One-step preparation of functionalized (E)-vinylsilanes from aldehydes

Article abstract of DOI:10.1021/ol201833u

Functionalized (E)-vinylsilanes have been prepared in one step from a wide range of aldehydes, via a chromium(II)-mediated olefination with novel dihalomethylsilane reagents, in moderate to excellent yields and with excellent stereoselectivity.

Full text of DOI:10.1021/ol201833u

ORGANIC
LETTERS
2011
Vol. 13, No. 18
4806–4809
One-Step Preparation of Functionalized
(E)-Vinylsilanes from Aldehydes
Diane S. W. Lim and Edward A. Anderson*
Chemical Research Laboratory, University of Oxford, 12 Mansfield Road, Oxford OX1
3TA, United Kingdom
edward.anderson@chem.ox.ac.uk
Received July 8, 2011
ABSTRACT
Functionalized (E)-vinylsilanes have been prepared in one step from a wide range of aldehydes, via a chromium(II)-mediated olefination with novel
dihalomethylsilane reagents, in moderate to excellent yields and with excellent stereoselectivity.
Organosilanes are versatile functionalities in the synthetic
chemist’s toolbox, offering a convenient and inexpensive
means to create new carbonÀcarbon1 or carbonÀhetero-
atom1b,2 bonds while remaining compatible with a range of
chemical manipulations.3 For example, vinylsilanes have been
employed with great efficacy in organic synthesis, as direct
mediators of carbonÀcarbon bond formation through the
nucleophilic capture of cations,1b,4 in Hiyama cross-coupling
reactions,1c,5 and also as substrates for Tamao-Fleming
oxidations in which they represent masked carbonyl groups.6
Hiyama cross-coupling in particular represents an extre-
mely important application, which usually requires the silane
to be functionalized with a heteroatom (or other readily
substituted group)7 so that it can be activated toward
transmetalation on treatment with a fluoride source or a
base.8 “Safety-catch” silanols, which are stable to conditions
(6) (a) Tamao, K.; Kumada, M.; Maeda, K. Tetrahedron Lett. 1984,
25, 321–324. (b) Tamao, K.; Maeda, K.; Tanaka, T.; Ito, Y. Tetrahedron
Lett. 1988, 29, 6955–6956. (c) Marshall, J. A.; Yanik, M. M. Org. Lett.
2000, 2, 2173–2175. (d) Sunderhaus, J. D.; Lam, H.; Dudley, G. B. Org.
Lett. 2003, 5, 4571–4573. (e) Trost, B. M.; Ball, Z. T.; Joge, T. Angew.
Chem., Int. Ed. 2003, 42, 3415–3418. (f) Spletstoser, J. T.; Zacuto, M. J.;
Leighton, J. L. Org. Lett. 2008, 10, 5593–5596.
(7) The seminal report of trimethylvinylsilane cross-coupling showed
limited scope with respect to the silane. See: Hatanaka, Y.; Hiyama, T. J.
Org. Chem. 1988, 53, 918–920.
(1) (a) Bracegirdle, S.; Anderson, E. A. Chem. Soc. Rev. 2010, 39,
4114–4129. (b) Fleming, I.; Dunogues, J.; Smithers, R. Organic Reac-
ꢀ
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Compounds in Cross-Coupling Reactions. In Metal-Catalyzed Cross-
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Walter, D. Chem. Rev. 1997, 97, 2063–2192. (f) Masse, C. E.; Panek, J. S.
Chem. Rev. 1995, 95, 1293–1316. (g) Langkopf, E.; Schinzer, D. Chem.
Rev. 1995, 95, 1375–1408.
(8) For comparisons of functionalized silanes in Hiyama couplings,
see: (a) Denmark, S. E.; Neuville, L.; Christy, M. E. L.; Tymonko, S. A.
