Iridium-Catalyzed Enantioselective Hydrogenation of Vinylsilanes

Article abstract of DOI:10.1002/adsc.201700162

We have screened a diverse array of iridium complexes derived from chiral N,P ligands as catalysts for the asymmetric hydrogenation of vinylsilanes, a transformation for which generally applicable catalysts were lacking. Several catalysts emerged from thi

Full text of DOI:10.1002/adsc.201700162

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DOI: 10.1002/adsc.201700162
Very Important Publication
Iridium-Catalyzed Enantioselective Hydrogenation of Vinylsilanes
Aie Wang,+a,b Maurizio Bernasconi,+b and Andreas Pfaltzb,*
a
Department of Chemistry, Fujian Provincial Key Laboratory of Chemical Biology, College of Chemistry, Xiamen
University, Xiamen, Fujian 361005, Peopleꢀs Republic of China
Department of Chemistry, University of Basel, St. Johanns-Ring 19, CH-4056 Basel, Switzerland
b
Fax : (+41)-61-267-1103; e-mail: andreas.pfaltz@unibas.ch
+
These authors contributed equally to this work.
Received: February 8, 2017; Revised: February 27, 2017; Published online: && &&, 0000
Supporting information for this article can be found under http://dx.doi.org/10.1002/adsc.201700162.
Abstract: We have screened a diverse array of iridi-
um complexes derived from chiral N,P ligands as
catalysts for the asymmetric hydrogenation of vinyl-
silanes, a transformation for which generally appli-
cable catalysts were lacking. Several catalysts
emerged from this study that enabled the highly
enantioselective hydrogenation of a wide range of
vinylsilanes with trisubstituted or disubstituted ter-
minal C=C bonds bearing aryl, alkyl, ethoxycarbon-
yl, or hydroxymethyl substituents. In addition to tri-
methylsilyl and dimethyl(phenyl)silyl derivatives,
trialkoxysilyl- and silacyclobutyl-substituted alkenes
were used as substrates.
conveniently accessible by copper-catalyzed enantio-
selective allylic substitution.[8] The most general and
widely used method for the synthesis of chiral silanes
is the asymmetric hydrosilylation of alkenes, pio-
neered by Hayashi.[9] High enantioselectivities were
achieved with monosubstituted or 1,2-disubstituted al-
kenes, whereas 1,1-disubstituted alkenes usually react
with lower enantioselectivity, especially those with
terminal dialkyl-substituted C=C bonds. Moreover,
regioselectivity is a frequently encountered problem.
In this respect, asymmetric hydrogenation of vinyl-
silanes provides a potentially attractive alternative.
However, examples of this approach to chiral silanes
are scarce. In 2006, Andersson and co-workers report-
ed the first enantioselective hydrogenation of vinylsi-
lanes by iridium catalysts derived from chiral phosphi-
no-thiazoline or phosphino-oxazoline ligands.[10a]
However, only in one case, using (E)-trimethyl(2-phe-
nylprop-1-en-1-yl)silane as substrate (structure 7c in
Table 1), high enantiomeric excesses of up to 98%
Keywords: asymmetric hydrogenation; iridium; N,P
ligands; vinylsilanes
Chiral organosilanes are synthetically valuable com- were obtained, while other vinylsilanes reacted with
pounds, which have found use as versatile precursors moderate to poor enantioselectivities of 28–58% ee.
for selective carbon-carbon bond formation[1] or as In subsequent studies additional Ir-N,P ligand com-
chiral catalysts[2] for asymmetric transformations. In plexes were tested, although solely in the hydrogena-
addition, C–Si bonds can be converted into C–O tion of 7c.[10b,c] After completion of our work de-
bonds in a stereospecific manner by Fleming–Tamao scribed herein, Li et al. reported a study on the asym-
oxidation.[3] Furthermore, due to their low toxicity metric hydrogenation of vinylsilanes with an Ir-Thre-
and favorable metabolic profiles, chiral organosilanes PHOX catalyst (ligand 3b in Figure 1).[10d] They ob-
have received increasing attention in medicinal tained high enantioselectivities in several cases but
chemistry.[4] Therefore, methods that enable the syn- the range of substrates investigated was limited to vi-
thesis of highly enantioenriched chiral organosilicon nylsilanes with a terminal C=C bond. Therefore, the
compounds are of great interest.
availability of other catalysts that enhance the sub-
Several enantioselective routes to chiral organosi- strate scope of this transformation is clearly desirable.
