Asymmetric transfer hydrogenation of imines in water/methanol co-solvent system and mechanistic investigation by DFT study

Article abstract of DOI:10.1039/c4ra07964g

Asymmetric transfer hydrogenation of various cyclic imines proceeded efficiently with water/methanol co-solvent media in 20 min with excellent yields and enantioselectivities by employing Rh-TsDPEN catalyst and sodium formate as a hydrogen donor. The role

Full text of DOI:10.1039/c4ra07964g

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Asymmetric transfer hydrogenation of imines in
water/methanol co-solvent system and
mechanistic investigation by DFT study†
Cite this: RSC Adv., 2014, 4, 46351
Vaishali S. Shende,a Savita K. Shingote,a Sudhindra H. Deshpande,a
Received 1st August 2014
Accepted 5th September 2014
b
b
a
*
*
Nishamol Kuriakose, Kumar Vanka and Ashutosh A. Kelkar
DOI: 10.1039/c4ra07964g
www.rsc.org/advances
Asymmetric transfer hydrogenation of various cyclic imines proceeded methanol was found to be the best co-solvent. Thus 97% yield of
efficiently with water/methanol co-solvent media in 20 min with 1a with 95% ee was achieved with H2O–MeOH (1 : 1, v/v) as a
excellent yields and enantioselectivities by employing Rh–TsDPEN solvent in just 20 min (TOF: 295 hꢀ1) (Scheme 1). The reactions
catalyst and sodium formate as a hydrogen donor. The role of the were carried out in air, and excellent enantioselectivities were
co-solvent in enhanced productivity of the reaction was investigated observed for various imine substrates.
by DFT. The mechanism for ATH of the imines has been discussed on
We initiated our study on ATH of model substrate, 1-methyl-
6,7-dimethoxy-3,4-dihydroxisoquinoline (1a) with Rh–TsDPEN as
the catalyst6 and HCOONa as a hydrogen donor in water at 40 ꢁC
with a substrate/catalyst (S/C) ratio of 100 : 1 (1a: 0.5 mmol,
HCOONa: 5 equiv., water: 2 ml). In all these studies Rh–TsDPEN
catalyst was generated in situ by reacting [Rh(Cp*)Cl2]2 and 1.5
equiv. of TsDPEN in 1 ml distilled water for 1 h at 40 ꢁC.7 Within
5 h ATH of 1a proceeded with 85% conversion and 95% enan-
tioselectivity (Table 1, Entry 1). Effect of temperature on the
activity and enantioselectivity was investigated (Table 1).
Conversion of 1a increased with increase in temperature (85%
at 40 ꢁC to 91% at 60 ꢁC) with the drop in enantioselectivity from
95% at 40 ꢁC to 88% at 60 ꢁC. Hence the temperature of reaction
the basis of the DFT study.
Optically active amines are important building blocks for bio-
logically active molecules in medical, pharmaceutical, agricul-
tural sciences, avor and fragrance industries.1 Different
methods have been developed for the preparation of enantio-
merically pure amines in the past few years.2 Asymmetric
transfer hydrogenation (ATH) of imines is one of the most
popular methods used for the preparation of chiral amines.
Various chiral catalysts have been investigated for ATH of
imines, but the most noteworthy to date are the ruthenium and
rhodium complexes with optically active N-toluenesulfonyl-1,2-
diphenylethylenediamine (TsDPEN) ligand3 in organic solvents
ꢁ
was maintained at 40 C for further study.
