Stereospecific cationic [1,2] silyl shift with retention of configuration at the migrating terminus

Full text of DOI:10.1021/ja9737937

1930
J. Am. Chem. Soc. 1998, 120, 1930-1931
neighboring silicon groups are fixed syn in a cyclic system (eq
1b). In this paper, we disclose a highly stereospecific skeletal
rearrangement involving the syn [1,2] silyl shift, which produces
enantiomerically enriched propargylsilane and allylsilanes, in the
reaction of oxasilacycloalkanes prepared by intramolecular bis-
silylation of propargylic and allylic alcohols.5
In a previous paper, we demonstrated the palladium(0)-
catalyzed intramolecular bis-silylation of secondary propargylic
alcohols leading to the stereospecific synthesis of chiral
allenylsilanes.5b Intermediary four-membered cyclic oxasiletanes
were spectroscopically identified but were not isolable due to
hydrolysis of the strained Si-O bond during chromatographic
purification. However, the bis-silylation of tertiary alcohols
provided stable four-membered cyclic oxasiletanes (eq 2). Tri-
Stereospecific Cationic [1,2] Silyl Shift with Retention
of Configuration at the Migrating Terminus
Michinori Suginome, Akira Takama, and Yoshihiko Ito*
Department of Synthetic Chemistry
and Biological Chemistry
Graduate School of Engineering
Kyoto UniVersity, Kyoto 606-01, Japan
ReceiVed NoVember 4, 1997
Nucleophilic [1,2] alkyl shift to cationic carbon center is a
fundamental process in organic chemistry, being involved in such
well-known reactions as Wagner-Meerwein and pinacol rear-
rangements.1 From the mechanistic point of view, stereochemical
investigation has been carried out to reveal that the cationic [1,2]
shift generally proceeds with inversion or racemization at the
migrating terminus.1,2 It has been reported that the silyl group
similarly migrates to an electron-deficient carbon center to provide
useful synthetic methods of stereodefined organosilicon com-
pounds.3,4 In a manner similar to that of the alkyl shift, inversion
at the migrating terminus (anti migration) was observed in the
reactions of stereochemically defined organosilicon compounds.3
In these cases, anti periplaner conformation of the leaving group
and the migrating silyl group may be responsible for the observed
anti migration (eq 1a). However, concerted, stereospecific syn
methyl-2,2-diphenyldisilanyl ethers 1a,b having an alkyl or a silyl
group at the sp carbon reacted in the presence of the palladium
catalyst under reflux in toluene to give the corresponding 4-exo
cyclization products 2a and b in high yields after column
chromatography on silica gel. Pentamethyldisilanyl derivative
1c also afforded 2c at 75 °C in good yield.6
Stirring of 2a in aqueous THF at room temperature resulted in
the formation of protiodesilylation product 3 presumably via
nucleophilic attack of water to the silicon in the four-membered
ring followed by cleavage of the silicon-carbon bond (eq 3).
migration could be possible if the syn periplaner conformation is
favorable due to structural reason, e.g., the leaving and the
(1) Hanson, J. R. In ComprehensiVe Organic Synthesis; Trost, B. M., Ed.;
Pergamon Press: Oxford, 1991; Vol. 3, pp 705-719. Rickborn, B. In
ComprehensiVe Organic Synthesis; Trost, B. M., Ed.; Pergamon Press: Oxford,
1991; Vol. 3, pp 721-732. For the stereochemical aspect, see: Kallmerten,
J. In StereoselectiVe Synthesis; Helmchen, G., Ed.; Thieme Verlag: Stuttgart,
Germany, 1996; Vol. 6, pp 3810-3832.
(2) A [1,2] alkyl shift with net retention of configuration at the migrating
terminus was reported for the skeletal rearrangement of a rigid bicyclic system.
For example, see: Uyehara, T.; Yamada, J.; Kato, T. Tetrahedron Lett. 1985,
26, 5069.
Simple hydrolysis at the Si-O bonds, which may usually take
place for strained silyl ethers, was not observed at all. In sharp
contrast, treatment of 2a with acidic aqueous THF gave tertiary
propargylsilanol 4 in high yield through [1,2] silyl migration in
high yield without formation of 3. It is presumed that silyl group
in the ring migrates to the tertiary carbocation, which was
generated by the carbon-oxygen bond cleavage with protonation,
to form vinylic cation stabilized by the three â-silyl groups. The
rearrangement reaction finishes with an elimination of the
trimethylsilyl group.
