In conclusion, we have developed a new selective synthetic
approach to phenols by aerobic oxidation of 1,1-diarylethanes
using NHPI as a key catalyst.This work was supported by a
Grant-in-Aid for Scientific Research on Priority Areas
‘‘Advanced Molecular Transformation of Carbon Resources’’
from the Ministry of Education, Culture, Sports, Science and
Technology, Japan, ‘‘High-Tech Research Center’’ Project for
Private Universities: matching fund subsidy from the Ministry
of Education, Culture, Sports, Science and Technology,
2005–2009, and Research Association for Ishii Oxidation
Technology.
Scheme 2
4-tert-butylphenol (3j) in 49%, 54% and 50% yields, respec-
tively, with good selectivities (78–87%) (entries 7–9). Further-
more,
the
oxidation
of
1-(3,4-dimethoxyphenyl)-1-
phenylethane (1k) gave 3,4-dimethoxyphenol (3k) (60% yield,
79% selectivity). From 1-(4-methoxyphenyl)-1-(4-tolyl)ethane
(1l) and 1-(4-chlorophenyl)-1-(4-tolyl)ethane (1m), the same
phenol 3a was formed in high selectivity (entries 11 and 12).
From 1-(3,5-dimethyl-4-methoxyphenyl)-1-phenylethane (1n),
3,5-dimethyl-4-methoxyphenol (3n) (68% yield, 93% selectiv-
ity) was obtained (entry 13). The results presented in Table 2
indicate that the 1,2-aryl shift is a nucleophilic process on the
electron deficient oxygen atom and the degradation of hydro-
peroxides was selectively induced to give more electron-rich
phenols in high selectivities.
Notes and references
1. (a) V. D. Luedeke, in Encyclopedia of Chemical Processing
and Design, ed. J. J. Mcketta, Marcel Dekker, New York, 1978,
p. 72; (b) H. Rademacher, in Ullmann’s Encyclopedia of Industrial
Chemistry, ed. W Gerhartz, Wiley, New York, 5th edn., 1987, vol.
A8, p. 201; (c) J. N. Weber, in Kirk-Othmer Encyclopedia of
Chemical Technology, ed. J. I. Kroschwitz, Wiley, New York,
4th ed., 1990, vol. 19, p. 500; (d) K. Wessermel and H.-J. Arpe,
Industrial Organic Chemistry, Wiley-VCH Verlag, Weinheim,
4th edn., 2003, p. 239; (e) W. B. Fisher and L. Crescentini,
in Kirk-Othmer Encyclopedia of Chemical Technology,
ed. J. I. Kroschwitz, Wiley, New York, 4th edn., 1990, vol. 4,
p. 827.
It is interesting to note that 1,1-diarylethanes bearing alkyl
substituents on their ortho-positions like 1-phenyl-1-(2,5-xylyl)-
ethane (1o) and 1-mesityl-1-phenylethane (1p) were reluctant
for the aerobic oxidation (entries 14-15). We thought that 1oꢁ
and 1pꢁ generated from 1o and 1p are sterically hindered stable
radicals which can serve as inhibitors in the radical chain-
transfer reaction. In particular, the 1pꢁ is a very stable radical
which can take many resonance forms (Scheme 2). Therefore,
these diarylethanes can act as chain-transfer inhibitors for the
aerobic oxidation of alkanes. Thus, we examined the aerobic
oxidation of toluene in the presence or absence of 1p (eqn (2)).
Toluene (7) was efficiently oxidized by NHPI combined with
Co(OAc)2 to give benzoic acid (8) in 83% yield, but no
oxidation of 7 was observed in the presence of a small amount
of 1p (5 mol%) (eqn (2)).
2. (a) H. Hock and S. Lang, Ber. Dtsch. Chem. Ges., 1944, B77, 257;
(b) W. Jordan, H. van Barneveld, O. Gerlich, M. K. Boymann and
J. Ullrich, in Ullmann’s Encyclopedia of Industrial Organic
Chemicals, ed. W Gerhartz, Wiley-VCH, Weinheim, 1985, vol.
A9, pp. 299–312.
3. (a) Y. Ishii, K. Nakayama, M. Takeno, S. Sakaguchi, T. Iwahama
and Y. Nishiyama, J. Org. Chem., 1995, 60, 3934; (b) Y. Ishii,
S. Sakaguchi and T. Iwahama, Adv. Synth. Catal., 2001, 343, 393.
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Y. Nishiyama, J. Org. Chem., 1996, 61, 4520.
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Chem., Int. Ed., 2001, 40, 222.
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2000, 122, 7390.
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343, 809.
8. Y. Aoki, S. Sakaguchi and Y. Ishii, Tetrahedron, 2005, 61,
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and R. A. Sheldon, Tetrahedron, 2002, 58, 9055.
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ð2Þ
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3717.
13. Treatment of the reaction mixture with PPh3 formed
Phenol and amine derivatives are well-known to serve as
radical inhibitors, however, the autoxidation is rarely inhibited
by a simple aromatic hydrocarbon such as 1p. Therefore, 1p is
a particular interesting compound as a new class of inhibitor
not involving amino or phenol group.
1-(4-methoxy-phenyl)-1-phenylethanol
(9)
exclusively.
However, the alcohol 9 could not be detected by GC analysis
due to thermal conversion of 9 to 5a during the GC measurement.
Therefore, we performed further treatment of the reaction
mixture with 0.3 M H2SO4 to produce 5a as a product
(in entry 6, Table 1).
ꢀc
This journal is The Royal Society of Chemistry 2008
Chem. Commun., 2008, 3417–3419 | 3419