At the start of experiment with water as a solvent, reaction
mixture became slightly turbid aer the addition of substrate
and sodium formate to pre-catalyst solution and as the reaction
progressed there was product separation with brown colored
globules oating on the aqueous layer and thus resulting in a
biphasic reaction system. In order to increase the solubility of
substrate and avoid product separation during the reaction,
methanol was added as a co-solvent in 1 : 1 (v/v) ratio and to our
surprise the reaction achieved 98% conversion and 95% enan-
tioselectivity within 20 min (Table 1, entry 4; See ESI, Fig. S1,†
with formic acid–triethylamine azeotrope as
a hydrogen
donor.2,4 There are very few reports on the ATH of imines in
water5 but major efforts were focused on the development of the
water soluble ligands5a–c,f and the use of additives/surfactants5a,d
to improve the activity of ATH of imines in water. However,
longer reaction time is the major problem of all these investi-
gations. Herein we wish to report the role of methanol as a co-
solvent in achieving rapid ATH of imines with [Rh(Cp*)Cl2]2–
TsDPEN catalyst system in water with sodium formate as a
hydrogen donor. Various co-solvents were screened and
aChemical Engineering and Process Development, National Chemical Laboratory, Pune
411008, India. E-mail: aa.kelkar@ncl.res.in; Fax: +91 20 25902621
bPhysical and Materials Chemistry Division, National Chemical Laboratory, Pune
411008, India. E-mail: k.vanka@ncl.res.in; Fax: +91 20 25902636
† Electronic supplementary information (ESI) available: Experimental procedures,
NMR spectra and HPLC traces of the products, computational details, electronic
energies in au and in kcal molꢀ1 (Tables S3–S6) and the xyz coordinates of all the
structures discussed in the manuscript. See DOI: 10.1039/c4ra07964g
Scheme 1 ATH of 1a in water using methanol as a co-solvent.
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Table 1 ATH of 1a in water and methanola
Entry Solvent
Temp (ꢁC) Time (min) Convd (%) eee (%)
1
2
H2O
H2O
H2O
H2O/MeOH 40
MeOH
MeOH
40
50
60
300
300
300
20
300
300
85
88
91
98
35
5
95
91
88
95
94
ndf
3
4b
5
40
40
6c
a
Reaction conditions: 1a (0.5 mmol), [Rh(Cp*)Cl2]2 (0.0025 mmol), (1S, 2S)-
TsDPEN(0.0075 mmol), HCOONa (2.5 mmol), solvent (2 ml). b H2O–MeOH
c
d
(v/v, 1 : 1). Reaction carried out without HCOONa. Determined by GC.
e
Determined by chiral HPLC. f Not determined.
Fig. 2 Co-solvent screening for ATH of 1a catalyzed by Rh–TsDPEN
Conv ( ), ee ( ), reactions were carried out on a 0.5 mmol scale in a 2
ml water–co-solvent (1 : 1, v/v) at 40 ꢁC, time: 30 min, * time: 20 min.
C-T prole of ATH of 1a). When ATH of imine 1a was carried out
in pure methanol with HCOONa as a hydrogen donor (Table 1,
entry 5) only 35% conversion with 94% enantioselectivity was
observed in 5 h. Experiment was carried out with only methanol
(without sodium formate) to check the role of methanol as a
hydrogen donor (Table 1, entry 6). In this reaction 5% conver-
sion of 1a was observed in 300 min indicating that methanol
worked just as a solvent in this reaction.
To gure out the optimum amount of methanol necessary as
a co-solvent for this reaction, ATH of 1a was carried out by
varying the amount of methanol in water from 25% to 75% and
the results are presented in Fig. 1. Total quantity of solvent was
kept constant at 2 ml. From the Fig. 1, it can be seen that
conversion of 1a increased with increase in methanol concen-
tration till 50% methanol (98% conversion in 20 min) and
reaction mixture was homogeneous throughout the course of
reaction. However, with further increase in methanol concen-
tration to 75% conversion of 1a decreased considerably (92% in
30 min). Enantioselectivity was not affected by a change in
methanol concentration (94–95%).
conversion from 88% to 95% and showed enantioselectivities
from 84% to 93% in 30 min reaction time. However, with THF
just 25% conversion was observed in 30 min with 92% ee. Use of
coordinating solvent THF leads to deactivation of catalytic
system resulting in decreased conversion.8 Best results (98%
conversion with 95% enantioselectivity in 20 min) were
obtained with water as solvent and methanol or ethanol as co-
solvents (water : co-solvent, 1 : 1 v/v) and further work was
carried out with methanol as a co-solvent.
