Asymmetric Magnesium-Catalyzed Hydroboration by Metal-Ligand Cooperative Catalysis

Article abstract of DOI:10.1002/anie.201908012

Asymmetric catalysis with readily available, cheap, and non-toxic alkaline earth metal catalysts represents a sustainable alternative to conventional synthesis methodologies. In this context, we describe the development of a first Mg^II-catalyzed enantioselective hydroboration providing the products with excellent yields and enantioselectivities. NMR spectroscopy studies and DFT calculations provide insights into the reaction mechanism and the origin of the enantioselectivity which can be explained by a metal-ligand cooperative catalysis pathway involving a non-innocent ligand.

Full text of DOI:10.1002/anie.201908012

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201908012
German Edition: DOI: 10.1002/ange.201908012
Hydroboration
Asymmetric Magnesium-Catalyzed Hydroboration by Metal-Ligand
Cooperative Catalysis
Alban Falconnet, Marc Magre,* Bholanath Maity,* Luigi Cavallo,* and Magnus Rueping*
Abstract: Asymmetric catalysis with readily available, cheap,
and non-toxic alkaline earth metal catalysts represents a sus-
tainable alternative to conventional synthesis methodologies.
In this context, we describe the development of a first MgII-
catalyzed enantioselective hydroboration providing the prod-
ucts with excellent yields and enantioselectivities. NMR
spectroscopy studies and DFT calculations provide insights
into the reaction mechanism and the origin of the enantiose-
lectivity which can be explained by a metal-ligand cooperative
catalysis pathway involving a non-innocent ligand.
best of our knowledge, the onward development of an
enantioselective hydroboration of polarized double bonds
using a chiral magnesium catalyst has not been reported.
Herein, we report an efficient method for the enantioselective
synthesis of chiral alcohols by employing a readily available
chiral magnesium complex (Scheme 1) and provide insight
into the reaction mechanism and the origin of the enantio-
selectivity.
T
he application of alkaline-earth metals in catalysis is
a topical area. Most of the group 2 metals are abundant in
the earthꢀs crust and are characterized by their low cost and
environmentally benign nature.[1] However, despite their
abundance and low toxicity, alkaline-earth metals such as
MgII have only been used in a limited number of catalytic
transformations.[2] This limited use may be due to their
propensity to undergo Schlenk-type equilibrium which results
in catalytically inactive species. To avoid these problems,
sterically demanding monoanionic ligands have been used to
stabilize the alkaline-earth metals. In particular, b-diketimine
ligands have been widely employed in group 2 catalysis as
their bulkiness inhibits any kind of ligand redistribution. As
a result, several magnesium-catalyzed transformations have
been developed as a green alternative to transition metal
catalyzed processes.[3,4] These transformation include the
impressive progress in the hydroboration of polarized and
nonpolarized unsaturated bonds by use of b-diketiminato-
stabilized magnesium complexes and the formation of
magnesium hydride species.[5] In addition, chiral magnesium-
(II) binaphtholates as cooperative Brønsted/Lewis acid–base
catalysts have been explored.[6,7] Based on their unexpectedly
high reactivity and sustainability, the use of chiral magnesium
catalysts for the accomplishment of an asymmetric hydro-
boration would therefore be a desirable goal. However, to the
Scheme 1. Magnesium-catalyzed asymmetric hydroboration of ketones.
The initial studies towards the development of an efficient
magnesium-catalyzed hydroboration of ketones[8] began with
the evaluation of different chiral magnesium complexes
derived from(R)-(+)-BINOL derivatives (1a–h), HBpin as
hydroborating agent, and acetophenone (2a) as a model
substrate (Table 1). With 1a as ligand at room temperature,
the transformation reached completion within less than
2 hours, affording, after protic quenching, the chiral alcohol
3a with moderate enantioselectivity (entry 1). We were
pleased to see that BINOL-Mg complexes were active in
the hydroboration of 2a given that to date only monoanionic
ligands have shown to be active in the non-selective hydro-
boration of unsaturated bonds. By decreasing the temper-
ature to ꢀ408C, the enantiomeric ratio increased to 93:7,
while the excellent activity was maintained (entry 1 vs. 2).
