Mechanism of the asymmetric hydrogenation of exocyclic α,β- unsaturated carbonyl compounds with an iridium/BiphPhox catalyst: NMR and DFT studies

Article abstract of DOI:10.1002/anie.201309677

The mechanism of the asymmetric hydrogenation of exocyclic α,β-unsaturated carbonyl compounds with the (aS)-Ir/iPr-BiphPhox catalyst was studied by NMR experiments and DFT computational analyses. Computed optical yields of the asymmetric hydrogenation proceeding by an iridium(I)/iridium(III) mechanism involving a transition state stabilized through two intramolecular hydrogen bonds are in good accordance with the experimental ee values. H in a fix: The title studies show an equilibrium of dinuclear iridium hydrides at low temperature after hydrogenation of the precatalyst. Computations of the conceivable R and S pathways allowed elucidation of the mechanism for enantioselection, thus affording computed optical yields which are consistent with the experimental values. The structure S1 features hydrogen bonding between the catalyst and the carbonyl group of the substrate. Copyright

Full text of DOI:10.1002/anie.201309677

Angewandte
Chemie
DOI: 10.1002/anie.201309677
Structure Elucidation
Mechanism of the Asymmetric Hydrogenation of Exocyclic a,b-
Unsaturated Carbonyl Compounds with an Iridium/BiphPhox Catalyst:
NMR and DFT Studies**
Yuanyuan Liu, Ilya D. Gridnev,* and Wanbin Zhang*
Abstract: The mechanism of the asymmetric hydrogenation of
exocyclic a,b-unsaturated carbonyl compounds with the (aS)-
Ir/iPr-BiphPhox catalyst was studied by NMR experiments
and DFT computational analyses. Computed optical yields of
the asymmetric hydrogenation proceeding by an iridium(I)/
iridium(III) mechanism involving a transition state stabilized
through two intramolecular hydrogen bonds are in good
accordance with the experimental ee values.
and heterocycles reported recently,[3,4] it is important to
understand the factors governing enantioselection in these
reactions.
As the (aS)-Ir/iPr-BiphPhox complex 1 demonstrated
excellent results in the iridium-catalyzed asymmetric hydro-
genation of exocyclic a,b-unsaturated carbonyl compounds[8]
(Scheme 1), we decided to use it in our mechanistic study.
M
etal-catalyzed asymmetric hydrogenation is a well-estab-
lished method of longstanding interest because of its high
efficiency for preparing enantiomerically enriched com-
pounds.[1–4] Recently, Ir/P,N complexes have attracted much
attention because of their ready availability, high reactivity,
and high enantioselectivity in asymmetric hydrogenation
reactions.[3,4] However, the mechanism of the iridium-cata-
lyzed asymmetric hydrogenation has not been studied in
sufficient detail[5] when compared to the corresponding
ruthenium-[6] and rhodium-catalyzed[7] reactions. Pfaltz et al.
described the quantitative formation of solvate dihydrides
upon hydrogenation of an iridium catalytic precursor at low
temperatures.[5a,g] Computationally the hydrogenation of
unfunctionalized olefins was studied by Andersson and co-
workers and Burgess and co-workers using DFT computa-
tions, thus revealing an interplay of iridium(III) and iri-
dium(V) intermediates,[5b,e,f,h] and the possibility of an iri-
dium(I)/iridium(III) mechanism was discussed and supported
by Chen et al.[5d,i,l] In view of the numerous successful iridium-
catalyzed hydrogenations of various functionalized olefins
Scheme 1. Asymmetric hydrogenation of exocyclic a,b-unsaturated
carbonyl compounds catalyzed by (aS)-Ir/iPr-BiphPhox. BArF =tetrakis-
[3,5-bis(trifluoromethyl)phenyl]borate.
A solution containing 17 mg of 1 in 0.5 mL of CD2Cl2 was
hydrogenated at À408C for 10 minutes.[9] The 1,5-cycloocta-
diene ligand of 1 is immediately hydrogenated to cyclooctane
upon introduction of hydrogen, even at decreased temper-
atures. The initial spectrum contained several species. After
storage of the sample for a week in a freezer (À188C), its
1H NMR spectrum (Figure 1) at 203 K contained two diaste-
[*] Y. Liu, Prof. W. Zhang
School of Chemistry and Chemical Engineering
Shanghai Jiao Tong University
800 Dongchuan Road, Shanghai 200240 (China)
E-mail: wanbin@sjtu.edu.cn
Homepage: http://wanbin.sjtu.edu.cn
Prof. I. D. Gridnev
Department of Chemistry, Graduate School of Science
Tohoku University
Aramaki 3-6, Aoba-ku, Sendai 9808578 (Japan)
E-mail: igridnev@m.tohoku.ac.jp
Figure 1. Section plot of the 1H NMR spectrum (700 MHz, CD2Cl2,
213 K) for the five hydrides of the dimer 2a.
[**] Computational results in this research were obtained using the
supercomputing resources at the Information Synergy Center, Tokyo
Institute of Technology. This work was partially supported by the
National Natural Science Foundation of China (21172143 and
21232004), Nippon Chemical Industrial Co., Ltd, Shanghai Jiao
Tong University, and CAMPUS Asia Program of Tohoku University.
