Chloride-Bridged Dinuclear Rhodium(III) Complexes Bearing Chiral Diphosphine Ligands: Catalyst Precursors for Asymmetric Hydrogenation of Simple Olefins

Article abstract of DOI:10.1002/anie.201601748

Efficient rhodium(III) catalysts were developed for asymmetric hydrogenation of simple olefins. A new series of chloride-bridged dinuclear rhodium(III) complexes 1 were synthesized from the rhodium(I) precursor [RhCl(cod)]₂, chiral diphosphine ligands, and hydrochloric acid. Complexes from the series acted as efficient catalysts for asymmetric hydrogenation of (E)-prop-1-ene-1,2-diyldibenzene and its derivatives without any directing groups, in sharp contrast to widely used rhodium(I) catalytic systems that require a directing group for high enantioselectivity. The catalytic system was applied to asymmetric hydrogenation of allylic alcohols, alkenylboranes, and unsaturated cyclic sulfones. Control experiments support the superiority of dinuclear rhodium(III) complexes 1 over typical rhodium(I) catalytic systems.

Full text of DOI:10.1002/anie.201601748

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201601748
German Edition: DOI: 10.1002/ange.201601748
Asymmetric Hydrogenation
Chloride-Bridged Dinuclear Rhodium(III) Complexes Bearing Chiral
Diphosphine Ligands: Catalyst Precursors for Asymmetric
Hydrogenation of Simple Olefins
Yusuke Kita, Shoji Hida, Kenya Higashihara, Himanshu Sekhar Jena, Kosuke Higashida, and
Kazushi Mashima*
Abstract: Efficient rhodium(III) catalysts were developed for
asymmetric hydrogenation of simple olefins. A new series of
chloride-bridged dinuclear rhodium(III) complexes 1 were
synthesized from the rhodium(I) precursor [RhCl(cod)]2,
chiral diphosphine ligands, and hydrochloric acid. Complexes
from the series acted as efficient catalysts for asymmetric
hydrogenation of (E)-prop-1-ene-1,2-diyldibenzene and its
derivatives without any directing groups, in sharp contrast to
widely used rhodium(I) catalytic systems that require a direct-
ing group for high enantioselectivity. The catalytic system was
applied to asymmetric hydrogenation of allylic alcohols,
alkenylboranes, and unsaturated cyclic sulfones. Control
experiments support the superiority of dinuclear rhodium(III)
complexes 1 over typical rhodium(I) catalytic systems.
complexes.[2b, 4, 7,8] Aiming to develop a rhodium(III) catalytic
system, we focused our attention on monohydride
rhodium(III) complexes. Our research was based on the
À
reported reactivity of ethene inserted into a Rh H bond of
monohydride rhodium(III) complexes bearing dicyclo-
hexyl(2-methoxyethyl)phosphine.[9] Previously, we developed
chloride-bridged dinuclear iridium complexes that dissociated
into the corresponding mononuclear monohydride
iridium(III) complexes.[10] Asymmetric hydrogenation of
heteroaromatics, such as quinolinium salts,[11] quinoxalines,[12]
isoquinolinium salts,[13] pyridinium salts,[14] and quinazolinium
salts,[15] was accomplished with the latter. Accordingly, we
envisioned that rhodium analogues could serve as efficient
new catalyst precursors, that could provide monohydride
rhodium(III) species capable of catalyzing the asymmetric
hydrogenation of simple olefins. We found that chiral
chloride-bridged dinuclear rhodium(III) complexes profi-
ciently catalyzed the asymmetric hydrogenation of simple
olefins, as well as allylic alcohols, alkenylboranes, and
unsaturated cyclic sulfones.
