Received: 28 March 2017 Revised: 30 June 2017 Accepted: 12 August 2017

REV I EW ART I C L E

DOI: 10.1002/chir.22768

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

Irshad Ahmad1 | Shagufta1 | Abdul Rahman AlMallah2

1Department of Mathematics and NaturalSciences, School of Arts and Sciences,American University of Ras Al Khaimah,Ras Al Khaimah, UAE2Department of Chemical and PetroleumEngineering, School of Engineering,American University of Ras Al Khaimah,Ras Al Khaimah, UAE

CorrespondenceIrshad Ahmad and Shagufta, Departmentof Mathematics and Natural Sciences,School of Arts and Sciences, AmericanUniversity of Ras Al Khaimah, Ras AlKhaimah, UAE.Emails: iahmad@aurak.ac.ae;shagufta.waseem@aurak.ac.ae

Funding informationSchool of Graduate Studies and Research,American University of Ras Al Khaimah,Ras Al Khaimah, UAE, Grant/AwardNumber: AAS/001/16

Chirality. 2017;1–13.

Abstract

Enantiomerically pure secondary alcohols are essential compounds in

organic synthesis and are used as chiral auxiliaries and synthetic intermedi-

ates in the pharmaceutical, agrochemical, and fine chemical industries. One

of the attractive and practical approaches to achieving optically pure second-

ary alcohols is oxidative kinetic resolution of racemic secondary alcohols

using chiral Mn(III) salen complexes. In the last decade, several chiral

Mn(III) salen complexes have been reported with excellent enantioselectivity

and activity in the homogeneous and heterogeneous catalysis of the oxida-

tive kinetic resolution of racemic secondary alcohols. This review article is

an overview of the literature on the recent development of chiral Mn(III)

salen complexes for oxidative kinetic resolution of racemic secondary alco-

hols. The catalytic activity of monomeric, dimeric, macrocyclic, polymeric,

and silica/resin supported chiral Mn(III) salen complexes is discussed

in detail.

KEYWORDS

chiral catalysts, Mn(III) salen complex, oxidative kinetic resolution, racemic secondary alcohols

1 | INTRODUCTION

The oxidation of alcohols is one of the most general andwell‐explored reactions in organic chemistry.1,2 Highlyefficient and selective oxidative methods of alcohols tothe corresponding ketones, aldehydes, and carboxylicacids have been developed.3-12 Application of these oxi-dation methods to the kinetic resolution of racemic alco-hols provides optically active (chiral) alcohols togetherwith carbonyl compounds. These enantiopure secondaryalcohols are important chiral auxiliaries and buildingblocks for the synthesis of important pharmaceutical,agrochemical, and fine chemicals.13-16 The use of enzymecatalysts for kinetic resolution through selective oxida-tion of one of the enantiomers is well known in the liter-ature.17-21 Recently, researchers have developed several

wileyonlinelibrary.com/journa

effective non–enzymatic catalysts for the oxidativekinetic resolution (OKR) of racemic alcohols.22-25 In thelast two decades, different chiral ligands with palla-dium,26-32 ruthenium,33-35 iridium,36,37 cobalt,38 iron,39

and manganese (Mn) metal ions have been used as cata-lysts for OKR of racemic secondary alcohols. In particu-lar, chiral Mn(III) salen complexes are considered to bethe most effective catalyst for the OKR of racemicsecondary alcohols (Figure 1) because of their facilesynthesis and catalytic reaction condition with high turn-over frequency.

To our knowledge, in the last ten years, no review arti-cle has been published on chiral Mn(III) salen complexcatalyzed asymmetric oxidative kinetic resolution of race-mic secondary alcohols. Therefore, we have compiled andreviewed the different chiral Mn(III) salen complexes

© 2017 Wiley Periodicals, Inc.l/chir 1

FIGURE 1 Oxidative kinetic resolution of racemic secondary

alcohols using chiral Mn(III) salen complexes

2 AHMAD ET AL.

reported in the last decade for the OKR of racemic second-ary alcohols. We believe that this review will provide themost complete insight into these complexes for use byresearchers working in this area and facilitate thedevelopment of novel catalysts for OKR of secondaryalcohols.

2 | CHIRAL Mn(III) SALENCOMPLEXES FOR OKR OF RACEMICSECONDARY ALCOHOLS

As evident from the literature, various types of chiralMn(III) salen complexes are utilized for the OKR of sec-ondary alcohols. Consequently, this review article isbroadly classified into two categories: homogeneous andheterogeneous chiral Mn(III) salen complexes, which arefurther subcategorized into nonrecyclable, recyclable,and silica/resin supported chiral Mn(III) salen complexes,respectively.

