Ru-Tethered (R,R)-TsDPEN with DMAB as an efficient catalytic system for high enantioselective one-pot synthesis of chiral β-aminolviaasymmetric transfer hydrogenation

Article abstract of DOI:10.1039/d0nj06108e

This work reflects Ru-tethered-TsDPEN as an active chiral catalyst for one pot selective synthesis of optically active α-substituted alcohols and its derivatives from α-bromo ketones in the presence of dimethylamine borane (DMAB) as the hydrogen source. Various Ru-chiral catalysts have been screened and the methodology proceededviaa (R,R) Ru-tethered TsDPEN catalyst through asymmetric transfer hydrogenation (ATH) of the in-situ formed ketones to the corresponding chiral β-aminol product. Thus, the Ru-tethered TsDPEN-DMAB catalytic system works efficiently with higher yield and high enantiomeric excess over others for the ATH process. Based on a study ofortho,metaandparasubstituted α-bromo acetophenone derivatives, effective enantioselectivity has been observed fororthosubstituted β-aminol. The mechanism has been optimized depending on product analysis with the help of its kinetic AT-IR study. This work also focusses on the synthesis of various β-amino alcohol derivatives where the effect of an EWG and EDG on enantio-selectivity has been studied.

Full text of DOI:10.1039/d0nj06108e

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Ru-Tethered (R,R)-TsDPEN with DMAB as an
efficient catalytic system for high enantioselective
one-pot synthesis of chiral b-aminol via
asymmetric transfer hydrogenation†
Cite this: New J. Chem., 2021,
45, 5357
Ashish A. Mishra
and Bhalchandra M. Bhanage
*
This work reflects Ru-tethered-TsDPEN as an active chiral catalyst for one pot selective synthesis of
optically active a-substituted alcohols and its derivatives from a-bromo ketones in the presence of
dimethylamine borane (DMAB) as the hydrogen source. Various Ru-chiral catalysts have been screened
and the methodology proceeded via a (R,R) Ru-tethered TsDPEN catalyst through asymmetric transfer
hydrogenation (ATH) of the in-situ formed ketones to the corresponding chiral b-aminol product. Thus,
the Ru-tethered TsDPEN-DMAB catalytic system works efficiently with higher yield and high
enantiomeric excess over others for the ATH process. Based on a study of ortho, meta and para
substituted a-bromo acetophenone derivatives, effective enantioselectivity has been observed for ortho
substituted b-aminol. The mechanism has been optimized depending on product analysis with the help
of its kinetic AT-IR study. This work also focusses on the synthesis of various b-amino alcohol
derivatives where the effect of an EWG and EDG on enantio-selectivity has been studied.
Received 15th December 2020,
Accepted 11th February 2021
DOI: 10.1039/d0nj06108e
rsc.li/njc
1,2-diols7 with good enantioselectivity (Scheme 1a). Furthermore,
Sekar et al. also synthesized b-amino alcohols from a-ketoamides
Introduction
a-Hetero substituted alcohols are important moieties in pharma- by the application of phosphine ligands along with a Cu salt
ceutical and natural products.1 Moreover, they are important (Scheme 1b). However, in both these methods hazardous acid and
intermediates in the development of drug molecules.2 Chiral base are used along with the phosphine ligands which hindered
b-amino alcohols are often used as chiral ligands in asymmetric the separation and purification of the product, and furthermore
synthesis for the development of complex chiral catalysts3 and the protocol also had limited substrate scope.8 In addition,
because of the broad use of chiral b-amino alcohols in asym- Xu et al., also reported the formation of b-amino alcohols from
metric synthesis they have been synthesized by various synthetic a-aminoketones via an ATH process in excellent %ee, by the use of
routes, like the reduction of a-amino acids, asymmetric amino a [Ru-(p-cymene) Cl2]2 catalyst along with (S,S)-TsDPEN ligands.9
hydroxylation of alkenes4a with good stereo-control of the However, this protocol showed the opposite configured product
b-amino alcohol, and resolution of the racemic starting with low enantioselectivity. Amongst all the hydrogen sources,
compound.4b The ring opening of epoxides by amines4c pro- DMAB served as a better candidate than the most commonly
duced a low yield of the aromatic b-amino alcohols so effective reported azeotropic mixture of FA : TEA in 5 : 2 ratio. Jung et al.
