Cyclodextrin-Catalyzed Organic Synthesis: Reactions, Mechanisms, and Applications

Review Cyclodextrin-Catalyzed Organic Synthesis: Reactions, Mechanisms, and Applications

Chang Cai Bai 1,† ID , Bing Ren Tian 1,†, Tian Zhao 1, Qing Huang 1,2,* and Zhi Zhong Wang 1,2,*

1 School of Pharmacy, Ningxia Medical University, Yinchuan 750004, China; [email protected] (C.C.B.); [email protected] (B.R.T.); [email protected] (T.Z.) 2 Key Laboratory of Hui Ethnic Medicine Modernization, Ministry of Education, Yinchuan 750004, China * Correspondence: [email protected] (Q.H.); [email protected] (Z.Z.W.); Tel.: +86-0951-688-0582 (Z.Z.W.) † These authors contributed equally to this work.

Received: 2 August 2017; Accepted: 2 September 2017; Published: 7 September 2017

Abstract: Cyclodextrins are well-known macrocyclic oligosaccharides that consist of α-(1,4) linked glucose units and have been widely used as artiﬁcial enzymes, chiral separators, chemical sensors, and drug excipients, owing to their hydrophobic and chiral interiors. Due to their remarkable inclusion capabilities with small organic molecules, more recent interests focus on organic reactions catalyzed by cyclodextrins. This contribution outlines the current progress in cyclodextrin-catalyzed organic reactions. Particular emphases are given to the organic reaction mechanisms and their applications. In the end, the future directions of research in this ﬁeld are proposed.

Keywords: cyclodextrin; catalysis; mechanism; green chemistry; organic reaction; asymmetric reaction

1. Introduction Supramolecular chemistry is defined as the chemistry of the intermolecular bond, and focuses on the chemical systems made up of a discrete number of assembled molecular subunits or components. It aims at developing highly complex chemical systems from molecular components reversibly held together by non-covalent intermolecular forces [1,2]. In recent years, the applications of supramolecular chemistry are of intriguing interest. The 2016 Nobel Prize in Chemistry was awarded to three organic chemists (Jean-Pierre Sauvage, Sir J. Fraser Stoddart, and Ben L. Feringa) for the designing and synthesis of molecular machines based on supramolecular chemistry [3–5]. In addition, reactive and catalysis represent significant features of the functional properties of supramolecular systems. Host molecules may chelate guest molecules (with definite stability, selectivity, and kinetic features), react with them (with definite rate, selectivity, turnover), and, finally, release the products, thus regenerating the reagents for a new cycle. Cyclodextrins (CDs), as a model of the host molecule in supramolecular chemistry, are well-known macrocyclic oligosaccharides consisting of α-(1,4) linked glucose units [6,7]. They are inexpensive, water-soluble natural products, non-toxic, easily functionalized, and commercially available, and have been widely used as artificial enzymes, chiral separators, chemical sensors, and drug excipients [8–14], owing to their hydrophobic and chiral interiors [15–17]. Due to their remarkable inclusion capabilities with small organic molecules, more recent interests focus on organic reactions catalyzed by cyclodextrins. Parent cyclodextrins can be used as catalysts for organic synthesis, and can also be modified with some transition metals to form new catalysts, which catalyze certain organic reactions [18]. In addition, cyclodextrins and their derivatives are widely used as phase transfer catalysts for the synthesis of organic compounds [19]. Cyclodextrin-based supramolecular chemistry creates many research areas, and the present review focuses on the current progress in cyclodextrin-catalyzed organic reactions, mechanisms, and applications.

Molecules 2017, 22, 1475; doi:10.3390/molecules22091475 www.mdpi.com/journal/molecules Molecules 2017, 22, 1475 2 of 17 MoleculesMolecules 20172017,, 2222,, 14751475 22 ofof 1616

2.2. ApplicationApplication ofof CyclodextrinCyclodextrin inin ConventionalConventional ReactionReaction 2. Application of Cyclodextrin in Conventional Reaction

2.1.2.1. ConventionalConventional OxidationOxidation ReactionReaction TheThe oxidationoxidation reactionreaction isis anan important important reactionreaction inin organicorganic chemistry. chemistry. InIn recent recent years, years, cyclodextrins cyclodextrins havehave been been widely widely used used inin thethethe oxidationoxidationoxidation reaction.reaction.reaction. Cyclodextrins,Cyclodextrins,Cyclodextrins, as as supramolecularsupramolecular catalysts, catalysts, hadhad the the effecteffect ofof anan acceleratedaccelerated reactionreaction reaction raterate rate soso so thatthat that thethe the reactionreaction reaction timetime time waswas was decreased.decreased. decreased. ChenChen Chen etet et al.al. al. [20][20] [20 ] studiedstudied studied thethe the oxidationoxidation reactionreaction ofofof cinnamicaldehyde cinnamicaldehydecinnamicaldehyde using usingusingβ -CDββ-CD-CD as asas the thethe catalyst catalystcatalyst (Scheme (Scheme(Scheme1a). 1a). Cinnamaldehyde1a). CinnamaldehydeCinnamaldehyde and andandβ-CD ββ-CD could-CD couldcould form form theform inclusion thethe inclusioninclusion complex complexcomplex with a molarwithwith aa ratio molarmolar of 1:1ratioratio in water,ofof 1:11:1 andinin water,water, the rate andand of nucleophilicthethe raterate ofof nucleophilicnucleophilicoxidation in oxidationoxidation the presence inin thethe of presencepresenceβ-CD was ofof faster ββ-CD-CD thanwaswas fasterfaster that in thanthan the thatthat absence inin thethe of absenceabsence the catalyst. ofof thethe In catalyst.catalyst. addition, InIn addition,addition,the yield the ofthe benzaldehyde yieldyield ofof benzaldehydebenzaldehyde was up to waswas 69% upup at 333toto 69%69% K. The atat 333 proposed333 K.K. TheThe mechanism proposedproposed ofmechanismmechanism the oxidation ofof thethe of oxidationoxidationcinnamicaldehyde ofof cinnamicaldehydecinnamicaldehyde was shown in waswas Scheme shownshown1b. inin SchemeScheme 1b.1b.

((aa)) ((bb)) Scheme 1. (a) The oxidation reaction of cinnamaldehyde catalyzed by β-CD; (b) the mechanism of Scheme 1. ((a)) The oxidation reaction of cinnamaldehyde catalyzed by β-CD;-CD; ( (b)) the mechanism of oxidationoxidation ofof cinnamaldehydecinnamaldehyde catalyzed catalyzed by by ββ-CD.-CD.

