Mechanism studies of Ir-catalyzed asymmetric hydrogenation of unsaturated carboxylic acids

Article abstract of DOI:10.1021/jacs.6b11655

The Ir-catalyzed asymmetric hydrogenation of olefins is widely used for production of value-added bulk and fine chemicals. The iridium catalysts with chiral spiro phosphine-oxazoline ligands developed in our group show high activity and high enantioselect

Full text of DOI:10.1021/jacs.6b11655

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Article
Mechanism Studies of Ir-Catalyzed Asymmetric
Hydrogenation of Unsaturated Carboxylic Acids
Mao-Lin Li, Shuang Yang, Xun-Cheng Su, Hui-Ling Wu, Liang-Liang Yang, Shou-Fei Zhu, and Qi-Lin Zhou
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 12 Dec 2016
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Mechanism Studies of Ir-Catalyzed Asymmetric Hydrogenation of
Unsaturated Carboxylic Acids
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MaoꢀLin Li1, ‡, Shuang Yang1, ‡, XunꢀCheng Su1, HuiꢀLing Wu1, LiangꢀLiang Yang1, ShouꢀFei Zhu*,1,
QiꢀLin Zhou*,1,2
1 State Key Laboratory and Institute of ElementoꢀOrganic Chemistry, Nankai University, Tianjin 300071, China
2 Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University, Tianjin 300071, China
ABSTRACT: The Irꢀcatalyzed asymmetric hydrogenation of olefins is widely used for production of valueꢀadded bulk and fine
chemicals. The iridium catalysts with chiral spiro phosphineꢀoxazoline ligands developed in our group show high activity and high
enantioselectivity in the hydrogenation of olefins bearing a coordinative carboxyl group, such as α,βꢀunsaturated carboxylic acids,
β,γꢀunsaturated carboxylic acids, and γ,δꢀunsaturated carboxylic acids. Here we conducted detailed mechanistic studies on these Irꢀ
catalyzed asymmetric hydrogenation reactions by using (E)ꢀ2ꢀmethylꢀ3ꢀphenylacrylic acid as a model substrate. We isolated and
characterized several key intermediates having Ir–H bonds under the real hydrogenation conditions. Particularly, an Ir(III) migratoꢀ
ry insertion intermediate was first isolated in an asymmetric hydrogenation reaction promoted by chiral Ir catalysts. That this interꢀ
mediate cannot undergo reductive elimination in the absence of hydrogen strongly supports the involvement of an Ir(III)/Ir(V) cycle
in the hydrogenation. On the basis of the structure of the Ir(III) intermediate, variableꢀtemperature NMR spectroscopy, and density
functional theory calculations, we elucidated the mechanistic details of the Irꢀcatalyzed hydrogenation of unsaturated carboxylic
acids and explained the enantioselectivity of the reactions. These findings experimentally and computationally elucidate the mechaꢀ
nism of Irꢀcatalyzed asymmetric hydrogenation of olefins with a strong coordinative carboxyl group and will likely inspire further
catalyst design.
INTRODUCTION
Owing to its perfect atom economy, operational simplicity,
and mild reaction conditions, transitionꢀmetalꢀcatalyzed
asymmetric hydrogenation of unsaturated compounds is beꢀ
coming a very important method for the production of chiral
valueꢀadded bulk and fine chemicals. Studies of the mechaꢀ
nism have attracted considerable interest1 because mechanistic
information and the isolation of key active intermediates can
guide the rational design of new chiral catalysts. The Rh comꢀ
plexes of chiral diphosphine ligands (chiral analogues of Wilꢀ
kinson catalyst) and Ir complexes of chiral P,Nꢀligands (chiral
analogues of Crabtree catalyst) are the most representative
chiral catalysts for the asymmetric hydrogenation of olefins.1
Figure 1. Schematic of the catalytic cycles for transitionꢀmetalꢀ
Generally speaking, Rhꢀ or Irꢀcatalyzed asymmetric hydroꢀ
catalyzed asymmetric hydrogenation reactions. M = Rh or Ir; L*
genation of olefins involves activation of both hydrogen and a
= chiral ligand.
