9-alkylanthracene radical cations. An alternate route to apparent products of deprotonation

Article abstract of DOI:10.1039/a704730d

Side-chain oxidation products formed in the oxidation of 9-alkylanthracenes in MeOH-MeCN do not arise from radical cation deprotonation.

Full text of DOI:10.1039/a704730d

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9-Alkylanthracene radical cations. An alternate route to apparent products of
deprotonation
J. M. Tanko* and Yonghui Wang
Department of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061-0212, USA
Side-chain oxidation products formed in the oxidation of
9-alkylanthracenes in MeOH–MeCN do not arise from
radical cation deprotonation.
+
+
MeOH2
The removal of an electron from a hydrocarbon dramatically
increases its acidity. For example, the pKa of toluene drops from
41 to ca. 213 with the removal of a single electron.
Consequently, there is a great deal of interest in the thermo-
dynamic and kinetic acidity of radical cations.1–4
MeOH
MeOH
•+
CH2 •
+
OMe
•
H
Me
For an alkylaromatic radical cation, there are two competing
pathways for reaction with a base (B:); side chain deprotonation
(yielding a benzyl radical), and/or nucleophilic addition to the
aromatic ring.5–8 For alkylbenzenes, the side-chain deprotona-
tion predominates. The effect of structure (of either the
alkylbenzene or the base) on the rate of deprotonation has been
extensively studied, and the deprotonation is believed to
proceed via an early (reactant-like) transition state. Rate
constants were successfully correlated to the Brønsted equation,
1a
Me
Scheme 1
balances > 95%). This dramatic effect of temperature on the
product ratio suggested that 2a was not arising from the direct
deprotonation of a radical cation.
In order to test this hypothesis, 9,10-dimethylanthracene 1b
(Scheme 2) was treated with 1.9 equiv. of CeIV under a variety
of conditions (Table 1), and essentially the same results were
obtained: at 0 °C, only ring-oxidation product 3b was found; at
45 °C approximately equal amounts of 2b and 3b were formed.
In order to determine whether 2b was formed from 3b, the
following control experiment was run: 1b was allowed to react
with a ≈ 0.25–0.30.4,9 The rate constant for deprotonation of
.
PhCH3 + (H2O as base) is approximately 1.0 3 107 m21 s21 6
.
In the case of 9-alkylanthracenes, the situation is more
complicated. Thermodynamically, these radical cations are
quite acidic (9-methylanthracene is estimated to have a pKa of
ca. 26.4),2 however the literature is somewhat contradictory in
terms of the extent to which these radical cations undergo
deprotonation. In 1970, Parker reported that radical cations
generated from 9-alkyl- and 9,10-dialkyl-anthracenes lead
mainly to products derived from nucleophilic addition of
solvent (water, alcohol, acetic acid, etc.) to the aromatic ring.10
Moreover, when HOAc was the solvent and at elevated
temperatures, the initially formed addition products were
observed to eliminate HOAc and rearrange to yield a ‘side-
chain oxidation’ product [eqn. (1)].10 Similarly, Camaioni and
R
Me
1a R = c -C3H5
b R = Me
CH2OAc
AcO
Me
Ce(NH4)2(NO3)6
MeCN
Heat
+
HOAc (1)
R
R
X
X
AcO
Me
Me
+
co-workers also found that a significant fraction of ‘side-chain
oxidation’ products were derived from HOAc-addition prod-
ucts.7,11 Others have generally ascribed the side-chain oxidation
products as arising exclusively from deprotonation of a radical
cation.12,13
Me
CH2X
2a R = c -C3H5; X = OMe
b R = Me; X = OMe
c R = Me; X = ONO2
3a R = c -C3H5; X = OMe
b R = Me; X = OMe
c R = Me; X = ONO2
We became interested in this problem as the result of a
continuing study of the chemistry of radical cations derived
from cyclopropylarenes.14 Initially, using the radical cation
generated from 9-cyclopropyl-10-methylanthracene 1a, we had
hoped to use the deprotonation reaction in an intramolecular
competition to clock the rate of methanol-induced cyclopropane
ring opening (Scheme 1).
Treatment of 1a with 1.9 equiv. of ceric ammonium nitrate in
MeCN–MeOH at 45 °C led to side-chain oxidation product 2a
and ring-oxidation product 3a (Scheme 2) in 37 and 57% yield,
respectively. However, when the reaction was conducted at
0 °C, 3a was formed in 91% yield, and no 2a was detected (mass
Scheme 2
Table 1 Products arising from treatment of 9,10-dimethylanthracene with
CeIV under various conditions
Reaction conditions
2b (%)
3b (%)
0 °C; 30 min
45 °C; 30 min
0 °C; 30 min
then
0.0
40.8
0.0
98.3
53.0
94.3
45 °C; 30 min
43.0
49.0
Chem. Commun., 1997
2387