FIGURE 2 Nonrecyclable chiral Mn(III) salen complexes for OKR of

2.1 | Homogeneous chiral Mn(III) salencomplexes

2.1.1 | Nonrecyclable chiral Mn(III) salencomplexes

In 1998, Katsuki and his coworkers utilized binol‐derivedMn(III) salen complexes for the asymmetric oxidation ofalcohols with iodosobenzene (PhIO) as the co‐oxidant.40

The reaction results were not very promising and showedlow yield and moderate enantioselectivities. Subsequentlyin 2003, W. Sun et al utilized chiral Mn(III) salen com-plexes as effective catalysts for the asymmetric oxidationof secondary alcohols to ketones in the presence of theco‐oxidant diacetoxyiodobenzene (PhI(OAc)2) and thuscontributed significantly to the research of OKR of race-mic secondary alcohols.41 When the reaction was carriedout with catalyst 1 (Figure 2) in the solvent dichlorometh-ane, only 2% ee (enantiomeric excess) was obtained.

However, change of solvent to water increases theenantiomeric excess to 8.9%. Considering the insolubilityof both the substrate and catalyst in water, a phase trans-fer catalyst such as tetraethylammonium bromide wasintroduced in the aqueous system, and results werepromising with 85.2% ee and 51.7% conversion (Table 1).The replacement of the tert‐butyl groups at the 5,5′‐posi-tions in complex 1 with methyl groups lowers theenantioselectivity; however, the Mn(III) salen complex 2

secondary alcohols

AHMAD ET AL. 3

(Figure 2), derived from (R,R)‐diphenylethylenediamine,provided 88.1% ee and 62.5% conversion (Table 1). It wasobserved that complex 2 was more effective with substrate1‐phenyl ethanol and 1‐phenyl‐2‐propanol. However thereplacement of one of the small substituents in secondary

TABLE 1 Oxidative kinetic resolution of various secondary alcohols u

Entry Catalysts Substrate Additive/ Oxidant

1 1 (2 mol%) N(C2H5)4Br (8 mol%)/

2 2 (2 mol%) N(C2H5)4Br (8 mol%)/

3 3 (2 mol%) KBr (4 mol%)/ PhI(OA

4 4 (2 mol%) N(C2H5)4Br (8 mol%)/

5 4 (1 mol%) N(C2H5)4Br (8 mol%)/

6 4 (1 mol%) 1‐Butyl‐2,6‐dimethyl pbromide (8 mol%)/

7 5a (1 mol%) Without additive/ NB

8 5b (1 mol%) Without additive/ NB

9 3 (1 mol%) Br2 (6 mol%)/ NaClO

10 3 (1 mol%) Br2 (6 mol%)/ NaClO

11 3 (1 mol%) NBS (8 mol%) / NaClO

12 6 (2 mol%) KBr (8 mol%)/ PhI(OA

alcohols frommethyl by an ethyl or a larger group resultedin a sharp decrease in the enantiomeric excess ofthe product. Additionally, with complex 1 higherenantioselectivities were perceived for substrates bearingsubstituents on the aromatic ring. The results revealed that

sing nonrecyclable chiral Mn(III) salen complexes

Conversion (%) ee (%) Reference

PhI(OAc)2 51.7 85.2 Sun et al41

PhI(OAc)2 62.5 88 Sun et al41

c)2 64 96 Li et al42

PhI(OAc)2 59 24.2 Cheng et al43

PhI(OAc)2 68 68 Cheng et al43

yridiniumPhI(OAc)2

77.8 98.6 Cheng et al43

S 64 99 Xu et al44

S 61 85 Xu et al44

46.9 84.3 Zhang et al45

64.5 95.4 Zhang et al45

62.1 >99.9 Zhang et al46

c)2 60 89 Han et al47

4 AHMAD ET AL.

the steric hindrance at the 5,5′‐positions of the catalyst andelectronic cooperation between the catalyst and the sub-strate plays important role on the enantioselectivity ofthe reaction. Next, a detailed study was performed on theeffect of additives, organic solvent, and the substituents ofchiral salen ligands on the enantioselectivities of the reac-tions.42 The study provided an effective system for the oxi-dative kinetic resolution of secondary alcohols by usingMn(III) salen complex (Jacobsen catalyst, 3) (Figure 2) ascatalyst and PhI(OAc)2 with potassium bromide (KBr) asan additive (yielding 64% conversion and 96% ee)(Table 1). In addition, the possiblemechanism of the oxida-tive kinetic resolution was explained with the help of UV/Vis spectroscopy and electrospray ionization mass spec-trometry (ESI‐MS). The results revealed the significanceof the bromide ion in the enantioselective oxidative reac-tion and proved the oxygen transfer from PhI(OAc)2 tothe Mn(III) salen via oxo‐manganese species formed dur-ing oxidative kinetic resolution.