J. Org. Chem. 2006, 71, 8500–8509. (b) Denmark, S. E.; Butler, C. R. J.
Am. Chem. Soc. 2008, 130, 3690–3704. (c) Montenegro, J.; Bergueiro, J.;
Saa, C.; Lopez, S. Org. Lett. 2009, 11, 141–144. For specific studies, see
Fluorosilanes: (d) Hatanaka, Y.; Hiyama, T. J. Org. Chem. 1989, 54,
265–270. Siloxanes: (e) Tamao, K.; Kobayashi, K.; Ito, Y. Tetrahedron
Lett. 1989, 30, 6051–6054. (f) Denmark, S. E.; Pan, W. T. Org. Lett.
2001, 3, 61–64. (g) Denmark, S. E.; Yang, S. M. Org. Lett. 2001, 3, 1749–
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11768–11778. Silyl hydrides: (i) Denmark, S. E.; Neuville, L. Org. Lett.
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Am. Chem. Soc. 2007, 129, 2774–2776. Silanolates: (m) Denmark, S. E.;
Sweis, R. F. J. Am. Chem. Soc. 2001, 123, 6439–6440. Disiloxanes: (n)
Denmark, S. E.; Pan, W. T. Org. Lett. 2002, 4, 4163–4166. For a recent
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r
10.1021/ol201833u
Published on Web 08/12/2011
2011 American Chemical Society

that heteroatom-substituted silanes may not tolerate but
which can nonetheless be activated toward cross-coupling
in a similar manner, are a particularly important subclass.8k,9
Regio- and stereoselective access to functionalized vinylsi-
lanes has primarily been achieved via the transition metal-
catalyzed hydrosilylation of alkynes10 and from vinyl halides
via metalÀhalogen exchange/carbanion silylation.11 As these
vinylsilane precursors are themselves often prepared from
aldehydes, a direct synthesis of functionalized vinylsilanes
from aldehydes would represent a valuable means to bypass
an additional functional group manipulation. To our knowl-
edge, the only direct preparation of functionalized vinylsi-
lanes from aldehydes has been reported by Yoshida and co-
workers and involves the formation of 2-pyridyldimethylsilyl
alkenes through the Peterson olefination of a bis-pyridylsilyl
carbanion.12 While this route affords (E)-vinylsilanes in good
yields and excellent stereoselectivity, the reaction is necessa-
rily conducted under strongly basic conditions, which may
limit functional group tolerance. In contrast, the preparation
of nonfunctionalized vinylsilanes (i.e., trialkylvinylsilanes)
from aldehydes has much greater precedent. In particu-
lar, the Takai silylolefination of aldehydes with chro-
mium(II) chloride and Me3SiCHBr2, which affords
Scheme 1. Preparation of Dihalomethyl Benzyldimethylsilane
Reagents 1 and 2
(E)-vinylsilanes with high stereoselectivity,13 is well-
recognized as a valuable and reliable tool for the stereo-
selective synthesis of alkenyl trimethylsilanes.
In the course of investigations into the total synthesis of
polyene-containing natural products, we required a mild and
selective method for the synthesis of a vinylsilane suitable for
Hiyama coupling and recognized that application of a
Takai-type olefination using a functional dihalomethylsi-
lane reagent might offer a solution to this problem. While
the Takai olefination has been extended to the synthesis of
vinylboronic esters using pinBCHBr2,13c,14 an analogous
“functionalized” silylation has not yet been realized but
would certainly represent a powerful means to access such
useful vinylsilanes. We report herein the development of this
process and its application to a wide range of aldehydes.
Our investigations began with the selection of the benzyl-
dimethylsilyl group as a representative “safety-catch” silanol,
which has been shown to possess stability toward acidic and
basic reaction conditions but can be activated by a fluoride
source to participate in cross-coupling8k,9aÀ9f and oxida-
tion reactions.6e,9c,15 The dihalomethyl benzyldimethyl-
silane reagents 1 and 2 (Scheme 1) were conveniently
prepared on a multigram scale by treating the correspond-
ing dihalomethanes with LDA, followed by trapping of
the lithium carbanion with benzylchlorodimethylsilane
(Scheme 1).16 Dibromide 1 could also be prepared from
the Grignard reagent formed in situ from treatment
of bromoform with iso-propylmagnesium chloride.17
Although 1 can be stored indefinitely at room tempera-
ture without decomposition, diiodide 2 gradually turns
(9) Benzyldimethylsilanes: (a) Trost, B. M.; Machacek, M. R.; Ball,
Z. T. Org. Lett. 2003, 5, 1895–1898. (b) Trost, B. M.; Frederiksen, M. U.;
Papillon, J. P. N.; Harrington, P. E.; Shin, S.; Shireman, B. T. J. Am.