Certainly,Certainly, itit isis importantimportant andand valuablevaluable thatthat thethe reactionreaction couldcould bebe carriedcarried outout underunder mildmild Certainly, it is important and valuable that the reaction could be carried out under mild conditions, conditions,conditions, avoidingavoiding somesome sideside reactions.reactions. JiJi etet al.al. [2[21]1] studiedstudied thethe reactionreaction ofof thethe alcoholsalcohols oxidizedoxidized toto avoiding some side reactions. Ji et al. [21] studied the reaction of the alcohols oxidized to aldehydes aldehydesaldehydes underunder mildmild conditions,conditions, choosingchoosing thethe effectiveeffective andand inexpensiveinexpensive reactantsreactants (Scheme(Scheme 2a).2a). under mild conditions, choosing the effective and inexpensive reactants (Scheme2a). Because of the BecauseBecause ofof thethe abilityability toto formform inclusioninclusion complexescomplexes bebetweentween cyclodextrinscyclodextrins andand thethe reactants,reactants, thethe reactionreaction ability to form inclusion complexes between cyclodextrins and the reactants, the reaction solution solutionsolution waswas homogeneous,homogeneous, whichwhich waswas beneficialbeneficial toto ththee progressprogress ofof thethe reaction.reaction. TheThe substratesubstrate scopescope ofof was homogeneous, which was beneﬁcial to the progress of the reaction. The substrate scope of the thethe alcoholsalcohols hadhad beenbeen widelywidely expandedexpanded toto includeinclude aromaticaromatic alcoholsalcohols inin goodgood yield.yield. TheThe advantagesadvantages ofof alcohols had been widely expanded to include aromatic alcohols in good yield. The advantages of thesethese reactionsreactions consistedconsisted inin inexpensiveinexpensive reactantsreactants andand recycledrecycled catalystscatalysts ofof cyclodextrins.cyclodextrins. OnOn thethe otherother these reactions consisted in inexpensive reactants and recycled catalysts of cyclodextrins. On the other hand,hand, thethe mechanismmechanism ofof thethe oxidationoxidation ofof alcoholsalcohols waswas proposedproposed (Scheme(Scheme 2b).2b). hand, the mechanism of the oxidation of alcohols was proposed (Scheme2b).

ClCl -cyclodextrin-cyclodextrin H H O H OHOH RR OHOH RR OO H OH HH H O HH OOH NaClO,NaClO, HH2O,O, 5050℃℃ OH 2 CHO O CHCH2OHOH H C H O H H HH CC OO HH CHO O 2 HH CC HH OO H C H O H CC H OO RR == Ar,Ar, CH=CHArCH=CHAr OH YeildYeild == >99%,>99%, >99%>99% ClOClO OH -OH-OH ClCl

((aa)) ((bb))

Scheme 2. (a) The oxidation reaction of alcohol catalyzed by CDs; (b) the proposed reaction mechanism SchemeScheme 2.2. ((aa)) TheThe oxidationoxidation reactionreaction ofof alcoholalcohol catalyzedcatalyzed byby CDs;CDs; ((bb)) thethe proposedproposed reactionreaction of alcohol oxidized to aldehyde. mechanismmechanism ofof alcoholalcohol oxidizedoxidized toto aldehyde.aldehyde.

NewNew cup-shapedcup-shaped cup-shaped αα-cyclodextrin-cyclodextrinα-cyclodextrin derivativesderivatives derivatives werewere weredesigneddesigned designed andand synthesizedsynthesized and synthesized forfor thethe oxidationoxidation for the reaction.reaction.oxidation LopezLopez reaction. etet al.al. [22][22] Lopez studiedstudied etal. aa seriesseries [22] studiedofof neneww blocking-integratedblocking-integrated a series of new blocking-integrated oror bridgedbridged cyclodextrinscyclodextrins or bridged whichwhich A D A hadhadcyclodextrins functionalfunctional whichgroups,groups, had suchsuch functional asas 66AA,, 66DD-di--di- groups,OO-(prop-2-methyl-1,3-dienyl),-(prop-2-methyl-1,3-dienyl), such as 6 , 6 - di-O-(prop-2-methyl-1,3-dienyl), 66AA,, 66DD-di--di-OO-(prop-2-hydroxy--(prop-2-hydroxy- 6 , D A D 2-hydroxymethyl-1,3-dienyl)-hexadeca-2-hydroxymethyl-1,3-dienyl)-hexadeca-6 -di-O-(prop-2-hydroxy-2-hydroxymethyl-1,3-dienyl)-hexadeca-OO-benzyl,-benzyl, 66AA,, andand 66DD-di--di-OO-(prop-2-formyl-2-hydroxy-1,3-dienyl)-(prop-2-formyl-2-hydroxy-1,3-dienyl)O-benzyl, 6 , and 6 -di-O-(prop-2- -hexadeca--hexadeca-formyl-2-hydroxy-1,3-dienyl)-hexadeca-OO-benzyl.-benzyl. TheThe resultsresults indicatedindicated ththOat-benzyl.at thethe derivativesderivatives The results couldcould indicated catalyzecatalyze the thatthe oxidationoxidation the derivatives reactions,reactions, could i.e.,i.e., 2-hydroxyaniline2-hydroxyanilinecatalyze the oxidation waswas reactions,oxidoxidizedized i.e.,intointo 2-hydroxyaniline 2-amino-phenoxazin-2-one,2-amino-phenoxazin-2-one, was oxidized catalyzedcatalyzed into 2-amino-phenoxazin-2-one, byby thethe derivativesderivatives ofof cyclodextrins.cyclodextrins.catalyzed by the derivatives of cyclodextrins.

