Self-Initiated Autoxidation of a Sterically Crowded Cycloheptatriene Derivative via Norcaradienyloxyl Radicals

Article abstract of DOI:10.1021/jo961321p

An extremely crowded cycloheptatriene derivative, 1,3,5-tri-tert-butyl-7-(9-phenyl-9-fluorenyl)-l,3,5-cycloheptatriene (1), underwent autoxidation at 25 °C in cyclohexane to give 4-tert-butyl-2-pivaloylphenol (3) (33%), 5-tert-butyl-2-pivaloylphenol (4) (11%), 1,3,5-tri-tert-butylbenzene (5) (5%), tert-butyl 9-phenyl-9-fluorenyl peroxide (6) (44%), bis(9-phenyl-9-fluorenyl) peroxide (7) (7%), 1,1′,3,3′,5,5′-hexa-tert-butyl-7,′-bicycloheptatriene (8) (2%), and carbon monoxide (4%). ESR studies showed that 1 dissociates into l,3,5-tri-tert-butyltropyl radical (11) and 9-phenylfluorenyl radical (12) at 25-85 °C. The enthalpy and entropy of dissociation were determined to be 23.3 kcal/mol and 22.0 cal/mol·K, respectively. The formation of 3-5 can be explained by a mechanism involving attack of molecular oxygen to 11 and subsequent valence tautomerism of cycloheptatrienyloxyl radicals to norcaradienyloxyl radicals.

Full text of DOI:10.1021/jo961321p

888
J . Org. Chem. 1997, 62, 888-892
Self-In itia ted Au toxid a tion of a Ster ica lly Cr ow d ed
Cycloh ep ta tr ien e Der iva tive via Nor ca r a d ien yloxyl Ra d ica ls
Toshikazu Kitagawa,* Atsushi Miyabo, Hideki Fujii, Takao Okazaki, Tetsuya Mori,
Masaki Matsudou, Takeshi Sugie, and Ken’ichi Takeuchi*
Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University,
Sakyo-ku, Kyoto 606-01, J apan
Received J uly 12, 1996X
An extremely crowded cycloheptatriene derivative, 1,3,5-tri-tert-butyl-7-(9-phenyl-9-fluorenyl)-1,3,5-
cycloheptatriene (1), underwent autoxidation at 25 °C in cyclohexane to give 4-tert-butyl-2-
pivaloylphenol (3) (33%), 5-tert-butyl-2-pivaloylphenol (4) (11%), 1,3,5-tri-tert-butylbenzene (5) (5%),
tert-butyl 9-phenyl-9-fluorenyl peroxide (6) (44%), bis(9-phenyl-9-fluorenyl) peroxide (7) (7%),
1,1′,3,3′,5,5′-hexa-tert-butyl-7,7′-bicycloheptatriene (8) (2%), and carbon monoxide (4%). ESR studies
showed that 1 dissociates into 1,3,5-tri-tert-butyltropyl radical (11) and 9-phenylfluorenyl radical
(12) at 25-85 °C. The enthalpy and entropy of dissociation were determined to be 23.3 kcal/mol
and 22.0 cal/mol‚K, respectively. The formation of 3-5 can be explained by a mechanism involving
attack of molecular oxygen to 11 and subsequent valence tautomerism of cycloheptatrienyloxyl
radicals to norcaradienyloxyl radicals.
In tr od u ction
ammonium nitrate,3 chromic acid,4 and selenium diox-
ide,5 have been reported to oxidize cycloheptatriene to
give various products, such as benzaldehyde, benzene and
carbon monoxide, and tropone.
In this paper, we report the synthesis of a crowded
cycloheptatriene derivative 1 and a study of its spontane-
ous autoxidation. The bulky phenylfluorenyl substituent
at C-7 facilitates the homolytic cleavage of a carbon-
carbon bond by extremely large intramolecular conges-
tion and by resonance stabilization of the resulting
radicals, initiating autoxidation at ambient temperature
without an added initiator.