A
symmetric hydrogenation of olefins is one of the most
efficient reactions for generating enantiomerically enriched
compounds.[1] A number of chiral transition metal complexes
have been used as catalysts for asymmetric hydrogenation of
olefins.[2] Among the chiral transition metal catalyst systems
used for asymmetric hydrogenation of olefins, chiral rhodium
catalysts have attracted particular attention because of their
efficiency, enantioselectivity, pressure tolerance, and solvent
profiles.[3] However, to achieve high asymmetric induction,
We initially synthesized chloride-bridged dinuclear
rhodium(III) complex (S)-1a, bearing (S)-BINAP, by adding
5 equiv of HCl in diethyl ether to a mixture of [RhCl(cod)]2
and 2.05 equiv of (S)-BINAP in toluene at room temperature
(Scheme 1). The 1H NMR spectrum of (S)-1a exhibited
a hydride signal at À15.9 ppm with two coupling constants
(JP-H of 22.7 and 15.5 Hz), indicating that the hydride was
located at a position cis with respect to the two phosphines of
(S)-BINAP. The 31P{1H} NMR spectrum of (S)-1a displayed
a typical doublet of doublets pattern at 48.0 and 36.2 ppm
(JRh-P = 137 Hz and JP-P = 27 Hz). Figure 1 shows the cationic
part of (S)-1a, which has a bifacial dinuclear structure with
three chloride atoms bridging two rhodium atoms (one outer-
sphere chloride anion is omitted for clarity). Application of
the same synthetic procedure to a series of chiral atropiso-
meric diphosphine ligands with an (S)-configuration led to
successful synthesis of the corresponding dinuclear rhodium
complexes (S)-1b–e in high yields, although (S)-1 f was
obtained in only moderate yield because of the steric
congestion imposed by the (S)-DTBM-SEGPHOS ligand.
We conducted asymmetric hydrogenation of a simple
olefin, (E)-prop-1-ene-1,2-diyldibenzene (2a), using (S)-1a–f
as catalysts. To our delight, 2a was hydrogenated using (S)-1a
as a catalyst with 30 bar hydrogen pressure in toluene at
1008C for 20 h, producing (S)-propane-1,2-diyldibenzene
[(S)-3a] in 30% yield with 70% ee (Table 1, entry 1). The
catalysts (S)-1b and (S)-1c, respectively bearing (S)-tol-
rhodium catalysts unavoidably require a coordinating group
[4]
=
next to the C C double bond. Such high enantioselectivity
was considered a consequence of the chelating coordination
of substrates. A typically proposed intermediate in asymmet-
ric hydrogenation of olefinic substrates bearing a coordinating
=
amide group involves, 1) both the C C double bond and the
coordinating functional group, 2) a dihydride rhodium(III)
species generated in situ by the reaction of a rhodium(I)
precursor with H2.[5] Thus, hydrogenation of simple olefins
with high enantioselectivity by chiral rhodium systems has
remained challenging,[6] despite the tremendous advances in
iridium-catalyzed asymmetric hydrogenation of unfunction-
alized olefins. In this pioneering work, Pfaltz demonstrated
the high catalytic performance of P,N-ligated iridium
[*] Dr. Y. Kita, S. Hida, K. Higashihara, Dr. H. S. Jena, K. Higashida,
Prof. Dr. K. Mashima
Department of Chemistry, Graduate School of Engineering Science,
Osaka University
1-3 Toyonaka, Osaka 560-8521 (Japan)
E-mail: mashima@chem.es.osaka-u.ac.jp
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201601748.
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Table 1: Optimization study.[a]
Entry Catalyst
x
Additive
Yield[b]
[%]
ee[c]
[%]
1
2
3
4
5
6
7
(S)-1a
(S)-1b
(S)-1c
(S)-1d
(S)-1e
(S)-1 f
(S)-1 f
4
4
4
4
4
4
2
–
–
–
–
–
–
30
48
24
67
>99
96
70
70
31
84
91
95
94
nBu4NCl
(20 mol%)
nBu4NCl
(10 mol%)
nBu4NCl
(5 mol%)
nBu4NCl
(2 mol%)
NaBArF
(4 mol%)
–
90
8
(S)-1 f
(S)-1 f
(S)-1 f
2
2
1
1
2
93
93
95
95
9
10[d]
11
12
96 (98)[e]
95
[RhCl(cod)]2/
(S)-DTBM-SEGPHOS
[RhCl((S)-DM-SEGPHOS)]2
88
27
<5
35
Scheme 1. Synthesis of chloride-bridged dinuclear rhodium complexes.