protocols are highly desired for the synthesis of this important reported that DMAB acts as the best hydrogen source along with
class of compounds.4d–i Furthermore, the amination of alcohol5 the Ru catalytic system.10 Furthermore, Goksu et al. reported
opens a way for the formation of the C-N bond, which can be DMAB as a non-toxic, non-flammable, cheap and easily available
used as a synthetic methodology for the synthesis of b-aminols. hydrogen source which can help in the transfer hydrogenation
However, one pot synthesis of b-amino alcohol has also been process of ketone and aldehyde.11 According to the literature,
achieved from a bifunctional base catalyst.6 Similarly, Yang et al. because of its high volume/mass hydrogen density, two moles of
carried out the synthesis of b-amino alcohols via amination of hydrogen gas are produced from one mole of DMAB via the
dehydrogenative method in the presence of
a favourable
catalyst.12 Furthermore, for many years, our lab has also been
working effectively in the asymmetric transfer hydrogenation
process,13 encouraged by which here we have designed a metho-
dology for the synthesis of highly enantioselective chiral b-amino
alcohols.
Department of Chemistry, Institute of Chemical Technology, Nathalal Parekh Marg,
Matunga, Mumbai 400019, India. E-mail: bm.bhanage@gmail.com,
bm.bhanage@ictmumbai.edu.in; Fax: +91-22-24145614;
Tel: +91-22-3361-1111/2222
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
d0nj06108e
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Table 2 Optimisation table of hydrogen source, time scale and tempera-
ture screening for the synthesis of chiral (ꢁ) b-aminola
Entry no.
Time (h)
Solvent
Temp. (1C)
%Yieldb
%eec
Solvent screening
14
15
16
17
18
19
20
24
24
24
24
24
24
24
MeOH
EtOH
IPA
CH3CN
DMF
THF
Toluene
RT (29)
RT (29)
RT (29)
RT (29)
RT (29)
RT (29)
RT (29)
60
52
68
58
20
32
12
64
68
75
34
14
30
03
Scheme 1 Ru-Tethered catalysed one pot synthesis of chiral (ꢁ)
b-aminol.
Time scale study
21
22
23
12
48
72
DCM
DCM
DCM
RT (29)
RT (29)
RT (29)
11
98
98
80
96
96
Herein we report an asymmetric transfer hydrogenation
process via nucleophilic substitution of a primary amine on
the a-carbon of a-bromoketone, where bromine acts as a good
leaving group. In the developed protocol, an efficient
Ru-tethered catalyst has been explored along with DMAB as
an excellent hydrogen source for achieving higher %ee. The
reaction involves nucleophilic substitution reaction, which is
Temperature screening
24
25
26
24
24
24
DCM
DCM
DCM
0
40
50
34
76
43
92
64
24
a
Conditions. 1 mmol of A10 and A20 taken with 1 mmol of DMAB and
1 mmol of base NaHCO3, 1 mol% of Ru-tethered catalyst dissolved in
b
c
different solvent. %Yield is based on GC-MS analysis. %ee are based
on HPLC analysis.
Table 1 Optimisation table of catalyst loading, catalyst and solvent
screening for the synthesis of chiral (ꢁ) b-aminola
followed by the ATH process for the synthesis of b-amino
alcohols. Although DMAB is highly sensitive to moisture, it
acts as a better hydrogen source for forming the hydride
catalyst as compared to other hydrogen sources with respect
to enantioselectivity achieved.
nMe2NHꢀBH3 - (MeNQBH)n + nH2
Entry
no.
Catalyst loading
Catalyst (mol%)
Hydrogen
source
%Yieldb %eec
Results and discussion
Catalyst screening
We began our optimization study with 2-bromo-1-
phenylethanone (a-bromo ketone) as the model substrate.
Initially, the model substrate was screened for one pot asym-
metric synthesis by the use of a unique Ru-chiral complex in
dichloro methane (DCM) solvent as mentioned in Table 1.