Moreover, to explore the effect of substituents on thechiral Mn(III) salen catalysts, a series of chiral Mn(III)salen complexes with various substituents on the aromaticrings and different diamine backbones were synthesizedand screened for OKR of racemic secondary alcohols.43

For the asymmetric oxidative kinetic resolution of race-mic 1‐phenyl‐1‐propanol in the presence of PhI(OAc)2and tetra‐butylammonium bromide [N(C4H9)4Br], themost active catalyst was chiral Mn(III) salen complex 4(Figure 2) having tert‐butyl groups at the 5,5′‐positionsof salicylaldehyde. Catalyst 4 exhibited only 24.2% ee and59% conversion (Table 1). The enantiomeric excess valuewas increased by using additives such as KBr and differentquaternary ammoniums with a bromonium ion. Themaximum increases in the enantiomeric excess valuewere observed when an additive (1‐butyl‐2,6‐dimethylpyridinium bromide) was used (68% ee and conversion)(Table 1). Then this methodology was applied toother secondary alcohols such as 1‐phenyl‐1‐butanol, 1‐phenyl‐1‐pentanol, 1‐cyclohexyl‐1‐propanol, and 1‐cyclohexyl‐1‐pentanol. Under the optimal condition,cyclohexyl‐1‐propanol accomplished the maximum enan-tiomeric excess (98.6%) and conversion (77.8%) (Table 1).

In 2010, E. J. Corey et al proposed the mechanism ofthe enantioselective oxidation of racemic secondary alco-hols catalyzed by chiral Mn(III) salen complexes.48 Theproposed pathway suggested the formation of positivebromine species HOBr under the reaction conditions byoxidation of bromide ion with PhI(OAc)2, and then anMn(V) salen complex is generated in the presence of thepositive bromine species. Bearing in mind these results,W. Sun and his coworkers in 2012 introduced N‐bromosuccinimide (NBS) as the oxidant to produce HOBr.44 Inthis study, benzylic alcohols with ortho/meta substituted

aromatic were also investigated for asymmetric oxidativekinetic resolution reaction. All the chiral Mn(III) salencatalysts in the presence of the NBS oxidant showed sig-nificant enantiomeric excess and conversion. Catalyst 5a(Figure 2) bearing a 5‐tert‐butyl group showed the bestresults with 99% ee and 64% conversion (Table 1) on sub-strate (±)‐1‐(2‐bromophenyl)ethanol. In comparison, cat-alyst 5b (Figure 2), without any ligand substituent in thearomatic ring and under similar reaction conditions,exhibited only 85% ee and 61% conversion (Table 1).Moreover, meta‐substituted benzylic alcohols affordedhigh enantioselectivities, a higher rate of reaction, com-pared to the ortho‐substituted substrates.

The main disadvantages of using PhI(OAc)2 as the oxi-dant during asymmetric oxidative kinetic resolution ofracemic secondary alcohols are that it is low atom‐effi-cient, expensive, and unfavorable to the environment,and thus, continuous efforts have been made to replace itwith a more efficient and less expensive oxidant. In thisrespect, Y. Zhang et al in 2014 performed the asymmetricoxidative kinetic resolution of racemic secondary alcoholscatalyzed by Jacobsen's chiral Mn(III) salen complex (3)(Figure 2) with sodium hypochlorite (NaOCl) as oxidant.45

The catalytic amount of Br2 was used as cycling agent, andthe reaction was conducted in the biphasic CH2Cl2/H2Omedium in the presence of potassium acetate, KOAc.The NaOCl used as oxidant is very inexpensive, readilyavailable, and showed promising results. The resultsrevealed that the electrostatic properties of the substitu-ents play an important role and have significant influenceon the reaction. The system works efficiently with all thescreened substrates, and good enantioselectivity wasobserved with 1‐phenyl ethanol (84.3% ee and 46.9% con-version) (Table 1) and its derivatives. The substrates bear-ing electron‐donating substituents at the para position,such as 1‐(4‐methoxyphenyl)‐1‐ethanol, were not veryeffective (19.7% ee and 29.8% conversion). The best result,ie, 95.4% ee and 64.5% conversion (Table 1), was obtainedwith substrate 1‐(4‐methylphenyl) ethanol. The system,under mild reaction conditions, was very effective withsecondary alcohols with fused‐rings. The main disadvan-tage of the system was the handling of the rather toxicand corrosive Br2, especially on a large scale. Hence, Br2was replaced with NBS as a cycling agent for the OKR ofalcohols catalyzed by 3.46 The reaction was performedwith NaClO as an oxidant in a biphasic CH2Cl2/H2Omedium in the presence of KOAc. The results revealed>99.9% ee and 62.1% conversion (Table 1) with the addi-tional advantages of low cost and easy product separationfrom the reaction mixture.

