Advances in Asymmetric Oxidative Kinetic Resolution of Racemic Secondary Alcohols Catalyzed by Chiral Mn(III) Salen Complexes

Received: 28 March 2017 Revised: 30 June 2017 Accepted: 12 August 2017 DOI: 10.1002/chir.22768

REVIEW ARTICLE

Advances in asymmetric oxidative kinetic resolution of racemic secondary alcohols catalyzed by chiral Mn(III) salen complexes

Irshad Ahmad1 | Shagufta1 | Abdul Rahman AlMallah2

1 Department of Mathematics and Natural Abstract Sciences, School of Arts and Sciences, American University of Ras Al Khaimah, Enantiomerically pure secondary alcohols are essential compounds in Ras Al Khaimah, UAE organic synthesis and are used as chiral auxiliaries and synthetic intermedi- 2 Department of Chemical and Petroleum ates in the pharmaceutical, agrochemical, and fine chemical industries. One Engineering, School of Engineering, American University of Ras Al Khaimah, of the attractive and practical approaches to achieving optically pure second- Ras Al Khaimah, UAE ary alcohols is oxidative kinetic resolution of racemic secondary alcohols using chiral Mn(III) salen complexes. In the last decade, several chiral Correspondence Irshad Ahmad and Shagufta, Department Mn(III) salen complexes have been reported with excellent enantioselectivity of Mathematics and Natural Sciences, and activity in the homogeneous and heterogeneous catalysis of the oxida- School of Arts and Sciences, American tive kinetic resolution of racemic secondary alcohols. This review article is University of Ras Al Khaimah, Ras Al Khaimah, UAE. an overview of the literature on the recent development of chiral Mn(III) Emails: [email protected]; salen complexes for oxidative kinetic resolution of racemic secondary alco- [email protected] hols. The catalytic activity of monomeric, dimeric, macrocyclic, polymeric, Funding information and silica/resin supported chiral Mn(III) salen complexes is discussed School of Graduate Studies and Research, in detail. American University of Ras Al Khaimah, Ras Al Khaimah, UAE, Grant/Award KEYWORDS Number: AAS/001/16 chiral catalysts, Mn(III) salen complex, oxidative kinetic resolution, racemic secondary alcohols

1 | INTRODUCTION effective non–enzymatic catalysts for the oxidative kinetic resolution (OKR) of racemic alcohols.22-25 In the The oxidation of alcohols is one of the most general and last two decades, different chiral ligands with palla- well‐explored reactions in organic chemistry.1,2 Highly dium,26-32 ruthenium,33-35 iridium,36,37 cobalt,38 iron,39 efficient and selective oxidative methods of alcohols to and manganese (Mn) metal ions have been used as cata- the corresponding ketones, aldehydes, and carboxylic lysts for OKR of racemic secondary alcohols. In particu- acids have been developed.3-12 Application of these oxi- lar, chiral Mn(III) salen complexes are considered to be dation methods to the kinetic resolution of racemic alco- the most effective catalyst for the OKR of racemic hols provides optically active (chiral) alcohols together secondary alcohols (Figure 1) because of their facile with carbonyl compounds. These enantiopure secondary synthesis and catalytic reaction condition with high turn- alcohols are important chiral auxiliaries and building over frequency. blocks for the synthesis of important pharmaceutical, To our knowledge, in the last ten years, no review arti- agrochemical, and fine chemicals.13-16 The use of enzyme cle has been published on chiral Mn(III) salen complex catalysts for kinetic resolution through selective oxida- catalyzed asymmetric oxidative kinetic resolution of race- tion of one of the enantiomers is well known in the liter- mic secondary alcohols. Therefore, we have compiled and ature.17-21 Recently, researchers have developed several reviewed the different chiral Mn(III) salen complexes

2.1 | Homogeneous chiral Mn(III) salen complexes 2.1.1 | Nonrecyclable chiral Mn(III) salen FIGURE 1 Oxidative kinetic resolution of racemic secondary complexes alcohols using chiral Mn(III) salen complexes In 1998, Katsuki and his coworkers utilized binol‐derived Mn(III) salen complexes for the asymmetric oxidation of reported in the last decade for the OKR of racemic second- alcohols with iodosobenzene (PhIO) as the co‐oxidant.40 ary alcohols. We believe that this review will provide the The reaction results were not very promising and showed most complete insight into these complexes for use by low yield and moderate enantioselectivities. Subsequently researchers working in this area and facilitate the in 2003, W. Sun et al utilized chiral Mn(III) salen com- development of novel catalysts for OKR of secondary plexes as effective catalysts for the asymmetric oxidation alcohols. of secondary alcohols to ketones in the presence of the

co‐oxidant diacetoxyiodobenzene (PhI(OAc)2) and thus contributed significantly to the research of OKR of race- 2 | CHIRAL Mn(III) SALEN mic secondary alcohols.41 When the reaction was carried COMPLEXES FOR OKR OF RACEMIC out with catalyst 1 (Figure 2) in the solvent dichlorometh- SECONDARY ALCOHOLS ane, only 2% ee (enantiomeric excess) was obtained. However, change of solvent to water increases the As evident from the literature, various types of chiral enantiomeric excess to 8.9%. Considering the insolubility Mn(III) salen complexes are utilized for the OKR of sec- of both the substrate and catalyst in water, a phase trans- ondary alcohols. Consequently, this review article is fer catalyst such as tetraethylammonium bromide was broadly classified into two categories: homogeneous and introduced in the aqueous system, and results were heterogeneous chiral Mn(III) salen complexes, which are promising with 85.2% ee and 51.7% conversion (Table 1). further subcategorized into nonrecyclable, recyclable, The replacement of the tert‐butyl groups at the 5,5′‐posi- and silica/resin supported chiral Mn(III) salen complexes, tions in complex 1 with methyl groups lowers the respectively. enantioselectivity; however, the Mn(III) salen complex 2

FIGURE 2 Nonrecyclable chiral Mn(III) salen complexes for OKR of secondary alcohols AHMAD ET AL. 3

(Figure 2), derived from (R,R)‐diphenylethylenediamine, alcohols from methyl by an ethyl or a larger group resulted provided 88.1% ee and 62.5% conversion (Table 1). It was in a sharp decrease in the enantiomeric excess of observed that complex 2 was more effective with substrate the product. Additionally, with complex 1 higher 1‐phenyl ethanol and 1‐phenyl‐2‐propanol. However the enantioselectivities were perceived for substrates bearing replacement of one of the small substituents in secondary substituents on the aromatic ring. The results revealed that

TABLE 1 Oxidative kinetic resolution of various secondary alcohols using nonrecyclable chiral Mn(III) salen complexes

Entry Catalysts Substrate Additive/ Oxidant Conversion (%) ee (%) Reference 41 1 1 (2 mol%) N(C2H5)4Br (8 mol%)/ PhI(OAc)2 51.7 85.2 Sun et al

