Oxidation of hydroquinones with meso-hexakispentafluorophenyl [26]hexaphyrin(1.1.1.1.1.1)

Article abstract of DOI:10.1039/b515740d

Treatment of various hydroquinones and catechols with meso- pentafluorophenyl [26]hexaphyrin(1.1.1.1.1.1) provided the corresponding quinones quantitatively. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b515740d

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/obc | Organic & Biomolecular Chemistry
Oxidation of hydroquinones with meso-hexakispentafluorophenyl
[26]hexaphyrin(1.1.1.1.1.1)
Chihiro Maeda, Hiroshi Shinokubo* and Atsuhiro Osuka*
Received 7th November 2005, Accepted 6th December 2005
First published as an Advance Article on the web 13th December 2005
DOI: 10.1039/b515740d
Treatment of various hydroquinones and catechols with
meso-pentafluorophenyl [26]hexaphyrin(1.1.1.1.1.1) provided
the corresponding quinones quantitatively.
view of sustainable processes and high enantiomeric induction, but
oxidation-organocatalysts based on their multi-redox behaviour
remain mainly unexplored.3 Here we wish to report a hexaphyrin-
mediated oxidation of hydroquinones, catechols, and pinacols.
Although metal–porphyrin complexes often serve as the catalyst
for various types of oxidation reactions both in natural and
artificial systems,4 this is the first example of non-metalated
porphyrinoid-mediated oxidation of organic molecules.
In recent years, a series of meso-aryl expanded porphyrins
have been synthesized in moderate yields under the modified
Rothemund–Lindsey conditions.5a We have also achieved an
improved synthesis of meso-aryl expanded porphyrins from
dipyrromethanes or tripyrromethanes as starting substrates.5b
Although a number of expanded porphyrins exhibit multi-redox
behaviour, we picked up hexaphyrin because of its availability, and
tested its ability for the oxidation of organic molecules.
Expanded porphyrins that are porphyrin analogues with more
than five pyrrolic subunits have attracted much attention because
of their unique properties, such as their interesting structural
features and unique metal-coordination capability.1 In addition,
expanded porphyrins often exhibit stable multi-redox states, while
porphyrins seldom show such behaviour. For instance, meso-
pentafluorophenyl hexaphyrin(1.1.1.1.1.1) consists of two com-
pounds 1 and 2, which have different oxidation states with 26p and
28p conjugation respectively, and they are easily interconvertible
upon reduction with NaBH4 and oxidation with 2,3-dichloro-5,6-
dicyano-1,4-benzoquinone (DDQ) (Scheme 1).2
We had noticed that the reduction of [26]hexaphyrin to [28]hexa-
phyrin with a brilliant colour change from red–purple to blue
occurs in the presence of acids and alcohols. Thus, [26]hexaphyrin
1 was treated with benzyl alcohol in the presence of a trace of
trifluoroacetic acid to provide 2 quantitatively, but benzaldehyde
1
could not be detected by the H NMR analysis. After several
experiments, we found that an addition of 1.0 equiv. of 1,4-
hydroquinone (3a) to a solution of [26]hexaphyrin 1 in CDCl3
provided 1,4-benzoquinone (4a) instantly and quantitatively at
room temperature (Scheme 2). 1H NMR analysis of the resulting
solution revealed the quantitative formation of [28]hexaphyrin
Scheme 1 Interconversion of [26]hexaphyrin and [28]hexaphyrin.
This multi-redox phenomenon reminds us of transition metal
ions, because multi-valency plays an essential role in a number
of metal-mediated oxidation processes. It then occurred to us
that expanded porphyrins can oxidize organic compounds such
as alcohols in a similar manner to a number of transition metal
ions, and hopefully may act as an organocatalyst for oxidation
reactions. Currently, organocatalysts for carbon–carbon bond
forming processes have been a hot topic in organic synthesis in
Department of Chemistry, Graduate School of Science, Kyoto University and
PRESTO & CREST, Japan Science and Technology Agency (JST), Kyoto,
606-8502, Japan
Scheme
[26]hexaphyrin.
2 Oxidation of hydroquinones and catechols with
200 | Org. Biomol. Chem., 2006, 4, 200–202
This journal is
The Royal Society of Chemistry 2006
©

