Highly efficient and chemoselective reductive bis-silylation of quinones by silyltellurides

Article abstract of DOI:10.1021/ol006581e

(matrix presented) Silyltellurides serve as new silicon-based chemoselective reducing agents and reduce quinones to the corresponding bis-silylated hydroquinones. The reaction proceeds under ambient thermal conditions without the need of any additional promoters or catalysts and gives the products in excellent yields. Several control experiments suggest that the reaction is initiated by a single electron transfer from silyltellurides to quinones.

Full text of DOI:10.1021/ol006581e

ORGANIC
LETTERS
2000
Vol. 2, No. 23
3671-3673
Highly Efficient and Chemoselective
Reductive Bis-silylation of Quinones by
Silyltellurides
Shigeru Yamago,* Hiroshi Miyazoe, Kazunori Iida, and Jun-ichi Yoshida*
Department of Synthetic Chemistry and Biological Chemistry, Graduate School of
Engineering, Kyoto UniVersity, Kyoto 606-8501, Japan
yamago@sbchem.kyoto-u.ac.jp
Received September 12, 2000
ABSTRACT
Silyltellurides serve as new silicon-based chemoselective reducing agents and reduce quinones to the corresponding bis-silylated hydroquinones.
The reaction proceeds under ambient thermal conditions without the need of any additional promoters or catalysts and gives the products in
excellent yields. Several control experiments suggest that the reaction is initiated by a single electron transfer from silyltellurides to quinones.
Silicon-based reducing agents have attracted a great deal of
attention recently because of their low toxicity compared to
the corresponding tin-based reagents.1 However, since the
currently used reagents are limited primarily to silyl hydride
reagents, development of new silicon-based reducing agents
with improved reactivity and selectivity is consequently
highly desirable. We have recently shown that the thermal
reaction of silyl tellurides with carbonyl compounds gener-
ates R-siloxy carbon radicals, which are useful intermediates
for C-C bond formations.2 While investigating the reactivity
of silyl tellurides with carbonyl compounds, we discovered
a new type of reductive bis-silylation of quinones.3,4 We
report here a highly efficient and chemoselective thermal
reaction of silyl tellurides with quinones (Figure 1) that is
(1) (a) Weber, W. P. In Silicon Reagents for Organic Synthesis; Springer-
Verlag: Berlin, 1983; pp 273-338. (b) Ojima, I. In The Chemistry of
Organic Silicon Compounds, Part 2; Patai, S., Rappoport, Z., Eds.; John
Wiley & Sons: New York, 1989; pp 1479-1526. (c) Brook, M. A. In
Silicon in Organic, Organometallic, and Polymer Chemistry; John Wiley
& Sons: New York, 2000, pp 171-188 and 401-422. (d) Chatgilialoglu,
C. Acc. Chem. Res. 1992, 25, 188. (e) Chatgilialoglu, C. Chem. ReV. 1995,
95, 1229.
Figure 1. Reductive bis-silylation of quinones.
the first example of the use of the silyltellurides as silicon-
based reducing agents.
The feasibility of the reaction was initially examined with
1,4-benzoquinone (1a, R ) H) and trimethylsilylphenyl-
(2) Miyazoe, H.; Yamago, S.; Yoshida, J. Angew. Chem., Int. Ed. 2000,
39, 3669.
(3) (a) Wiberg, N.; Weith, M. Chem. Ber. 1971, 104, 3191. (b)
Matsumoto, H.; Koike, S.; Matsubara, I.; Nakano, T.; Nagai, Y. Chem.
Lett. 1982, 533. (c) Lecea, B.; Aizpurua, J. M.; Palomo, C. Tetrahedron
1985, 41, 4657. (d) Bakola-Christianopoulou, M. N. J. Organomet. Chem.
1986, 308, C24. (e) Komoriya, H.; Kako, M.; Nakadaira, Y.; Mochida, K.;
Tonogaki-Kubota, M.; Kobayashi, T. Chem. Lett. 1994, 1439. (f) Yamashita,
H.; Reddy, N. P.; Tanaka, M. Organometallics 1997, 16, 5223.
(4) Stepwise bis-silylation by the reduction of quinones followed by
silylation has been also reported. (a) Rasmussen, J. K.; Krepski, L. R.;
Heilmann, S. M.; Smith, H. K., II; Tumey, M. L. Synthesis 1983, 457. (b)
Boudjouk, P.; So, J. H. Synth. Commun. 1986, 16, 775.
10.1021/ol006581e CCC: $19.00 © 2000 American Chemical Society
Published on Web 10/26/2000

