Reaction of δ-silyl-γ,δ-epoxy-α,β-unsaturated acylsilanes with cyanide ion: Possibility of the formation of silicate intermediate in anion-induced ring opening of epoxysilanes

Article abstract of DOI:10.1016/j.tetlet.2005.07.120

Reaction of trans- and cis-δ-silyl-γ,δ-epoxy-α, β-unsaturated acylsilanes 1 and 9 with cyanide ion in the presence of an electrophile affords double Brook rearrangement products 10 in an E/Z ratio depending on the cis/trans geometry of the epoxysilanes.

Full text of DOI:10.1016/j.tetlet.2005.07.120

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 6429–6432
Reaction of d-silyl-c,d-epoxy-a,b-unsaturated acylsilanes
with cyanide ion: possibility of the formation of silicate
intermediate in anion-induced ring opening of epoxysilanes
Koudai Tanaka,a Hyuma Masu,b Kentaro Yamaguchib and Kei Takedaa,*
aDepartment of Synthetic Organic Chemistry, Graduate School of Medical Sciences, Hiroshima University, 1-2-3 Kasumi,
Minami-Ku, Hiroshima 734-8551, Japan
bTokushima Bunri University, Shido, Sanuki, Kagawa 769-2193, Japan
Received 1 July 2005; revised 20 July 2005; accepted 25 July 2005
Abstract—Reaction of trans- and cis-d-silyl-c,d-epoxy-a,b-unsaturated acylsilanes 1 and 9 with cyanide ion in the presence of an
electrophile affords double Brook rearrangement products 10 in an E/Z ratio depending on the cis/trans geometry of the
epoxysilanes.
Ó 2005 Elsevier Ltd. All rights reserved.
During our investigation of chirality transfer in twofold
Brook rearrangement-mediated tandem reactions of
d-silyl-c,d-epoxy-a,b-unsaturated acylsilanes 1a1 with a
cyanide ion (1a!2) (Scheme 1), we observed that gener-
ation of an alkoxide, a possible intermediate in the reac-
tion, by treatment of a-silyl alcohol (2Z,4E)-31 with
KN(SiMe3)2 (KHMDS) in the presence of MeI afforded
O-silyl cyanohydrin derivatives 4 in 78% yield with an
5E:5Z ratio of 1:5.0, in sharp contrast to the fact that
only (3E,5E)-2 was obtained from the reaction of 1a
with KCN/MeI (Scheme 2).1,2 A similar trend was also
observed for other bases and for the addition of 18-
crown-6. Interestingly, the E/Z ratio in 4 was dependent
upon the olefin geometries in 3. Thus, (2E,4E)-3 affor-
ded 4 in an 5E:5Z ratio of 1:33. These striking results
prompted us to study this in some detail.3,4
tion structures in the Brook rearrangement5 (Scheme
3). X-ray analysis of (2Z,4E)-3 indicates that a
ground-state conformation has a silyl group aligned
almost perpendicularly to the plane of the olefin and
an OH group in an inside position (Fig. 1).6,7 Assuming
that the conformation in the solid state is similar to the
conformation in solution, the most plausible pathway
from 3 to 4 may involve the formation of a silicate inter-
mediate 6a,8 which can be directly derived from the most
stable conformation of 3, and the C–Si bond adopts a
coplanar arrangement with the p-orbitals of the double
bond, followed by Brook rearrangement and reaction
with an electrophile. These findings are in good agree-
ment with those reported by Panek9 on the catalytic
osmylation of C1-oxygenated allylsilanes and with those
described in the literature for allylsilanes.10
The observed Z-selectivities can be rationalized in terms
of the ground-state conformation at C6 and the transi-
On the other hand, the result for epoxysilane 1a, in
which only (E)-2 was formed, suggests that an alkoxide
KCN
O
OSiMe2But
O
18-crown-6
R
tBuMe2Si
SiMe2But
tBuMe2SiO
CN
El, THF, 0˚C
1a
2
El = MeI, ClCO2R', NCCO2R'
Scheme 1. Tandem reaction of 1a with an electrophile.
Keywords: Brook rearrangement; Tandem reactions; Epoxysilanes; Allyl silanes; Electrophilic substitution.
*
Corresponding author. Tel./fax: +81 82 257 5184; e-mail: takedak@hiroshima-u.ac.jp
0040-4039/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.07.120

