, H, and O analysis (expected:
85.0% C, 8.66% H, 6.30% O; found: 85.6% C, 8.17% H, 6.23%
O).
(ShI + SdI)/(XIShI - SdI)
(ShII + SdII)/(XIIShII - SdII
(XII - Q)
(XI - Q)
R )
)
(7)
)
Using the data from our most successful separation
(Table 1) in eq 6 yields a separation factor of R ) 1.11 (
0.02 when Cs+ serves as the counterion and R ) 0.846 (
0.030 when Na+ serves as the counterion. The data show
that there is no doubt that there is chiral recognition
between the verbenoxy group on the COT dianion and
the ibuprofoxy moiety on the cosolvent. The separation
(11) Stevenson, C. D.; McElheny, D. J .; Kage, D. E.; Ciszewski, J .
T.; Reiter, R. C. Anal. Chem. 1998, 70, 3830.
(12) Linder, W. In Chirality- From Weak Bosons to the R-Helix;
J anoschek, R., Ed.; Springer-Verlag: New York, 1991; Chapter 9.
Ibu pr ofoxym eth oxyeth oxyeth an e: (()-Ibuprofenol-d2 (4.7
g, 24 mmol) was obtained in nearly 100% yield by the reduction

7592 J . Org. Chem., Vol. 65, No. 22, 2000
Stevenson and Cashion
F igu r e 2. 1H NMR (400 MHz) spectrum (y-axis in ppm) of the S cosolvent [(S)- ibuprofoxymethoxyethoxyethane] in CDCl3.
Sch em e 1
Sch em e 2
of 5.0 g of (()-4-isobutyl-R-methylphenylacetic acid (ibuprofen)
(24 mmol) with 1.0 g of LiAlD4 (98 atom % D) (32 mmol) at 0
°C in anhydrous diethyl ether. After 1 h, the reaction mixture
was allowed to come to ambient temperature. Wet ether (20
mL) was then added to the reaction mixture, followed by the
addition of 10 mL of a 5% HCl solution. The ether solution
was separated and concentrated under reduced pressure. The
resulting (()-ibuprofenol-d2 (4.7 g, 24 mmol) was refluxed with
a molar excess of sodium metal in THF for approximately 36
h to form the respective sodium alkoxide. The sodium alkoxide
was not isolated but was added to a separate reaction vessel
containing 13.0 g of 1-bromo-2-(2-methoxyethoxy)ethane
(BrMEE, 71 mmol) in THF and left to react at 50 °C for 24 h
(Scheme 2). The THF was distilled from the reaction vessel
while remaining under an N2 (g) atmosphere. Deionized water
(20 mL) was added to the reaction mixture, followed by
addition of a 5% NaOH solution until the reaction mixture was
basic to litmus. The resulting mixture was extracted with
diethyl ether several times and dried with MgSO4. Most of the
unreacted BrMEE was then removed under reduced pressure
to increase the efficiency of the following chromatographic step.
The resulting (()-ibuprofoxymethoxyethoxyethane-d2 [(()-
ibuMEE-d2, 0.8 g, 2.7 mmol] was purified via alumina flash
chromatography and recovered in ∼11% yield. Deionized water
(6 mL) was added to 200 mL of alumina gel (activated, neutral,
Brockman I) in a slurry of 10% ethyl acetate in hexane. The
addition of 6 mL of water to each 100 mL of dry alumina was
necessary to reduce the activity of the alumina. The Rf values
of (()-ibuprofenol-d2, (()-ibuMEE-d2, and BrMEE under these
chromatographic conditions were 0.11, 0.32, and 0.51, respec-
tively. (S)-(+)-ibuMEE was synthesized using LiAlH4 in place

