The rate enhancement effect of protodesilylation of arylsilane derivatives

Article abstract of DOI:10.1246/bcsj.74.753

Protodesilylation of the aryl moiety in (p-methoxyphenyl)dimethylsilyl and (2,6-dimethoxyphenyl)dimethylsilyl groups proceeds much faster than that in a phenyldimethylsilyl group under acidic conditions; these aryl groups can be substituted with an F atom to give organosilyl fluoride even in the presence of an acid-sensitive group such as an alkenyl group.

Full text of DOI:10.1246/bcsj.74.753

c. Jpn.
Bull. Chem. Soc. Jpn., 74, 753—754 (2001)
753
e Chemical
ciety of Ja-
n
Short Articles
e Chemical
ciety of Ja-
n
The Rate Enhancement Effect of
Protodesilylation of Arylsilane
Derivatives
K. Utimoto et al
01
Kiitiro Utimoto, Yasuyuki Otake, Hideaki Yoshino,
Eiko Kuwahara, Koichiro Oshima, and
Seijiro Matsubara*
3
4
SJA8
09-2673
0036
tober
Department of Material Chemistry, Graduate School of Engi-
neering, Kyoto University, Yoshida, Sakyo, Kyoto 606-8501
00
(Received October 19, 2000)
Scheme 2.
an aryl group, phenyl, p-methoxyphenyl, and 2,6-dimethox-
yphenyl were examined. As shown in Scheme 2, a methoxy
group on the benzene ring accelerated the substitution reac-
tion; 90% of dimethyl(octyl)phenylsilane was recovered un-
changed even after 60 min, whereas (p-methoxyphenyl)dime-
thyl(octyl)silane was converted into 5 in 95% yield after 50
min. In the case of the reaction of (2,6-dimethoxyphenyl)dim-
ethyl(octyl)silane, it was converted into 5 quantitatively within
5 min.
00
0
Protodesilylation of the aryl moiety in (p-methoxyphen-
yl)dimethylsilyl and (2,6-dimethoxyphenyl)dimethylsilyl
groups proceeds much faster than that in a phenyldimethylsi-
lyl group under acidic conditions; these aryl groups can be
substituted with an F atom to give organosilyl fluoride even in
the presence of an acid-sensitive group such as an alkenyl
group.
The booster effect on protodesilylation by a methoxy group
benefits the selective protodesilylation even in the presence of
an acid-sensitive group. As shown in Scheme 3, allenyl (6a),
allyl (9a), and 1-alkynyl (13a) groups were intact during the
reaction. Protodesilylation of the 1-alkenyl group competed
with that of the p-methoxyphenyl group (10a); use of a 2,6-
dimethoxyphenyl group improved this situation (11a, 12a).
The fluorodimethylsilyl group is recognized as a hydroxy
group precursor1 and an important intermediate for the Hiyama
coupling reaction via a hypercoordinated silicon intermediate.3
The methoxy-substituted aryldimethylsilyl group will be a use-
ful intermediate in the case of the selective preparation of 1-
alkenyl-, allenyl-, 1-alkynyl-, or allyldimethylsilyl fluoride.
Oxidation of a C–Si bond to a C–O bond developed by
Tamao, Kumada,1a and Fleming1b created the concept of using
a silyl group as a masked hydroxy group.1 A silyl group as a
masked hydroxy group has been widely used for organic trans-
formations, as it is stable over a wide range of reaction condi-
tions. Especially, Fleming used a stable dimethyl(phenyl)silyl
group which is planned to be oxidized in two steps:1b protodes-
ilylation and oxidation. The first step, protodesilylation is per-
formed with protic acid and a fluoride ion to afford a fluo-
rodimethylsilyl group which is easily oxidized with alkaline
hydrogen peroxide or perbenzoic acid. This first step, protode-
silylation, sometimes becomes a crucial reaction, as a protic
reaction condition often causes a decomposition of the sub-
strate. For example, alkenyldimethyl(phenyl)silane cannot be
converted into alkenyldimethylsilyl fluoride under protic con-
ditions, as the alkenyl group is protodesilylated faster than the
phenyl group. A booster on the phenyl group should be intro-
duced to achieve effective protodesilylation of the aryl moiety.
Considering the mechanism of protodesilylation in a dime-
thyl(phenyl)silyl group proposed by Eaborn,2 the crucial step
is the equilibrium between 1 and 2 (Scheme 1). For the effec-
tive protonation, introduction of an electron rich methoxy sub-
stituent on the phenyl group might be favored. As shown in
Scheme 2, aryldimethyl(octyl)silane was treated with boron
trifluoride–acetic acid complex (2/3) and chloroacetic acid. As
Experimental
All solvents and chemicals were used as obtained from com-
mercial suppliers. Chromatographic purification of products was
accomplished using forced-flow chromatography on Kanto Chem-
ical Co., Inc. Silica gel 60 N (spherical, neutral). 1H NMR spectra
were recorded on Varian Gemini-2000 (300 MHz) instrument and
are internally referenced to residual protic solvent signals.
General Procedure. To a solution of aryldimethylsilyl sub-
stituted compound (4, 6a–13a, 1.0 mmol) in hexane (2.0 mL),
2BF3•3AcOH (0.18 g, 0.5 mmol) and ClCH2CO2H (0.085 g, 1.0
mmol) were added at the indicated temperature in Scheme 3. The
resulting mixture was stirred for 10–30 min. The reaction was
quenched by an addition of sat. NaHCO3 aq (20 ml). The mixture
was extracted with hexane, dried over Na2SO4, and concentrated
1
in vacuo. The obtained residue was characterized by H NMR
analysis.4
(E)-4-Fluorodimethylsilyl-7-phenyl-1,2,6-heptatriene (6b).
1H NMR (CDCl3, 20 ˚C) δ 0.30 (d, J ꢀ 7.5 Hz, 3H), 0.31 (d, J ꢀ
7.5 Hz, 3H), 1.70–1.82 (m, 1H), 2.30–2.60 (m, 2H), 4.79 (dd , J ꢀ
Scheme 1.