View Article Online
2. An equimolar amount of 4a and 2 were detected, and this
indicates that [26]hexaphyrin acts as a two-electron oxidant for
hydroquinones. Treatment of 3b, 3c and 3d with 1 also afforded the
corresponding quinones 4b, 4c and 4d in more than 90% yields in
10 min. However, oxidation of electron-deficient hydroquinone 3e
with an electron-withdrawing acyl group proceeded very slowly.6
Furthermore, oxidation of catechols 3f and 3g to o-benzoquinones
was also effected with [26]hexaphyrin efficiently.
electron transfer steps from 1 to hydroquinone involving a semi-
quinone radical intermediate.8
Oxidative cleavage of 1,2-diols with some oxidants such as
lead tetraacetate is known to proceed in a radical mechanism
involving an alkoxyl radical.9 We then examined the reaction
of benzopinacol with 1 (Scheme 3). To our delight, treatment
of benzopinacol with [26]hexaphyrin in N,N-dimethylformamide
provided benzophenone quantitatively in 30 min at 120 ◦C.
Oxidation of phenols with [26]hexaphyrin 1 turned out to be
inefficient. Thus, the reaction of 2,4,6-tri-tert-butylphenol (3h)
with [26]hexaphyrin 1 afforded a small amount of [28]hexaphyrin
in less than 5% yield, which was detected by the TLC and 1H NMR
analysis of the reaction mixture, but none of oxidized product was
observed. Oxidation of electron-rich phenol, 2,6-di-tert-butyl-4-
methoxyphenol (3i), was still sluggish to afford 2,6-di-tert-butyl-
1,4-benzoquinone (4i) in 25% yield in 10 days via the cleavage of
the ether linkage. The ESR spectrum of the reaction mixture of
2,4,6-tri-tert-butylphenol (3h) and [26]hexaphyrin in benzene at
room temperature confirmed the formation of a radical species,
which we tentatively assigned as the corresponding 2,4,6-tri-tert-
butylphenoxyl radical, although the signal was weak probably due
to inefficient formation of the radical (Fig. 1).7 In contrast, both
[26]hexaphyrin and 3h were silent for the ESR analysis before
mixing. On the basis of these observations, we assume that this
oxidation reaction would proceed through two sequential one-
Scheme 3 Oxidative C–C bond cleavage of benzopinacol.
We also attempted re-oxidation of [28]hexaphyrin to [26]hexa-
phyrin with molecular oxygen as an oxidant. This would enable a
hexaphyrin-catalyzed oxidation reaction of organic molecules with
dioxygen as the terminal oxidant. We found that 2 was oxidized
to 1 quantitatively in 3 days with air at room temperature in basic
aqueous isopropyl alcohol (Scheme 4).
In summary, we have found that treatment of hydroquinones
and catechols with [26]hexaphyrin provided p- and o-quinones,
most likely through a radical mechanism. [26]Hexaphyrin also
mediates oxidative C–C bond cleavage of benzopinacol to ben-
zophenone. We also found an aerobic oxidation of [28]hexaphyrin
to [26]hexaphyrin. These results demonstrate that multi-redox
behaviour of expanded porphyrins can be utilized for oxidation
of organic compounds. Further effort on the use of hexaphyrin as
an oxidation catalyst is currently under way in our laboratory.
We thank Professors T. Nishinaga (Tokyo Metropolitan Uni-
versity), N. Takeda (Kyoto University) and T. Ikeue (Shimane
University) for the help with the ESR measurement.
Fig. 1 ESR spectrum of a mixture of 2,4,6-tri-tert-butylphenol (3h) and
[26]hexaphyrin 1 in benzene at room temperature.
Scheme 4 Oxidation of [28]hexaphyrin to [26]hexaphyrin.
The Royal Society of Chemistry 2006 Org. Biomol. Chem., 2006, 4, 200–202 | 201
This journal is
©