Amongst all the chiral catalysts screened, the Ru-tethered
catalyst dominates the protocol as compared to untethered
catalysts. A high %ee was obtained by (R,R)-Cat D as compared
to (R,R)-Cat A, B and C (Table 1, entries 1–4). Furthermore, with
the perfect catalyst design, the hydrogen source was also
screened which showed surprising results. Amongst all the
hydrogen sources, different ratios of the azeotropic mixture of
1
2
3
4
(R,R) A
(R,R) B
(R,R) C
(R,R) D
3
3
3
3
FA : TEA (5 : 2) 94
FA : TEA (5 : 2) 92
FA : TEA (5 : 2) 94
FA : TEA (5 : 2) 95
72
71
75
79
Hydrogen source screening
5
6
7
8
9
10
(R,R) D
(R,R) D
(R,R) D
(R,R) D
(R,R) D
(R,R) D
3
3
3
3
3
3
FA : TEA (3 : 2) 60
FA : TEA (1 : 5) 80
64
24
96
65
—
12
DMAB
IPA
HCOONa
98
68
—
H2O/IPA (1 : 1) 23
Catalyst loading
11
12
13
(R,R) D
(R,R) D
(R,R) D 0.5
2
1
DMAB
DMAB
DMAB
98
98
91
96
96
79
formic acid/triethylamine (F/T) (Table
1
entries 4–6),
2-propanol (IPA) (Table 1 entry 8), sodium formate (Table 1,
entry 9), and IPA/H2O (1 : 1) (a mixture of 2-propanol and water)
(Table 1, entry10) were optimized for the protocol, and
a
Conditions. 1 mmol of A10 and A20 taken with same amt. of base
b
NaHCO3 in 4 mL of DCM for 24 h. %Yield is based on GC-MS analysis.
%ee are based on HPLC analysis.
c
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dimethylamine borane (DMAB) (Table
1 entry 7) shows
excellent 96%ee. Again, the catalyst loading was also screened
from 3 mol% to 0.5 mol% (Table 1, entries 10–13) amongst
which 1 mol% showed better enantioselectivity as compared to
other sources (Table 1, entry 12).
A solvent study was also conducted in the tethered complex
which showed that DCM is the most suitable solvent as
compared to the others. However, from the solvent study, the
relationship between the nature of the solvent and the
asymmetricity of the product has been confirmed - for example,
for a protic solvent with a lower dielectric constant, a higher
%ee is achieved. DCM (e = 9.1) shows better enantioselectivity
than IPA (e = 17.9), thus the dielectric constant, which signifies
the polarity of the solvent,14 decreases as mentioned below for
polar protic solvents: CH3CN (e = 37.5) 4 MeOH (e = 32.6) 4
EtOH (e = 22.4) 4 IPA (e = 17.9) 4 DCM (e = 9.1), whereas %ee
follows the inverse pattern for the given dielectric constant
value. DCM shows (96%ee) (Table 1, entry 12) 4 IPA (75%ee) 4
EtOH (68%ee) 4 MeOH (64%ee) 4 CH3CN (34%ee) (Table 2,
entries 14–17). Similarly, for polar aprotic solvents, THF with a
lower dielectric constant shows a higher %ee as compared to
the higher dielectric constant of DMF (Table 2, entries 18 and
19). However, toluene having a very low dielectric constant
produces a racemic compound rather than a chiral one
(Table 2, entry 20), which may be because of its non-polar
nature. Thus, a polar aprotic solvent behaves as a better solvent
for achieving enantioselectivity as compared to the other
solvents.
Scheme 2 Substrate scope for the one pot synthesis of chiral (ꢁ) b-aminol.
Conditions: 1 mmol of A10 and 1 mmol of A20 taken with 1 mmol of DMAB and
1 mmol of base NaHCO3 along with 1 mol% of Ru–tethered catalyst dissolved
in 4 mL of DCM. %Yield is based on GC-MS analysis; %ee is based on HPLC
analysis.
Furthermore, time scale has been studied where decreasing
the time from 24 h to 12 h shows a decrease in the yield from
98% to 11% as well as a decrease in %ee from 96% to 80%,
whereas increasing time does not show much fluctuation in
%yield and %ee. (Table 2, entries 21–23). Henceforth, the
reaction was performed at different temperatures where lower
%ee was observed at a higher temperature (Table 2, entries 25
and 26), whereas at lower temperature a small decrease in %ee
was observed (Table 2, entry 24). Thus for 1 mmol of substrate,
DCM acts as a suitable solvent with 1 mol% of Ru-tethered (R,R)
catalyst and DMAB as a hydrogen source for obtaining b-aminol
at RT after 24 h.
