Efficient and rapid C-Si bond cleavage in supercritical water

Article abstract of DOI:10.1021/ja034227g

Arylsilanes, alkenylsilanes, allylic silanes, and alkylsilanes were found to undergo extremely facile and rapid C-Si bond cleavage in supercritical water. The rapid C-Si bond cleavage occurred even with robust unactivated tetraalkylsilanes. The control experiments revealed the dramatic difference between supercritical and subcritical conditions and that between supercritical water and supercritical methanol, attesting to a unique reactivity of supercritical water in C-Si bond cleavage. Copyright

Full text of DOI:10.1021/ja034227g

Published on Web 04/24/2003
Efficient and Rapid C-Si Bond Cleavage in Supercritical Water
Kenichiro Itami,*,† Koji Terakawa,† Jun-ichi Yoshida,*,† and Okitsugu Kajimoto*,‡
Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto UniVersity,
Yoshida, Kyoto 606-8501, Japan, and Department of Chemistry, Graduate School of Science, Kyoto UniVersity,
Kitashirakawa-Oiwakecho, Kyoto 606-8502, Japan
Received January 17, 2003; E-mail: yoshida@sbchem.kyoto-u.ac.jp
Supercritical water (scH2O), the critical temperature and pressure
of which are 374 °C and 22.1 MPa, respectively, has been attracting
increasing attention because of its unique physical and chemical
properties that are very different from those of ambient water.1 For
example, because the dielectric constant is much lower, many
organic compounds are miscible in scH2O. Furthermore, the ion
dissociation constant of water initially increases as the temperature
rises and drops dramatically at the critical point under the constant-
pressure condition. Although scH2O has tended to be used as a
medium for the degradation of toxic materials, waste synthetic
polymers, and biomass, chemists have recently set out to use scH2O
as a medium for chemical synthesis.2
Organosilicon compounds have been extensively utilized in
modern organic synthesis.3 However, their use in scH2O has not
been reported to date. Taking the high oxophilicity of silicon into
account, water would be expected not only as a new reaction
medium but also as a Lewis basic donor (activator) for silicon under
supercritical conditions. We report herein an extremely facile C-Si
bond cleavage of organosilicon compounds in scH2O (eq 1), which
serves as a starting point toward the development of silicon-based
organic synthesis in scH2O.
°C, 5 min), a mixture of dodecenes was obtained in 67% yield (eq
3). The initial desilylation product (1-dodecene) seemed to isomerize
into thermodynamically more stable inner alkenes under the reaction
conditions.7 Indeed, the ratio of 1-dodecene/other dodecenes
increased (84/16) as the reaction time was shortened to 1 min.
Similarly, allylic silane 4 also underwent rapid desilylation in
scH2O (eq 4). However, it was surprising to observe that 47% of
1-dodecene remained even after a prolonged reaction time (3 h).
Shortening the reaction time resulted in the recovery of starting
allylic silane 4. Quite interestingly, the competitive reaction of 3
and 4 indicated the faster desilylation of 3 in scH2O, although the
opposite reactivity order is generally observed in the reaction with
electrophiles.8 In fact, we found that 4 was more reactive than 3
under typical desilylation conditions (HI in C6D6 at room temper-
ature).9 Therefore, the preliminary results demonstrated herein imply
a unique desilylation in scH2O.
Electrophilic cleavage of arylsilanes has been extensively utilized
in organic synthesis because a coming electrophile always occupies
the position where the silyl group has been attached (ipso
substitution).4 Thus, we began our research by subjecting arylsilanes
to scH2O (eq 2).5 When 1 and 2 were employed, protodesilylation
occurred within 30 min at 390 °C to give benzene derivatives in
high yields. As expected, the use of D2O resulted in the introduction
of deuterium (>95% D) at the position where the silyl group was
previously bonded.4
We next examined the reaction of alkylsilanes, which are known
to be much less reactive toward C-Si bond cleavage than the
organosilanes mentioned before (Table 1).3 We found that the C-Si
bond cleavage of alkylsilanes having a heteroatom on silicon took
place efficiently.10 For example, silanol 5 underwent efficient C-Si
bond cleavage at 390 °C, giving dodecane in 68% yield (run 1).
Raising the temperature had little effect on the product yield (run
2). However, the desilylation was extremely sluggish at 350 °C
(subcritical conditions), and the corresponding siloxane 8 was
formed in 30% yield (run 3). Interestingly, it was found that the
addition of either HCl or NaOH had a beneficial effect on the C-Si
bond cleavage (runs 4 and 5). When 5 was subjected into
supercritical methanol, no C-Si bond cleavage occurred, and only
the corresponding methoxysilane was detected (run 6). The reaction
using chlorosilane 6 gave dodecane in 89% yield (run 7). The higher
yield as compared to that with 5 may be due to the effect of HCl
generated in situ. Although the desilylation was less efficient when
ethoxysilane 7, siloxane 8, and disilane 9 were used (runs 8, 10,
and 12), the addition of HCl gave rise to higher yields of the
desilylation products (runs 9, 11, and 13).
