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

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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 Chirality. 2017;1–13. wileyonlinelibrary.com/journal/chir © 2017 Wiley Periodicals, Inc. 1 2 AHMAD ET AL. 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.
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