41 2 2 (2 mol%) N(C2H5)4Br (8 mol%)/ PhI(OAc)2 62.5 88 Sun et al

42 3 3 (2 mol%) KBr (4 mol%)/ PhI(OAc)2 64 96 Li et al

43 4 4 (2 mol%) N(C2H5)4Br (8 mol%)/ PhI(OAc)2 59 24.2 Cheng et al

43 5 4 (1 mol%) N(C2H5)4Br (8 mol%)/ PhI(OAc)2 68 68 Cheng et al

6 4 (1 mol%) 1‐Butyl‐2,6‐dimethyl pyridinium 77.8 98.6 Cheng et al43

bromide (8 mol%)/ PhI(OAc)2

7 5a (1 mol%) Without additive/ NBS 64 99 Xu et al44

8 5b (1 mol%) Without additive/ NBS 61 85 Xu et al44

45 9 3 (1 mol%) Br2 (6 mol%)/ NaClO 46.9 84.3 Zhang et al

45 10 3 (1 mol%) Br2 (6 mol%)/ NaClO 64.5 95.4 Zhang et al

11 3 (1 mol%) NBS (8 mol%) / NaClO 62.1 >99.9 Zhang et al46

47 12 6 (2 mol%) KBr (8 mol%)/ PhI(OAc)2 60 89 Han et al 4 AHMAD ET AL. the steric hindrance at the 5,5′‐positions of the catalyst and aromatic were also investigated for asymmetric oxidative electronic cooperation between the catalyst and the sub- kinetic resolution reaction. All the chiral Mn(III) salen strate plays important role on the enantioselectivity of catalysts in the presence of the NBS oxidant showed sig- the reaction. Next, a detailed study was performed on the nificant enantiomeric excess and conversion. Catalyst 5a effect of additives, organic solvent, and the substituents of (Figure 2) bearing a 5‐tert‐butyl group showed the best chiral salen ligands on the enantioselectivities of the reac- results with 99% ee and 64% conversion (Table 1) on sub- tions.42 The study provided an effective system for the oxi- strate (±)‐1‐(2‐bromophenyl)ethanol. In comparison, cat- dative kinetic resolution of secondary alcohols by using alyst 5b (Figure 2), without any ligand substituent in the Mn(III) salen complex (Jacobsen catalyst, 3) (Figure 2) as aromatic ring and under similar reaction conditions, catalyst and PhI(OAc)2 with potassium bromide (KBr) as exhibited only 85% ee and 61% conversion (Table 1). an additive (yielding 64% conversion and 96% ee) Moreover, meta‐substituted benzylic alcohols afforded (Table 1). In addition, the possible mechanism of the oxida- high enantioselectivities, a higher rate of reaction, com- tive kinetic resolution was explained with the help of UV/ pared to the ortho‐substituted substrates.

Vis spectroscopy and electrospray ionization mass spec- The main disadvantages of using PhI(OAc)2 as the oxi- trometry (ESI‐MS). The results revealed the significance dant during asymmetric oxidative kinetic resolution of of the bromide ion in the enantioselective oxidative reac- racemic secondary alcohols are that it is low atom‐effi- tion and proved the oxygen transfer from PhI(OAc)2 to cient, expensive, and unfavorable to the environment, the Mn(III) salen via oxo‐manganese species formed dur- and thus, continuous efforts have been made to replace it ing oxidative kinetic resolution. with a more efficient and less expensive oxidant. In this Moreover, to explore the effect of substituents on the respect, Y. Zhang et al in 2014 performed the asymmetric chiral Mn(III) salen catalysts, a series of chiral Mn(III) oxidative kinetic resolution of racemic secondary alcohols salen complexes with various substituents on the aromatic catalyzed by Jacobsen's chiral Mn(III) salen complex (3) rings and different diamine backbones were synthesized (Figure 2) with sodium hypochlorite (NaOCl) as oxidant.45 43 and screened for OKR of racemic secondary alcohols. The catalytic amount of Br2 was used as cycling agent, and For the asymmetric oxidative kinetic resolution of race- the reaction was conducted in the biphasic CH2Cl2/H2O mic 1‐phenyl‐1‐propanol in the presence of PhI(OAc)2 medium in the presence of potassium acetate, KOAc. and tetra‐butylammonium bromide [N(C4H9)4Br], the The NaOCl used as oxidant is very inexpensive, readily most active catalyst was chiral Mn(III) salen complex 4 available, and showed promising results. The results (Figure 2) having tert‐butyl groups at the 5,5′‐positions revealed that the electrostatic properties of the substitu- of salicylaldehyde. Catalyst 4 exhibited only 24.2% ee and ents play an important role and have significant influence 59% conversion (Table 1). The enantiomeric excess value on the reaction. The system works efficiently with all the was increased by using additives such as KBr and different screened substrates, and good enantioselectivity was quaternary ammoniums with a bromonium ion. The observed with 1‐phenyl ethanol (84.3% ee and 46.9% con- maximum increases in the enantiomeric excess value version) (Table 1) and its derivatives. The substrates bear- were observed when an additive (1‐butyl‐2,6‐dimethyl ing electron‐donating substituents at the para position, pyridinium bromide) was used (68% ee and conversion) such as 1‐(4‐methoxyphenyl)‐1‐ethanol, were not very (Table 1). Then this methodology was applied to effective (19.7% ee and 29.8% conversion). The best result, other secondary alcohols such as 1‐phenyl‐1‐butanol, 1‐ ie, 95.4% ee and 64.5% conversion (Table 1), was obtained phenyl‐1‐pentanol, 1‐cyclohexyl‐1‐propanol, and 1‐ with substrate 1‐(4‐methylphenyl) ethanol. The system, cyclohexyl‐1‐pentanol. Under the optimal condition, under mild reaction conditions, was very effective with cyclohexyl‐1‐propanol accomplished the maximum enan- secondary alcohols with fused‐rings. The main disadvan- tiomeric excess (98.6%) and conversion (77.8%) (Table 1). tage of the system was the handling of the rather toxic

