Phase Separation-Promoted Redox Deracemization of Secondary Alcohols over a Supported Dual Catalysts System

Article abstract of DOI:10.1002/cctc.202100738

Unification of oxidation and reduction in a one-pot deracemization process has great significance in the preparation of enantioenriched organic molecules. However, the intrinsic mutual deactivation of oxidative and reductive catalysts and the extrinsic incompatible reaction conditions are unavoidable challenges in a single operation. To address these two issues, we develop a supported dual catalysts system to overcome these conflicts from incompatibility to compatibility, resulting in an efficient one-pot redox deracemization of secondary alcohols. During this transformation, the TEMPO species onto the outer surface of silica nanoparticles catalyze the oxidation of racemic alcohols to ketones, and the chiral Rh/diamine species in the nanochannels of the thermoresponsive polymer-coated hollow-shell mesoporous silica enable the asymmetric transfer hydrogenation (ATH) of ketones to chiral alcohols. To demonstrate the general feasibility, a series of orthogonal oxidation/ATH cascade reactions are compared to prove the compatible benefits in the elimination of their deactivations and the balance of the cascade directionality. As presented in this study, this redox deracemization process provides various chiral alcohols with enhanced yields and enantioselectivities relative to those from unsupported dual catalysts systems. Furthermore, the dual catalysts can be recycled continuously, making them an attractive feature in the application.

Full text of DOI:10.1002/cctc.202100738

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Phase Separation-Promoted Redox Deracemization of
Secondary Alcohols over a Supported Dual Catalysts
System
Zhitong Zhao,[a] Chengyi Wang,[a] Qipeng Chen,[a] Yu Wang,[a] Rui Xiao,[a] Chunxia Tan,*[a] and
Guohua Liu*[a]
Unification of oxidation and reduction in a one-pot deracemiza-
tion process has great significance in the preparation of
enantioenriched organic molecules. However, the intrinsic
mutual deactivation of oxidative and reductive catalysts and
the extrinsic incompatible reaction conditions are unavoidable
challenges in a single operation. To address these two issues,
we develop a supported dual catalysts system to overcome
these conflicts from incompatibility to compatibility, resulting in
an efficient one-pot redox deracemization of secondary alco-
hols. During this transformation, the TEMPO species onto the
outer surface of silica nanoparticles catalyze the oxidation of
racemic alcohols to ketones, and the chiral Rh/diamine species
in the nanochannels of the thermoresponsive polymer-coated
hollow-shell mesoporous silica enable the asymmetric transfer
hydrogenation (ATH) of ketones to chiral alcohols. To demon-
strate the general feasibility, a series of orthogonal oxidation/
ATH cascade reactions are compared to prove the compatible
benefits in the elimination of their deactivations and the
balance of the cascade directionality. As presented in this study,
this redox deracemization process provides various chiral
alcohols with enhanced yields and enantioselectivities relative
to those from unsupported dual catalysts systems. Furthermore,
the dual catalysts can be recycled continuously, making them
an attractive feature in the application.
Introduction
products. However, this sequential oxidation/reduction process
is thermodynamically unfavorable, involving a decreased en-
tropic cost by converting two racemic alcohols into a single
alcohol and a kinetically microscopic reversibility used in
thermal reactions. Therefore, to obtain an efficient redox
deracemization process, two obligatory events must be over-
come. The first is that the oxidation and reduction work
independently to avoid the quench between oxidants and
reductants since it is thermodynamically favorable. The second
is that this cascade process must match a kinetic demand of the
microscopic reversibility, which requires an additional driving
force to impart on reaction directionality.