[a] Reaction conditions: a mixture of 2a (0.20 mmol), rhodium catalyst
(2.0 mmol), and toluene (3.0 mL) under H2 (30 bar) was heated at 1008C
for 20 h. [b] Determined by 1H NMR analysis. [c] Determined by HPLC
analysis. [d] Run at 808C. [e] Isolated yield of products obtained by
a 1 mmol scale reaction at 1008C.
Development of this method was based on our previous
observation that a chloride anion reacted with cationic triply
chloride-bridged dinuclear iridium complexes, analogous to 1,
to give the corresponding mononuclear anionic iridium
complexes.[13b] In fact, the addition of 20 mol% of nBu4NCl
increased catalyst activity, resulting in decreased catalyst
loading (2 mol%; entry 7). Decreasing the amount of
nBu4NCl led to an increase in the yield of (R)-3a, reaching
96% using 1 mol% of (S)-1 f and 2 mol% of nBu4NCl,
without loss of enantioselectivity (entries 8–10). Finally, we
determined that the optimized hydrogenation conditions
required 1 mol% (S)-1 f, 2 mol% nBu4NCl in toluene, and
30 bar hydrogen pressure at 808C for 20 h. A comparison with
typical rhodium catalysis suitable for asymmetric hydrogena-
tion of olefins bearing directing groups was subsequently
carried out. Asymmetric hydrogenation of 2a was performed
using [RhCl(cod)]2, (S)-DTBM-SEGPHOS, and sodium
Figure 1. Molecular structure of the cationic part of (S)-1a. Hydrogen
atoms are omitted for clarity. Selected bond distances (ꢀ) and angles
À
À
À
(8): Rh(1) P(1)=2.273(3), Rh(1) P(2)=2.268(3), Rh(1) Cl(1)=2.451-
À
À
(2), Rh(1) Cl(2)=2.449(2), Rh(1) Cl(3)=2.5743(16); P(1)-Rh(1)-P-
(2)=92.27(8), Cl(1)-Rh(1)-Cl(2)=80.50(7), Cl(1)-Rh(1)-Cl(3)=79.15-
(6), Cl(2)-Rh(1)-Cl(3)=79.11(6), P(1)-Rh(1)-Cl(1)=91.74(8), P(1)-Rh-
(1)-Cl(2)=167.68(7), P(1)-Rh(1)-Cl(3)=108.99(7).
BINAP and (S)-H8-BINAP, were ineffective (entries 2 and 3).
In contrast, (S)-SEGPHOS-type ligands showed high cata-
lytic activity and high enantioselectivity: (S)-1d resulted in
a 67% yield of products with 84% ee (entry 4), and the
reaction using bulkier diphosphines, such as (S)-DM-
SEGPHOS and (S)-DTBM-SEGPHOS, produced (S)-3a in
excellent yields with 91% ee and 95% ee, respectively
(entries 5 and 6).
Toluene was the best among the screened solvents, based
on the balance of yield and enantioselectivity (see Supporting
Information). The chloride-bridged dinuclear structure is
robust; therefore, we examined the effects of adding ammo-
nium chloride to facilitate the dissociation of dinuclear
complexes into catalytically active mononuclear complexes.
tetrakis[3,5-bis(trifluoromethyl)phenyl]borate
(NaBArF),
resulting in almost racemic 3a (entry 11). The poor results
obtained using a chiral chelating arylphosphine ligand are
consistent with the previous results, in which only chiral
chelating alkylphosphine ligands worked well for asymmetric
hydrogenation catalyzed by cationic rhodium catalysts.[16]
Additionally, asymmetric hydrogenation catalyzed by
[RhCl((S)-DM-SEGPHOS)]2 furnished 3a in low yield and
low enantioselectivity (entry 12). These findings clearly
demonstrate that the activity of dinuclear rhodium complexes
1 differ from that of the typical rhodium(I) catalysts.
