Borohydride intermediates pave the way for magnesium-catalysed enantioselective ketone reduction

Article abstract of DOI:10.1039/c9cc09111d

A magnesium precatalyst for the highly enantioselective hydro-boration of CO bonds is reported. The mechanistic basis of the unprecedented selectivity of this transformation has been investi-gated experimentally by isolation of catalytic intermediates and theoretically by DFT calculations. The facile formation of a magnesium borohydride species is critical in overcoming competing pathways in the selectivity-determining insertion step.

Full text of DOI:10.1039/c9cc09111d

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DOI: 10.1039/C9CC09111D
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Borohydride Intermediates Pave the Way for Magnesium-
Catalysed Enantioselective Ketone Reduction
Vladislav Vasilenko,a Clemens K. Blasius,a Hubert Wadepohl,a Lutz H. Gadea,*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
A magnesium precatalyst for the highly enantioselective hydro-
During the course of our previous work in the field of
boration of C=O bonds is reported. The mechanistic basis of the enantioselective ketone reduction catalysis with 3d metal
unprecedented selectivity of this transformation has been investi- complexes,12,13 we observed the potential of chiral
gated experimentally by isolation of catalytic intermediates and bis(oxazolinyl-methylidene)isoindoline (“boxmi”) manganese
theoretically by DFT calculations. The facile formation of a complexes to undergo a borane-assisted hydrogen transfer
magnesium borohydride species is critical in overcoming with an excellent degree of enantiocontrol.13 We speculated
competing pathways in the selectivity-determining insertion step.
that such a concept could be transferred to other metal
catalysts in which a direct metal hydride insertion might
otherwise prove troublesome. Our approach has now led to a
magnesium-based precatalyst for the highly enantioselective,
hydroboration of ketones and, in addition, we explored key
intermediates relevant to the catalytic cycle and provide
evidence for a magnesium borohydride complex as the
selectivity-determining species in the hydroboration cycle.
Boxmi-magnesium alkyl complexes could be readily
generated in situ by mixing the protioligand R,R’boxmi-H with
the magnesium dialkyl precursor [Mg(CH2SiMe3)2THF] which
Alkaline earth metal catalysis has seen a tremendous growth in
recent years, driven by the successful design of stable, but
highly reactive new molecular catalysts1 for various known as
well as unprecedented transformations.2 These cover a broad
range of chemical patterns from conjugate addition to
standard reduction and oxidation chemistry. Magnesium-
catalysed hydroelementations of C=X bonds have received
considerable attention due to the wide substrate scope and
the wealth of ancillary ligands suitable for this
transformation.2a,2b,3
allowed
a rapid assessment of their potential in the
On the other hand, the number of highly enantioselective
reactions of this type is still very limited.4 To our knowledge,
only a single example of notable stereoselectivities has been
very recently reported for the magnesium-catalysed reduction
of carbonyl groups based on metal-ligand cooperative effects.5
Particularly, in the case of hydrosilylations, this drawback has
been attributed to the tendency of alkaline earth metal
hydrides to form contact-ion pairs, consisting of a cationic LAe+
fragment and an anionic pentacoordinate silicate (e.g.
hydroelementation of acetophenone as the model substrate
(Table 1). Whereas hydrosilylation catalysis turned out to
occur sluggishly and with low enantioselectivity (#1,2), instant
completion of the reaction was observed with HBPin as the
reducing agent, producing a high ee of 89% with Ph as the
oxazoline unit (#3,4). This strong dependence of the activity
and the selectivity on the reducing agent suggested the
participation of the borane in the selectivity-determining step
– reminiscent
of
the
manganese-catalysed
ketone
[PhSiH4]-)
– thus, effectively eliminating a handle for
hydroboration (vide infra).13
enantiocontrol.6 In contrast, adduct formation between
borohydrides and alkaline earth metal centers has been well-
documented particularly in the case of pinacol derivatives and
we envisioned this motif to be more suitable for the transfer of
chirality in alkaline earth metal-based enantioselective
catalysis.7-11
We were unable to further improve the selectivity of the
insertion step by variation of the oxazoline and backbone
substituents (#3-7) but observed an incremental increase in
the enantiomeric excess upon using an isolated precatalyst 1
(see SI) and by warming the reaction from 40°C to rt over a
period of 2 h (3 and 7 percentage points, respectively). Solvent
screening revealed the remarkable robustness of this catalytic
system (89-95%ee) in most common solvents (see SI), whilst a
significant drop in enantiomeric excess was only observed in
strongly coordinating solvents (MeCN, 70%ee) and in the
presence of chelating donors (TMEDA, 10%ee). Although the
catalyst content could in principle be reduced to 0.5mol% (see
SI), we maintained our original 5 mol% catalyst loading in the
a. Anorganisch-Chemisches Institut, Universität Heidelberg, Im Neuenheimer Feld
270, 69120 Heidelberg.
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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evaluation of the substrate scope to uncover the full potential
of this catalytic platform (Scheme 1).
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Figure 1. Solid-state molecular structure of precatalyst 1 obtained by X-ray
DOI: 10.1039/C9CC09111D
diffraction. Only one of the two independent molecules is shown, displacement
ellipsoids set at 30 % probability level, hydrogen atoms omitted for clarity.
Selected bond lengths (Å) and angles (°), values for the second independent
molecules are given in square brackets: Mg1-C29 2.162(4) [2.159(4)], Mg1-O3
2.125(3) [2.117(3)], Mg1-N1 2.215(3) [2.232(3)], Mg1-N2 2.147(4) [2.147(4)],
Mg1-N3 2.195(4) [2.200(3)].
Table 1. Screening of reaction conditions for the magnesium-catalysed
hydroelementation of acetophenone.
O
R,R'boxmi-H (5 mol%)
O
OH