Upon examining the reported redox deracemizations to
overcome these drawbacks as shown in Scheme 1,[9] the William
group designed a two-step operation to solve this thermody-
namic problem. The initial non-selective oxidation of achiral
alcohol using cyclohexanone as a hydrogen acceptor and the
subsequent asymmetric hydrogenation with the in-situ gener-
ated Noyori complex as a catalyst can produce chiral alcohols
(Scheme 1A1).[9a,b] Meanwhile, the Nishibayashi group also used
a similar method to perform this redox deracemization via a
combination of two chiral ruthenium catalysts (Scheme 1A2).[9c]
By utilizing a site-isolated method,[10] the Weck group elegantly
assembled TEMPO (2,2,6,6-tetramethylpiperidine-N-oxyl) and
chiral diamine-based Rh-complex within the different parts of
Deracemization represents a promising method for the con-
struction of enantioenriched organic molecules, wherein a
racemic mixture can be ideally transformed into two single
enantiomers.[1] Recent studies have presented some notable
examples for deracemizations, involving photocatalyst,[2]
enzymes,[3] supramolecular interactions,[4] and so on.[5] Despite
its attractiveness, successful deracemizations through a sequen-
tial chemical process in a single operation remain challenging.[6]
Optically pure alcohols, as important synthetic motifs, have
been used extensively in the pharmaceutical, agrochemical, and
fine chemical industries.[7] Generally, a biocatalytic system
through an enzymatic redox deracemization of racemic alcohols
can prepare enantioenriched alcohols.[8] However, due to the
restricted substrate specificity of enzymes, a chemical redox
deracemization is highly desirable. An ideally chemical trans-
formation, the oxidation of racemic alcohols to ketones and the
reduction of ketones to chiral alcohols, can greatly broaden the
applications in the construction of enantioenriched alcohol
[a] Z. Zhao, C. Wang, Q. Chen, Y. Wang, R. Xiao, C. Tan, Prof. G. Liu
International Joint Laboratory on Resource Chemistry of
Ministry of Education
Shanghai Engineering Research Center of
Green Energy Chemical Engineering
Shanghai Normal University
Shanghai 200234 (P. R. China)
Fax: (+) (0086) 216432280
cross-linked micelles to construct
a supported catalyst
(Scheme 1A3),[9d,10d] which enabled an efficient redox deracem-
ization in a one-step operation. But, due to the complicated
preparation procedure, it is highly desirable to explore a new
method for a highly efficient redox deracemization in a single
operation.
E-mail: tanchx@shnu.edu.cn
ghliu@shnu.edu.cn
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/cctc.202100738
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ization of secondary alcohols provides various secondary
alcohols with enhanced yields and enantioselectivities com-
pared with those from unsupported dual catalysts systems.
Results and Discussion
Syntheses and Structural Characterizations of the
Heterogeneous Catalysts
A simple procedure for the anchoring of the TEMPO species
onto the outer surface of the silica nanoparticles, abbreviated
as [TEMPO]@NPs (3) (TEMPO:[11] 2,2,6,6-tetramethylpiperidine-N-
oxyl), was performed via a postgrafting process, as outlined in
Scheme 2. Firstly, the silica nanoparticles (SiO2@NPs (1)) were
obtained according to the reported method.[11a] 3-(triethoxysilyl)
propanamine was then postgrafted in the outer surface of the
nanoparticles to produce NH2@NPs (2), followed by the
reductive amination with 4-oxo-TEMPO in the presence of
NaBH3CN, affording the crude 3.[11a] Finally, this crude material
was subjected to strict Soxhlet extraction for the removal of the
unreacted reagents to give the pure catalyst 3 as a white
powder. A thermogravimetric (TG) analysis showed that the
propyl-linked TEMPO-loading amounts were 4.79% (see Fig-
ure S1 of ESI). Solid-state 13C cross-polarization/magic angle
spinning (CP/MAS) NMR spectroscopy (see Figure S2 of ESI)
disclosed the successful immobilization of TEMPO species onto
the outer surface of silica nanoparticles. Besides general signals
observed at δ=12.5, 29.7, and 46.5 ppm for the chained alkyl
carbon atoms in the propyl moiety, the characteristic signals at
δ between 59.4 and 65.3 ppm corresponded to the carbon
atoms connected with the nitrogen atoms in the cyclic TEMPO
moiety, whereas those at δ=19.5 and 26.1 ppm could be
ascribed to the chained or cyclic alkyl carbon atoms without
connection to the nitrogen atoms. All these observations
demonstrated the well-defined single-site TEMPO active species
because these chemical shifts were the same as those reported
in the literature.[11b] In addition, the silicate network and
composition of catalyst 3 were proven by the solid-state 29Si
Scheme 1. (A) Reported redox deracemizations and (B) a schematic illustra-
tion for one-pot redox deracemization in this work
Inspired by a compartmentalization-featured dual catalysts
system in our previous report,[10c] we hypothesize that the phase
separation of the biphasic co-solvents in a dual catalysts system
can drive a redox deracemization of secondary alcohols to chiral
alcohols, wherein two heterocatalysts have different hydro-
philicity/hydrophobicity (Scheme 1B). During this transforma-
tion, the in-situ generated organic ketones via an oxidation
process of alcohols in the aqueous phase (H2O phase) could be
pumped into the ATH process in the organic phase (CH2Cl2
phase). If both oxidant and/or reductant keep enough low
concentrations in its corresponding opposite solvent, it can
ensure the oxidation and reduction reactions work independ-
ently, thereby leading to the desirable chiral alcohols. Herein,
we report a rational design of a compartmentalization-type
supported dual catalysts system with the hydrophilic supported
TEMPO as an oxidative catalyst, and the hydrophobic supported
chiral Rh/diamine as a reductive catalyst. As we envisage, the
TEMPO species onto the outer surface of mesoporous silica
nanoparticles can catalyze the oxidation of racemic secondary
alcohols into ketones, and the chiral Rh/diamine-species within
the nanochannels of the thermoresponsive polymer-coated
hollow-shell mesoporous silica nanoparticles can convert the in-
situ generated ketones into chiral alcohols. Furthermore, the
polymer-coating layer in the latter catalyst also offers an
additional thermoresponsive driving force to impart on the
cascade directionality, meeting a kinetic match of the chiral
alcohols to promote this deracemization process. As presented
in this study, this phase separation-promoted redox deracem-
Scheme 2. Assembly of catalysts 3 and 5 used in the dual catalysts system.
