Synthesis of (Z)-vinylsilanes with high diastereoselectivity by using samarium diiodide

Article abstract of DOI:10.1021/ol015601p

matrix presented β-Elimination of O-acetyl 1-chloro-1-trimethylsilylalkan-2-ols 1 was achieved by using samarium diiodide as a metaling reagent and afforded the corresponding (Z)-vinylsilanes with high stereoselectivity. The starting compounds 1 were easi

Full text of DOI:10.1021/ol015601p

ORGANIC
LETTERS
2001
Vol. 3, No. 6
937-939
Synthesis of (Z)-Vinylsilanes with High
Diastereoselectivity by Using Samarium
Diiodide
Jose´ M. Concello´n,* Pablo L. Bernad, and Eva Bardales
Departamento de Qu´ımica Orga´nica e Inorga´nica,
Facultad de Qu´ımica UniVersidad de OViedo, Julia´n ClaVer´ıa, 8, 33071 OViedo, Spain
jmcg@sauron.quimica.unioVi.es
Received January 23, 2001
ABSTRACT
â-Elimination of O-acetyl 1-chloro-1-trimethylsilylalkan-2-ols 1 was achieved by using samarium diiodide as a metaling reagent and afforded
the corresponding (Z)-vinylsilanes with high stereoselectivity. The starting compounds 1 were easily prepared by treatment of different aldehydes
with (chlorolithiomethyl)trimethylsilane and further acetylation.
Alkenylsilanes serve as important intermediates in organic
synthesis.1 In the literature, there are various methodologies
for their preparation.2 These methods utilize, generally,
alkynes,3 alkenyl halides,4 or carbonyl compounds5 as starting
materials. More specifically, using carbonyl compounds as
starting materials. several methodologies have been published
to promote the elimination reaction, but the use of samarium
diiodide remain unreported.
1,1-dihaloalkan-2-ols6 and the synthesis of (E)-R,â-unsatur-
ated esters with total stereoselectivity from the corresponding
2-halo-3-hydroxy esters.7 In both cases, elimination reactions
were promoted by samarium diiodide at room temperature,
the methodology being easy, simple, and general. Here, we
wish to report a new synthesis of (Z)-vinylsilanes with high
stereoselectivity by treatment of the easily available O-acetyl
1-chloro-1-trimethylsilylalkan-2-ols with samarium diiodide.
The reaction of several O-acetyl 1-chloro-1-trimethylsi-
lylalkan-2-ols 1 with SmI2 in THF at reflux gave the
corresponding (Z)-vinylsilane with high stereoselectivity and
in high yield (Scheme 1, Table 2).8
The starting material 1 can be easily prepared by reaction
of different aldehydes with (chlorolithiomethyl)trimethyl-
silane at -65 °C and further O-acetylation with acetic
anhydride (Scheme 2, Table 1).
The R-silyl carbanion was generated by lithiation of
(chloromethyl)trimethylsilane with sec-butyllithium and tet-
Recently, we have reported the preparation of (Z)-vinyl
halides with high stereoselectivity (ed >80%) from O-acetyl
(1) For a review, see: (a) Colvin, E. W. In ComprehensiVe Organo-
metallic Chemistry; Abel, E. W., Stone, F. G. A., Wilkinson, G., Eds.;
Pergamon Press: Oxford, 1995; Vol. 2, pp 5-8; Vol. 11, pp 315-317. (b)
Fleming, I.; Dunogues, J.; Smithers, R. H. Org. React. 1989, 37, 57-575.
(c) Chan, T. A.; Fleming, I. Synthesis 1979, 761-786.
(2) For a review, see: (a) Ojima, I. In The Chemistry of Silicon
Compounds; Patai, S., Rappoport, Z., Eds.; Wiley: New York, 1989; pp
1479-1520. (b) Takeda, T. In Synthesis of Organometallic Compounds;
Komiya, S., Ed.; Wiley: New York, 1997; p 391.
(3) (a) Dash, A. K.; Wang, J. Q.; Eisan, M. S. Organometallics 1999,
18, 4724-4741. (b) Bratovanov, S.; Ko´zm´ınski, W.; Fa¨ssler, J.; Molnar,
Z.; Nanz, D.; Bienz, S. Organometallics 1997, 16, 3128-3134. (c) Asao,
N.; Sudo, T.; Yamamoto, Y. J. Org. Chem. 1996, 61, 7654-7655. (d)
Soderquist, J. A.; Santiago, B. Tetrahedron Lett. 1990, 31, 5113-5116.
(e) Mitchell, T. N.; Wickenkamp, R.; Amamria, A.; Dicke, R.; Schneider,
U. J. Org. Chem. 1987, 52, 4868-4874.
Scheme 1. Synthesis of (Z)-Vinylsilanes
(4) (a) Murata, M.; Watanabe, S.; Masuda, Y. Tetrahedron Lett. 1999,
40, 9255-9257. (b) Fugami, K.; Oshima, K.; Utimoto, K.; Nozaki, H.
Tetrahedron Lett. 1986, 27, 2161-2164.
(5) (a) Itami, K.; Nokami, T.; Yoshida, J. Org. Lett. 2000, 2, 1299-
1302. (b) Ager, D. J. Org. React. 1990, 38, 1-223. (c) Soderquist, J. A.;
Anderson, C. L. Tetrahedron Lett. 1988, 29, 2425-2428.
10.1021/ol015601p CCC: $20.00 © 2001 American Chemical Society
Published on Web 02/21/2001

