Chiral Iron(II)-Catalysts within Valinol-Grafted Metal-Organic Frameworks for Enantioselective Reduction of Ketones

Article abstract of DOI:10.1021/acscatal.1c02529

The development of highly efficient and enantioselective heterogeneous catalysts based on earth-abundant elements and inexpensive chiral ligands is essential for environment-friendly and economical production of optically active compounds. We report a strategy of synthesizing chiral amino alcohol-functionalized metal-organic frameworks (MOFs) to afford highly enantioselective single-site base-metal catalysts for asymmetric organic transformations. The chiral MOFs (vol-UiO) were prepared by grafting of chiral amino alcohol such as l-valinol within the pores of aldehyde-functionalized UiO-MOFs via formation of imine linkages. The metalation of vol-UiO with FeCl2 in THF gives amino alcohol coordinated octahedral FeII species of vol-FeCl(THF)3 within the MOFs as determined by X-ray absorption spectroscopy. Upon activation with LiCH2SiMe3, vol-UiO-Fe catalyzed hydrosilylation and hydroboration of a range of aliphatic and aromatic carbonyls to afford the corresponding chiral alcohols with enantiomeric excesses up to 99%. Vol-UiO-Fe catalysts have high turnover numbers of up to 15 ?000 and could be reused at least 10 times without any loss of activity and enantioselectivity. The spectroscopic, kinetic, and computational studies suggest iron-hydride as the catalytic species, which undergoes enantioselective 1,2-insertion of carbonyl to give an iron-alkoxide intermediate. The subsequent σ-bond metathesis between Fe-O bond and Si-H bond of silane produces chiral silyl ether. This work highlights the importance of MOFs as the tunable molecular material for designing chiral solid catalysts based on inexpensive natural feedstocks such as chiral amino acids and base-metals for asymmetric organic transformations.

Full text of DOI:10.1021/acscatal.1c02529

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Chiral Iron(II)-Catalysts within Valinol-Grafted Metal−Organic
Frameworks for Enantioselective Reduction of Ketones
Neha Antil,† Naved Akhtar,† Rajashree Newar, Wahida Begum, Ajay Kumar, Manav Chauhan,
and Kuntal Manna*
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ABSTRACT: The development of highly efficient and enantioselec-
tive heterogeneous catalysts based on earth-abundant elements and
inexpensive chiral ligands is essential for environment-friendly and
economical production of optically active compounds. We report a
strategy of synthesizing chiral amino alcohol-functionalized metal−
organic frameworks (MOFs) to afford highly enantioselective single-
site base-metal catalysts for asymmetric organic transformations. The
chiral MOFs (vol-UiO) were prepared by grafting of chiral amino
alcohol such as L-valinol within the pores of aldehyde-functionalized
UiO-MOFs via formation of imine linkages. The metalation of vol-
UiO with FeCl2 in THF gives amino alcohol coordinated octahedral
FeII species of vol-FeCl(THF)3 within the MOFs as determined by X-
ray absorption spectroscopy. Upon activation with LiCH2SiMe3, vol-
UiO-Fe catalyzed hydrosilylation and hydroboration of a range of aliphatic and aromatic carbonyls to afford the corresponding chiral
alcohols with enantiomeric excesses up to 99%. Vol-UiO-Fe catalysts have high turnover numbers of up to 15 000 and could be
reused at least 10 times without any loss of activity and enantioselectivity. The spectroscopic, kinetic, and computational studies
suggest iron-hydride as the catalytic species, which undergoes enantioselective 1,2-insertion of carbonyl to give an iron-alkoxide
intermediate. The subsequent σ-bond metathesis between Fe−O bond and Si−H bond of silane produces chiral silyl ether. This
work highlights the importance of MOFs as the tunable molecular material for designing chiral solid catalysts based on inexpensive
natural feedstocks such as chiral amino acids and base-metals for asymmetric organic transformations.
