Dinuclear salen cobalt complex incorporating Y(OTf)3: enhanced enantioselectivity in the hydrolytic kinetic resolution of epoxides

Article abstract of DOI:10.1039/c5ra12408e

The activation of inactive Jacobsen's chiral salen Co(ii) (salen = N,N′-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediamine) compound is attained by dinuclear chiral salen Co(iii)-OTf complex formation with yttrium triflate. The yttrium metal not only displays a promoting effect on electron transfer, but also assists in forming two stereocentres of a Lewis acid complex with Co(iii)-OTf. We found that the binuclear Co-complex significantly enhanced reactivity and enantioselectivity in the hydrolytic kinetic resolution of terminal epoxides compared to its analogous monomer and kinetic data are also consistent with these results.

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Dinuclear salen Cobalt complex incooperated Y(OTf)3: Enhanced
enantioselectivity in hydrolytic kinetic resolution of epoxides
Deepak Patel,a Ganesh Ram Kurrey,a Sandip S. Shinde,b Pradeep Kumar,b Geon-Joong Kimc
and Santosh Singh Thakura*
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
The activation of inactive Jacobsen chiral salen Co(II)
(salen =N, N′-bis(3,5-di-tert-butylsalicylidene)-1,2-
Recently, we have developed the dinunlear chiral salen Co
complex bearing Lewis acids of different salts of Al, Ga, In and
cyclohexane-diamine) compound is attained by dinuclear 50 Tl which showed very good chiral catalyst in asymmetric ring
10 chiral salen Co(III)-OTf complex formation with yttrium
triflate. The yttrium metal not only displayed the promoting
effect in the electron transfer reaction, but also it assisted to
form two stereocentres of Lewis acid complex with Co(III)-
opening and closing reactions.11
In continuation of our interest in activation and developing of
dinuclear chiral salen complex, herein reporting the synthesis and
application of functional dinuclear salen CoIII complex (Fig. 1).
OTf. We found that the binuclear Co-complex significantly 55 Interestingly, the Y(OTf)3 not only works as a linker, but also
15 enhanced reactivity and enantioselectivity in the hydrolytic
kinetic resolution of terminal epoxides compared to its
monomer analogous and kinetic data is also consistent with
these results.
cooperatively activates the incoming nucleophiles and thus
reduce the barrier heights in the HKR reaction of racemic
terminal epoxides, which ultimately provides enhanced activity
and enantioselectivity to its monomer analogous. The
60 monometallic and bimetallic chiral salen Co linked with Y(OTf)3
can be prepared by reacting the chiral salen CoII and Y(OTf)3
with mole ratio of 1:1.33 and 2:1.66 respectively in THF solvent
at room temperature and open atmosphere for 40 minutes. After
completion of reaction the solvent was removed by rotary
20 Chiral salen metal complexes proved as potential catalysts in
various asymmetric syntheses.1ꢀ4 Catalytic asymmetric ring
opening of racemic terminal epoxides with water (Hydrolytic
Kinetic Resolution, HKR) using Jacobsen’s chiral (salen)CoIIIꢀ
OAc complex as chiral catalyst has been widely applied and 65 evaporation and crude green solid residue was obtained and dried.
25 highly efficient for the preparation of enantioenriched epoxides in
academic and industrial sector.5ꢀ6
The dried crude green solid was dissolved in CH2Cl2 and washed
by H2O at least three times. After concentration and evaporation
of separated CH2Cl2, afforded dark green solid powder (m. p. >
380oC) Yield = 98ꢀ99 %. The chiral salen (R, R)ꢀ(ꢀ)ꢀN, N′ꢀbis
The catalytically inactive chiral salen CoII complex is activated
by aerobic oxidation via one electron transfer reaction to get
chiral salen CoIII complex promoted by many Brϕnsted acids, 70 (3,5ꢀdiꢀtertꢀbutylsalicylidene)ꢀ1,2ꢀcyclohexane diamino cobalt
30 which was mechanistically found to exhibit second order
dependence on salen [CoIII] catalyst unit routed through
cooperative bimetallic mechanism.7ꢀ8 The metal valence change
from CoIII to CoII during HKR of racemic epoxides has been
(II) (Fig. 1) and Y(OTf)3 (purity= 98%) were procured from
Aldrich and used as such.