The autoxidation of an unsaturated hydrocarbon is
generally thought to be a radical chain reaction in which
the substrate molecule undergoes hydrogen abstraction
at the allylic position followed by an attack by molecular
oxygen on the resulting delocalized radical. In the
initiation step a hydrogen atom is abstracted by a radical
that is generated from an added initiator.
The thermal cleavage of a weak C-C bond is another
possible initiation step for the autoxidation of hydrocar-
bons. A typical example is the well-known formation of
bis(triphenylmethyl) peroxide from the triphenylmethyl
dimer.1 This mechanism is of interest since the reaction
does not require an added initiator and can proceed
efficiently by nonchain mechanisms.
Resu lts a n d Discu ssion
Sterically crowded cycloheptatrienes 1 and 2 were
synthesized by the cation-anion combination reaction of
1,3,5-tri-tert-butyltropylium ion and unsubstituted tro-
pylium ion, respectively, with 9-phenyl-9-fluorenyllithi-
um in THF at -78 °C.
1,3,5-Cycloheptatriene derivatives having a bulky sub-
stituent at the C-7 position are also capable of producing
a resonance-stabilized tropyl radical via cleavage of the
C-C bond. Although reactions of cycloheptatrienes with
singlet oxygen to form endoperoxides have been inves-
tigated intensively from both mechanistic and synthetic
points of view,2 there have been no systematic studies of
the products and mechanism of their reaction with
ground-state oxygen. Other oxidants, such as ceric
We have reported that cycloheptatriene (CHT)-nor-
caradiene (NCD) valence tautomerism is significantly
shifted toward NCD by the presence of bulky substitu-
ents, especially tert-butyl groups on C-2 and C-5.6 Com-
pound 1 also showed NMR spectra that are characteristic
of CHT-NCD equilibrium: the doublet for H-6 of the
cycloheptatrienyl moiety at δ 3.83 at 25 °C disappeared
when the CD2Cl2 solution of 1 was cooled to -60 °C and
was split into two signals (δ 5.59, CHT; 2.27, NCD) at
* To whom correspondence should be addressed. Phone: +81-75-
753-5713. Fax: +81-75-761-0056. E-mail: kitagawa@scl.kyoto-u.ac.jp.
X Abstract published in Advance ACS Abstracts, J anuary 15, 1997.
(1) Gomberg, M. J . Am. Chem. Soc. 1900, 22, 757; Ber. 1900, 33,
3150.
(2) (a) Adam, W.; Peters, E.-M.; Peters, K.; Rebollo, H.; Rosenthal,
R. J .; Schnering, H. G. v. Chem. Ber. 1984, 117, 2393. (b) Adam, W.;
Du¨rr, H.; Pauly, K.-H.; Peters, E.-M.; Peters, K.; Rebollo, H.; Schnering,
H. G. v. J . Org. Chem. 1984, 49, 1333. (c) Adam, W.; Adamsky, F.;
Kla¨rner, F.-G.; Peters, E.-M.; Peters, K.; Rebollo, H.; Ru¨ngeler, W.;
Schnering, H. G. v. Chem. Ber. 1983, 116, 1848. (d) Adam, W.; Balci,
M. J . Am. Chem. Soc. 1979, 101, 7537, 7542. (e) Adam, W.; Balci, M.
Angew. Chem. 1978, 90, 1014. (f) Asao, T.; Yagihara, M.; Kitahara, Y.
Bull. Chem. Soc. J pn. 1978, 51, 2131. (g) Mori A.; Takeshita, H. Chem.
Lett. 1978, 395.
(3) (a) Trahanovsky, W. S.; Young, L. B.; Robbins, M. D. J . Am.
Chem. Soc. 1969, 91, 7084. (b) Mu¨ller, P.; Katten, E.; Rocˇek, J . J . Am.
Chem. Soc. 1971, 93, 7116.
(4) Mu¨ller, P.; Rocˇek, J . J . Am. Chem. Soc. 1974, 96, 2836.
(5) Radrick, P. J . Org. Chem. 1964, 29, 960.