In 2010, E. J. Corey et al proposed the mechanism of and corrosive Br2, especially on a large scale. Hence, Br2 the enantioselective oxidation of racemic secondary alco- was replaced with NBS as a cycling agent for the OKR of hols catalyzed by chiral Mn(III) salen complexes.48 The alcohols catalyzed by 3.46 The reaction was performed proposed pathway suggested the formation of positive with NaClO as an oxidant in a biphasic CH2Cl2/H2O bromine species HOBr under the reaction conditions by medium in the presence of KOAc. The results revealed oxidation of bromide ion with PhI(OAc)2, and then an >99.9% ee and 62.1% conversion (Table 1) with the addi- Mn(V) salen complex is generated in the presence of the tional advantages of low cost and easy product separation positive bromine species. Bearing in mind these results, from the reaction mixture. W. Sun and his coworkers in 2012 introduced N‐bromo Considering the significance of sugar‐based chiral succinimide (NBS) as the oxidant to produce HOBr.44 In Mn(III) salen complexes in the asymmetric epoxidation this study, benzylic alcohols with ortho/meta substituted of unfunctional olefins,49,50 F. Han et al in 2008 explored AHMAD ET AL. 5 the catalytic applicability of Mn(III) salen complexes with of the complex. A moderate to good enantioselectivity of sugar moieties at C‐5(5′) of the salicylaldehyde on OKR of rating of 73‐85% was obtained with bulkier secondary racemic secondary alcohols.47 The synthesized chiral alcohols such as 1‐naphthyl ethanol and 2‐naphthyl etha- catalysts showed good results for the OKR of 1‐ nol; however, the rate of these reactions was very low. The phenylethanol using PhI(OAc)2 as an oxidant. Catalyst 6 effect of solvent and various additives was examined by (Figure 2) exhibited promising results with 89% of enan- using complex 7 for OKR of racemic secondary alcohols tiomeric excess and 60% conversion (Table 1). The effects of 1‐phenyl ethanol as a representative substrate. Using of solvent and additive on the asymmetric oxidative water alone as solvent showed poor results (36% ee and kinetic resolution of 1‐phenylethanol using catalyst 6 47% conversion), whereas the 1:2 of CH2Cl2 and water sol- were studied, and in the results, the 1:2 CH2Cl2‐water vent system showed the best results (99% ee and 62% con- emerged as the best solvent and KBr as the best additive. version). The most effective additive was LiBr, which The sugars at the C‐5(5′) moiety of salicylaldehyde produced 97% ee and 64% conversion. The catalyst was showed a significant effect on the enantioselectivity of easily recovered by addition of a nonpolar solvent such OKR of 1‐phenylethanol, and it was established that the as n‐hexane, which precipitated out complex 7 because sugar with same rotation direction of polarized light as of its higher molecular weight and was recycled up to five the Diimine Bridge in the complex could enhance the chi- times with the retention of enantioselectivity. ral induction, whereas the sugars with the opposite one Additionally, R. I. Kureshy et al utilized the chiral poly- would reduce that of the corresponding complex. meric Mn(III) salen complexes for OKR of racemic second- 61 ary alcohols using PhI(OAc)2 as oxidant. The reaction was also examined with different additives in water/organic 2.1.2 | Recyclable chiral Mn(III) salen solvent mixtures at room temperature. Catalyst 8 (Figure 3) complexes showed promising enantioselectivity and conversion with Homogeneous chiral Mn(III) salen complexes are different secondary alcohols, and the best results were reported with high enantioselectivity for OKR of racemic obtained with 4‐fluorophenylethanol (>99% ee and 63% con- secondary alcohols, yet their application is associated with version) and 1‐phenyl ethanol (98% ee and 62% conversion) several disadvantages, which includes difficulty in the sep- (Table 2). To study the effect of catalyst loading the 1‐ aration of product and catalyst, as well as low stability. phenylethanol was used as a substrate. With the increase Because of the expensive nature of chiral catalysts, there in catalyst loading form 2 to 5 mol%, only slight decrease is a clear requirement for catalyst recycling. In recent in enantioselectivity was observed (98% to 94% ee,respec- years, chiral dimeric Mn(III) salen complexes have been tively). In comparison to catalyst loading of 2 mol%, the 0.6 widely used as recyclable catalysts in asymmetric epoxida- and 0.2 mol% exhibited reduction in enantioslectivity (96% tion reactions by researchers.51-59 Encouraged with these and 91% ee, respectively). The additive and solvent nature results, S. H. R. Abdi and his coworkers in 2007 utilized showed a notable effect on the activity and enantioselectivity the chiral dimeric Mn(III) salen complexes in asymmetric of the chiral Mn(III) salen complex. For the OKR of racemic oxidative kinetic resolution of racemic secondary alcohols secondary alcohols of 1‐phenyl ethanol using complex 8 as using PhI(OAc)2 as an oxidant with KBr as an additive at catalyst and KBr as an additive, good to excellent room temperature.60 Among all the studied chiral Mn(III) enantioselectivity (83‐94%) was observed when a solvent like salen complexes for the OKR of racemic 1‐phenyl ethanol, toluene, 1, 2‐dichloromethane and chloroform mixed with the maximum enantioselectivity (99%) and conversion water was used. The use of different bromide salts such as (62%) (Table 2) was observed with dimeric‐Mn(III) salen NaBr, LiBr, and hexyl pyridinium bromide as an additive complex (7) (Figure 3) with 2 mol% catalyst loading. How- with a dichloromethane and water solvent system showed ever, a further reduction in enantioselectivity was good enantioselectivity (87‐89%). The reaction did not work observed with 5 mol% (95% ee) and 0.2 mol% (90% ee) cat- in the absence of an additive as well as with KCl as an alyst loading with increase in reaction time. The catalytic additive, thus indicating the significance of the bromide effect of complex 7 for various racemic secondary alcohols ion for OKR. Catalyst 8 was easily and efficiently recyclable was studied. It was observed that the presence of substitu- up to five times with retention of enantioselectivity. ent at the para position of 1‐phenyl ethanol favors the In 2007 Sun et al also screened polymeric salen manga- reaction with enantioselectivity 95‐97%. Additionally, the nese (III)‐type complexes for the oxidative kinetic resolu- substituent at the ortho position of the phenyl ring or tion of racemic secondary alcohols.62 The polymeric extension of the alkyl chain reduces the enantioselectivity chiral Mn(III) salen catalyst 8 showed excellent results because of the steric hindrance of the substituent group, (95.2% ee and 58.8% conversion) (Table 2) in the OKR of which prohibits the interaction of the substrate with the 1‐phenyl ethanol when H2OandCH2Cl2 were used as sol- chiral center having the catalytically active metal center vent. Catalyst 8 was tested for different secondary aromatic 6 AHMAD ET AL.

TABLE 2 Oxidative kinetic resolution (OKR) of various secondary alcohols using recyclable chiral Mn(III) salen complexes

Entry Catalysts Substrate Additive/ Oxidant Conversion (%) ee (%) Reference 60 1 7 (2 mol%) KBr (4 mol%)/ PhI(OAc)2 62 99 Pathak et al

