Radical-mediated synthesis of substituted quinones with organotellurium compounds

Article abstract of DOI:10.1246/cl.2000.1234

Carbon-centered radicals generated from the corresponding organotellurium compounds react with a variety of quinones under photo-thermal conditions to give the monoaddition product in good to excellent yield. The reaction can be used for the synthesis of polyprenyl quinoid natural products.

Full text of DOI:10.1246/cl.2000.1234

1234
Chemistry Letters 2000
Radical-Mediated Synthesis of Substituted Quinones with Organotellurium Compounds
Shigeru Yamago,* Masahiro Hashidume, and Jun-ichi Yoshida*
Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering
Kyoto University, Sakyo-ku, Kyoto 606-8501
(Received July 27, 2000; CL-000715)
Carbon-centered radicals generated from the corresponding
hydrone was also isolated. The reaction in the dark was slow,
but eventually gave the same product after heating at 100 °C for
7 h (57%). The effect of the solvent is marginal, and the reac-
tion in polar solvents, such as acetonitrile and ethanol, produced
virtually identical results. The insensitivity to the solvent is
consistent with the involvement of neutral radical species rather
than polar intermediates, such as radical ion species.
organotellurium compounds react with a variety of quinones
under photo–thermal conditions to give the monoaddition prod-
uct in good to excellent yield. The reaction can be used for the
synthesis of polyprenyl quinoid natural products.
A great deal of attention has been focused on the synthesis
of substituted quinones because of their unique and diverse bio-
logical functions.1 Since a variety of quinones are available,
the attachment of the carbon chain to the preexistent quinone
skeleton is one of the most attractive and straightforward syn-
thetic approaches.2 Indeed, enormous effort has been devoted
to the realization of this process by organometallic-mediated
reactions and by cycloaddition.3 While radical-mediated syn-
thesis seems to be a desirable alternative, only a few examples
have been reported so far,4 and its synthetic scope has been lim-
ited. This is probably because the most conventional reaction
system using alkyl halide and tin hydride could not be applied
due to the high reactivity of the tin hydride toward quinones.5
We have already reported that carbon-centered radicals are
reversibly generated from organotellurium compounds.6,7
Therefore, we envisaged that the radicals thus generated would
be useful for the functionalization of quinones.8 We report
herein a new synthesis of substituted quinones by the radical-
mediated reaction of organotellurium compounds with various
quinones. The reaction proceeds under photo–thermal condi-
tions, and affords a monoaddition product in good to excellent
yield (eq 1). Since organotelluriums are readily available,9 the
current method provides a new approach for the radical-mediat-
ed synthesis of substituted quinones.
The synthetic scope of the current reaction was examined,
and the results are shown in Table 1.11 A variety of substituted
quinones react with organotellurium compounds to give the
monoaddition product in good to excellent yield (entries 2–9).
The reaction conditions have been examined in detail by
using benzyl tolyl telluride (1a)10 and 1,4-benzoquinone (2A).
Thus, 1a was heated with 2A (2.0 equiv) in C6D6 at 100 °C
under irradiation with a 250-W high pressure Hg lamp in a
Pyrex tube,7a and the 2-benzyl-1,4-benzoquinone (3aA) was
formed in 57% yield. The tellurium moiety was recovered
quantitatively as ditolyl ditellulide, and about 40% of the quin-
Copyright © 2000 The Chemical Society of Japan