S0022-3263(96)01321-7 CCC: $14.00 © 1997 American Chemical Society

Autoxidation of a Sterically Crowded Cycloheptatriene
J . Org. Chem., Vol. 62, No. 4, 1997 889
Sch em e 1
F igu r e 1. ESR spectra measured in degassed m-xylene at 85
°C. A: Radical 11 generated from 8. B: Radical 12 generated
from 9,9′-diphenyl-9,9′-bifluorene. C: The sum of the spectra
A and B. D: Compound 1.
Sch em e 2
-100 °C. From the chemical shifts of H-6 at 25 and -100
°C the CHT:NCD ratio at 25 °C was calculated to be
47:53.
radical 11 and 9-phenylfluorenyl radical 12.7 The facile
homolysis was demonstrated by ESR measurements for
Although 1 was stable at room temperature in de-
gassed solutions, it was oxidized rapidly in the presence
of oxygen. When a cyclohexane solution of 1 was stirred
in the presence of air at 25 °C for 22 h, a mixture
consisting of substituted phenols 3 (33%) and 4 (11%),
tri-tert-butylbenzene 5 (5%), peroxides 6 (44%) and 7
(7%), and bitropyl 8 (2%, mixture of meso and dl isomers)
was obtained (Scheme 1). In addition, formation of
carbon monoxide (4%) was detected by GC analysis. On
the other hand, less crowded cycloheptatrienes 2 and 8
were inert to oxygen under the same conditions.
degassed m-xylene or cyclohexane solutions (∼10-2 M)
of 1. A weak signal was observed at 25 °C, which
increased in intensity as the temperature was increased.
The spectra agreed with the sum of the spectra of 11 and
12 generated by thermal dissociation of the corresponding
dimers (Figure 1). By assuming that the homolysis of 1
had reached equilibrium, the enthalpy and entropy of
dissociation were determined from the temperature
dependence of the equilibrium constants, Khomo, observed
for cyclohexane solution at 25-85 °C, to be 23.3 kcal/
mol and 22.0 cal/mol‚K, respectively (Figure 2).8
Scheme 3 shows plausible pathways for the formation
of 3-5. The radical 11 undergoes addition of oxygen to
one of the carbons of the seven-membered ring. The
resulting peroxyl radicals 13a -c are transformed into
cycloheptatrienyloxyl radicals 14a -c by a well-known,
The products were identified by comparison of their
NMR spectra with those of independently prepared
authentic samples (Scheme 2). Phenols 3 and 4 were
synthesized by Friedel-Crafts acylation of p- and m-tert-
butylanisole, respectively, with pivaloyl chloride/AlCl3
and subsequent demethylation with AlCl3 in benzene.
Peroxide 6 was synthesized by CuCl-catalyzed oxidation
of 9-phenylfluorene with tert-butyl hydroperoxide.
The formation of products in Scheme 1 can be ex-
plained in terms of a mechanism involving the homolytic
cleavage of a C-C bond in 1 into tri-tert-butyltropyl
(6) (a) Takeuchi, K.; Arima, M.; Okamoto, K. Tetrahedron Lett. 1981,
22, 3081. (b) Takeuchi, K.; Fujimoto, H.; Okamoto, K. Tetrahedron Lett.
1981, 22, 4981. (c) Takeuchi, K.; Kitagawa, T.; Toyama, T.; Okamoto,
K. J . Chem. Soc., Chem. Commun. 1982, 313. (d) Takeuchi, K.;
Fujimoto, H.; Kitagawa, T.; Fujii, H.; Okamoto, K. J . Chem. Soc.,
Perkin Trans. 2 1984, 461. (e) Takeuchi, K.; Senzaki, Y.; Okamoto, K.
J . Chem. Soc., Chem. Commun. 1984, 111. (f) Takeuchi, K.; Kitagawa,
T.; Ueda, A.; Senzaki, Y.; Okamoto, K. Tetrahedron 1985, 41, 5455.
(7) It was shown, however, that 1 undergoes heterolytic fission of
the same carbon-carbon bond to form an ion pair in polar solvents
such as methanol. Controlling factors of the dissociation mode will be
reported elsewhere.
(8) Attempts to detect cage recombination of 11 and 12 by 1H CIDNP
(at 100 °C in CDCl3 under argon) were not successful, probably owing
to low concentration of polarized molecule and/or small difference
between the g values of these radicals.