telluride (2).5 Thus, when 1a and 2 (2.2 equiv) were mixed
in THF at room temperature, the red color corresponding to
the diphenyl ditelluride appeared immediately, and the bis-
silylated 1,4-dihydroquinone (3a, R ) H) and diphenyl
ditelluride (4) were isolated in quantitative yields (Table 1,
To further clarify the synthetic utility of the current
reaction, we next examined the compatibility of functional
groups. The reaction of duroquinone (1.0 equiv) and 2 (2.0
equiv) was conducted in the presence of 2 equiv of the
compounds listed in Figure 2. We found that the reaction
Table 1. Reductive Bis-Silylation of Quinones with
Silyltelluridesa
Ered
(V vs SCE)b
yield
(%)
entry
1
quinone
2,3-dichloro-4,5-dicyano-
1,4-benzoquinone
2,3,5,6-tetrachloro-
1,4-benzoquinone
0.49
0.00
66c
Figure 2. Functional groups that are compatible with the reductive
bis-silylation.
2
96
3
4
5
6
2,6-dichloro-1,4-benzoquinone
1,2-naphthoquinone
1,4-benzoquinone
2,3-dimethoxy-5-methyl-
1,4-benzoquinone
1,4-naphthoquinone
duroquinone
anthraquinone
-0.20
-0.39
-0.54
-0.67
95
82
100
100
occurred selectively with the quinone and that the additives
were recovered quantitatively in all cases (>90%), while the
reaction of 2 with duroquinone took place in high yield
(>90% yield). The results indicate that the reaction is
compatible with various functional groups, e.g., alkyl and
aryl halides, aldehydes, R,â-unsaturated carbonyl compounds,
and nitro groups. The selective reduction of quinones in the
presence of an alkyl iodide is worth noting, because the
reactivity of the free triethylsilyl radical toward duroquinone
and primary alkyl iodides is known to be almost identical (k
) 109 M-1 s-1).7
7
8
9
-0.72
-0.86
-0.96
98
91
97
a Typical procedures: a THF solution of quinone (ca. 0.5 M) and 2 (2.2
equiv) was stirred for 0.5-1 h at room temperature. b Reduction potential
was measured in a 0.1 M Bu4NClO4 solution of CH3CN by cyclic
voltammetry using glassy carbon as working electrodes. c Isolated as the
corresponding dihydroquinone because the silylated product was hydro-
lytically unstable.
Since we have already reported that the reaction of 2 with
carbonyl compounds generates the R-siloxy carbon radicals,
we propose the mechanism as depicted in Scheme 1. An
entry 5).6 When 1 equiv of 2 was used, half of the quinone
was converted to 3a, indicating that the second silylation
reaction was faster than the first. While the reaction
proceeded in various solvents, the rate of the reaction was
faster in polar solvents than in nonpolar ones. For example,
the reaction in CD3CN was completed in about 0.5 h, while
that in C6D6 required 2 h for completion.
Scheme 1. Possible Reaction Mechanism
The scope of the current reaction was examined, and the
results are summarized in Table 1. Several kinds of quinones
with various substitution patterns were found to react with
2 to afford 3 in good to excellent yields. In all cases, mono-,
di-, tri-, and tetrasubstituted quinones gave the desired
adducts. It is worth noting that not only 1,4-quinones but
also 1,2-quinones were reduced to give the corresponding
bis-silylated 1,2-hydroquinones (entry 4). The reaction rate
was found to be sensitive to the electronic character of the
substituents, and the quinones of higher reduction potential
tend to react faster than those bearing electron-releasing
substituents. Indeed, thin-layer chromatography analysis
indicated that the reaction proceeded in about 10 min with
quinones whose reduction potential is higher than -0.4 V
vs SCE (entries 1-4). The reactivity of the quinones whose
reduction potential is lower than -0.4 V was slightly less,
but in all cases the reaction was completed within 0.5 h
(entries 5-9).
initial electron transfer (ET) from 2 to the quinone generates
a radical ion pair, and the subsequent O-Si bond formation
in the ion pair generates the phenoxy radical 5.8 The radical
5 further reacts with 2 to give 3 and 4.9 An alternative
possibility, which includes the generation of trimethylsilyl
radical from 2 and the subsequent reaction of the radical
(7) (a) Chatgilialoglu, C.; Ingold, K. U.; Scaiano, J. C. J. Am. Chem.
Soc. 1982, 104, 5119. (b) Chatgilialoglu, C.; Ingold, K. U.; Scaiano, J. C.
J. Am. Chem. Soc. 1982, 104, 5123.
(8) When s a 1:1 mixture of 2 and 2,6-di-tert-butyl-1,4-benzoquinone
was dissolved in EtCN at 20-30 °C, a characteristic ESR signal corre-
sponding to the phenoxy radical species was observed at g ) 2.0044 with
aH ) 0.092 mT. For such radical species, see: (a) Chen, K. S.; Foster, T.;
Wan, J. K. S. J. Chem. Soc., Perkin Trans. 2 1979, 1288. (b) Alberti, A.;
Chatgilialoglu, C. Tetrahedron 1990, 46, 3963.
(5) (a) Drake, J. E.; Hemmings, R. T. Inorg. Chem. 1980, 19, 1879. (b)
Sasaki, K.; Aso, Y.; Otsubo, T.; Ogura, F. Tetrahedron Lett. 1985, 26, 453.
(6) Typical Reaction Procedure. A mixture of 1a (54.2 mg, 0.50 mmol)
and 2 (302.4 mg, 1.09 mmol) in THF (1.0 mL) was stirred at room
temperature for 0.5 h. Removal of the solvent followed by purification on
silica gel afforded 3a as a white solid (127.2 mg, 0.50 mmol, 100%).
(9) See ref 2 for the analogous reaction of TEMPO free radical with 2.
3672
Org. Lett., Vol. 2, No. 23, 2000