Molecules 2017, 22, 1475 3 of 16 Molecules 2017, 22, 1475 3 of 17 Molecules 2017, 22, 1475 3 of 16 Saghatforoush et al. [23] designed and prepared β-cyclodextrin-graphene oxide-SO3H composite-modifiedSaghatforoush et glassy al. carbon [[23]23] designeddesigned electrode andand (β-CD/GO-SO preparedprepared 3βH–GCE)β-cyclodextrin-graphene-cyclodextrin-graphene as an electrochemical oxide-SOoxide-SO sensor,33H composite-modifiedcomposite-modiﬁedwhich could catalyze glassy the oxidation carbon electrodereaction of ((β βca-CD/GO-SOdaverine to3H–GCE) 35-aminopentaH–GCE) as as an annal electrochemical electrochemical (Scheme 3a). Becausesensor, sensor, whichthe derivative couldcould catalyzecatalyze of β-CD thetheoxidation oxidationcould accommodate reaction reaction of of cadaverine cacadaverinedaverine to to 5-aminopentanal and 5-aminopenta promote nalits (Scheme (Schemeelectron-transfer,3a). 3a). Because Because thethe thederivativecomplexes derivative ofprovidedβ -CDof β could-CD a highercould accommodate accommodateelectroactive cadaverine surfacecadaverine and promotearea. and The promote its main electron-transfer, advantageits electron-transfer, theof complexesusing thethe complexesprovidedβ-CD/GO-SO a higherprovided3H–GCE electroactive awas higher easy surface andelectroactive quick area. surface The surface main renewal advantage area. after The of each usingmain use. theadvantage Onβ-CD/GO-SO the other of usinghand,3H–GCE thethe βwaselectrochemical-CD/GO-SO easy and3 quickH–GCE sensor surface was was easy renewal successfully and afterquick eachused surface use. to Ondeterminerenewal the other after cadaverine hand, each the use. electrochemicalin On fish the samples, other sensorhand, and wasthethe electrochemicalsuccessfullyelectrode exhibited used sensor to determinegood was repeatability successfully cadaverine and inused fishstabilit samples,to y.determine Those and results the cadaverine electrode might exhibited inopen fish up samples, good a new repeatability approach and the electrodeandfor stability.the use exhibited Thoseof the resultsgood β-CD/GO-SO repeatability might open3H/GCE upand a stabilitin new the approach y.biochemical Those forresults the and use might ofmedical the openβ-CD/GO-SO analysisup a new fields. approach3H/GCE The forinmechanism thethe biochemical use ofof the the oxidation and β-CD/GO-SO medical of cadaverine analysis3H/GCE ﬁelds. wasin theproposed The biochemical mechanism (Scheme ofand 3b). the medical oxidation analysis of cadaverine fields. wasThe mechanismproposed (Scheme of the oxidation3b). of cadaverine was proposed (Scheme 3b). O Catalyst H2NNH2 H2N H NH3 20℃,Buffers O Catalyst OSO3H H2NNH2 H2N H NH3 20℃,Buffers O OSO3H OSO3H H NNHH NH+ NH 2 2 2 O 3 H 3 O O OSO3H S + NH HO SO H2NNH2 H2NH- 3 3 OSO3H O H OSO H G 3 O 3 - O HO SO O S 3 OSO3H - O G OSO3H - O H2O O

O H2O

(a) (b) (a) (b) Scheme 3. (a) The oxidation reaction of cadaverine catalyzed by β-CD/GO-SO3H–GCE; (b) the proposed mechanism of the oxidation of cadaverine. Scheme 3. ((aa)) The oxidation reaction ofof cadaverinecadaverine catalyzedcatalyzed byby ββ-CD/GO-SO-CD/GO-SO33H–GCE;H–GCE; ( (bb)) the proposed mechanism of the oxidation of cadaverine. In general, the aldehyde was directly oxidized to the carboxylic acid in the heterogeneous system.In general,However, the Shialdehyde et al. was[24] directlystudied oxidized the oxidation to the carboxylicreaction with acid βin-cyclodextrin the heterogeneous in the In general, the aldehyde was directly oxidized to the carboxylic acid in the heterogeneous system. system.homogeneous However, system. Shi Benzaldehyde et al. [24] studied could be the accommodated oxidation reaction in the β -cyclodextrinwith β-cyclodextrin cavity to inform the a However, Shi et al. [24] studied the oxidation reaction with β-cyclodextrin in the homogeneous system. homogeneouscomplex, which system. was dissolved Benzaldehyde in the couldaqueous be phase.accommodated Using the in mild the βoxidizing-cyclodextrin agents cavity (NaClO/HCl), to form a Benzaldehyde could be accommodated in the β-cyclodextrin cavity to form a complex, which was complex,benzaldehyde which could was dissolved be oxidized in the to aqueous benzoic phase. acid, Usingand thethe mildoptimum oxidizing reaction agents conditions (NaClO/HCl), were dissolved in the aqueous phase. Using the mild oxidizing agents (NaClO/HCl), benzaldehyde could be benzaldehydeexplored (Scheme could 4a). be Under oxidized the optimized to benzoic condition, acid, and the theyield optimum was up toreaction 98%. In conditionsthis reaction, were the oxidized to benzoic acid, and the optimum reaction conditions were explored (Scheme4a). Under the exploredreactants (Schemewere inexpensive 4a). Under and the optimizedβ-cyclodextrin condition, could thebe recycled.yield was Inup addition, to 98%. In the this corresponding reaction, the optimized condition, the yield was up to 98%. In this reaction, the reactants were inexpensive and reactantsmechanism were was inexpensive that the oxidizing and β-cyclodextrin agents oxidized could benzaldehyde be recycled. to In carboxylic addition, acidthe correspondingwhen the acid β-cyclodextrin could be recycled. In addition, the corresponding mechanism was that the oxidizing mechanismprovided protons was that (Scheme the oxidizing 4b). agents oxidized benzaldehyde to carboxylic acid when the acid agents oxidized benzaldehyde to carboxylic acid when the acid provided protons (Scheme4b). provided protons (Scheme 4b).