(3) For the [1,2] silyl shift to the cationic centers generated from sp3 carbon,
see: (a) Hudrlik, P. F.; Hudrlik, A. M.; Nagendrappa, G.; Yimenu, T.; Zellers,
E.; Chin, E. J. Am. Chem. Soc. 1980, 102, 6896-6898. (b) Miura, K.; Hondo,
T.; Saito, H.; Ito, H.; Hosomi, A. J. Org. Chem. 1997, 62, 8292-8293. (c)
Ooi, T.; Kiba, T.; Maruoka, K. Chem. Lett. 1997, 519-520. (d) Tanino, K.;
Yoshitani, N.; Moriyama, F.; Kuwajima, I. J. Org. Chem. 1997, 62, 4206-
4207.
(4) For examples of the reactions involving a similar cationic [1,2] silyl
shift, see: (a) Danheiser, R. L.; Carini, D. J.; Basak, A. J. Am. Chem. Soc.
1981, 103, 1604-1606. (b) Danheiser, R. L.; Carini, D. J.; Fink, D. M.; Basak,
A. Tetrahedron 1983, 39, 935-947. (c) Danheiser, R. L.; Kwasigroch, C. A.;
Tsai, Y.-M. J. Am. Chem. Soc. 1985, 107, 7233-7235. (d) Danheiser, R. L.;
Carini, D. J.; Kwasigroch, C. A. J. Org. Chem. 1986, 51, 3870-3878. (e)
Kno¨lker, H.-J.; Jones, P. G.; Pannek, J.-B. Synlett 1990, 429. (f) Danheiser,
R. L.; Dixon, B. R.; Gleason, R. W. J. Org. Chem. 1992, 57, 6094-6097. (g)
Panek, J. S.; Beresis, R. J. Org. Chem. 1993, 58, 809-811. (h) Panek, J. S.;
Jain, N. F. J. Org. Chem. 1993, 58, 2345-2348. (i) Yamazaki, S.; Tanaka,
M.; Yamaguchi, A.; Yamabe, S. J. Am. Chem. Soc. 1994, 116, 2356-2365.
(j) Brengel, G. P.; Rithner, C.; Meyers, A. I. J. Org. Chem. 1994, 59, 5144-
5146. (k) Akiyama, T.; Ishikawa, K.; Ozaki, S. Chem. Lett. 1994, 627-630.
The [1,2] silyl migration reaction of 2a was more effectively
catalyzed by catalytic amount of trimethylsilyl triflate (Me3SiOTf)
(5) (a) Suginome, M.; Matsumoto, A.; Ito, Y. J. Am. Chem. Soc. 1996,
118, 3061-3062. (b) Suginome, M.; Matsumoto, A.; Ito, Y. J. Org. Chem.
1996, 61, 4884-4885. (c) Suginome, M.; Iwanami, T.; Matsumoto, A.; Ito,
Y. Tetrahedron: Asymmetry 1997, 8, 859-862.
(6) The reaction of 1c under reflux in toluene resulted in the formation of
a complex mixture.
S0002-7863(97)03793-1 CCC: $15.00 © 1998 American Chemical Society
Published on Web 02/14/1998

Communications to the Editor
J. Am. Chem. Soc., Vol. 120, No. 8, 1998 1931
Table 1. Stereospecific Silyl Migration Producing Enantioenriched
Propargylsilanesa
8
Me3SiOTf
(equiv)
entry 6 (%ee (config))
yield (%)b %ee (config)c
1
2
3
4
a (97.2 (R))
b (92.8 (S))
c (95.3 (S))
d (93.2 (S))
1.5
1.5
1.5
0.2
70
67
70
89
97.0 (R)
89.7 (S)
94.4 (S)
90.9 (S)
a Pd(acac)2 (2 mol %) and 1,1,3,3-tetramethylbutyl isocyanide (8 mol
%) were used for the bis-silylation step (reflux in toluene for 1 h). The
reaction mixtures were then treated with Me3SiOTf at -78 °C. b Isolated
yields for the two steps. c Determined by HPLC.