The continuous monitoring for the formation of oxidized salen
studied in detail.9 Further, The high reactivity, recyclability and 75 Co dinuclear complex (Fig. 2) and quantitative estimation were
35 DFT calculation of counter ion/axial ligand linked to
monometallic salen CoIII unit has also been demonstrated
including triflate ion.10
performed by UVꢀvis spectroscopy by using the following
equations.12
II
III
εCo
εCo
nm x [CoII]t + εCo
nm x [CoII]t + εCo
nm x [CoIII]t = Abs, 359 nm
nm x [CoIII]t = Abs, 419 nm
Eq. 1
Eq. 2
Eq. 3
359
359
419
80
II
III
419
[CoII]t + [CoIII]t = [CoII]0
aDepartment of Chemistry. Guru Ghasidas Vishwavidyalaya , Koni,
40 Bilaspur-495001, Chhattisgarh, India Fax: +917752 260435; Tel:
+919981209738; E-mail: santosh.chirality@gmail.com
bOrganic Chemistry Division; National Chemical Laboratory, Dr.Homi
Bhabha Road, Pune-411008, India
85 The value of ε for the characteristic peak of salen CoII were
calculated to be 1.168 x10 4 Mꢀ1 cmꢀ1 at 359 nm and 1.209 x 104
Mꢀ1 cmꢀ1 at 419 nm by taking various concentration of salen CoII
complex and using linear regression method whereas for salen
CoIII we have calculated using numerical method based on the
90
cDepartment of Chemical Engineering, Inha University, Incheon,
45 Republic of Korea
† Electronic Supplementary Information (ESI) available: [Catalyst
prepartion and charactization data]. See DOI: 10.1039/b000000x/
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50
55
60
65
70
75
5
10
15
20
25
30
UVꢀvis spectrum of salen CoII and salen CoIII in different
concentrations of these reactions with the help of above
mentioned equations. We observed that there is complete
The full range general XPS survey data provides valence states
80 of elements and very useful structural information of bimetallic
salen Co bearing Y(OTf)3 shown in Fig 3a.The oxidation state of
cobalt (III) in bimetallic salen catalyst B was further confirmed
by Co 2p XPS spectrum where its binding energy of Co 2p3/2
increases with the increase in the formal oxidation state of the
85 cobalt ion.13 It shows a Co 2p3/2 and Co 2p1/2 core level peak at a
binding energy of 780.5 and 796.2 eV respectively (Fig 3b)
without exhibiting any complex satellite peak always observed in
the Co(II) 2p spectra . The spin –orbit splitting is found to be
15.7 eV, indicates the Co (III) oxidation state which is similar to
90 the reported literature.13 The slight shifting of binding energy
peak might be possibly due to the difference in coordination
environment of salen ligand oxygen which is coordinated to
Y(OTf)3 (Fig. 1). In general, analysis of the XPS survey data of
bimetallic catalyst B revealed the presence of significant peaks
95 associated with carbon (1s = 285 eV), nitrogen (1s = 400 eV),
oxygen (1s = 532 eV), fluorine (1s= 689 eV), cobalt (3s= 106
eV), yttrium (4s= 42 eV; 3d5=154 eV) sulphur (2p= 169 eV, 2s=
230 eV) (Fig. 3a)
35 conversion of salen CoII to salen CoIII and the disappearance of
characteristic peak at 419 nm of salen CoII (see kinetic profle ESI
Fig S6).
19F NMR confirms the presence of OTf, shows the chemical shift
1
100 at δ= ꢀ62.70; ꢀ191.63 and ꢀ62.70; ꢀ170.93 and ꢀ191.63. The H,
13C NMR, XRD, HRMS and FTIR support the formation of
complex A and B (see ESI).
The catalytic activities of the salen Coꢀcomplex A and B were
evaluated in the HKR of terminal epoxides and results were
105 summarized in Table 1. The loading of the monomer A and
Fig. 2 Monitoring of continuous UVꢀvis spectra for the formation of
40
chiral [salenCoꢀOTf]2Y(OTf)3 dimer.
2
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binuclear B catalyst were based on per [Co] salen unit. All HKR
reactions were performed under solvent free conditions except for
epoxides with aryl ether (entry 7 and 8). Entry 2 of propylene
55 performed and it showed the two term rate equation involving
both intraꢀ and intermolecular reaction (eq 4) 11b, 14
Rate ∝ kintra [catalyst] + kinter[catalyst]2
Eq. 4
5
60 plots of rate/[catalyst] vs. [catalyst] provides linear with slopes
equal to kinter and yꢀintercepts corresponding to kintra with positive
value for dimer catalyst B i. e. 36.3 Mꢀ1 x minꢀ1 and 68.0 x 10ꢀ2
minꢀ1 respectively. The monomeric catalyst A provided yꢀ
intercepts of nearly zero, revealing the absence of any firstꢀorder
65 pathway in this HKR reaction (Fig. 4). These results are more
10
Table 1 : HKR of terminal epoxides catalyzed by the mono and biꢀ
metallic catalyst A and Ba
15
70
20
25
30
Entry
Epoxides
Catal. Time
ee of
Yield
(%)d
Typeb
(h) recovered
epoxides
(%)c
1e
2
A
B
1
12
4
16
56
99.2
>99
63
20
46
43
23
A
6
B
A
B
1.5
16
10
99.4
74
99.6
42
26
43
3
4
A
B
12
2
53
99.7
21
44
5
A
B
12
3
61
99.4
23
42
35
6
A
B
12
4
47
99.3
19
44
Fig. 3 (a) XPS survey data of the bimetallic salen CoꢀY(OTf)3 catalyst (b)
Co 2p high resolution region.