 H O H OH  COOH  H O H OH  COOH H ClO H2O

H ClO H2O

(a) (b) (a) (b) Scheme 4.4.( a()a The) The oxidation oxidation reaction reaction of benzaldehyde of benzaldehyde catalyzed catalyzed by β-CD; by (b )β the-CD; proposed (b) the mechanism proposed Schemeofmechanism the oxidation 4. (ofa) theThe reaction oxidation oxidation of benzaldehyde reaction reaction of benzaldehyde of to benzoicbenzaldehyde acid. to benzoic catalyzed acid. by β-CD; (b) the proposed mechanism of the oxidation reaction of benzaldehyde to benzoic acid. In the aspect of polymer of cyclodextrins, Yang et al. [25,26] designed and synthesized a new In the aspect of polymer of cyclodextrins, Yang et al. [25,26] designed and synthesized a new β-cyclodextrinIn the aspect cross-linked of polymer with of cychitosanclodextrins, (β-CDP) Yang for et the al. selective[25,26] designed oxidation and of cinnamaldehydesynthesized a new in β-cyclodextrin cross-linked with chitosan (β-CDP) for the selective oxidation of cinnamaldehyde in βthe-cyclodextrin presence of cross-linked bicarbonate with and chitosanhydrogen (β peroxide-CDP) for (Scheme the selective 5a). Theoxidation experimental of cinnamaldehyde results clearly in the presence of bicarbonate and hydrogen peroxide (Scheme5a). The experimental results clearly theshowed presence that βof-CDP bicarbonate could form and thehydrogen complexes peroxide with cinnamaldehyde(Scheme 5a). The through experimental the weak results interaction clearly showed that β-CDP could form the complexes with cinnamaldehyde through the weak interaction showedof the hydrogen that β-CDP bond, could and form chitosan the complexes played an withimportant cinnamaldehyde role in the complexes. through the Under weak theinteraction optimal of the hydrogen bond, and chitosan played an important role in the complexes. Under the optimal ofcondition, the hydrogen benzaldehyde bond, and was chitosan obtained played in 78% an important yield, compared role in theto thecomplexes. parent cyclodextrinUnder the optimal as the condition, benzaldehyde was obtained in 78% yield, compared to the parent cyclodextrin as the condition,catalyst. In benzaldehyde addition, the reusability was obtained of the in catalyst78% yield, was comparedevaluated, toand the it couldparent be cyclodextrin recycled six astimes, the catalyst. In addition, the reusability of the catalyst was evaluated, and it could be recycled six times,

Molecules 2017, 22, 1475 4 of 17

Molecules 2017, 22, 1475 4 of 16 catalyst. In addition, the reusability of the catalyst was evaluated, and it could be recycled six times, whichwhich indicated indicated that thatβ β-CDP-CDP waswas anan effectiveeffective catalystcatalyst for the selective oxidation oxidation of of cinnamaldehyde. cinnamaldehyde. TheThe advantages advantages ofof thethe polymerpolymer ofof cyclodextrinscyclodextrins consistedconsisted in simple preparation and and convenient convenient recycling,recycling, which which made made them them useful useful for manufacturingfor manufacturing natural natural benzaldehyde benzaldehyde in an industrialin an industrial process. Theprocess. proposed The mechanismproposed mechanism of the oxidation of the of oxidation cinnamicaldehyde of cinnamicaldehyde was shown inwas Scheme shown5b, in where Schemeβ-CDP 5b, actedwhere as β PTC-CDP in acted the two-phase as PTC in solutionthe two-phase medium. solution medium.

O  O OH O  O O H O O H H H H O H O O  O O O O O O O O O O

HCO4 HCO4 HCO4

O O HO O O O O H HO O HO HO HO HO

O  O OH O O O  O O O H O O H H HO H H H O O  O O O O O O O O HCO4 HCO4 HCO4

-HCO3 -HCO3 O O O O HO O O HO HO HO HO HO

(a) (b)

SchemeScheme 5.5. ((aa)) TheThe oxidationoxidation reactionreaction ofof cinnamaldehydecinnamaldehyde to benzaldehyde catalyzed catalyzed by by ββ-CDP;-CDP; (b(b) The) The proposed proposed mechanism mechanism of of the the oxidation oxidation ofof cinnamaldehyde.cinnamaldehyde.

Similarly, Yang et al. [27] conjugated lignin to cyclodextrin to prepare a new material as a reverseSimilarly, phase Yangtransfer et al. catalyst [27] conjugated for the oxidation lignin to reac cyclodextrintion of different to prepare alcohols a new to material aldehydes as ain reverse good phaseyield. transferThe catalyst catalyst was foreco-friendly, the oxidation and can reaction be recy ofcled. different In addition, alcohols Chen to aldehydes et al. [28] found in good that yield. the Thedegrees catalyst of substitution was eco-friendly, of 2-HP- andβ-CD can behad recycled. an effect In on addition, the hydrolysis Chen et of al. cinnamaldehyde. [28] found that the When degrees the ofdegree substitution of substitution of 2-HP- increased,β-CD had the an effectrate of on hydrolysis the hydrolysis decreased. of cinnamaldehyde. When the degree of substitution increased, the rate of hydrolysis decreased. 2.2. Conventional Reduction Reaction 2.2. Conventional Reduction Reaction The processes of oxidation and reduction occur simultaneously, and the reduction reaction is The processes of oxidation and reduction occur simultaneously, and the reduction reaction is also also an important reaction in organic chemistry. an importantUnder non-aqueous reaction in organic conditions, chemistry. Pospisil et al. [29] studied the reduction reaction of 3-(3,5- dichlorophenyl)-5-methyl-5-vinyl-1,3-oxazolidine-2,4-dioneUnder non-aqueous conditions, Pospisil et al. [29] studied using thecatalyzed reduction by reactionβ-CD using of 3-(3,5- the β dichlorophenyl)-5-methyl-5-vinyl-1,3-oxazolidine-2,4-dioneelectrochemical method (Scheme 6a). The mechanism of the usingreduction catalyzed of vinclozoline by -CD through using the the electrochemicalinclusion complex method with β (Scheme-CD was6 a).proposed The mechanism (Scheme 6b). of the reduction of vinclozoline through the inclusion complexp with β-CD wasg proposed (Schemey 6b). HO CH3 HO CH3 -2Cl 70% C CH3 -2Cl 50% C CH3 N N H H2 -2.15V 2 -2.35V O O Cl O Cl HO CH3 CH3 -Cl -CO2 24% -Cl, -CO2 50% N N

Cl O CH3 + C CH N 2 H Cl HO CH3 O Cl O N

HO HO CH3 CH3 Cl HO CH -CO 3 -2.65V C CH C CH 2 3 N 3 N H2 H2 N -2Cl 97% (a) O (b) O Scheme 6. (a) The reduction reaction of 3-(3,5-dichlorophenyl)-5-methyl-5-vinyl-1,3-oxazolidine- 2,4-dione catalyzed by β-CD; (b) the reduction mechanism of 3-(3,5-dichlorophenyl)-5-methyl- Scheme 6. (a) The reduction reaction of 3-(3,5-dichlorophenyl)-5-methyl-5-vinyl-1,3-oxazolidine-2,4-dione 5-vinyl-1,3-oxazolidine-2,4-dione catalyzed by β-CD. catalyzed by β-CD; (b) the reduction mechanism of 3-(3,5-dichlorophenyl)-5-methyl-5-vinyl-1,3-oxazolidine- 2,4-dione catalyzed by β-CD. Guo et al. [30] modified cyclodextrin with palladium to synthesize a new material (DACH-Pd-β-CD) for the reduction reaction of a serial of nitrobenzene to aromatic amine compoundsGuo et in al. excellent [30] modiﬁed yield (Scheme cyclodextrin 7a). On the with other palladium hand, the catalyst to synthesize could be aeasily new separated material β (DACH-Pd-from the reaction-CD) for solution, the reduction and reactionstill maintained of a serial high of nitrobenzene catalytic activity to aromatic after aminefive cycles. compounds The mechanism of the reduction of nitrobenzene through the inclusion complex with DACH-Pd-β-CD was proposed (Scheme 7b).