890 J . Org. Chem., Vol. 62, No. 4, 1997
Kitagawa et al.
tions,11 thus providing conditions favorable for this
conversion. If radical 11 is present in abundance, as
would be expected for zinc reduction, significant amount
of intermediate radicals 14-16 would escape these
pathways, before the addition of the second oxygen, by
coupling with other radicals. These results demonstrate
the utility of the slow thermal dissociation of sterically
congested molecules as systems for studies of the chemi-
cal behavior of free radicals.
Exp er im en ta l Section
Melting points are uncorrected. IR spectra were recorded
on a Perkin-Elmer Model 1600 spectrophotometer. 1H NMR
spectra were obtained with a J EOL EX400 (400 MHz), GSX270
(270 MHz), or FX90 (90 MHz) instrument. 13C NMR spectra
were measured with a J EOL EX400 (100 MHz), GSX270 (68
MHz), or FX90 (22.5 MHz) instrument. Elemental analyses
were performed by the Microanalytical Center, Kyoto Univer-
sity. 9-Phenylfluorene,12 1,3,5-tri-tert-butyltropylium perchlo-
rate,13 tropylium tetrafluoroborate,14 9,9′-diphenyl-9,9′-biflu-
orene,15 and 1,1′,3,3′,5,5′-hexa-tert-butyl-7,7′-bicycloheptatriene
(8)16 were prepared by literature methods.
1,3,5-Tr i-ter t-bu tyl-7-(9-p h en yl-9-flu or en yl)-1,3,5-cyclo-
h ep ta tr ien e (1). A solution of (9-phenyl-9-fluorenyl)lithium
was prepared from 1.24 g (5.13 mmol) of 9-phenylfluorene and
butyllithium (1.58 M in hexane, 3.41 mL) in THF (60 mL) at
-78 °C. This solution was added to a stirred suspension of
1,3,5-tri-tert-butyltropylium perchlorate (2.11 mg, 5.88 mmol)
in 60 mL of THF at -78 °C. The mixture was stirred at 0 °C
for 15 min, and the solvent was evaporated under reduced
pressure. The residue was extracted with chloroform (60 mL).
Evaporation of the chloroform afforded a faintly yellow semi-
solid, which was dissolved in 150 mL of methanol and kept at
-20 °C overnight to yield colorless crystals of 1 (1.90 g, 74%):
mp 145 °C dec; IR (KBr) 2960, 1477, 1450, 1361, 746 cm-1; 1H
NMR (400 MHz, CD2Cl2) δ 7.65-7.00 (m, 13H), 5.61 (s, 1H),
5.50 (s, 1H), 3.83 (d, J ) 9.8 Hz, 1H, H-6), 3.35 (d, J ) 9.8 Hz,
1H, H-7), 1.18 (s, 9H), 0.80 (s, 9H), 0.75 (s, 9H); 13C NMR (68
MHz, CDCl3) δ 152.0, 149.1, 147.6, 145.2, 144.0, 141.2, 140.2,
88.8, 60.8, 36.2, 36.0, 34.9 (C); 128.5, 127.7, 127.4, 126.8, 126.7,
126.52, 126.48, 125.9, 125.2, 120.0, 119.4, 119.0, 117.0, 70.1,
31.0 (CH); 30.3, 30.0, 28.8 (CH3). The 13C signals at δ 88.8
and 70.1, which were assigned to C-1 and C-6, respectively,
were extremely broadened due to CHT-NCD interconversion.
The assignments of H-6 and H-7 are based on the H-C COSY
spectrum, which indicated a strong coupling of the H-7 signal
with the C-7 signal at δ 31.0. Anal. Calcd for C38H44: C, 91.14;
H, 8.86. Found: C, 91.17; H, 8.86.