With the optimized parameters in hand, various substrates
have been screened where an EWG and EDG are substituted at
the aryl ring of both the ketone and amine parts. Various
starting materials viz. a-bromo ketones have been prepared
by the known method15 which was then taken along with a
primary amine for synthesizing different derivatives of b-amino
alcohol. It is observed from the substrate study that an EWG
showed high %ee because of its electronic effect on the
carbonyl functional moiety, whereas an EDG showed good
%ee because of the steric effect offered by the –ortho functional
group. EWGs like –F (D2), –Cl (D3), and –Br (D4) at the –ortho
position showed higher %ee, which indicates that these electro-
negative atoms on the substrate favour the adjacent position to
Z6-arene of the catalyst, thereby making the carbonyl moiety
electronically favourable for the ATH process resulting in
higher %ee. Similarly –OMe (D5) and –Me (D6) at the –ortho
position showed higher %ee, which indicates that these groups
exhibit steric hindrance at the adjacent position for the binding
of the catalyst with the substrate from one face, thus favouring
the interaction of the substrate with the catalyst from the other
face resulting in higher %ee. At this stage, the study was further
extended to substitution on the aryl ring attached to the amine
part, i.e., aryl amine, which signifies no decrease in %ee; rather
it yielded a b-amino alcohol with higher %ee. –Me (D9), –F
(D11), –Cl (D12) and –Br (D13) at the para position showed
excellent enantioselectivity with a higher %ee. Again, encouraged
by these results, a substrate study with separate di-substitution on
both the aryl rings was carried out. Initially, the 2nd and 5th
positions of the aryl ketone ring (D7), as well as the 2nd and 3rd
positions (D14), got substituted with the –OMe group, wherein
both showed 97%ee. Furthermore, di-substitution was carried out
at the aryl amine part of the substrate, where the 2nd position
substituted by –OMe and the 5th position occupied by –Me group
(D8) showed 95%ee. Similarly, when an EWG like –I was sub-
stituted at the 2nd position and –Cl at the 4th position (D15), it
showed 99%ee. However, when –Me was occupied at the 3rd and
4th position (D10) then it showed 88%ee. Similarly, a small
decrease in enantioselectivity to 86%ee was also observed when
the 3rd position was occupied by –Me and the 4th position was
occupied by a –Br group (D16) which indicates that the 3rd and
4th positions of the aryl ring on the amine part hinder favourable
interaction between the substrate and the catalyst, thus resulting
in lowering the %ee, whereas –I at the ortho position favoured
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showed a further decrease in enantioselectivity to 81%ee. These
results showed good yield but variation in %ee is also observed
which indicates that at the –ortho position, steric hindrance, as
well as the electronic effect, favours the interaction of the
carbonyl moiety with the catalyst from one face, whereas
a –para substituted –Cl group showed smaller hindrance while
the same group at the –meta position showed more electronic
hindrance and as a result %ee decreased to 81% from 92%ee.
Furthermore, a mechanistic investigation was carried out for
the one pot asymmetric synthesis of b-amino alcohol. Thus, for
tracing the product, GC-MS, as well as AT-IR of the product, was
analysed at a different time of reaction (Fig. 1). Initially, an
a-bromo keto compound (A1) showed a nucleophilic substitution
Scheme 3 Products of ATH of the –ortho, –meta and –para substituted
aryl ketones. Conditions: 1 mmol of A10 and 1 mmol of A20 taken with
1 mmol of DMAB, 1 mmol of base NaHCO3, and 1 mol% of Ru-tethered
catalyst dissolved in 4 mL of DCM. %Yield is based on GC-MS analysis; %ee
is based on HPLC analysis.
interaction of the substrate with the catalyst from one face, thus reaction with aniline (A2) in the presence of base producing
yielding higher %ee of the product (Scheme 2). active species a-aminoketone (C) which can be confirmed by
Again, to extend the efficiency of the protocol ortho, meta the AT-IR spectroscopic study. Initially a –CQO peak was
and para substituted ketones have been processed towards ATH observed at 1690 cmꢁ1 for compound A1 and an –NH2 peak
which showed some interesting facts as mentioned in was observed at 3435 cmꢁ1 and 3350 cmꢁ1 (peak for primary
Scheme 3.
amine) for compound A2 which after 3 h can be seen decreasing
From the results obtained, D3 containing a –Cl group at the without any disturbance in the –CQO peak. However, after 12 h,
ortho position of the aryl ketone showed 92%ee whereas it it can be observed clearly from AT-IR spectroscopy that the
decreases to 87%ee when a –Cl group occupied the para primary amine was converted to a secondary amine, and as a
position (D18). Again a –Cl group at the meta position (D17) result, conversion of two peaks into one peak was observed at
Fig. 1 Represents kinetic study for the consumption of substrate and development of the one pot enantioselective product along with AT-IR
spectroscopy.
