Radical Ion Probes. 9. The Chemistry of Radical Cations Derived from 9-Cyclopropylanthracene and 9-Bromo-10-cyclopropylanthracene

Article abstract of DOI:10.1021/jo971524t

Reactions of radical cations generated from 9-cyclopropylanthracene (1) and 9-bromo-10-cyclopropylanthracene (2) in the presence of methanol have been investigated electrochemically. The major products arising from oxidation of both substrates are attributable to CH₃OH attack at the aromatic ring (occurring at the radical cation stage for 2 and the dication stage for 1) rather than CH₃OH-induced cyclopropane ring opening, which is estimated to be exothermic by 20 kcal/mol. Although ring-opened products are detected in some instances, these are found to arise from subsequent reaction of the primary oxidation products. These observations are consistent with a proposed product-like transition state for the nucleophile-induced ring opening of cyclopropylarene radical cations in which the positive charge is localized on the cyclopropyl group and nucleophile, and thus unable to derive stabilization from the aromatic ring.

Full text of DOI:10.1021/jo971524t

628
J . Org. Chem. 1998, 63, 628-635
Ra d ica l Ion P r obes. 9. Th e Ch em istr y of Ra d ica l Ca tion s Der ived
fr om 9-Cyclop r op yla n th r a cen e a n d
9-Br om o-10-cyclop r op yla n th r a cen e
Yonghui Wang, Karen H. McLean, and J . M. Tanko*
Department of Chemistry, Virginia Polytechnic Institute and State University,
Blacksburg, Virginia 24061-0212
Received August 13, 1997
Reactions of radical cations generated from 9-cyclopropylanthracene (1) and 9-bromo-10-cyclopro-
pylanthracene (2) in the presence of methanol have been investigated electrochemically. The major
products arising from oxidation of both substrates are attributable to CH3OH attack at the aromatic
ring (occurring at the radical cation stage for 2 and the dication stage for 1) rather than CH3OH-
induced cyclopropane ring opening, which is estimated to be exothermic by 20 kcal/mol. Although
ring-opened products are detected in some instances, these are found to arise from subsequent
reaction of the primary oxidation products. These observations are consistent with a proposed
product-like transition state for the nucleophile-induced ring opening of cyclopropylarene radical
cations in which the positive charge is localized on the cyclopropyl group and nucleophile, and
thus unable to derive stabilization from the aromatic ring.
In tr od u ction
naphthalene.10 While oxidation of these substrates in the
presence of methanol leads to cyclopropane ring-opened
products, the rate constant for ring opening was excep-
tionally slow (<20 M-1 s-1 for R-cyclopropylnaphthalene
radical cation) despite the fact that the ring-opening
reaction is exothermic by nearly 30 kcal/mol. These
observations were interpreted on the basis of a late,
product-like transition state for ring opening in which
the positive charge is effectively localized on the cyclo-
propyl group and thus unable to enjoy stabilization by
the aromatic ring.10
To further test this hypothesis, utilizing electrochemi-
cal methods, the chemistry of radical cations generated
from 9-cyclopropylanthracene (1) and 9-bromo-10-cyclo-
propylanthracene (2) was examined. The results of these
experiments are described herein.
Cyclopropyl-containing substrates have frequently been
utilized to “probe” for radical cation intermediates in a
number of important chemical and biochemical
oxidations.,1-4 It is frequently assumed that incorpora-
tion of a cyclopropyl group into a substrate will result in
ring-opened products if a radical cation is an important
reactive intermediate in the reaction pathway. A dif-
ficulty associated with this approach is that there is
surprisingly little actually known about the chemistry
of these cyclopropane radical cations.
In a series of papers,5-8 Dinnocenzo et al. have dem-
onstrated that ring opening of a phenylcyclopropane
radical cation occurs via a bimolecular, SN2-type process
(eq 1).9 The rate constant for reaction of phenylcyclo-
propane radical cation and methanol has been deter-
mined to be 9.5 × 107 M-1 s-1 in CH3CN.7
Recently, we reported results pertaining to the reactiv-
ity of radical cations derived from R- and â-cyclopropyl-
Radical cations have been shown to be reactive inter-
mediates produced during the anodic oxidation of an-
thracene and related compounds.11-13 The follow-up
chemistry of anthracene radical cations, which generally
involves dimerization and/or reaction with nucleophiles,
has been the subject of numerous investigations.14-16
(1) Silverman, R. B.; Hoffman, S. J . J . Am. Chem. Soc. 1980, 102,
884. Silverman, R. B.; Hoffman, S. J .; Catus, W. B., III. J . Am. Chem.
Soc. 1980, 102, 7126.
(2) Hanzlik, R. P.; Tullman, R. H. J . Am. Chem. Soc. 1982, 104,
2048. Riley, P.; Hanzlik, R. P. Tetrahedron Lett. 1989, 30, 3015.
(3) Macdonald, T. L.; Zirvi, K.; Burke, L. T.; Peyman, P.; Guengerich,
F. P. J . Am. Chem. Soc. 1982, 104, 2050.
(4) Castellino, A. J .; Bruice, T. C.; J . Am. Chem. Soc. 1988, 110,
1313. Castellino, A. J .; Bruice, T. C. J . Am. Chem. Soc. 1988, 110, 7512.
(5) Dinnocenzo, J . P.; Todd, W. P.; Simpson, T. R.; Gould, R. R. J .
Am. Chem. Soc. 1990, 112, 2462.
(6) Dinnocenzo, J . P.; Simpson, T. R.; Zuilhof, H.; Todd, W. P.;
Heinrich, T. J . Am. Chem. Soc. 1997, 119, 987.
(7) Dinnocenzo, J . P.; Lieberman, D. R.; Simpson, T. R. J . Am. Chem.
Soc. 1993, 115, 366.
(8) Dinnocenzo, J . P.; Zuilhof, H.; Lieberman, D. R.; Simpson, T.
R.; McKechney, M. W. J . Am. Chem. Soc. 1997, 119, 994.
(9) This bimolecular mechanism for cyclopropane ring opening was
originally proposed by Rao and Hixson. See: Rao, V. R.; Hixson, S. S.
J . Am. Chem. Soc. 1979, 101, 6458.
(10) Wang, Y.; Tanko, J . M. J . Am. Chem. Soc. 1997, 119, 8201.
(11) Marcoux, L. S.; Fritsch, J . M.; Adams, R. N. J . Am. Chem. Soc.
1967, 89, 5766.
(12) Phelps, J .; Santhanam, K. S. V.; Bard, A. J . J . Am. Chem. Soc.
1967, 89, 1752.