The efficiency of the water/methanol (1 : 1, v/v) solvent
media was demonstrated for ATH of various cyclic imines using
Rh–TsDPEN catalyst system with excellent yields and enantio-
selectivities as presented in Table 2. The chain length of the
alkyl substituent at the 1-position of imine group had little
effect on the enantioselectivity (Table 2, Entries 1–3). Imine
derivatives having bulky phenyl group on imino carbon are hard
to reduce (Table 2, Entries 8, 9), where second aromatic system
apparently interferes with selective catalyst binding; resulting in
low yield and enantioselectivity.2b Notably the enantioselectiv-
ities for ATH of imines achieved with Rh–TsDPEN catalyst
system in water–methanol solvent in the present work (95–99%
ee) are better than those reported in literature with FA–TEA
hydrogen donor in organic solvents (89–99% ee).4
Co-solvents are known to have hydrogen-bond donor and/or
acceptor groups for aqueous solubility and a small hydrocarbon
region that serves to disrupt the strong hydrogen-bond network
of pure water, thereby increasing the solubility of reactants/
products.9 In the present work methanol helps in solubilising
the reactant 1a and product 2a in aqueous reaction mixtures by
forming hydrogen bonds with the imino and amino nitrogen
atoms. Several DFT calculations have been performed for ATH
reaction of ketones and aldehydes which explain the role of
solvent in the transition state of hydrogen transfer through
hydrogen bonding and thereby ATH in water to proceed with
accelerated rate.10–12 It can be feasible for imines also; that the
solvent present in the reaction mixture helps in accelerating the
ATH of 1a by forming local hydrogen-bonding network.13
In order to understand the co-solvent effect of methanol on
the activity of the ATH of 1a, density functional theory (DFT)
calculations were performed, at the B3LYP/TZVP//PBE/TZVP
level of theory. In order to ensure that the results are reliable; all
Various co-solvents were screened (co-solvent: water ratio
1 : 1) for ATH of 1a and the results are presented in Fig. 2 (for
details see ESI, Table S1†). Among all co-solvents screened,
methanol achieved 98% conversion with 95% enantioselectivity
in 20 min. Alcoholic solvents like propanol, butanol and iso-
propanol also showed conversions ranging from 93% to 95%
within 30 min reaction time with enantioselectivity value of 91–92%.
Polar aprotic solvents like DMF, DMSO and NMP achieved
Fig. 1 Effect of methanol content in water on ATH of 1a: 25% MeOH in
water (:), 50% MeOH in water (A), 75% MeOH in water (-); 1a (0.5
mmol); HCOONa (2.5 mmol); temp: 40 ꢁC, solvent: 2 ml.
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Table 2 ATH of imines catalyzed by Rh–TsDPEN in H2O–MeOH (1/1,
For pathway (iii), without the aid of any hydrogen bonding
solvent molecule, the complex formed between the rhodium
complex 3a and imine 1a (Scheme 2, A1) is 7.1 kcal molꢀ1 less
stable than the analogous complex that would be formed with
the aid of the hydrogen bonding methanol solvent (Scheme 2,
C1). Also, for pathway (i) the complex formed with the aid of the
hydrogen bonding water molecule (Scheme 2, B1) is 4.3 kcal
molꢀ1 less stable than the hydrogen bonded methanol complex
(Scheme 2, C1). This result indicates that the choice of the
pathway that would be most likely to be followed would be
determined at the inception of the reaction (Scheme 2). By
employing the formula DG ¼ ꢀRT ln K, it can be estimated that
the possible reason for the increased rate of reaction in water–
methanol (1 : 1) solvent media is the pathway which involves
TS-C and it is 137 600 times more likely to occur over pathway
which involves TS-A and 1300 times more likely to occur over
pathway which involves TS-B (see Scheme 2) (see computational
details in the ESI† le for the calculation details). Moreover, the
presence of hydrogen bonding methanol molecule leads to
greater stability of the product complex (Scheme 2, C2), by 1.8
kcal molꢀ1 in pathway (ii) over the product complex with
hydrogen bonded water (Scheme 2, B2) in pathway (i) and by 6.8
kcal molꢀ1 over the product complex without solvent molecule
(Scheme 2, A2) in pathway (iii) (Scheme 2).