silyl group sterically as well as by stabilizing the resultant cation
by the â-silicon effect, which leads to the elimination of the
trimethylsilyl group to furnish the product 8. The fact that the
phenyl group, which may stabilize the generated cation and often
result in racemization, did not affect the stereospecificity in the
reaction of 7d indicated that the concerted [1,2] silyl shift is a
highly favorable process.
at -78 °C to give the corresponding propargylsilane 5a with
formation of a siloxane bond in high yield (eq 4). Me3SiOTf
To our surprise, the stereospecific [1,2] silyl shift was observed
not only for the four-membered rings but also for optically active
six-membered disiladioxanes (R)-9a,b, which were readily pre-
pared by intramolecular bis-silylation of optically active (R)-(E)-
and (R)-(Z)-3-decen-2-ol, respectively.5a,c A treatment of (R)-9a
(99.6% ee) with Me3SiOTf at -78 °C immediately provided cis-
allylsilane (Z)-10 of >99.0% ee with (R) configuration in high
yield (eq 7).7 The (R)-configuration unambiguously manifested
also catalyzed the rearrangement of 2b and c to give the
corresponding propargylic siloxanes in moderate yields (42 and
58%, respectively).
Next, the stereochemical course of the present rearrangement
reaction was investigated with optically active secondary oxasi-
letanes 7a-d, which were formed by the bis-silylation with the
corresponding disilanyl ethers 6a-d of optically active propar-
gylic alcohols of 92.8-97.2% enantiomeric excess (ee) (eq 5).
that the [1,2] silyl shift also proceeded with retention of
configuration at the migrating terminus. Furthermore, the highly
selective formation of (Z) isomer indicated that the elimination
of the Me3Si group, following the migration reaction, took place
exclusively from a conformation anti to the migrating silyl group.
Similarly, nearly complete syn [1,2] silyl shift was observed for
the reaction of its epimer (R)-9b, giving (R)-10 in good yield (eq
8). In this case, however, the olefin geometry was found to be
As 7a-d thus produced were not isolable, the reaction mixtures
containing 7a-d were treated with Me3SiOTf at -78 to 0 °C.
For alkyl derivatives 7a-c, 1.5 equiv of Me3SiOTf were required
for the completion of the reaction to give rearranged propargylic
siloxane 8a-c (Table 1, entries 1-3). Noteworthy is that the
rearrangement took place in high stereospecificity with retention
of stereochemistry at the migrating termini.7 In addition, phenyl-
substituted 7d also underwent the rearrangement in the presence
of catalytic amount of Me3SiOTf to give optically active 8d with
retention of configuration in good yield (entry 4).
The stereochemical outcome indicates that the [1,2] silyl shift
takes place in a concerted manner, i.e., the oxygen atom activated
by Me3SiOTf leaves from the secondary carbon atom with the
concurrent growing of the vacant p orbital, which is stabilized
by the R-Si-C bond (eq 6). The syn periplaner stabilization
should be responsible for the stereospecific syn migration.
Presumably, the two trimethylsilyl groups at the methylidene
terminal assist the reaction effectively by pushing the migrating
4:1 (Z/E), suggesting that the syn elimination of the trimethylsilyl
group proceeded predominantly. These results conclude that the
highly stereospecific syn [1,2] silyl shift was followed by the
nonstereospecific elimination of the Me3Si group from the
thermodynamically favorable conformations.
In summary, we described that the stereospecific cationic syn
[1,2] silyl shift is able to occur with the retention of configuration
of the stereochemistry at the migrating terminus through the syn
periplanar conformation of the leaving and migrating silyl groups
in cyclic system.
Supporting Information Available: Detailed experimental proce-
dures, involving the determination of enantiomeric excesses and absolute
configurations, and characterization of the new compounds (7 pages).
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instructions.
(7) The enantiomeric excesses and absolute configurations of 8a-d and
10 were determined by HPLC analyses after derivatization by hydrogenation
of the carbon-carbon multiple bond with diimide and subsequent oxidation
of the silicon-carbon bond with hydrogen peroxide. The slight loss of the
enantiomeric excesses might be due to partial racemization during the diimide
reduction (TsNHNH2, Et3N, dioxane reflux).5c For derivatization, see the
Supporting Information.
JA9737937