(13) Peover, M. E.; White, B. S. J . Electroanal. Chem. 1967, 13, 93.
(14) Parker, V. D. Acc. Chem. Res 1984, 17, 243.
(15) Hammerich, O.; Parker, V. D. In Advances in Physical Organic
Chemistry, Vol. 20; Academic Press: London 1984; pp 55-189.
(16) Eberson, L.; Utley, J . H. P.; Hammerich, O. In Organic
Electrochemistry, An Introduction and a Guide, Third Edition, Revised
and Expanded; Lund, H., Baizer, M. M., Eds.; Marcel Dekker, Inc.:
New York, 1990; pp 505-533.
S0022-3263(97)01524-7 CCC: $15.00 © 1998 American Chemical Society
Published on Web 01/14/1998

Radical Ion Probes
J . Org. Chem., Vol. 63, No. 3, 1998 629
Ta ble 2. Yield s of P r od u cts P r od u ced in th e
Con tr olled -Cu r r en t Oxid a tion of
9-Cyclop r op yla n th r a cen e (1)
Sch em e 1
yield, %a
[CH3OH], electrons,
workup
method
M
equiv
7
3
4
5
8
9
1
4.1
2.5
2.5
0.25
2.0
3.0
3.0
2.5
aqueous
aqueous
37.0 19.8 1.2 0.0 13.9 b 20.4
3.6 27.6 32.0 0.0 3.6 b 17.8
nonaqueous 0.0 2.8 41.4 20.4 13.0 b
nonaqueous 0.0 12.8 16.7 8.0
6.2
b
b
b
a
b
Yield determined by GC or 1H NMR. Trace.
Sch em e 2
Ta ble 1. Yield s of P r od u cts P r od u ced in th e
Con tr olled -Cu r r en t Oxid a tion of
9-Br om o-10-cyclop r op yla n th r a cen e (2)
yield (%)
[CH3OH], electrons,
workup
method
M
equiv
3
4
5
6
4.1
2.5
4.1
2.5
0.25
2.8
2.7
2.5
2.7
2.5
aqueous
aqueous
nonaqueous
nonaqueous
nonaqueous
62a
68c
b
b
b
b
b
b
early in the electrolysis but was converted to other
products upon further electrolysis. (The yield of 7
reached a maximum of 37% after 2 equiv of electrons was
transferred). A small quantity of anthraquinone (8) was
detected, even at the early stages of the electrolysis.
Bianthrone (9) was also detected in runs at low methanol
concentration.
0.0 7.4a
0.0 9.3c
0.0 5.8c
21.8a
41.0c
12.6c
23.2a
12.3c
4.3c
Isolated yield. Trace. c Yield determined by 1H NMR.
a
b
Resu lts
1. P r ep a r a tive Electr olyses. 9-Br om o-10-cyclo-
p r op yla n th r a cen e (2). The preparative (constant cur-
rent) electrolysis of 2 was conducted in CH3CN/CH3OH
with 0.1 M LiClO4 as the supporting electrolyte. The
products isolated from this electrolysis (Scheme 1) varied
dramatically as a function of the workup procedure
employed. Aqueous workup (extraction with H2O/ether)
yielded exclusively 9-cyclopropyl-9-methoxyanthracene
(3). In contrast, nonaqueous workup (removing solvent
by rotary evaporation followed by extraction with CH2-
Cl2) yielded mainly cyclopropyl ring-opened products (4,
5, and 6). The yields of several representative runs are
shown in Table 1.
Although substantially different products resulted, the
mass balances observed for both the aqueous and non-
aqueous workup procedures were similar (60-70%),
leading to the suspicion that the reaction products might
be interconverting during workup. Indeed, monitoring
of the electrolysis prior to workup by either GC or TLC
revealed that only 3 was produced during the electrolysis,
suggesting that this compound was the precursor to 4,
5, and 6.
In separate experiments this hypothesis was validated
by subjecting 3 to conditions designed to emulate the
conditions of the nonaqueous workup procedure. For
example, treatment of 3 with HBr in CH3CN led to
formation of ring-opened bromide 6 in 80% yield (unop-
timized). Similarly, treatment of 3 with HClO4 in CH3-
CN yielded perchlorate ester 5 in 46% yield (unopti-
mized). These results substantiate the contention that
cyclopropyl ring-opened products 4, 5, and 6 are produced
during workup.
9-Cyclop r op yla n th r a cen e (1). Controlled-current
electrolysis of 1 in CH3CN/CH3OH followed by a non-
aqueous workup resulted in the formation of cyclopro-
pane ring-opened products (in analogy to the results
observed in the oxidation of 2). However, ring-opened
products were also found when an aqueous workup was
employed (Table 2). Periodic monitoring of the reaction
mixture during the electrolysis by TLC and GC revealed
that 9-cyclopropyl-10-methoxyanthracene (7) was formed
2. Lin ea r Sw eep , Cyclic, a n d Der iva tive Cyclic
Volta m m etr y. The mechanism and kinetics of decay of
radical cations generated from 1 and 2 were studied
electrochemically. Voltammetric techniques such as
cyclic voltammetry, derivative cyclic voltammetry (CV,
DCV), or linear sweep voltammetry (LSV) permit assign-
ment of the rate law for the decay of species generated
(reversibly) by heterogeneous electron transfer (Scheme
2), where A represents the neutral substrate, B the
radical cation, and X another chemical entity in solution
which may be involved in the reaction (e.g., nucleophile).
In cases where no reverse wave is observed in the cyclic
voltammogram, linear sweep voltammetry (LSV) is a
powerful technique for studying the follow-up chemistry
of the electrogenerated intermediate.17,18 Put briefly, the
observed variation in the forward peak potential (Ep) as
a function of sweep rate ν, substrate concentration [A],
and auxiliary reagent (nucleophile) concentration [X]
provides useful information regarding the rate law for
decay of electrogenerated species B.
When a reverse wave is observed, cyclic and derivative
cyclic voltammetry (CV, DCV) and the “reaction order
approach” advocated by Parker et al. are applicable.18 The
reaction order approach provides a means of assessing
the rate law for radical ion decay by observing the
variation of the cathodic to anodic derivative peak current
ratio (Ipc′/Ipa′) as a function of [A], [X], and ν.18
(17) Nadjo, L.; Save´ant, J . M. Electroanal. Chem. 1973, 48, 113.
(18) Parker, V. D. in Topics in Organic Electrochemistry; Fry, A. J .,
Britton, W., Eds.; Plenum Press: New York, 1986; pp 35-79. Parker,
V. D. In Comprehensive Chemical Kinetics, Vol. 26, Electrode Kinet-
ics: Principles and Methodology; Bamford, C. H., Compton, R. G., Eds.;
Elsevier: New York, 1986; pp 145-202.