KEYWORDS: asymmetric catalysis, iron, metal−organic frameworks, hydrosilylation, hydroboration
drawn immense interest in recent years because of the lower
price and toxicity of the base-metals than late transition
metals.1−11 The chiral auxiliary ligands play the most crucial
role in designing chiral base-metal catalysts as the chiral ligands
control the electronic and steric properties of the metal center
and create enantioinduction for asymmetric catalysis. Bulky
chiral nitrogen-donor or phosphine ligands are typically
employed to constitute robust and rigid base-metal catalysts
for good enantioselectivity and to prevent intermolecular
decomposition.1,3,4,12−16 However, intricate design and multi-
step synthesis of many such ligands increase their production
cost that compromise the economic gain of cheap earth-
abundant metal usage. Additionally, several other factors such
as commercial unavailability, difficulty of larger scale synthesis,
INTRODUCTION
■
The development of chiral earth-abundant metal catalysts for
the sustainable production of optically active compounds has
Received: June 5, 2021
Revised: July 19, 2021
Figure 1. Design of amino alcohol-functionalized metal−organic
frameworks to develop robust single-site earth-abundant metal
catalysts for heterogeneous asymmetric catalysis.
https://doi.org/10.1021/acscatal.1c02529
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Figure 2. (a) Synthesis of chiral valinol-functionalized UiO-67 MOF (vol-UiO-67) and its metalation with iron. (b) The synthesis of aldehyde-
functionalized UiO-68-MOF, its postsynthetic functionalization with L-valinol, and the metalation of chiral valinol-functionalized UiO-68 MOF
(vol-UiO-68) with iron.
poor recyclability, and patent issues make the application of
many such chiral ligands impractical in industrial chemical
synthesis, which becomes the major drawback in commercial
supply chain planning.13,17,18 Therefore, the development of
cheap and affordable chiral ligands from cheap feedstocks for
preparing chiral heterogeneous base-metal catalysts is crucial
for sustainable asymmetric organic transformations.
Herein, we report a strategy of synthesizing chiral mono
amino alcohol ligated single-site iron catalysts supported by
porous MOFs for highly enantioselective reduction of ketones
(Figure 1). We demonstrate that the active-site isolation within
MOFs not only stabilize solution inaccessible mono 1,2-amino
alcohol coordinated iron species but also afford catalysts
having more open metal centers, which facilitates substrates
binding to the metal. This work highlights the rational design
and synthesis of isoreticular chiral MOFs as the tunable
heterogeneous catalyst platforms derived from inexpensive
chiral feedstocks and earth-abundant metals for sustainable
asymmetric catalysis.
Chiral 1,2-amino alcohols are versatile structural motifs
widely present in biologically active molecules.19−22 Although
chiral 1,2-amino alcohols are easily affordable by reducing
naturally occurring amino acids, their application as chiral
bidentate ligands in base-metal catalysis is very limited,
presumably due to the absence of sterically bulky substituents
that leads to the formation of oligomeric metal species or
intermolecular decomposition.23−25 In asymmetric catalysis,
amino acid-derived 1,2-amino alcohols have been primarily
employed as chiral auxiliaries26 or the source of a stereogenic
center in the ligand component of the base-metal cata-
lysts,27−34 or ligands after multistep derivatization.31,35,36
Grafting of amino alcohols onto porous solid supports
followed by metalation could provide robust single-site earth-
abundant metal catalysts that would impose excellent chiral
induction within pores for enantioselective catalysis. As a
porous and tunable molecular material, metal−organic frame-
works (MOFs) have emerged as an interesting class of
supports to prepare heterogeneous and robust chiral base-
metal catalysts via active-site isolation.37−72 Owing to their
modular and tunable properties, the chemoselectivity and
enantioselectivity of chiral MOF-catalysts could be easily
optimized by adjusting pore sizes and postsynthetic
modification techniques.42,73,74 In addition, the precise knowl-
edge of crystalline MOF structures by X-ray crystallography
provides distinct advantages over other supported chiral
catalysts by enabling rational tuning of catalytic activities/
selectivities through the use of tailor-made building blocks and
direct observation of structure−activity relationships.