N
R
R
R'
R'

Mg(CH2SiMe3
)
2(THF) (5 mol%)
NH
reductant (2 equiv.)
solvent, Temp., 2 h
basic or acidic work-up
N
R,R'boxmi-H
O
R[a]
R’
reductant
solvent
%c.[b]
%ee[b]
#
1[c]
2[c]
3[c]
4[c]
5[c]
6[c]
Ph
Ph
Ph
Ph
iPr
tBu
Bn
Ph
Ph
H
H
(EtO)2MeSiH
Ph2SiH2
HBPin
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
36
0
33 (R)
n.d.
H
>99
>99
>99
>99
>99
>99
>99
89 (S)
87 (S)
5 (S)
Me
Me
H
HBPin
HBPin
HBPin
11 (R)
4 (R)
An X-ray diffraction study of precatalyst 1 featuring phenyl
7[c]
8[c,e]
Me
H
HBPin
oxazoline substituents (Figure 1) revealed
a molecular
structure containing a THF ligand bound to the magnesium
centre, while the ancillary pincer ligand generates a defined
chiral pocket with the typical short isoindoline N-M bond
HBPin
92 (S)
96 (S)
9[d,e]
H
HBPin
[a] (S,S)-enantiomers of all ligands were employed. [b] Determined by HPLC length and somewhat longer oxazoline N-M distances.14
analysis. [c] Performed at rt. [d] Warmed from 40°C to rt over 2 h. [e] Isolated
precatalyst 1 was used. Basic work-up (K2CO3/MeOH) for hydrosilylation, acidic
work-up (SiO2) for hydroboration.
Remarkably, this precatalyst turned out to be thermally stable
in the absence of other reagents, with no decomposition or
deactivation by the formation of an inactive “homoleptic”
complex [M(boxmi)2] under catalytic conditions over extended
periods even in boiling toluene.
High enantioselectivities were observed for all aryl alkyl
ketones tested (up to 98%ee), regardless of electron-donating
or electron-withdrawing groups attached to the arene ring.
We then set out to explore potential intermediates of the
catalytic cycle and initially focused on the characterisation of
Notably, under the chosen reaction conditions no reduction of
magnesium alkoxides which are the anticipated result of an
the ester moiety in ketone S3 was observed and only occurred
after prolonged reaction times (days) at room temperature
and in the presence of a higher excess of HBPin. In addition,
the catalytic system allows the efficient reduction of -
insertion process of a ketone into a metal-hydride bond. At a
temperature of 40°C alcoholysis of magnesium alkyl 1 in THF
gave the corresponding alkoxide 2. We could obtain single
crystals of this species by maintaining the low temperature
substituted and cyclic alkyl aryl ketones as well as dialkyl
throughout the process of isolation.
ketones, albeit with slightly diminished enantiodiscrimination.
The absence of any reactivity towards epoxides, imines or
alkenes further highlights the functional group tolerance of
this system and its selectivity towards carbonyl groups (see SI).
Scheme 2. Formation of an labile alkoxide 2 by alcoholysis of precatalyst 1 at low
temperature. ArF denotes a 4-fluorophenyl group.
O
O
OH