Considering the significance of sugar‐based chiralMn(III) salen complexes in the asymmetric epoxidationof unfunctional olefins,49,50 F. Han et al in 2008 explored

AHMAD ET AL. 5

the catalytic applicability of Mn(III) salen complexes withsugar moieties at C‐5(5′) of the salicylaldehyde on OKR ofracemic secondary alcohols.47 The synthesized chiralcatalysts showed good results for the OKR of 1‐phenylethanol using PhI(OAc)2 as an oxidant. Catalyst 6(Figure 2) exhibited promising results with 89% of enan-tiomeric excess and 60% conversion (Table 1). The effectsof solvent and additive on the asymmetric oxidativekinetic resolution of 1‐phenylethanol using catalyst 6were studied, and in the results, the 1:2 CH2Cl2‐wateremerged as the best solvent and KBr as the best additive.The sugars at the C‐5(5′) moiety of salicylaldehydeshowed a significant effect on the enantioselectivity ofOKR of 1‐phenylethanol, and it was established that thesugar with same rotation direction of polarized light asthe Diimine Bridge in the complex could enhance the chi-ral induction, whereas the sugars with the opposite onewould reduce that of the corresponding complex.

2.1.2 | Recyclable chiral Mn(III) salencomplexes

Homogeneous chiral Mn(III) salen complexes arereported with high enantioselectivity for OKR of racemicsecondary alcohols, yet their application is associated withseveral disadvantages, which includes difficulty in the sep-aration of product and catalyst, as well as low stability.Because of the expensive nature of chiral catalysts, thereis a clear requirement for catalyst recycling. In recentyears, chiral dimeric Mn(III) salen complexes have beenwidely used as recyclable catalysts in asymmetric epoxida-tion reactions by researchers.51-59 Encouraged with theseresults, S. H. R. Abdi and his coworkers in 2007 utilizedthe chiral dimeric Mn(III) salen complexes in asymmetricoxidative kinetic resolution of racemic secondary alcoholsusing PhI(OAc)2 as an oxidant with KBr as an additive atroom temperature.60 Among all the studied chiral Mn(III)salen complexes for the OKR of racemic 1‐phenyl ethanol,the maximum enantioselectivity (99%) and conversion(62%) (Table 2) was observed with dimeric‐Mn(III) salencomplex (7) (Figure 3) with 2 mol% catalyst loading. How-ever, a further reduction in enantioselectivity wasobserved with 5 mol% (95% ee) and 0.2 mol% (90% ee) cat-alyst loading with increase in reaction time. The catalyticeffect of complex 7 for various racemic secondary alcoholswas studied. It was observed that the presence of substitu-ent at the para position of 1‐phenyl ethanol favors thereaction with enantioselectivity 95‐97%. Additionally, thesubstituent at the ortho position of the phenyl ring orextension of the alkyl chain reduces the enantioselectivitybecause of the steric hindrance of the substituent group,which prohibits the interaction of the substrate with thechiral center having the catalytically active metal center

of the complex. A moderate to good enantioselectivity ofrating of 73‐85% was obtained with bulkier secondaryalcohols such as 1‐naphthyl ethanol and 2‐naphthyl etha-nol; however, the rate of these reactions was very low. Theeffect of solvent and various additives was examined byusing complex 7 for OKR of racemic secondary alcoholsof 1‐phenyl ethanol as a representative substrate. Usingwater alone as solvent showed poor results (36% ee and47% conversion), whereas the 1:2 of CH2Cl2 and water sol-vent system showed the best results (99% ee and 62% con-version). The most effective additive was LiBr, whichproduced 97% ee and 64% conversion. The catalyst waseasily recovered by addition of a nonpolar solvent suchas n‐hexane, which precipitated out complex 7 becauseof its higher molecular weight and was recycled up to fivetimes with the retention of enantioselectivity.