F igu r e 2. Temperature dependence of the equilibrium con-
stant of the homolysis of 1 (1.45 × 10-2 M) in cyclohexane.
self-reaction mechanism through tetroxides.9 Tautomer-
ization of 14a -c to the corresponding norcaradienyloxyl
radicals 15a -c and subsequent ring opening to form
cyclohexadienyl radicals 16a -c are based on analogy to
the mechanism proposed for the oxidation of cyclohep-
tatriene with ceric ammonium nitrate3 or chromic acid.4
In the present case, the valence isomerization of 14a -c
to 15a -c is promoted by the steric effect of the tert-butyl
groups on olefinic carbons.6 The addition of a second
oxygen molecule to the tertiary radical center of 16a and
16b, followed by elimination of a tert-butyl radical, yields
tert-butylpivaloylphenols 3 and 4. In path C, however,
radical 16c yields 5 by the loss of formyl radical, which,
upon hydrogen abstraction by a radical in the reaction
mixture, forms carbon monoxide.
The 9-phenylfluorenyl radical 12 is also attacked by
oxygen molecule to form the (9-phenyl-9-fluorenyl)peroxyl
radical 20 and when combined with the tert-butyl radical
or another molecule of 12 yields peroxides 6 and 7,
respectively (Scheme 4).
The agreement of the yields of phenols (3 + 4) and
peroxides (6 + 7) as well as those of 5 and carbon
monoxide is consistent with mechanisms shown in
Schemes 3 and 4.
Single-electron reduction of 1,3,5-tri-tert-butyltropy-
lium ion under oxygen with zinc dust in 50% aqueous
acetonitrile also gave 3-5, consistent with the formation
of the tri-tert-butyltropyl radical as the initiation step in
the autoxidation of 1. However, the yields of substituted
phenols 3 and 4 were much lower (8 and 4%, respectively)
than those observed for the autoxidation of 1.10 Possibly
7-(9-P h en yl-9-flu or en yl)-1,3,5-cycloh ep t a t r ien e (2).
Compound 2 was synthesized as described above from 9-phe-
nylfluorene (0.221 g, 0.913 mmol) and tropylium tetrafluo-
roborate (0.162 mg, 0.910 mmol). Recrystallization from
ethanol gave 2 as white crystals (0.148 g, 49%): mp 193-194
°C; IR (KBr) 3021, 1447, 741, 706, 636 cm-1 1H NMR (400
;
MHz, CDCl3) δ 7.81 (d, J ) 7.8 Hz, 2H), 7.37 (t, J ) 7.3 Hz,
2H), 7.33-7.12 (m, 7H), 7.10 (d, J ) 8.3 Hz, 2H), 6.66 (s, 2H),
6.08 (d, J ) 9.8 Hz, 2H), 5.00 (t, J ) 7.5 Hz, 2H), 2.91 (br s,
1H); 13C NMR (68 MHz, CDCl3) δ 150.7, 143.4, 141.1, 60.0 (C);
130.6, 128.5, 127.8, 127.7, 126.6, 125.1, 125.0, 124.8, 119.9,
45.6 (CH). Anal. Calcd for C26H20: C, 93.94; H, 6.06. Found:
C, 93.71; H, 6.06.
(11) The ESR spectrum of a 1.45 × 10-2
M solution of 1 in
a high [O2]/[11] concentration ratio is important for the
efficient conversion of radical 11 to 3 and 4 through
pathways involving the consecutive addition of two
oxygen molecules (Scheme 3, path A and B). Thermal
homolysis of 1 generates radicals of very low concentra-
cyclohexane indicated that the concentration of 11 was only 9.3 × 10-8
M at 25 °C.
(12) Rundel, W. Chem. Ber. 1966, 99, 2707.
(13) Komatsu, K.; Takeuchi, K.; Arima, M.; Waki, Y.; Shirai, S;
Okamoto, K. Bull. Chem. Soc. J pn. 1982, 55, 3257.
(14) Conrow, K. Organic Syntheses; J ohn Wiley and Sons: New
York, 1973; Collect. Vol. 5, p 1138.
(9) Ingold, K. U. Free Radicals; Kochi, J . K., Ed.; J ohn Wiley and
Sons: New York, 1973; Vol. 1, p 59.
(10) Dimer 8, which is the major product of the reaction under inert
atmosphere, was not detected.