substrate, and the catalytic cycle consists of three basic steps:
However, although extensive mechanistic studies of Irꢀ
oxidative addition of hydrogen to the transition metal atom M
catalyzed asymmetric hydrogenation of unfunctionalized oleꢀ
(Rh or Ir), migratory insertion of the unsaturated group of the
fins have been conducted through isolation of active intermeꢀ
substrate into the M–H bond, and reductive elimination of the
diates or calculations,3 the migratory insertion intermediates of
metal from the migratory insertion intermediate (Figure 1).
Migratory insertion intermediates I are particularly valuable
chiral Ir catalysts have, to the best of our knowledge, never
been isolated and characterized.4 As a result, the mechanism of
for mechanistic studies because they contain both of the actiꢀ
Irꢀcatalyzed asymmetric hydrogenation still lacks a critical
vated reaction components (hydrogen and the olefin substrate)
jigsaw. Because of containing highly active M–H bond and
and can therefore provide the evidence for understanding chiꢀ
M–C bond, the migratory insertion intermediates of the Irꢀ
ral induction by the catalyst. Benefiting from the characterizaꢀ
catalyzed hydrogenation of olefins are very unstable and diffiꢀ
cult to isolate and characterize.
tion of migratory insertion intermediates of chiral Rhꢀ
diphosphine catalysts through NMR at low temperature, the
catalytic cycle of Rhꢀcatalyzed asymmetric hydrogenation of
We recently developed iridium catalysts with chiral spiro
phosphineꢀoxazoline (SIPHOX) ligands (Figure 2).5 Unlike
the other chiral iridium catalysts with phosphineꢀoxazoline
ligands, which exhibit high activity as well as enantioselectiviꢀ
ty in the hydrogenation of unfunctionalized olefins, the Irꢀ
olefins has been wellꢀestablished.2
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SIPHOX catalysts showed excellent activity and enantioselecꢀ mation, SI, Figures S1 and S5). This result indicated that the
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tivity in the hydrogenation of olefins bearing a coordinative
carboxyl group, such as α,βꢀunsaturated carboxylic acids,6 β,γꢀ
unsaturated carboxylic acids,7 and γ,δꢀunsaturated carboxylic
acids.8 The high stability of these catalysts under hydrogenaꢀ
tion conditions5 allows us to probe the mechanism of the Irꢀ
catalyzed asymmetric hydrogenation by trapping the active
intermediates. Herein we report our findings in the study of
the mechanism of asymmetric hydrogenations catalyzed by Irꢀ
SIPHOX catalysts: specifically, we describe the first isolation
and identification of an active Ir(III) migratory insertion inꢀ
termediate. That the Ir(III) migratory insertion intermediate
cannot undergo reductive elimination in the absence of hydroꢀ
gen gas provides strong evidence to support the involvement
of an Ir(III)/Ir(V) cycle in these reactions. On the basis of the
reactivity of the Ir(III) migratory insertion intermediate, variaꢀ
bleꢀtemperature nuclear magnetic resonance (NMR) spectrosꢀ
copy experiments, and density functional theory (DFT) calcuꢀ
lations, we explain the enantioselectivity of these reactions.
Our findings elucidate the mechanism of Irꢀcatalyzed asymꢀ
metric hydrogenation of olefins with strong coordinative
groups.
oxidative addition of the H–H bond to the Ir atom generated
an iridium dihydride in which the two hydrides are oriented cis
to the phosphorus atom of the ligand because the strong trans
effect of the phosphorus atom weakened the trans Ir–H bond.10
DFT calculations showed that the distances between the axial
hydride and one of the hydrogen atoms on the phenyl ring of
the ligand differ in the two Irꢀdihydride species (2.30 and 2.05
Å in 2a and 2b, respectively) and that 2a is more stable than
2b.11 The calculations also suggested that 2a and 2b may be in
kinetic equilibrium during the hydrogenation. However, beꢀ
cause 2a is the more thermodynamically stable product and
because its calculated structure is more consistent with the
NMR results, we suggest that 2a is the most likely product.