Other solvents were tested, however, the selectivity did not
improve (entries 2 vs. 3–6). From the different chiral (R)-
(+)-BINOL scaffolds (1a–h) tested, magnesium complexes of
ligand 1a provided the best enantioselectivity (entries 2 vs. 7–
13). Finally, we investigated the effect of alkali salt additives
as they can have an influence on the performance. However,
only a slight increase in enantioselectivity was observed
(entry 14 vs. 2).
[*] M. Sc. A. Falconnet, Dr. M. Magre, Prof. Dr. M. Rueping
Institute of Organic Chemistry, RWTH Aachen
Landoltweg 1, 52074 Aachen (Germany)
E-mail: marc.magre@oc.rwth-aachen.de
magnus.rueping@rwth-aachen.de
Dr. B. Maity, Prof. Dr. L. Cavallo, Prof. Dr. M. Rueping
KAUST Catalysis Center (KCC), King Abdullah University of Science
and Technology (KAUST)
Thuwal, 23955-6900 (Saudi Arabia)
E-mail: bholanath.maity@kaust.edu.sa
luigi.cavallo@kaust.edu.sa
To evaluate the scope of our novel enantioselective
magnesium-catalyzed hydroboration reaction a broad range
of prochiral ketones were applied as substrates (Scheme 2).
The use of differently substituted arylmethyl ketones afforded
the corresponding chiral secondary alcohols 3a–e in excellent
yields and comparable selectivities. When other electron-
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.201908012.
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Table 1: Optimization of the magnesium catalyzed hydroboration of
acetophenone.[a]
Entry
Solvent
Ligand
Conv. [%][b]
er[c]
1[d]
2
3
4
5
6
7
8
9
10
11
12
13
14[e]
toluene
toluene
Et2O
THF
DCM
hexane
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
1a
1a
1a
1a
1a
1a
1b
1c
1d
1e
1 f
>99
>99
>99
>99
>99
50
>99
>99
>99
>99
>99
>99
>99
76:24
93:7
71:29
54:46
89:11
50:50
74:26
59:41
88:12
89:11
84:16
63:37
84:16
95:5
1g
1h
1a
>99 (98)
[a] 2a (0.5 mmol), HBpin (1.5 equiv), MgBu2 (7 mol%, 1m in heptane),
1a–1h (7.5 mol%), solvent [0.4m] at ꢀ408C for 2 h. [b] Determined by
GC using n-dodecane as an internal standard. Yield after isolation in
given within parentheses. [c] Determined by chiral-phase HPLC. Abso-
lute configuration was assigned by comparison with literature values.
[d] At 258C. [e] LiCl (20 mol%).
withdrawing groups were present on the aromatic ring (2 f–g),
enantioselectivities decreased while maintaining the excellent
yields. It is important to highlight the excellent chemo- and
enantioselectivities observed when 2h, containing a methyl-
ester group, was tested. We were pleased to see that
substituents in the ortho-position of the aromatic ring were
also tolerated (3i–k). Furthermore, different acetophenone
derivatives containing polycyclic substituents (2l) and hetero-
cycles (2m,n) could be reacted to the corresponding alcohols
with high selectivities. Branched alkyl substituents (2o) as
well a range of electronically different 1-indanones (2p–s)
were hydroborated in excellent yields and enantioselectiv-
ities.
Generally, the hydroboration of a,b-unsaturated ketones
is more challenging as chemoselectivity as well as enantiose-
lectivity has to be taken into account. Therefore, to date only
few successful examples for the selective 1,2-hydroboration of
a,b-unsaturated ketones have been accomplished.[8d,e] We
were delighted to see that with our newly developed
magnesium catalysis, only selective 1,2-addition occurred
and that the corresponding allylic alcohols (3t–z) were
obtained in good yields and with excellent enantioselectiv-
ities. It is important to notice that in the presence of
a substituent in a-position (3t vs. 3u), higher enantioselectiv-
ities are obtained while maintaining the excellent yields and
regioselectivities. Likewise, different propargylic ketones
Scheme 2. Magnesium-catalyzed asymmetric hydroboration of ketones.