61 2 8 (0.6 mol%) KBr (1.2 mol%)/ PhI(OAc)2 63 >99 Kureshy et al

61 3 8 (2 mol%) KBr (1.2 mol%)/ PhI(OAc)2 62 98 Kureshy et al

62 4 8 (2 mol%) KBr (8 mol%)/ PhI(OAc)2 58.8 95.2 Sun et al

63 5 9 (2 mol%) KBr (4 mol%)/ PhI(OAc)2 63 83 Bera et al

63 6 9 (2 mol%) KBr (4 mol%)/ PhI(OAc)2 58 99 Bera et al

7 10 (1.5 mol%) NBS (0.7 equiv) 66 95 Maity et al64

8 10 (1.5 mol%) NBS (0.7 equiv) 65 97 Maity et al64

65 9 11 (1 mol%) KBr (6 mol%)/ PhI(OAC)2 62 >99 Li et al

65 10 11 (1 mol%) KBr (6 mol%)/ PhI(OAC)2 61 95 Li et al

66 11 12 (1.5 mol%) KBr (6 mol%)/ PhI(OAC)2 63 96 Tan et al

66 12 12 (1.5 mol%) KBr (6 mol%)/ PhI(OAC)2 63 94 Tan et al

and aliphatic racemic alcohols for the OKR reaction. The Subsequently S. H. R. Abdi and his coworkers in 2013 aromatic alcohols bearing various para substituents exhib- tried to reduce the cost of catalysts for practical applica- ited high enantioselectivity (up to 98%) whereas with tions by increasing their reusability. In this regard, a ortho‐substituted aromatic alcohols no enantioselectivity series of macrocyclic Mn(III) salen complexes were syn- was observed. The polymeric catalyst also showed good thesized and evaluated for the OKR of racemic secondary enantioselectivity (up to 99%) with secondary aliphatic alcohols with PhI(OAc)2 and NBS as co‐oxidant in water‐ racemic alcohols. The polymeric catalyst was easily recov- dichloromethane solvent mixture.63 The maximum ered from the reaction mixture by adding hexane and enantioselectivity (up to 99%) for the tested racemic sec- reused four times with only slight loss in enantioselectivity. ondary alcohols was observed when homopiperazine AHMAD ET AL. 7

FIGURE 3 Recyclable chiral Mn(III) salen complexes for OKR of secondary alcohols macrocyclic complex 9 (Figure 3) was used as the catalyst observed when the catalyst loading was 1.5 mol% with PhI(OAc)2 and N‐bromo succinimide (NBS) as oxi- (Table 2). However, either increasing or decreasing the dants and in presence of KBr as an additive and dichloro- loading of catalyst from 1.5 mol% resulted in a decrease methane/water solvent mixture. With catalyst 9 and in enantioselectivity. No improvement in enatioselectivity

PhI(OAc)2 as an oxidant, the 1‐phenylethanol substrate was observed by changing the solvent from dichlorometh- showed 83% ee and 63% conversion (Table 2); and the best ane to chloroform or dichloroethane. Therefore, the opti- result was obtained with the aliphatic alcohol 2‐butanol, mum reaction condition for the OKR of various secondary which yielded 99% ee and 58% conversion (Table 2). In alcohols was considered to be a catalyst loading of comparison to PhI(OAc)2, using NBS as an oxidant was 1.5 mol% in a biphasic solvent mixture of dichlorometh- more effective and exhibited higher enantioselectivity ane/water at ambient temperature (20°C). Catalyst 10 effi- even for the OKR of sterically hindered racemic secondary ciently catalyzed the para‐substituted benzylic secondary alcohols. Catalyst 9 was easily precipitated out by adding alcohols with good enantioselectivity. It was observed hexane and was recycled efficiently seven times without that for the electron‐withdrawing groups, such as any loss in activity and enantioselectivity. fluoro and chloro groups, the catalyst provided high Later a series of macrocyclic diethyl tartrate linked enantioselectivity, that is, 96% and 93% ee, respectively, Mn(III) salen complexes were synthesized and utilized whereas when the electron‐donating methyl (84% ee) for asymmetric epoxidation of olefins. Catalyst 10 and very bulky bromo (85% ee) groups were present, little (Figure 3) showed promising results for asymmetric epox- decrease in enantioselctivity was noticed. Among the idation of olefins, and therefore, it was studied for OKR of non–benzylic secondary alcohols, the 1‐phenyl‐2‐ racemic secondary alcohols.64 The effect of catalyst load- propanol showed high enantioselectivity (97% ee) with ing and solvent was examined for asymmetric oxidative 65% conversion (Table 2). Catalyst 10 recycling studies kinetic resolution of racemic 1‐phenylethanol. The result was performed only for epoxidation reaction of alkenes, revealed that on substrate 1‐phenylethanol, the best and results showed that it can be reused up to four times enantioselectivity (95%) and conversion (66%) rating was with no loss in activity. 8 AHMAD ET AL.

The higher catalytic efficacy and recovery of ionic liquids (ILs) bonded to the chiral Mn(III) salen complex for enantioselective epoxidation of styrene67,68 motivated C. Li et al in 2011 to synthesize three novel ionic liquid‐ bridged chiral dimeric Mn(III) salen complexes and uti- lize them for asymmetric oxidative kinetic resolution of racemic secondary alcohols.65 The N,N‐dialkylimidazo- lium ionic liquids were used to bridge chiral dimeric FIGURE 5 PNIPAAm‐functionalized chiral Mn(III) salen ‐ Mn(III) salen complexes at the C 5 position of the complex for OKR of secondary alcohols salicylaldehyde moieties. Among all the complexes tested, the IL‐bridged salen Mn(III) complex 11 (Figure 4) exhib- grafting onto the metal center. The catalysts were exam- ited the most promising enantioselectivity (>99%) and ined for OKR of various racemic secondary alcohols with ‐ conversion (62%) in the OKR of 1 phenyl ethanol using PhI(OAc)2 and KBr. When the OKR reaction was per- PhI(OAc)2 as an oxidant (Table 2). It was proposed that formed with pure water, the thermoresponsive catalysts the higher enantioselectivity could be due to the good sol- worked efficiently and thus eliminated the need for any ubility of complex 11 in the solvent, ie, dichloromethane, organic solvent. The OKR of 1‐phenyl ethanol in the aque- and the interaction of the two catalytic active sites of com- ous medium with complex 12 (1.5 mol%) provided plex 11. Further catalytic activities of this complex for the enantioselectivity up to 96% ee and conversion 63% OKR of different racemic secondary alcohols were stud- (Table 2). The results were promising, and the values were ied. The result suggested the importance of steric hin- quite high when compared to the results obtained over drance around the hydroxyl group of substrate in the neat complex and in solvent water (56% ee and 54% overall efficiency of the kinetic resolution. The presence conversion). The best result was obtained with 1.5 mol% of substituted groups causes steric hindrance and prevents catalyst, and further changes in catalyst loading exhibited the hydroxyl group in the substrate approaching the cata- no enhancement in the enantioselectivity. lytic metal center of complex 11 and therefore resulted in The presence of thermoresponsive PNIPAAm groups the decrease in enantioselectivity. Complex 11 effectively provided the inverse temperature‐dependent water solu- catalyzed the OKR of aliphatic alcohol 2‐pentanol with bility to the PNIPAAm‐based complexes. During oxidative 95% enantioselectivity and 61% conversion (Table 2). On kinetic resolution (OKR) in water, the catalyst showed the other hand, for the heterocyclic compounds such as significant thermoregulated phase‐separable catalysis 2‐furylethanol, the chiral Mn(III) salen complex 11 was (TPSC), and by simply regulating the temperature of totally inactive. The complexes showed promising recycla- aqueous medium, it was easily recovered for reuse. The ble results and were easily recycled for five successive cat- complexes showed excellent recycling efficiency of up to alytic runs and considered as promising chiral catalysts four times without noticeable loss of enantioselectivity for the OKR of racemic secondary alcohols. and activity. The study revealed that the steric hindrance Further to reduce the catalyst separation and solubility around the hydroxyl group of the substrates played a vital limitation problems in aqueous reactions, R. Tan et al role in the overall efficiency of the OKR of racemic sec- in 2013 reported the synthesis of a new series of ondary alcohols. The aliphatic alcohol 2‐pentanol showed novel PNIPAAm‐functionalized chiral Mn(III) salen promising results (94% ee and 63% conversion) (Table 2) complexes possessing a thermoregulated phase‐transfer whereas with heterocyclic compound 2‐furylethanol no function.66 The thermoresponsive polymer PNIPAAm oxidation was observed. [poly(Nisopropylacrylamide)] was prepared through free‐ radical polymerization and then introduced to the frame- 2.2 | Heterogeneous chiral Mn(III) salen work of the chiral Mn(III) salen complexes. In complex 12 complexes (Figure 5), the thermoresponsive polymer PNIPAAm was covalently connected to one side of the 5‐position in the Continuous efforts are being made by researchers to salen ligand, whereas in other complexes by axially immobilize the catalyst on solid supports, which increase