Scheme 2. Synthesis of Starting Compounds
Table 1. Yields of Starting Compounds 1 and 5
yield (%)
isomeric
entry
R
composition
5b
1c
1
2
2
4
5
6
7
8
9
Bu
71/29
75/25
68/32
67/33
71/29
74/26
85/15
69/31
82
81
81
79
85
78
77
84
79
81
86
85
86
84
88
85
84
87
88
90
Buc
i-Bu
Ph
p-ClC6H4
p-MeOC6H4
cyclohexyl
C7H15
used. (6) When 1-chloro-1-trimethylsilylalkan-2-ols (no
O-acetylated) were utilized no elimination reaction was
detected. (7) When the reaction was performed at room
temperature, neither â-elimination was observed.
MeCH(Ph)
Me2CH(CH2)2CH(Me)CH2
80/20
37/3613/12
The observed stereochemistry and the high Z-selectivity
of the elimination reaction can be explained by the formation
of the intermediate 2, in which the oxophilic Sm (III) center
is chelated with the carbonyl oxygen atom of the acetoxy
group producing a six-membered ring (Scheme 2).12 This
chelation increases the ability of the acetoxy group as a
leaving group. We surmise that the chairlike transition-state
model I (Scheme 3) might be involved with an equatorial R
10
a
Isomeric composition determined by 13C NMR of the crude products.
b Yield based on starting aldehyde 4. c Yield based on chlorohydrin 5.
ramethylethylenediamine (TMEDA) at temperatures ranging
between -78 and -65 °C.9
The diastereoisomeric excess was determined on the crude
1
reaction products by H NMR spectroscopy (200 or 300
MHz) and GC-MS. The results are summarized in Table
2. The Z stereochemistry in the double bond CdC of
products 3 was assigned on the basis of the value of the 1H
NMR coupling constant between the olefinic protons10 and
by comparison of their NMR spectra with those previously
reported (3a,3d 3d,3e,11b 3g11a).
Scheme 3. Proposed Mechanism
Several comments are worth noting. (1) Although the
elimination reaction was carried out using a mixture of
diastereoisomers (roughly 1:1) of 1, the corresponding
vinylsilanes 3 were obtained with high diastereoisomeric
excess (de). (2) The reaction was general to obtain vinylsi-
lanes derived from aliphatic (linear, branched or cycic)
aldehydes. (3) The synthesis of vinylsilanes can be carried
out at higher scale: 89% of 3h was obtained starting from
1 mmol of 1h. (4) Substitution at the silicon atom of 3 could
also be changed using a different R-silyl carbanion to prepare
compound 1 (entry 2). (5) Elimination took place with
moderate stereoselectivity when aromatic aldehydes were
group (to avoid 1,3-diaxial interactions) and an axial trim-
ethylsilyl group (to avoid interactions with the samarium
coordination sphere and taking into account that no 1,3-
diaxial interactions are present). Elimination from I gives
(Z)-vinylsilanes 3.
(6) Concello´n, J. M.; Bernad, P. L.; Pe´rez-Andre´s, J. A. Angew. Chem.
1999, 111, 2528-2530; Angew. Chem., Int. Ed. Engl. 1999, 38, 2384-
2386.
(7) Concello´n, J. M.; Pe´rez-Andre´s, J. A.; Rodr´ıguez-Solla, H. Angew.
Chem. 2000, 112, 2773-2775; Angew. Chem., Int. Ed. Engl. 2000, 39,
2866-2868.
(8) Representative procedure for the synthesis of (Z)-vinylsilanes 3: A
solution of SmI2 (2 mmol) in THF (24 mL) was added, under nitrogen
atmosphere, to a solution of the corresponding O-acetyl 1-chloro-1-
trimethylsilylalkan-2-ol 1 (0.4 mmol) in THF (4 mL). After being stirred
at reflux for 3.5 h, the mixture was treated with aqueous HCl (0.1 M, 10
mL), and after usual workup, the corresponding crude vinylsilanes 3 were
obtained, which were examined by 1H NMR and GC/MS to give the
diastereoisomeric excess reported in Table 2. Distillation at reduced pressure
provided pure (Z)-vinylsilanes 3.
(9) Representative procedure for the synthesis of staring compounds 1:
To a stirred solution of (chloromethyl)trimethylsilane (14.95 mL, 14 mmol)
in THF (21 mL) were successively added sec-butillithium (11.9 mL of 1.3
M solution in cyclohexane, 15.4 mmol) and TMEDA (2.1 mL, 14 mmol)
at -78 °C under nitrogen atmosphere. Stirring was continued for 40 min,
allowing the temperature to rise to -65 °C, and then a solution of the
corresponding carbonyl compound (10 mmol) in THF (3 mL) was added.
The resulting mixture was stirred for 2 h at temperature ranging between
-60 and -45 °C. The reaction was treated with a saturated aqueous solution
of NH4Cl (25 mL), and after usual workup, the corresponding crude
1-chloro-1-trimethylsilylalkan-2-ol was obtained. This alcohol (8 mmol)
was treated with pyridine (25 mL), acetic anhydride, and 4-(dimethylamino)-
pyridine (5 mg). After being stirred for 12 h, the mixture was treated with
ice, and after usual workup, the corresponding crude O-acetyl 1-chloro-1-
trimethylsilylalkan-2-ol 1 was obtained. The crude products 1 were used
without further purification.
Table 2. Synthesis of (Z)-Vinylsilanes 3
entry 3a
R
Z/Eb
yield (%)
1
2
3a Bu
3b Buc
>98/-
>98/-
97/3
80/20
68/32
15/85
97/3
>98/-
>98/-
95/5
96
94
95
95
95
94
95
96
92
94
3
4
3c i-Bu
3d Ph
5
3e p-ClC6H4
6
3f
p-MeOC6H4
7
8
3g Cyclohexyl
3h C7H15
9
10
3i
3j
MeCH(Ph)
Me2CdCH(CH2)2CH(Me)CH2
a Yield based on the starting crude acetate 1; all vinylsilanes were fully
characterized by spectroscopy methods (IR, NMR and MS). bDiastereo-
isomeric excess determined by GC/MS and 200 or 300 MHz H and 13C
1
c
NMR analysis of the crude products. (Z).-1-hexenyldimethylphenylsilane
obtained using (chlorolithiomethyl)dimethylphenylsilane.
938
Org. Lett., Vol. 3, No. 6, 2001