630 J . Org. Chem., Vol. 63, No. 3, 1998
Wang et al.
F igu r e 2. Cyclic voltammogram of 9-bromo-10-cyclopropyl-
anthracene (2) at 2000 mV/s (CH3CN solvent, LiClO4 support-
ing electrolyte, 0.1 M Ag+/Ag reference).
F igu r e 1. Cyclic voltammogram of 9-cyclopropylanthracene
(1) at 200 mV/s (CH3CN solvent, LiClO4 supporting electrolyte,
0.1 M Ag+/Ag reference).
Ta ble 3. LSV An a lysis of th e Electr och em ica l Oxid a tion
of 9-Cyclop r oyp la n th r a cen e (1)
rate law
∂Ep/∂ log(ν)
∂Ep/∂ log[A]
∂Ep/∂ log[X]a
k[B]
29.6b
19.7b
19.7b
21 ( 1
0b
0b
0b
k[B]2
-19.7b
k[B]2[X]/[A]
observed
a
0b
-19.7b
0 ( 1
-21 ( 1
b
X ) CH3OH. Theoretical response, see refs 18 and 19.
9-Cyclop r op yla n th r a cen e (1). The cyclic voltam-
mogram of 1 (Figure 1) is characterized by an initial
oxidation wave (Ep ≈ +780 mV at 500 mV/s) and several
additional oxidation waves at more positive potentials.
In the presence of CH3OH the peaks at more positive
potentials are perturbed but the initial oxidation wave
is unaffected (i.e., the peak potential and current remain
the same, Figure 1). The initial oxidation wave is
irreversible at all accessible sweeprates (up to 50 V/s for
our system). Consequently, LSV was an appropriate
technique for studying this system.
Ep was found to vary both as a function of sweeprate
and substrate concentration but was independent of the
concentration of methanol (Table 3). These observations
are in excellent agreement with a second-order mecha-
nism for radical cation decay: -d[1•+]/dt ) k[1•+]2.
9-Br om o-10-cyclop r op yla n th r a cen e (2). The cyclic
voltammogram of 2 (Figure 2) reveals an initial oxidation
wave (Ep ≈ +950 mV at 2000 mV/s) and subsequent
oxidation waves at more positive potentials. However,
unlike 1, at higher sweeprates the initial oxidation wave
becomes reversible (Figure 3). At the higher sweeprates,
the initial wave corresponds to a one-electron process (2
f 2•+ + e-). In the presence of methanol, the initial
oxidation wave becomes irreversible and corresponds to
a 2 e- process. (Details regarding electron stoichiometry
are provided in the Supporting Information.)
F igu r e 3. Cyclic voltammogram of 9-bromo-10-cyclopropyl-
anthracene (2) at 8000 mV/s (CH3CN solvent, LiClO4 support-
ing electrolyte, 0.1 M Ag+/Ag reference).
are summarized in Table 4. Two rate laws are consistent
with the observed DCV results. (The DCV reaction order
approach does not allow deconvolution of the individual
reaction orders in A and B.18) However, both CV and
LSV permit the separation of the individual reaction
orders in A and B. LSV is applicable at lower scanrates
where no reverse (cathodic) wave is observed. At low
scan rates, Ep was found to vary as a function of
At the higher sweeprates, the DCV reaction order
approach is applicable. The results of these experiments

Radical Ion Probes
J . Org. Chem., Vol. 63, No. 3, 1998 631
Ta ble 4. DCV An a lysis of th e Electr och em ica l Oxid a tion
of 9-Br om o-10-cyclop r op yla n th r a cen e (2)
Sch em e 3
∂ log(νC)/∂ log[X]a
RA/B
RX
a
rate law
∂ log(νC)/∂ log[A]
k[B][X]
0.0b
1.0b
1
2
2
2
1
1
1
1
k[B]2[X]
k[A][B][X]
observed
1.0b
1.0b
1.0b
1.1 ( 0.1
1.0b
1.1 ( 0.1
a
b
X ) CH3OH. Theoretical response, see ref 18.
Ta ble 5. LSV An a lysis of th e Electr och em ica l Oxid a tion
of 9-Br om o-10-cyclop r op yla n th r a cen e (2)
rate law
∂Ep/∂ log(ν)
∂Ep/∂ log[A]
∂Ep/∂ log[X]
k[A][X]
k[A][B]
k[A][B][X]
observed
29.6a
29.6a
29.6a
31 ( 1
0.0a
-29.6a
-29.6a
-30 ( 1
-29.6a
0.0a
-29.6a
-30 ( 1
a
Theoretical response, see refs 18 and 19.
Sch em e 4
reported by Fujita and Fukuzumi.24 This mechanism is
further supported by the bulk electrolysis results which
reveal that 7 is the product initially produced during the
oxidation. The isolation of 7 and 3 as the only detectable
products provides direct evidence that the cyclopropyl
group survives both the radical cation and dication stages
of oxidation.
F igu r e 4. Variation of the cathodic to anodic current ratio
(-ipc/ipa) with sweeprate for the oxidation of 9-bromo-10-
cyclopropylanthracene (2, 0.25 M CH3OH, CH3CN solvent,
LiClO4 supporting electrolyte).
sweeprate and both the concentrations of 2 and CH3OH
(Table 5), supporting a rate law that is first order each
in 2•+, 2, and CH3OH: -d[2•+]/dt ) k [2•+] [2][CH3OH].
At higher sweeprates where the CV becomes partly
reversible, differentiation between the rate laws k[A]]B]-
[X] vs k[B]2[X] can also be achieved by examining the
variation of the cathodic to anodic current ratio (ipc/ipa)
as a function of sweeprate.19 In Figure 4, the theoretical
response for these two rate laws20 and the experimental
Further oxidation of 725 forms 7•+ which likely under-
goes oxidative methyl transfer (Scheme 4), as proposed
in Parker’s studies of the anodic oxidation of dimethoxy-
durene,26 to form 10. Further oxidation yields 11 which
leads to the isolated product (3) after nucleophilic attack
of methanol.
results obtained at 0.25 M CH3OH are compared.21
A
(19) Andrieux, C. P.; Save´ant, J . M. In Investigation of Rates and
Mechanisms of Reactions, Part II, 4th ed.; Bernasconi, C., Ed.; Wiley:
New York, 1986; pp 305-390. See also: Andrieux, C. P.; Nadjo, L.;
Save´ant, J . M. J . Electroanal. Chem. 1970, 26, 147.
substantially better fit to the curve corresponding to the
rate law first order each in A and B is observed.22
(20) Simulations were performed using DigiSim 2.0, (Bioanalytical
Systems, Inc., W. Lafayette, IN) for the rate laws kobs[A][B] and kobs[B]2,
where kobs ) k[CH3OH].