View Article Online
4 (a) Metalloporphyrins in Catalytic Oxidation, ed. R. A. Sheldon, Marcel
Dekker, New York, 1994; (b) D. Dolphin, T. G. Traylor and L. Y. Xie,
Acc. Chem. Res., 1997, 30, 251; (c) J. T. Groves, K. Shalyaev and J. Lee,
in The Porphyrin Handbook, Vol. 4, ed. K. M. Kadish, K. M. Smith and
R. Guilard, Academic Press, San Diego, 2000, p. 119.
5 (a) J.-Y. Shin, H. Furuta, K. Yoza, S. Igarashi and A. Osuka, J. Am.
Chem. Soc., 2001, 123, 7190–7191; (b) R. Taniguchi, S. Shimizu, M.
Suzuki, J.-Y. Shin, H. Furuta and A. Osuka, Tetrahedron Lett., 2003, 44,
2505–2507.
6 An addition of trifluoroacetic acid to the reaction mixture did not
improve the yield of the product 4e.
7 Resolution of this spectrum was not sufficient as to analyze hyperfine
constants, but the splitting may be assigned for the coupling between the
phenoxyl radical and protons at the 3-, and 5-positions.
References
1 (a) J. L. Sessler, A. Gebauer and S. J. Weghorn, in The Porphyrin
Handbook, Vol. 2, ed. K. M. Kadish, K. M. Smith and R. Guilard,
Academic Press, San Diego, 2000, p. 55; (b) J. L. Sessler and S. J.
Weghorn, Expanded, Contracted & Isomeric Porphyrins, Pergamon,
Oxford, 1997; (c) E. Vogel, J. Heterocycl. Chem., 1996, 33, 1461; (d) A.
Jasat and D. Dolphin, Chem. Rev., 1997, 97, 2267; (e) T. D. Lash,
Angew. Chem., Int. Ed., 2000, 39, 1763; (f) J.-i. Setsune and S. Maeda,
J. Am. Chem. Soc., 2000, 122, 12405; (g) H. Furuta, H. Maeda and A.
Osuka, Chem. Commun., 2002, 1795; (h) T. K. Chandrashekar and S.
Venkatraman, Acc. Chem. Res., 2003, 36, 676; (i) J. L. Sessler and D.
Seidel, Angew. Chem., Int. Ed., 2003, 42, 5134; (j) A. Ghosh, Angew.
Chem., Int. Ed., 2004, 43, 1918.
2 M. G. P. M. S. Neves, R. M. Martins, A. C. Tome, A. J. D. Silvestre,
A. M. S. Silva, V. Felix, M. G. B. Drew and J. A. S. Cavaleiro, Chem.
Commun., 1999, 385–386.
3 For reviews, see: (a) P. I. Dalko and L. Moisan, Angew. Chem., Int. Ed.,
2001, 40, 3726; (b) E. R. Jarbo and S. J. Miller, Tetrahedron, 2002, 58,
2481; (c) B. List, Synlett, 2001, 1675; (d) P. I. Dalko and L. Moisan,
Angew. Chem., Int. Ed., 2004, 43, 5138.
8 Cyclic voltammetry of 1 in CH2Cl2 containing tetrabutylammonium
tetrafluoroborate as an electrolyte showed two electrochemical waves at
−0.52 and −0.85 V (vs ferrocene/ferrocenium ion). See S. Mori and A.
Osuka, J. Am. Chem. Soc., 2005, 127, 8030.
9 M. L. Mihailovic´ and Z. Cekovic´, Lead Acetate in Handbook of
Reagents for Organic Synthesis, ed. D. Burke and R. L. Danheiser, Wiley,
Chichester, 1999, p. 190.
ˇ
ˇ
202 | Org. Biomol. Chem., 2006, 4, 200–202
This journal is
The Royal Society of Chemistry 2006
©