Iridium-catalysed asymmetric hydrogenation of vinylsilanes as a route to optically active silanes

Article abstract of DOI:10.1002/adsc.200600279

The first use of vinylsilanes as substrates in the asymmetric iridium-catalysed hydrogenation is reported, providing products with enantioselectivities of up to 98%.

Full text of DOI:10.1002/adsc.200600279

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DOI: 10.1002/adsc.200600279
Iridium-Catalysed Asymmetric Hydrogenation ofVinylsilanes as
a Route to Optically Active Silanes
a
a
a
a,
Klas Källstrçm, Ian J. Munslow, Christian Hedberg, and Pher G. Andersson *
a
Department of Biochemistry and Organic Chemistry, Box 576, 751 23 Uppsala, Sweden
Fax : (+46)-18-471-3818; e-mail: Pher.Andersson@kemi.uu.se
Received: June 14, 2006; Accepted: October 5, 2006
Supporting information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
Abstract: The first use of vinylsilanes as substrates
in the asymmetric iridium-catalysed hydrogenation
is reported, providing products with enantioselectiv-
ities of upto 98%.
rate and enantioselectivity we started with a compari-
son of three different ligand classes, our newly devel-
oped thiazole complex 1, Pfaltz Ir-PHOX complex 2
and another phosphine-oxazoline complex
3
[
3,11,12]
(
Figure 1).
Keywords: asymmetric; hydrogenation; iridium; vi-
nylsilanes
The hydrogenation of functionalised olefins is one of
[
1]
the most studied enantioselective catalytic reactions.
While the ruthenium- and rhodium-catalysed asym-
metric hydrogenations of chelating olefins have a long
history, unfunctionalised olefins still represent a chal-
[
2]
Figure 1. Ir complexes used in asymmetric hydrogenation of
lenging class of substrates. During the last few years,
À
[
3a–e]
[4a–i]
vinyl silanes. The anion in all cases is B
A
H
R
U
G
A
H
R
U
Pfaltz
and others
have used phosphine-oxazo-
[5]
(BAr ).
F
line ligands as chiral mimics of Crabtreeꢀs catalyst,
which have been used successfully for asymmetric hy-
drogenations of arylalkenes. Recently, the composi-
All of the catalysts evaluated gave full conversion
tion of ligands has expanded to also include oxazo- within 12 h, with complex 1 proving the most selective
[
6]
line-carbene ligands and, chiral-at-sulphur, sulphox- catalyst for the evaluated substrates (Entries 1 and 2,
[
7]
ide-phosphine ligands. However, iridium-catalysed Table 1). Complexes 2 and 3 hydrogenated substrate 4
asymmetric hydrogenation is still highly substrate de- (Entry 1) with slightly lower selectivities 25% and
pendent and the development of new efficient chiral 26% ee, respectively, when compared to complex 1,
ligands that tolerate a broad range of substrates re- 28% ee. When the trimethylsilyl (TMS) groupwas
mains a challenge. With the exception of a few recent- moved into the non-prochiral position (Entry 2) com-
ly reported examples of the asymmetric reduction of plex 1 still proved to be the most selective catalyst, al-
[
8,9]
some substituted N-heteroaromatics,
further devel- though surprisingly complex 2 resulted in a racemic
opment of new substrate classes has not yet been product, whereas complex 3 gave a slightly lower ee
[2]
fully exploited. As part of our ongoing development (96%). Complex 1 was therefore evaluated with a
of Ir-catalysed asymmetric hydrogenation, we have range of vinylsilane substrates, which were readily
[
13–15]
expanded our research to include new substrate prepared according to published procedures.
Cat-
classes in parallel with catalyst development. alyst 1 can be prepared according to our previously
[
11]
Organosilanes are important organic intermediates published procedure, these complexes have the ad-
and a number of innovative new organosilicon drugs vantage over, e.g., Rh complexes, of being air- and
[
10a–c]
are in development,
ated the Ir-catalysed reduction of a new class of sub- freezer without any detectable decomposition.
strates, vinylsilanes. In order to find a catalyst that Primarily, we were interested in substrates of the
can hydrogenate the substrates with both satisfactory type 4 due to the TMS groupbeing located on the
in this report we have evalu- moisture-stable; they can be stored for months in the
Adv. Synth. Catal. 2006, 348, 2575 – 2578
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Klas Källstrçm et al.
Product
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[
a]
Table 1. Asymmetric reduction of vinylsilanes.
[b]
Entry
Substrate
Complex
Conversion [%]
ee [%]
[
c]
[16]
[17]
1
2
3
>99
>99
>99
28 (S)
25 (R)
26 (R)
1
[c]
1
2
3
>99
>99
>99
98 (R)
rac.
2
3
96 (S)
[
c]
[18]
[f]
1
>99
58 (S)
[c]
4
5
1
1
>99
>99
48 (À)
[
d]
[f]
55 (+)
[
e]
[19]
6
1
>99
55 (S)
[
[
[
[
[
a]
b]
c]
d]
e]
Conditions: pressure, 30 bar; all reactions were run at room temperature for 12 h in CH Cl ; catalyst loading, 0.5 mol%.
2
2
1
Determined by H NMR.
GC-MS (G-TA, 608C, 30 min, 58Cmin , 758C, 100 kPa).
GC (CP-Chirasil-Dex, 608C, 1.58Cmin , 1208C).
À1
À1
GC-MS (G-TA, 908C, 20 min, 100 kPa). Absolute configurations were determined by optical rotation and comparison
with literature data.
Absolute configuration unknown.
[
f]
prochiral carbon, however, with all the catalysts
tested only relatively poor enantioselectivities were
obtained. Changing the substitution pattern of the
double bond, i.e., exchanging the silyl and methyl
groups (substrate 5), resulted in a product with 98%
ee, using catalyst 1. Disappointingly the remaining
substrates tested, 6, 7, 8 and 9, only gave rise to mod-
Scheme 1. Converting the -SiMe Ph moiety into an alcohol
2
erate ees, 58, 28, 55 and 55%, respectively. Interest- function (10).
ingly substrate 8 is one of the few examples of a sub-
strate lacking an aromatic ring being used in asym-
[
20]
metric Ir-catalysed hydrogenations. Substrate 9 is a
Our previous findings have suggested that sub-
potentially appealing compound as the resulting re- strates possessing a polarised double bond, like a,b-
duced vinylsilane can be converted into its corre- unsaturated esters, introduce an electronic effect into
[
11]
sponding alcohol. Oxidative cleavage of the dimethyl- the selectivity of the system. DFT calculations on
phenylsilyl group, using a modified protocol of the trans-b-methyl cinnamate suggest a strong preference
[
21a–b]
original Fleming–Tamao oxidation (Scheme 1),
resulted in (S)-10 being isolated in 55% ee.
for b-addition of the hydride in the migratory inser-
tion step, whereas for trans-a-methyl cinnamate this
We attempted to rationalise the obtained selectivi- preference is substantially lower leading to reduced
[
11]
ties using our previously proposed selectivity selectivity for this substrate. Since silyl groups are
[11]
model.
When the substrates are arranged so that also electron-withdrawing one may also expect to see
the vinylic proton resides in the hindered quadrant a similar outcome for the substrates used in this study
[
22]
and the bulky silyl and phenyl groups occupy the (substrates 4, 6, 7, 8 and 9). This may explain why
open quadrants (Figure 2), the model correctly pre- only 5 results with high selectivity, since the other
dicts the stereochemical outcome for all substrates substrates will have a mismatch between sterics and
having known absolute configuration.
electronics.
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Adv. Synth. Catal. 2006, 348, 2575 – 2578