General reaction conditions: 2 (0.5 mmol), HBpin (1.5 equiv), MgBu2
(7 mol%, 1m in heptane), 1a (7.5 mol%), solvent [0.4m] at ꢀ408C for
3 h. [a] 24 h [b] MgBu2 (10 mol%), ligand (10.5 mol%). [c] Determined
from the 3ad-Bz, 3ae-Bz and 3af-Bz protected product. Absolute
configuration was assigned upon the assumption of a common
stereochemical mechanism.
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underwent the magnesium-catalyzed hydroboration well and
species (d = 34.6 ppm). Moreover, 1H NMR spectroscopy
afforded the alkynols 3aa–ae in excellent yields and enantio-
selectivities. This result surpassed the best results in the
literature,[9] showing the great efficiency of the enantioselec-
tive magnesium catalysis. Regarding the dialkyl ketone 3af,
excellent yield was obtained, albeit with very poor enantio-
selectivity, showing a limitation on the scope of this magne-
sium system.
showed that the signal of HBpin disappeared. However,
a Mg H species was not formed.[10] The groups of Nozaki and
ꢀ
Hill reported several Mg B activation modes.[11] Based on the
ꢀ
literature 11B NMR values and the Lewis acidity/basicity
affinity, the shift in the 11B NMR activation of the HBpin is
ꢀ
ꢀ
most probably due to a dual Mg O and O B coordination. In
this regard, recently Graves and co-workers reported a similar
activation mode of HBpin by tripodal trisnitroxide aluminum
catalyst.[12] Following the experimental results, we performed
DFT calculations to gain insight into the reaction pathway
and to define the origin of the enantioselectivity. Based on the
calculated results, we propose the following reaction pathway.
The reaction starts with the formation of the active catalyst by
coordination of the ligand to MgBu2 and synchronous
elimination of butane, a step exergonic by 66.4 kcalmolꢀ1
(see Scheme S1 in the Supporting Information). There is an
equilibrium between A and the corresponding oligomeric
species An as reported before. Coordination of the ketone
results into intermediate C by two successive exergonic steps
of 20.5 and 14.5 kcalmolꢀ1 (Figure 1).
To gain more information about the reaction mechanism,
we decided to identify the catalytically active species.
BINOLate-magnesium complexes form an equilibrium of
monomeric BINOL-Mg and oligomeric MgII species.[7a,d,g]
The oligomer can be dissociated into the active mono-
meric complex in the presence of any coordinating species. In
our NMR spectroscopy studies we also observed this phe-
nomenon (for detailed characterization, see the Supporting
Information).
Importantly, in the previously reported MgII-catalyzed
ꢀ
hydroborations of unsaturated bonds, a Mg H species was
been observed and can be formed from the alkyl magnesium
precatalyst.[5] However, in our case the reaction of dibutyl
magnesium with 1a did not lead to an Mg-alkyl precursor
To proceed further along the reaction pathway a HBpin
catalyst. For this reason, the formation of Mg H is less likely
molecule is coordinated to C, endergonic by 1.1 kcalmolꢀ1, to
ꢀ
=
and the reaction mechanism must be different. Thus, we
decided to examine the mechanism in more detail.
generate D (see Figure S1). The hydroboration of the C O
bond could now occur from the coordinated HBpin molecule
via a [2s + 2p]-addition-type transition state. This step
requires an activation free energy of 33.0 kcalmolꢀ1, which
is too high to be consistent with the reaction conditions used
(see Figure S2). Thus, an alternative reaction pathway needs
to be considered. Based on the above considerations the
BINOL ligand could act as a non-innocent ligand and could
In our 1a/Mg catalytic system only one molecule of
HBpin is necessary for the hydroboration and provides 3a
quantitatively in 15 minutes. To gain information about
HBpin activation, we mixed 1a/Mg with an equimolar
amount of HBpin. 11B NMR spectroscopy showed the com-
plete conversion of HBpin (d = 28.7 ppm) into a new boron
Figure 1. Computed energy profile for the magnesium catalyzed enantioselective hydroboration of ketones. Free energy values at M06(SMD)/
Def2-TZVPP//BP86/Def2-SVP level of theory are presented.
