Highly Stereoselective Intramolecular Addition of a Hydroxyl Group to Vinylsilanes via 1,2-Silyl Migration

Full text of DOI:10.1021/jo971847p

8292
J . Org. Chem. 1997, 62, 8292-8293
of DTBP gave 2a in 80% yield although it took a longer
reaction time (72 h). These results strongly support that
TiCl4 functions as the actual catalyst rather than HCl.
High ly Ster eoselective In tr a m olecu la r
Ad d ition of a Hyd r oxyl Gr ou p to
Vin ylsila n es via 1,2-Silyl Migr a tion 1
The change in the geometry of 1a resulted in a marked
decrease in both the reactivity and stereoselectivity.5
Under the standard conditions shown in Table 1, the (E)-
isomer of 1a was cyclized to 2a in 17% yield (trans/cis )
41/59) after being stirred for 4 days. The substituent on
silicon also affected the reactivity of 1 (entries 1-7). The
vinylsilane 1b, bearing a trimethylsilyl group, was more
sensitive to protiodesilylation than 1a . To suppress the
desilylation, a more sterically bulky silyl group (Si) such
as SiMePh2 or SiMe2-t-Bu was employed.3h Contrary to
our expectation, 1c underwent desilylation to a signifi-
cant extent, and the reactivity of 1c was lower than that
of 1a . On the other hand, the cyclization of 1d effected
not only a high yield of 2d but also fast reaction rate,7
although it exhibited a lower trans-selectivity (trans/cis
) 95/5) in addition to the formation of 3d as a minor
product at room temperature. However, the stereose-
lectivity (trans/cis ) 97/3) was improved by the lowering
of the reaction temperature to 0 °C, and the direct
cyclization to 3d and the protiodesilylation of 1d were
restrained. We further conducted the cyclization of the
vinylsilanes 1e-g to investigate the electronic effects of
the substituent R1 on the reactivity. As a result, it turned
out that electron-donating p-tolyl and p-anisyl groups
accelerated the protiodesilylation while the electron-
withdrawing p-trifluoromethylphenyl group considerably
diminished the reactivity of the carbon-carbon double
bond to prevent the conversion of 1g. The latter result
implies that proton addition to the R-carbon is the rate-
determining step in the present cyclization (vide infra).
Katsukiyo Miura, Takeshi Hondo, Hiroshi Saito,
Hajime Ito, and Akira Hosomi*
Department of Chemistry, University of Tsukuba,
Tsukuba, Ibaraki 305, J apan
Received October 7, 1997
The development of new synthetic methods utilizing
the 1,2-silyl migration of â-silyl carbenium ions is of
considerable current interest.2-4 Previously, we have
reported the acid-catalyzed cyclization of the vinylsilanes
1 (R ) H) to the tetrahydrofurans 3 via a â-silyl
carbenium ion intermediate (eq 1).5 We first report
herein that the acid-catalyzed cyclization of 1 (R ) alkyl)
gives the tetrahydropyrans 2 with high trans-selectivity,
but not 3, and also describe the mechanistic aspects of
this novel cyclization via 1,2-silyl migration.
Treatment of the (Z)-vinylsilane 1a with a catalytic
amount of TiCl4 (5 mol %) in CHCl3 stereoselectively gave
the 2,3-disubstituted tetrahydropyran 2a (trans/cis )
>99/<1)6 along with a desilylated product, (E)-4-nonen-
1-ol (4a ) (entry 1 in Table 1). While HCl gas and AcCl
(5 mol %) as well as TiCl4 were good catalysts (HCl, 24
h, 67%; AcCl, 30 h, 67%), CH3CO2H, SnCl4, BF3•OEt2,
and Sc(OTf)3 hardly induced the cyclization. AcCl would
serve as a source of HCl by the reaction with the hydroxyl
group because the presence of 2,6-di-tert-butylpyridine
(DTBP, 5 mol %), a proton scavenger, prevented the AcCl-
catalyzed cyclization of 1a . Similarly, in the TiCl4-
catalyzed system, there is a possibility that HCl gener-
ated from TiCl4 is an actual catalyst. However, TiCl4
exhibited a higher catalytic activity than HCl gas and
AcCl, and the TiCl4-catalyzed cyclization in the presence
The TiCl4-catalyzed reaction tolerated polar function-
alities such as ether and ester groups (entries 9 and 10).