2
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With the optimized conditions in hand, we examined the
The applicability of the rhodium catalyst system for
scope of the reaction using a range of alkenes without any
directing groups (Table 2). Olefins with differing substituents
on the phenyl ring were subjected to asymmetric hydro-
genation (entries 1–7). The reaction was compatible with
several functional groups, including trifluoromethyl, fluoro,
and chloro groups, affording the hydrogenated products in
high yield with high enantioselectivity (entries 1–3). Sub-
strates with an electron-donating group, such as a methyl or
methoxy group, required higher catalyst loading or a longer
reaction time to achieve high conversion, and the correspond-
ing hydrogenated products (R)-3 f and (À)-3g were obtained
in 87% and 94% ee, respectively (entries 5 and 6). As the
meta-methyl substituent on the phenyl group significantly
decreased the yield of (À)-3h, we used a more reactive
catalyst (S)-1e to complete conversion to give (À)-3h in 98%
ee (entry 7).
asymmetric hydrogenation of different kinds of alkenes
bearing a functional group with rather weak coordination
ability is noteworthy (Table 3). Typical allylic alcohols were
used as substrates for this catalytic reaction.[17] 3-Phenyl-
methallyl alcohol (4a) was hydrogenated at 308C to furnish
the corresponding product in quantitative yield with 99% ee
(entry 1). The reaction of 3-aryl substituted 3-methylallyl
alcohols proceeded with high enantioselectivity regardless of
the substitution pattern on the phenyl ring (entries 2–4).
Alkenyl boranes were also suitable substrates, and chiral
centers on the a- or b-position of borane were readily
introduced (entries 5 and 6). Furthermore, we applied our
Table 3: Scope of alkenes.[a]
Table 2: Scope of alkenes lacking a directing group.[a]
Entry
1[d]
4
5
Yield[b] ee[c]
[%]
[%]
4a
(À)-5a
99
99(À)
Entry
1[d]
2
3
Yield[b] ee[c]
2[e]
4b
4c
(À)-5b
(À)-5c
99
94
87(À)
96(À)
[%]
[%]
2b
(À)-3b
94
97(À)
3
2
2c
2d
2e
2 f
(À)-3c
(À)-3d
(À)-3e
(À)-3 f
(R)-3g
(À)-3h
99
94
98
76
80
99
98(À)
95(À)
96(À)
87(À)
94(R)
98(À)
4
4d
4e
(À)-5d
(R)-5e
87
99
90(À)
3
5[f]
85(R)[g]
4
6[f]
4 f
(R)-5 f
99
96
93(R)[g]
95(À)
5[e]
6[f]
7[d]
7[h]
4g
(À)-5g
2g
2h
8[i]
4h
(R)-5h
61
94(R)
[a] Reaction conditions: a mixture of 4 (0.20 mmol), (S)-1 f (2.0 mmol),
nBu4NCl (4.0 mmol), and toluene (3.0 mL) under H2 (30 bar) was heated
at 308C for 20 h. [b] Isolated yield of products. [c] Determined by HPLC
[a] Reaction conditions: a mixture of 2 (0.20 mmol), (S)-1 f (2.0 mmol),
nBu4NCl (4.0 mmol), and toluene (3.0 mL) under H2 (30 bar) was heated analysis. [d] (S)-1e was used as the catalyst. [e] Run for 24 h. [f] 2 mol%
at 808C for 20 h. [b] Isolated yield of products. [c] Determined by HPLC
analysis. [d] (S)-1e was used as the catalyst. [e] 2 mol% of (S)-1 f was
used. [f] Run for 24 h.
of (S)-1e was used. Run at 808C for 24 h. [g] Determined with the
corresponding alcohol obtained after oxidation. [h] Run at 808C. [i] Run at
1008C. Bpin=4,4,5,5-tetramethyl-1,3,2-dioxaborolanyl.