lanes have been described, such as the 1,4-addition of
As part of our long-term studies of Ir-catalyzed
silicon nucleophiles to prochiral a,b-unsaturated car- asymmetric hydrogenation,[11–24] we explored the po-
bonyl compounds[5] or the 1,4-addition of carbon[6] or tential of various classes of chiral Ir-N,P ligand com-
hydrogen[7] nucleophiles to silyl-substituted a,b-unsa- plexes for the enantioselective hydrogenation of vi-
turated carbonyl compounds leading to chiral b-silyl nylsilanes. Ir complexes of this type have considerably
carbonyl compounds. Enantioenriched allylsilanes are enhanced the scope of asymmetric hydrogenation of
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the saturated silane 8a within 2 h at room tempera-
ture (entries 1–7, Table 1). In general, complexes with
phosphinite-based N,P ligands gave higher enantiose-
lectivities (86–98% ee) than PHOX or PyrPHOX li-
gands (Table 1, entries 3–7 vs. entries 1 and 2). The
best enantioselectivity (98% ee) was achieved with
the bicyclic pyridine-phosphinite ligand 6b.
In the hydrogenation of (E)-(2-cyclohexylbut-1-en-
1-yl)trimethylsilane (7b) lacking an aromatic substitu-
ent, most catalysts that had given high enantioselec-
tivities for 7a performed poorly (entries 10–12), with
the exception of complexes derived from pyridine-
based ligands 6a and 6b (entries 13 and 14). The latter
clearly performed best, affording the desired product
8b with full conversion and 96% ee.
The corresponding methyl-substituted vinylsilanes
7c and 7d as well gave high enantioselectivities with
pyridine-based ligands 6a–c (entries 15–20). The
chloroalkyl-substituted vinylsilane 7e reacted with
lower but still respectable enantioselectivity. In this
case oxazoline- and imidazoline-based ligands 4a and
5a also performed well, inducing ee values of 83%
and 80%, respectively, comparable to those with pyri-
dine-phosphinites 6a and 6b (entries 21–24).
Next, we studied the hydrogenation of silane (Z)-7f
with a TMS group at the trisubstituted C atom, which
leads to a product with a silyl-substituted stereogenic
center. In contrast to Anderssonꢀs catalysts,[10a] which
gave low enantioselectivity (28% ee) for this sub-
strate, all of our phosphinite ligands 3a, 4a, 5a, 6a,
and 6b performed well (entries 27–31). Especially pyr-
idine-based ligands 6a and 6b stood out with enantio-
selectivities of >99% ee. Hydrogenation of the
Figure 1. Chiral N,P ligands used in this study.
olefins because they do not require the assistance of isomer (E)-7f proved to be more challenging
a coordinating group in the vicinity of the C=C bond (entries 32–39). Most catalysts gave low conversion
like Rh and Ru diphosphine complexes. The range of and only moderate to poor enantioselectivities
substrates that have been successfully hydrogenated (entries 33–34). Only the PyrPHOX complex
with these catalysts comprises a wide variety of func- [Ir(COD)(2a)]BArF produced the product with full
tionalized and unfunctionalized olefins.[11,19–24] Even conversion and an ee value of 88% after a prolonged
purely alkyl-substituted alkenes were found to react reaction time of 4 h.
with excellent enantioselectivities and high turnover
In further studies we focused on the hydrogenation
numbers. Therefore, we thought that alkenes bearing of vinylsilanes with terminal C=C bonds. As terminal
a silyl instead of an alkyl group should be feasible olefins are known to react with higher enantioselec-
substrates as well. Here we report the results of a sys- tivity at lower pressure,[30] catalyst screening was per-
tematic hydrogenation study of a diverse range of vi- formed under 1 bar of hydrogen gas.
nylsilanes using Ir complexes derived from N,P li-
First, we tested a-trimethylsilylstyrene (7g) as sub-
gands 1–6 as catalysts (see Figure 1).