Additionally, R. I. Kureshy et al utilized the chiral poly-meric Mn(III) salen complexes for OKR of racemic second-ary alcohols using PhI(OAc)2 as oxidant.61 The reactionwas also examined with different additives in water/organicsolvent mixtures at room temperature. Catalyst 8 (Figure 3)showed promising enantioselectivity and conversion withdifferent secondary alcohols, and the best results wereobtainedwith 4‐fluorophenylethanol (>99% ee and 63% con-version) and 1‐phenyl ethanol (98% ee and 62% conversion)(Table 2). To study the effect of catalyst loading the 1‐phenylethanol was used as a substrate. With the increasein catalyst loading form 2 to 5 mol%, only slight decreasein enantioselectivity was observed (98% to 94% ee, respec-tively). In comparison to catalyst loading of 2 mol%, the 0.6and 0.2 mol% exhibited reduction in enantioslectivity (96%and 91% ee, respectively). The additive and solvent natureshowed a notable effect on the activity and enantioselectivityof the chiral Mn(III) salen complex. For the OKR of racemicsecondary alcohols of 1‐phenyl ethanol using complex 8 ascatalyst and KBr as an additive, good to excellentenantioselectivity (83‐94%)was observedwhen a solvent liketoluene, 1, 2‐dichloromethane and chloroform mixed withwater was used. The use of different bromide salts such asNaBr, LiBr, and hexyl pyridinium bromide as an additivewith a dichloromethane and water solvent system showedgood enantioselectivity (87‐89%). The reaction did not workin the absence of an additive as well as with KCl as anadditive, thus indicating the significance of the bromideion for OKR. Catalyst 8 was easily and efficiently recyclableup to five times with retention of enantioselectivity.

In 2007 Sun et al also screened polymeric salen manga-nese (III)‐type complexes for the oxidative kinetic resolu-tion of racemic secondary alcohols.62 The polymericchiral Mn(III) salen catalyst 8 showed excellent results(95.2% ee and 58.8% conversion) (Table 2) in the OKR of1‐phenyl ethanol when H2O and CH2Cl2 were used as sol-vent. Catalyst 8 was tested for different secondary aromatic

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

1 7 (2 mol%) KBr (4 mol%)/ PhI(OAc)2 62 99 Pathak et al60

2 8 (0.6 mol%) KBr (1.2 mol%)/ PhI(OAc)2 63 >99 Kureshy et al61

3 8 (2 mol%) KBr (1.2 mol%)/ PhI(OAc)2 62 98 Kureshy et al61

4 8 (2 mol%) KBr (8 mol%)/ PhI(OAc)2 58.8 95.2 Sun et al62

5 9 (2 mol%) KBr (4 mol%)/ PhI(OAc)2 63 83 Bera et al63

6 9 (2 mol%) KBr (4 mol%)/ PhI(OAc)2 58 99 Bera et al63

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

9 11 (1 mol%) KBr (6 mol%)/ PhI(OAC)2 62 >99 Li et al65

10 11 (1 mol%) KBr (6 mol%)/ PhI(OAC)2 61 95 Li et al65

11 12 (1.5 mol%) KBr (6 mol%)/ PhI(OAC)2 63 96 Tan et al66

12 12 (1.5 mol%) KBr (6 mol%)/ PhI(OAC)2 63 94 Tan et al66

6 AHMAD ET AL.

and aliphatic racemic alcohols for the OKR reaction. Thearomatic alcohols bearing various para substituents exhib-ited high enantioselectivity (up to 98%) whereas withortho‐substituted aromatic alcohols no enantioselectivitywas observed. The polymeric catalyst also showed goodenantioselectivity (up to 99%) with secondary aliphaticracemic alcohols. The polymeric catalyst was easily recov-ered from the reaction mixture by adding hexane andreused four times with only slight loss in enantioselectivity.

Subsequently S. H. R. Abdi and his coworkers in 2013tried to reduce the cost of catalysts for practical applica-tions by increasing their reusability. In this regard, aseries of macrocyclic Mn(III) salen complexes were syn-thesized and evaluated for the OKR of racemic secondaryalcohols with PhI(OAc)2 and NBS as co‐oxidant in water‐dichloromethane solvent mixture.63 The maximumenantioselectivity (up to 99%) for the tested racemic sec-ondary alcohols was observed when homopiperazine

FIGURE 3 Recyclable chiral Mn(III)

salen complexes for OKR of secondary

alcohols

AHMAD ET AL. 7

macrocyclic complex 9 (Figure 3) was used as the catalystwith PhI(OAc)2 and N‐bromo succinimide (NBS) as oxi-dants and in presence of KBr as an additive and dichloro-methane/water solvent mixture. With catalyst 9 andPhI(OAc)2 as an oxidant, the 1‐phenylethanol substrateshowed 83% ee and 63% conversion (Table 2); and the bestresult was obtained with the aliphatic alcohol 2‐butanol,which yielded 99% ee and 58% conversion (Table 2). Incomparison to PhI(OAc)2, using NBS as an oxidant wasmore effective and exhibited higher enantioselectivityeven for the OKR of sterically hindered racemic secondaryalcohols. Catalyst 9 was easily precipitated out by addinghexane and was recycled efficiently seven times withoutany loss in activity and enantioselectivity.