However, the vinylsilane 1k (R ) Ph) underwent no
cyclization because of fast desilylation (entry 11). In the
case of 1l (R ) SiMe3), the 1,2-silyl migration did not
occur at all, and 3l was exclusively obtained as a ca. 1:1
diastereomeric mixture (entry 12).
On the basis of our previous and present results,5,8
a
plausible mechanism for the formation of trans-2 is
shown in Scheme 1. It consists of the following five
steps: (1) the attachment of a proton or TiCl4 to the
hydroxyl group of 1 forms the oxonium ion 5, (2) the
proton on the oxygen atom in 5 shifts to the R-carbon,
(3) the resultant â-silyl carbenium ion 6 turns to its
conformer 7 stabilized by σ-π conjugation,9,10 (4) a 1,2-
silyl migration converts 7 into another â-silyl carbenium
ion 8,11 and (5) intramolecular attack of the oxygen from
the side opposite to the silyl group gives trans-2 and
regenerates the proton or TiCl4. The presence of an alkyl
group as R is essential to the 1,2-silyl migration in step
4. This is probably reasonable because the alkyl group
(1) Studies on Organosilicon Chemistry. 137.
(2) (a) Masse, C. E.; Panek, J . S. Chem. Rev. (Washington, D.C.)
1995, 95, 1293. (b) Panek, J . S. In Comprehensive Organic Synthesis;
Trost, B. M. Ed.; Pergamon Press: Oxford, 1991; Vol. 1, p 579.
(3) [3 + 2] cycloaddition of allylsilanes and the related compounds
via the 1,2-silyl migration. See: (a) Brengel, G. P.; Meyers, A. I. J .
Org. Chem. 1996, 61, 3230. (b) Schinzer, D.; Panke, G. J . Org. Chem.
1996, 61, 4496. (c) Suginome, M.; Matsunaga, S.; Ito, Y. Synlett 1995,
941. (d) Kno¨lker, H.-J .; Graf, R. Synlett 1994, 131. (e) Akiyama, T.;
Ishikawa, K.; Ozaki, S. Chem. Lett. 1994, 627. (f) Panek, J . S.; J ain,
N. F. J . Org. Chem. 1994, 59, 2674. (g) Stahl, A.; Steckhan, E.; Nieger,
M. Tetrahedron Lett. 1994, 35, 7371. (h) Kno¨lker, H.-J .; Foitzik, N.;
Goesmann, H.; Graf, R. Angew. Chem., Int. Ed. Engl. 1993, 32, 1081.
(i) Danheiser, R. L.; Takahashi, T.; Berto´k, B.; Dixon, B. R. Tetrahedron
Lett. 1993, 34, 3845 and references cited in these references. For a
review of allylsilanes in organic synthesis, see: (j) Hosomi, A. Acc.
Chem. Res. 1988, 21, 200.
(4) For other reactions via the 1,2-silyl migration, see: (a) Tanino,
K.; Yoshitani, N.; Moriyama, F.; Kuwajima, I. J . Org. Chem. 1997,
62, 4206. (b) Yamazaki, S.; Tanaka, M.; Yamabe, S. J . Org. Chem. 1996,
61, 4046 and references therein. (c) Angle, S. R.; Boyce, J . P.
Tetrahedron Lett. 1994, 35, 6461. (d) Kang, K.-T.; Lee, J . C.; U, J . S.
Tetrahedron Lett. 1992, 33, 4953. (e) Hudrlik, P. F.; Hudrlik, A. M.;
Nagendrappa, G.; Yimenu, T.; Zellers, E. T.; Chin, E. J . Am. Chem.
Soc. 1980, 102, 6896; correction: ibid. 1982, 104, 1157.