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MAS NMR spectroscopy, wherein 3 possessed an inorganic silica
network with the strongest Q3 species (RO-Si(OSi)3: R=propyl-
linked TEMPO groups) as its main silica wall of the silica
nanoparticles (see Figure S3 of ESI).[12]
Immobilization of the chiral rhodium/diamine-species
within the nanochannels of the silica shell in the water-
soluble thermoresponsive polymer-coating hollow-shell mes-
oporous silica nanoparticles, abbreviated as Polymer@[Rh]-
@HSMSNPs (5) (Polymer:[13] poly(ethene-co-acrylamide-co-
acrylonitrile), [Rh]=Cp*RhArDPEN:[14] Cp*=pentamethyl cy-
denoted at asterisk could be assigned to the spinning side-
bands for the phenyl group and the alkyl carbon atoms of the
residual structure-directing molecules. Moreover, the solid-state
29Si MAS NMR spectra also confirmed its organic silica network,
wherein the silica wall in 5 was mainly comprised of the
strongest T3 species (R-Si(OSi)3: R=ethylene-bridged groups
and alkyl-linked Ru/diamine complexes) owing to the obviously
strong T signals relative to Q signals (see Figure S3 of ESI).
The hollow-shell-structured morphology of the catalyst 5
with ordered pore arrangements was further characterized
using scanning electron microscopy (SEM) and transmission
electron microscopy (TEM), as shown in Figure 2. The SEM
image revealed that the nanoparticles in 5 were uniformly
dispersed with an average size of 385 nm (Figure 2a) (also see
Figure S6 of ESI for the SEM image of 3). The TEM image of 5
also disclosed that each silica nanosphere consisted of a
nanocage with an average size of 275 nm in diameter and a
thin shell of 55 nm in thickness (Figure 2b). In particular, the
clopentadiene,
phenylsulfonyl-1,2-diphenylethylene-diamine),
ArDPEN=(S,S)-4-((trimethoxysilyl)ethyl)
was per-
formed via a sequential polymerization-coordination proce-
dure (Scheme 2). In the free-radical polymerization step, the
obtained as-made 4 reacted with acrylamide and acrylonitrile
to produce the polymer-coating material Polymer@HSMSNPs
(4’),[10c] wherein the polymer-loading amounts were 2.01%, as
determined by the TG analysis (see Figure S4 of ESI). In the
°
coordination step, the reaction of 4’ and (Cp*RhCl2)2 at 40 C
in CH2Cl2 formed a coarse catalyst, wherein a strict Soxhlet
extraction afforded a pure catalyst 5 as a light-yellow powder
(see Figure S5 of ESI). For comparison, a parallel analog (5’) of
the catalyst without the outer polymer-coating layer, abbre-
viated as [Rh]@HSMSNPs (5’), was also prepared following a
similar coordination procedure (see experimental part of ESI).
Figure 1 showed the solid-state 13C CP/MAS NMR spectra of
4 and 5, which revealed the well-defined single-site rhodium/
diamine centers within the silica shell in 5. In general, catalyst 5
presented some common signals observed at δ=192.4 and
8.4 ppm, corresponding to the carbonyl carbon atoms of the
polymeric moiety and the ethyl carbon atoms of the ethane-
bridged organic silica, respectively. In particular, catalyst 5
exhibited some characteristic signals at δ=13.0 and 99.4 ppm,
which were ascribed to the methyl and aromatic carbon atoms
in the Cp* ring. These observed characteristic signals were
similar to those attained with the corresponding parent
Cp*RhTsDPEN,[14a] proving the well-defined single-site rhodium/
diamine centers in 5. Furthermore, the other carbon signals
Figure 2. (a) SEM images of catalyst 5, (b) TEM images of catalyst 5, and
(c) TEM image with the chemical mapping of 5 showing the distribution of Si
(white) and Rh (red). Dispersive situations in CH2Cl2/water (V/V=1/2) co-
solvents for 3 in upper water (d), for 5 in lower CH2Cl2 (e), for a mixture of 3
and 5 with 3 in upper water and 5 in below CH2Cl2 (f), and for a mixture of 3
and 5’ with both in upper water (g).
Figure 1. Solid-state 13C CP/MAS NMR spectra of 4 and catalyst 5.
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TEM image with a chemical mapping demonstrated that the
rhodium centers in 5 were uniformly distributed (Figure 2c).