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system to the hydrogenation of unsaturated cyclic sulfones 4g
and 4h, which resulted in smooth formation of chiral
sulfolanes (À)-5g and (R)-5h with 95% ee and 94% ee,
respectively (entries 7 and 8).
Scientific Research (A) (No. 26248028) from Japan Society
for the Promotion of Science (JSPS).
Keywords: asymmetric catalysis · hydrogenation ·
Notably, the catalytic activity of dinuclear rhodium
complexes differs from that of well-established rhodium(I)
catalysts, including mononuclear cationic rhodium com-
plexes[5,16] and Wilkinson type complexes,[18] which generate
dihydride rhodium complexes under hydrogen gas. On this
basis, we assumed that the active species was a dichloro-
monohydride species derived from the dinuclear rhodium
complexes (S)-1, though we could not detect any defined
species in spectral measurements. Dissociation of (S)-1 into
dichloro-monohydride complexes can be accelerated by
reversible interaction with nBu4NCl. Thus, we propose the
following mechanism based on previous reports of dinuclear
iridium catalysts for asymmetric hydrogenation.[10–15] Initially,
the dinuclear complexes (S)-1 dissociate into the correspond-
ing mononuclear dichloro-monohydride rhodium(III) com-
reaction mechanisms · rhodium
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À
=
plexes, in which a Rh H bond readily adds to a C C bond.
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cannot rule out the possibility that the catalytic reaction
proceeds through a monochloro-dihydride rhodium com-
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rhodium complex with H2.[19]
In summary, we prepared new chiral dinuclear rhodium-
(III) complexes as active catalysts for asymmetric hydro-
genation of simple olefins. These complexes were applied to
the asymmetric hydrogenation of allylic alcohols, alkenylbor-
anes, and cyclic unsaturated sulfones. In sharp contrast to the
widely used cationic rhodium(I) catalysts, we propose that the
monohydride rhodium(III) complex acted as an active
species, which incorporates hydride attack of simple olefins
as its key step. Further investigations involving the newly
developed triply halide-bridged dinuclear rhodium(III) and
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Experimental Section
A glass tube was charged with a rhodium complex (0.010 mmol,
1 mol%) and nBu4NCl (0.020 mmol, 2 mol%) in a glove box.
Subsequently, an olefin (1.0 mmol) in toluene (6.0 mL) was added
into the glass tube in the stainless autoclave reactor using an inlet.
After three vacuum/argon cycles, the reactor was charged with H2,
and hydrogen pressure subsequently increased to 30 bar. The reaction
mixture in the reactor was stirred at 808C for 20 h. After release of H2,
solvent was removed by evaporation. Enantiomeric excess (ee) was
determined by HPLC analysis of the crude product. The product was
purified by filtration through a short pad of silica gel (eluted with
hexane), and analyzed by spectroscopic methods.
Acknowledgements
The authors thank Dr. Hiroshi Tadaoka and Franziska
Dorothea van Krꢀechten for the initial experimental results.
This work was financially supported by a Grant-in-Aid for
4
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Communications
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Received: February 19, 2016
Revised: March 23, 2016
Published online: && &&, &&&&
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Communications
Asymmetric Hydrogenation
Dinuclear rhodium(III) complexes were
highly catalytically active for asymmetric
hydrogenation of simple olefins in con-
trast to widely utilized rhodium(I) cata-
lytic systems.
Y. Kita, S. Hida, K. Higashihara,
H. S. Jena, K. Higashida,
K. Mashima*
&&&& — &&&&
Chloride-Bridged Dinuclear Rhodium(III)
Complexes Bearing Chiral Diphosphine
Ligands: Catalyst Precursors for
Asymmetric Hydrogenation of Simple
Olefins
6
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