Molecules 2017, 22, 1475 5 of 17

in excellent yield (Scheme7a). On the other hand, the catalyst could be easily separated from the reaction solution, and still maintained high catalytic activity after ﬁve cycles. The mechanism of the reduction of nitrobenzene through the inclusion complex with DACH-Pd-β-CD was proposed

(SchemeMolecules 201720177b).,, 2222,, 14751475 55 ofof 1616

N Pd NH Pd NH 2 NH 2 HAcO OAc AcO OAc

(a) (b) (a) (b) Scheme 7.7. ((a)) The The reductionreduction reactionreaction ofof nitroarenenitroarenenitroarene catalyzedcatalyzedcatalyzed bybyby DACH-Pd-DACH-Pd-DACH-Pd-ββ-CD;-CD; ( (b)) the the proposedproposed mechanism pathways ofof nitroarenenitroarene reduction.reduction.

Kakroudi et al. [31] constructed the host-guest system using β-cyclodextrin and nanosized-titania Kakroudi etet al.al. [[31]31] constructedconstructed the the host-guest host-guest system system using usingβ-cyclodextrin β-cyclodextrin and and nanosized-titania nanosized-titania for for the photocatalytic reduction of nitroaromatic compound under sunlight (Scheme 8a). This system, forthe the photocatalytic photocatalytic reduction reduction of nitroaromatic of nitroaromatic compound compound under under sunlight sunlight (Scheme (Scheme8a). This 8a). system, This system, called called “Green Nest”, was eco-friendly, and highly efficient to the reduction reaction. Also, more “Greencalled “Green Nest”, wasNest”, eco-friendly, was eco-friendly, and highly and efficient highly to efficient the reduction to the reaction. reduction Also, reaction. more interestingly,Also, more interestingly, nitroaromatic compounds could be reduced to aromatic amine compounds through interestingly,nitroaromatic nitroaromatic compounds could compounds be reduced could to aromaticbe reduced amine to aromatic compounds amine through compounds one-pot reaction.through one-pot reaction. The mechanism of the reduction of the nitroaromatic compound through the Theone-pot mechanism reaction. of The the reductionmechanism of theof nitroaromaticthe reduction compoundof the nitroaromatic through the compound inclusion complexthrough with the inclusion complex with β-cyclodextrin was proposed (Scheme 8b). inclusionβ-cyclodextrin complex was with proposed β-cyclodextrin (Scheme 8wasb). proposed (Scheme 8b).

NO2 NO2

O O NO 2 NH2 NO2 NH TiO2 2 TiO2 H2O H2O O sunlight O O sunlight O

(a) (b) (a) (b)

Scheme 8. (a) The photoreduction of 3-nitroacetophenone catalyzedβ by β-CD-TiO2; (b) the Scheme 8.8.( a()a The) The photoreduction photoreduction of 3-nitroacetophenone of 3-nitroacetophenone catalyzed catalyzed by -CD-TiO by β2;(-CD-TiOb) the mechanism2; (b) the mechanism of the photoreduction of 3-nitroacetophenone catalyzedβ by β-CD-TiO2. mechanismof the photoreduction of the photoreduction of 3-nitroacetophenone of 3-nitroacetophenone catalyzed by catalyzed-CD-TiO by2. β-CD-TiO2.

Lu etet al. al. [32 [32]] studied studied the the reduction reduction reaction reaction of nitrobenzene of nitrobenzene in the presence in the presence of sodium of hydroxide sodium hydroxideand β-cyclodextrin and β-cyclodextrin-cyclodextrin (Scheme9a). (Scheme(Schemeβ-Cyclodextrin 9a).9a). β-Cyclodextrin-Cyclodextrin could form thecouldcould inclusion formform thethe complex inclusioninclusion with complexcomplex nitrobenzene. withwith nitrobenzene.In the aqueous In solution, the aqueous the mechanism solution, the of mechanis the reductionm of the of nitrobenzene reduction of nitrobenzene through the one-electronthrough the one-electrontransfer process transfer was proposedprocess was (Scheme proposed9b). (Scheme 9b).

((a)) ((b)) Scheme 9. (a) The reduction reaction of nitrobenzene catalyzed by β-CD; (b) the proposed Scheme 9.9.( a(()a The)) TheThe reduction reductionreduction reaction reactionreaction of nitrobenzene ofof nitrobenzenenitrobenzene catalyzed catalyzedcatalyzed by β-CD; byby (b )β the-CD;-CD; proposed ((b)) thethe mechanism proposedproposed pathwaysmechanism of pathways nitrobenzene of nitrobenzene reduction. reduction.

2.3. Conventional Addition Reaction An addition reaction, one of the important reactionstions inin organicorganic chemistry,chemistry, makesmakes twotwo oror moremore molecules combine to form a larger one. Krishnaveni et al. [33,34] studied the addition reaction of thiols with olefins in the presence of β-cyclodextrin-cyclodextrin underunder roomroom temperaturetemperature (Scheme(Scheme 10a)10a),, andand thethe yieldyield waswas higherhigher (up(up toto 98%)98%) thanthan thatthat inin thethe absenceabsence ofof β-cyclodextrin.-cyclodextrin. InIn addition,addition, thethe advantadvantages of this method consisted in a