Furthermore, the nitrogen adsorption-desorption isotherms of 5
also confirmed its mesoporous structure, since a typical IV
character with an H1 hysteresis loop was observed that was the
same as those reported for hollow-shell-structured mesoporous
silica (see Figure S7 of ESI).[10c]
the mixed TEMPO and Cp*RhTsDPEN gave a decreased yield
and diminished ee when compared with that with the only
Cp*RhTsDPEN (entry 7 versus entry 5, Table 1). Although this
situation could be slightly improved in the reaction with the
mixed TEMPO and supported 5 (entry 8 versus entry 5, Table 1),
the still low yield and ee value demonstrated that the TEMPO
also disturbed the reactivity and enantioselectivity of
Cp*RhTsDPEN to a certain degree. Through the evaluation of
two single-step reactions, a conclusion could be drawn, wherein
the TEMPO and Cp*RhTsDPEN species are incompatible.
Catalytic Property
In the evaluation of the cascade reaction (III) stage, we
compared three kinds of different dual catalysts systems for the
model redox deracemizations of racemic 6a in a one-pot
manner. In the case of the unsupported TEMPO and
Cp*RhTsDPEN as a dual catalysts system, we found that the
cascade reaction gave the mixed 7a (76% yield) and nearly
racemic alcohols (23% yield) (entry 9, Table 1). This observation
demonstrated that there was an obvious mutual deactivation
behavior in a one-pot manner. This phenomenon indicated that
two issues, the inner incompatible nature between TEMPO and
Cp*RhTsDPEN discussed above and the outer disturbances
coming from the aqueous components (NaOCl and KBr for the
With the as-made catalysts 3 and 5 prepared, a series of control
experiments based on two single-step reactions (I and II) and a
cascade (III) were evaluated for the optimization of the redox
deracemization, as shown in Table 1. In the evaluation of the
single-step reaction stage, we found that the single-step
oxidation (I) of the racemic 1-phenylethan-1-ol (6a) catalyzed
°
by 3 at 0 C could produce the intermediate acetophenone (7a)
in a similar yield to that obtained with the homogeneous
TEMPO, demonstrating a maintainable reactivity after the
immobilization of TEMPO (entry 2 versus entry 1, Table 1).
However, the single-step oxidation with a mixture of TEMPO
plus Cp*RhTsDPEN or a mixture of 3 plus Cp*RhTsDPEN
afforded 7a with decreased yields relative to those catalyzed by
just TEMPO or 3 (entries 3 and 4 versus entries 1 and 2, Table 1).
This comparison indicated that the Cp*RhTsDPEN affected the
reactivity of TEMPO.
oxidation), imparted
a negative influence on this redox
deracemization in this one-pot process. In the case of the partly
supported dual catalysts systems, it was found that the cascade
reaction catalyzed by the integrated 3 and Cp*RhTsDPEN gave
the final product ((S)-6a) with a similar result to that attained
with the unsupported TEMPO and Cp*RhTsDPEN (entry 10
versus entry 9, Table 1). This finding indicated that the negative
effect coming from the outer aqueous NaOCl could destroy the
reduction process since the single-step reduction without the
NaOCl and KBr gave the markedly enhanced (S)-6a (entry 10
In the case of the single-step reduction (II) of 7a, the ATH
°
catalyzed by 5 at 40 C could also give (S)-1-phenylethan-1-ol
((S)-6a) in 97% yield and 98% ee, which was comparable to
that attained with its homogeneous Cp*RhTsDPEN (entry 6
versus entry 5, Table 1). However, this ATH transformation with
Table 1. Optimizations of the dual catalysts used in the redox deracemization.[a]
Entry
Reaction
Catalysts
7a, [%] Yield
(S)-6a, [%] Yield [% ee]
1
2
3
4
5
6
7
8
I
I
I
I
II
II
II
II
III
III
III
III
TEMPO
3
TEMPO+[Rh]
3+[Rh]
[Rh]
97
97
79
81
/
/
/
/
/
/
/
98 (98)
97 (98)
72 (91)
85 (93)
23 (5)
37 (5)
82 (91)
97 (98)
5
TEMPO+[Rh]
TEMPO+5
TEMPO+[Rh]
3+[Rh]
TEMPO+5
3+5
/
9
76
62
17
trace
10
11
12
[a] Reaction conditions: For the reaction I, catalysts (1.0 mol% of TEMPO, and/or 1.0 mol% of Rh), 6a (0.10 mmol), KBr (11.90 mg, 1 equivalents) and NaOCl
°
(0.70 mL, 1.0 M, 7 equivalents) in 3.0 mL of CH2Cl2/H2O (v/v=1/2), 0 C, 10h. For reaction II, catalysts (1.0 mol% of Rh, and/or 1.0 mol% of TEMPO), 7a
°
(0.10 mmol), and HCOONa (15 equiv.) in 3.0 mL of CH2Cl2/H2O (v/v=1/2), 40 C, 18 h. For cascade reaction III, dual catalysts (1.0 mol% of TEMPO, 1.0 mol%
°
of Rh), 6a (0.10 mmol), KBr (11.90 mg, 1 equivalents) and NaOCl (0.70 mL, 1.0 M, 7 equivalents) in 3.0 mL of CH2Cl2/H2O (v/v=1/2) at 0 C for the first 10 h,
followed by addition of HCOONa (10 equivalents) at 40 C for the second 18 h. All yields were determined using 1H-NMR spectroscopy and the ee values
°
were determined using chiral HPLC analysis. [Rh]=Cp*RhTsDPEN.
