One-pot transformation of RCHO to (E)-RCH=CHSiMe3 using CHI3, Mn, Me3SiCl, and a catalytic amount of CrCl2

Article abstract of DOI:10.1055/s-1999-2948

Iodoform is reduced with manganese in the presence of Me₃SiCl to give Me₃SiCHI₂ (1). A one-pot transformation of aldehydes to (E)-1- alkenyltrimethylsilanes is then performed with iodoform, manganese, Me₂SiCl, and a catalytic amount of chromium(II) chloride in THF via in situ formation of 1.

Full text of DOI:10.1055/s-1999-2948

LETTER
1769
One-Pot Transformation of RCHO to (E)-RCH=CHSiMe3 Using CHI3, Mn,
Me3SiCl, and a Catalytic Amount of CrCl2
Kazuhiko Takai,* Shintaro Hikasa, Tetsuya Ichiguchi, Naoki Sumino
Department of Applied Chemistry, Faculty of Engineering, Okayama University, Tsushima, Okayama 700-8530, Japan
Fax +81-86-251-8094; E-mail: ktakai@cc.okayama-u.ac.jp
Received 14 July 1999
Fürstner succeeded in reducing the amount of chromi-
Abstract: Iodoform is reduced with manganese in the presence of
um(II) by using manganese metal and Me3SiCl in the re-
Me3SiCl to give Me3SiCHI2 (1). A one-pot transformation of alde-
action of allyl- and alkenylchromium reagents.8
Therefore, the two disadvantages can be overcome using
the manganese metal.
hydes to (E)-1-alkenyltrimethylsilanes is then performed with iodo-
form, manganese, Me3SiCl, and a catalytic amount of chromium(II)
chloride in THF via in situ formation of 1.
Keywords: geminal dichromium reagent, geminal diiodo com-
pound, (E)-1-alkenylsilane, chromium(II), chromium(III), manga-
nese
Ph
Ph
CHO
SiMe3
THF, 25 °C
2
Me3SiCHX2 (2.0), CrCl2 (8.0)
X = Br
X = I
24 h 86% (E / Z = >99 / <1)
4 h 90% (E / Z = >99 / <1)
Manganese metal has a reducing ability between that of
samarium(II) and zinc, and is employed in organic synthe-
sis after appropriate activation.1 One of the typical meth-
ods for activating metals such as manganese is by
treatment with Me3SiCl.2 When active halogen com-
pounds were treated with manganese and Me3SiCl, reduc-
tion of the halogen and successive silylation was found to
occur. For example, treatment of iodoform with manga-
nese in THF in the presence of an excess amount of
Me3SiCl afforded Me3SiCHI2 (1) in 33% yield (eq 1).3 In
this reaction, a trimethylsilyl group selectively replaced
one of the three iodine atoms. The yield of 1 improved to
59%, when DME was used as a solvent and NaI was add-
ed.
24 h
CHI3 (3.0), Mn (9.0), Me3SiCl (9.0), CrCl2 (0.16)
74% (E / Z = >99 / <1)
Me3SiCHI2 (2.0), Mn (6.0)
Me3SiCl (6.0), CrCl2 (0.16)
24 h 87% (E / Z = >99 / <1)
Equation 2
A THF solution of iodoform was added to a mixture of
manganese, Me3SiCl, and a catalytic amount of CrCl2 in
THF. The mixture was stirred at 25 °C for 5 min, and 3-
phenylpropanal was added to the mixture. The resulting
mixture was stirred at 25 °C for 24 h. After usual workup
and purification, (E)-alkenylsilane 2 was produced stereo-
selectively in 74% yield along with 4-phenyl-1-butene (3)
in 11% yield (eq. 2); 4-Phenyl-1-iodo-1-butene (4), which
is produced by a reaction of iodoform and chromium(II)
chloride,9 was not detected.10 The transformations from
some aldehydes are summarized in Table 1. (E)-Isomers
of alkenylsilanes were selectively produced in all cases. A
ketone group was untouched during the transformation
(run 4).
Mn, Me3SiCl
CHI3
Me3SiCHI2
1
THF, 0 °C, 10 min
33%
DME, NaI, 25 °C, 1 h
59%
Equation 1
The selective iodine-silicon exchange of iodoform
prompted us to improve a chromium-based transforma-
tion of aldehydes into (E)-alkenylsilanes reported previ-
ously.4,5 In this transformation, a geminal dichromium
reagent was prepared by reduction of Me3SiCHBr2 with
chromium(II) chloride, and the reagent added to alde-
hydes without affecting the coexisting ketone and ester
groups due to the mild nucleophilicity of the organochro-
mium reagent.6 However, there are two disadvantages:
First, Me3SiCHBr2 should be prepared in advance by
deprotonation and silylation from dibromomethane,
which requires using a strong base under a carefully con-
trolled low temperature.7 Second, chromium(II) chloride
is a one-electron reductant, thus, 8 equiv. of the salt is usu-
ally necessary to obtain reasonable yields. Recently,
Synlett 1999, No. 11, 1769–1771 ISSN 0936-5214 © Thieme Stuttgart · New York