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Figure 3. Preparation of an iridium dihydride species.
Treatment of 2a with 1 equivalent of sodium (E)ꢀ2ꢀmethylꢀ
3ꢀphenylacrylate in methanol at 25 °C generated migratory
insertion product 3 in 97% yield within 5 min (Figure 4, Eq.
a). All of the organic components of 3, including the SIPHOX
ligand and the partially hydrogenated substrate, were unamꢀ
biguously identified by means of NMR spectroscopy. We asꢀ
signed the coordination geometry of this chiral octahedral
iridium complex on the basis of ligand field theory. The obꢀ
servation of a doublet at −31.51 ppm (see SI, Figure S12, 2JPꢀH
= 29.2 Hz) in the 1H NMR spectrum indicated that the hydride
was opposite to a vacant orbital or to a coordinated solvent
molecule (MeOH); this determination was made on the basis
of previously reported data for Irꢀhydride complexes.3n,12 The
1H NMR spectrum also showed a doublet (3JHꢀH = 6.8 Hz) for
the methyl group of the partially hydrogenated substrate, conꢀ
firming that a hydride in 2a had been transferred to the αꢀ
carbon of the carboxyl group in the substrate. In the 13C NMR
(see SI, Figures S14 and S15) and twoꢀdimensional NMR
spectra of 3 (see SI, Figures S16–S20), the coupling constants
Figure 2. IrꢀSIPHOXꢀcatalyzed enantioselective hydrogenation of
olefins containing a coordinative carboxy group.
RESULTS AND DISCUSSION
Isolation of Iridium Migratory Insertion Interme-
diate. Treatment of chiral IrꢀSIPHOX catalyst (Sa)ꢀ19 with
hydrogen gas in methanol solution at 25 °C produced an Irꢀ
dihydride species in quantitative yield (Figure 3). NMR specꢀ
troscopy and highꢀresolution mass spectrometry clearly indiꢀ
cated that this species was either 2a or 2b. This assignment
1
was based on the observation of the H NMR signal for P–Ir–
H as a double doublet at –20.23 and –31.01 parts per million
(ppm) (JPꢀH = 20.8 and 24.8 Hz, where J is the coupling conꢀ
stant between the phosphorus and the hydride) and the 31P
NMR signal as a singlet at 22.82 ppm (see Supporting Inforꢀ
between
the
phosphorus
and
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Figure
4.
Structural
analysis
and
transformations
of
migratory
insertion
intermediate
3.
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2
the Ir–C (δ = 17.8 ppm, JPꢀC = 76 Hz) and between the phosꢀ
phorus and the αꢀcarbon of the carboxyl group (δ = 49.5 ppm,
3JPꢀC = 10 Hz) implied that the Ir–C was located trans to the
phosphorus atom. Nuclear Overhauser effect spectroscopy
clearly showed interactions between the hydrogen on the benꢀ
zene ring of the substrate and the hydrogen on the oxazoline
ring of the ligand, and these results also supported the trans
location of the Ir–C relative to the phosphorus atom. Additionꢀ
ally, the exact formula of 3 was confirmed by highꢀresolution
mass spectrometry (calcd for C58H72IrNO3P+: 1054.4874;
found: 1054.4860). Hence, the threeꢀdimensional structure of
3 was assigned as shown in Figure 4. The carboxyl group of 3
played an important role in stabilizing the structure, which
enabled us to isolate this Ir(III) migratory insertion intermediꢀ
ate.
corresponding alkane, and an Ir(V) species might be involved
in this process. This experimental result rules out the involveꢀ
ment of an Ir(I)/Ir(III) cycle and strongly supports an
Ir(III)/Ir(V) cycle for the hydrogenation of olefins under these
conditions.