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versus entries 5 and 6, Table 1). This phenomenon could be
further confirmed by a single-step parallel experiment, wherein
the ATH of 7a catalyzed by Cp*RhTsDPEN in the presence of
NaOCl and KBr only provides (S)-6a with trace yield. Interest-
ingly, this kind of outer disturbance could be markedly
improved in the case of the integrated TEMPO and 5 as a dual
catalysts system, since the cascade could provide (S)-6a in an
82% yield and 91% ee. This observation illustrated that the
addition of the excess HCOONa could destroy the residual
aqueous NaOCl component, thereby reducing the outer dis-
turbances imparted on Cp*RhTsDPEN center and leading to an
enhanced yield and enantioselectivity (entry 11 versus entries 9
and 10, Table 1). In the case of the supported 3 and 5 as a dual
catalysts system, the cascade reaction could reach the reaction
completion, providing (S)-6a in a 97% yield and 98% ee, which
was similar to the corresponding two single-step transforma-
tions (entry 12 versus entries 2 and 6, Table 1). This finding
demonstrated that the designed dual catalysts realized the
efficient redox deracemization process from incompatibility to
compatibility, indicating the advantage of the phase separation
in this dual catalysts system.
To determine the phase separation-promoted redox dera-
cemization, three aspects of the effecting factors were further
investigated, as shown in Table 2. In the investigation of the
role of the phase separation based on the hydrophilicity/
hydrophobicity, the model one-pot redox deracemization of the
racemic 6a catalyzed by the integrated 3 and 5’ (a parallel
analog of the catalyst 5 without the outer polymer-coating
layer) was compared. It was found that a 68% yield (86% ee) of
(S)-6a was observed, which was obviously lower than a 97%
yield (98% ee) in the supported 3 and 5 dual catalysts system
(entry 2 versus entry 1, Table 2). This comparison indicated a
lack of the phase separation ability in this integrated 3 and 5’
dual catalysts system proven by an investigation of the
dispersive situations in the mixed co-solvents (CH2Cl2/H2O: v/v=
1:2). As shown in 2d–2g of Figure 2, catalyst 3 was hydrophilic
with being highly dispersed in H2O (Figure 2d), and 5 was
hydrophobic with being highly dispersed in CH2Cl2 (Figure 2e),
whereas catalysts 3 and 5 presented an obvious layer-isolated
behavior (Figure 2f). However, both catalysts 3 and 5’ were
hydrophilic that were highly dispersed in the upper H2O layer
(Figure 2g). Due to no layer-isolated behavior in this integrated
3 and 5’ dual catalysts system, the in-situ generated oxidative
ketone (7a) goes through the phase separation to enter the
lower CH2Cl2 layer for the ATH transformation (a biphasic
driving shift of 7a into CH2Cl2 layer), thereby leading to a poor
reactivity.
In the investigation of a biphasic driving force, we further
tested the partition coefficient of the intermediate ketone (7a)
and the racemic alcohol (6a) in the co-solvents (CH2Cl2/H2O: v/
v=1:2). It was found that the partition coefficient of 7a in the
below CH2Cl2 layer is more than 99.9% detected by HPLC
analysis, confirming that this phase separation driving force
makes 7a completely enter the organic layer for the second-
step reduction. This demonstrated the obvious difference in the
investigation of the hydrophilicity/hydrophobicity for the above
two dual catalysts systems. Also, we analyzed the partition
coefficient of the racemic 6a since the unreacted residual 6a
could make the total ee value of (S)-6a to be diminished. The
result showed that the partition coefficient of 6a in a mixture of
CH2Cl2 and H2O is 85/15. Interestingly, the first-step oxidation
reaction during this cascade process in the only H2O as a
solvent did not occur under the optimal reaction conditions,[11a]
wherein the ee value was still the same as that of the racemate
(entry 3, Table 2). This finding demonstrated that, to avoid a
diminished ee value of (S)-6a in this co-solvents system (CH2Cl2/
H2O: v/v=1:2), the first-step oxidation during the cascade
process must be converted completely, which might be an
obligatory demand for high enantioselectivity. This finding
indicated that a cascade directionality played a crucial function
during this one-pot transformation.