FIGURE 4 Ionic liquid‐bridged chiral dimeric Mn(III) salen complex for OKR of secondary alcohols AHMAD ET AL. 9 its handling and recycling compared to their homoge- separation and recycling of catalyst more convenient. neous counterparts. M. L. Kantam and coworkers in Therefore, S. B. Halligudi and his coworkers in 2008 2007 used the resin‐supported sulfonato‐Mn (salen) com- immobilized chiral Mn(III) salen complex onto an ionic plex [(R,R)‐1] (13) (Figure 6) for the asymmetric oxidative liquid‐modified mesoporous silica SBA‐15 through SILP kinetic resolution of racemic secondary alcohols.69 The 1‐ strategy.74 The ionic liquid modified mesoporous silica, phenyl ethanol was oxidized in the presence of resin (R, SBA‐15 catalyst (14) (Figure 7) with long range order,

R)‐1 catalyst (13) and PhI(OAc)2 as an oxidant at room large monodispersed mesopores and thick walls was temperature. The influence of solvent and additive on synthesized. The use of catalyst 14 was examined for the OKR of 1‐phenyl ethanol was examined, and the max- the OKR of racemic secondary alcohols and exhibited imum enantioselectivity (55.3%) and conversion (61.5%) excellent enantioselectivity (99%) and conversion (63%) (Table 3) of the product was observed when water was with 1‐phenyl ethanol as substrate and KBr as an used as solvent and KBr as an additive. It was suggested additive (Table 3). The catalyst was economically viable, that the low enantioselectivity was due to the introduction very effective, and recovered by simple filtration. No of –SO3Na groups at the 5,5′‐position. The resin (R,R)‐1 significant difference in the conversion and enan- catalyst (13) was recovered by simple filtration and reused tioselectivity was observed after reusing catalyst in five three times with consistent activity. consecutive cycles. The supported ionic liquid phase (SILP) catalysts are Later the effects of different solvent and support on examined for various organic reactions such as the OKR of racemic secondary alcohols with immobilized hydroformylation, hydrogenation, asymmetric aldol con- catalyst 14 were examined.75 To optimize the reaction densation and asymmetric epoxidation.70-73 SILP allows the application of fixed‐bed reactors and makes the

FIGURE 7 Chiral Mn(III) salen complex immobilized on ionic FIGURE 6 Resin supported sulfonato‐Mn(III) salen complex liquid modified mesoporous silica SBA‐15 for OKR of secondary [(R,R)‐1] (13) for the OKR of racemic secondary alcohols alcohols

TABLE 3 Oxidative kinetic resolution of various secondary alcohols using heterogeneous chiral Mn(III) salen complexes

Entry Catalysts Substrate Additive/ oxidant Conversion (%) ee (%) Reference 30 1 13 (1 mol%) KBr (4 mol%)/ PhI(OAc)2 61.5 55.3 Kantam et al

32 2 14 (0.12 mol%) KBr (4 mol%)/ PhI(OAc)2 63 99 Sahoo et al

34 3 16a (2 mol%) KBr (8 mol%)/ PhI(OAc)2 54 88 Ren et al

34 4 16a (2 mol%) KBr (8 mol%)/ H2O2 49 85 Ren et al 10 AHMAD ET AL.

FIGURE 8 Mesoporous helical silica immobilized Mn(III) salen complex for OKR of secondary alcohols condition, 1‐phenyl ethanol was used as substrate and The synthesized catalysts were screened for OKR of

PhI(OAc)2 as the oxidant. Different additives were racemic secondary alcohols mainly in water and with screened for this reaction, and the best result was iodobenzene diacetate and aqueous hydrogen peroxide obtained with additives having bromide anions such as (30 wt%) as oxidant. Mn(III) salen complexes exhibited

N(CH3)4Br and KBr. When bromide ion ionic liquid reasonable conversion, enantioselectivity, and selectivity (which can act both as support and the additive) was used, factor for OKR of different secondary alcohols. It was the results were promising. The bromide ion‐containing observed that, in comparison to attached Mn(III) salen, ionic liquids were water soluble, and the Mn(III) salen the configuration of silica supports significantly affect complex was less soluble in this ionic liquid; therefore, the major configuration of the resolution product of sec-