1770
K. Takai et al.
LETTER
Under standard reaction conditions (Table 1, run 1), com- tained in 74% yield (E / Z = 81 / 19) as a major product
pounds were recovered in the following order: 1- and alkenylsilane 2 was produced in only 9% yield.13 Ad-
dodecene (98%); 1-dodecyne (99%); 1-chlorododecane dition of iodoform and 3-phenylpropanal at the same time
(99%); ethyl octanoate (94%); nonanenitrile (92%). In in this catalytic CrCl2 system also increased the amount of
contrast to a stoichiometric reaction with CrCl2 in which iodoalkene 4. Iodoalkene 4, alkenylsilane 2, and alkene 3
nonanal ethylene acetal was recovered in 97% yield, the were produced in 52%, 21%, and 9% yields, respectively.
compound was recovered in only 24% yield during the Because the catalytic cycle A proceeds quickly,14 simulta-
transformation. This was probably due to the formation of neous addition of iodoform and an aldehyde produced a
Me3SiI.
similar effect as increasing the amount of CrCl2. Forma-
tion of iodoalkene 4 was suppressed by pre-stirring the
mixture of iodoform, Me3SiCl, manganese, and a catalytic
amount of CrCl2 at 25 °C for 5 min before addition of 3-
phenylpropanal, probably due to the consumption of iodo-
form leading to Me3SiCHI2.
A plausible mechanism for the CrCl2-manganese-promot-
ed formation of alkenylsilane is shown in Scheme 1.11 Io-
doform is reduced with 1 equiv. of manganese in THF to
form IMnCHI2, which is trapped with Me3SiCl to give
Me3SiCHI2. Reduction of Me3SiCHI2 with 4 equiv. of
chromium(II) gives the corresponding geminal dichromi-
um reagent, which reacts with an aldehyde followed by
elimination of X2Cr-O-CrX2 to afford an alkenylsilane.
The formed X2Cr-O-CrX2 is converted to chromium(III)
halide with Me3SiCl, and the chromium(III) is then re-
duced with manganese to reproduce chromium(II).12
Thus, 3 equiv. of both manganese and Me3SiCl are neces-
sary for this transformation.
Typical Procedure
Under an argon atmosphere, Me3SiCl (2.3 mL, 18 mmol)
was added at 25 °C to a suspension of CrCl2 (39 mg, 0.32
mmol),15 and manganese (0.99 g, 18 mmol)16 in THF (12
mL). After stirring the mixture at 25 °C for 30 min, a so-
lution of iodoform (2.4 g, 6.0 mmol) in THF (8 mL) was
added to the mixture at 25 °C over a period of 5 min, and
the mixture was stirred for 5 min. A solution of 3-phenyl-
propanal (0.27 g, 2.0 mmol) in THF (8 mL) was added to
the mixture at 25 °C over a period of 5 min and the result-
ing mixture was stirred at 25 °C for 24 h. The color of the
mixture gradually turned from dark red to brown while
stirring. The reaction mixture was poured into water (50
mL) and the mixture was extracted with hexane (3x40
mL). The organic extracts were washed with aqueous
Na2S2O3 and brine, dried over anhydrous MgSO4 and con-
centrated. Purification by column chromatography on sil-
ica gel (hexane) gave trimethyl((E)-4-phenyl-1-
butenyl)silane (2) in 74% yield (0.30 g, E / Z = >99 / <1)
as a colorless oil along with 4-phenyl-1-butene (3, 29 mg,
11%).
2 Me3SiCl
2 Mn
R
2 MnX2
(Me3Si)2O
I
(X2Cr)2O
CrX2
2 CrX3
2 CrX3
R
4 CrX2
I
OCrX2
H
I
CrX2
CrX2
CHI3
C
C
Catalytic Cycle A
Me3SiCl
Mn
RCHO
MnX2
CrX2
CrX2
Me3Si
Catalytic Cycle B
Me3SiCHI2
H
X= I or Cl
Acknowledgement
CrX2
SiMe3
OCrX2
Financial support by a Grant-in-Aid for Scientific Research on Prio-
rity Area No. 283 from the Ministry of Education, Science, Sports
and Culture of Japan is gratefully acknowledged.
2 CrX3
R
4 CrX2
2 CrX3
References and Notes
(X2Cr)2O
2 Me3SiCl
2 MnX2
(Me3Si)2O
R
(1) (a) Hiyama, T.; Sawahata, M.; Obayashi, M. Chem. Lett.
1983, 1237; Nippon Kagaku Kaishi, 1984, 1022. (b) Cahiez,
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Tetrahedron Lett. 1996, 37, 7094. (d) Takai, K.; Ueda, T.;
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Ikeda, N. J. Org. Chem. 1997, 62, 8612. (g) Li, C.-J.; Meng,
Y.; Yi, X.-H. J. Org. Chem. 1997, 62, 8632.
2 Mn
SiMe3
Scheme 1
Two important factors for the preferential formation of an
alkenylsilane over an iodoalkene are shown to be 1) a cat-
alytic amount of CrCl2, and 2) a mixture of iodoform,
Me3SiCl, manganese, and CrCl2, which is stirred for 5 min
before addition of an aldehyde. For example, when the
amount of CrCl2 was increased to stoichiometric (9.0
equiv.) in the typical procedure (eq. 2, line 3), the product
distribution changed markedly; Iodoalkene 4 was ob-
(2) (a) Gaudemar, M. Bull. Soc. Chim. Fr. 1962, 974.
(b) Knochel, P.; Yeh, M. C. P.; Berk, S. C.; Talbert, J. J. Org.
Chem. 1988, 53, 2390. (c) Fürstner, A.; Hupperts, A. J. Am.
Chem. Soc. 1995, 117, 4468.
Synlett 1999, No. 11, 1769–1771 ISSN 0936-5214 © Thieme Stuttgart · New York