Discu ssion
1. Oxid a tion of 9-Cyclop r op yla n th r a cen e (1).
LSV results for 1 show that decay of 1•+ in CH3CN/CH3-
OH is second order in radical cation and zero order in
CH3OH. Consequently, CH3OH attack must occur after
the rate-limiting step. On the basis of these results, a
disproportionation mechanism for decay of the radical
cation from 1 is proposed (Scheme 3).23 Because the
dication will invariably be more reactive toward nucleo-
philes than the radical cation, it is reasonable to suppose
that k2[CH3OH] > k-1[1] so that the overall rate law
reduces to k1[1•+].2 A nearly identical mechanism for
decay of 9-alkylanthracene radical cations in CH3CN/
H2O, studied by stopped-flow kinetics, was recently
(21) For the data in Figure 4, the cathodic current (ipc) is measured
from the zero-current axis.
(22) Similar results were obtained at other concentrations of
methanol (Supporting Information). The pseudo-second-order rate
constant kobs was found to vary with [CH3OH]: kobs ) 4.3 × 103, 1.1 ×
104, and 2.6 × 104 M-1 s-1 at 0.25, 0.5, and 1.0 M CH3OH, respectively.
Thus, based upon CV analysis, the overall rate law is k[A][B][X] with
k ) 2.9 × 104 M-2 s-1
.
(23) The factors which affect whether radical cation decay might
occur via disproportionation have been extensively discussed by Parker,
see ref 14.
(24) Fujita, M.; Fukuzumi, S. Chem Lett. 1993, 1911.
(25) As might be expected, the electron-donating methoxy in 7
causes the molecule to be more easily oxidized than starting compound
1. The CV of 7 exhibits a peak potentail of +676 mV (0.69 mM in
CH3CN/LiClO4). Under analogous conditions, Ep for 1 is ca. 750 mV.

632 J . Org. Chem., Vol. 63, No. 3, 1998
Wang et al.
Sch em e 5
of the intrinsic stability of the anthracene radical cations,
ring opening is thermodynamically (and thus kinetically)
disfavored. Similar arguments have been advanced to
explain the extremely low rate of ring opening of several
cyclopropane-containing radical anions.30
Previously, we devised a thermodynamic cycle (Scheme
6) to estimate ∆G° for the CH3OH-induced ring opening
of a cyclopropylarene radical cation in CH3CN.10 The
pertinent ∆G°s for reactions i f vi were obtained as
follows: (i) the oxidation potential of Ar-c-C3H5,31 (ii) the
C-C bond dissociation energy of cyclopropane (BDEC-C
) 61 kcal/mol) corrected for the radical stabilization
energy (RSE) of the different aryl groups,32 (iii) the bond
dissociation energy of a 1° R-OCH3 bond (BDEC-O ) 82
kcal/mol),33 (iv) the H-O bond strength of methanol
(BDEO-H ) 104 kcal/mol), (v) the standard potential of
the H+/H• couple in CH3CN (reported by Parker to be
-1.88 V vs NHE),34 and (vi) the difference in pKa between
CH3CN and the ether oxygen (pKa(CH3CN) ) -10.12;35
pKa(CH3CH2OCH2CH3) ) -3.59).36 The results of this
analysis are summarized in Table 6.
As the data in Table 6 show, these ring-opening
reactions are extremely exothermic for Ar ) phenyl,
R-naphthyl, or 9-anthryl. Thus the fact that 9-cyclopro-
pylanthracene radical cations do not undergo ring open-
ing is not because the ring opening is endothermic.
4. Na tu r e of th e Tr a n sition Sta te for Rin g Op en -
Because the anodic process produces H+, some of 3 is
converted to cyclopropyl-ring-opened products either
during the electrolysis or upon workup.
2. Oxid a tion of 9-Br om o-10-cyclop r op yla n th r a -
cen e (2). CV, DCV, and LSV results for 2 are all
consistent with a rate law for decay of 2•+ which is first
order each in radical cation, parent compound, and
methanol (-d[2•+]/dt ) k[2•+][2][CH3OH]. The reason 2•+
does not decay via a disproportionation mechanism (as
was proposed for 1•+) may be due to the fact that the
bromine substituent deactivates the aromatic ring, mak-
ing the removal of a second electron from 2•+ energetically
prohibitive. (Parker found that E° for 9-bromoan-
thracene was more positive than anthracene by 90 mV).27
As a consequence of having the disproportionation path-
way effectively “turned-off,” decay of the radical cation
follows a different pathway, presumably involving nu-
cleophilic attack of CH3OH prior to or during the rate-
limiting step. However, the appearance of 2 in the rate
law was somewhat unexpected.
Two plausible mechanisms account for the presence of
2 in the rate law. The first involves formation of
π-complex between 2 and 2•+ (12, Scheme 5). Radical
cation/neutral molecule complexes involving aromatic
radical cations have been characterized spectroscopically
and are usually formulated as a π-dimer with the two
molecules oriented face to face, with charge and spin
delocalized into both rings.28 (Radical cation 7•+ presum-
ably undergoes the same follow-up chemistry outlined in
Scheme 4.)
•+
in g. Methanol-induced ring opening of Ar-c-C3H5 is
substantially exothermic for Ar ) phenyl, R-naphthyl,
and 9-anthryl (-39, -28, and -20 kcal/mol, respectively).
For such exothermic reactions, an early (reactant-like)
transition state would normally be anticipated on the
basis of the Hammond postulate. However, the change
from Ar ) phenyl to naphthyl results in a 6 order of
magnitude decrease in rate, corresponding to ∂∆Gq/∂∆G°
g 0.77 and thus suggesting a late (product like) transition
state.10
There are two important contributors to ∆G° for ring
opening of cyclopropylarene radical cations: (a) the
ability of Ar to stabilize the ring-closed radical cation (i.e.,
Eo /Ar), and b) the ability of Ar to stabilize the radical
Ar•+
portion of the ring-opened, distonic radical cation (RSE-
(Ar)). Of these two, the former is far more important.
(29) Norrsell, F.; Handoo, K. L.; Parker, V. D. J . Org. Chem. 1993,
58, 4929.