(7) The high reactivity of 1d is probably due to electron-donating
ability of the tert-butyl group, which accelerates proton addition to the
R-carbon (step 2 in Scheme 1). The phenyl group of 1a or 1c would
rather act as an electron-withdrawing substituent and reduce the
reactivity of the double bond toward protonation. Mayr, H.; Patz, M.
Angew. Chem., Int. Ed. Engl. 1994, 33, 938.
(8) We have shown that the cyclization of 1 (R ) H) to 3 proceeds
in a syn addition mode of the hydroxyl group and proposed the oxonium
ion intermediate from the stereochemical outcome. See ref 5.
(9) For the activating and directive effects of silicon, see: Bassindale,
A. R.; Taylor, P. G. The Chemistry of Organic Silicon Compounds;
Patai, S., Rappoport, Z., Ed.; Wiley: Chichester, 1989; Part 2, p 893.
(10) (a) Schweig, A.; Weidner, U.; Manuel, G. J . Organomet. Chem.
1974, 67, C4. (b) Brown, R. S.; Eaton, D. F.; Hosomi, A.; Traylor, T.
G.; Wright, J . M. J . Organomet. Chem. 1974, 66, 249.
(5) (a) Miura, K; Okajima, S.; Hondo, T.; Hosomi, A. Tetrahedron
Lett. 1995, 36, 1483. (b) Miura, K; Hondo, T.; Okajima, S.; Hosomi, A.
Tetrahedron Lett. 1996, 37, 487.
(6) No cis isomer was detected at all within the limitation of 270
MHz 1H NMR analysis.
S0022-3263(97)01847-1 CCC: $14.00 © 1997 American Chemical Society

Communications
J . Org. Chem., Vol. 62, No. 24, 1997 8293
Ta ble 1. TiCl4-Ca ta lyzed Cycliza tion of Vin ylsila n es 1a
substrate (1)
yield/%
entry
R
R1
R2
time/h
2b (trans/cis)c
3
4d,e
1d (Z/E)c
1
2
3
4f
5
6
7
8
9
Bu
Bu
Bu
Bu
Bu
Bu
Bu
Me
Ph
Me
Ph
t-Bu
p-MeC6H4
p-MeOC6H4
p-CF3C6H4
Ph
Ph
Ph
Ph
Ph
Me
Me
Ph
1a
1b
1c
1d
1e
1f
1g
1h
1i
9.5
7.5
24
2.3
9.5
9.5
9.5
15
15
47
24
13
75 (>99/<1)
66 (>99/<1)
58 (>99/<1)
93 (97/3)g
57 (>99/<1)
53 (>99/<1)
39 (>99/<1)
76 (>99/<1)
72 (>99/<1)
72 (>99/<1)
0
0
0
15
26
13
0
20
28
6
10
27
20
29h
8j
6 (83/17)
0
25 (92/8)
0
12 (>99/<1)
15 (>99/<1)
49 (>99/<1)
3 (81/19)
0
4 (>99/<1)
30 (82/18)
4 (>99/<1)
0
Me
Me
Me
Me
Me
Me
Me
Me
Me
3g
0
0
0
0
0
-(CH2)3OBn
-(CH2)3OAc
Ph
10
11
12
1j
1k
1l
0
0
SiMe3
0
67i
a
b
All reactions were performed with 1 (0.50 mmol), TiCl4 (0.025 mmol), and CHCl3 (2.5 mL) at rt unless otherwise noted. Isolated
yield of a purified product except for entry 4. c Determined by 1H NMR analysis. The ratio >99/<1 means that no isomer is detected by
1H NMR analysis. d4 and 1 were obtained as a mixture. The yields and the isomeric ratios of the recovered 1 were determined by 1H
NMR analysis. e E/Z ) >99/<1. f At 0 °C. The yields and ratio were estimated by GC analysis of a mixture of 2d and (2R*,1′S*)-3d . The
g
h
reaction at rt for 0.75 h afforded 2d , 3d , (E)-nonen-1-ol (4a ), and 1d in 85% (trans/cis ) 95/5), 5%, 8%, and 0% yields, respectively.