Molecules 2017, 22, 1475 6 of 17

2.3. Conventional Addition Reaction An addition reaction, one of the important reactions in organic chemistry, makes two or more molecules combine to form a larger one. Krishnaveni et al. [33,34] studied the addition reaction of thiols with olefins in the presence of Moleculesβ-cyclodextrin 2017, 22, under1475 room temperature (Scheme 10a), and the yield was higher (up to 98%) than that6 of 16 in Molecules 2017, 22, 1475 6 of 16 the absence of β-cyclodextrin. In addition, the advantages of this method consisted in a lower reaction lower reaction time and the recycling of the catalyst. The mechanism of the addition of thiols with timelower and reaction the recycling time and of the the recycling catalyst. Theof the mechanism catalyst. The of the mechanism addition ofof thiolsthe addition with olefins of thiols through with olefins through the inclusion complex with β-cyclodextrin was proposed (Scheme 10b). Similarly, theolefins inclusion through complex the inclusion with β-cyclodextrin complex with was β-cyclodextrin proposed (Scheme was proposed 10b). Similarly, (SchemeSrinivas 10b). Similarly, et al. [35] Srinivas et al. [35] studied the Michael addition reaction of benzeneselenol with olefins, and the yield studiedSrinivas theet al. Michael [35] studied addition the reactionMichael ofaddition benzeneselenol reaction of with benzeneselenol olefins, and thewith yield olefins, could and achieve the yield up could achieve up to 88% under the optimal condition. In those addition reactions mediated by β-CD, tocould 88% achieve under theup optimalto 88% under condition. the optimal In those condit additionion. In reactions those addition mediated reactions by β-CD, mediated the process by β was-CD, the process was economical and eco-friendly. economicalthe process andwas eco-friendly.economical and eco-friendly.

-CD/6-8h R-SH + X -CD/6-8h X X X R-SH + ℃ R-S H2O/25 R-S H2O/25℃ R=aryl, cyclohexyl R=aryl, cyclohexyl X=CHO, COMe, CN, CO2Me, CONH2 X=CHO, COMe, CN, CO Me, CONH Yeild= 95%, 98%, 97%, 96%,2 96% 2 Yeild= 95%, 98%, 97%, 96%, 96% (a) (b) (a) (b) Scheme 10. ((aa)) The The addition addition reaction reaction of of thiols thiols with with olefins oleﬁns catalyzed catalyzed by by β-CD;-CD; ( (bb)) the the proposed Scheme 10. (a) The addition reaction of thiols with olefins catalyzed by β-CD; (b) the proposed mechanism of thiols with olefins oleﬁns catalyzed by β-CD.-CD. mechanism of thiols with olefins catalyzed by β-CD. In the aspect of the Aza-Michael addition reaction, Surendra et al. [36] studied the addition InIn thethe aspectaspect ofof thethe Aza-MichaelAza-Michael additionaddition reaction,reaction, SurendraSurendra etet al.al. [[36]36] studiedstudied thethe additionaddition reaction of amines with alkenes in the presence of β-cyclodextrin (Scheme 11a). When the contents reactionreaction ofof aminesamines withwith alkenes alkenes in in the the presence presence of ofβ -cyclodextrinβ-cyclodextrin (Scheme (Scheme 11 11a).a). When When the the contents contents of of β-cyclodextrin increased, the yields were improved. More interestingly, β-cyclodextrin (as the βof-cyclodextrin β-cyclodextrin increased, increased, the yieldsthe yields were were improved. improved. More interestingly,More interestingly,β-cyclodextrin β-cyclodextrin (as the catalyst) (as the catalyst) could be recycled. The mechanism of the addition of amines with alkenes mediated by couldcatalyst) be recycled.could be Therecycled. mechanism The mechanism of the addition of the of aminesaddition with of alkenesamines mediatedwith alkenes by β -cyclodextrinmediated by β-cyclodextrin was proposed (Scheme 11b). wasβ-cyclodextrin proposed (Scheme was proposed 11b). (Scheme 11b).

O O 1 R NH2 H2C 1 R NH2 H2C O Me O Me H

2 C H 2 C Aqueous phase O 1 Aqueous phase

R =Ph,Ph-F(p), Tol, Bn O R1 =Ph,Ph-F(p), Tol, Bn O Yeild = 88%, 86%, 86%, 80% O

Me Yeild = 88%, 86%, 86%, 80% Me

(a) (b) (a) (b) Scheme 11. (a) The addition reaction of amines with alkenes catalyzed by β-CD; (b) the proposed Scheme 11. (a) The addition reaction of amines with alkenes catalyzed by ββ-CD; (b) the proposed mechanismScheme 11. of(a addition) The addition reaction reaction of amines of amines with alkenes. with alkenes catalyzed by -CD; (b) the proposed mechanism ofof additionaddition reactionreaction ofof aminesamines withwith alkenes.alkenes. Regarding the Diels–Alder reaction, Alupei et al. [37] reported the addition reaction of Regarding the Diels–Alder reaction, Alupei et al. [37] reported the addition reaction of cyclopentadieneRegarding thewith Diels–Alder unsaturated reaction, polyester Alupei in the et presence al. [37] reportedof methylated- the additionβ-cyclodextrin reaction and of cyclopentadiene with unsaturated polyester in the presence of methylated-β-cyclodextrin and synthesizedcyclopentadiene a new with type unsaturatedof polypseudorotaxane polyester in(Scheme the presence 12a). The of results methylated- indicatedβ-cyclodextrin that the complex and synthesized a new type of polypseudorotaxane (Scheme 12a). The results indicated that the complex ofsynthesized cyclopentadiene a new with type cyclodextrin of polypseudorotaxane was formed in (Scheme water under 12a). room The temperature, results indicated which that greatly the of cyclopentadiene with cyclodextrin was formed in water under room temperature, which greatly enhancedcomplex ofthe cyclopentadiene stability and solubility with cyclodextrinof the monome wasr in formed water. Thus, in water the Diels–Alder under room reaction temperature, could enhanced the stability and solubility of the monomer in water. Thus, the Diels–Alder reaction could bewhich successfully greatly enhanced carried out the in stability an environmentally and solubility friendly of the monomermanner by in using water. the Thus, resulting the Diels–Alder complex of be successfully carried out in an environmentally friendly manner by using the resulting complex of cyclopentadienereaction could beand successfully methylated- carriedβ-cyclodextrin. out in an The environmentally mechanism of friendlythe addition manner reaction by using of cyclopentadiene and methylated-β-cyclodextrin. The mechanism of the addition reaction of cyclopentadienethe resulting complex with of cyclopentadieneunsaturated polyes and methylated-ter throughβ-cyclodextrin. the inclusionThe mechanismcomplex ofwith the cyclopentadiene with unsaturated polyester through the inclusion complex with methylated-addition reactionβ-cyclodextrin of cyclopentadiene was proposed with (Scheme unsaturated 12b). polyester through the inclusion complex with methylated-β-cyclodextrin was proposed (Scheme 12b). methylated-β-cyclodextrin was proposed (Scheme 12b).

water RT + water RT +

+ O O O 9 O n O + O 9 O n O 9 O 9 O O n O n O 9 O O n O O 9 O O O O n O O O O 9 O n O O + O O O 9 9 + O n O O n 9 O O n O O O O (a) (b) (a) (b) Scheme 12. (a) The addition reaction of the unsaturated polyester catalyzed by β-CD; (b) the Scheme 12. (a) The addition reaction of the unsaturated polyester catalyzed by β-CD; (b) the proposed mechanism of addition reaction of unsaturated polyester. proposed mechanism of addition reaction of unsaturated polyester.