Chem. Soc. 2005, 127, 3666–3667. (c) Denmark, S. E.; Fujimori, S. J.
Am. Chem. Soc. 2005, 127, 8971–8973. (d) Denmark, S. E.; Liu, J. H. C.
J. Am. Chem. Soc. 2007, 129, 3737–3744. (e) Zhang, Y.; Panek, J. S. Org.
Lett. 2007, 9, 3141–3143. (f) Morrill, C.; Mani, N. S. Org. Lett. 2007, 9,
1505–1508. Siletanes: (g) Denmark, S. E.; Choi, J. Y. J. Am. Chem. Soc.
1999, 121, 5821–5822. (h) Denmark, S. E.; Wehrli, D.; Choi, J. Y. Org.
Lett. 2000, 2, 2491–2494. 2-Pyridylsilanes: (i) Itami, K.; Nokami, T.;
Ishimura, Y.; Mitsudo, K.; Kamei, T.; Yoshida, J.-i. J. Am. Chem. Soc.
2001, 123, 11577–11585. Intramolecular activation of vinylsilanes: (j)
Shindo, M.; Matsumoto, K.; Shishido, K. Synlett 2005, 176–178. (k)
Nakao, Y.; Imanaka, H.; Sahoo, A. K.; Yada, A.; Hiyama, T. J. Am.
Chem. Soc. 2005, 127, 6952–6953.
(10) (E)-selective hydrosilylation: (a) Denmark, S. E.; Wang, Z. G.
Org. Lett. 2001, 3, 1073–1076. (b) Takahashi, T.; Bao, F. Y.; Gao, G. H.;
Ogasawara, M. Org. Lett. 2003, 5, 3479–3481. (c) Aneetha, H.; Wu, W.;
Verkade, J. G. Organometallics 2005, 24, 2590–2596. (d) Marko, I. E.;
Berthon-Gelloz, G.; Schumers, J. M.; De Bo, G. J. Org. Chem. 2008, 73,
4190–4197. See also refs, 6b, 6c, 8g, and 8h. (Z)-selective: (e) Na, Y. G.;
Chang, S. B. Org. Lett. 2000, 2, 1887–1889. (f) Maifeld, S. V.; Tran,
M. N.; Lee, D. Tetrahedron Lett. 2005, 46, 105–108. R-selective: (g)
Trost, B. M.; Ball, Z. T. J. Am. Chem. Soc. 2005, 127, 17644–17655. (h)
Kawanami, Y.; Sonoda, Y.; Mori, T.; Yamamoto, K. Org. Lett. 2002, 4,
2825–2827. Tuneable selectivity: (i) Faller, J. W.; D’Alliessi, D. G.
Organometallics 2002, 21, 1743–1746. (j) Katayama, H.; Taniguchi, K.;
Kobayashi, M.; Sagawa, T.; Minami, T.; Ozawa, F. J. Organomet.
Chem. 2002, 645, 192–200. (k) Mori, A.; Takahisa, E.; Yamamura, Y.;
Kato, T.; Mudalige, A. P.; Kajiro, H.; Hirabayashi, K.; Nishihara, Y.;
Hiyama, T. Organometallics 2004, 23, 1755–1765. For a recent review,
see: (l) Trost, B. M.; Ball, Z. T. Synthesis 2005, 853–887.