RESULTS AND DISCUSSION
■
Synthesis and Characterization of Valinol-Function-
alized UiO-MOFs and Their Postsynthetic Metalation.
Isoreticular and chiral valinol-functionalized MOFs; vol-UiO-
67 and vol-UiO-68 are constructed from L-valinol function-
alized linear dicarboxylate linkers and Zr6O4(OH)4 secondary
building units (SBUs) to afford UiO-frameworks.75−79 Both
vol-UiO-67 and vol-UiO-68 were synthesized following a
similar procedure via synthesizing the aldehyde-functionalized
UiO-67-CHO and UiO-68-CHO MOFs, respectively, followed
by the grafting of L-valinol within these MOFs (Figure 2). The
solvothermal reaction between 2-formyl-[1,1′-biphenyl]-4,4′-
dicarboxylic acid and ZrCl4 in DMF at 80 °C afforded UiO-67-
CHO.80 Likewise, heating a mixture of 2′-formyl-[1,1′:4′,1′′-
terphenyl]-4,4′′-dicarboxylic acid and ZrCl4 in DMF at 90 °C
produced UiO-68-CHO. The presence of aldehyde groups at
the linkers of MOFs was confirmed by 1H NMR spectra of the
digested MOFs (Figure S59, SI). The UiO-topology of both
UiO-67-CHO and UiO-68-CHO was assigned by the
similarity of their PXRD patterns to those of simulated UiO-
67 and UiO-68 MOFs, respectively. The condensation reaction
between the aldehyde moiety of MOF and amino group of L-
valinol furnished the valinol-grafted chiral vol-UiO MOFs
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Figure 3. (a) PXRD patterns of simulated UiO-67 MOF (black),85 freshly prepared pristine UiO-67-CHO (red), vol-UiO-67 (green), vol-UiO-67-
FeCl (magenta), vol-UiO-67-Fe recovered after hydrosilylation (blue) and hydroboration (purple) of 4-methoxyacetophenone. (b) PXRD patterns
of simulated UiO-68 MOF (black),86 freshly prepared pristine UiO-68-CHO (red), vol-UiO-68 (blue), vol-UiO-68-FeCl (magenta), vol-UiO-68-
Fe recovered after hydrosilylation (green) and hydroboration (violet) of 4-methoxyacetophenone. (c) XANES spectra of FeCl2(black), vol-UiO-
68-FeCl (red), vol-UiO-68-FeH (blue) and vol-UiO-68-Fe after hydrosilylation of 4-methoxy-acetophenone (magenta) at the Fe K-edge. (d) Fe 2p
XPS spectrum of vol-UiO-68-FeCl. (e) Zr 3d XPS spectrum of vol-UiO-68-FeCl. (f) BET nitrogen sorption isotherms (77 K) of vol-UiO-67, vol-
UiO-68, vol-UiO-67-FeCl, and vol-UiO-68-FeCl. (g) DFT optimized structure of vol-FeCl(THF)3 moiety within vol-UiO-FeCl MOFs. (h) Fe K-
edge EXAFS spectra of vol-UiO-68-Fe and its fits in R-space, showing the real component (blue hollow triangles) and magnitude (blue solid
triangles) of the Fourier transformation. The fitting range is 1.0−4.0 Å in R-space (within the gray lines).
through the formation of imine linkages.81−84 The H NMR
spectra of the digested vol-UiO-67 and vol-UiO-68 showed the
characteristic peaks of imine and valinol moieties and the
disappearance of aldehyde peaks suggesting complete post-
synthetic modification of UiO-CHO MOFs. The deprotona-
tion of hydroxyl group of valinol moiety by LiN[Si(Me)3]2
(LiHMDS) followed by salt metathesis reaction with FeCl2 in
THF afforded iron-metalated vol-UiO-FeCl as a light yellow
solid. Inductively coupled plasma optical emission spectrosco-
py (ICP-OES) showed Zr/Fe ratios of 1.66 and 1.44,
corresponding to the Fe-loadings of 60% and 71% with
respect to the valinol moiety of vol-UiO-67 and vol-UiO-68,
respectively. The crystallinity and structure of MOFs remained
intact upon metalation as evidenced by the similarity in the
PXRD patterns of freshly prepared vol-UiO-FeCl with those of
pristine vol-UiO (Figure 3a,b).