Iridium-Catalysed Asymmetric Hydrogenation of Vinylsilanes
COMMUNICATIONS
(
5-Chloro-2-methylpentyl)trimethylsilane
Compound 8 was hydrogenated via the typical protocol
After 12 h hydrogen pressure was vented and reaction con-
tents were filtered through a short silica plug, eluting with
pentane. Solvent was removed under vacuum, providing
pure (5-chloro-2-methylpentyl)trimethylsilane; yield: 96 mg
2
D
5
1
(
>99%); [a] : +3.308 (c 2.0, CHCl ); H NMR (400 MHz,
3
CDCl ): d=3.53 (t, J=7 Hz, 2H), 1.74–1.81 (m, 2H), 1.59–
3
1
.65 (m, 1H), 1.39–1.45 (m, 1H), 1.29–1.35 (m, 1H), 0.93 (d,
J=7 Hz, 3H), 0.64 (dd, J=5, 15 Hz, 1H), 0.43 (dd, J=8,
1
3
1
4
(
5 Hz, 1H), 0.02 (s, 9H); C NMR (400 MHz, CDCl ): d=
3
5.7, 40.0, 30.7, 29.4, 25.3, 23.0, À0.4; IR (neat): n =2955.6
max
À1
s), 2894.0, 1248.7 (s), 909.4, 838.0 (s), 735.2 (s), 692.3 cm ;
+
MS (EI): m/z=177 (M ÀMe), 135, 114, 95, 93; anal. calcd.
for C H ClSi: C 56.07, H 10.98; found: C 56.12, H 11.02.
9
21
Figure 2. Schematic diagram of the selectivity model with
substrates 5 (left) and 9 (right) coordinating onto the cata-
lyst.
Acknowledgements
In conclusion, we have reported the first successful
hydrogenation of a range of vinylsilanes and shown
This workwas supported by grants from SSF SELCHEM
that the reduction of simple olefins need not be re- graduate program and the Swedish Research Council (VR).
stricted to the model substrates that have so far domi- We wish to thankthe Wenner-Gren Foundation for the gener-
nated the field of Ir-catalysed asymmetric hydrogena- ous funding of I. J. M. The authors are very grateful to Dr.
Peter Brandt for fruitful discussions concerning this article.
tions.
Experimental Section
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www.asc.wiley-vch.de
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