(11) For other classical routes to vinylsilanes, see: (a) Chan, T. H.;
Fleming, I. Synthesis 1979, 761–786. For other transition metal-
catalyzed routes to vinylsilanes, see: (b) Yamashita, H.; Roan, B. L.;
Tanaka, M. Chem. Lett. 1990, 2175–2176. (c) Pietraszuk, C.; Fischer, H.;
Rogalski, S.; Marciniec, B. J. Organomet. Chem. 2005, 690, 5912–5921.
For the silylcupration of alkynes, see: (d) Loh, T. P.; Wang, P.; Yeo,
X. L. J. Am. Chem. Soc. 2011, 133, 1254–1256. (e) Barbero, A.; Pulido,
F. J. Acc. Chem. Res. 2004, 37, 817–825.
(12) (a) Itami, K.; Nokami, T.; Yoshida, J. Org. Lett. 2000, 2, 1299–
1302. (b) Itami, K.; Nokami, T.; Yoshida, J. Tetrahedron 2001, 57, 5045–
5054. (c) Itami, K.; Mitsudo, K.; Nokami, T.; Kamei, T.; Koike, T.;
Yoshida, J. J. Organomet. Chem. 2002, 653, 105–113.
(14) (a) Takai, K.; Shinomiya, N.; Kaihara, H.; Yoshida, N.; Moriwake,
T.; Utimoto, K. Synlett 1995, 963–964. For recent applications in synthesis,
see: (b) Onodera, Y.; Suzuki, T.; Kobayashi, S. Org. Lett. 2011, 13, 50–53. (c)
Inoue, A.; Kanematsu, M.; Yoshida, M.; Shishido, K. Tetrahedron Lett.
2010,51, 3966–3968. (d) Kanematsu, M.; Shindo, M.; Yoshida, M.; Shishido,
K. Synthesis 2009, 2893–2904. (e) Cheung, L. L.; Marumoto, S.; Anderson,
C. D.; Rychnovsky, S. D. Org. Lett. 2008, 10, 3101–3104.
(15) Trost, B. M.; Ball, Z. T.; Laemmerhold, K. M. J. Am. Chem. Soc.
2005, 127, 10028–10038.
(16) (a) Inoue, A.; Kondo, J.; Shinokubo, H.; Oshima, K. Chem.;
Eur. J. 2002, 8, 1730–1740. (b) Charette, A. B.; Bull, J. A. J. Org. Chem.
2008, 73, 8097–8100.
(17) Seyferth, D.; Lambert, R. L.; Hanson, E. M. J. Organomet.
Chem. 1970, 24, 647–661.
(13) (a) Takai, K.; Kataoka, Y.; Okazoe, T.; Utimoto, K. Tetrahe-
dron Lett. 1987, 28, 1443–1446. (b) Takai, K.; Hikasa, S.; Ichiguchi, T.;
Sumino, N. Synlett 1999, 1769–1771. (c) Takai, K.; Kunisada, Y.;
Tachibana, Y.; Yamaji, N.; Nakatani, E. Bull. Chem. Soc. Jpn. 2004,
77, 1581–1586.
Org. Lett., Vol. 13, No. 18, 2011
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from a colorless to brown oil on storage for a month in the
dark at 4 °C.
Table 2. Preparation of (E)-Vinylsilanes from Aromatic and
Heteroaromatic Aldehydes
Reaction optimization studies were performed on ben-
zaldehyde (Table 1), which we initially subjected to stan-
dard Takai olefination conditions of 2 equiv of 1 and 8
equiv of chromium(II) chloride in dry THF.13a,18 To our
delight, this resulted in complete consumption of benzal-
dehyde after 16 h and provided the desired vinylsilane 3a in
74% isolated yield, with exclusive formation of the (E)-
vinylsilane (entry 1). Reducing the equivalents of 1 and
CrCl2 to 1.3 and 6.0, respectively, resulted in complete
conversion on the same time scale and a slightly improved
yield (entry 2), although further efforts to lower the
stoichiometry of either reagent led to incomplete conver-
sion even after 24 h (entries 3 and 4).19
In an effort to reduce the reaction time scale, we next tested
the olefination at elevated temperatures. Pleasingly, reaction
at 50 °C led to complete conversion in just two hours, with no
detriment to the stereoselectivity (E:Z > 99:1, entry 5).