Later a series of macrocyclic diethyl tartrate linkedMn(III) salen complexes were synthesized and utilizedfor asymmetric epoxidation of olefins. Catalyst 10(Figure 3) showed promising results for asymmetric epox-idation of olefins, and therefore, it was studied for OKR ofracemic secondary alcohols.64 The effect of catalyst load-ing and solvent was examined for asymmetric oxidativekinetic resolution of racemic 1‐phenylethanol. The resultrevealed that on substrate 1‐phenylethanol, the bestenantioselectivity (95%) and conversion (66%) rating was

observed when the catalyst loading was 1.5 mol%(Table 2). However, either increasing or decreasing theloading of catalyst from 1.5 mol% resulted in a decreasein enantioselectivity. No improvement in enatioselectivitywas observed by changing the solvent from dichlorometh-ane to chloroform or dichloroethane. Therefore, the opti-mum reaction condition for the OKR of various secondaryalcohols was considered to be a catalyst loading of1.5 mol% in a biphasic solvent mixture of dichlorometh-ane/water at ambient temperature (20°C). Catalyst 10 effi-ciently catalyzed the para‐substituted benzylic secondaryalcohols with good enantioselectivity. It was observedthat for the electron‐withdrawing groups, such asfluoro and chloro groups, the catalyst provided highenantioselectivity, that is, 96% and 93% ee, respectively,whereas when the electron‐donating methyl (84% ee)and very bulky bromo (85% ee) groups were present, littledecrease in enantioselctivity was noticed. Among thenon–benzylic secondary alcohols, the 1‐phenyl‐2‐propanol showed high enantioselectivity (97% ee) with65% conversion (Table 2). Catalyst 10 recycling studieswas performed only for epoxidation reaction of alkenes,and results showed that it can be reused up to four timeswith no loss in activity.

FIGURE 5 PNIPAAm‐functionalized chiral Mn(III) salen

complex for OKR of secondary alcohols

8 AHMAD ET AL.

The higher catalytic efficacy and recovery of ionic liq-uids (ILs) bonded to the chiral Mn(III) salen complex forenantioselective 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 ofracemic secondary alcohols.65 The N,N‐dialkylimidazo-lium ionic liquids were used to bridge chiral dimericMn(III) salen complexes at the C‐5 position of thesalicylaldehyde moieties. Among all the complexes tested,the IL‐bridged salen Mn(III) complex 11 (Figure 4) exhib-ited the most promising enantioselectivity (>99%) andconversion (62%) in the OKR of 1‐phenyl ethanol usingPhI(OAc)2 as an oxidant (Table 2). It was proposed thatthe higher enantioselectivity could be due to the good sol-ubility of complex 11 in the solvent, ie, dichloromethane,and the interaction of the two catalytic active sites of com-plex 11. Further catalytic activities of this complex for theOKR of different racemic secondary alcohols were stud-ied. The result suggested the importance of steric hin-drance around the hydroxyl group of substrate in theoverall efficiency of the kinetic resolution. The presenceof substituted groups causes steric hindrance and preventsthe hydroxyl group in the substrate approaching the cata-lytic metal center of complex 11 and therefore resulted inthe decrease in enantioselectivity. Complex 11 effectivelycatalyzed the OKR of aliphatic alcohol 2‐pentanol with95% enantioselectivity and 61% conversion (Table 2). Onthe other hand, for the heterocyclic compounds such as2‐furylethanol, the chiral Mn(III) salen complex 11 wastotally inactive. The complexes showed promising recycla-ble results and were easily recycled for five successive cat-alytic runs and considered as promising chiral catalystsfor the OKR of racemic secondary alcohols.

Further to reduce the catalyst separation and solubilitylimitation problems in aqueous reactions, R. Tan et alin 2013 reported the synthesis of a new series ofnovel PNIPAAm‐functionalized chiral Mn(III) salencomplexes possessing a thermoregulated phase‐transferfunction.66 The thermoresponsive polymer PNIPAAm[poly(Nisopropylacrylamide)] was prepared through free‐radical polymerization and then introduced to the frame-work of the chiral Mn(III) salen complexes. In complex 12(Figure 5), the thermoresponsive polymer PNIPAAm wascovalently connected to one side of the 5‐position in thesalen ligand, whereas in other complexes by axially

grafting onto the metal center. The catalysts were exam-ined for OKR of various racemic secondary alcohols withPhI(OAc)2 and KBr. When the OKR reaction was per-formed with pure water, the thermoresponsive catalystsworked efficiently and thus eliminated the need for anyorganic solvent. The OKR of 1‐phenyl ethanol in the aque-ous medium with complex 12 (1.5 mol%) providedenantioselectivity up to 96% ee and conversion 63%(Table 2). The results were promising, and the values werequite high when compared to the results obtained overneat complex and in solvent water (56% ee and 54%conversion). The best result was obtained with 1.5 mol%catalyst, and further changes in catalyst loading exhibitedno enhancement in the enantioselectivity.