Variable-Temperature NMR Experiments. With
these key intermediates in hand, we conducted variableꢀ
temperature 1H and 31P NMR experiments (see SI, Figure S42)
to track the progress of the hydrogenation reaction (Figure 5).
When a solution of 2a in CD3OD under argon in an NMR tube
was treated with sodium (E)ꢀ2ꢀmethylꢀ3ꢀphenylacrylate at 243
K, a set of new iridium hydride signals appeared (Figure 5b).
By analogy to Pfaltz’s observations,3n we attributed these new
signals to Ir(III) dihydride olefin complex 6S (see its structure
in Figures 6 and 7), an intermediate that formed prior to miꢀ
gratory
Intermediate 3 cannot undergo reductive elimination sponꢀ
taneously in the absence of hydrogen gas. However, when 3
was heated or kept in solution for 24 h, it underwent β–H
elimination and subsequent formation of binuclear complex 4,
which has only one unsaturated carboxylate (Figure 4, Eq. b).
However, our attempts to grow a single crystal of 3 resulted in
the formation of a single crystal of 4 instead. Xꢀray diffraction
analysis of 4 showed that the carboxyl group bridges two iridꢀ
ium centers, each of which has an axial terminal hydride (H1
and H2, crystal 4, Figure 4).13 Note that a triflate anion has
been omitted from all the structures for clarity.
In contrast, under 1 atm of hydrogen, 3 reacted smoothly at
25 °C to form complex 5 within 0.5 h (Figure 4, Eq. c). Xꢀray
diffraction analysis of a crystal of 5 (the ligands in 5 have R
configurations) indicated that it, like 4, has a binuclear strucꢀ
ture, but with a fully hydrogenated carboxylate as the bridging
ligand. Both 4 and 5 can be used as catalyst precursor for the
hydrogenation of (E)ꢀ2ꢀmethylꢀ3ꢀphenylacrylic acid. Under 1
atm of deuterium, intermediate 3 was hydrogenated to a prodꢀ
uct containing 50% D at the newly formed C–H(D) bond at
the βꢀposition (see SI, Figures S39–S40), which implies that
the H atoms involved in the reductive elimination step came
from both the hydride of 3 and the hydrogen molecule (see the
discussion below). Clearly, the addition of hydrogen gas was
necessary to convert migratory insertion intermediate 3 to the
1
Figure 5. Variableꢀtemperature H NMR analysis of the hydroꢀ
genation in CD3OD.
insertion. At 288 K, the signals of 6S disappeared, and signals
of 3 and 4 appeared (Figure 5c). When the temperature was
increased to 298 K and kept there for 10 min, most of the 4
was transformed to migratory insertion intermediate 3 (conꢀ
taining about 5% of 4) (Figure 5d). The solution of 3 was then
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treated with hydrogen gas as the temperature was increased the catalytic cycle shown in Figure 7. The cycle starts with
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from 243 to 298 K (Figures 5e–g). The transformation of 3 to
5 started at 288 K and was complete by the time the temperaꢀ
ture reached 298 K.
iridium dihydride complex 6S, which was formed from a coꢀ
ordination of unsaturated carboxylate substrate to the iridium
dihydride 2a, which in turn undergoes migratory insertion of
an Ir–H bond to give intermediate 3. In addition, bridging of
two molecules of Ir(III) species 2a with a single unsaturated
carboxylate molecule can result in the formation of dimer 4.
Intermediate 3 is further oxidized by hydrogen to Ir(V) species
9c, which undergoes a reductive elimination to give an interꢀ
mediate 10. The ligand exchange of intermediate 10 with unꢀ
saturated carboxylate substrate releases the hydrogenation
product and regenerates the iridium dihydride complex 6S.