BiphPhox = 2-(2’-(diphenylphosphino)-[1,1’-biphenyl]-2-yl)-4,5-
dihydrooxazole.
reomers of a pentahydride complex (2a,b) with three bridging
hydride ligands in a 5:1 ratio (see the Supporting Informa-
tion). A broadening of the peaks of the minor 2b was
observed when the temperature was increased. Even upon
a slight rise in temperature from 203 K to 213 K the peaks of
2b broadened dramatically and became difficult to distin-
guish.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201309677.
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cis dihydride can accommodate the sub-
strate in four possible manners). However,
it is a well-established fact in for transition
metal catalyzed hydrogenations that the
hydride trans to the phosphorus atom must
be transferred in the migratory insertion
stage.[5e] Therefore, we restricted our anal-
ysis to eight different pathways (four S and
four R) initiated by the different modes of
the substrate coordination coplanar to the
P-Ir-H bond (Figure 2). In the consider-
Scheme 2. Equilibrium of dimers found at low temperature.
Each isomer exhibited five hydride resonances in the
d range from À4 to À30 ppm. The relatively low-field
hydrides of 2a and 2b resonated as a doublet and triplet
with the 1JIr-H coupling constants of 98 and 74 Hz, respectively.
These data confirm the relative configuration of the two
chelate cycles in 2a and 2b shown in Scheme 2. In total there
are seven possible isomers of 2 and it is difficult to assign the
structures of 2a,b precisely by NMR spectroscopy alone. We
hypothesize that the dimer pentahydrides 2a,b have the
structures shown in Scheme 2, and they are based on 2D
NMR (1H-1H COSY, 1H-1H ROESY and 1H-31P HMBC) and
computational data.[9] In recent reports similar structure was
also found in rhodium-catalyzed hydrogenation.[10]
If the NMR spectrum was taken directly after low-
temperature hydrogenation, other species were observed.
Two major peaks at d = À17.60 ppm (d, JIr-H = 41.3 Hz) and
Figure 2. Eight possible pathways with the substrate 5 coordinated
coplanar to the P-Ir-H moiety. CSH=catalyst/hydride/substrate com-
plex.
1
at d = À29.78 ppm (brs) were seen in the hydride region of
the 1H NMR spectrum. Additionally, three sets of smaller
signals of a similar shape, but lower intensity, were observed
in close proximity to the two main signals. These observations
correspond to the initial formation of several diastereomers
of dinuclear iridium tetrahydrides (3) which are in equilibri-
um with the monomer solvate complexes 4. Computational
analysis showed that the formation of the dimers 3 from the
monomers 4 is significantly exogonic. The symmetry of the
spectrum of the major isomer of 3 corresponds well to the
structure of the most stable among the computed structures of
diastereomers. If the temperature of the hydrogenated sample
was raised over 08C, a rapid loss of hydrogen accompanied by
the decomposition of the catalyst occurred.
ation of an iridium(III)/iridium(V) mechanism,[5b,c,e,f,h,j] we
have also tried to coordinate another molecule of dihydrogen
to iridium either in the dihydride complexes or in the
monohydride intermediates formed after the migratory
insertion step. However, hydrogen invariably escaped
during the computational optimizations. Hence, we were
unable to confirm any involvement of a iridium(III)/iri-
dium(V) mechanism in our case.
The energies of the transition states, for the migratory
insertion steps of the pathways with hydride initially trans-
ferred to the double-linked carbon atom on the five-
membered ring, were calculated to be more than 6 kcalmolÀ1
higher than those in the other four pathways. From a chemical
point of view, this means an easier hydride transfer to the
protonated carbon atom of the double bond. Therefore we
further analyzed the pathways S1, S2, R1, and R2 (Figure 3).
The migratory insertion step of the pathway 1 was calculated
to be S stereoselective (the difference in stabilities of the
migratory insertion transition states of S1 and R1 pathways is
2.1 kcalmolÀ1). In contrast, the transition states of the S2 and
R2 pathways were found to be almost equally stable. Hence,
the experimentally observed S-enantioselective reaction
means that the migratory insertion by the R2 pathway does
not actually take place.