The presence of thermoresponsive PNIPAAm groupsprovided the inverse temperature‐dependent water solu-bility to the PNIPAAm‐based complexes. During oxidativekinetic resolution (OKR) in water, the catalyst showedsignificant thermoregulated phase‐separable catalysis(TPSC), and by simply regulating the temperature ofaqueous medium, it was easily recovered for reuse. Thecomplexes showed excellent recycling efficiency of up tofour times without noticeable loss of enantioselectivityand activity. The study revealed that the steric hindrancearound the hydroxyl group of the substrates played a vitalrole in the overall efficiency of the OKR of racemic sec-ondary alcohols. The aliphatic alcohol 2‐pentanol showedpromising results (94% ee and 63% conversion) (Table 2)whereas with heterocyclic compound 2‐furylethanol nooxidation was observed.

2.2 | Heterogeneous chiral Mn(III) salencomplexes

Continuous efforts are being made by researchers toimmobilize 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-neous counterparts. M. L. Kantam and coworkers in2007 used the resin‐supported sulfonato‐Mn (salen) com-plex [(R,R)‐1] (13) (Figure 6) for the asymmetric oxidativekinetic resolution of racemic secondary alcohols.69 The 1‐phenyl ethanol was oxidized in the presence of resin (R,R)‐1 catalyst (13) and PhI(OAc)2 as an oxidant at roomtemperature. The influence of solvent and additive onthe OKR of 1‐phenyl ethanol was examined, and the max-imum enantioselectivity (55.3%) and conversion (61.5%)(Table 3) of the product was observed when water wasused as solvent and KBr as an additive. It was suggestedthat the low enantioselectivity was due to the introductionof –SO3Na groups at the 5,5′‐position. The resin (R,R)‐1catalyst (13) was recovered by simple filtration and reusedthree times with consistent activity.

The supported ionic liquid phase (SILP) catalysts areexamined for various organic reactions such ashydroformylation, hydrogenation, asymmetric aldol con-densation and asymmetric epoxidation.70-73 SILP allowsthe application of fixed‐bed reactors and makes the

FIGURE 6 Resin supported sulfonato‐Mn(III) salen complex

[(R,R)‐1] (13) for the OKR of racemic secondary alcohols

TABLE 3 Oxidative kinetic resolution of various secondary alcohols u

Entry Catalysts Substrate Additive/ oxidan

1 13 (1 mol%) KBr (4 mol%)/ PhI

2 14 (0.12 mol%) KBr (4 mol%)/ PhI

3 16a (2 mol%) KBr (8 mol%)/ PhI

4 16a (2 mol%) KBr (8 mol%)/ H2

separation and recycling of catalyst more convenient.Therefore, S. B. Halligudi and his coworkers in 2008immobilized chiral Mn(III) salen complex onto an ionicliquid‐modified mesoporous silica SBA‐15 through SILPstrategy.74 The ionic liquid modified mesoporous silica,SBA‐15 catalyst (14) (Figure 7) with long range order,large monodispersed mesopores and thick walls wassynthesized. The use of catalyst 14 was examined forthe OKR of racemic secondary alcohols and exhibitedexcellent enantioselectivity (99%) and conversion (63%)with 1‐phenyl ethanol as substrate and KBr as anadditive (Table 3). The catalyst was economically viable,very effective, and recovered by simple filtration. Nosignificant difference in the conversion and enan-tioselectivity was observed after reusing catalyst in fiveconsecutive cycles.

Later the effects of different solvent and support onthe OKR of racemic secondary alcohols with immobilizedcatalyst 14 were examined.75 To optimize the reaction