(30) (a) Tanko, J . M.; Drumright, R. E. J . Am. Chem. Soc. 1990,
112, 5362. (b) Tanko, J . M.; Drumright, R. E. J . Am. Chem. Soc. 1992,
114, 1844. (c) Tanko, J . M.; Drumright, R. E.; Suleman, N. K.;
Brammer, L. E., J r. J . Am. Chem. Soc. 1994, 116, 1785. (d) Tanko, J .
M.; Brammer, L. E., J r.; Hervas’, M.; Campos, K., J . Chem. Soc., Perkin
Trans. 2 1994, 1407.
(31) Because the CV of 1 is irreversible, it was not possible to
determine E° experimentally. However, ionization potentials derived
from semiempirical MO theory (AM1) have been shown to accurately
predict E°s for a series of aromatic hydrocarbons: E° (V vs NHE) )
0.9121 × IP - 5.8255 (ref 10). For 1, the AM1-calculated IP is 8.01 eV
leading to an E° of 1.48 V. For 2, E° is predicted to be 1.599 V (based
upon a calculated IP of 8.14 eV). This calculated value compares very
favorably to the experimentally determine value of 1.498 V. (Note: At
higher sweeprates, the CV of 2 is reversible, permitting determination
of E°.)
An alternative possibility is that CH3OH attacks the
radical cation directly (prior to the rate-limiting step)29
and that the neutral substrate serves as a “shuttle” in a
subsequent bromine atom or H+ transfer step. Our
results do not allow differentiation between these two
possibilities.
3. Th er m od yn a m ic Con sid er a tion s. It might be
argued that the failure of 1•+ and 2•+ to undergo ring
opening may be thermodynamic in origin, i.e., because
•
(32) RSEs for ArCH2 were taken as the difference in the bond
strength of ArCH2-H (88.0, 85.1, and 81.8 kcal/mol for Ar ) C6H5,
R-C10H7, and 9-C14H9, respectively) and CH3CH2-H (98.2 kcal/mol).
Bond strengths were taken from McMillen, D. F.; Golden, D. M. Annu.
Rev. Phys. Chem. 1982, 33, 493.
(33) Benson, S. W. Thermochemical Kinetics; Wiley: New York,
1976; p 309.
(34) Parker, V. D. J . Am. Chem. Soc. 1992, 114, 7458.
(35) Deno, N. C.; Gaugler, R. W.; Wisotsky, M. J . J . Org. Chem. 1966,
31, 1967.
(26) Parker, V. D. J . Chem. Soc., Chem. Commun. 1968, 610.
(27) Norrsell, F.; Handoo, K. L.; Parker, V. D. J . Org. Chem. 1993,
58, 4929
(28) Eberson, L.; Hartshorn, M. P.; Persson, O. J . Chem. Soc., Perkin
Trans. 2 1995, 409 and references therein.
(36) Deno, N. C.; Turner, J . O. J . Org. Chem. 1966, 31, 1969.

Radical Ion Probes
J . Org. Chem., Vol. 63, No. 3, 1998 633
Sch em e 6
Ta ble 6. ∆G° for th e Meth a n ol-In d u ced Rin g Op en in g of
stabilization afforded by the aromatic ring. In the
transition state (13), however, although the radical
portion of the developing distonic radical ion can still
derive stabilization from the aromatic ring, the cation
portion becomes highly localized at C-2 and at the oxygen
of the attacking nucleophile.
Cyclop r op yla r en e Ra d ica l Ca tion s in CH3CN
•+/Ar
(V vs NHE)
E°Ar
RSE
(kcal/mol)
∆G°
aryl group
(kcal/mol)a
phenyl
R-naphthyl
9-anthryl
2.58b
1.99b
1.48
10.2b
13.1b
16.4
-39.1b
-28.4b
-20.0
a
•+/Ar
∆G° ) 30.7 - 23.1EAr
- RSE(Ar) in kcal/mol, see text and
b
Scheme 8. Reference 10.
5. Ster eoelectr on ic Con sid er a tion s. Stereoelec-
tronic factors may also contribute to the high barrier
associated with the ring opening of 9-cyclopropylan-
thracene radical cations. Two conformational extremes
are important for cyclopropane rings attached to a
π-system, bisected (θ ) 0°) and perpendicular (θ ) 90°),
where θ is the angle defined by the cyclopropyl methine
C-H bond with respect to the atoms of the adjacent
π-system. In general, the bisected conformation is pre-
ferred because overlap between the cyclopropyl HOMO
and LUMO of the π-system is maximal in this conforma-
tion37 and is thus expected to be the reactive conformation
for the ring opening reaction.
Earlier studies have found that 9-cyclopropylan-
thracene (neutral molecule) adopts the perpendicular
conformation because the normally preferred bisected is
destabilized by steric interactions between the cyclopro-
pyl group and the peri-hydrogens.38 The conformational
preference(s) of 9-cyclopropylnaphthalene radical cation
was explored using SCF-MO theory (AM1, C.I. ) 1).39
AM1 calculations suggest that for 9-cyclopropylan-
thracene the perpendicular conformation is preferred by
4.5 kcal/mol.
F igu r e 5. Effect of different aryl groups on the reaction
coordinate diagram for the methanol-induced ring opening of
•+
Ar-c-C3H5
On this basis, the implication of a product-like transition
state for ring opening is that the aromatic ring is
expected to have little effect on the free energy of the
transition state but a large effect on the free energy of
the reactants (Figure 5). These arguments are reason-
able in light of the mechanism associated with these ring-
opening reactions. In the reacting radical cation, charge
and spin are highly delocalized and able to enjoy the
(37) For an excellent discussion of qualitative MO theory pertaining
to arylcyclopropanes, see Takahashi, Y.; Ohaku, H.; Nishioka, N.;
Ikeda, H.; Miyashi, T.; Gormin, D. A.; Hillinski, E. F. J . Chem. Soc.,
Perkin Trans. 2 1997, 303.
(38) Drumright, R. E.; Mas, R. H.; Merola, J . S.; Tanko, J . M. J .
Org. Chem. 1990, 55, 4098.

634 J . Org. Chem., Vol. 63, No. 3, 1998
Wang et al.
However, in 1969, Bauld, et al. reported the ambient
Con clu sion s
temperature EPR spectrum of 1•+ (generated from treat-
ment of 1 with H2SO4).40 The observed hyperfine cou-
Radical cations generated from 9-cyclopropylanthracene
(and derivatives) do not undergo nucleophile-assisted ring
opening, despite the fact that these reactions are highly
exothermic. These results are consistent with a model
for the transtion state for ring opening of cyclopropyl-
arene radical cations, which in terms of the distribution
of charge and spin is more product-like than reactant-
like. Thus, the effect of the aromatic ring on reaction
rate is mainly due to changes in the free energy of the
reactant(s), with only a small effect on the free energy of
the transtion state for ring opening. These results
suggest that caution should be exercised when using
cyclopropyl-containing substrates as probes for radical
cation intermediates in that they demonstrate that a
potent thermodynamic driving force for ring opening does
not guarantee that ring opening will occur.