Encouraged by these promising results, we undertook the
investigation of C-Si bond cleavage of the other class of nucleo-
philic organosilicon compounds, alkenylsilanes and allylic silanes,
in scH2O.6 When alkenylsilane 3 was subjected into scH2O (390
The successful desilylation of functionalized alkylsilanes finally
led us to the ultimate challenge, the C-Si bond cleavage of
unactivated tetraalkylsilanes.11 Beyond our imagination, this trans-
† Department of Synthetic Chemistry and Biological Chemistry, Graduate School
of Engineering, Kyoto University.
‡ Department of Chemistry, Graduate School of Science, Kyoto University.
9
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J. AM. CHEM. SOC. 2003, 125, 6058-6059
10.1021/ja034227g CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
Table 1. C-Si Bond Cleavage of Functionalized Alkylsilanes in
The realization of C-Si bond cleavage of robust unactivated
tetraalkylsilane in scH2O is interesting because it does not require
strong Lewis acids such as AlCl3 as promoters.11 Moreover, the
control experiments revealed the dramatic difference between
supercritical and subcritical conditions (run 1 vs run 3) and that
between scH2O and scMeOH (run 1 vs run 5), attesting to a unique
reactivity of scH2O in C-Si bond cleavage.
In summary, aryl-, alkenyl-, allyl-, and alkylsilanes were found
to undergo extremely facile C-Si bond cleavage in scH2O.
Particularly, the finding that robust tetraalkylsilanes could also
undergo rapid C-Si bond cleavage is extremely intriguing and
would help to alter the chemist’s impression that the trialkylsilyl
group may be a dead-end for manipulating alkylsilanes.11 With this
desilylation in hand, the trialkylsilyl group can now be regarded
as a removable group that is expected to exert steric and electronic
controls over numerous reactions. The effort to develop silicon-
based organic synthesis in scH2O is now in progress.
scH2O
run
X
temp (°C)
additivea
yield (%)b
1
2
3
4
5
6
7
8
9
10
11
12
13
OH (5)
OH (5)
OH (5)
OH (5)
OH (5)
OH (5)
Cl (6)
OEt (7)
OEt (7)
OSiMe2C12H25 (8)
OSiMe2C12H25 (8)
SiMe3 (9)
390
410
350
390
390
390
390
390
390
390
390
390
390
68
66
11c
88
85
0d
89
23
72
46
88
56
91
HCl
NaOH
HCl
HCl
HCl
SiMe3 (9)
Acknowledgment. This work was supported by CREST of the
Japan Science and Technology Corporation. We thank Dr. Hiroyuki
Oka for technical assistance and valuable suggestions.
a 1.0 equivalent to alkylsilanes. b Determined by GC analysis using
tetradecane as an internal standard. c Siloxane 8 was formed in 30% yield.
d Reaction was performed in supercritical methanol. C12H25SiMe2OMe was
formed in 53% yield.
Supporting Information Available: Experimental procedures and
analytical and spectroscopic data of compounds (PDF). This material
is available free of charge via the Internet at http://pubs.acs.org.
Table 2. C-Si Bond Cleavage of Tetraalkylsilanes in scH2O
References
(1) (a) Kajimoto, O. Chem. ReV. 1999, 99, 355. (b) Savage, P. E. Chem. ReV.
1999, 99, 603. (c) Bro¨ll, D.; Kaul, C.; Kra¨mer, A.; Krammer, P.; Richter,
T.; Jung, M.; Vogel, H.; Zehner, P. Angew. Chem., Int. Ed. 1999, 38,
2998. (d) Siskin, M.; Katrinzky, A. R. Chem. ReV. 2001, 101, 825.
(2) Selected examples. Diels-Alder reaction: (a) Korzenski, M. B.; Kolis,
J. W. Tetrahedron Lett. 1997, 38, 5611. (b) Harano, Y.; Sato, H.; Hirata,
F. J. Am. Chem. Soc. 2000, 122, 2289. Friedel-Crafts alkylation: (c)
Chandler, K.; Deng, F.; Dillow, A. K.; Liotta, C. L.; Eckert, C. A. Ind.
Eng. Chem. Res. 1997, 36, 5175. Beckmann and pinacol rearrangements:
(d) Ikushima, Y.; Hatakeda, K.; Sato, O.; Yokoyama, T.; Arai, M. J. Am.
