Chiral Cyclopentadienyl Iridium(III) Complexes Promote Enantioselective Cycloisomerizations Giving Fused Cyclopropanes

Article abstract of DOI:10.1002/anie.201506483

The cyclopentadienyl (Cp) group is a very important ligand for many transition-metal complexes which have been applied in catalysis. The availability of chiral cyclopentadienyl ligands (Cp^x) lags behind other ligand classes, thus hampering the investigation of enantioselective processes. We report a library of chiral Cp^xIr^III complexes equipped with an atropchiral Cp scaffold. A robust complexation procedure reliably provides Cp^xIr^III complexes with tunable counterions. In a proof-of-concept application, the iodide-bearing members are shown to be highly selective for enyne cycloisomerization reactions. The dehydropiperidine-fused cyclopropane products are formed in good yields and enantioselectivities.

Full text of DOI:10.1002/anie.201506483

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201506483
German Edition: DOI: 10.1002/ange.201506483
Asymmetric Catalysis
Chiral Cyclopentadienyl Iridium(III) Complexes Promote
Enantioselective Cycloisomerizations Giving Fused Cyclopropanes
Michael Dieckmann, Yun-Suk Jang, and Nicolai Cramer*
Abstract: The cyclopentadienyl (Cp) group is a very important
ligand for many transition-metal complexes which have been
applied in catalysis. The availability of chiral cyclopentadienyl
ligands (Cpx) lags behind other ligand classes, thus hampering
the investigation of enantioselective processes. We report
a library of chiral CpxIrIII complexes equipped with an
atropchiral Cp scaffold. A robust complexation procedure
reliably provides CpxIrIII complexes with tunable counterions.
In a proof-of-concept application, the iodide-bearing members
are shown to be highly selective for enyne cycloisomerization
reactions. The dehydropiperidine-fused cyclopropane products
are formed in good yields and enantioselectivities.
ation between a chiral cyclopentadiene 1 and simple IrIII
starting materials, in analogy to the well-established reactions
of Cp*H and IrCl3, failed to provide any clean complexes.[7]
We therefore adapted a procedure previously established for
the [CpxRh(C2H4)2] complexes.[4] Optimally, the useful but
rather labile species [{Ir(C2H4)2Cl}2] is generated transiently
by briefly bubbling ethylene into a solution of [{Ir(coe)2Cl}2]
(Scheme 1; coe = cyclooctene). The solution containing
T
he design of new ligand systems for transition-metal
complexes is important to expand current reactivity bounda-
ries in catalysis. In this respect, cyclopentadienyl (Cp) and
pentamethylcyclopentadienyl (Cp*) are essential ligands for
many complexes with numerous applications in catalysis.[1]
The limited availability of chiral versions of Cp ligands
(Cpx)[2] and their corresponding metal complexes[3] has
hampered progress in asymmetric catalysis with this ligand
type. We recently introduced two classes of chiral Cpx
ligands.[4] Their disubstituted nature places them intermediate
between Cp and Cp* in terms of their electronic and steric
properties. For the most part, the use of these ligand series in
asymmetric catalysis has been limited to rhodium-catalyzed
[4,5]
À
C H bond functionalizations.
The exploitation of chiral
Scheme 1. Synthesis of the chiral [{CpxIrI2}2] complexes.
Cpx ligands in conjunction with other important and fre-
quently used transition metals is an important and highly
promising task. However, unlike phosphine ligands, for
example, which often can be simply used by in situ formation
procedures to form the required complexes, Cp metal
complexes have to be preassembled prior to their use in
catalysis.[6] Therefore, of primary importance is the develop-
ment of robust complexation and purification techniques for
each specific transition metal. Herein, we report a set of chiral
CpxIrIII complexes and demonstrate their potential in enan-
tioselective cycloisomerization reactions as a proof-of-princi-
ple application.
formed [{Ir(C2H4)2Cl}2] was directly added to the cyclo-
pentadienyl anion at ambient temperature yielding CpxIrI
complexes 2a–2h.
To access the
corresponding
IrIII complexes, a suitable oxidation method was needed. In
contrast to the corresponding CpxRhI complexes which fail to
react with molecular iodine, the more easily oxidizable CpxIrI
complexes 2 smoothly provide [{CpxIrI2}2] dimers 3a–3h.[8]
The dark-red and completely air-stable powders 3 are
versatile intermediates to which any other desired counterion
can be readily introduced by simple halide-abstraction
methods.
First, a reliable method to prepare the desired iridium
complexes was developed. Unfortunately, direct complex-
In the X-ray crystal structure, the IrI ethylene complex 2d
(Figure 1a) shows identical bond lengths and geometries as its
RhI congener (Figure 1b),[4b] suggesting that complex 2d and
its analogues might be promising candidates in asymmetric
transformations.[9]
[*] Dr. M. Dieckmann, Y.-S. Jang, Prof. Dr. N. Cramer
Laboratory of Asymmetric Catalysis and Synthesis
EPFL SB ISIC LCSA, BCH 4305
1015 Lausanne (Switzerland)
E-mail: nicolai.cramer@epfl.ch
Homepage: http://isic.epfl.ch/lcsa
As a proof-of-concept application for the chiral CpxIrIII
complexes, we selected the cycloisomerization of enynes to
form cyclopropanes [Eq. (1)] reported by Malacria, Fenster-
bank, and co-workers.[10]
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201506483.