â
pling constant to the cyclopropyl methine hydrogen (aH
)
was reported to be 4.0 G, and the spectrum was inter-
preted on the basis of a freely rotating cyclopropyl group
with a slight preference for the bisected conformation.40
However, variable temperature experiments were not
performed, and it is difficult to assess whether the EPR
spectrum is more consistent with a time-averaged signal
arising from both conformations or a single, discrete
conformation (AM1 calcuations predict aHâ to be 0.7 and
7.0 G for the bisected and perpendicular conformations,
respectively.)41 At present, it is not possible to assess
whether the transition state for ring opening is even
higher than that depicted in Figure 5 because of unfavor-
able stereoelectronic factors. 9-Cyclopropylanthracene
radical cations do not appear to be a good test case for
ascertaining the role of stereoelectronic factors on the
rate of these nucleophile-induced ring-opening reactions.
6. Im p lica tion s Rega r d in g th e Use of Cyclop r o-
p yl-Con ta in in g Su bstr a tes a s P r obes for Ra d ica l
Ca tion In ter m ed ia tes. The fact that these radical
cations do not undergo cyclopropane ring opening is
especially significant because cyclopropyl-containing com-
pounds are often utilized as probes for radical cation
intermediates in a number of important chemical and
biochemical oxidations (e.g. MAO-A, c-P450).1-4 The
assumption is that radical cation intermediacy can be
inferred if ring-opened products are formed. These
results demonstrate that for some substrates (a) ring-
opened products might not be produced despite the fact
that radical cations are bona fide intermediates of the
oxidation and/or (b) that other intermediates or products
produced during oxidation may lead to ring opening.
7. Note on th e F or m a tion of P er ch lor a te Ester
5. Suprisingly, perchlorate ester 5 was successfully
isolated and characterized despite the unstable and
extremely explosive nature of alkyl perchlorates. Be-
cause of the novelty of this compound,42 some discussion
of its characterization is warranted. The IR spectrum
of 5 exhibited two peaks at 1230 and 1260 cm-1 (Cl-O
asymmetric stretching) and a peak at 1040 cm-1 (Cl-O
symmetric stretching), which are a characteristic of
Exp er im en ta l Section
Gen er a l. Melting points are uncorrected. 1H NMR and 13C
NMR were run at 270 and 400 MHz. All chemical shifts are
reported in δ units relative to TMS in CDCl3. LiClO4 (Aldrich)
was dried under vacuum before use. Acetonitrile (Mallinck-
rodt, HPLC grade, 99+%) was refluxed over calcium hydride
for at least 1 h and then distilled slowly, discarding the first
5 and last 10% of distillate. Methanol (Baker Analyzed HPLC
grade) was dried by stirring over calcium hydride, followed
by distillation. 9-Cyclopropylanthracene was prepared ac-
cording to published procedures.40 9-Bromo-10-cyclopropylan-
thracene was prepared from 9-cyclopropylanthracene via
reaction with NBS.43
Electr och em ica l Exp er im en ts. Electrochemical mea-
surements were performed on an EG&G Princeton Applied
Research model 273 potentiostat/galvanostat interfaced to an
MS-DOS computer. The details regarding this instrument and
data collection software were described earlier.30 Voltammet-
ric measurements were performed on solutions which con-
tained 0.5 M LiClO4 in CH3CN. The solutions were prepared
by weighing LiClO4 into an oven-dried 10 mL volumetric flask
and then placing the volumetric flask, together with all
voltammetry cell pieces, in a vacuum-drying oven under
vacuum (30-40 mmHg) at 40 °C for at least 8 h CH3CN and
the desired amount of CH3OH were added to the septum-
sealed volumetric flask via syringe. The resulting solution was
transferred to a vacuum oven-dried, argon-purged voltamme-
try cell. The electroactive substance was added, and the
resulting solution was purged with argon. A three-electrode
voltammetry cell was utilized: A Pt microdisk working
electrode (0.32 mm diameter) was prepared for use by polish-
1
covalent organic perchlorates.42 The H NMR spectrum
of 5 possesses a triplet shifted unusually downfield (δ )
4.8 ppm vs TMS) corresponding to the CH2 R to the
perchlorate group. Finally, the molecular weight (and
formula) were both confirmed using FAB-MS, HRMS,
and elemental analysis.
(42) Recent studies have demonstrated that perchlorate ion can
manifest nucleophilic properties in the presence of extraneous nucleo-
philes (e.g., halide ions). See: Zefirov, N. S.; Zhdankin, V. V.; Koz′min,
A. S. Russian Chem. Rev. 1988, 57, 1041, and references therein.
(43) The following procedure was taken from Mas, R. H., Ph.D.
dissertation, Virginia Polytechnic Institute and State University, 1989.
A 20 mL carbon tetrachloride solution of bromine (35 µL, 0.69 mmol)
was added dropwise to a mixture of 9-cyclopropylanthracene (0.30 g,
1.38 mmol) and NBS (0.12 g, 0.70 mmol) inside a 100 mL round-
bottomed flask wrapped with Al foil and equipped with a magnetic
stir bar. The reaction mixture was maintained at 15 °C with a water
bath. After 1 h, 30 mL of saturated sodium bisulfite was added to
quench the reaction. The organic layer was isolated, dried over
anhydrous sodium sulfate, filtered, and evaporated. The resulting
residue was dissolved in 40 mL of hexane, chilled in an ice bath, and
filtered to remove any remaining NBS or succinimide. After column
chromatography using neutral alumina and 1:30 benzene/hexane, 0.35
mg (85% yield) of 9-bromo-10-cyclopropylanthracene was obtained and
subsequently recrystallized from ethanol: mp 113-114 °C; 1H NMR
δ 0.78 (2H, m), 1.48 (2H, m), 2.46 (1H, m), 7.50-7.61 (m, 4H), 8.56
(2H, m), 8.78 (2H, m); IR 2996, 2987, 1438, 1361, 1310, 1274, 1254,
1029, 1007, 954, 907, 865, 846, 817, 797, 757, 750, 699, 655, 612, 597,
578, 523; UV (C2H5OH) 399 (10, 243), 378 (10, 392), 359 (6, 176), 341
(1, 900), 257 (135 683); EI-MS m/e (%) 298 (11.8, M + 2), 296 (12.0,
M+), 217 (100), 215 (55), 202 (54), 189 (18), 176 (10), 108 (18).