1
Transmission electron micrograph of vol-UiO-68-FeCl
displayed octahedron particles having the average diameter
of 1.0 μm (Figure S8a, SI). The oxidation state of the Fe-
centers of vol-UiO-FeCl was determined by X-ray absorption
near-edge structure (XANES) and X-ray photoelectron
spectroscopy (XPS), and the coordination environment of
iron was investigated by X-ray absorption fine structure
(EXAFS) spectroscopy and density functional theory (DFT)
studies. The XANES spectroscopy of vol-UiO-FeCl indicates
the FeII oxidation state as its pre-edge position aligned well
with that of FeCl2 (Figure 3c). The assignment of +2-oxidation
state of iron was further supported by XPS spectroscopy based
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2.07, and 2.16 Å. The EXAFS feature of the iron-centers in vol-
UiO-FeCl fitted well with this DFT optimized structure to
obtain the nearly identical local coordination environment and
bond lengths, thus revealing the formation of vol-FeCl(THF)3
species within the MOFs (Figure 3h). Vol-UiO-67-FeCl and
vol-UiO-68-FeCl had a BET surface area of 785 and 1382 m2/
g, respectively, and the corresponding average pore sizes are
0.7 and 1.4 nm, respectively (Figure 3f). In order to make the
pores of the metalated MOFs more spacious for facile mass
transport, we have also prepared m-vol-UiO-67-Fe by doping
of about 50% of its functionalized linkers with the
unfunctionalized one (section 2.3, SI). ICP-OES analysis
revealed a Zr/Fe ratio of 3.54, corresponding to the Fe-loading
of 56% with respect to the valinol moiety of m-vol-UiO-67.
Asymmetric Hydrosilylation of Ketones Catalyzed by
Vol-UiO-Fe MOFs. Upon activation with LiCH2SiMe3, all vol-
UiO-Fe MOFs became active in catalyzing asymmetric
hydrosilylation of a range of aromatic and aliphatic ketones
to afford the corresponding silyl ethers with excellent
enantiopurity. Asymmetric hydrosilylation using homogeneous
Fe-catalysts typically requires high catalyst loading (0.3−5%)
and also chromatographic purification to afford pure
products.4,7,8,87−93 The hydrosilylation reaction was performed
by stirring a mixture of carbonyl substrate, 1.1 equiv of silane
such as (EtO)2MeSiH or PhSiH3 and catalytic amount of vol-
UiO-Fe in THF at room temperature, and pure silyl ether was
obtained in most cases by the simple removal of solid MOF
and volatiles from the crude product mixture. The enantio-
meric excess of the silyl ether product was determined by GC-
analysis of the corresponding secondary alcohol obtained after
hydrolysis of the silyl ether with K2CO3 in methanol. At a 0.05
mol % of Fe-loading, vol-UiO-67-Fe catalyzed hydrosilylation
of 4-methoxyacetophenone (1a) using (EtO)2MeSiH in THF
at room temperature gave full conversion within 10 min, and
diethoxy((1-(4-methoxyphenyl)ethyl)peroxy)(methyl)silane
(2a) was obtained in quantitative yield with 99% ee. Under the
identical reaction conditions, m-vol-UiO-67-Fe and vol-UiO-
68-Fe also produced 2a in excellent yields with 99% ee. In
general, m-vol-UiO-67-Fe and vol-UiO-68-Fe had similar
activities but displayed higher turnover frequency (TOF)
than vol-UiO-67-Fe likely due to the facile diffusion of
substrates and product molecules through their larger channels
(entries 1−3, 5−7, Table 1). Hydrosilylation of 1a remarkably
gave 15 000 TON with UiO-68-Fe. Several other substituted
acetophenones bearing different functional groups such as
methoxy (1a), hydroxy (1b and 1c), and nitro (1d) were
reduced to their corresponding alcohols in good yields with ee
up to 99% (entries 2−9, Table 1). Halogenated acetophenone