(15) Staab, H. A.; Rao, K. S.; Brunner, H. Chem. Ber. 1971, 104,
2634.
(16) Vincow, G.; Morrell, M. L.; Hunter, F. R.; Dauben, H. J ., J r. J .
Chem. Phys. 1968, 48, 2876.

Autoxidation of a Sterically Crowded Cycloheptatriene
J . Org. Chem., Vol. 62, No. 4, 1997 891
Sch em e 3
9H); 13C NMR (68 MHz, CDCl3) δ 212.1, 161.3, 140.2, 116.8,
44.4, 34.1 (C); 132.9, 127.1, 118.7 (CH); 31.4, 28.9 (CH3). Anal.
Calcd for C15H22O2: C, 76.88; H, 9.46. Found: C, 77.01; H,
9.75.
Sch em e 4
5-ter t-Bu tyl-2-p iva loyla n isole (10). 3-tert-Butylanisole
was synthesized by adding dimethyl sulfate (8.4 g, 67 mmol)
to a solution of 3-tert-butylphenol (10.0 g, 67 mmol) in 1.0 M
NaOH (67 mL) and refluxing the mixture at 120 °C for 30 min.
Extraction with ether and subsequent distillation gave a
colorless liquid (bp 79.5-82 °C/5 mmHg, 8.8 g, 81%).
To a solution of 3-tert-butylanisole (2.03 g, 12.4 mmol) and
pivaloyl chloride (1.49 g, 12.4 mmol) in CS2 (5 mL) was added
AlCl3 (1.62 g, 12.1 mmol). After initial exothermic reaction,
the mixture was stirred at room temperature for 1 h. To the
cooled mixture was cautiously added 10 mL of 5% NaHCO3.
The mixture was extracted with ether, and the ether solution
was washed with 5% NaHCO3 and 10% NaCl and dried
(MgSO4). Evaporation of the ether afforded a yellow liquid,
which on MPLC (SiO2, hexane-ether 9:1) gave 3-tert-buty-
lanisole (1.37 g) and 10 (orange crystals, 0.81 g). The orange
crystals were recrystallized from hexane to give 0.53 g (17%)
of 10 as white crystals: mp 45.5-46 °C; IR (KBr) 2967, 1690,
1610, 1234, 1030, 964 cm-1; 1H NMR (400 MHz, CDCl3) δ 6.95
4-ter t-Bu tyl-2-p iva loyla n isole (9). To a solution of 4-tert-
butylanisole (5.0 g, 30 mmol) and pivaloyl chloride (3.7 g, 31
mmol) in CS2 (8 mL) was added AlCl3 (4.1 g, 31 mmol). After
initial exothermic reaction, the mixture was stirred at room
temperature for 1 h. To the cooled mixture was cautiously
added 100 mL of 0.6 M NaHCO3. The mixture was extracted
with ether, and the ether solution was dried (MgSO4). Evapo-
ration of the ether afforded a yellow liquid, which on MPLC
(SiO2, hexane-ether 97:3) gave 9 (1.9 g, 25%): white crystals;
mp 30-31 °C (after recrystallization from hexane); IR (liquid
film) 2964, 1697, 1499, 1250 cm-1; 1H NMR (90 MHz, CDCl3)
δ 7.32 (dd, J ) 8.7, 2.5 Hz, 1H), 7.01 (d, J ) 2.5 Hz, 1H), 6.82
(s, 2H), 6.91 (s, 1H), 3.79 (s, 3H), 1.32 (s, 9H), 1.21 (s, 9H); 13
C
NMR (100 MHz) δ 213.6, 155.0, 153.3, 128.3, 125.8, 117.0,
108.1, 55.1, 44.7, 34.8, 31.1, 26.6. Anal. Calcd for C16H24O2:
C, 77.38; H, 9.74. Found: C, 77.27; H, 9.99.
(d, J ) 8.7 Hz, 1H), 3.76 (s, 3H), 1.29 (s, 9H), 1.21 (s, 9H); 13
C
NMR (22.5 MHz, CDCl3) δ 213.0, 152.6, 142.3, 130.2, 126.0,
122.8, 110.1, 54.8, 44.2, 33.6, 31.0, 26.4. Anal. Calcd for
C16H24O2: C, 77.38; H, 9.74. Found: C, 77.09; H, 9.81.