v/v) with sodium formate as hydrogen donora
Entry
Imine
Time (min)
Yieldb (%)
eec (%)
1
2
3
4
5
6
7
8
9
1a
1b
1c
1d
1e
1f
1g
1h
1i
20
20
20
20
20
20
20
180
180
97
95
94
94
94
95
94
40d
15d
95
94
96
99
93
97
99
17
14
a
Reaction conditions: 1 (0.5 mmol), [Rh(Cp*)Cl2]2 (0.0025 mmol),
(1S, 2S)-TsDPEN (0.0075 mmol), HCOONa (2.5 mmol), H2O–
b
c
MeOH(1 : 1, v/v, 2 ml), temp: 40 ꢁC. Isolated yield. Determined by
Interestingly, as the values along the three potential energy
surfaces indicate, the difference between the barriers in the
three cases is signicantly closer than in the case of the ruthe-
nium based catalyst studied by Xiao et al., where the barrier
heights dropped by 6.21 kcal molꢀ1 when a solvent molecule
chiral HPLC. d Conversion determined by GC.
the calculations were performed with the real system 3a (see was considered.12 For the current case, inclusion of the
Scheme 2 and ESI† for details on DFT study). Previous studies on hydrogen bonding solvent molecule was seen to actually
the Ru-catalyzed ATH of formaldehyde10 and ketones14 suggests increase the barrier by 2.1 kcal molꢀ1, in comparison to the
a sequential mechanism for transfer of hydrogen to the non-hydrogen bonded case (see Scheme 2) for pathway (i), and
substrate molecule by the incorporation of implicit/explicit by 4.6 kcal molꢀ1 for pathway (ii) (see Scheme 2). The expla-
solvent molecules which involves a highly unstable alkoxide-like nation for this is that the favourable hydrogen bond between
intermediate. However, in the present study of imines such a the two reactant species 1a and 3a is broken at the transition
step-wise mechanism with an analogous intermediate was not state, which leads to the observed higher barriers. Thus the
observed. Based on the DFT calculations Xiao et al. have driving factor in determining the productivity of the systems for
proposed that, a solvent water molecule could hydrogen bond ATH of 1a in water–methanol (1 : 1) mixture is the thermody-
with the substrate and thereby affect both the thermodynamics namics, and not the kinetics. It is likely that the initiation of the
and the kinetics of the ATH of ketones.12 Based on the above reaction, via the formation of the complex between metal–
study three possible transition states shown in Fig. 5 were ligand complex 3a and imine 1a, is a signicant factor in
considered for imines with three different pathways (i) the determining its success, and the likelihood of initiation is
presence of hydrogen bonding water molecule TS-B (ii) the signicantly enhanced in the presence of methanol (potential
presence of hydrogen bonding methanol molecule TS-C (iii) the energy surfaces shown in Scheme 2).
absence of hydrogen bonding solvent molecule TS-A (Scheme 2).
It may be noted that the reaction with pure methanol as a
We have also considered the possibility that the solvent (water or solvent was not taken into consideration because of the low
methanol) can be a direct part of the hydrogen transfer pathway solubility of the sodium formate in pure methanol. This would
via an eight membered transition state. We have compared this signicantly impact in situ formation of the rhodium complex.
possibility with a concerted pathway involving a six-membered Also, results clearly suggest that the reaction is facile in a 1 : 1
transition state that would be formed between the rhodium water–methanol mixture than in pure methanol or water. For
catalyst system and the substrate (Fig. 3). As the gure indicates, the product structures, we have considered the possibility of
the barriers are signicantly lower for the six-membered tran- hydrogen bonding between N–H of the product amine (2a) and
sition state without participation of the solvent in the transition the sulfonyl oxygen of the rhodium complex (see B and D of
state. Hence, the reaction pathway involving the six-membered Fig. 4) and also the hydrogen bonding between the nitrogen
transition state has been considered, with the solvent molecule atom of the product amine (2a) and the protic hydrogen of the
playing the role of a hydrogen bonding agent.
methanol or water solvent molecules (see A and C of Fig. 4). It
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Scheme 2 Energy profiles for asymmetric transfer hydrogenation of imine calculated by the DFT method (energy difference in kcal molꢀ1).