(4-bromoacetophenone) was also efficiently reduced to give
the corresponding silyl ether (2e) with 89% ee (entries 10 and
11, Table 1). Hydrosilylation of aliphatic aromatic ketones
such as 2-methyl-1-phenylpropan-1-one (1f), unsymmetric
benzophenone such as phenyl(o-tolyl)methanone) (1g) gave
the reduced products with 99% ee under identical reaction
conditions (entries 12−14, Table 1). A smaller-sized substrate,
methyl-1-phenylpropan-1-one (1f), showed a higher TOF than
a larger one such as phenyl(o-tolyl)methanone) (1g) (entries
12 and 13, Table 1).94
Table 1. Asymmetric Hydrosilylation of Ketones Catalyzed
by vol-UiO-Fe MOFs
a
a
Reaction conditions: 0.05 mol % Fe, 0.585 mmol ketone, 0.643
b
mmol (EtO)2MeSiH or PhSiH3, 1 mL THF, 25 °C, 10 min. Isolated
yield, TOF is calculated on the basis of the GC-yield of 2 for the first
10% conversion. %ee was determined by chiral GC or HPLC,
c
absolute configurations were assigned on the basis of the literature
d
reports. %ee was determined by chiral HPLC.
on FeII 2p3/2 binding energy of 709.4 eV and 2p1/2 binding
energy of 722.9 eV (Figure 3d). The oxidation state of Zr4+ at
SBUs was unchanged upon postsynthetic functionalization of
vol-UiO as indicated by XPS of vol-UiO-FeCl (Figure 3e). The
DFT-calculation with the B3LYP/6-311G (d,p) basis set
suggests an octahedral iron cation coordinated with one
neutral imine nitrogen, one monoanionic oxo, one chloride,
and three THF molecules (Figure 3g). The calculated bond
lengths of Fe−N, Fe−O(vol), and Fe−Cl are 2.07, 1.93, and
2.48 Å, respectively, while the Fe−O(THF) distances are 2.06,
At a 0.05 mol % Fe-loading, both vol-UiO-67-Fe and vol-
UiO-68-Fe efficiently hydrosilylated heterocyclic aromatic
ketones such as 1-(pyridin-2-yl)ethanone (1i), 1-(pyridin-4-
yl)ethanone (1j) and 1-(thiophen-2-yl)ethanone (1k) to
afford corresponding chiral alcohols (3i-k) with 99% ee
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Figure 4. (a) % GC-yield and the corresponding %ee of silyl ether (2a) at several runs in the recycle of vol-UiO-68-Fe for hydrosilylation of 4-
methoxyacetophenone. (b) % GC-yield and the corresponding %ee of borate ester (4a) at several runs in the recycle of vol-UiO-68-Fe for
hydroboration of 4-methoxyacetophenone. (c) Plots of initial rates -(d[substrate]/dt) for hydrosilyaltion of 4-methoxyacetophenone versus initial
concentrations of iron and 4-methoxyacetophenone for the first 5 min. (d) Fe 2p XPS spectrum of vol-UiO-68-FeH. (e) Zr 3d XPS spectrum of
vol-UiO-68-FeH. (f) EXAFS spectra and fits in R-space at the Fe K-edge of vol-UiO-68-Fe after hydrosilylation of 4-methoxyacetophenone
showing the magnitude (blue solid triangles) and real component (blue hollow triangles) of the Fourier transformation. The fitting range is 1.1−4.0
Å in R-space (within the gray lines).
(entries 16−20, Table 1). Moreover, aliphatic ketone such as
6-methylhept-5-en-2-one (1l) was reduced by 0.05 mol % of
vol-UiO-68-Fe or vol-UiO-67 using (EtO)2MeSiH to afford
(R)-6-methylhept-5-en-2-ol with 99% ee (entries 21 and 22,
Table 1). Importantly, vol-UiO-68-Fe could be recycled and
reused for at least 11 times for reduction of 4-methoxyaceto-
phenone (1a) using (EtO)2MeSiH without diminishing its
activity and enantioselectivity (Figure 4a). The leaching of iron
and zirconium in supernatant after recycling were 0.03% and
0.21% after run 1, 0.08% and 0.5% after run 8, respectively, as
analyzed by ICP-OES (Table S1, SI). The crystallinity of the
MOFs remained intact after the reduction of 4-methoxyace-
tophenone as evidenced by the PXRD pattern (Figure 3a,b).