Chiral Recognition
J . Org. Chem., Vol. 65, No. 22, 2000 7593
of LiAlD4 in an analogous manner. The structure was verified
by NMR (Figure 2), mass spectral, and C and H analysis
(expected: 73.97% C, 9.58% H, 16.45% O; found: 74.0% C,
9.55% H, 16.4% O).
Sep a r a tion s. Experiments involving the cesium cation
were carried out in an apparatus such as that shown in Figure
3. In a typical experiment, a cosolvent mixture was prepared
by dissolving 150 µL (0.508 mmol) of (()-ibuMEE-d2 and 150
µL (0.508 mmol) of (S)-(+)-ibuMEE in 10 mL of hexane
distilled from NaK2. This cosolvent mixture in hexane was
dried over calcium hydride under an argon atmosphere. Into
bulb A, Figure 3, was placed 20 µL (0.085 mmol) of verbenoxy-
COT. Under an argon atmosphere, the cosolvent mixture in
hexane was then placed in bulb A. The apparatus was then
attached to a vacuum line, and the hexane was removed with
stirring under vacuum. The apparatus was removed from the
vacuum line and opened under an inert argon atmosphere. A
sealed glass ampule containing 0.27 g of Cs metal (2.0 mmol)
was then opened and placed in bulb A. The apparatus was
reattached to the vacuum line, and 10 mL of THF were
distilled into bulb A from NaK2. The mixture in bulb A was
then maintained at -78 °C and allowed to stir for 24 h. The
solution in bulb A changed color to a dark burgundy, signifying
that the verbenoxy-COT had been reduced to the dianion,
F igu r e 3. Apparatus used for the partial separations of the
(R) and (S) enantiomers of the cosolvent using Cs metal as
the reducing agent. When Na served as the reducing metal, a
sidearm was attached to tube A. The sodium metal was then
distilled into tube A (forming a mirror) while the contents of
tube A were maintained at liquid nitrogen temperature.
F igu r e 4. (Upper) Mass spectrum of dideuterioibuprofoxymethoxyethoxyethane (IV-d2). Note the intense peak at m/z ) 176
corresponding to C13H16D2+. The fragility of the ether linkages renders the molecular ion unobservable. (Lower) Mass spectrum
of ibuprofoxymethoxyethoxyethane (IV-h2). These spectra were obtained using a GC-mass spectrometer. The elution time for the
material rendering the mass spectral data appears above the respective spectrum.

7594 J . Org. Chem., Vol. 65, No. 22, 2000
Stevenson and Cashion
reaction 2. The verbenoxy-COT dianion appeared as a solid
dark burgundy salt at the bottom of tube A. The THF was
then removed under vacuum while the mixture was main-
tained at e-40 °C. Hexane (10 mL) was then distilled from
NaK2 into tube A, and the mixture was stirred for several
minutes. Allowing the solid salt mixture to settle to the bottom
of tube A for several minutes resulted in a clear hexane phase
over the solid salt. This clear hexane phase was decanted from
the solid verbenoxy-COT dianion salt and poured through the
porous glass frit into tube B. An additional 10 mL of dry
hexane was distilled into tube A, and the above hexane
extraction process was repeated. The resulting mixture in tube
B was labeled Phase I. A saturated solution of I2 in 10 mL of
hexane was then added to tube A, reoxidizing the dianion to
The mass spectrometer was operated in single ion mode to
detect only fragment ions of m/z of 174.10 ( 0.9 and 176.20 (
0.9. These ions were chosen because the m/z ) 174 peak is
extremely small in the spectrum of the dideuterated material
(Figure 4). The ratio of the fragment ions 176.20:174.10 in each
sample is the same as the ratio of ibuMEE-d2:ibuMEE in each
sample. Multiple (5-10) injections of each sample were made,
and an average ratio ( standard deviation for each sample
was recorded. These ratios were then used to calculate the
separation factor (R) obtained from each experiment.
In str u m en ta tion . Typical X-band EPR acquisition param-
eters; data were obtained at 9.8 GHz. Modulation frequency:
60.00 kHz, modulation amplitude: 0.05 G.
the neutral state. (C10H14-O-COT2-,2M+:2IV + I2 f C10H14
-
O-COT + 2MI + 2IV). This reoxidation freed the cosolvent
in tube A. The resulting mixture in tube A was labeled Phase
II.
Ack n ow led gm en t. We thank the National Science
Foundation (Grant 9617066) for support of this work.
Samples of the original ibuMEE mixture, Phase I, and
Phase II were then subjected to GC mass spectral analysis.
J O000972P