was found that the latter case was energetically favorable by 1.3
kcal molꢀ1 for water and 3.0 kcal molꢀ1 for methanol respec- outer-sphere pathway, proposed for carbonyl reduction12,14 or an
tively (see Fig. 4). inner-sphere mechanism in which imine coordinates to the
In ATH of imines, hydrogen transfer can follow either an
In this way methanol used as a co-solvent reduces the acti- metal prior to hydrogen transfer.15 Blacker et al. have found that
vation barrier in transfer hydrogenation of imine and overall the hydride and the proton are delivered to the substrate in a
result is the increased reaction rate for ATH of imine 1a. Since stepwise manner.16 Nova et al. have explained the importance of
all the calculations were carried out by taking the temperature two different substituents on the chelating bis(amido) ligand
as T ¼ 298.15 K, we have also calculated the effect of tempera- for transfer hydrogenation by bifunctional Cp*Rh(III) catalysts
ture in the thermodynamic and the kinetic energies by adding where the 16-electron Rh complex + formic acid are shown to be
the temperature correction at T ¼ 313.15 K. The calculations in equilibrium with the formate complex, but the latter lies
suggest that the energy values changes only by 0.1–0.9 kcal outside the pathway for dehydrogenation.17 Blackmond et al.
molꢀ1 (Table S1 in the ESI†). The current calculations therefore have suggested that reductions of imines with Rh–di-amine
provide insights into the nature of the ATH mechanism in catalysts might proceed via the unprotonated imine.18 Wills
different solvents, and showcase the importance of the role of et al. have proposed that the reductions of cyclic imines proceed
hydrogen bonding in improving the activity of the process and by the ionic “anti” mechanism and a transition state structure
stability of the products formed.
for imine reduction is also proposed.19 Based on literature
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Fig. 3 Structures of six membered and eight membered (8M) transi-
tion state for asymmetric transfer hydrogenation of imine (energy
difference in kcal molꢀ1).
reports14–19 and results presented in DFT study we have
proposed a plausible mechanistic pathway for the ATH of 1a in
an aqueous-methanolic solution (Scheme 3) and possible
proposed transition states are depicted in Fig. 5.
Precatalyst Rh–TsDPEN 3 is generated in situ from [Rh(Cp*)
Cl2]2 and TsDPEN in water and then it reacts with HCOONa
forming formato complex 4. The reduction proceeds via
the formato intermediate 4, followed by decarboxylation to give
Scheme 3 Proposed mechanism for ATH of imine 1a with Rh–TsDPEN
in water–methanol (1 : 1, v/v) solvent system.
Rh–hydride intermediate 5 and the coordinatively unsaturated
intermediate 7 (Scheme 3). It has been conrmed by DFT
calculations that transition state in which the methanol solvent
molecule provides activation of imine substrate via hydrogen
bonding i.e. TS-C (Fig. 3) will be operating in mechanism for
ATH of 1a and in this way reductions proceed by the ionic “anti”
mechanism proposed by Wills et al.19 Thus TS-C in DFT study
Fig.
4 Optimized geometry structures of product; (A): N–H of
product 2a hydrogen bonding only with methanol, (B): N–H of
product 2a hydrogen bonding with sulfonyl oxygen of the TsDPEN Fig. 5 Proposed transition states without participation of solvent
ligand, (C): N–H of product 2a hydrogen bonding only with water, (D): molecules (TS-A) and with participation of water (TS-B) and with
N–H of product 2a hydrogen bonding with sulfonyl oxygen of the participation of methanol (TS-C) (hydrogen atoms have been omitted
TsDPEN ligand; energy difference in kcal molꢀ1
.
for the purpose of clarity).
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resembles with intermediate 6 in the catalytic cycle (Scheme 2).
Although the N–H bond of the catalyst already activates the
imine, the methanol solvent molecule may provide additional
activation through hydrogen bonding (Scheme 2). This obser-
vation is also supported by computational study of transfer
hydrogenation in methanol in which it has been shown that
methanol as a solvent could lower the thermodynamic free
energy of hydrogen transfer and even alter the mechanism.10
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Conclusions
In summary, we have developed a simple protocol for ATH of
imines in homogeneous H2O–MeOH solvent media with Rh–
TsDPEN catalyst system and HCOONa as a hydrogen donor
which affords rapid access to highly enantioselective chiral
amines. Excellent enantioselectivities and good conversions
were achieved for various imine derivatives. The DFT calcula-
tions provide insights into the nature of the ATH mechanism
and showcase the signicance of the role of hydrogen bonding
through solvent molecule in improving the activity of the
process. This simple and more efficient aqueous-methanolic co-
solvent media with Rh–TsDPEN catalyst system provides a
valuable alternative to modied water soluble ligands and it
also offers several advantages, including mild reaction condi-
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