Compared with the reported homogeneous iron catalysts for
asymmetric hydrosilylation of ketones, vol-UiO-Fe catalysts
require low Fe-loading (0.05 mol %) and could be easily
reusable.4,7,8,87−93 Furthermore, pure products were afforded
after catalysis without any chromatographic purification due to
the heterogeneous nature of the MOFs.
Mechanistic Investigation of Vol-UiO-Fe-Catalyzed
Asymmetric Hydrosilylation of Ketones. To study the
reaction mechanism of vol-UiO-Fe catalyzed asymmetric
hydrosilylation of ketones, we have performed several
experiments such as identification of key intermediates and
resting states of the catalyst, kinetics, and DFT calculations.
The treatment of LiCH2SiMe3 with vol-UiO-FeCl(THF)3 in
THF presumably forms vol-UiO-Fe(CH2SiMe3) precatalyst,
which produces vol-UiO-FeH catalyst upon addition of
(OEt)2MeSiH via σ-bond metathesis at room temperature.
The heating of a mixture of vol-UiO-FeH with H2O produced
nearly one equivalent of H2 as analyzed by gas chromatography
suggesting the formation of iron-hydride as the active catalytic
species (section 3.3, SI). The XANES and XPS analysis of vol-
UiO-FeH indicated the presence of FeII-centers (Figures 3c
and 4d). We have also identified the resting state of the catalyst
by EXAFS studies of vol-UiO-68-Fe recovered after hydro-
silylation of 4-methoxyacetophenone in THF. The EXAFS fit
at the Fe K-edge revealed a distorted tetrahedral geometry of
FeII coordinated to a vol-ligand, O[S−CH(Ar)(Me)] and one
THF molecule with no feature corresponding to the reduced
metallic Fe-particles (Figure 4f). The EXAFS fitting of vol-
UiO-68−Fe-O[S-CH(Ar)(Me)](THF) resting state gave a
Fe−N(vol) distance of 1.99 Å, Fe−O(vol) of 1.89 Å, Fe−
O(THF) of 2.09 Å and Fe−O(alkoxide) of 1.91 Å. The iron-
alkoxide resting state suggests that the enanatioselective
insertion of carbonyl into the Fe−H likely occurs in the
catalytic cycle. The lack of any characteristic reflection peaks at
higher 2θ angles in the PXRD spectra of vol-UiO-68-FeH and
vol-UiO-68-Fe after catalysis also precludes the formation of
Fe-particulates upon treatment of silane and during the
catalysis (Figure S5, SI).
Importantly, the homogeneous control, prepared from
(S,E)-2-(benzylideneamino)-3-methylbutan-1-ol and FeCl2,
was far less active and enantioselective than vol-UiO-68-Fe
presumably because of the detrimental intermolecular
interaction (section 4.7, SI). We thus believe that the chiral
single-site iron-species (vol-FeH) is stabilized within MOFs via
active site isolation that prevents intermolecular decomposi-
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Figure 6. DFT-calculated Gibbs free enthalpy reaction profile diagram
of vol-UiO-Fe catalyzed hydrosilylation of 4-methoxyacetophenone
(1a) using (OEt)2MeSiH at 298 K. Structural models of the two
stereodetermining transition states, TS-1(S) and TS-1(R) are shown
at the top. The carbon, nitrogen, oxygen, hydride, and iron atoms of
TS-1(S) and TS-2(R) are shown in gray, blue, red, whitish, and
purple color, respectively.