N
Ph
SiMe3
Ph
THF
O
N
ArF
ArF
N
Mg
N
N
THF
Mg
THF/Et2O, -40°C
THF
Ph
Scheme 1. Substrate scope of the magnesium-catalysed hydroboration of
ketones. All reactions were performed at 5 mol% catalyst loading with isolated 1.
N
Ph


O
O
HBPin (2 equiv.)
toluene, 40 °C to rt, 2 h
1
2
O
OH
Mg

R
R'
R
R'
then SiO2
P
S
Figure 2. Solid state molecular structure of alkoxide intermediate 2. Only one of
the two disordered conformations of the thf ligand is shown, displacement
ellipsoids set at 30 % probability level, hydrogen atoms omitted for clarity.
Selected bond lengths (Å) and angles (°): Mg-O3 1.9420(15), Mg-O4 2.1402(15),
Mg-O5 2.156(9), Mg-N1 2.2222(18), Mg-N2 2.2137(17), Mg-N3 2.2426(18).
OH
OH
OH
OH
MeO2C
Ph
MeO
P1
P2
P3
P4
>99%, 96%ee
>99%, 94%ee
>99%, 97%ee
>99%, 94%ee
OH
OH
OH
OH
MeO
P5
P6
P7
P8
>99%, 98%ee
>99%, 95%ee
>99%, 96%ee
>99%, 90%ee
OH
OH
OH
OH
Ph
C6H13
Ph
Ph
P9
P10
P11
P12
>99%, 75%ee
>99%, 80%ee
>99%, 80%ee
>99%, 69%ee
2 | J. Name., 2012, 00, 1-3
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carbon atom (Figure 4). A rapid conversion of complex 3 in the
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Scheme 3. Formation of a temperature sensitive borohydride 3 by reaction of
precatalyst 1 with an excess of HBPin.
presence of a ketone – even at temperaDtuOrI:e1s0.a1s03l9o/wC9aCsC0981011°DC
was observed. Upon addition of the substrate at this
temperature, the original 11B resonance at 0 ppm (A) vanished
immediately in favour two new downfield signals (B,C) in its
proximity, which overtime converged into a single resonance
(B → C). The 11B NMR chemical shift suggests that the resulting
O
O



Ph
SiMe3
Ph
O
N
N
O
HBPin (3 equiv.)
B
H
N
Mg
N
N
Mg
N
H
Et2O, -40°C to -10°C
THF
Ph
THF
Ph