[BMIM] PF6 was selected as the ionic liquid in the sup- ondary alcohols. The best chiral induction was obtained ported ionic liquid catalysis. A number of silica supports with the combination of (S,S)‐diammonium salt‐doped such as MCM‐41, MCM‐48, SBA‐15, and amorphous silica silica and Mn(III)‐(R,R) salen. Both iodobenzene acetate were examined; however, no significant difference in and hydrogen peroxide (30%) act as effective terminal oxi- activity was observed, suggesting that there was no effect dant. For the OKR of 1‐phenyl ethanol, the best result was because of the support on the OKR of racemic secondary obtained with catalyst 16a, which showed 85% ee and 54% alcohols. Among all the solvents screened, the hexane conversion when PhI(OAc)2 was used as the additive and water mix, along with diethyl ether and water mix- (Table 3). A slight decrease in enantioselectivity (85%) ture, showed good activity and selectivity for OKR and conversion (49%) was observed when H2O2 was used because of reduced leaching of the complex into the as an additive (Table 3). In terms of recycling, the most organic phase. The best oxidant for OKR was found to efficient systems were heterogeneous lipophilic Mn(III) be PhI(OAc)2, and it was observed that enantiomeric salen with PhI(OAc)2 and heterogeneous hydrophilic excess increased with the increase in conversion and time. Mn(III) salen with H2O2. This report explored the applica- The catalytic system was used for various racemic second- tion of new chiral silica materials in catalytic resolution of ary alcohols and showed good results. The α‐methyl‐p‐ secondary alcohols. chlorobenzyl alcohol and 2‐decanol showed excellent enantioselectivity (99% and 94%, respectively) whereas α‐methyl‐p‐methoxy benzyl alcohol exhibited low 3 | CONCLUSION enantioselectivity (30%). This catalytic system was thus not effective for heteroatoms containing aromatic second- During the past decade, there have been significant ary alcohols such as furyl and pyridyl substituted alcohols. advances in the development of chiral Mn(III) salen com- In 2016, L. Ren et al attempted to construct an effi- plexes and their applications in the asymmetric oxidative cient and environmentally benign system for asymmetric kinetic resolution of racemic secondary alcohols. The oxidative kinetic resolution of secondary alcohols using aim of this review was to present an overview of the liter- new chiral silica materials and reported the synthesis of ature on the recent chiral Mn(III) salen complexes mesoporous helical silica through doping of chiral reported for OKR of racemic secondary alcohols. Mono- diammoniumcyclohexane mono‐tartrate salt in a sol‐gel meric, dimeric, macrocyclic, polymeric, silica/resin process.76 Two series of catalysts, ie, catalysts 15 and 16 Mn(III) salen complexes are discussed for OKR of racemic (Figure 8), were prepared by immobilization of different secondary alcohols. Most of the complexes exhibited Mn(III) salen complexes into helical silica efficiently excellent enantioselectivity, activity, stability and recycled using different linkages. successfully under the oxidizing reaction conditions. We AHMAD ET AL. 11 anticipate that this review will inspire the researchers 12. Shenci L, Bei PS, Woon‐Yew W, Yu Z. Kinetic resolution of 3‐ around the world and supports in the development of hydroxy‐3‐substituted oxindoles through NHC‐catalyzed oxida- ‐ more efficient catalysts for asymmetric oxidation of alco- tive esterification. Synlett. 2013;24:1165 1169. hols and other organic reactions. 13. Laumen K, Breitgoff D, Schneider MP. Enzymic preparation of enantiomerically pure secondary alcohols. Ester synthesis by irreversible acyl transfer using a highly selective ester hydrolase ACKNOWLEDGMENTS from pseudomonas sp.; an attractive alternative to ester hydroly- sis. J Chem Soc Chem Commun. 1988;22:1459‐1461. The author appreciate the financial support provided by 14. Singh VK. Practical and useful methods for the enantioselective the School of Graduate Studies and Research, American reduction of unsymmetrical ketones. Synthesis. 1992;7:605‐617. University of Ras Al Khaimah, Ras Al Khaimah, UAE 15. Wills M. Asymmetric catalysis using air: clean kinetic resolution through seed grant funded project no. AAS/001/16. of secondary alcohols. Angew Chem Int Ed. 2008;47:4264‐4267. 16. Patel RN. Biocatalytic synthesis of chiral alcohols and amino ORCID acids for development of pharmaceuticals. Biomolecules. 2013;3:741‐777. Irshad Ahmad http://orcid.org/0000-0001-7902-4715 17. Faber K. Biotransformations in Organic Chemistry. Heidelberg: Shagufta http://orcid.org/0000-0003-1028-7888 Springer; 1992. 18. Persson BA, Larsson ALE, Ray ML, Backvall JE. Ruthenium‐ REFERENCES and enzyme‐catalyzed dynamic kinetic resolution of secondary alcohols. J Am Chem Soc. 1999;121:1645‐1650. 1. Sheldon RA, Kochi JK. Metal‐catalyzed Oxidations of Organic Compounds. New York: Academic Press; 1984. 19. Jesus PC, Rezende MC, Nascimento MG. Enzymatic resolution of alcohols via lipases immobilized in microemulsion‐based gels. 2. Hudlicky M. Oxidations in Organic Chemistry. Washington, DC: Tetrahedron: Asymmetry. 1995;6:63‐66. ACS Monograph Series 186; American Chemical Society; 1990. 20. Stampfer W, Kosjek B, Faber K, Kroutil W. Biocatalytic oxidative ' ‐ 3. Marko IE, Giles PR, Tsukazaki M, Brown SM, Urch CJ. Copper kinetic resolution of sec‐alcohols: stereocontrol through sub- catalyzed oxidation of alcohols to aldehydes and ketones: an strate‐modification. Tetrahedron: Asymmetry. 2003;14:275‐280. efficient, aerobic alternative. Science. 1996;274:2044‐2046. 21. Jacobsen EE, Andersen LS, Anthonsen T. Immobilization does 4. Sato K, Aoki M, Takagi J, Noyori R. Organic solvent‐ and halide‐ not influence the enantioselectivity of CAL‐B catalyzed kinetic free oxidation of alcohols with aqueous hydrogen peroxide. JAm resolution of secondary alcohols. Tetrahedron: Asymmetry. Chem Soc. 1997;119:12386‐12387. 2005;16:847‐850. 5. Brink GJ, Arends IWCE, Sheldon RA. Green, catalytic oxidation 22. Sigman MS, Jensen DR. Ligand‐modulated palladium‐catalyzed of alcohols in water. Science. 2000;287:1636‐1639. aerobic alcohol oxidations. Acc Chem Res. 2006;39:221‐229. 6. Adam W, Gelalcha FG, Saha‐Moller CR, Stegmann VR. 23. Bagdanoff JT, Ferreira EM, Stoltz BM. Palladium‐catalyzed Chemoselective C−H oxidation of alcohols to carbonyl com- enantioselective oxidation of alcohols: a dramatic rate accelera-

pounds with iodosobenzene catalyzed by (salen) chromium tion by Cs2CO3/t‐BuOH. Org Lett. 2003;6:835‐837. ‐ complex. J Org Chem. 2000;65:1915 1918. 24. Nishibayashi I, Takei I, Uemura S, Hidai M. Extremely high 7. Adam W, Hajara S, Herderich M, Saha‐Moller CR. A highly enantioselective redox reaction of ketones and alcohols catalyzed

chemoselective oxidation of alcohols to carbonyl products with by RuCl2(PPh3)(oxazolinylferrocenylphosphine). Organometal- iodosobenzene diacetate mediated by chromium(III) (salen) lics. 1999;18:2291‐2293. complexes: synthetic and mechanistic aspects. Org Lett. 25. Hashiguchi S, Fujii A, Haack KJ, Matsumura K, Ikariya T, ‐ 2000;2:2773 2776. Noyori R. Kinetic resolution of racemic secondary alcohols by 8. Dijksman A, Gonza' lez AM, Payeras AM, AIWCE, Sheldon RA. RuII‐catalyzed hydrogen transfer. Angew Chem Int Ed Engl. Efficient and selective aerobic oxidation of alcohols into alde- 1997;36:288‐290. hydes and ketones using ruthenium/TEMPO as the catalytic 26. Ferreira EM, Stoltz BM. The palladium‐catalyzed oxidative system. J Am Chem Soc. 2001;123:6826‐6833. kinetic resolution of secondary alcohols with molecular oxygen. 9. Son YC, Makwana VD, Howell AR, Suib SL. Efficient, catalytic, J Am Chem Soc. 2001;123:7725‐7726. aerobic oxidation of alcohols with octahedral molecular sieves. 27. Jensen DR, Pugsley JS, Sigman MS. Palladium‐catalyzed Angew Chem Int Ed. 2001;40:4280‐4283. enantioselective oxidations of alcohols using molecular oxygen. ‐ 10. Mori K, Yamaguchi K, Hara T, Mizugaki T, Ebitani K, Kaneda J Am Chem Soc. 2001;123:7475 7476. K. Controlled synthesis of hydroxyapatite‐supported palladium 28. Chen T, Jiang JJ, Xu Q, Shi M. Axially chiral NHC−Pd(II) com- complexes as highly efficient heterogeneous catalysts. JAm plexes in the oxidative kinetic resolution of secondary alcohols Chem Soc. 2002;124:11572‐11573. using molecular oxygen as a terminal oxidant. Org Lett. ‐ 11. Yamaguchi K, Mizuno N. Supported ruthenium catalyst for the 2007;9:865 868. heterogeneous oxidation of alcohols with molecular oxygen. 29. Breuning M, Steiner M, Mehler C, Paasche A, Hein D. A flexible Angew Chem Int Ed. 2002;41:4538‐4542. route to chiral 2‐endo‐substituted 9‐oxabispidines and their 12 AHMAD ET AL.