We anticipated that the diiodosilane reagent 2 might offer
benefits over 1 in terms of reactivity and indeed found that
Table 1. Silylolefination Optimization Studies
silane CrCl2 temp time
entry (equiv) (equiv) (°C) (h)
yield (%)a
(conversion (%))b E:Zb
1
2
3
4
5
6
7
1 (2.0)
1 (1.3)
1 (1.2)
1 (1.3)
1 (1.3)
2 (1.3)
2 (1.3)
8.0
6.0
6.0
4.0
6.0
6.0
6.0
25
25
25
25
50
25
50
16
74 (100)
82 (100)
74 (92)
>99:1
> 99:1
>99:1
>99:1
> 99:1
>99:1
> 99:1
16
24
a Reaction conditions: A: BnMe2SiCHBr2 (1.3 equiv), CrCl2
(6.0 equiv), THF, 25 °C; B: BnMe2SiCHBr2 (1.3 equiv), CrCl2
(6.0 equiv), THF, 50 °C; C: BnMe2SiCHI2 (1.3 equiv), CrCl2 (6.0 equiv),
THF, 50 °C. b Isolated yields; c Determined by 1H NMR spectroscopic
analysis of the crude reaction mixture. d Z:E = 13:1. e Quenched with
aqueous EDTA.Na2.
24
53 (71)
2
75 (100)
76 (100)
77 (100)
6.5
0.75
a Isolated yield. b Determined by 1H NMR spectroscopic analysis of
the crude reaction mixture.
generally afforded good to excellent yields of the corre-
sponding vinylsilanes 3bÀf when treated with 1 (entries
1À5), with electron-rich, electron-poor, halogen, and boro-
nic ester substituents all tolerated by the mild reaction
conditions. In the latter case, heating of the reaction mixture
(Conditions B) led to a slightly improved yield, and in all
instances, complete selectivity for the (E)-vinylsilane was
observed. The exception to this selection of substrates was
p-nitrobenzaldehyde, for which no product was detected in
the crude reaction mixture despite complete consumption of
the starting material (entry 6), likely reflecting the suscept-
ibility of the nitro group to reduction by chromium(II).20
this reagent maintained both the yield and selectivity of the
olefination, but significantly reduced the reaction time at
room temperature (entry 6). The most rapid silylolefination
could be achieved using 2 at 50 °C, which reduced the
reaction time to just 45 min (77%, E:Z > 99:1, entry 7).
In our subsequent substrate evaluations, conditions corre-
sponding to entries 2, 5, and 7 were employed.
With this set of optimal reaction conditions established, a
range of aldehydes were tested to determine the scope of the
silylolefination. Initial screening of aromatic aldehydes
(Table 2) revealed that para-substituted benzaldehydes
(20) Akita, Y.; Inaba, M.; Uchida, H.; Ohta, A. Synthesis 1977, 792–
794.
(21) The geometry of the alkene was characterised by a 3JHH value of
15.0 Hz, which is typical for (Z)-vinylsilanes; see ref 10e.
(22) Auge, J.; Boucard, V.; Gil, R.; Lubin-Germain, N.; Picard, J.;
Uziel, J. Synth. Commun. 2003, 33, 3733–3739.
(18) Takai, K.; Nitta, K.; Utimoto, K. J. Am. Chem. Soc. 1986, 108,
7408–7410.
(19) Chromium(II) chloride from a variety of sources (Sigma Aldrich
(anhydrous, 99.99%) and Alfa Aesar (anhydrous, 97%)) was used.
(23) See the Supporting Information for details.
4808
Org. Lett., Vol. 13, No. 18, 2011

and 5). Vinylsilane 3r, bearing a terminal alkyne and which
would likely prove challenging to prepare via hydrosilyla-
tion, was isolated from hex-5-ynal in good yield (entry 6).