A
i
j
PhMe2Si ether of 4k was also obtained in 22% yield. A 51:49 diastereomeric mixture. The total yield of (E)-5-(trimethylsilyl)-4-penten-
1-ol (2%) and (E)-5-(dimethylphenylsilyl)-4-penten-1-ol (6%).
Sch em e 1
We have further disclosed that the vinylsilanes 9 also
undergo the present cyclization via 1,2-silyl migration to
provide the 2,3-disubstituted tetrahydrofurans 10 with
high trans-selectivity, although 9 was much less reactive
than 1 (eq 3). Similar to the cyclization of 1, the
introduction of SiMe2-t-Bu remarkably improved both the
reaction rate and the yield of 10.
stabilizes the rearranged carbenium ion 8.3i In the
cyclization of 1l, the lower stabilizing ability of the TMS
group than that of ordinary alkyl groups9 would inhibit
the 1,2-silyl migration of 7.
The preferred formation of 2 to 3 in entries 1-10 is
attributable to acid-catalyzed isomerization of 3 to 2 by
thermodynamic control.12 Indeed, the isomerizations of
(2R*,1′S*)-3d and (2R*,1′R*)-3d to 2d rapidly proceeded
with inverse stereoselectivity13,14 (eq 2). The stereospe-
Finally, we carried out oxidative removal of the silicon
moiety of the cyclized products to enhance the synthetic
utility of the present reaction. According to the reported
procedure,16 2a and 10a could be converted to the
corresponding alcohols in 88% and 89% yields, respec-
tively, with stereochemical retention.13
We are now studying application of this method to the
stereoselective synthesis of trisubstituted tetrahydropy-
rans and tetrahydrofurans, and the results will be
reported in due course.
Ack n ow led gm en t. The present work was partly
supported by Grants-in-Aid for Scientific Research,
Grants-in-Aid for Scientific Research on Priority Areas
from the Ministry of Education, Science, Sports and
Culture, J apan, Teikoku Chemical Industries, Inc., and
Pfizer Pharmaceuticals Inc. T.H. acknowledges support
from the Research Fellowships of the J apan Society for
the Promotion of Science for Young Scientists.
cific conversion of (2R*,1′S*)-3d into trans-2 well agrees
with the mechanism shown in Scheme 1. We could also
observe the partial isomerization of trans-2d to (2R*,1′S*)-
3d . These results demonstrate that the present isomer-
ization is reversible, and therefore, trans-2d is a ther-
modynamically favored product.15
Su p p or tin g In for m a tion Ava ila ble: Experimental pro-
cedures and spectral data for the substrates and the products
(15 pages).
(11) There is a longstanding dispute on the structure of â-silyl
carbenium ions, which can take a hyperconjugatively stabilized open
form or a bridged form. Although the open forms 7 and 8 are employed
in Scheme 1, when R is an alkyl group, they can be displaced to one
bridged intermediate without any problems. Lambert, J . B.; Zhao, Y.
J . Am. Chem. Soc. 1996, 118, 7867.
J O971847P
(12) Although the fast cyclization of 8 compared with 7, that is,
kinetic control, can be responsible for the exclusive formation of trans-
2d , we do not have any reliable evidence that trans-2d is a kinetically
favored product.
(13) See the Supporting Information.
(14) Recently, Kuwajima et al. have reported novel ring expansion
reactions with the 1,2-silyl migration. See ref 4a.
(15) The MM2 calculation of the steric energy using the CAChe
system (Sony/Tektronix Co. Ltd.) indicates that trans-2d is more stable
by 3.5 kcal/mol than (2R*,1′S*)-3d in their optimized structures.
(16) (a) Tamao, K.; Ishida, N. J . Organomet. Chem. 1984, 269, C
37. (b) Murakami, M.; Suginome, M.; Fujimoto, K.; Nakamura, H.;
Andersson, P. G.; Ito, Y. J . Am. Chem. Soc. 1993, 115, 6487.