Scheme 4. Divergent Chemoselectivity of
Silyloxyhexylsiletane 7
Scheme 5. Competition between a Siletane and a
Dichlorosilane
alcohols. Entries 9-13 cover the synthesis of phenols from
a range of ortho-, meta-, and para-substituted arylsiletanes,
including electron-rich (entry 10) and electron-deficient
(entry 12) species.
Note that many of the more widely used Tamao-Fleming
substrates require initial arene protonolysis (or other cleavage,
e.g., with bromine) to set up the oxidation reaction.3 This
complicating factor must be addressed if one wishes to
oxidize an aryl group.15,16 That the siletanes readily give way
to phenols with no evidence of arene protodesilylation17 is
a key feature of this chemistry. Indeed, phenol synthesis is
not a trivial process, and this siletane oxidation may
constitute an attractive option.
Entry 7 presents another key advantage of the siletane
reagents: oxidation of a carbon-silicon bond in the presence
of a silyl ether. To the best of our knowledge, such selectivity
is unique for a tetraalkylsilane oxidation with Tamao’s
standard conditions.18,19 In fact, although examples are
known,3 Tamao-Fleming protocols are rarely conducted in
the presence of silyl ethers.20 Furthermore, cleavage of the
silyl ether with mild acid leaves the siletane untouched
(Scheme 4). This selectivity would likely be impossible with
the more labile heteroatom-substituted silanes required for
the traditional Tamao oxidation.
This remarkable selectiVity inspired us to challenge the
siletanes against heteroatom-substituted silanes in terms of
reactiVity. For the sake of comparison, we conducted the
competition experiment outlined in Scheme 5. Phenethylsi-
letane 2 and (4-methylphenethyl)silyl dichloride 23 were
mixed in equimolar ratios and subjected to the reaction
conditions for 2 h.
Importantly, neither oxidation was complete within 2 h,
1
at which time we analyzed the crude mixture by H NMR
spectroscopy. The observed ratio of product alcohols 3 and
24 (1:1.2) indicates that the oxidation of the siletane is
virtually as fast as the analogous dichloride, despite being
much less prone to incidental hydrolysis.
In conclusion, we report our initial findings on the facile
oxidation of easy to handle siletanes. This chemistry, which
merges the mildness and efficiency of the standard Tamao
oxidation with the practicality of tetraalkylsilanes, capitalizes
on the unique reactivity of strained silacycles. We are
currently looking at a broader range of substrates, at
alternative means of their preparation, and at employing the
siletanes in multistep synthetic sequences. We will report
our findings in due course.
(14) The siletane ring reacts rapidly, followed by oxidation of the pendant
alkyl groups. In a reaction monitored by 1H NMR spectroscopy, 1,1-
dimethylsiletane13 was oxidized to propanediol and propanol in a 1.5:1 ratio.
Presumably double oxidation of the siletane ring generates propanediol,
whereas hydrolysis followed by oxidation produces propanol.
(15) For prior examples of arylsilane oxidations that yield phenols, see
ref 4 and Suginome, M.; Matsunaga, S.; Ito, Y. Synlett 1995, 941-942. (b)
For a recent study on the protonolysis of arylsilanes, see: Utimoto, K.;
Otake, Y.; Yoshino, H.; Kuwahara, E.; Oshima, K.; Matsubara, S. Bull.
Chem. Soc. Jpn. 2001, 74, 753-754.
(16) Woerpel’s anhydrous, basic conditions (t-BuOOH, KH, ∆) may
effect arylsilane oxidations without initial arene protonolysis, although the
isolation of phenols is not specifically mentioned: Smitrovich, J. H.;
Woerpel, K. A. J. Org. Chem. 1996, 61, 6044-6046. We thank a referee
for bringing this report to our attention.
Acknowledgment. We thank the FSU Department of
Chemistry and Biochemistry and the FSU-FYAP award
program for support of this work. Postdoctoral support from
the MDS Research Foundation for H.L. is gratefully ac-
knowledged. We thank Prof. Marie E. Krafft for helpful
guidance and discussion.
(17) In particular, no evidence was found for the formation of anisole in
entry 10 or biphenyl in entry 11 (TLC or 1H NMR of crude reaction
mixture).
Supporting Information Available: Full experimental
detail and characterization data for all compounds. This
material is available free of charge via the Internet at
(18) For the oxidation of a trialkylsilyl hydride with hydrogen peroxide
in the presence of a silyl ether under more forcing conditions, see: Tamao,
K.; Yamauchi, T.; Ito, Y. Chem. Lett. 1987, 171-174. (b) 2-Pyridylsilanes
can be oxidized with hydrogen peroxide by heating under similar conditions,
but tolerance of silyl protecting groups has not been demonstrated; see:
Itami, K.; Kamei, T.; Mitsudo, K.; Nokami, T.; Yoshida, J. J. Org. Chem.
2001, 66, 3970-3976.
(19) For examples of multistep, one-pot dearylation/oxidations, see: (a)
Fleming, I.; Ghosh, S. K. J. Chem. Soc., Chem. Commun. 1992, 1775-
1777. (b) Hunt, J. A.; Roush, W. R. J. Org. Chem. 1997, 62, 1112-1124.
OL035695Y
(20) For a detailed study in which tetraalkylsilanes ultimately proved
incompatible with a silyl ether protecting group, see: Barrett, A. G. M.;
Head, J.; Smith, M. L.; Stock, N. S.; White, A. J. P.; Williams, D. J. J.
Org. Chem. 1999, 64, 6005-6018.
Org. Lett., Vol. 5, No. 24, 2003
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