strate (Table 2). In all cases full conversion was ob-
For our studies we selected representative examples tained with enantioselectivities ranging from 13% to
of Ir catalysts derived from oxazoline-based N,P li- 88% ee. The best catalyst for this substrate was the
gands PHOX,[13] PyrPHOX,[14] ThreePHOX,[15] and complex derived from the pyridine-phosphinite ligand
SimplePHOX,[16] an imidazoline analogue of Simple- 6a (entry 6). Even better results were obtained with
PHOX,[17] and a series of pyridine-based N,P ligands the corresponding cyclohexyl-substituted vinylsilane
6a–d[18] (Figure 1), which we had successfully applied 7h. Notably, besides the pyridine-phosphinite 6a, the
in previous studies. For an initial screening, we chose oxazoline- and imidazoline-based ligands 4a and 5a
(E)-trimethyl(2-phenylbut-1-en-1-yl)silane (7a) as also performed well with this substrate (entries 11–
substrate. Using 0.5 mol% catalyst under 50 bar of hy- 13). Clearly the highest enantioselectivity (97% ee)
drogen gas, all catalysts tested gave full conversion to was induced by ligand 4a. As observed before for vi-
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Table 1. Asymmetric hydrogenation of vinylsilanes with tri-
substituted C=C bonds.
Table 2. Asymmetric hydrogenation of terminal vinylsilanes.
Entry Substrate
L
Conv. [%][a] ee [%][b]
Entry Substrate
L
Conv. [%][a] ee [%][b]
1
2
3
4
5
6
7
(S)-1a
(S)-2a
(4S,5S)-3a >99
(S)-4a
(R)-5a
(S)-6a
(S)-6b
>99
>99
30 (S)[27]
27(S)
74 (S)
20 (R)
67(S)
1
2
3
4
5
6
7
(S)-1a
(S)-2a
(4S,5S)-3a
(S)-4a
(R)-5a
(S)-6a
>99
>99
>99
>99
>99
>99
>99
8 (À)
31 (À)
94 (+)
87 (À)
86 (+)
91 (+)
98 (À)
>99
>99
>99
>99
88 (S)
13 (R)
(R)-6b
8
9
(S)-1a
(S)-2a
(4S,5S)-3a >99
(S)-4a
(R)-5a
(S)-6a
(S)-6b
93
>99
20 (+)
36 (+)
48 (À)
97 (+)
88 (À)
90 (À)
47 (À)
69 (R)[28]
65 (S)
46 (S)
8
9
(S)-1a
(S)-2a
(4S,5S)-3a
(S)-4a
(R)-5a
(S)-6a
58
44
18 (À)
10
11
12
13
14
8 (À)
>99
>99
>99
40
10
11
12
13
14
>99
>99
>99
>99
>99
28 (+)
50 (+)
20 (+)
85 (À)
96 (À)
(S)-6b
15
16
17
(S)-4a
(R)-5a
(S)-6a
>99
>99
>99
15
16
17
(S)-6a
(R)-6b
(R)-6c
>99
>99
>99
92 (R)[25]
90 (S)
94 (S)
18
19
20
21
(S)-4a[c]
(R)-5a[c]
(S)-6a
>99
51
68
81[d] (R)[29]
50 (S)
91 (S)
18
19
20
(S)-6a[c]
(R)-6b[c]
(R)-6c[c]
>99
>99
>99
92 (À)
91 (+)
95 (+)
(S)-6a[c]
>99
91 (S)
[a]
Determined by GC analysis of the reaction mixture after
removal of the catalyst.
Determined by GC analysis on a chiral stationary phase.
Absolute configurations are assigned based on the sign
of the optical rotation reported in the cited references.
Under 5 bar of H2.
21
22
23
24
(S)-4a
(R)-5a
(S)-6a
(S)-6b
>99
>99
>99
>99
83 (À)
80 (+)
77 (+)
80 (+)
[b]
[c]
[d]
25
26
27
28
29
30
31
(S)-1a[d]
(S)-2a[d]
>99
>99
3 (R)[26]
23 (S)
98 (R)
87 (S)
89 (R)
Determined by GC analysis after conversion to its corre-
sponding alcohol.
(4S,5S)-3a[d] >99
(S)-4a[d]
(R)-5a[e]
(S)-6a
>99
>99
>99
>99
>99 (R)
>99 (R)
nylsilanes with trisubstituted C=C bonds (Table 1), re-
placement of a cyclohexyl substituent by a less steri-
cally demanding n-alkyl group resulted in a loss of
enantioselectivity (entries 15–17). SimplePHOX 4a
was again the best performing ligand with an ee of
69%.