(39) Dewar, M. J . S.; Zoebisch, E. G.; Healy, E. F.; Stewart, J . J . F.
J . Am. Chem. Soc. 1985, 107, 3902.
(40) Bauld, N. L.; McDermed, J . D.; Hudson, C. E.; Rim, Y. S.;
Zoeller, J ., J r.; Gordon, R. D.; Hyde, J . S. J . Am. Chem. Soc. 1969, 91,
6666.
(41) Spin desnities for the H(1s) orbital were determined using the
keywords UHF and ESR and converted to hyperfine coupling constants
using the relationship: aH ) FH, 1s x 382 G. (The constant 382 G is
R
derived from the experimentally observed aH for the methyl radical
(23.0 G) and the AM1-calculated spin density in H(1s) (0.0602).
Hyperfine coupling constants calculated in this manner nicely duplicate
experimental values for several cyclopropyl-containing radicals and
radical ions. For example, the observed hyperfine couplings for the
cyclopropylcarbinyl radical (which adopts the bisected conformation)
are aHR ) 20.74 G, aHâ ) 2.55 G, aHø )2.01, 2.98 G (Kochi, J . K.; Krusic,
P. J .; Eaton, D. R. J . Am. Chem. Soc. 1969, 91, 1877). The AM1-derived
R
â
ø
values are aH ) 21.8 G, aH ) 2.65 G, aH )0.68, 1.6 G. For
9-cyclopropylanthracene radical anion (which adopts the perpendicular
conformation), AM1 predicts aH ) 5.9 G (experimentally, aH ) 6.64
â
â
G.; ref 40).

Radical Ion Probes
J . Org. Chem., Vol. 63, No. 3, 1998 635
1
ing with alumina slurry as outlined in the BAS electrode
polishing kit (part no. MF-2056). An Ag/Ag+ (0.10 M in CH3-
CN) electrode was used as a reference (+0.337 V vs SCE). A
Pt wire (2 cm in length, 2 mm in diameter) was used as
auxiliary electrode. The voltammetry cell was placed in a
Fisher FS-14 solid state/ultrasonic bath filled with water.
Between runs, the ultrasonic system was activated for 30 s to
clean the working electrode surface and agitate the solution.
Positive-feedback iR compensation was set as described previ-
ously (90% of oscillation value). All experiments were per-
formed at ambient temperature (23 °C).
Preparative-scale electrolyses were performed on solutions
which contained 0.1 M LiClO4 in CH3CN containing CH3OH.
The solutions were prepared as described for the voltammetry
experiments. A conventional H-cell, with two compartments
separated by a medium glass frit (22 mm in diameter), was
utilized. The electrolyte solution (60 mL) was partitioned
equally between the two compartments under argon. The
electroactive substrate was added to the anodic compartment,
and both anodic and cathodic compartments were purged for
at least 10 min with argon before electrolysis. The working
electrode was fabricated from Pt gauze (45 mesh, 30 mm ×
20 mm). For the cathodic compartment, a coiled copper wire
(2 mm in diameter, 5 dm in length) was utilized as the
auxiliary electrode. The reference electrode was Ag/Ag+. All
electrolysis experiments were performed at ambient temper-
ature (23 °C). Constant current electrolyses were performed
at currents ranging from -25 to -40 mA. Both the anodic
and cathodic compartments were purged with argon and
agitated via ultrasound during electrolysis. GC and TLC were
used to monitor the progress of the electrolyses.
P er ch lor a te ester 5: mp 98 °C; H NMR δ 3.20 (q, 2H, J
) 7 Hz), 4.73 (t, 2H, J ) 7 Hz), 6.42 (t, 1H, J ) 7 Hz), 7.50-
7.66 (m, 5H), 7.80 (d, 1H), 8.28 (q, 2H); 13C NMR δ 29.2 (t),
74.4 (t), 123.4 (d), 127.0 (d), 127.3 (d), 127.7 (d), 128.1 (d), 128.5
(d), 130.5 (s), 131.9 (d), 132.3 (s), 133.1 (d), 134.6 (s), 136.1 (s),
140.4 (s), 184.5 (s); IR (cm-1) 1664, 1598, 1474, 1382, 1317,
1289, 1269 (s, νs ClO3), 1235 (s, νas ClO3), 1098, 1037 (s, νs
ClO3); CI-MS m/e (%) 251 (MH+ + 2 - ClO3, 7.5), 249 (MH+
-
ClO3, 5.4), 233 (MH+ - ClO4, 22.8), 231 (33.6), 195 (100), 194
(96.6); FAB-MS m/e (%) 335 (MH+ + 2, 17.4), 333 (MH+, 48.6),
251 (MH+ + 2 - ClO3, 16.5), 233 (MH+ - ClO4, 19.8), 231
(22.2), 220 (48.6), 219 (40.8), 83 (ClO3+, 34.2), 73 (65.4), 69
(HClO2+, 56.4), 55 (100); HRMS (CI) for C17H14O5Cl (MH+),
calcd 333.0529, found 333.0512, error -5.0 ppm. Anal. Calcd
for C17H13O5Cl: C, 61.44; H, 3.95. Found: C, 60.63; H, 4.21.
Br om id e 6: 1H NMR δ 3.26 (q, 2H, J ) 7 Hz), 3.57 (t, 2H,
J ) 7 Hz), 6.54 (t, 1H, J ) 7 Hz), 7.45-7.56 (m, 2H), 7.59-
7.68 (m, 3H), 7.83 (d, 1H), 8.28 (q, 2H); 13C NMR δ 31.9 (t),
33.95 (t), 126.9 (d), 127.4 (d), 127.5 (d), 127.7 (d), 128.2 (d),
133.0 (s), 130.4 (s), 131.8 (d), 132.2 (s), 132.24 (d), 132.5 (d),
132.9 (d), 136.6 (s), 140.7 (s), 184.6 (s); IR (cm-1) 1662, 1598,
1474, 1383, 1315, 1287, 1098; EI-MS m/e (%) 314 (M + 2, 16.8),
312 (M+, 16.5), 233 (40.8), 231 (25.8), 219 (45.6), 215 (100);
HRMS (EI) for C17H13BrO, calcd 312.0150, found 312.0139,
error -3.4 ppm.
Electr olysis of 9-Cyclop r op yla n th r a cen e (1). 47.6 mg
(0.218 mmol) of 1 in CH3CN containing 2.5 M CH3OH was
electrolyzed at -30 mA for 35 min (3.0 equiv of electrons).