Figure 5. Mechanistic proposal for vol-UiO-Fe catalyzed asymmetric
hydrosilylation of ketones.
stereodetermining transition state TS-1(S). The conversion of
INT-2(S) to INT-3(S) is exergonic by 9.1 kcal/mol with an
energy barrier of 12.4 kcal/mol. Subsequently, the σ-bond
metathesis of Fe−O bond of INT-3(S) with Si−H bond of
(OEt)2MeSiH, associating with an activation free energy of 5.5
kcal/mol, gives rise to the formation of 2a(S) and reproduces
vol-FeH. The free energy diagram also reveals that the
coordination of THF to INT-3(S) leads to the formation of
lowest energy intermediate INT-3(S)-THF, which is also
identified as the resting state of the vol-UiO-68-Fe catalyst by
our EXAFS studies. The DFT-calculated energy profile
diagram revealed that the Re-face attack in TS-1(S), leading
to the generation of (S)-2a, is energetically favored by 2.8 kcal/
mol (ΔΔG#TS‑1) than the Si-face attack in TS-1(R), producing
(R)-2a, which is consistent with the experimentally observed
enantioselectivity at 298 K. The structural models of TS-1(R)
and TS-1(S) in Figure 6 showed that Si-face attack is
steroelectronically disfavored due to the steric repulsion
between the isopropyl group of the vol-ligand and the aryl
substituent of the ketones in the R-pathway.
Asymmetric Hydroboration of Ketones Catalyzed by
Vol-UiO-Fe MOFs. Upon activation with LiCH2SiMe3, both
UiO-67-Fe and UiO-68-Fe became the active catalysts for the
asymmetric hydroboration of aliphatic and aromatic ketones
using pinacolborane (Table 2).96,97 The reaction of ketones
with 1.1 equiv of pinacolborane in the presence of 0.05 mol %
Fe loading, vol-UiO-Fe for 12 min in THF at room
temperature produced borate esters in excellent yields and
enantiopurity. In most cases, the borate esters were obtained in
quantitative yields by simple removal of the solid MOF-catalyst
and volatiles. A wide range of aromatic ketones with different
tion. The enantiomeric excesses of the product 2a did not
change by varying the concentration of the ketone 1a, which
indicated that the catalyst’s structure remain unchanged during
1
the catalysis. H NMR spectrum of the digested vol-UiO-Fe
after hydrosilylation of 1a showed the retention of vol-moiety
of the linkers. On the basis of these experimental results, we
surmise that the coordination of the carbonyl to the iron
followed by 1,2-isertion of carbonyl group into the Fe−H bond
gives Fe-alkoxide intermediate. Subsequently, the σ-bond
metathesis of Fe-alkoxide and silane furnish the silyl ether
product and regenerates Fe-hydride (Figure 5).95 The
empirical rate law for hydrosilylation of 1a using (OEt)2MeSiH
was also determined by the initial rate method for the first 5
min of the catalysis (section 5, SI). The initial substrate
conversion rates had first-order dependency on both 1a and
iron (Figure 4c) but were independent of silane concentrations
(Figure S10, SI). The empirical rate law suggests that a
carbonyl molecule and iron-catalyst are involved in the
turnover-limiting step. We thus infer a reversible coordination
of carbonyl to the iron followed by insertion of the carbonyl
into the Fe−H as the turnover-limiting step in the catalytic
cycle.
To gain additional insight into the proposed mechanism and
investigate the origin of the enantioselectivity, we have
modeled the whole catalytic cycle using DFT methods at
298 K (Figure 6). In the case of the S-pathway, the DFT-
optimized structures reveal that the carbonyl (1a) coordinates
to iron from the opposite side of the bulky isopropyl-group of
vol-Fe catalyst to give intermediate INT-2(S) and then the
insertion takes place at the Re-face of the carbonyl in the
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compatibility at room temperature to afford optically active
reduced products with ee up to 99%. The enantioselective
hydrosilylation and hydroboration of prochiral ketones provide
chiral alcohols with wide range applications in organic
synthesis, materials science, and pharmaceutical chemistry.