Molecules 2017, 22, 1475 6 of 16

lower reaction time and the recycling of the catalyst. The mechanism of the addition of thiols with olefins through the inclusion complex with β-cyclodextrin was proposed (Scheme 10b). Similarly, Srinivas et al. [35] studied the Michael addition reaction of benzeneselenol with olefins, and the yield could achieve up to 88% under the optimal condition. In those addition reactions mediated by β-CD, the process was economical and eco-friendly.

-CD/6-8h R-SH + X X R-S H2O/25℃

R=aryl, cyclohexyl X=CHO, COMe, CN, CO2Me, CONH2 Yeild= 95%, 98%, 97%, 96%, 96% (a) (b)

Scheme 10. (a) The addition reaction of thiols with olefins catalyzed by β-CD; (b) the proposed mechanism of thiols with olefins catalyzed by β-CD.

In the aspect of the Aza-Michael addition reaction, Surendra et al. [36] studied the addition reaction of amines with alkenes in the presence of β-cyclodextrin (Scheme 11a). When the contents of β-cyclodextrin increased, the yields were improved. More interestingly, β-cyclodextrin (as the catalyst) could be recycled. The mechanism of the addition of amines with alkenes mediated by β-cyclodextrin was proposed (Scheme 11b).

O 1 R NH2 H2C O Me H 2 C Aqueous phase O R1 =Ph,Ph-F(p), Tol, Bn O Yeild = 88%, 86%, 86%, 80% Me

(a) (b) Scheme 11. (a) The addition reaction of amines with alkenes catalyzed by β-CD; (b) the proposed mechanism of addition reaction of amines with alkenes.

Regarding the Diels–Alder reaction, Alupei et al. [37] reported the addition reaction of cyclopentadiene with unsaturated polyester in the presence of methylated-β-cyclodextrin and synthesized a new type of polypseudorotaxane (Scheme 12a). The results indicated that the complex of cyclopentadiene with cyclodextrin was formed in water under room temperature, which greatly enhanced the stability and solubility of the monomer in water. Thus, the Diels–Alder reaction could be successfully carried out in an environmentally friendly manner by using the resulting complex of cyclopentadiene and methylated-β-cyclodextrin. The mechanism of the addition reaction of cyclopentadiene with unsaturated polyester through the inclusion complex with Molecules 2017, 22, 1475 7 of 17 methylated-β-cyclodextrin was proposed (Scheme 12b).

water RT +

+ O O O 9 n O O 9 n O 9 O O n O O O O O O O 9 + O n 9 n O O O O

(a) (b)

Scheme 12.12.( a()a The) The addition addition reaction reaction of the of unsaturated the unsaturated polyester polyester catalyzed catalyzed by β-CD; by (b )β the-CD; proposed (b) the mechanismproposed mechanism of addition of reaction addition of re unsaturatedaction of unsaturated polyester. polyester. Molecules 2017, 22, 1475 7 of 16 Molecules 2017, 22, 1475 7 of 16

CondensationCondensation polymerization polymerization was was a asimple simple method method forfor the thesynthesis synthesis of the of sequence the sequence chain. chain. Tao Condensation polymerization was a simple method for the synthesis of the sequence chain. Tao etTao al. et [38] al. [ 38synthesized] synthesized a hydrop a hydrophilichilic periodic periodic polymer polymer using using the the nitrosonitroso benzene/cyclodextrinbenzene/cyclodextrin et al. [38] synthesized a hydrophilic periodic polymer using the nitroso benzene/cyclodextrin complexcomplex (Scheme 13a).13a). Firstly, the water-soluble inclusion complexes of CD/NBCD/NB were were formed and complex (Scheme 13a). Firstly, the water-soluble inclusion complexes of CD/NB were formed and thenthen polymerized polymerized with with poly poly (ethylene glycol) glycol) bis bis ( α-bromoisobutyrate) to to produce macromolecular then polymerized with poly (ethylene glycol) bis (α-bromoisobutyrate) to produce macromolecular polymers in good yield. The The me mechanismchanism of this reaction was proposedproposed (Scheme 1313b).b). The The process polymers in good yield. The mechanism of this reaction was proposed (Scheme 13b). The process provided a a simple method method to to synthesize the the sequen sequencece chain and was beneﬁcialbeneficial to the application provided a simple method to synthesize the sequence chain and was beneficial to the application inin industry. industry. in industry.

O O O O O N Cu/Ligand O N PEG O Cu/Ligand N Br + H2O PEG O PEG N Br + H2O PEG Br O Br O O O n n (a) (b) (a) (b) Scheme 13. (a) The macromolecular addition reaction catalyzed β-CD; (b) the mechanism of the SchemeScheme 13.13. (a) TheThe macromolecularmacromolecular additionaddition reaction catalyzedcatalyzed β-CD; ( b) the mechanismmechanism ofof thethe macromolecular addition reaction. macromolecularmacromolecular additionaddition reaction.reaction. Girish et al. [39] reported the synthesis of 2-phenyl imidazole derivatives by the addition GirishGirish etet al. al. [39 [39]] reported reported the the synthesis synthesis of 2-phenyl of 2-phenyl imidazole imidazole derivatives derivatives by the additionby the addition reaction reaction using nano ZrO2-β-cyclodextrin as a supramolecular catalyst in good yield (Scheme 14a). In usingreaction nano using ZrO nano-β-cyclodextrin ZrO2-β-cyclodextrin as a supramolecular as a supramolecular catalyst in cata goodlyst yield in good (Scheme yield 14 (Schemea). In addition, 14a). In addition, the catalyst2 can be recycled to facilitate resource conservation. The mechanism of the theaddition, catalyst the can catalyst be recycled can tobe facilitate recycled resource to facilitate conservation. resource The conservation. mechanism The of the mechanism addition reaction of the addition reaction of 2-phenyl imidazole derivatives was proposed (Scheme 14b). ofaddition 2-phenyl reaction imidazole of 2-ph derivativesenyl imidazole was proposed derivatives (Scheme was proposed 14b). (Scheme 14b).