In the investigation of the phase separation-promoted
cascade directionality, we compared the model cascade reac-
tion at the different reaction temperatures (entries 4 and 5,
Table 2). It was found that the thermoresponsive polymer-
coating layer in 5 was necessary because the cascade reaction
°
°
either at 20 C or 40 C gave the final (S)-6a with the obviously
decreased yields and ee values. This was attributed to the
Table 2. Identification of phase separation-promoted redox deracemization.[a]
[%] Yield and [% ee][b]
°
Entry
Catalysts
Solvent
X [ C]
°
1
2
3
4
5
6
7
3+5
3+5’
3+5
3+5
3+5
3+5
3+5
CH2Cl2/H2O (1/2)
CH2Cl2/H2O (1/2)
H2O
CH2Cl2/H2O (1/2)
CH2Cl2/H2O (1/2)
toluene/H2O (1/2)
Et2O/H2O (1/2)
0 C
97/98
68/86
ND/0
53/68
45/56
91/95
38/93
°
0 C
0 C
°
°
20 C
°
40 C
°
0 C
°
0 C
[a] Reaction conditions: Catalysts (1.0 mol% of TEMPO, 1.0 mol% of Rh), 6a (0.10 mmol), KBr (11.90 mg, 1 equivalent) and NaOCl (0.70 mL, 1.0 M,
°
°
7 equivalents) in 3.0 mL of solvents at X C for the first 10 h, followed by addition of HCOONa (10 equivalents) at 40 C for the second 18 h. [b] Yields were
determined using 1H-NMR spectroscopy and the ee values were determined using chiral HPLC analysis. ND=not detected.
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swelling conformational state of the thermoresponsive poly-
redox deracemization because of the highest yield and ee value
(entry 1 versus entries 6 and 7, Table 2).
°
mer-coating layer in 5 at >10 C proven by a turbidity
investigation of 5 (see Figure S8 of ESI).[10c,13a] During this
process, the first-step oxidation and the second-step reduction
simultaneously proceeded. As a result, the partly final (S)-6a
was repeatedly oxidized into 7a for an not useful consumption
of oxidative reagents, leading to an uncompleted conversion of
racemic 6a thereby a poor yield with ee value. It is noteworthy
that, among those mixed co-solvents, a mixture of CH2Cl2 and
H2O was the optimal co-solvent system used in this one-pot
To clarify the role of the thermoresponsive polymer-coating
layer of 5 in manipulating the phase separation-promoted
cascade directionality during this one-pot redox deracemization
process, a kinetic process by detecting the time-course of this
supported dual catalysts system was also investigated, as
shown in Figure 3. Initially, the oxidation of the racemic 6a
°
smoothly proceeds at 0 C as the concentration of 6a rapidly
decreases. After the transformation over the first 10 h, the
intermediate 7a reaches a maximum yield of 97%. Subse-
quently, after the addition of HCOONa at room temperature
°
and the adjustment of the reaction temperature to 40 C, the
ATH of 7a occurs to sharply yield (S)-6a as the concentration of
7a gradually decreases between 10 and 20 h. Finally, the
reaction smoothly proceeds to afford (S)-6a in a 97% yield after
a further 8 h. This investigation disclosed a controllable cascade
directionality, which was comprised of an oxidation followed by
the ATH process.
Based on the feasible one-pot redox deracemization proc-
ess, a series of aryl-substituted secondary alcohols were further
examined, as shown in Table 3. The results showed that all the
tested achiral substrates could be steadily converted into the
corresponding chiral products in high yields and enantioselec-
tivities, which were comparable to those attained with the
single-step enantioselective ATH transformation.[14a] No signifi-
cant stereoelectronic properties of the substituents on the
aromatic rings were observed because their enantioselectivities
with the electron-withdrawing (F, Cl, Br, and NO2) or -donating
substituents (MeO, and Me) on the aromatic rings were equally
efficient (entries 2–5 versus entries 6–9). Furthermore, the one-
pot redox deracemizations of the naphthyl-, furyl-, and thienyl-
substituted substrates also produced the corresponding chiral
products with high enantioselectivities (entries 10–12).
Figure 3. The time-course of the dual catalysts 3 and 5 for the redox
deracemization of racemic 1-phenylethan-1-ol (6a) (The error bars represent
the standard deviation). The reaction was performed with 1.0 mol% of
TEMPO in 3 and 1.0 mol% Rh in 5, 1 equivalent of 6a, in 3.0 mL of CH2Cl2/
°
H2O (v/v=1/2) containing 7 equivalents of NaOCl, 1 equivalent of KBr at 0 C
°
for the first 10 h, followed by addition of 10.0 equivalent of HCOONa at 40 C
for the second 18 h).