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K. Tanaka et al. / Tetrahedron Letters 46 (2005) 6429–6432
OSiMe2But
CN
tBuMe2SiO
KHMDS
OSiMe2But
CH3
OH
CH3I, THF
tBuMe2Si
CN
-80 ˚C, 1 min
(2Z,4E)-3
H
4
base
4, yield (%) (E/Z)
KHMDS
78 (1/5.0)
KHMDS/18-crown-6 54 (1/4.0)
NHMDS
LDA
78 (1/8.3)
23 (1/1.8)
OH
CN
OSiMe2But
CH3
KHMDS
tBuMe2SiO
CH3I, THF
tBuMe2Si
OSiMe2But
CN
-80 ˚C, 1 min
(2E,4E)-3
H
4 (83% (E/Z = 1/33))
Scheme 2. Treatment of 3 with a base.
CN
H
CN
H
R3SiO
H
R3SiO
H
SiR3
H
H
H
6
O
O
H
5b SiR3
5a
CN
R3SiO
CN
H
R3SiO
H
H
H
OSiR3
CH3
OSiR3
CH3
R3SiO
H
SiR3
H
H
R3SiO
CN
CN
O
O
(3E,5Z)-7
(3E,5E)-7
SiR3
6a
6b
H
SiR3
CN
H
O
O
H
H
SiR3
8
Scheme 3.
silicates 6a and b, leading to (5Z)-7 and (5E)-7, respec-
tively.11 Although the migrating silyl groups in both
6a and b are located in the position to ensure best over-
lap with the p-bond, the latter seems to be formed faster
with rotation about the C5–C6 bond from 8.12 To test
this possibility and to preclude the possibility that the
difference in the stereochemical outcome between the
reactions of 1a and 3 is due to the difference in the reac-
tion conditions used,13 we conducted the reaction using
cis-epoxide derivatives 9a, which would give an (5Z)-7
as a major product if the above assumption is true.
When 9a was treated with KCN/NCCO2Et under the
same conditions as those employed for 1a, (5Z)-10a
was obtained as a major isomer (E/Z = 1/2.8),14 sup-
porting our assumption mentioned above (Scheme 4).
The lower selectivity observed for the cis derivative
may be explained by eclipsing interaction between the
hydrogen at C-4 and the tert-butyldimethylsilyl group
in the formation of silicate as shown in 11.
Figure 1. ORTEP drawing of (2Z,4E)-3.
5 was not involved as an intermediate in the major path-
way or that it was involved but was too short-lived to
change its conformation to the most stable one 5a.
One possible explanation of this result is that the C–O
bond cleavage of epoxide and Si–O bond formation in
silicate intermediate 8, generated from addition of cya-
nide ion to 1a, occur in a concerted fashion to provide
Next, to obtain further support for our assumption of
silicate intermediacy, we carried out reactions using