O
O
1
3
products most likely also feature
a
tetracoordinate
environment around the B nucleus, as opposed to a significant
downfield shift expected for the free, tricoordinate boric ester
product.
Complex 2 was identified by X-ray analysis as a monomeric
octahedral boxmi alkoxide, containing two THF co-ligands
(Scheme 2 and Figure 2). The observed lability of this
compound is thought to be due to a rapid decomposition upon
Thus, species B is most likely a ketone borohydride
complex. Predominant alkoxide and minor ketone resonances
in the 19F NMR are in agreement with a fast THF/ketone
exchange (B) followed by an almost complete and instant
reduction of the carbonyl moiety (C). Unfortunately, the
pronounced dynamics between various alkoxide/borate
arrangements present in this system precluded the growth and
isolation of single crystals of either of the two species.
ligand exchange which precluded
a systematic further
characterization of this compound. Subsequently, we looked at
the conversion of the precatalyst under catalytic conditions, i.
e., in presence of pinacol borane (Scheme 3). At rt we
observed an instant release of the corresponding metathesis
product Me3SiCH2BPin, accompanied by the formation of a
new thermally labile species. From 1H and 11B NMR, we
deduced the existence of a tetracoordinate boron species
containing two diastereotopic hydrogen atoms which could
also be exchanged for deuterium (δB = 0.9 ppm and δH = 4.16
ppm, see SI). The latter finding was eventually confirmed by
isolation and crystallisation of this compound at low
temperatures (40°C to 10°C), exposing its zwitterionic
nature consisting of a cationic [boxmi-Mg]+ and an anionic
[PinBH2]- fragment combined with an additional THF co-ligand
(Figure 3).8b,9
Figure 4. 11B and 19F NMR spectra of the stoichiometric reaction between
borohydride 3 (corresponding to resonance A) and 4’-fluoroacetophenone at
80°C to 30°C in toluene. Ketone and alkoxide regions of the 19F NMR spectrum
have been highlighted in blue and red, respectively. The asterisk (*) denotes a
tri-coordinate boron species, potentially the boric ester product.
Here, the pinacol borate unit is bound to the magnesium
center by one of the pinacol oxygen atoms, and both borate
hydrogens point away from the metal. Employing this isolated
species in the catalytic hydrosilylation and hydroboration of
acetophenone we observed a comparable performance in
both cases (89%ee for HBPin and 35%ee for (EtO)2MeSiH) as
for the in situ generated catalyst.
Figure 3. Solid state molecular structure of borohydride intermediate 3 obtained
by X-ray diffraction. Only one of the two disordered conformations of the thf
ligand is shown, displacement ellipsoids set at 30 % probability level, hydrogen
atoms omitted for clarity. Selected bond lengths (Å) and angles (°): Mg-O3
1.9695(16), Mg-O5 2.0947(16), Mg-N1 2.1628(19), Mg-N2 2.1107(18), Mg-N3
2.1734(19).
To obtain additional insight into factors determining the
enantioselectivity in magnesium-catalysed ketone reductions,
DFT modeling was employed to study potential insertion
mechanisms. Specifically, we investigated the equilibrium
between simple magnesium hydrides and zwitterionic
borohydrides underlying this transformation combined with a
detailed evaluation of the hydrogen transfer steps associated
with these species (Figure 5).
Figure 5. Gibbs free enthalpies (kcal mol–1) of various species related to the
reduction of the carbonyl group by a magnesium catalyst. Values obtained from
DFT calculations for 233 K and 1 atm at the BP86/def2TZVP//
PBE0/def2QZVPP/SMD(toluene) level of theory. L denotes H,Phboxmi.
H
LMg
H-1
H
O
(16.8)
H
B
O
S
Ph
H
We subsequently studied the stoichiometric reaction of the
borohydride complex with a fluorinated ketone substrate
(4’-fluoroacetophenone) by 11B and 19F NMR spectroscopy at
low temperatures (in the range of 80°C to 30°C) which
allowed the simultaneous monitoring of the coordination
environment of the borane, and the oxidation state at the C1
O
B
O
Me
O
LMg
LMg
H
B-ts
(8.6)
(H-1)2
(7.9)
O
O
H
B
O
MgL
Ph
B-2
(2.8)
H
LMg
H
H
LMg
B-1
O
(2.5)
B-3
H
B
O
(0.0)
H
O
Me
LMg
Ph
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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As expected for the highly Lewis-acidic magnesium centre, a
dimeric structure (H-1)2 was found to be favoured over a
monomeric magnesium hydride H-1 (ΔG = 8.9 kcal/mol). More
importantly, binding of HBPin is strongly exergonic relative to
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both intermediates, highlighting the prevalence of
a
magnesium borohydride B-1 during hydroboration catalysis.
The borane coordination mode appears to be regulated by
the presence of a co-ligand, with the true zwitterionic
structure as encountered the solid state only being favoured
upon introduction of an additional ketone donor (B-1/2 vs B-
3). In this regard, the THF ligand in the crystal structure can be
considered a ketone surrogate and a rapid equilibrium
between both states is expected during catalysis in solution.
We could locate a low-energy transition state manifold B-ts
following the coordination of the ketone which preferentially
produced the (S)-product (ΔG‡ = 8.6 kcal/mol, ΔΔG‡ = 2.2
kcal/mol). In contrast, no direct insertion path was found for
the dimeric structure and only enolization steps leading to the
release of H2 were identified for an Mg-H monomer. As a
consequence, not only is the zwitterionic magnesium
hydridoborate more stable than its hydride congener, it also
facilitates a productive conversion of the ketone substrate.
In conclusion, a magnesium-based catalyst has been
developed for the enantioselective hydroboration of ketones
under mild reaction conditions. In situ NMR spectroscopy and
isolation of proposed catalytic intermediates, as well as DFT
modeling have established a zwitterionic structure formed
from a magnesium hydride and HBPin as the critical motif to
turn the poor characteristics of a slow and unselective direct
Mg-H insertion into a fast and highly selective transformation.
This study may serve as an inspiration for other reductive
transformations in alkaline earth metal catalysis and beyond.15
The authors acknowledge a predoctoral fellowship to V. V.
(LGF Funding Program of the state of Baden-Württemberg)
3
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641.
and
generous
funding
by
the
Deutsche
Forschungsgemeinschaft (DFG, Ga488/9-2) as well as support
by the bwHPC initiative and the bwHPC-C5 project.
9
D. Mukherjee, A. Ellern and A. D. Sadow, Chem. Sci., 2014, 5,
959–964.
Notes and references
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14 For comparison of other boxmi metal complexes: see ref. [10b]
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