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Table 1: Initial optimization of the CpxIrIII-catalyzed cyclopropanation.[a]
Entry
X
Solvent
T [8C]
Conv. [%][b]
5a [%][b]
e.r.[c]
1
2
3
4
5
6
7
8
Cl
Br
I
OBz
SbF6
I
I
I
I
I
I
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
(CH2Cl)2
CH2Cl2
CHCl3
toluene
THF
60
60
60
60
23
23
23
23
23
23
23
40
100
100
0
100
83
100
100
74
20
43
40
0
75:25
80:20
82:18
–
0
–
Figure 1. X-ray crystal structure of the CpxIrI complex 2d and its CpxRhI
analogue.
32
36
50
19
16
6
81:19
82:18
85:15
83:17
79:21
76:23
9
10
11
70
53
CH3CN
[a] Conditions: 4a (25 mmol), 3d (1.25 mmol), 0.1m in the indicated
solvent, 24 h. [b] Determined by 1H NMR spectroscopy using an internal
standard. [c] Determined by HPLC on a chiral stationary phase.
Bz=benzoyl.
Cyclization reactions of this type are induced by an
activation of the alkyne rendering it more electrophilic,[11]
a reaction for which carbophilic p-acidic metals, such as Au
and Pt, are typically employed.[12–14] Related asymmetric
versions of the reaction were reported first with IrI com-
plexes[15] and recently using gold(I),[16] platinum(II),[17] and
rhodium(I/II)[18] catalysts. However, to our knowledge, no
chiral catalytic system efficiently accommodates terminal
alkyne substrates,[19] making this transformation a challenging
and complementary application for the CpxIrIII system. First,
the key contributing reaction parameters of the cyclization to
form cyclopropane 5a were evaluated using the parent ligand
1d (Table 1), focusing, in this case, on the critical influence of
the counterion on both reactivity and selectivity. The
reactivity of the IrIII complexes increased substantially by
changing from chloride to heavier halides (Table 1, entries 1–
3). Moreover, the iodide complex provided a product with an
increased enantioselectivity. A cationic or carboxylate bear-
ing complex was not suitable, leading to unspecific decom-
position or recovered starting material (Table 1, entries 4–5).
The reaction proceeds well with the iodide complex at
ambient temperature in CHCl3 with an increased e.r. value
of 85:15 (Table 1, entry 8). Other solvents were significantly
less efficient (Table 1, entries 6–11).
Table 2: Screening of different CpxIrIII catalysts 3.[a]
Entry
3
R
T [8C]
Conv. [%][b]
5a [%][b]
e.r.[c]
1
2
3
4
5
6
7
3a
3b
3d
3c
3e
3 f
3g
3h
3h
Ph
Bn
23
23
23
23
23
23
23
23
0
72
66
100
76
100
70
100
100
100
11
11
50
38
61
44
43
64
76
75:25
85:15
85:15
82.5:17.5
90:10
89:11
88:12
93.5:6.5
96:4
MeO
PhO
iPrO
iPentO
CypO
tBuO
tBuO
8
9[d]
[a] Conditions: 4a (25 mmol), 3 (1.25 mmol), 0.1m in CHCl3, 24 h.
[b] Determined by 1H NMR spectroscopy using an internal standard.
[c] Determined by HPLC on a chiral stationary phase. [d] 120 h.
Bn=benzyl; Cyp=cyclopentyl.
Subsequently, we explored the influence of substituents
on the chiral Cpx structure on the progress of the reaction
(Table 2). Although a phenyl or benzyl R group at the 3,3’-
position instead of the methoxy group conserved mostly the
selectivity, conversion and yield dropped (Table 2, entries 1–
3). Therefore, we focused on different ether groups. Typically,
bulkier ethers gave rise to better enantioselectivities and
improved the yield (Table 2, entries 4–7). The highest selec-
tivity was achieved with complex 3h containing a tert-butoxy
ligand giving 5a in 93.5:6.5 e.r. (Table 2, entry 8). Finally,
lowering the reaction temperature to 08C increased the
selectivity to 96:4 e.r. (Table 2, entry 9).