K. Tanaka et al. / Tetrahedron Letters 46 (2005) 6429–6432
6431
OSiMe2But
CO2Et
atory Research, No. 14657563, 2002 and by the Naito
Foundation. We thank the Research Center for Mole-
cular Medicine, Faculty of Medicine, Hiroshima Uni-
versity and the Natural Science Center for Basic
Research and Development (N-BARD), Hiroshima
University for the use of their facilities.
tBuMe2SiO
tBuMe2Si
O
KCN
18-crown-6
O
SiMe2But
CN
NCCO2Et
Et2O
0 ˚C, 2 h
9a
H
10a (95%, E/Z = 1/2.8)
SiR3
O
CN
H
4
O
H
Supplementary data
R3Si
11
H
Supplementary data associated with this article can be
found, in the online version at doi:10.1016/j.tet-
let.2005.07.120. Information available: full experimental
details and spectral data.
Scheme 4. Reaction of 9a with ethyl cyanoformate in the presence of
KCN/18-crown-6.
NCCO2Et (1.1 eq)
OSiMe2But
O
KCN (0.1 eq)
References and notes
O
CO2Et
18-crown-6 (0.1 eq)
R3Si
SiMe2But
R3Si
R3SiO
CN
1. Tanaka, K.; Takeda, K. Tetrahedron Lett. 2004, 45, 7859–
7861.
2. Reaction of (2Z,4E)-3 with NCCO2Et resulted in the
formation of an O-carbamoyl derivative.
Et2O, 0 ˚C
2 h
1b,c, 9b,c
H
10b,c
E/Z
E
yield (%)
93
1b
i-Pr3Si
3. For discussions on stereochemistry in the reactions in
which enol silyl ethers are formed from a-silyl alkoxides
bearing a b-leaving group via the Brook rearrangement,
see: (a) Reich, H. J.; Holtan, R. C.; Bolm, C. J. Am. Chem.
Soc. 1990, 112, 5609–5617; (b) Okugawa, S.; Takeda, K.
Org. Lett. 2004, 6, 2973–2975; (c) Clayden, J.; Watson, D.
W.; Chambers, M. Tetrahedron 2005, 61, 3195–3203.
4. To the best of our knowledge, there is no precedent for a
stereochemical discussion concerning Brook rearrange-
ment in a-silyl allylalkoxides.
5. For reviews on the Brook rearrangement, see: (a) Brook,
M. A. Silicon in Organic, Organometallic, and Polymer
Chemistry; John Wiley and Sons, 2000; (b) Brook, A. G.;
Bassindale, A. R. In Rearrangements in Ground and
Excited states; de Mayo, P., Ed.; Academic Press: New
York, 1980, pp 149–221; (c) Brook, A. G. Acc. Chem. Res.
1974, 7, 77–84; For the use of the Brook rearrangement in
tandem bond formation strategies, see: (d) Moser, W. H.
Tetrahedron 2001, 57, 2065–2084; Also, see: (e) Ricci, A.;
DeglÕInnocenti, A. Synthesis 1989, 647–660; (f) Bulman
Page, P. C.; Klair, S. S.; Rosenthal, S. Chem. Soc. Rev.
1990, 19, 147–195; (g) Qi, H.; Curran, D. P. In Compre-
hensive Organic Functional Group Transformations; Katri-
tzky, A. R., Meth-Cohn, O., Rees, C. W., Moody, C. J.,
Eds.; Pergamon: Oxford, 1995, pp 409–431; (h) Cirillo, P.
F.; Panek, J. S. Org. Prep. Proced. Int. 1992, 24, 553–582;
(i) Patrocinio, A. F.; Moran, P. J. S. J. Braz. Chem. Soc.
2001, 12, 7–31.
1c
9b
9c
E
t-BuPh2Si
i-Pr3Si
97
96
99
1/3.3
1/1.5
t-BuPh2Si
Scheme 5. Reaction of 1b,c and 9b,c with ethyl cyanoformate in the
presence of KCN/18-crown-6.
1b,c and 9b,c, which bear electronically and sterically
different silyl groups, with the expectation that in the
case of 9c, the silicate ion 6a would have a sufficiently
long lifetime for conversion to rotamer 6b, leading to
a thermodynamically stable E-isomer, by introduction
of a phenyl group on the silyl group, and consequently
the E/Z ratio in 10 would increase (Scheme 5).15 While
in the case of triisopropylsilyl derivatives 1b and 9b,
more sterically demanding than tert-butyldimethylsilyl
group, essentially the same stereochemical outcome as
that observed for tert-butyldimethylsilyl derivatives 1a
and 9a was observed, tert-butyldiphenylsilyl derivative
9c, more sterically demanding but more silicate ion-sta-
bilizing than the tert-butyldimethylsilyl group, afforded
10c in an increased ratio (E/Z: 1/1.5 vs 1/2.8) of E-iso-
mer to Z-isomer, supporting the above assumption.
In conclusion, we demonstrated the possibility that a
c-anion-induced ring opening in a,b-epoxysilanes pro-
vides an allylsilicate intermediate in a concerted fashion
and demonstrated that the intermediate does not have a
sufficient lifetime to allow it to rotate freely about the
C(a)–C(b) bond and undergoes Brook rearrangement
according to the least motion principle16 to give prod-
ucts in an E/Z ratio depending on the geometry of the
epoxysilanes.
6. X-ray data for (2Z,4E)-3: C18H35NO2Si2; formula weight,
353.65; crystal system, monoclinic; space group, P2(1)/c;
˚
unit cell dimensions, a = 10.745(3) A, a = 90°, b =
˚
˚
7.6785(18) A, b = 94.078(4)°, c = 27.391(6) A, c = 90°;
3
˚
volume, 2254.2(9) A ; Z = 4; R = 0.0702.
7. To the best of our knowledge, this is the first X-ray crystal
structure of a-trialkylsilyl allylic alcohol. For an X-ray
structure of (1-hydroxyprop-2-eny1)silanol, see: Buynak,
J. D.; Strickland, J. B.; Lamb, G. W.; Khasnis, D.; Modi,
S.; Williams, D.; Zhang, H. J. Org. Chem. 1991, 56, 7076–
7083.
8. For reviews of hypervalent silicon compounds, see: (a)
Kost, D.; Kalikhman, I. Hypervalent Silicon Compounds.
In The Chemistry of Organic Silicon Compounds; Rappo-
port, Z., Apeloig, Y., Eds.; Wiley: Chichester, UK, 1998;
Vol. 2, Chapter 23; (b) Chuit, C.; Corriu, R. J. P.; Reye,
C.; Young, J. C. Chem. Rev. 1993, 93, 1371–1448;
Acknowledgements
This research was partially supported by the Ministry of
Education, Science, Sports and Culture, Grant-in-Aid
for Scientific Research (B), No. 15390006, 2003, Explor-