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R
R
Ce+4
Ce+3
NuH
H+
to nucleophilic addition to the aromatic ring, and that little of the
observed side-chain oxidation product can be attributed to
deprotonation of the radical cation. The observed side-chain
oxidation products actually arise from further reaction of the
nucleophillic addition product. Conversion of this initially
formed adduct to the side-chain oxidation product is facile in the
presence of a Lewis acid and at higher reaction temperatures.
Consequently, it is dangerous to draw mechanistic inferences
pertaining to the deprotonation of 9-alkylanthracene radical
cations based upon product analyses.‡
R
Nu
+
•
•
Me
Me
Me
Ce+4
Ce+3
H+
Nu
We gratefully acknowledge the National Science Foundation
(CHE-9412814) for financial support
R
Nu
R
Nu
R
Nu
–NuH
higher
temperatures
+
Footnotes and References
Me
Nu
CH2
Me
* E-mail: jtanko@vt.edu
H+
† The fact that deprotonation of radical cations is sluggish despite a potent
thermodynamic driving force has been noted in the literature, see refs. 9 and
12.
‡ A reviewer has noted that our reaction conditions are non-aqueous, and
that these observations may not apply to aqueous systems.
NuH
R
+
R
R
NuH
H+
1 A. M. Nicholas and D. R. Arnold, Can. J. Chem., 1982, 60, 2165;
A. M. Nicholas, R.J. Boyd and D. R. Arnold, Can. J. Chem., 1982, 60,
3011.
2 F. G. Bordwell and J. Cheng, J. Am. Chem. Soc., 1989, 111, 1792;
X. Zhang and F. G. Bordwell, J. Org. Chem., 1992, 57, 4163; X. Zhang,
F. G. Bordwell, J. E. Bares, J. Cheng and B. C. Petrie, J. Org. Chem.,
1993, 58, 3051.
3 V. D. Parker, Y. Chao and B. Reitsto¨en, J. Am. Chem. Soc., 1991, 113,
2336.
4 E. Baciocchi, T. D. Giacco and F. Elisei, J. Am. Chem. Soc., 1993, 115,
12 290, and references therein.
5 V. D. Parker and L. Eberson, Acta. Chem. Scand., 1970, 24, 3542.
6 K. Sehested and J. Holcman, J. Phys. Chem., 1978, 82, 651.
7 L. A. Deardurff, M. S. Alnajjar and D. M. Camaioni, J. Org. Chem.,
1986, 51, 3686.
8 E. Baciocchi, C. Rol and G. V. Sebastiani, Gazz. Chim. Ital., 1982, 112,
513.
9 C. J. Schlesener, C. Amatore and J. K. Kochi, J. Am. Chem. Soc., 1984,
106, 3567; C. J. Schlesener, C. Amatore and J. K. Kochi, J. Am. Chem.
Soc., 1984, 106, 7472, and references therein.
+
CH2
CH2
CH2Nu
Scheme 3 Note: To conserve space, only the central ring of the
9,10-disubstituted anthracene is shown
with CeIV at 0 °C for 30 min, and the reaction mixture was
divided into two portions. The first portion was subject to work-
up, and analyzed by H NMR spectroscopy. The only product
1
detected was 3b. The second portion, which was warmed to
45 °C for 30 min and subsequently subject to the same work-up,
was found to contain 2b and 3b in approximately the same ratio
as when 1b was reacted with CeIV at 45 °C.
The conversion 3b ? 2b is apparently favored by higher
reaction temperatures, and requires the presence of a Lewis
acid. (3b is stable at 50 °C indefinitely. However, in the
presence of a small amount of H+, 3b is quantitatively converted
into 2b within 15 min.)
10 V. D. Parker, Acta Chem. Scand., 1970, 24, 3455; V. D. Parker, J. Chem.
Soc., Chem. Commun., 1969, 873.
In the absence of MeOH, oxidation of 1b at 0 °C, followed by
work-up, leads exclusively to side-chain oxidation product 2c.
However, the failure to isolate 3c is likely because of its
instability. Indeed, when the reaction was conducted in CD3CN
and analyzed by 1H NMR prior to work-up, we found no
resonances attributable to either 1b or 2c. We tentatively assign
the observed resonances to 3c. After work-up of the NMR
sample, 2c was the only observed product (85% yield). The
presumed mechanism for these reactions is summarized in
Scheme 3.
11 D. M. Camaioni, personal communication.
12 L. M. Tolbert and R. K. Khanna, J. Am. Chem. Soc., 1987, 109, 3477;
L. M. Tolbert, R. K. Khanna, A. E. Popp, L. Gelbaum and
L. A. Bottomley, J. Am. Chem. Soc., 1990, 112, 2373; P. Anzenbacker,
T. Niwa, L. M. Tolbert, S. R. SIrimanne and F. P. Guengerich,
Biochemistry, 1996, 35, 2512.
13 Z. Li and L. M. Tolbert, Gaodeng Xuexiao Huaxue Xuebao, 1995, 16,
380; Z. Li and L. M. Tolbert, Acta Chimica Sinica, 1995, 53, 1204; Z. Li
and L. M. Tolbert, Youji Huaxue, 1995, 15, 303.
14 Y. Wang and J. M. Tanko, J. Am. Chem. Soc., 1997, 119, 8201.
These results demonstrate that deprotonation (base = MeOH
or NO32) of 9-alkylanthracene radical cations is slow† relative
Received in Corvallis, OR, USA, 3rd July 1997; 7/04730D
2388
Chem. Commun., 1997