7f
A
B
12
6
32
99.4
12
45
epoxide shows highest reactivity among the examined epoxides,
40 afford >99% ee in short time (1.5h). Overall obtained results
with rest of epoxides, demonstrate that the chiral salen Co
dimeric complex B catalyzed HKR of various terminal epoxides
containing functionalities of chloro, ether, ester and aliphatic,
have been shown good to excellent enantioselectivity (>97ꢀ99%
45 ee ) except the compound 2ꢀnaphthyl glycidyl ether (entry 8).
8g
A
B
48
24
28
97.1
10
46
aThe reaction were carried out 10.84 mmol scale by taking 0.55 equiv.
H2O with respect to epoxide.bCatalyst loading based on racemic epoxides
and for A =0.4 mol%; B=0.2 mol%. c % of ee was determined by chiral
GC or chiral HPLC. dIsolated yield is based on racemic epoxides
On the contrary, the analogous monometallic salen Co catalyst
A showed less reactivity and selectivity in comparison to its
dimeric catalyst B. All the epoxides taken in the present study the
50 krel were also observed high enough which led to enhanced
enantioselectivity for the recovered epoxides and diol products.
75 (theoretical maximum=50%). For catalyst 1 reaction performed at 0ꢀ4oC
e
f
with 0.5 mol% catalyst loading (Ref.8a). Solvent THF: CH2Cl2 2:1 (v/v)
with respect to epoxides. gTHF is used as solvent.
encouraging than our previous report.11b,11c In order to check the
80 stability of the dimeric catalyst B during HKR reaction, a control
experiment was performed and treated with excess of water and
To elucidate the higher reactivity and enantioselectivity of the
dimer catalyst B the kinetic study of epicholorohydrin (ECH) was
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reported yttrium triflate mediated asymmetric catalysis ring
opening of aziridines.15 and other asymmetric reaction.16
worked up and extracted by using CH2Cl2 solvent, it showed the
same reactivity and enantioselectivity in this HKR reaction.
Additionally, the catalyst B can be regenerated and reꢀused at
least up to three cycles without any appreciable loss in activity
and enantioselectivity (see ESI Fig. S35).
5
55
These data indicates the catalyst
B
exhibits better
stereochemical communication, optimal transition state geometry
and possibly the cooperativity of Lewis acid Y(OTf)3 assist to
activate nucleophile which leads enhanced enantioselectivity.
10
60
15
65 Scheme 2: Postulated structure for the activation of nucleophile mediated
by Y(OTf)3
In conclusion, the dimeric salen Co catalyst bearing Y(OTf)3
shows high reactivity and enantioselectivity in the hydrolytic
kinetic resolution with various terminal epoxides than its
70 monomer analogous. Advantage of this new catalyst is its water
tolerance property due to the reasonable moisture stability of
Y(OTf)3. The role of different rare earth metal triflate with chiral
metal complex in various asymmetric reactions is under progress
in our laboratory.
20
75 Acknowledgements
25
Fig.4 Kinetic study of the asymmetric HKR of the epichlorohydrin
catalyzed by the monomer and dimmer catalysts.
The corresponding author (ssthakur) sincerely acknowledges the
University Grant Commission, New Delhi for financial assistance
as UGC startꢀup grant [No.F.20ꢀ1/2012 (BSR)/20ꢀ1(3)ꢀ2012
(BSR)] for this work. We are thankful to Prof. ChiꢀHowꢀ
80 Peng,Taiwan for helpful discussion and Dr. S. P. Borikar, NCL,
India for GCꢀanalysis.
The HKR reaction catalyzed by dimeric catalyst B follows the
cooperative bimetallic mechanism in a rate determining step
30 where one salen Co unit acts as chiral Lewis acid centre which
selectively binds with matched epoxides and another serving to
deliver the hydroxide nucleophile. As proposed by the Jacobsen
et al.,8b these interactions are mediated by the chiral, stepped
conformations of the salen ligand (Scheme I).
Notes and references
35
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In the present HKR reaction, we propose that before forming
CoꢀOH or CoꢀOH2 intermediate the Y(OTf)3 support to activate
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salen Co centre and provides enhanced enantioselectivity in
50 comparison to its monomer analogous (Scheme 2). The proposed
mechanism shows complete agreement with the previously
100
Gmbh & Co.,
KgaA Weinheim, Germany 2004, pp. 165ꢀ199.
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