6432
K. Tanaka et al. / Tetrahedron Letters 46 (2005) 6429–6432
(c) Deerenberg, S.; Schakel, M.; de Keijzer, A. H. J. F.;
Kranenburg, M.; Lutz, M.; Spek, A. L.; Lammertsma, K.
Chem. Commun. 2002, 348–349. Also, see: Ref. 5a.
12. For a similar explanation for other systems, see: (a)
Fleming, I.; Newton, T. W. J. Chem. Soc., Perkin Trans. 1
1984, 119–123; (b) Miller, R. B.; McGarvey, G. J. Org.
Chem. 1978, 43, 4424–4431; (c) Brook, A. G.; Duff, J. M.;
Hitchcock, P.; Mason, R. J. Organomet. Chem. 1976, 113,
C11–C12; (d) Koenig, K. E.; Weber, W. P. Tetrahedron
Lett. 1973, 14, 2533–2536.
13. Treatment of 3 with KHMDS (1.1 equiv) in the presence
of MeI and 18-crown-6 (0.05 equiv) in THF at 0 °C for
5 min, conditions that roughly mimic those which can be
encountered in the reaction from 1a, resulted in the
formation of 4 (52% yield; E/Z = 1/2.5) and protonated
derivative 2 (El = H) (28% yield; E/Z = 1/6.7).
9. (a) Panek, J. S.; Cirillo, P. F. J. Am. Chem. Soc. 1990, 112,
4873–4878; (b) Lambert, J. B. Tetrahedron 1990, 46, 2677–
2689.
10. (a) Buckle, M. J. C.; Fleming, I.; Gil, S.; Pang, K. L. C.
Org. Biomol. Chem. 2004, 2, 749–769; (b) Masse, C. E.;
Panek, J. S. Chem. Rev. 1995, 95, 1293–1316; (c) Fleming,
I.; Barbero, A.; Walter, D. Chem. Rev. 1997, 97, 2063–
´
2192; (d) Fleming, I.; Dunogues, J.; Smithers, R. Org.
React. 1989, 37, 57–575.
11. We have previously reported the possibility that C–O
bond cleavage and Si–O bond formation in base-pro-
moted ring opening of metalated O-silyl cyanohydrins of
b-silyl-a,b-epoxyaldehydes proceed in a concerted manner.
See: Sasaki, M.; Kawanishi, E.; Nakai, E.; Matsumoto, T.;
Yamaguchi, K.; Takeda, K. J. Org. Chem. 2003, 68, 9330–
9339.
14. Reaction of 1a with KCN/NCCO2Et provided (5E)-10a in
93% yield. See Ref. 1.
15. The use of less bulky silyl groups such as trimethylsilyl and
dimethylphenylsilyl resulted in the hydrolysis of the enol
silyl ether to the corresponding aldehydes.
16. Hine, J. Adv. Phys. Org. Chem. 1977, 15, 1–61.