Chem. Soc. 2000, 122, 1908. Ester hydrolysis: (e) Oka, H.; Yamago, S.;
Yoshida, J.; Kajimoto, O. Angew. Chem., Int. Ed. 2002, 41, 623. Pd-
catalyzed Mizoroki-Heck reaction: (f) Reardon, P.; Metts, S.; Crittendon,
C.; Daugherity, P.; Parsons, E. J. Organometallics 1995, 14, 3810.
run
silane
media
temp (°C)
time (h)
additivea
yield (%)b
1
2
3
4
5
6
7
8
9
10
10
10
10
10
11
11
12
12
13
13
14
H2O
H2O
H2O
H2O
MeOH
H2O
H2O
H2O
H2O
H2O
H2O
H2O
390
390
350
110
390
390
390
390
390
390
390
390
0.5
0.5
0.5
14
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
72 (100)
88 (100)
11 (35)c
0 (4)
HCl
HCl
1 (20)
0.4 (8)
74 (100)
4 (11)
79 (100)
2 (9)
HCl
HCl
10
11
12
HCl
HCl
80 (100)
368
(3) Brook, M. A. Silicon in Organic, Organometallic, and Polymer Chemistry;
John Wiley & Sons: New York, 2000.
(4) Eaborn, C. Pure Appl. Chem. 1969, 19, 375.
a 1.0 equivalent to alkylsilanes. b Determined by GC analysis using
tetradecane as an internal standard. The figure in parentheses is the
conversion (%) of tetraalkylsilane. c Silanol 5 (9%) was detected.
(5) To exclude any positive or negative effect of the stainless steel reactor,
most of the reactions in this study were performed in a sealed quartz tube
placed in a stainless steel reactor (SUS316).
(6) Fleming, I.; Dunogue`s, J.; Smithers, R. Org. React. 1989, 37, 57.
(7) When 1-dodecene was subjected in scH2O (390 °C), complete isomer-
formation was found to be extremely rapid in scH2O (Table 2).
Primary alkyltrimethylsilane 10 underwent expeditious C-Si bond
cleavage at 390 °C within 30 min, giving dodecane in 72% yield
(run 1).12 By adding HCl, we increased the yield to 88% (run 2).
The desilylation was extremely inefficient in subcritical water (run
3) or entirely sluggish in refluxing HCl/H2O (run 4) or in
supercritical methanol (run 5). The desilylation of secondary
alkylsilane 11 was less efficient (0.4%) at 390 °C (run 6). This
may be due to the steric hindrance around the silicon atom. The
same tendency was observed when Et3Si and i-Pr3Si groups were
attached in place of the Me3Si group (runs 8 and 10). Such a
considerable reactivity difference may be useful for the selective
desilylation from a compound bearing several different silyl groups.
However, in all cases, the addition of HCl again accelerated the
cleavage of these sterically hindered C-Si bonds within 30 min
(runs 7, 9, and 11). When 14 was used as a substrate, 368% of
1-dodecane was formed (run 12). This result unambiguously verifies
that all C-Si bonds of tetraalkylsilanes are cleaved in scH2O.
ization into inner alkenes was observed within 3 h.
(8) (a) Reference 6. (b) Laguerrf, M.; Grignon-Dubois, M.; Dunogue`s, J.
Tetrahedron 1981, 37, 1161.
(9) Utimoto, K.; Kitai, M.; Nozaki, H. Tetrahedron Lett. 1975, 2825.
(10) (a) Kumada, M.; Tamao, K.; Yoshida, J. J. Organomet. Chem. 1982, 239,
115. (b) Tamao, K. AdVances in Silicon Chemistry; JAI Press Inc.:
Greenwich, CT, 1996; Vol. 3, p 1.
(11) Armitage, D. A. In ComprehensiVe Organometallic Chemistry; Wilkinson,
G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon: New York, 1982; Vol.
2, Chapter 9.1. Armitage has noted in this comprehensive review:
“Tetraalkylsilanes are remarkably stable compounds. The silicon-carbon
bond is strong and almost nonpolar. It is therefore only broken under the
most vigorous conditions, unless assisted by an alkyl group possessing
an activating substituent suitably placed.” Representative methods of C-Si
bond cleavage of tetraalkylsilane are collected in this review. For relatively
recent examples on this subject, see: Kakiuchi, F.; Furuta, K.; Murai, S.;
Kawasaki, Y. Organometallics 1993, 12, 15. Smitrovich, J. H.; Woerpel,
K. A. J. Org. Chem. 1996, 61, 6044.
(12) When the reaction was performed directly in a stainless reactor, desilylation
was quite inefficient (57% conversion; dodecane 19%; 5 29%). Therefore,
in this particular class of compounds (tetraalkylsilanes), the inhibiting effect
of the stainless steel reactor (SUS316) and/or the promoting effect of the
quartz tube might be involved for C-Si bond cleavage.
JA034227G
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J. AM. CHEM. SOC. VOL. 125, NO. 20, 2003 6059