Next, the enantioselective cycloisomerization was applied
to different enyne substrates. A variety of aromatic substitu-
ents R1 are well tolerated and give cyclopropanes 5 in high
enantioselectivity (Table 3, entries 1–10). Electron-rich
arenes display a significantly higher reactivity, suggesting
the importance of stabilization of positive charge during the
reaction. Electron-poor arenes such as p-nitrophenyl need
a slightly higher reaction temperature of 238C (Table 3,
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Table 3: Reaction scope for the synthesis of cyclopropanes 5.[a]
Entry
4
5
Yield
[%][b]
e.r.[c]
1
2
3
4
4a
4b
4c
4d
5a
5b
5c
5d
R1 =Ph, R2 =H
76
52
72
66
96:4
R1 =4-MeOC6H4, R2 =H
R1 =4-(F3CO)C6H4, R2 =H
R1 =5-benzo[d][1,3]dioxolyl,
R2 =H
96.5:3.5
96.5:3.5
96:4
5
6
7
8
9
4e
4 f
4g
4h
4i
5e
5 f
5g
5h
5i
5j
5k
5l
R1 =4-MeC6H4, R2 =H
R1 =3,5-MeC6H3, R2 =H
R1 =4-BrC6H4, R2 =H
R1 =3-ClC6H4, R2 =H
R1 =2-naphthyl, R2 =H
R1 =4-NO2C6H4, R2 =H
R1 =2-thienyl, R2 =H
R1 =2-benzothienyl, R2 =H
65
58
80
58
67
70
47
79
87
67
86
87
78
45
94:6
93.5:6.5
97:3
95:5
92.5:7.5
94:6
94:6
97:3
96.5:3.5
94:6
91:9
Scheme 2. Proposed catalytic cycle for the cyclization.
10[d] 4j
11
12
13
4k
4l
to the terminal alkyne of 4 gives p-complex 6. We believe that
the orientation of the alkyne with respect to the chiral Cpx
ligand is critical for the enantioselectivity. A 6-endo-dig
cyclization would lead to cyclic structures represented in their
different mesomeric forms. The carbocationic character of
these intermediates provides a rationale for the detected
increase in reaction rates with electron-rich arene substituents
R1. Finally, a 1,2-hydride shift yields 7, which in turn releases
product 5 and the IrIII complex, completing the catalytic cycle.
Alternatively, a mechanism involving an iridium vinylidene
intermediate[20] could be considered, although this pathway is
less likely as an internal alkyne substrate also cyclizes.[21]
In summary, we report enantiopure cyclopentadienyl IrIII
complexes drawing their chirality from an atropchiral Cpx
4m 5m R1 =2-benzofuryl, R2 =H
14[e] 4n
15[d] 4o
5n
5o
5p
5q
5r
R1 =3-furyl, R2 =H
R1 =CH2OBn, R2 =H
R1 =Ph, R2 =Me
R1 =H, R2 =Ph
16
4p
91:9
91:9
89:11
17[f] 4q
18[f] 4r
R1 =H, R2 =5-benzo[d]-
[1,3]dioxolyl
19[f] 4s
5s
R1 =H, R2 =Me
87
0
81.5:18.5
–
cis-
4a
epi-
5a
20
[a] Conditions: 4 (0.1 mmol), 3h (5.0 mmol), 0.1m in CHCl3, 5 days at
08C. [b] Yield of isolated product. [c] Determined by HPLC on a chiral
stationary phase. [d] 3h (10 mmol). [e] 3h (7.5 mmol). [f] 3d (5.0 mmol),
08C, 48 h.
ligand.
A robust complexation using commercial [{Ir-
(coe)2Cl}2] and iodine as oxidant provides [{CpxIrI2}2]
dimers. As the first application of the obtained complexes in
asymmetric catalysis, we developed cycloisomerization reac-
tions of enynes to give fused cyclopropanes with high
entry 10). Electron-rich heterocycles also display good reac-
tivities and high enantioselectivities (Table 3, entries 11–14).
The cycloisomerization is not limited to arene groups and
a functionalized alkyl substituent R1 provides comparable
results (Table 3, entry 15). Trisubstituted alkene 4p (R1 = Ph,
R2 = Me) cyclizes with slightly lower selectivity (Table 3,
entry 16). 1,1-disubstitued olefins (R1 = H) react more selec-
tively with methoxy-substituted complex 3d (Table 3,
enantioselectivities.
The
discovered
3,3’-tert-butoxy
Cpx ligand displayed superior selectivity. Importantly, com-
plementary to asymmetric Au catalysis, the IrIII-catalyzed
process is well suited to terminal alkynes. Further work
encompassing the application of CpxIrIII complexes as cata-
À
lysts, in particular for enantioselective C H functionalization
reactions, is ongoing.
entries 17–19). In stark contrast,
a cis-1,2-disubstituted
olefin (cis-4a) is not suitable for the cyclization and is
completely recovered (Table 3, entry 20). Related substrates
that have the NTs group replaced by an oxygen atom or
a malonate group react very sluggishly. The absolute config-
uration of the cyclopropane products was unambiguously
established by X-ray crystallographic analysis of 5g to be
(S,S,S).[9]
Acknowledgements
This work is supported by the European Research Council
(ERC Grant agreement number 257891). M.D. thanks the
DAAD for a postdoctoral fellowship. We thank Dr. R.
Scopelliti for X-ray crystallographic analysis of 2d and 5g.
In analogy to related reactions with Au and Pt catalyst-
s,[13a,16c,d] the following catalytic cycle is proposed (Scheme 2).
After initial dissociation of dimeric complex 3, coordination
Keywords: asymmetric catalysis · cycloisomerization ·
cyclopentadienyl ligands · enantioselectivity · iridium
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Received: July 15, 2015
Published online: August 27, 2015
12152 www.angewandte.org
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