with the quinone to generate 5, was less likely for the
following reasons. First, since the reactivity of the trimeth-
ylsilyl radical toward duroquinone and primary alkyl iodidies
is almost the same, the observed chemoselectivity ruled out
the possibility of generation of the free trimethylsilyl radical.
Indeed, the calculated homolytic bond dissociation energy
of 2, 232 kJ/mol, is too high for the Si-Te bond to undergo
homolysis under the reaction conditions.10 Second, the
observed solvent effect is consistent with the involvement
of the polar intermediate, e.g., radical ion pairs. Third, the
reactivity of the quinones correlates well with their reduction
potential; the quinones of higher reduction potential react
faster than those with lower reduction potential. The oxida-
tion potential of 2 is 0.94 V vs SCE, which is higher than
the reduction potential of the quinones. However, the
preassociation of 2 and the quinones may considerably
enhance the ET processes.11 Obviously, further studies are
needed to clarify the detail reaction mechanism.
In conclusion, the silyltellurides serve as highly reactive
and chemoselective silicon-based reducing agents for the
reduction of quinones and provide easy access to the bis-
silylated hydroquinones. Since protected hydroquinones are
important intermediates for the synthesis of complex quinoid
compounds,12,13 the current method provides a new and
efficient approach for the synthesis of this class of com-
pounds.
(10) Calculation was carried out with Gaussian 98 programs. Geometry
optimizations were performed with the hybrid B3LYP density functional
with a Hey-Wadt effective core potential (ECP) and outermost valence
electron basis set for tellurium and 6-31G(d) basis set for the rest. Gaussian
98, ReVision A.7; Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria,
G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J.
A., Jr.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels,
A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.;
Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.;
Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick,
D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.;
Ortiz, J. V.; Baboul, A. G.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz,
P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-
Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe,
M.; Gill, P. M. W.; Johnson, B.; W. Chen, W.; Wong, M. W.; Andres, J.
L.; Gonzalez, C.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian,
Inc.: Pittsburgh, PA, 1998.
Acknowledgment. This work was partly supported by a
Grant-in-Aid for Scientific Research from the Ministry of
Education, Science, Sports, and Culture, Japan. We thank
Professor S. Okazaki and Professor M. Oyama, Department
of Material Chemistry of Kyoto University, for the ESR
measurement and valuable discussions and Dr. C. Chatgil-
ialoglu, I.Co.C.E.A. of Consiglio Nazionale delle Ricerche,
Bologna, for valuable discussions and suggestions.
OL006581E
(11) For an excellent review of such processes, see: Fukuzumi, S. Bull.
Chem. Soc. Jpn. 1997, 70, 1.
(12) Greeves, N. In ComprehensiVe Organic Synthesis; Trost, B. M.,
Fleming, I., Eds.; Pergamon Press: Oxford, 1991; Vol. 8, pp 1-24.
(13) Naruta, Y.; Maruyama, K. In The Chemistry of the Quinonoid
Compounds; Patai, S., Rappoport, Z., Eds.; Wiley: New York, 1988; Vol.
2, Chapter 8.
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