Due to the porous and reticular nature of the MOFs, the
activity and selectivity of the MOFs were tuned by varying the
pore sizes and exploiting a mixed ligand strategy. The
coordination of iron within MOFs was elucidated using
EXAFS with DFT models. Spectroscopic, kinetic, and
computational studies explored the identification of the
catalytic species and the catalyst’s resting state, the mechanism
of hydrosilylation reactions, and the origin of the enantiose-
lectivity. This work highlights the importance of MOFs as the
tunable molecular material for developing chiral earth-
abundant metal catalyst ligated with inexpensive chiral amino
alcohols for sustainable asymmetric organic transformations.
Table 2. Asymmetric Hydroboration of Ketones Catalyzed
by Vol-UiO-Fe MOFs
a
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acscatal.1c02529.
■
sı
Synthesis and characterization of MOF-Fe materials,
procedures for catalytic reactions, details for kinetics,
EXAFS and GC-analysis, and DFT calculations (PDF)
AUTHOR INFORMATION
Corresponding Author
■
Kuntal Manna − Department of Chemistry, Indian Institute of
Technology Delhi, Hauz Khas, New Delhi 110016, India;
orcid.org/0000-0002-2924-0353; Email: kmanna@
chemistry.iitd.ac.in
Authors
Neha Antil − Department of Chemistry, Indian Institute of
Technology Delhi, Hauz Khas, New Delhi 110016, India
Naved Akhtar − Department of Chemistry, Indian Institute of
Technology Delhi, Hauz Khas, New Delhi 110016, India
Rajashree Newar − Department of Chemistry, Indian Institute
of Technology Delhi, Hauz Khas, New Delhi 110016, India
Wahida Begum − Department of Chemistry, Indian Institute
of Technology Delhi, Hauz Khas, New Delhi 110016, India
Ajay Kumar − Department of Chemistry, Indian Institute of
Technology Delhi, Hauz Khas, New Delhi 110016, India
Manav Chauhan − Department of Chemistry, Indian Institute
of Technology Delhi, Hauz Khas, New Delhi 110016, India
a
Reaction conditions: 0.05 mol % Fe, 0.585 mmol ketone, 0.643
b
mmol pinacolborane, 1 mL THF, 25 °C, 12 min. %ee were
determined by chiral GC after oxidizing the borate esters (4) to the
corresponding alcohols, and absolute configurations were assigned on
the basis of the literature reports.
functional groups have been successfully hydroborated to
afford the corresponding borate esters with enantiopurity up to
99% and TONs up to 12 000. Hydroboration of heteroarenes
such as 2-acetylpyridine and 4-acetylpyridine produced 4i and
4j, respectively, in quantitative yields with 99% ee (entries 8−
10, Table 2). Under the identical reaction conditions, vol-UiO-
Fe also efficiently reduced aliphatic ketone such as 6-
methylhept-5-en-2-one (1l) to afford (R)-6-methylhept-5-en-
2-ol (4l) with 99% ee (entry 11, Table 2).
Complete contact information is available at:
https://pubs.acs.org/10.1021/acscatal.1c02529
Author Contributions
†Neha Antil and Naved Akhtar contributed equally.
Notes
CONCLUSIONS
The authors declare no competing financial interest.
■
In conclusion, we have demonstrated a strategy of developing
heterogeneous single-site chiral base metal catalysts using
easily affordable amino alcohols as the chiral ligands and
MOFs as the porous support. The amino alcohol ligated iron
catalysts were prepared by grafting L-valinol within the pores of
MOFs followed by postsynthetic metalation. The resultant
MOF-Fe catalysts were highly active and enantioselective in
the hydrosilylation and hydroboration of a range of aromatic
and aliphatic ketones with excellent functional group
ACKNOWLEDGMENTS
■
This research is funded by the Science and Engineering
Research Board (SERB), India (project ECR/2017/001931).
N.A., R.N., and M.C. acknowledge CSIR, and W.B. acknowl-
edges UGC for financial support. We thank Prof. B. Jayaram
for his help in our computational studies. We acknowledge
Raja Ramanna Centre for Advanced Technology-Indore
(RRCAT) for providing Indus-2 SRS facility for XAFS
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