754 Bull. Chem. Soc. Jpn., 74, No. 4 (2001)
© 2001 The Chemical Society of Japan
Scheme 3.
6.3, 2.7 Hz, 2H), 5.15 (dt, J ꢀ 8.4, 6.3 Hz, 1H), 6.28 (dt, 15.9, 6.9
Hz, 1H), 6.45 (d, J ꢀ 15.9 Hz, 1H), 7.16–7.40 (m, 5H).
4-Fluorodimethylsilyl-1-pentadecen-6-yne (7b). 1H NMR
(CDCl3, 20 ˚C) δ 0.27 (d, J ꢀ 7.8 Hz, 3H), 0.28 (d, J ꢀ 7.8 Hz,
3H), 0,88 (t, J ꢀ 6.6 Hz, 3H), 1.02–1.13 (m, 1H), 1.20–1.38 (m,
12H), 2.06–2.40 (m, 6H), 5.00–5.11 (m, 2H), 5.70–5.84 (m, 1H).
4-Fluorodimethylsilyl-4-phenyl-1-butene (8b). 1H NMR
(CDCl3, 20 ˚C) δ 0.15 (d, J ꢀ 7.5 Hz, 3H), 0.16 (d, J ꢀ 7.5 Hz,
3H), 2.26–2.34 (m, 1H), 2.58–2.65 (m, 2H), 4.88–5.04 (m, 2H),
5.66–5.80 (m, 1H), 7.05–7.32 (m, 5H).
(m, 10H), 1.48–1.60 (m, 2H), 2.24 (td, J ꢀ 7.2, 2.4 Hz, 1H).
This work was supported by Grants-in-Aid for Scientific
Research on Priority Areas (No. 10208208), No. 12305058,
No. 12450363, and No. 12875176 from The Ministry of Edu-
cation, Science, Sports, and Culture.
(E)-4-Fluorodimethylsilyl-6-phenyl-1,5-hexadiene (9b). 1H
NMR (CDCl3, 20 ˚C) δ 0.29 (d, J ꢀ 7.5 Hz, 3H), 0.30 (d, J ꢀ 7.5
Hz, 3H), 1.98–2.10 (m, 1H), 2.30–2.52 (m, 2H), 5.00–5.18 (m,
2H), 5.81–5.94 (m, 1H), 6.13 (dd, J ꢀ 15.9, 9.3 Hz, 1H), 6.36 (d,
J ꢀ 15.9 Hz, 1H), 7.18–7.40 (m, 5H).
References
1
a) K. Tamao, T. Kakui, M. Akita, T. Iwahara, R. Kanatani,
J. Yosida, and M. Kumada, Tetrahedron, 32, 983 (1983); K.
Tamao, N. Ishida, N. Tanaka, and M. Kumada, Organometallics,
2, 1694 (1983). b) I. Fleming, R. Henning, and H. Plaut, J. Chem.
Soc., Chem. Commun., 1984, 29; I. Fleming, R. Henning, D. C.
Parker, H. E. Plaut, and P. E. J. Sanderson, J. Chem. Soc., Perkin
Trans 1, 1995. 317. c) G. R. Jones and Y. Landais, Tetrahedron,
52, 7599 (1996).
(Z)-1-Fluorodimethylsilyl-1-decene (10b). 1HNMR(CDCl3,
20 ˚C) δ 0.31 (d, J ꢀ 7.5 Hz, 6H), 0.94 (t, J ꢀ 6.6 Hz, 3H), 1.20–
1.45 (m, 12H), 2.10–2.22 (t, J ꢀ 7.2 Hz, 2H), 5.44 (dd, J ꢀ 14.1,
7.8 Hz, 1H), 6.43 (dt, J ꢀ 14.1, 7.5 Hz, 1H).
(E)-1-Fluorodimethylsilyl-1-decene (12b). 1HNMR(CDCl3,
20 ˚C) δ 0.27 (d, J ꢀ 7.5 Hz, 6H), 0.84 (t, J ꢀ 6.6 Hz, 3H), 1.20–
1.42 (m, 8H), 2.10–2.17 (t, J ꢀ 7.2 Hz, 2H), 5.61–5.69 (m, 1H),
6.27 (ddt, J ꢀ 18.9, 6.3, 1.2 Hz, 1H).
2
C. Eaborn, J. Organomet. Chem., 100, 43 (1975); B. Ben-
netau and J. Dunoguès, Synlett, 1993, 171.
3
4
Y. Hatanaka and T. Hiyama, Synlett, 1991, 845.
The starting substrates were prepared according to the re-
1-Fluorodimethylsilyl-1-decyne (13b). 1H NMR (CDCl3, 20
˚C) δ 0.34 (d, J ꢀ 6.9 Hz, 6H), 0.95 (t, J ꢀ 6.6 Hz, 3H), 1.22–1.42
ported procedure: S. Matsubara, Y. Otake, T. Morikawa, and K.
Utimoto, Synlett, 1998, 1315.