sing heterogeneous chiral Mn(III) salen complexes

t Conversion (%) ee (%) Reference

(OAc)2 61.5 55.3 Kantam et al30

(OAc)2 63 99 Sahoo et al32

(OAc)2 54 88 Ren et al34

O2 49 85 Ren et al34

FIGURE 7 Chiral Mn(III) salen complex immobilized on ionic

liquid modified mesoporous silica SBA‐15 for OKR of secondary

alcohols

FIGURE 8 Mesoporous helical silica

immobilized Mn(III) salen complex for

OKR of secondary alcohols

10 AHMAD ET AL.

condition, 1‐phenyl ethanol was used as substrate andPhI(OAc)2 as the oxidant. Different additives werescreened for this reaction, and the best result wasobtained with additives having bromide anions such asN(CH3)4Br and KBr. When bromide ion ionic liquid(which can act both as support and the additive) was used,the results were promising. The bromide ion‐containingionic liquids were water soluble, and the Mn(III) salencomplex was less soluble in this ionic liquid; therefore,[BMIM] PF6 was selected as the ionic liquid in the sup-ported ionic liquid catalysis. A number of silica supportssuch as MCM‐41, MCM‐48, SBA‐15, and amorphous silicawere examined; however, no significant difference inactivity was observed, suggesting that there was no effectbecause of the support on the OKR of racemic secondaryalcohols. Among all the solvents screened, the hexaneand water mix, along with diethyl ether and water mix-ture, showed good activity and selectivity for OKRbecause of reduced leaching of the complex into theorganic phase. The best oxidant for OKR was found tobe PhI(OAc)2, and it was observed that enantiomericexcess increased with the increase in conversion and time.The catalytic system was used for various racemic second-ary alcohols and showed good results. The α‐methyl‐p‐chlorobenzyl alcohol and 2‐decanol showed excellentenantioselectivity (99% and 94%, respectively) whereasα‐methyl‐p‐methoxy benzyl alcohol exhibited lowenantioselectivity (30%). This catalytic system was thusnot effective for heteroatoms containing aromatic second-ary alcohols such as furyl and pyridyl substituted alcohols.

In 2016, L. Ren et al attempted to construct an effi-cient and environmentally benign system for asymmetricoxidative kinetic resolution of secondary alcohols usingnew chiral silica materials and reported the synthesis ofmesoporous helical silica through doping of chiraldiammoniumcyclohexane mono‐tartrate salt in a sol‐gelprocess.76 Two series of catalysts, ie, catalysts 15 and 16(Figure 8), were prepared by immobilization of differentMn(III) salen complexes into helical silica efficientlyusing different linkages.

The synthesized catalysts were screened for OKR ofracemic secondary alcohols mainly in water and withiodobenzene diacetate and aqueous hydrogen peroxide(30 wt%) as oxidant. Mn(III) salen complexes exhibitedreasonable conversion, enantioselectivity, and selectivityfactor for OKR of different secondary alcohols. It wasobserved that, in comparison to attached Mn(III) salen,the configuration of silica supports significantly affectthe major configuration of the resolution product of sec-ondary alcohols. The best chiral induction was obtainedwith the combination of (S,S)‐diammonium salt‐dopedsilica and Mn(III)‐(R,R) salen. Both iodobenzene acetateand hydrogen peroxide (30%) act as effective terminal oxi-dant. For the OKR of 1‐phenyl ethanol, the best result wasobtained with catalyst 16a, which showed 85% ee and 54%conversion when PhI(OAc)2 was used as the additive(Table 3). A slight decrease in enantioselectivity (85%)and conversion (49%) was observed when H2O2 was usedas an additive (Table 3). In terms of recycling, the mostefficient systems were heterogeneous lipophilic Mn(III)salen with PhI(OAc)2 and heterogeneous hydrophilicMn(III) salen with H2O2. This report explored the applica-tion of new chiral silica materials in catalytic resolution ofsecondary alcohols.

3 | CONCLUSION

During the past decade, there have been significantadvances in the development of chiral Mn(III) salen com-plexes and their applications in the asymmetric oxidativekinetic resolution of racemic secondary alcohols. Theaim of this review was to present an overview of the liter-ature on the recent chiral Mn(III) salen complexesreported for OKR of racemic secondary alcohols. Mono-meric, dimeric, macrocyclic, polymeric, silica/resinMn(III) salen complexes are discussed for OKR of racemicsecondary alcohols. Most of the complexes exhibitedexcellent enantioselectivity, activity, stability and recycledsuccessfully under the oxidizing reaction conditions. We

AHMAD ET AL. 11

anticipate that this review will inspire the researchersaround the world and supports in the development ofmore efficient catalysts for asymmetric oxidation of alco-hols and other organic reactions.

ACKNOWLEDGMENTS

The author appreciate the financial support provided bythe School of Graduate Studies and Research, AmericanUniversity of Ras Al Khaimah, Ras Al Khaimah, UAEthrough seed grant funded project no. AAS/001/16.

ORCID

Irshad Ahmad http://orcid.org/0000-0001-7902-4715Shagufta http://orcid.org/0000-0003-1028-7888

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How to cite this article: Ahmad I, Shagufta,AlMallah AR. Advances in asymmetric oxidativekinetic resolution of racemic secondary alcoholscatalyzed by chiral Mn(III) salen complexes.Chirality. 2017;1–13. https://doi.org/10.1002/chir.22768