The addition of 1 equivalent of the prochiral substrate 5 to
a solution containing 2 at À188C resulted in slow quantitative
formation of the hydrogenation product 6 (Figure 2). The
enantiomeric excess of 6 was determined to be 90% (S), that
is, of the same sign and value for the optical yield of the
preparative hydrogenation of 5 catalyzed by 1 [93% (S)]. We
have thoroughly investigated, computationally, the possibility
of a dimeric catalyst (either 2 or 3) and were convinced that
the steric repulsion strictly precludes coordination of the
substrate in either case. Hence, we concluded that the
hydrogenation is effectuated by the monomeric dihydride
complexes 4, and the formation of the pentahydrides 2 from
tetrahydride dimers 3 is also reversible.
There are a total of sixteen possible ways in which a C1-
symmetric prochiral substrate can coordinate to a C1-sym-
metric cis dihydride (each of the four diastereomers of
In search of a possible reason for this, we have simulated
the approach of the substrate to the catalyst, thus resulting in
the coordination of the substrate appropriate for the occur-
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Figure 3. Reaction pathways S1, S2, R1, and R2 for the migratory
insertion step in the hydrogenation of 5 catalyzed by 1.
Figure 5. Optimized structures of catalyst/hydride/substrate com-
plexes for the pathways R1, S1, and S2.
hydrogen bonding between the axial hydride and the carbonyl
group of the substrate.
Table 1 shows the calculated differences in the free
energies of the migratory insertion stage for the S1 and R1
pathways and the comparison to the experimentally observed
ee values in the catalytic reactions. One can see that in
complete accordance with the experimental data, the compu-
tational results reliably predict the S stereoselectivity in all
cases. Moreover, the substrate dependence of the optical
yields is also reasonably reproduced. Analysis of the tran-
sition-state structures suggests that the evident decrease of
the ee values in the case of the substrates containing six-
membered rings, is explained by the destabilization of the S
pathways as a result of the additional CH2 group of the
substrate.
Thus, we conclude that the asymmetric hydrogenation of
exocyclic a,b-unsaturated carbonyl compounds catalyzed by
a (aS)-Ir/iPr-BiphPhox catalyst takes place by initial forma-
tion of iridium dihydrides which reversibly form various
dimeric species, but can be recovered to effect the catalysis.
The enantiodetermining step is the migratory insertion which
preferably takes place in the catalyst–substrate adducts with
Figure 4. Energy scans simulating the approach of 5 to the catalytic
hydride. The final minima correspond to the completely optimized
structures of the adducts.
rence of the migratory insertion by S2 and R2 pathways. The
corresponding energy scans are shown in the Figure 4.
When approaching the catalyst by the S2 pathway, the
substrate rapidly finds a minimum through hydrogen-bond
formation with the axial hydride. In contrast, for the case of
the R2 pathway such an interaction is reliably switched off by
one of the phenyl substituents of the catalyst.[11] As a result,
only the pathway R1 (Figure 5) must be considered as
a competing pathway to the formation of the S product by
either of the S1 or S2 pathways.
=
The optimized structures of catalyst/hydride/substrate
complexes for the R1, S1, and S2 pathways are shown in
Figure 5. The S pathways have hydrogen-bonding effects
between the carbonyl group of the substrate and the hydrogen
atom on the phenyl ring of the catalyst. In contrast, in the
R pathway there is steric hindrance between the CH2 group of
the five-membered ring of the substrate and the phenyl ring of
the catalyst. The transition state of the pathway S1 is slightly
more stable than that of S2, and may be due to the additional
the C CHPh group oriented coplanar to the trans-P-Ir-H
fragment. One of the possible R pathways is effectively
switched off at the stage of the substrate coordination, and
the order of enantioselection is regulated by the difference in
the stabilities of the transition states of migratory insertion for
two S pathways and one R pathway.
Received: November 6, 2013
Published online: January 21, 2014
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[a]
Substrates
DG(S1ÀR1)
Experimental
DGexp[c]
[kcalmolÀ1
]
ee [%][b]
[kcalmolÀ1
]
2.1
1.2
2.9
3.6
95
92
98
99
2.16
1.87
2.71
3.12
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0.6
1.1
71
53
1.05
0.70
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[a] DG(S1-R1) =free-energy differences of the transition states for the
migratory insertion step in pathways S1 and R1. Computed under 20 atm
at 298.15 K. [b] Enantioselectivities of experimental catalytic hydrogena-
tion reaction. [c] DGexp =expected values derived from the experimental
ee values.[12]
Keywords: density functional calculations · hydrogenation ·
.
iridium · NMR spectroscopy · reaction mechanisms
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52.6 Hz), À17.66 (s), À25.02 (t, 27.6 Hz), À29.92 ppm (s);
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1H NMR hydride region (700 MHz, CD2Cl2, 203 K): d = À3.93
(t, 74.2 Hz), À10.61 (brs), À15.80 (brs), À17.75 (brs),
À21.57 ppm (brs); 31P NMR (283 MHz, CD2Cl2, 203 K): d =
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