The more sterically hindered dimethylphenylsilyl
substituent in substrate 7j led to a strong decrease in
reactivity compared to the TMS analogue (entries 18–
21). Only 68% conversion was obtained under stan-
dard conditions with the catalyst derived from ligand
6a, although high levels of enantioselectivity were
achieved. To speed up the reaction, the hydrogen
pressure was raised to 5 bar. Under these conditions
both the SimplePHOX (4a) and pyridine-phosphinite
(6a) complexes produced the product with full con-
version and enantioselectivities of 81% and 91% ee,
(S)-6b
32
33
34
35
36
37
38
39
(S)-1a
40
88
>99
75
42
8
69 (R)
86 (R)
88 (R)
25 (S)
67 (R)
63 (S)
62 (S)
75 (S)
(S)-2a
(S)-2a[c]
(4S,5S)-3a
(S)-4a
(R)-5a
(S)-6a
(S)-6b
17
4
[a]
Determined by GC analysis of the reaction mixture after
removal of the catalyst.
Determined by GC analysis on a chiral stationary phase.
Absolute configurations are assigned based on the sign
of the optical rotation reported in the cited references.
Reaction time: 4 h.
[b]
[c]
[d]
[e]
Reaction time: 6 h.
Reaction time: 24 h.
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respectively, while the complex with the imidazoline- (Table 3, entry 1). As the side product 9 likely results
based ligand 5a showed poor reactivity and selectivity. from an acid-promoted[33] condensation, 5 mol% of
In view of the promising results obtained with tri- potassium carbonate was added to the reaction mix-
methylsilyl- and dimethylphenylsilyl-substituted al- ture. As hoped, the reaction proceeded smoothly to
kenes, we decided to explore further silane deriva- give the saturated triethoxysilane 8k as the sole prod-
tives.
uct (entries 2–7). The best enantioselectivity (70% ee)
Trialkoxysilyl-substituted alkenes were chosen be- was achieved with complexes derived from Simple-
cause the hydrogenation products can be readily PHOX ligand 4b and phosphino-imidazoline
transformed to enantioenriched alcohols[3] or tri- (PHIM)[17] 10a (Figure 2).
ACHTUNGTRENNUNG
fluorosilanes,[31] the latter being of interest as starting
compounds for stereoselective sp2–sp3 Hiyama cross-
coupling reactions.[32]
Initial hydrogenations of triethoxy(vinyl)silane 7k
under standard conditions (0.5 mol% catalyst loading,
50 bar H2) led to an inseparable mixture of the de-
sired product 8k and a side product, to which we
assigned structure 9 based on MS and NMR data
Table 3. Asymmetric hydrogenation of trialkoxy(vinyl)si-
lanes.
Figure 2. Ligands 10 and 11.
The trimethoxysilyl-substituted analogue 7l gave
similar results (entries 8–11). However, a different
ligand, the ThrePHOX derivative 3a, performed best
in this case. As generally observed for terminal ole-
fins, the enantioselectivity was higher at low hydrogen
pressure, however, the difference was small (3a: 77%
ee at 50 bar vs. 80% ee at 5 bar; entries 8 and 9). Con-
sistent with the results obtained for the TMS ana-
logues 7h and 7i (Table 2), better enantioselectivity
(88% ee) was achieved with the cyclohexyl-substitut-
ed vinylsilane 7m compared to the corresponding n-
alkyl-substituted substrate 7l. In this case, the optimal
ligand was the SimplePHOX derivative 4b (entry 13).
Due to ring strain, siletanes (silacyclobutanes) have
unique properties that distinguish them from analo-
gous acyclic alkylsilanes. They have been shown to
readily undergo Tamao–Fleming oxidation and there-
fore can be used as hydroxy surrogates.[34] Moreover,
they can be converted to structurally diverse products
by transition metal-catalyzed reactions involving in-
sertion and ring expansion.[35] We therefore decided
to study the asymmetric hydrogenation of vinylsile-
tanes as a possible approach to enantioenriched alkyl-
Entry Substrate
L
Conv.
[%][a]
ee
[%][b]
1
2
3
4
5
6
7
(S)-4b[c]
(4S,5S)-3a >99
(S)-4a
(S)-4b
(S)-6a
(R)-6b
(S)-10a
53
n.d.[d]
64 (À)
44 (+)
70 (+)
20 (+)
16 (À)
70 (+)
>99
>99
>99
>99
>99
8
9
(4S,5S)-3a >99
77 (À)
80 (À)
(4S,5S)-
>99
3a[e]
10
11
(S)-4b
(S)-10a
>99
90
51 (+)
65 (+)
12
13
14
(4S,5S)-3a >99
(S)-4b
(S)-10b
44 (+)
88 (À)
79 (À)
AHCTUNGTRENNUNG
>99
>99
ing. With most catalysts complex, mixtures of the de-
sired product 8n and several unidentified products
were formed. Better results were finally obtained with
PHOX complexes, especially with [Ir(COD)(1d)]
BArF, which gave 93% conversion to the hydrogena-
tion product 8n in 75% ee and only minor amounts of
an unidentified by-product (Scheme 1). When the
temperature was lowered to 08C, the ee increased to
83%.