application in the enantioselective oxidation of secondary complexes with N‐bromosuccinimide as a powerful oxidant. alcohols. J Org Chem. 2009;74:1407‐1410. Org Biomol Chem. 2012;10:2730‐2732. 30. Ebner DC, Trend RM, Matthew CG, McGrath J, O'Brien P, Stoltz 45. Zhang Y, Zhou Q, Ma W, Zhao J. Enantioselective oxidation of BM. Palladium‐catalyzed enantioselective oxidation of chiral racemic secondary alcohols catalyzed by chiral Mn(III)‐salen secondary alcohols: access to both enantiomeric series. Angew complex with sodium hypochlorite as oxidant. Catal Commun. Chem Int Ed. 2008;47:6367‐6370. 2014;45:114‐117. 31. Ebner DC, Bagdanoff JT, Ferreira EM, et al. The palladium‐cat- 46. Zhang Y, Gao B, Zhou Q, Zhao J. Oxidative kinetic resolution of – alyzed aerobic kinetic resolution of secondary alcohols: reaction secondary alcohols with salen Mn(III)/NBS/NaClO system. ‐ development, scope, and applications. Chem A Eur J. Catal Lett. 2014;144:1797 1802. 2009;15:12978‐12992. 47. Han F, Zhao J, Zhang Y, Wang W, Zuo Y, An J. Oxidative kinetic ‐ ‐ – 32. Liu SJ, Liu L, Shi M. Preparation of novel axially chiral NHC‐ resolution of 1 phenylethanol catalyzed by sugar based salen ‐ Pd(II) complexes and their application in oxidative kinetic reso- Mn(III) complexes. Carbohydr Res. 2008;343:1407 1413. lution of secondary alcohols. Appl Organomet Chem. 48. Brown MK, Blewett MM, Colombe JR, Corey EJ. Mechanism of 2009;23:183‐190. the enantioselective oxidation of racemic secondary alcohols catalyzed by chiral Mn(III)‐salen complexes. J Am Chem Soc. 33. Nishibayashi Y, Yamauchi A, Onodera G, Uemura S. Oxidative 2010;132:11165‐11170. kinetic resolution of racemic alcohols catalyzed by chiral ferrocenyloxazolinylphosphine−ruthenium complexes. J Org 49. Yan SH, Klemm D. Mild and efficient synthesis of 5,6‐diamino‐ Chem. 2003;68:5875‐5880. 5,6‐dideoxy‐1,2‐O‐isopropylidene‐3‐O‐methyl‐β‐L‐idofuranose: precursor of the first carbohydrate‐derived chiral Mn(III)–salen 34. Nakamura Y, Egami H, Matsumoto K, Uchida T, Katsuki T. Aer- complex. Tetrahedron. 2002;58:10065‐10071. obic oxidative kinetic resolution of racemic alcohols with bidentate ligand‐binding Ru(salen) complex as catalyst. Tetrahe- 50. Chatterjee D, Basak S, Riahi A, Muzart J. Asymmetric epoxida- ‐ dron. 2007;63:6383‐6387. tion of alkenes with tert butyl hydroperoxide catalyzed by a novel chiral complex of manganese(III) containing a sugar based 35. Tanaka H, Nishikawa H, Uchida T, Katsuki T. Photopromoted tridentate Schiff‐base ligand. Catal Commun. 2007;8:1345‐1348. Ru‐catalyzed asymmetric aerobic sulfide oxidation and epoxida- 51. Kureshy RI, Ahmad I, Khan NH, Abdi SHR, Pathak K, Jasra RV. tion using water as a proton transfer mediator. J Am Chem Soc. Chiral Mn(III) salen complexes covalently bonded on modified 2010;132:12034‐12041. MCM‐41 and SBA‐15 as efficient catalysts for enantioselective epox- 36. Li YY, Zhang XQ, Dong ZR, Shen WY, Chen G, Gao JX. Kinetic idation of nonfunctionalized alkenes. J Catal. 2006;238(1):134‐141. resolution of racemic secondary alcohols catalyzed by chiral 52. Kureshy RI, Ahmad I, Khan NH, Abdi SHR, Pathak K, Jasra RV. diaminodiphosphine−Ir(I) complexes. Org Lett. 2006;8:5565‐5567. Encapsulation of a chiral MnIII(salen) complex in ordered meso- 37. Arita S, Koike T, Kayaki Y, Ikariya T. Aerobic oxidative kinetic porous silicas: an approach towards heterogenizing asymmetric resolution of racemic secondary alcohols with chiral bifunctional epoxidation catalysts for non‐functionalized alkenes. Tetrahe- amido complexes. Angew Chem. 2008;120:2481‐2483. dron: Asymm. 2005;16:3562‐3569. 38. Yamada T, Higano S, Yano T, Yamashita Y. Cobalt‐catalyzed 53. Kureshy RI, Khan NH, Abdi SHR, et al. Enantioselective epoxi- oxidative kinetic resolution of secondary benzylic alcohols with dation of nonfunctionalized alkenes catalyzed by recyclable molecular oxygen. Chem Lett. 2009;38:40‐41. new homochiral dimeric Mn(III) salen complexes. J Catal. 2004;224:229‐235. 39. Kunisu T, Oguma T, Katsuki T. Aerobic oxidative kinetic resolution of secondary alcohols with naphthoxide‐bound iron(salan) 54. Kureshy RI, Khan NH, Abdi SHR, Singh S, Ahmad I, Jasra RV. complex. J Am Chem Soc. 2011;133:12937‐12939. Catalytic asymmetric epoxidation of non‐functionalised alkenes using polymeric Mn(III) salen as catalysts and NaOCl as oxidant. 40. Hamada T, Irie R, Mihara J, Hamachi K, Katsuki T. J Mol Catal A‐Chemical. 2004;218:141‐146. Highly enantioselective benzylic hydroxylation with concave type of (salen)manganese (III) complex. Tetrahedron. 55. Kureshy RI, Khan NH, Abdi SHR, et al. Chiral Mn(III) ‐ 1998;54:10017‐10028. salen complex catalyzed enantioselective epoxidation of nonfunctionalized alkenes using urea‐H O adduct as oxidant. ‐ ‐ ‐ 2 2 41. Sun W, Wang H, Xia C, Li J, Zhao P. Chiral Mn(Salen) complex J Catal. 2003;219(1):1‐7l. catalyzed kinetic resolution of secondary alcohols in water. 56. Kureshy RI, Khan NH, Abdi SHR, Ahmad I, Singh S, Jasra RV. Angew Chem Int Ed. 2003;42(9):1042‐1044. Immobilization of dicationic Mn(III) salen in the interlayers of 42. Li Z, Tang ZH, Hu XX, Xia CG. Insight into the mechanism of montmorillonite clay for enantioselective epoxidation of non‐ oxidative kinetic resolution of racemic secondary alcohols by functionalised alkenes. Catal Lett. 2003;91:207‐210. using manganese(III)(salen) complexes as catalysts. Chemistry 57. Liu X, Tang N, Liu W, Tan M. Synthesis, catalytic activity and A Eur J. 2005;11:1210‐1216. recycle of novel dimeric salen‐Mn(III) complexes in asymmetric 43. Cheng Q, Deng F, Xia C, Sun W. The Mn(Salen)‐catalyzed oxidative epoxidation. J Mol Catal A: Chem. 2004;212:353‐358. kinetic resolution of secondary alcohols: reaction development 58. Sun Y, Tang N. Enantioselective epoxidation of olefins catalyzed and scope. Tetrahedron: Asymmetry. 2008;19:2359‐2362. by chiral dimeric and partially water‐soluble monomeric salen‐ 44. Xu D, Wang S, Shen Z, Xia C, Sun W. Enantioselective oxidation Mn(III) complexes in the presence of novel co‐catalysts. J Mol of racemic secondary alcohols catalyzed by chiral Mn(III)‐salen Catal A: Chem. 2006;255:171‐179. AHMAD ET AL. 13