To explain the synthesis of 3 with high stereoselectivity
from a mixture of diastereoisomers, we assume that after
metalation by samarium(II) initially two diastereoisomers are
obtained. Of the two, the diastereoisomer in which the
coordination of the samarium(III) center with the carbonyl
oxygen atom is the most favored could eliminate directly,
while the other diastereoisomer epimerizes13 before the
elimination.
aromatic aldehyde with an electron-releasing substituent
(entry 6), since the presence of this substituent favored the
formation of the carbocation intermediate.
In conclusion, we have described a simple, general
synthesis of vinylsilanes, which takes place in high yield
and with high stereoselectivity from easily available starting
compounds.
The lower stereoselectivity observed when aromatic al-
dehydes were used can be explained by assuming a non-
concerted elimination reaction: the stabilization of the
positive charge by resonance would induce prior cleavage
of the C-O bond II (Scheme 3). Indirect support for this is
provided by the lower Z-selectivity obtained from an
Acknowledgment. We thank II Plan Regional de Inves-
tigacio´n del Principado de Asturias (PBP-PGI99-01) and the
Ministerio de Educacio´n y Cultura (PB97-1278) for financial
support. E.B. thanks the Universidad de Oviedo for a
predoctoral fellowship.
(10) Gu¨nter, H. NMR spectroscopy; Wiley: England, 1996; p 119.
(11) (a) Miller, R. B.; McGarvey, G. J. J. Org. Chem. 1978, 43, 4424-
4430. (b) Eisch, J. J.; Foxton, M. W. J. Org. Chem. 1971, 36, 3520-3525.
(12) A similar chairlike transition-state model has been proposed to
explain the selectivity in other SmI2 reactions: see ref 6 and references
therein.
(13) Reactions of SmI2 with enantiopure halides producing racemization
have been reported: (a) Kagan, H. B.; Girard, P.; Namy, J. L. Tetrahedron
1981, 109, 175-180. (b) Matsubara, S.; Yoshioka, M.; Utimoto, K. Angew.
Chem. 1997, 109, 631-633; Angew. Chem., Int. Ed. Engl. 1997, 36, 617-
618.
Supporting Information Available: Spectroscopic data
for all new compounds 1 and 3, H NMR spectra of 1 and
3, GC of compounds 3, and representative procedure for the
synthesis of compounds 1 by using 1 mmol of starting
material. This material is available free of charge via the
Internet at http://pubs.acs.org.
1
OL015601P
Org. Lett., Vol. 3, No. 6, 2001
939