The carboxyl group of the hydrogenation product, saturated
carboxylate can also serve as a bridging ligand to generate
dimer 5, which serves as a resting catalyst for the next cycle.
Intermediates 2a, 3, 4, 5, and 6S were unequivocally identified
experimentally. The structures of Ir(V) intermediate 9c and
Ir(III) intermediate 10, chiral induction transition state TS-S,
and transition state TS-III/V were obtained by means of DFT
calculations.
Computational studies. To improve our understanding
of the details of the reaction mechanism, we performed DFT
studies using the reaction of iridium dihydride 2a with (E)ꢀ2ꢀ
methylꢀ3ꢀphenylacrylic acid as a model. First, we calculated
the chiral induction step, conversion of 2a to 3 (Figure 6a).14
There are two possible pathways for the formation of migratoꢀ
ry insertion product 3 from 2a, which are designated the R
pathway and the S pathway (Figure 6a) and which proceed via
different olefin coordination models (6S and 7R, respectively)
with the equilibration between 6 and 7.15 The calculated enerꢀ
gy of transition state TS-S was 4.3 kcal/mol lower than that of
transition state TS-R, which suggests that the reaction should
proceed with perfect enantioselectivity to afford the S product,
which is consistent with the experimental result (99% ee, with
S configuration).
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Numerous attempts to isolate an Ir(V) intermediate were
unsuccessful. We attribute this to the high thermal instability
of such intermediates. Fortunately, DFT studies can provide
insight into the formation and reactions of Ir(V) intermediates
(Figures 6b and 6c). According to our calculations, coordinaꢀ
tion of a hydrogen molecule to Ir(III) complex 3 can give eiꢀ
ther of the two isomeric Ir(III) dihydrogen complexes, 8a or
8b. Our calculations suggest that 8b, which is more stable than
8a, has a lower energy barrier in the subsequent oxidative
addition transition state TS-8b/9. Interestingly, 8b cannot diꢀ
rectly form Ir(V) trihydride intermediate 9c via TS-8b/9; inꢀ
stead, 8b rearranges to 9a and then 9b, which in turn underꢀ
goes oxidative addition to afford 9c. This high valent structure
readily undergoes reductive elimination via transition state
TS-III/V with an 11.6 kcal/mol energy barrier, which is 5.3
kcal/mol lower than that for transition state TS-I/III. Like the
experimental results, the DFT results suggest that the hydroꢀ
genation reaction preferentially proceeds via an Ir(III)/Ir(V)
pathway. The unexpected hydrogen rearrangement sequence
from 8b to 9c (Figure 6b) explains the deuterium distribution
in the product of the deuteration experiment (Figure 4, Eq. d).
In the reductive elimination of 9c, only the hydrogen or the
deuterium in the N–O–C–Ir plane can participate in the reacꢀ
tion. Because the N–O–C–Ir plane in 9c contains one hydroꢀ
gen and one deuterium, the reductive elimination reaction
gives a product containing 50% D at the newly formed C–H
bond at the βꢀposition. Another two alternative mechanisms to
generate Ir(V) including a concerted oxidative addition of
sideꢀon bound H2 with concomitant reductive elimination
mechanism (see SI, Figures S45–S46) were also calculated but
both exhibit unfavorable energy barrier comparing with the
pathway of Figure 6b.
It is noteworthy that an Irꢀdihydrideꢀolefin complex identiꢀ
fied as an intermediate in the hydrogenation of an unfunctionꢀ
alized substrate3n does not undergo migratory insertion in the
absence of H2, in contrast to the findings in this study. In anꢀ
other study using a lactone with an exocyclic C=C bond as
substrate3o evidence for an Ir(I)/(III) mechanism was found.
This mechanistic divergence indicates that the mechanism of
Irꢀcatalyzed hydrogenation of alkenes may vary depending on
the type of substrate and/or catalyst.
Figure 7. Catalytic cycle for the hydrogenation reaction.
