Novel desilylation of alpha-dimethylsilyl esters by electrochemically generated superoxide ion

Article abstract of DOI:10.1016/S0040-4039(03)01396-0

Electrochemical reduction of oxygen in the presence of an alpha-dimethylsilyl ester results in removal of the dimethylsilyl group. The reaction presumably proceeds by a mechanism involving electrochemically-generated superoxide ion.

Full text of DOI:10.1016/S0040-4039(03)01396-0

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 5859–5861
Novel desilylation of alpha-dimethylsilyl esters by
electrochemically generated superoxide ion
Christos Kaimakliotis and Albert J. Fry*
Chemistry Department, Wesleyan University, Middletown, CT 06459, USA
Received 15 May 2003; revised 2 June 2003; accepted 4 June 2003
Abstract—Electrochemical reduction of oxygen in the presence of an alpha-dimethylsilyl ester results in removal of the
dimethylsilyl group. The reaction presumably proceeds by a mechanism involving electrochemically-generated superoxide ion.
© 2003 Elsevier Ltd. All rights reserved.
As part of a series of investigations into the electro-
chemical behavior of organosilanes,1–3 we have synthe-
sized a series of alpha-dimethylsilyl esters (2) (Eq. (1)).4
Although our initial interest was in other facets of the
chemistry of these substance, it was of interest to
examine their cathodic behavior. As we have earlier
pointed out,5,6 the ease of electrochemical removal of
the alpha-substituent of alpha-X-substituted carbonyl
compounds, including esters, (Eq. (2)) is directly related
to the leaving group ability of X−, i.e. the acidity of
HX.
more, esters themselves are reduced only at very nega-
tive potentials6 and the electron-supplying silyl
substituent should reinforce the resistance of the car-
bonyl group of 2 to reduction by cathodic electron
transfer. Indeed, cyclic voltammograms of the alpha-
silyl esters 2 in nitrogen-purged acetonitrile containing
0.1 M Bu4N+BF4 exhibit no reduction current at any
−
potential before solvent discharge (Fig. 1, curve A). A
cyclic voltammogram of acetonitrile saturated with
oxygen, on the other hand, exhibits the well-known9
reversible wave due to reduction of oxygen to superox-
(1)
ide (curve B) at −1.3 V versus the Ag/AgNO3 reference
electrode.10 Addition of any one of the silyl esters 2 to
the solution of voltammogram B results in both an
increase in the height of the reduction wave and elimi-
nation of the anodic wave for reoxidation of superoxide
(curve C). We interpret this to be due to an ECE
process involving reaction of superoxide with 2. The
nature of the current-consuming process was estab-
lished by preparative controlled-potential or constant
electrolysis in acetonitrile at the oxygen reduction
potential while bubbling air through the solution. In
every case, conversion to the parent ester (1) took place
cleanly and in 95–100% yield. Although superoxide can
(2)
In view of the highly basic nature of silyl anions,7,8 it
should not be possible to desilylate species 2 by direct
cathodic cleavage of the carbonꢀsilicon bond. Further-
* Corresponding author. Tel.: 860-685-2622; fax: 860-685-2211;
e-mail: afry@wesleyan.edu
0040-4039/$ - see front matter © 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/S0040-4039(03)01396-0