59. Wang D, Wang M, Zhang R, et al. Asymmetric epoxidation of 69. Kantam ML, Ramania T, Chakrapani L, Choudary BM. Oxida- styrene and chromenes catalysed by dimeric chiral (pyrrolidine tive kinetic resolution of racemic secondary alcohols catalyzed salen)Mn(III) complexes. Appl Catal Gen. 2006;315:120‐127. by resin supported sulfonato‐Mn(salen) complex in water. J Mol Catal A: Chem. 2007;274:11‐15. 60. Pathak K, Ahmad I, Abdi SHR, Kureshy RI, Khan NH, Jasra RV. Oxidative kinetic resolution of racemic secondary alcohols cata- 70. Riisager A, Fehrmann R, Flicker S, Van Hal R, Haumann M, lyzed by recyclable chiral dimeric Mn(III) salen catalysts. J Mol Wassercheid P. Very stable and highly regioselective supported Catal A: Chemical. 2007;274:120‐126. ionic‐liquid‐phase (SILP) catalysis: continuous‐flow fixed‐bed hydroformylation of propene. Angew Chem Int Ed. 2005;44:815‐819. 61. Kureshy RI, Ahmad I, Pathak K, Khan NH, Abdi SHR. Easily recyclable chiral polymeric Mn(III) salen complexes for oxida- 71. Mehnert CP, Mozeleski EJ, Cook RA. Supported ionic liquid tive kinetic resolution of racemic secondary alcohols. Chirality. catalysis investigated for hydrogenation reactions. Chem ‐ 2007;19:352‐357. Commun. 2002;3010 3011. 72. Gruttadauria M, Riela S, Meo PL, D'Anna F, Noto R. Supported 62. Sun W, Wu X, Xia C. Enantioselective oxidation of secondary ionic liquid asymmetric catalysis. A new method for chiral cata- alcohols catalyzed by soluble chiral polymeric [N′,N′‐ lysts recycling. The case of proline‐catalyzed aldol reaction. Bis(salicylidene)ethane‐1,2‐diaminato(2−)]manganese(III) Tetrahedron Lett. 2004;45:6113‐6116. ([MnIII(salen)]) type complexes in a biphasic system. Helv Chim Acta. 2007;90:623‐626. 73. Lou L, Yu K, Ding F, Zhou W, Peng X, Liu S. An effective approach for the immobilization of chiral Mn(III) salen com- 63. Bera PK, Maity NC, Abdi SHR, Khan NH, Kureshy RI, Bajaj HC. plexes through a supported ionic liquid phase. Tetrahedron Macrocyclic Mn(III) salen complexes as recyclable catalyst for Lett. 2006;47:6513‐6516. oxidative kinetic resolution of secondary alcohols. Appl Catal A: General. 2013;467:542‐551. 74. Sahoo S, Kumar P, Lefebvre F, Halligudi SB. Oxidative kinetic resolution of alcohols using chiral Mn–salen complex 64. Maity NC, Bera PK, Saravanan S, et al. Diethyl tartrate immobilized onto ionic liquid modified silica. Appl Catal A: Gen- – linked chiral macrocyclic manganese(III) salen complex for eral. 2009;354:17‐25. enantioselective epoxidation of olefins and oxidative kinetic res- 75. Sahoo S, Kumar P, Lefebvre F, Halligudi SB. A chiral Mn(III) olution of racemic secondary alcohols. ChemPlusChem. salen complex immobilized onto ionic liquid modified mesopo- 2014;79:1426‐1433. rous silica for oxidative kinetic resolution of secondary 65. Li C, Zhao J, Tan R, et al. Recyclable ionic liquid‐bridged chiral alcohols. Tetrahedron Lett. 2008;49:4865‐4868. dimeric salen Mn(III) complexes for oxidative kinetic resolution 76. Ren L, Li L, Li Y, et al. Mesoporous helical silica immobilizing of racemic secondary alcohols. Catal Commun. 2011;15:27‐31. manganese(III) salen complex for oxidative kinetic resolution 66. Tan R, Dong Y, Peng M, Zheng M, Yin D. Thermoresponsive chi- of secondary alcohols. J Porous Mater. 2016;23:19‐33. ral salen Mn(III) complexes as efficient and reusable catalysts for the oxidative kinetic resolution of secondary alcohols in water. Appl Catal A: General. 2013;458:1‐10. How to cite this article: Ahmad I, Shagufta, 67. Tan R, Yin D, Yu N, Jin Y, Zhao H, Yin D. Ionic liquid‐function- AlMallah AR. Advances in asymmetric oxidative alized salen Mn(III) complexes as tunable separation catalysts for kinetic resolution of racemic secondary alcohols enantioselective epoxidation of styrene. JCatal. 2008;255:287‐295. catalyzed by chiral Mn(III) salen complexes. – 68. Tan R, Yin D, Yu N, Zhao H, Yin D. Easily recyclable polymeric Chirality. 2017;1 13. https://doi.org/10.1002/ ionic liquid‐functionalized chiral salen Mn(III) complex for chir.22768 enantioselective epoxidation of styrene. JCatal. 2009;263:284‐291.