The Ir(III)/Ir(V) Catalytic Cycle. Taken together, our
results suggest that the hydrogenation reaction proceeds via
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Figure 6. Results of DFT calculations for a model reaction. a, Calculated Gibbs energy profile for the chiral induction step. The migraꢀ
tory insertion reactions of the two Ir(III) dihydride olefin complexes (6S and 7R) proceed via different fourꢀmemberedꢀring transition
states, TS-R and TS-S, respectively, leading to different products. b, Calculated Gibbs energy profile for the Ir(III)/Ir(V) and Ir(I)/Ir(III)
pathways. The Ir(III)/Ir(V) pathway requires additional H2 for the oxidative addition reaction of 3 to form 9c. The calculated energies for
reductive elimination reactions via transition states TS-III/V and TS-I/III are also shown. c, Oxidative addition of and rearrangement of
the H–H bond in going from 8b to 9c along the Ir(III)/Ir(V) pathway. Hydrogen atoms bound to carbon are omitted for clarity.
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(2) (a) Halpern, J. Science 1982, 217, 401. (b) Brown, J. M. Organo-
metallics 2014, 33, 5912.
CONCLUSION
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In summary, detailed mechanistic studies on the enantioseꢀ
lective hydrogenation of an unsaturated carboxylic acid cataꢀ
lyzed by an iridium complex with a chiral SIPHOX ligand
were conducted. The high stability of the IrꢀSIPHOX catalyst
enabled us to isolate and fully characterize key intermediates
in the hydrogenation process, including intermediate 3. To our
knowledge, this is the first report of the isolation of a migratoꢀ
ry insertion intermediate in an asymmetric hydrogenation reꢀ
action promoted by chiral Crabtreeꢀtype catalysts. That 3
could not undergo reductive elimination in the absence of hyꢀ
drogen but smoothly gave the hydrogenation product in the
presence of hydrogen rules out the Ir(I)/Ir(III) cycle and
strongly supports an Ir(III)/Ir(V) cycle. With this information
about the intermediates, we tracked the hydrogenation process
by means of variableꢀtemperature NMR, and we performed
DFT calculations to illustrate the full catalytic cycle and raꢀ
tionalize the enantioselectivity of the reaction. The migratory
insertion intermediate contains both of the activated compoꢀ
nents of the reaction (the hydrogen and the unsaturated subꢀ
strate) and is the key for studying the reaction pathway and
chiral induction by the catalyst. The isolation and characterizaꢀ
tion of 3 elucidated the mechanism of Irꢀcatalyzed asymmetric
hydrogenation of olefins with a coordinative carboxyl group
and will likely inspire further catalyst design.
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(4) An alkyl Ir hydride complex with a chiral P,N ligand was isolated
by Maurer and Kazmaier; however, it proven to be an inactive comꢀ
ponent towards the hydrogenation. (a) Maurer, F.; Kazmaier, U. J.
Org. Chem. 2013, 78, 3425. A migratory insertion intermediate of Ru
catalyst has been isolated and characterized in asymmetric hydrogenaꢀ
tion reaction. However, the mechanism is different; it contains a hetꢀ
erolytic H2 activation. (b) Wiles, J. A.; Bergens, S. H.; Young, V. G.
J. Am. Chem. Soc. 1997, 119, 2940.
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ASSOCIATED CONTENT
Supporting Information.
Metrical parameters for the structures are available free of charge
from the Cambridge Crystallographic Data Centre under reference
numbers CCDCꢀ1496895 and 1496894. Experimental procedures
and spectral data. The Supporting Information is available free of
charge via the Internet at http://pubs.acs.org.
(5) Zhu, S.ꢀF.; Xie, J.ꢀB.; Zhang, Y.ꢀZ.; Li, S.; Zhou, Q.ꢀL. J. Am.
Chem. Soc. 2006, 128, 12886.
(6) (a) Li, S.; Zhu, S.ꢀF.; Zhang, C.ꢀM.; Song, S.; Zhou, Q.ꢀL. J. Am.