Chemistry Letters 2000
1235
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In the reaction of prenyl phenyl telluride, the prenyl moiety
selectively coupled with quinones at the less hindered α-posi-
tion, and the corresponding γ-coupling products could not be
detected (entries 8 and 9). Not only benzyl and prenyl radicals,
but also alkyl radicals, e.g., isopropyl radical, could be used for
the functionalization of quinones (entry 10). The regioselectivi-
ty of the reaction deserves comment; the reaction of the phenyl-
substituted quinone 2D occurred at the C-3 position, which is
conjugated with the phenyl group, to give the 2,3-disubstituted
quinone 3aD (entry 4). However, in the reaction of the t-butyl-
substituted quinone 2E, the selectivity was controlled by steric
factors, and the reaction afforded a 7:3 mixture of the 2,6- and
2,5-disubstituted quinones in a good combined yield (entry 5).
Since the reaction proceeds via radical intermediates, the free
hydroxy groups were compatible under the reaction conditions
(entry 7).
The current method can be applied to the synthesis of
polyprenyl quinonoids as shown in Scheme 1. Thus, geranyl
tolyl telluride 5 (a 73:23 mixture of the trans and cis isomers),
which was prepared by SmI2-mediated coupling of geranyl bro-
mide and ditolyl ditelluride,12 was allowed to react with 2C and
4 to obtain the corresponding geranyl-substituted quinones 6
and 7, respectively, in moderate yields. Analysis of the stereo-
chemistry revealed that a 7:3 mixture of the trans and cis iso-
mers was formed in both cases.13
3
4
5
6
R. J. Klingler, K. Mochida, and J. K. Kochi, J. Am. Chem. Soc.,
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7
8
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For our earlier work on free radical reactions in organic synthe-
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9
N. Petragnani, “Tellurium in Organic Synthesis,” Academic
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10 J. V. Comasseto, J. T. B. Ferreira, and J. A. F. Val, J.
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As the radical-mediated carbotelluration reaction to
alkynes and alkenes is well known,6b–d,8b the formation of 3
may be explained by the initial carbotelluration to quinones fol-
lowed by the elimination of aryltellurol. Aryltellurols thus
formed reduce quinones to hydroquinones with the generation
of corresponding diarylditelluride.14 However, several attempts
to observe the reaction intermediate are unsuccessful at the
present time. Experiments to ascertain the precise mechanism
are now being conducted, and further synthetic explorations are
currently under investigation.
11 Typical experimental procedures (entry 6); a solution of 1a (111
mg, 0.36 mmol) and 4 (125 mg, 0.72 mmol) in benzene (0.6
mL) in a Pyrex tube was irradiated with a 250-W high pressure
Hg lamp at 100 °C for 1 h. After removal of the solvent under
reduced pressure, the desired product was isolated by silica gel
column chromatography in 87% yield (81.7 mg). IR (KBr) 1663
1
(s), 1619 (w), 1592 (m), 1333 (s), 1293 (s), 716 (s); H NMR
(300 MHz, CDCl3) 2.25 (s, 3 H), 4.04 (s, 2 H), 7.16–7.30 (m, 5
H), 7.67–7.73 (m,2 H), 8.06–8.12 (m, 2 H); 13C NMR (75 MHz,
CDCl3) 13.22 (CH3), 32.40 (CH2), 126.42 (CH3), 126.57 (CH),
126.63 (CH), 128.74 (CH, 2 C), 128.80 (CH, 2 C), 132.19 (C),
132.28 (C), 133.61 (CH), 133.64 (CH), 138.23 (C), 144.59 (C),
145.52 (C), 184.90 (C=O), 185.64 (C=O); HRMS (EI) m/z
Found: M+, 262.0994. Calcd for C18H14O2: 262.0994. Anal.
Calcd for C18H14O2,: C, 82.42; H, 5.38%. Found: C, 82.26; H,
5.52%.
Financial support from Grant-in-Aid for Scientific
Research from the Ministry of Education, Science, Sports, and
Culture, Japan is gratefully acknowledged.
12 S. Fukuzawa, Y. Niimoto, T. Fujinami, and S. Sakai, Heteroat.
Chem., 1, 491 (1990).
13 Neryl tolyl telluride, which was prepared from neryl chloride
and ditolyl ditelluride, isomerized to 5 (ca. 7:3 mixture) before
the coupling reaction with quinones.
14 Y. Aso, T. Nishioka, M. Osuga, K. Nagakawa, K. Sasaki, T.
Otsubo, and F. Ogura, Nippon Kagaku Kaisi, 1987, 1490.
References and Notes
1
2
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Y. Naruta and K. Maruyama, in “The chemistry of the
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