Table 3. One-pot redox deracemization of racemic secondary alcohols.[a]
Entry
(S)-6
Ar
[%] Yield[b]
[% ee][c]
1
2
3
4
5
6
7
8
(S)-6a
(S)-6b
(S)-6c
(S)-6d
(S)-6e
(S)-6f
(S)-6g
(S)-6h
(S)-6i
(S)-6j
(S)-6k
(S)-6l
Ph
4-FPh
4-ClPh
4-BrPh
4-NO2Ph
4-MePh
3-MePh
4-OMePh
3-OMePh
2-naphthyl
2-furyl
97
83
81
84
77
87
93
93
84
93
78
76
98
93
91
93
83
90
90
93
95
92
96
96
9
10
11
12
2-thienyl
[a] Reaction conditions: Dual catalysts (1.0 mol% of TEMPO, 1.0 mol% of Rh), 6a (0.10 mmol), KBr (11.90 mg, 1 equivalent) and NaOCl (0.70 mL, 1.0 M,
°
°
7 equivalents) in 3.0 mL of CH2Cl2/H2O (v/v=1/2) at 0 C for the first 10 h, followed by addition of HCOONa (10 equivalents) at 40 C for the second 18 h.
[b] Isolated yields. [c] The ee values were determined using chiral HPLC analysis (see Figure S9 and Figure 11 of ESI).
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In addition to the one-pot redox deracemization studied in
be recycled six times. In the sixth run, the supported dual
catalysts could still give the chiral (S)-6a in an 86% yield with a
90% ee value (see Table S1 and Figure S10 of ESI).
Table 3, the preparation of the other chiral alcohols used in this
supported dual catalysts system was also performed, as shown
in Scheme 3. It was found that a gram-scale 1,2,3,4-tetrahydro-
naphthalen-1-ol (6m) could be converted into (S)-6m in 89%
yield and 99% ee (equation 1). Moreover, the one-pot redox Conclusion
deracemization with the large-sized 1-(pyren-1-yl)ethan-1-ol
(6n) and small-sized 6a as the mixed substrates mainly gave
(S)-6a (93% yield and 99% ee) along with a small amount of (S)-
6n (equation 2). However, a normal reduction of 7n catalyzed
by the homogeneous Cp*RhTsDPEN gave (S)-6n in 91% yield
and 93% ee. This observation suggested that the reaction with
the large-sized substrate is more difficult to pass through the
nanopores than with the small-sized substrate thereby a poor
reactivity.
In conclusion, through the use of two different types of silica
nanoparticles as dual-supports, we have successfully immobi-
lized TEMPO onto the outer surface of a silica nanoparticles,
and the Rh/diamine species in the nanochannels of silica shell
of another mesoporous silica. By integrating them into a dual
catalysts system, an efficient redox deracemization of secondary
alcohols is realized, where the TEMPO species catalyzes the
oxidation of racemic secondary alcohols into ketones, and the
chiral Rh/diamine species converts the in-situ generated inter-
mediate to the corresponding enantioenriched secondary
alcohols. As presented in this study, this dual catalysts system
can efficiently solve the problem of mutual interactions
between TEMPO and Rh/diamine of dual-species, providing
various chiral products in high yields and enantioselectivities.
Furthermore, a time-course study discloses a sequential reaction
sequence in the one-pot redox deracemization process. The
presented study also offers an avenue for the exploration of
multi-step sequential enantioselective organic transformation
through the use of this kind of integrated dual catalysts system.
Catalyst’s Recyclability
Another consideration for the design of this supported dual
catalysts system expects high recyclability, wherein the dual
catalysts could be recovered and recycled via a simple filtration.
As shown in Figure 4, in a consecutive one-pot redox deracem-
ization of 6a, we found that the supported dual catalysts could
Experimental Section
Preparation of Catalyst 3
In a typical synthesis. The SiO2@NPs (1) (1.0 g) were dispersed in
20.0 mL of dry toluene, and 3-(triethoxysilyl)propanamine
(200.0 mg, 0.90 mmol) was added to this suspension at room
°
temperature. The resulting mixture was stirred at 25 C for 24 h.