LETTER
One-Pot Transformation of RCHO
1771
(3) Seyferth, D.; Lambert, R. L., Jr.; Hanson, E. M. J. Org. Chem.
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recovered in 66% yield along with alkene 3 in 20% yield. The
result suggests that alkenylsilane 2 was not produced via
reductive iodine-silicone exchange of iodoalkene 4.
(12) A catalytic amount of CrCl3 can be used for the
transformation, however the yields are somewhat lower than
those with CrCl2 (Table 1). Chromium(III) hexahydrate
(99.995% purity) was purchased from Aldrich Chemical Co.
and dehydrated by treatment with thionyl chloride. Pray, A. R.
Inorg. Synth. 1954, 5, 153.
(6) For some reviews of organochromium reagents, see:
(a) Takai, K.; Utimoto, K. J. Synth. Org. Chem., Jpn. 1988, 46,
66; (b) Saccomano, N. A. In Comprehensive Organic
Synthesis, Vol. 1; Trost B. M., Ed; Pergamon Press: Oxford,
1991; p 173; (c) Cintas, P. Synthesis 1992, 248.
(d) Wessjohann, L. A.; Scheid, G. Synthesis 1999, 1.
(e) Fürstner, A. Chem. Rev. 1999, 99, 991.
(13) The reaction was finished within 1 h at 25 °C.
(14) The chromium(II)-reduction of Me3SiCHI2 to the
corresponding geminal dichromium reagent (Catalytic Cycle
B) proved to proceed slower than that of iodoform (Catalytic
Cycle A). For example, addition of 3-phenylpropanal to a
mixture of iodoform (0.1 equiv.), Me3SiCHI2 (0.1 equiv.), and
CrCl2 (0.4 equiv.) at 25 °C, followed by stirring for 3 h
produced iodoalkene 4 in 73% yield (based on CrCl2);
Alkenylsilane 2 was not detected. Recovery of iodoform and
Me3SiCHI2 were in 7% and 79% yields, respectively.
(15) Chromium(II) chloride (99.9% purity) was purchased from
Aldrich Chemical Co.
(7) Villieras, J.; Bacquet, C.; Normant, J.-F. Bull. Soc. Chim. Fr.
1975, 1797.
(8) Fürstner, A.; Shi, N. J. Am. Chem. Soc. 1996, 118, 2533;
Fürstner, A.; Shi, N. J. Am. Chem. Soc. 1996, 118, 12349.
(9) (a) Takai, K.; Nitta, K.; Utimoto, K. J. Am. Chem. Soc. 1986,
108, 7408. (b) Takai, K.; Ichiguchi, T.; Hikasa, S. Synlett
1999, 1268.
(10) 4-Phenyl-1-iodo-1-butene (4) was reduced to 4-phenyl-1-
butene (3) under the reaction conditions (25 °C, 24 h).
(11) Stirring a mixture of iodoalkene 4, iodoform, manganese, and
Me3SiCl, and a catalytic amount of chromium(II) chloride at
25 °C for 24 h did not afford alkenylsilane 2; iodoalkene 4 was
(16) Manganese powder was purchased from Kojundo Chemical
Laboratory Co. (99.9% purity, -50 mesh). The powder was
used for reactions without purification.
Article Identifier:
1437-2096,E;1999,0,11,1769,1771,ftx,en;Y14999ST.pdf
Synlett 1999, No. 11, 1769–1771 ISSN 0936-5214 © Thieme Stuttgart · New York