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be involved in the HBpin activation. Indeed, inserting the
ketones. Excellent yields and enantioselectivities have been
achieved for a wide range of ketones derived from acetophe-
none and 1-indanone. Moreover, the asymmetric magnesium
catalysis can be applied to a,b-unsaturated ketones whereby
the 1,2-addition is observed exclusively. Furthermore, even
propargylic ketones could be used to provide the correspond-
ing alcohols in excellent yields. Thus, the new methodology
compares favorably with previously developed protocols.
Following detailed spectroscopic and computational studies
a plausible reaction pathway has been established which, in
contrast to our expectations and previously described hydro-
borations, does not involve a magnesium hydride intermedi-
ate. Instead, the activation involves a new metal-ligand
cooperative catalysis pathway in which BINOL acts as
a non-innocent ligand. Thus, the newly developed magnesium
catalysis protocol is not only of interest because of its wide
scope and improved sustainability, but additionally because of
the mechanistic insights which allow the design of further
enantioselective procedures employing readily available
magnesium catalysts.
°
ꢀ
HBpin into the Mg O(BINOL) bond via [D-ES] with a free
energy barrier of 9.3 kcalmolꢀ1 results in the intermediate ES.
With the BINOL coordinating the HBpin becomes more
=
electron rich and facilitates the hydride transfer to the C O
via the transition state [ES-FS]°, with an energy span of
14.0 kcalmolꢀ1 (with reference to intermediate C) to give
intermediate FS. The next step is the migration of the alkoxide
from the magnesium to the boron center with a moderate
activation barrier of 13.0 kcalmolꢀ1. The transition state [FS-
°
ꢀ
GS] is characterized by concomitant B O(alkoxide) and Mg-
O(BINOL) bond formation. Finally, the liberation of the
product PS from the resulting intermediate GS closes the
catalytic cycle and regenerates the active catalyst.
The computed energy profile (Figure 1) reveals that the
hydride transfer (ES!FS) is predicted to be the rate-
determining step. Therefore, the enantiocontrol occurs at
the transition-state [ES-FS]°. To clarify the origin of the
enantioselectivity we also investigated the competitive tran-
sition state [ER-FR]° differing in the approach of the ketone
towards the formation of the other enantiomer PR (Figure 2).
In comparison, [ES-FS]° is 2.5 kcalmolꢀ1 more stable than
[ER-FR]°, and explains the preference for the PS formation.
The DFT result is in good agreement with the experimental
findings of 93:7 er (Table 1, entry 2). A closer inspection of
the transition-state geometries shows that the less favorable
[ER-FR]° transition state arises from steric repulsion between
the aryl residue of BINOL (the substituent at 3’ position) and
the aryl substituent of the ketone oriented perpendicularly
(Figure 2). This particular repulsion is absent in the case of
[ES-FS]°.
Acknowledgements
B.M. and L.C. acknowledge King Abdullah University of
Science and Technology (KAUST) for computational resour-
ces of the supercomputer Shaheen II.
Conflict of interest
In summary, we have developed a new magnesium-
catalyzed enantioselective hydroboration of prochiral
The authors declare no conflict of interest.
Keywords: asymmetric reduction · DFT calculation ·
hydroboration · ketones · magnesium
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1
ꢀ
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ꢀ
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Manuscript received: June 28, 2019
Version of record online: && &&, &&&&
Angew. Chem. Int. Ed. 2019, 58, 1 – 6
ꢀ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Hydroboration
Mg-ligand cooperative activation of
HBpin: The enantioselective magnesium-
catalyzed hydroboration of ketones using
a (R)-(+)-BINOL derivative as a chiral
ligand affords excellent yields and enan-
tioselectivities. Experimental investiga-
tions together with DFT calculations
provide insight into the reaction mecha-
nism and the origin of enantioselectivity.
A. Falconnet, M. Magre,* B. Maity,*
L. Cavallo,* M. Rueping*
&&&& — &&&&
Asymmetric Magnesium-Catalyzed
Hydroboration by Metal-Ligand
Cooperative Catalysis
6
www.angewandte.org
ꢀ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2019, 58, 1 – 6
These are not the final page numbers!