Ionic Liquid-functionalized Chiral salen Mn(Ⅲ) Complex for Asymmetric Oxidative Kinetic Resolution of Racemic Secondary Alcohols†

doi:10.7503/cjcu20130374

Abstract

Abstract:

Hydrophilic imidazolium-based ionic liquid(IL) was introduced into the framework of the chiral salen Mn(Ⅲ) complex by covalent linkage, which provided a ionic liquid-functionalized chiral salen Mn(Ⅲ) complex. Fourier transform infrared spectrometry, UV-Vis and thermal analysis results showed that the chiral salen Mn(Ⅲ) complex was modified with the IL moiety with the intact active site. The catalytic performance of IL-functionalized chiral salen Mn(Ⅲ) complex in the asymmetric oxidation kinetic resolution(OKR) of secondary alcohols was investigated. It was found that the surfactant-type complex showed higher catalytic activity than neat chiral salen Mn(Ⅲ) complex. Secondary alcohol conversion was more than 63%, enantioselective could reach 99% and separation efficiency was 18.3%. Catalyst could be facilely recovered from the reaction system for reuse 3 times by controlling solvent. The hydrophilic imidazolium-based IL was thought to improve the mass transfer associated with the aqueous reaction and stabilize the transition state, which in turns enhanced the activity and stability of the catalysts.

Key words: Enantioselectivity, Ionic liquid, Chiral salen Mn(Ⅲ) complex, Oxidation kinetic resolution, Secondary alcohol

CLC Number:

O621
O643

TrendMD:

LI Chengyong, TAN Rong, ZHAO Jiangfeng, YIN Donghong. Ionic Liquid-functionalized Chiral salen Mn(Ⅲ) Complex for Asymmetric Oxidative Kinetic Resolution of Racemic Secondary Alcohols^†[J]. Chem. J. Chinese Universities, 2014, 35(2): 297.

Figures/Tables 7

Fig.1 UV-Vis spectra of complex 1(a) and traditional chiral salen Mn(Ⅲ) complex(b)

Table 1 Oxidation kinetic resolution of α-methyl benzyl alcohol over complex 1a

Entry	Catalyst	Catalyst amount(%)	Conversion^b(%)	e.e.^c(%)	$k rel d$
1			60	0
2	Complex 1	0.25	58	69	5.9
3	Complex 1	0.5	62	90	10.0
4	Complex 1	1	63	>99	18.3
5	Complex 1	1.5	63	>99	18.3
6	Complex 2	1	59	56	3.8
7	Neat complex	1	61	97	16.9

Table 1 Oxidation kinetic resolution of α-methyl benzyl alcohol over complex 1a

Entry	Catalyst	Catalyst amount(%)	Conversion^b(%)	e.e.^c(%)	$k rel d$
1			60	0
2	Complex 1	0.25	58	69	5.9
3	Complex 1	0.5	62	90	10.0
4	Complex 1	1	63	>99	18.3
5	Complex 1	1.5	63	>99	18.3
6	Complex 2	1	59	56	3.8
7	Neat complex	1	61	97	16.9

Table 2 Oxidation kinetic resolution of α-methyl benzyl alcohol over complex 1 with various additivesa

Entry	Additive	Additive amount(%)	Conversion^b(%)	e.e.^c(%)	$k rel d$
1			16	0
2	KBr	4	55	69	7.2
3	KBr	6	63	>99	18.3
4	KBr	8	63	>99	18.3
5	N(CH₃)₄Br	6	64	99	16.8
6		6	64	98	14.6
7	CTAB	6	64	60	3.5
8	KCl	6	11	0
9		6	15	10	4.0

Table 2 Oxidation kinetic resolution of α-methyl benzyl alcohol over complex 1 with various additivesa

Entry	Additive	Additive amount(%)	Conversion^b(%)	e.e.^c(%)	$k rel d$
1			16	0
2	KBr	4	55	69	7.2
3	KBr	6	63	>99	18.3
4	KBr	8	63	>99	18.3
5	N(CH₃)₄Br	6	64	99	16.8
6		6	64	98	14.6
7	CTAB	6	64	60	3.5
8	KCl	6	11	0
9		6	15	10	4.0

Table 3 Oxidation kinetic resolution of α-methyl benzyl alcohol over the complex 1 in different solventa

Entry	Solvent(Volume/mL)	Conversion^b(%)	e.e.^c(%)	$k rel d$
1	H₂O(1.5)	51	70	10.5
2	CH₂Cl₂(1.5)	21	8	2.0
3	H₂O/CH₂Cl₂(1/0.5)	63	>99	18.3
4	H₂O/n-C₆H₁₄(1/0.5)	41	34	4.0
5	H₂O/CH₃CN(1/0.5)	61	52	3.2
6	H₂O/CH₃COOCH₂CH₃(1/0.5)	58	85	11.0
7	H₂O/CH₃COCH₃(1/0.5)	65	67	4.0

Table 3 Oxidation kinetic resolution of α-methyl benzyl alcohol over the complex 1 in different solventa

Entry	Solvent(Volume/mL)	Conversion^b(%)	e.e.^c(%)	$k rel d$
1	H₂O(1.5)	51	70	10.5
2	CH₂Cl₂(1.5)	21	8	2.0
3	H₂O/CH₂Cl₂(1/0.5)	63	>99	18.3
4	H₂O/n-C₆H₁₄(1/0.5)	41	34	4.0
5	H₂O/CH₃CN(1/0.5)	61	52	3.2
6	H₂O/CH₃COOCH₂CH₃(1/0.5)	58	85	11.0
7	H₂O/CH₃COCH₃(1/0.5)	65	67	4.0

Table 4 Oxidation kinetic resolution of various secondary alcohols over complex 1a

Entry	Conversion^b(%)	e.e.^c(%)	k_rel ^d
1	63	>99	18.3
2	60	28	1.9
3	63	95	12.2
4	33	3	1.3

Table 5 Reusability of complex 1a

Entry	Run times	Conversion^b(%)	e.e.^c(%)	k_rel ^d
1	Fresh	63	>99	18.3
2	2nd	62	98	17.2
3	3rd	62	97	15.5
4	4th	62	91	4.3

Table 6 Oxidation kinetic resolution of α-methyl benzyl alcohol over different IL-functionalized complexesa

Entry	Catalyst	Catalyst loading(%)	Conversion^b(%)	e.e.^c(%)	$k rel d$
1	Neat complex	2	55	40	3.5
2	Complex 1	1	53	52	5.8
3	Complex 1	1.5	55	77	10.0
4	Complex 1	2	54	88	19.9
5	Complex 1	3	54	88	19.9

Table 6 Oxidation kinetic resolution of α-methyl benzyl alcohol over different IL-functionalized complexesa

Entry	Catalyst	Catalyst loading(%)	Conversion^b(%)	e.e.^c(%)	$k rel d$
1	Neat complex	2	55	40	3.5
2	Complex 1	1	53	52	5.8
3	Complex 1	1.5	55	77	10.0
4	Complex 1	2	54	88	19.9
5	Complex 1	3	54	88	19.9

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