Portions of the reaction mixture were subjected to aqueous and
nonaqueous workup (vide supra) and quantitative 1H NMR
analysis. The results of this and other analogous runs are
summarized in Table 2.
Electr oylsis of 9-Br om o-10-cyclop r op yla n th r a cen e (2).
2 56.0 mg (0.189 mmol) in CH3CN containing 2.5 M methanol
was electrolyzed at -30 mA for 27 min (2.7 equiv of electrons).
The anodic solution (30 mL) was divided into two portions,
and two workup procedures were employed. Aqueous work-
up: one portion of electrolytic solution was extracted with H2O/
ether for three times. The ether layers were combined, dried
over anhydrous MgSO4, and concentrated. Quantitative 1H
NMR analysis with [(CH3)3SiOSi(CH3)3] as internal standard
was used to determine the yield of 3. An analytical sample of
3 was obtained for characterization via flash column chroma-
tography with CH2Cl2 as eluting solvent. Analogous proce-
dures were followed for electrolyses in the presence of 4.1 and
0.25 M CH3OH. The results are summarized in Table 1.
Nonaqueous workup: The second portion of the electroytic
solution was transferred into a 50 mL flask and evaporated.
CH2Cl2 was added to extract the organic materials, and the
resulting solution was filtered to remove LiClO4. Yields of 4,
5, and 6 were determined by 1H NMR analysis. Analytical
samples of these compounds were obtained as follows: Flash
column chromatography of the nonaqueous workup solution
with CH2Cl2 as eluting solvent gave pure 4 and a mixture of
5 and 6, which was subsequently separated via flash chroma-
tography using a solvent gradient 5 f 20% EtOAc/hexane.
9-Cyclop r op yl-9-m eth oxya n th r on e (3): 1H NMR δ 0.304
(m, 2H), 0.334 (m, 2H), 1.20 (m, 1H), 2.99 (s, 3H), 7.94 (t, 2H),
7.67 (t, 2H), 7.76 (d, 2H), 8.29 (d, 2H); 13C NMR δ 2.44 (t),
28.1 (d), 52.3 (q), 77.6 (s), 126.7 (d), 127.3 (d), 127.9 (d), 132.1
(s), 133.1 (s), 144.2 (s), 183.4 (s); IR (cm-1) 1665, 1601, 1458,
1319, 1272, 1072; EI-MS m/e (%) 265 (M + 1, 3.38), 264 (M+,
18.7), 236 (M+ - 28, 92), 223 (M+ - C3H5, 100), 215 (17.8),
208 (15.3); HRMS (EI) for C18H16O2, calcd 264.1150, found
264.1158, error 2.9 ppm.
Meth oxy eth er 4: 1H NMR δ 2.95 (q, 2H, J ) 7 Hz), 3.38
(s, 3H), 3.59 (t, 2H, J ) 7 Hz), 6.62 (t, 1H, J ) 7 Hz), 7.26-
7.50 (m, 2H), 7.53-7.64 (m, 2H), 7.76-7.85 (q, 2H), 8.21-8.32
(q, 2H); 13C NMR δ 31.7 (t), 58.8 (q), 71.9 (t), 123.5 (d), 126.8
(d), 127.3 (d), 127.4 (d), 127.7 (d), 127.8 (d), 130.3 (s), 131.7
(d), 132.1 (s), 132.2 (s), 132.8 (d), 133 (d), 136.8 (s), 141.1 (s),
184.8 (s); IR (cm-1) 1665, 1598, 1320, 1122; EI-MS m/e (%) 265
(M + 1, 7), 264 (M+, 32.7), 249 (M - CH3, 5.4), 219 (M - CH2-
OCH3, 100), 45 (CH2OCH3+, 91); HRMS (EI) for C18H16O2, calcd
264.1150, found 264.1146, error -1.6 ppm.
9-Cyclop r op yl-10-m eth oxya n th r a cen e (7). 1 (50.5 mg,
0.232 mmol) was electrolyzed at -30 mA for 25 min (2.0 equiv
e-’s). The progress of the electrolysis was monitored by GC
and TLC (3:1 hexane:CH2Cl2) so as to stop the electrolysis
when the yield of 7 was maximal. The anodic solution (30 mL)
was extracted with ether and water several times, dried
(MgSO4), and concentrated. Yields (based upon 1H NMR
analysis) are summarized in Table 2. Flash column chroma-
tography with a solvent gradient starting at 0% and finishing
at 25% CH2Cl2/hexane yielded 18% (10.4 mg) of 7: 1H NMR δ
0.80 (m, 2H), 1.44 (m, 2H), 2.43 (m, 1H), 4.14 (s, 3H), 7.46-
7.55 (m, 4H), 8.34 (m, 2H), 8.77 (m, 2H); 13C NMR δ 9.3 (t),
10.2 (d), 63.2 (q), 122.6 (d), 124.2 (s), 124.7 (d), 125.1 (d), 126.3
(d), 130.7 (s), 132.3 (s), 151.7 (s). IR (cm-1) 1664, 1620, 1652,
1452, 1390, 1330, 1282, 1116, 1088, 1027; EI-MS m/e (%) 249
(M + 1, 8.5), 248 (M+, 49), 247 (30), 233 (M - CH3, 47), 217
(M - OCH3, 100), 215 (89); HRMS (EI) for C18H16O, calcd
248.1201, found 248.1202, error 0.3 ppm.
Con tr ol Exp er im en ts. A 30 mL CH3CN solution of 3 (27.7
mg) was divided into three portions: 1 mL of CH3OH (2.5M)
and 0.1 of g (0.1 M) of LiClO4 were added to the first portion
and the reaction was subsequently subjected to nonaqueous
workup. TLC and GC of the resulting solution revealed only
unreacted 3. To the second portion were added 0.1 g of LiClO4
and 1 drop of HBr (32%). The reaction mixture was subjected
to nonaqueous workup. TLC and GC of resulting solution
showed the complete conversion of 3 f 6. The yield of 6 based
on 1H NMR analysis was 80%. To the third portion were
added 0.1 g of LiClO4 and 1 drop of HClO4 (60-70%).
Nonaqueous workup yielded 46% of 5 on the basis of 1H NMR
analysis.
Ack n ow led gm en t. Financial support from the Na-
tional Science Foundation (CHE-9412814) is acknowl-
edged and sincerely appreciated.
Su p p or tin g In for m a tion Ava ila ble: Discussion of elec-
tron stoichiometry and LSV, CV, and DCV plots for the
oxidation of 1 and 2 (9 pages). This material is contained in
libraries on microfiche, immediately follows this article in the
microfilm version of the journal, and can be ordered from the
ACS; see any current masthead page for ordering information.
J O971524T