After the filtration, the collected white solid (2) (1.01 g) was
suspended in 20.0 mL of dry methanol, 4-oxo-TEMPO (0.34 g,
2.0 mmol), and NaBH3CN (0.13 g, 2.0 mmol) in dry methanol
°
(10 mL) was added, and the resulting mixture was stirred at 25 C
for 24 h. The mixture was filtered and then washed with excess
water and methanol. After Soxhlet extraction for 24 h in CH2Cl2, the
solid was dried at room temperature under vacuum overnight to
afford 3 (1.03 g) as a white powder. Thermogravimetric analysis
(TG) showed the TEMPO-loadings were 3.45% (0.22 mmol,
34.57 mg) per gram of catalyst (3). 13C CP/MAS NMR (161.9 MHz):
70.5 (À CH2N+À in the surfactant cetyltrimethylammonium bromide
(CTAB) moiety), 65.3–59.4 (À CÀ and À CHÀ of cyclic alkyl carbon
atoms connected to the nitrogen atom), 57.3 (À N+(CH3)3 in the
surfactant cetyltrimethylammonium bromide (CTAB) moiety), 46.5
(À SiCH2CH2CH2NÀ ), 33.3 (À CH2À in CTAB moiety), 29.7
(À SiCH2CH2CH2NÀ , and À CH2CH2N+À in CTAB moiety), 26.1 (À CH2À
of cyclic alkyl carbon atoms and CH3CH2À in CTAB moiety), 19.5
(CH3À in cyclic moiety, and CH3À in CTAB moiety), 12.5 (À SiCH2À ).
29Si MAS/NMR (79.4 MHz): Q2 (δ=À 89.9 ppm), Q3 (δ=À 102.0 ppm),
Q4 (δ=À 112.5 ppm).
Scheme 3. Preparation of (S)-1,2,3,4-tetrahydronaphthalen-1-ol and investi-
gation of the substrate selectivity in the redox deracemization.
Preparation of Catalyst 5
In a typical synthesis. (First step for the polymerization) The obtained
Vinyl@SiO2@HSMSNPs (4) (0.50 g) were dispersed in 10 mL of
Figure 4. Reusability of the dual catalysts used in the redox deracemization
of 6a (The error bars represent the standard deviation).
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distilled DMSO under a nitrogen atmosphere and acrylamide
(0.10 g, 1.41 mmol), acrylonitrile (0.019 g, 0.36 mmol), and 2,2-
azobisisobutyronitrile (AIBN) (6.64 mg, 2% mol) added to the
resulting solution at room temperature. After degassing the
reaction mixture via three freeze-pump-thaw cycles, the flask was
Keywords: Asymmetric
Heterogeneous catalysis · Mesoporous material · Supported
catalysts
catalysis
·
Deracemization
·
°
placed into an oil bath to polymerize at 60 C for 6 h. (Second step
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for the coordination) After cooling to room temperature, the
resulting white solid 4’ (0.56 g) was collected by centrifugation and
suspended in 20.0 mL of dry CH2Cl2. Then, 50.0 mg of (Cp*RhCl2)2
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General Procedure for the Redox Deracemization Process
A typical procedure was as follows. A suspension of catalysts 3
(4.55 mg, 1.0 mol% of TEMPO, based on TG analysis) and 5
(11.92 mg, 1.0 mol% of Rh, based on ICP analysis), and alcohols
(0.10 mmol) in 1.0 mL of CH2Cl2 was added to the 2.0 mL of an
aqueous solution containing KBr (11.90 mg, 1 equivalent) and
NaOCl (0.70 mL, 1.0 M, 7 equivalents) in a 10.0 mL round-bottom
°
°
flask at 0 C. The resulting mixture was then stirred at 0 C for 6–
12 h. After the completion of the oxidation reaction detected by
TCL, HCO2Na (1.0 mmol) was added to this suspension, and the
°
resulting mixture was stirred at 40 C for 6–24 h. During this period,
the reaction was monitored by TLC. After completion of the
reaction, the catalysts were separated by centrifugation
(10,000 rpm) for the recycling experiment. The aqueous solution
was extracted with ethyl ether (3×3.0 mL). The combined ethyl
ether extracts were washed with aqueous Na2CO3 and then
dehydrated with Na2SO4. After evaporation of the solvent, the
resulting residue was purified by silica gel flash column chromatog-
raphy to afford the desired product. The ee values were determined
using HPLC analysis using a UV-Vis detector and Daicel chiral-cel
column (Φ 0.46×25 cm).
Acknowledgements ((optional))
We are grateful to the China National Natural Science Foundation
(22071154, 22001171), and the Shanghai Sciences and Technolo-
gies Development Fund (20070502600), and Shanghai Sailing
Program (2020YF1435200) for financial supports.
Conflict of Interest
The authors declare no conflict of interest.
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Manuscript received: May 19, 2021
Revised manuscript received: July 20, 2021
Version of record online: ■■■, ■■■■
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Z. Zhao, C. Wang, Q. Chen, Y. Wang,
R. Xiao, C. Tan*, Prof. G. Liu*
1 – 10
Phase Separation-Promoted
Redox Deracemization of
Secondary Alcohols over a
Supported Dual Catalysts System
Redox deracemization to chiral
alcohols: A supported dual catalysts
system by overcoming the mutually
deactivated TEMPO and Rh/diamine
dual-species is developed. Dual
catalysts perform an efficient phase
separation-promoted redox deracem-
ization of secondary alcohols to
provide various chiral alcohols with
high yields and enantioselectivities.