To demonstrate the applicability of the reaction in more
complex settings, a series of aldehydes containing other
functional groups or stereogenic centers were subjected to
the olefination. Vinylsilanes 3sÀ3u were obtained with
excellent stereoselectivity (E/Z > 99:1), with little or no
epimerization of the adjacent stereocenters,23 suggesting
the reaction to be well suited for applications in synthesis.
Finally, we aimed to demonstrate the utility of the silylo-
lefination through an application of a vinylsilane in cross-
coupling. We selected the boronic ester product 3f, which we
could subject to sequential Hiyama and Suzuki cross-cou-
plings. The orthogonal nature of the activation of
silicon and boron cross-coupling partners enabled us
to perform these operations in either order (Scheme 2).
Thus, Suzuki coupling of 3f with 4-iodotoluene
gave the intermediate silane 4 in 79% yield; subse-
quent fluoride activation of the benzyldimethylsilyl
group and Hiyama coupling with 4-iodoanisole pro-
vided the arylated product 5 (73%). Switching the
order of these events resulted in an equally efficient
synthesis (76 and 73% for the Hiyama and Suzuki
couplings, respectively).
Table 3. Preparation of (E)-Vinylsilanes from Conjugated and
Aliphatic Aldehydes
In conclusion, we have developed a mild and highly
stereoselective method for the preparation of vinylsilanes
suitable for cross-coupling or oxidation chemistry, via the
chromium-mediated coupling of dihalomethylsilanes with
aldehydes. Applications of this method in complex mole-
cule synthesis will be reported in due course.
a Reaction conditions: A: BnMe2SiCHBr2 (1.3 equiv), CrCl2 (6.0
equiv), THF, 25 °C; B: BnMe2SiCHBr2 (1.3 equiv), CrCl2 (6.0 equiv),
THF, 50 °C. b Isolated yields. c Determined by 1H NMR spectroscopic
analysis of the crude reaction mixture. d 99% ee. e 74% ee; the aldehyde
was prepared in 80% ee . f 60% ee.
In a manner reminiscent of other Takai olefinations, the
reaction proved selective for aldehydes in the presence of
ketones, as illustrated by the conversion of 3-acetylbenzal-
dehyde to 3h (Table 2, entry 7). The presence of ortho
substituents on the arene necessitated the use of heated
reaction conditions (entries 8 and 9), with mesitaldehyde
giving a low yield of silane 3j, which intriguingly was
formed in a 13:1 (Z:E) ratio.21
Scheme 2. Applications of Benzyldimethylvinylsilanes
The reaction of heteroaromatic aldehydes proved more
challenging; furfural required an extended reaction time to
achieve a satisfactory yield of 3k (Table 2, entry 10),
although this could be improved to 2 h by employing
Conditions B. In the case of nicotinaldehyde (entry 11),
no product was detected despite complete consumption of
1
starting material. Broadening of signals in the H NMR
spectrum of the crude reaction mixture suggested complexa-
tion of the pyridine to chromium, and pleasingly, quenching
the reaction mixture with aqueous EDTA-Na2 to sequester
the chromium(III) salts successfully liberated the pyridyl
group,22 enabling the isolation of 3l in moderate yields.
We next screened a range of nonaromatic aldehydes in
the silylolefination (Table 3). Simple aliphatic aldehydes
gaveexcellent yields (entries 1À3) withpivaldehyde being a
particularly notable substrate that underwent complete
conversion to 3o. R,β-Unsaturated aldehydes readily pro-
vided dienes 3p and 3q, which are valuable building blocks
for the synthesis of polyunsaturated compounds (entries 4
Acknowledgment. We thank the EPSRC for an Ad-
vanced Research Fellowship (E.A.A.) (EP/E055273/1)
and A*STAR for a National Science Scholarship (D.L.).
Supporting Information Available. Experimental pro-
cedures and spectroscopic data for reagents 1 and 2 and
vinylsilanes. This material is available free of charge via
the Internet at http://pubs.acs.org.
Org. Lett., Vol. 13, No. 18, 2011
4809