[a]
Determined by GC analysis of the reaction mixture after
removal of the catalyst.
Determined by GC or HPLC analysis on a chiral station-
ary phase.
[b]
[c]
[d]
In the absence of K2CO3, 16 h.
Not determined because of overlapping peaks of one
enantiomer and the side product.
Under 5 bar of H2.
[e]
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bis(ortho-tolyl)phosphine group, showed higher levels
of activity and enantioselectivity than their analogues
4b and 11b. The best result (>99% conversion, 86%
ee) was achieved with the catalyst derived from Neo-
PHOX ligand 11a.
In conclusion, in this study that included a variety
of vinylsilanes with trisubstituted or disubstituted ter-
minal C=C bonds and a diverse array of Ir-N,P ligand
complexes as catalysts, we have shown that asymmet-
ric hydrogenation provides an efficient access to
a wide range of chiral organosilanes with high enan-
Scheme 1. Asymmetric hydrogenation of vinylsiletanes.
Finally, we investigated the hydrogenation of func- tioselectivity. Both substrates with or without coordi-
tionalized vinylsilanes bearing a carboxylic ester or nating groups at the C=C bond were successfully hy-
hydroxymethyl group at the C=C bond (Figure 3).
drogenated. Depending on the substrate structure,
different ligand complexes emerged as the catalysts of
choice. The best catalysts identified in our study sig-
nificantly enhance the substrate scope in the asym-
metric hydrogenation of silyl-substituted C=C bonds,
opening up an attractive enantioselective route to
chiral organosilanes.
Experimental Section
General Procedure
A 2-mL glass vial was charged with a cylindrical stirring bar
(0.7 cm in length), the relevant iridium catalyst (0.5–
1.0 mol%) and the substrate (100 mmol). The mixture was
dissolved in CH2Cl2 (0.5 mL, 0.2M) and placed in an auto-
clave. The autoclave was attached to a high pressure hydro-
gen line and purged with H2 three times before being sealed
under the appropriate H2 pressure. The mixture was stirred
for the appropriate reaction time at room temperature.
After release of H2, the solution was concentrated under
a stream of nitrogen. The crude reaction mixture was then
dissolved in 0.5 mL of a 4:1 n-hexane:MTBE mixture and
the catalyst removed by filtration through a plug of SiO2 in
a Pasteur pipette. After washing with ca. 5 mL of a 4:1 n-
hexane:MTBE mixture, the solvent was evaporated under
vacuum to yield the hydrogenation product.
Figure 3. Asymmetric hydrogenation of a,b-unsaturated
ester 7o and allylic alcohol 7p.
The a,b-unsaturated ester 7o was first tested under
standard conditions (0.5 mol% catalyst loading, 50 bar
H2, 4 h). Most catalysts that we evaluated yielded the
desired product 8o with full conversion, with the ex-
ception of Ir complexes derived from pyridine-phos-
phinite ligands 6a, 6b and 6d, which were poorly
active. However, the highest enantioselectivity (90%
ee) was achieved with ligand 6d, which bears a sterical-
ly demanding aryl substituent at the pyridine ring
(Figure 3). With higher catalyst loading (1 mol%) and
over a prolonged reaction time of 12 hours, the reac-
tion went to completion with essentially the same
enantioselectivity (89% ee).
For analytical data and determination of enantioselectivi-
ties, see the Supporting Information.
Acknowledgements
Financial support by the Swiss National Science Foundation
is gratefully acknowledged.
Compared to the a,b-unsaturated ester 7o the allyl-
ic alcohol 7p was less reactive. Using 1.0 mol% of cat-
alyst at 50 bar H2 at room temperature, most catalysts
did not give any conversion after a reaction time of
12 h, except for complexes derived from Simple-
PHOX[16] and NeoPHOX[36] ligands. The substituents
on the phosphorus atom seem to play a crucial role in
these catalysts. Ligands 4a and 11a, both bearing a
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