2484
A. Grajewska et al.
LETTER
simple methodology is effective for the silylene protec-
tion of 1,3- and 1,4- but not 1,2-diols. The reaction condi-
tions are usually mild, also making it a practical tool for
GLC derivatization. The chemical stability of diphenyl-
silylene-protected diols is however not as good as that of
diols protected with a di-tert-butylsilylene group, and we
observed hydrolysis upon extended exposure to silica gel
during purification by flash chromatography.
Table 3 Cs2CO3-Catalyzed Silylene Protection of Selected 1,3- and
1,4-Diolsa
Ph Ph
Cs2CO3
(10 mol%)
OH OH
Si
Ph Ph
Si
O
O
+
R1
R2
THF
r.t.
H
H
n
R1
R2
n
1a–c (n = 1)
5a–b (n = 2)
2a
(1.2 equiv)
3aa–ca (n = 1)
6aa–ba (n = 2)
Entry Diol R1
R2
n
Silylene Time Conv. Yield
(h) (%)b (%)c
Supporting Information for this article is available online at
1
2
3
4
5
1a
1b
1c
5a
5b
Ph
Ph
Ph
H
Me
Ph
1
1
3aa
3ba
3ca
6aa
6ba
1
1
1
1
1
100
100
100
100
100
89
74
80
55d
70
Acknowledgment
t-Bu
H
1
2
2
A.G. thanks the Alexander von Humboldt-Foundation for a post-
doctoral fellowship (2009–2010).
Ph
H
References and Notes
a All reactions were conducted using Cs2CO3 (10 mol%) and Ph2SiH2
(2a, 1.2 equiv) with a substrate concentration of 0.11 M in THF at r.t.
b Conversion determined by 1H NMR analysis.
(1) (a) Wuts, P. G. M.; Greene, T. W. Greene´s Protective
Groups in Organic Synthesis, 4th ed.; Wiley: New York,
2007, 299. (b) Kocieński, P. J. Protecting Groups, 3rd ed.;
Thieme: Stuttgart, 2004, 119.
(2) Cragg, R. H.; Lane, R. D. J. Organomet. Chem. 1984, 267, 1.
(3) Trost, B. M.; Caldwell, C. G. Tetrahedron Lett. 1981, 22,
4999.
c Isolated yield of analytically pure material after purification by flash
chromatography on silica gel (cf. Supporting Information).
d Extended exposure to silica gel during purification by flash chroma-
tography resulted in decomposition. A sufficiently pure sample of 6aa
was obtained by evaporation of unreacted 5a under reduced pressure
(octaphenylcyclotetrasiloxane was a minor impurity).
(4) Corey, E. J.; Hopkins, P. B. Tetrahedron Lett. 1982, 23,
4871.
(5) Mai, K.; Patil, G. J. Org. Chem. 1986, 51, 3545.
(6) (a) For late-transition-metal catalysis, see: Corey, J. Y. In
Advances in Silicon Chemistry, Vol. 1; Larson, G., Ed.; JAI
Press: Greenwich, 1991, 327. (b) For Brønsted and Lewis
acid/base catalysis, see: Lukevics, E.; Dzintara, M.
J. Organomet. Chem. 1985, 295, 265.
(7) Nakano, T.; Nagai, Y. Chem. Express 1990, 5, 21.
(8) Bedard, T. C.; Corey, J. Y. J. Organomet. Chem. 1992, 428,
315.
(9) Lorenz, C.; Schubert, U. Chem. Ber. 1995, 128, 1267.
(10) Blackwell, J. M.; Foster, K. L.; Beck, V. H.; Piers, W. E.
J. Org. Chem. 1999, 64, 4887.
(11) Weickgenannt, A.; Oestreich, M. Chem. Asian J. 2009, 4,
406.
We also tested 1,2-diols but the reaction outcome was
quite different from that of the 1,3- and 1,4-diols. For ex-
ample, protection of 7a appeared to be low-yielding under
standard reaction conditions, forming only traces of de-
sired 8aa. However, a two-fold excess of Ph2SiH2 (2a) in
the same reaction produced a seven-membered ring in
good isolated yield (7a → 9aa, Scheme 2). Compound
9aa is considerably more stable than 8aa,17 which decom-
posed during purification on silica gel. The routine setup
[Cs2CO3 (10 mol%) without molecular sieves] is however
complicated by the formation of appreciable quantities of
octaphenylcyclotetrasiloxane, which is why the use of
molecular sieves with Cs2CO3 (50 mol%) is recommend-
ed (vide supra). We explain the origin of the silicon-bridg-
ing oxygen atom in 9aa by the presence of water, despite
its rigorous exclusion and the use of molecular sieves.
Sterically more hindered diols, e.g., pinacol, yielded in-
tractable product mixtures (not shown).
(12) Weickgenannt, A.; Mewald, M.; Muesmann, T. W. T.;
Oestreich, M. Angew. Chem. Int. Ed. 2010, 49, 2223; Angew.
Chem. 2010, 122, 2269.
(13) van Look, G.; Simchen, G.; Heberle, J. Silylating Agents;
Fluka Chemie AG: Buchs, 1995, 111.
(14) General Procedure for Silylene Protection A flame-dried
Schlenk flask was charged with Cs2CO3 (10 mol%). The
flask was evacuated and backfilled with argon prior to the
successive addition of the diol (1.0 equiv) dissolved in THF
(0.15 M) and the dihydrosilane (1.2 equiv) dissolved in THF
(0.50 M). The reaction mixture was maintained at r.t. for the
indicated time, and the base was then filtered off. The filtrate
was evaporated under reduced pressure, affording the crude
silylene-protected diols, which were purified by flash
column chromatography on silica gel.
Cs2CO3
(50 mol%)
Ph
Ph
Ph
O
2a
Ph
OH
Si
(2.1 equiv)
Si
O
O
Ph
Ph
+
OH
O
Ph
Si
3 Å
THF
12 h at r.t.
H
Ph
Ph
O
H
H
7a
8aa (traces)
9aa (70%)
(15) Cragg, R. H.; Lane, R. D. J. Organomet. Chem. 1985, 289,
23.
(16) The reaction of Ph2SiH2 with H2O in the absence of base
yields disiloxane (Ph2HSi)2O (cf. ref. 8).
Scheme 2 Cs2CO3-catalyzed protection of a 1,2-diol
In summary, we have demonstrated that Cs2CO3 catalyzes
the dehydrogenative silicon–oxygen coupling of dihy-
drosilanes and diols to form 1,3-dioxo-2-silacycles. This
(17) (a) Silcox, C. M.; Zuckerman, J. J. J. Organomet. Chem.
1966, 5, 483. (b) Yang, M.-H.; Yeh, F.-J. J. Chin. Chem.
Soc. 1991, 38, 57.
Synlett 2010, No. 16, 2482–2484 © Thieme Stuttgart · New York