Chem. Soc. 2008, 130, 8584. (b) Li, S.; Zhu, S.ꢀF.; Xie, J.ꢀH.; Song,
S.; Zhang, C.ꢀM.; Zhou, Q.ꢀL. J. Am. Chem. Soc. 2010, 132, 1172. (c)
Song, S.; Zhu, S.ꢀF.; Pu, L.ꢀY.; Zhou, Q.ꢀL. Angew. Chem. Int. Ed.
2013, 52, 6072. (d) Song, S.; Zhu, S.ꢀF.; Li, Y.; Zhou, Q.ꢀL. Org.
Lett. 2013, 15, 3722.
AUTHOR INFORMATION
Corresponding Author
(7) Song, S.; Zhu, S.ꢀF.; Yang, S.; Li, S.; Zhou, Q.ꢀL. Angew. Chem.
Int. Ed. 2012, 51, 2708.
* sfzhu@nankai.edu.cn
* qlzhou@nankai.edu.cn
(8) (a) Song, S.; Zhu, S.ꢀF.; Yu, Y.ꢀB.; Zhou, Q.ꢀL. Angew. Chem. Int.
Ed. 2013, 52, 1556. (b) Yang, S.; Zhu, S.ꢀF.; Guo, N.; Song, S.; Zhou,
Q.ꢀL. Org. Biomol. Chem. 2014, 12, 2049.
Author Contributions
‡These authors contributed equally.
(9) The catalysts with BArFꢀ or OTfꢀ as the anion gave the same reꢀ
sults (99% yield and 99% ee) in the hydrogenation of (E)ꢀ2ꢀmethylꢀ3ꢀ
phenylacrylic acid. For simplifying the NMR analysis, we used [(S)ꢀ
1]+OTfꢀ in our study.
Notes
The authors declare no competing financial interest.
(10) (a) Mazet, C.; Smidt, S. P.; Meuwly, M.; Pfaltz, A. J. Am. Chem.
Soc. 2004, 126, 14176. (b) Crabtree, R. H. The Organometallic Chem-
istry of the Transition Metals, 6th Edition. Wiley, 2014.
(11) (a) Bondi, A. J. Phys. Chem. 1964, 68, 441. (b) Rowland R. S.;
Taylor R. J. Phys. Chem. 1996, 100, 7384.
(12) (a) Scott, N. M.; Pons, V.; Stevens, E. D.; Heinekey, D. M.;
Nolan, S. P. Angew. Chem. Int. Ed. 2005, 44, 2512. (b) Castillo, M.
R.; Martín, M.; Fraile, J. M.; Mayoral, J. A.; Sola, E. Angew. Chem.
Int. Ed. 2011, 50, 3240.
(13) For other dinuclear iridium hydride complexes, see: Gruber, S.;
Neuburger, M.; Pfaltz, A. Organometallics 2013, 32, 4702.
(14) Different isomeric transition states have been calculated (see SI,
Figure S44), and the one with lowest energy is presented in Figure 6.
(15) For a similar equilibration, see: Gruber, S. Organometallics
ACKNOWLEDGMENTS
We thank Dr. XiaoꢀSong Xue for his constructive discussions on
DFT calculations. We thank the National Natural Science Foundaꢀ
tion of China, the National Basic Research Program of China
(2012CB821600), the “111” project (B06005) of the Ministry of
Education of China, and the National Program for Support of
Topꢀnotch Young Professionals for financial support.
REFERENCES
(1) (a) de Vries, J. G.; Elsevier, C. J. (Eds). The Handbook of Homo-
geneous Hydrogenation, WileyꢀVCH: Weinheim, 2007. (b) Osborn, J.
A.; Jardine, F. H.; Young, J. F.; Wilkinson, G. J. Chem. Soc. A 1966,
1711. (c) Crabtree, R. Acc. Chem. Res. 1979, 12, 331.
2016,
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