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nucleophilic attack of the primary amine on the a-carbon of
a-bromoketone is preferred followed by an ATH process, which
is also confirmed from GC-MS study. Furthermore, the effect of
ortho, meta and para substitution towards enantioselectivity has
also been studied which confirms that the ortho effect is more
favourable for high %ee as compared to meta and para.
Different derivatives of b-amino alcohol have been synthesized
and characterized by 1H & 13C-NMR spectroscopy, HPLC
(on OJ-H & AD-H chiral column) and optical polarimetry. Based
on the literature, a possible mechanism and transition state have
been deduced to determine the configuration of the product.
Experimental section
General procedure for the asymmetric synthesis of b-aminol
Scheme 4 Plausible mechanistic pathway for the synthesis of chiral
b-aminol.
In a 15 ml dried pressure tube cylinder, 1 mmol of 2-
bromoacetophenone and its derivatives along with 1 mmol of
aniline and its different derivatives are taken along with
1 mol% of catalyst Ru-Teth-TsDPEN (R,R) which was then
dissolved in 4 ml of DCM along with the addition of 1 mmol
of NaHCO3. After 15 min, again 1 mmol of DMAB was added
slowly under a nitrogen atmosphere. Furthermore, after 24 h,
the presence of the product is assessed by Thin Layer Chromato-
graphy (TLC), after which the compound is extracted with DCM
and water, and the organic layer is dried over anhydrous Na2SO4
followed by filtration and distillation. The final mixture is then
purified using silica gel column chromatography (eluent: n-hexane-
ethyl acetate, 90–10) to obtain pure chiral (ꢁ)-b-aminol.
3394 cmꢁ1 with the existence of the –CQO peak at 1685 cmꢁ1
.
This clearly indicates that one pot synthesis of enantio-selective
product ‘‘D’’ proceeds via ‘‘C’’ i.e.; a-aminoketone but not via ‘‘B’’
as observed in the literature.16 After 18 h, it can be again
visualized that the –CQO peak decreases and it disappears after
24 h along with broadening of the peak observed at 3400 cmꢁ1
,
which indicates the existence of an –OH bond, thereby confirming
the formation of chiral product ‘‘(ꢁ) D’’.
Based on the obtained results and AT-IR spectroscopic
study, a possible mechanism has been proposed (Scheme 4).
a-Bromo keto compound (A1) shows nucleophilic substitution
reaction with aniline (A2) in the presence of base producing
active species a-aminoketone (C). There are two possible pathways
for the same either via path A in which base NaHCO3 removes a
proton from aniline which then attacks the acidic carbon of ‘‘A1’’
producing compound ‘‘C’’ or via path B in which the lone-pair of
aniline attacks the acidic carbon producing compound ‘‘A4’’ from
which loss of a proton yields compound ‘‘C’’. Furthermore,
compound (C) fits into the ‘‘window’’ of (R,R) Ru-Teth-TsDPEN
hydride complex (3) showing favourable ‘‘edge-face’’ interaction
amongst themselves which leads to generating the targeted
product (D) via complex ‘‘4’’ whereas complex ‘‘5’’ does not show
favourable –CH/p interaction between the aromatic substrate and
Z6 aromaticity of the catalyst together with the possibility of steric
hindrance. Thus, an ‘‘S’’ configured (ꢁ) b-amino alcohol is
produced which can also be confirmed from its [a] value i.e.;
optical polarimetry.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
Author AAM is thankful to the Department of Science and
Technology (DST-INSPIRE), New Delhi, India, for providing a
Senior Research Fellowship (SRF).
Notes and references
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Conclusion
In summary, we have developed an efficient protocol for the
synthesis of chiral b-amino alcohols where the effect of the
(R,R) Ru-tethered-TsDPEN catalyst has been investigated over
its untethered catalyst. Further, DMAB emerges as an efficient
hydrogen source which also favours high %ee as compared to
other hydrogen sources. AT-IR study has been performed
kinetically, to confirm the change in functional moiety
throughout the reaction from which it can be confirmed that
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