4-ter t-Bu tyl-2-p iva loylp h en ol (3). A mixture of 9 (0.55
g, 2.2 mmol) and AlCl3 (0.31 g, 2.3 mmol) in benzene (10 mL)
was stirred at 50 °C for 27 h. 5% NaHCO3 (25 mL) was added,
and the mixture was extracted with ether. The ether solution
was washed with 5% NaHCO3 and dried (MgSO4). Evapora-
tion of the ether afforded a yellow oil, which on MPLC (SiO2,
hexane-ether 95:5) gave 3 (0.33 g, 63%): colorless oil; IR (film)
5-ter t-Bu tyl-2-p iva loylp h en ol (4). A mixture of 10 (0.43
g, 1.8 mmol) and AlCl3 (0.23 g, 1.8 mmol) in benzene (10 mL)
was stirred at 50 °C for 43 h. 5% NaHCO3 (25 mL) was added,
and the mixture was extracted with ether. The ether solution
was washed with 5% NaHCO3 and dried (MgSO4). Evapora-
tion of the ether afforded a yellow oil, which on MPLC (SiO2,
hexane-ether 95:5) gave 4 (0.14 g, 34%) and 10 (0.26 g). 4:
1
colorless oil; IR (liquid film) 2967, 1627, 1187, 970 cm-1; H
NMR (400 MHz, CDCl3) δ 12.79 (s, 1H), 7.95 (d, J ) 8.8 Hz,
1H), 7.01 (d, J ) 2.0 Hz, 1H), 6.88 (dd, J ) 8.8, 2.0 Hz, 1H),
1.44 (s, 9H), 1.30 (s, 9H); 13C NMR (68 MHz, CDCl3) δ 211.4,
163.8, 159.6, 115.0, 44.4, 35.1 (C); 130.6, 115.8, 115.4 (CH);
1
2963, 1635, 1482, 1299, 1167, 977 cm-1; H NMR (270 MHz,
CDCl3) δ 12.46 (s, 1H), 8.02 (d, 1H, J ) 2.6 Hz), 7.48 (dd, 1H,
J ) 8.8, 2.6 Hz), 6.95 (d, 1H, J ) 8.8 Hz), 1.47 (s, 9H), 1.32 (s,

892 J . Org. Chem., Vol. 62, No. 4, 1997
Kitagawa et al.
30.7, 28.7 (CH3). Anal. Calcd for C15H22O2: C, 76.88; H, 9.46.
Found: C, 77.12; H, 9.66.
hexane (50 mL) was added, and the stopcock was closed under
air. After the solution had been stirred at 25 °C for 22 h, the
stopcock was opened, and the gas in the flask (0.5 mL) was
taken with a syringe. GC analysis (molecular sieves 5A, 3 mm
i.d. × 2 m, 80 °C) for the gas indicated the formation of carbon
monoxide (4% based on 1). The cyclohexane was evaporated
under reduced pressure. NMR analysis of the residue showed
the formation of 3, 4, and 6 in 33, 11, and 44% yields,
respectively. The product mixture was dissolved in hexane
(5 mL) and cooled to -20 °C. After 2 h, 7 (7.0 mg, 7.0%) was
obtained as a white precipitate. The solvent was evaporated
to give an oil. The oil was separated by MPLC (SiO2, hexane-
ether 99:1) to give a mixture of 5 and 8 (5 and 2% yields,
respectively). Further elution gave a mixture of 3, 4, and 6,
which were separated by HPLC (Waters, µ-Porasil, hexane).
The products were identified by comparison of their 1H and
13C NMR spectra with those of authentic samples.
ter t-Bu tyl 9-P h en yl-9-flu or en yl P er oxid e (6). To a
solution of 9-phenylfluorene (1.00 g, 4.12 mmol) in chloroform
(5 mL) were added 5.0 mg of CuCl and 1.40 g (15.5 mmol) of
tert-butyl hydroperoxide. The mixture was refluxed for 6 h.