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C. Kaimakliotis, A. J. Fry / Tetrahedron Letters 44 (2003) 5859–5861
ment was only 2 mM. The concentration of silane was
higher (62 mM) in the preparative experiment, but
oxygen was continually bubbled through the solution to
maintain a saturated solution throughout.
A reviewer raised the question whether alpha-trialkylsi-
lyl esters would also undergo this transformation.
Because there is no good synthetic route to such sub-
stances, we are unable to answer this question with
certainty. We believe, however, that they would, since
we do not believe that SiꢀH bond breakage is involved
in this transformation. GC–Mass spectrometry showed
that in addition to 1, a substance whose mass and
fragmentation pattern correspond to that of bis-
[dimethylsilyl] ether (7) was also produced, but it was
too volatile to isolate from the mixture.
Figure 1. Curve A: cyclic voltammogram of 2 mM silyl ester
2a in acetonitrile/0.1
M
tetrabutylammonium tetra-
fluoroborate at a glassy carbon electrode versus Ag/0.1 M
AgNO3 (scan rate 100 mV/s). Curve B: cyclic voltammogram
of oxygen-saturated acetonitrile under the conditions of curve
A. Curve C: same as curve B with 2 mM 2a added.
This oxygen-mediated desilylation process has no close
precedent. However, Fattakhova, et al., have shown
that cathodic reduction of oxygen in the presence of a
dialkyldichlorosilane or diaryldichlorosilane leads to
the production of cyclosiloxanes (10), apparently via
intermediate silanones (9) (Eq. (3)).15 The reaction pro-
ducing 9 could also involve a pentacovalent intermedi-
ate formed by addition of superoxide ion to the central
silicon atom (Eq. (4)).
cleave esters,11,12 this did not take place under our
reaction conditions (acetonitrile, room tempera-
ture).13,14
We suggest the mechanism of Scheme 1 for the conver-
sion of 2 into 1. In aprotic media oxygen is reduced to
(3)
−
9
’
superoxide ion (O2 ). Addition of superoxide ion to
the silicon atom of 2 would afford pentacovalent inter-
mediate 3. Decomposition of 3 into a silylhydroperoxy
species (4) and enolate (5), followed by protonation of
5, would lead to 2. Reduction of 4 to the SiMe2HOO−
anion (6) and attack of 6 upon a second molecule of 1
would result in a second desilylation. A possible variant
on this mechanism would be fragmentation of 3 into 6
and a carbonyl-substituted radical (8), followed by
reduction of 8 to 5.
(4)
In an apparently related process, Park, et al., reported
that reaction of trialkylsilanes with potassium superox-
ide in acetonitrile affords hexaalkyldisiloxanes (11)
while dialkylsilanes were found to afford primarily
tetracyclosiloxanes (12), presumably via silanones (9)
(Scheme 2).16
Sufficient oxygen is available in the solution to sustain
the stoichiometry of Scheme 1. The solubility of oxygen
in acetonitrile at room temperature is 8.1 mM,9 whereas
the concentration of silane in the voltammetric experi-
Scheme 1.

C. Kaimakliotis, A. J. Fry / Tetrahedron Letters 44 (2003) 5859–5861
5861
chemistry For Chemists, 2nd ed.; Wiley: New York, 1995;
pp. 358–371.
10. This reference electrode is + 0.35 V versus SCE.
11. Kornblum, N.; Singaram, S. J. Org. Chem. 1979, 44,
4727–4729.
12. San Filippo, J., Jr.; Romano, L. J.; Chern, C.-I.; Valen-
tine, J. S. J. Org. Chem. 1976, 41, 586–588.
13. Representative preparative electrolysis: Electrolyses were
carried out in a divided electrolysis cell of standard
design14 using Union Carbide X2014 WCA grade carbon
cloth anode and cathode of 4 cm2 area each. The anode
compartment contained 75 mL of 0.1 M 0.1 M Bu4N+
Scheme 2.
Acknowledgements
−
BF4 in acetonitrile; the cathode compartment contained
50 mL of 0.1 M Bu4N+BF4 in DMF above a methylcel-
−
Financial support by the National Science Foundation
under grant cCHE-01-00727 is gratefully acknowl-
edged. C.K. received a summer research grant from an
award to Wesleyan University under the Undergradu-
ate Biological Sciences Education Program of the
Howard Hughes Medical Institute.
lulose gel14 to prevent mass transfer through the coarse
frit dividing the two compartments. One gram (4.63
mmol) of ester 1c was added to the cathode compartment
and electrolysis was commenced at −1.4 V versus Ag/0.1
M AgNO3 (using a Bioanalytical Systems PWR-3 poten-
tiostat) for controlled-potential electrolyses or at 100 mA
(using a Kepco ATE 150-0.7M power supply) for con-
stant current electrolyses. When the calculated current
(one Faraday per mole of silyl ester) had been passed, the
reaction mixture was evaporated, the residue extracted
with hexane, and the hexane evaporated to afford 0. 72
gm (4.6 mmol, 99% yield) of colorless liquid. Its mass
spectrum, GC retention time, and 300 MHz 1H NMR
spectrum were identical to those of an authentic sample
of ethyl heptanoate (1c) prepared by esterification of
heptanoic acid. Esters 1a, 1b, and 1d were produced in
95, 97, and 100% yield, respectively, using the same
procedure. GC-mass spectrometry also demonstrated the
formation of a substance of mass 134, believed to be
bis-[dimethylsilyl] ether (7), but this material was lost
during evaporation of the electrolysis mixture.
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