The CuCl was filtered off, and the solution was washed with
10% NaCl and dried (MgSO4). The solvent was evaporated,
and the residue was subjected to MPLC (SiO2, hexane-ether
98:2) to give 6 (1.17 g, 86%) as white crystals: mp 98.5-99.5
°C (after recrystallization from hexane); IR (KBr) 2979, 1452,
1360, 1194, 1011, 732 cm-1; 1H NMR (270 MHz, CDCl3) δ 7.66
(d, J ) 7.3 Hz, 2H), 8.42-8.17 (m, 11H), 1.07 (s, 9H); 13C NMR
(68 MHz, CDCl3) δ 146.7, 141.5, 140.7, 92.1, 79.8 (C); 129.0,
128.0, 127.4, 127.3, 126.7, 126.3, 119.7 (CH); 26.4 (CH3). Anal.
Calcd for C23H22O2: C, 83.61; H, 6.71. Found: C, 83.90; H,
6.68.
Zin c Red u ction of 1,3,5-Tr i-ter t-bu tyltr op yliu m ion
u n d er O2. In a two-necked flask equipped with a gas inlet
tube was saturated a solution of 1,3,5-tri-tert-butyltropylium
perchlorate (52.2 mg, 0.145 mmol) in 50% aqueous acetonitrile
(100 mL) with O2 by bubbling O2 gas with stirring. Zinc dust
(5.0 g, 16.0 mmol) was added in portions over 30 min, and
bubbling and stirring were continued for 1.8 h. The zinc was
filtered off, and the solution was extracted with CH2Cl2. The
extract was dried (MgSO4). Evaporation of the solvent gave
a yellow solid, which was subjected to NMR analysis to
determine the product yields: 3 (8%), 4 (4%), and 5 (12%).
Bis(9-p h en yl-9-flu or en yl) P er oxid e (7). A solution of
9,9′-diphenyl-9,9′-bifluorene (92 mg, 0.19 mmol) in cyclohexane
(50 mL) was stirred at 60 °C for 2 h under O2 atmosphere.
Evaporation of the solvent gave reddish crystals. The crystals
were dissolved in 2 mL of CH2Cl2, and the solution was diluted
with 5 mL of hexane and allowed to stand for 30 min to form
white crystals of 7 (45 mg, 46%): mp 189.5-191 °C (lit.17 194-
195 °C); IR (KBr) 3063, 1449, 1169, 1019 cm-1; 1H NMR (270
MHz, CDCl3) δ 7.58 (d, J ) 7.7 Hz, 4H), 7.28 (td, J ) 7.4, 1.4
Hz, 4H), 7.21-7.11 (m, 14H), 7.05 (td, J ) 7.4, 1.1 Hz, 4H);
13C NMR (68 MHz, CDCl3) δ 146.0, 141.0, 140.6, 93.2 (C);
129.0, 128.0, 127.5, 127.3, 126.5, 126.1, 119.6 (CH).
ESR Sp ectr a of 1. ESR spectra were recorded on a J EOL
SR2EX spectrometer equipped with a standard variable-
temperature controller using degassed solutions of 1 in cyclo-
hexane or m-xylene. Total radical concentrations were deter-
mined from double integration of the first derivative ESR
spectra. Signal intensities were calibrated with an 8.91 × 10-6
M 4-hydroxy-TEMPO solution in benzene.
Ack n ow led gm en t. This work was supported by a
Grant-in-Aid for Scientific Research from the Ministry
of Education, Science, Sports and Culture, J apan.
Su p p or t in g In for m a t ion Ava ila b le: 1H and 13C NMR
1
and H-C COSY spectra of 1 (including H NMR spectrum at
-100 °C) and a table of temperature dependence of equilibrium
constant of the homolysis of 1 (5 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.
Au toxid a tion of 1. Compound 1 (194 mg, 0.388 mmol) was
placed in a 250-mL round-bottomed flask equipped with a two-
way stopcock that was capped with a rubber septum. Cyclo-
(17) Bassey, M.; Buncel, E.; Davies, A. G. J . Chem. Soc. 1955, 2550.
J O961321P