(a) (b) (a) (b) Scheme 14. (a) The addition reaction of 2-phenyl imidazole derivatives catalyzed by Scheme 14. (a) The addition reaction of 2-phenyl imidazole derivatives βcatalyzed by ZrOScheme2-β-cyclodextrin; 14. (a) The addition (b) the reactionmechanism of 2-phenyl of addition imidazole reaction derivatives of 2-phenyl catalyzed imidazole by ZrO derivatives.2- -cyclodextrin; ZrO2-β-cyclodextrin; (b) the mechanism of addition reaction of 2-phenyl imidazole derivatives. (b) the mechanism of addition reaction of 2-phenyl imidazole derivatives. Ge et al. [40] studied the synthesis of glycidyl monostearate using HP-β-CD as a biomimic catalyst Ge et al. [40] studied the synthesis of glycidyl monostearate using HP-β-CD as a biomimic catalyst (phase transfer catalyst) (Scheme 15a). HP-β-CD showed the highest catalyticβ activity, and the yield was (phaseGe transfer et al. [40 catalyst)] studied (Scheme the synthesis 15a). HP- of glycidylβ-CD showed monostearate the highest using catalytic HP- -CDactivity, as a biomimicand the yield catalyst was up to 92.6% under the optimized reaction conditions.β The kinetic profiles indicated that HP-β-CD acted (phaseup to 92.6% transfer under catalyst) the optimized (Scheme reaction15a). HP- conditions.-CD showed The kinetic the highest profiles catalytic indicated activity, that HP- andβ-CD the yieldacted as the catalyst by accommodating small molecules to form inclusion complexes (Scheme 15b). β wasas the up catalyst to 92.6% by underaccommodating the optimized small reaction molecules conditions. to form inclusion The kinetic complexes proﬁles indicated(Scheme 15b). that HP- -CD acted as the catalyst by accommodating small molecules to form inclusion complexes (Scheme 15b).

O O Cl HP CD O O + O HP CD Cl + 8 O 8 60℃/water O 8 8 ONa 60℃/water O O ONa O (a) (b) (a) (b) Scheme 15. (a) The synthesis of glycidyl monostearate catalyzed by HP-β-CD; (b) the proposed Scheme 15. (a) The synthesis of glycidyl monostearate catalyzed by HP-β-CD; (b) the proposed mechanism of the synthesis of glycidyl monostearate. mechanism of the synthesis of glycidyl monostearate. Regarding polymerization, Galia et al. [41] reported the ring opening polymerization (ROP) of Regarding polymerization, Galia et al. [41] reported the ring opening polymerization (ROP) of ε-caprolactone in the presence of β-cyclodextrin in batch reactors. It was interesting that the rate of ε-caprolactone in the presence of β-cyclodextrin in batch reactors. It was interesting that the rate of the polymerization was enhanced, and the monomer conversion was up to 99% with wet β-CD the polymerization was enhanced, and the monomer conversion was up to 99% with wet β-CD under pressure. On the other hand, MALDI-TOF analyses indicated that a major fraction of polymer under pressure. On the other hand, MALDI-TOF analyses indicated that a major fraction of polymer chains formed under pressure were initiated by water molecules. The experimental results indicated chains formed under pressure were initiated by water molecules. The experimental results indicated that the reaction mechanism in the presence of wet β-CD, similar to that for lipase-catalyzed ROP of that the reaction mechanism in the presence of wet β-CD, similar to that for lipase-catalyzed ROP of cyclic esters, is operative, i.e., the secondary hydroxyl group of β-CD could mimic the role of the cyclic esters, is operative, i.e., the secondary hydroxyl group of β-CD could mimic the role of the hydroxyl group of serine 105. The hydroxyl groups of β-CD brought a nucleophilic attack on the hydroxyl group of serine 105. The hydroxyl groups of β-CD brought a nucleophilic attack on the

Molecules 2017, 22, 1475 7 of 16

Condensation polymerization was a simple method for the synthesis of the sequence chain. Tao et al. [38] synthesized a hydrophilic periodic polymer using the nitroso benzene/cyclodextrin complex (Scheme 13a). Firstly, the water-soluble inclusion complexes of CD/NB were formed and then polymerized with poly (ethylene glycol) bis (α-bromoisobutyrate) to produce macromolecular polymers in good yield. The mechanism of this reaction was proposed (Scheme 13b). The process provided a simple method to synthesize the sequence chain and was beneficial to the application in industry.

O O O N Cu/Ligand PEG O N Br + H2O PEG Br O

O n (a) (b)

Scheme 13. (a) The macromolecular addition reaction catalyzed β-CD; (b) the mechanism of the macromolecular addition reaction.

Girish et al. [39] reported the synthesis of 2-phenyl imidazole derivatives by the addition reaction using nano ZrO2-β-cyclodextrin as a supramolecular catalyst in good yield (Scheme 14a). In addition, the catalyst can be recycled to facilitate resource conservation. The mechanism of the addition reaction of 2-phenyl imidazole derivatives was proposed (Scheme 14b).

(a) (b)

Scheme 14. (a) The addition reaction of 2-phenyl imidazole derivatives catalyzed by ZrO2-β-cyclodextrin; (b) the mechanism of addition reaction of 2-phenyl imidazole derivatives.

Ge et al. [40] studied the synthesis of glycidyl monostearate using HP-β-CD as a biomimic catalyst (phase transfer catalyst) (Scheme 15a). HP-β-CD showed the highest catalytic activity, and the yield was upMolecules to 92.6%2017, under22, 1475 the optimized reaction conditions. The kinetic profiles indicated that HP-β-CD acted8 of 17 as the catalyst by accommodating small molecules to form inclusion complexes (Scheme 15b).

O O HP CD O Cl + 8 8 60℃/water O ONa O

(a) (b)

Scheme 15. ((aa)) The The synthesis synthesis of of glycidyl monostearate catalyzed catalyzed by HP-β-CD;-CD; ( b) the proposed mechanism of the synthesis of glycidyl monostearate.

Regarding polymerization, Galia et al. [41] reported the ring opening polymerization (ROP) of Regarding polymerization, Galia et al. [41] reported the ring opening polymerization (ROP) of ε-caprolactone in the presence of β-cyclodextrin in batch reactors. It was interesting that the rate of ε-caprolactone in the presence of β-cyclodextrin in batch reactors. It was interesting that the rate of the the polymerization was enhanced, and the monomer conversion was up to 99% with wet β-CD polymerization was enhanced, and the monomer conversion was up to 99% with wet β-CD under under pressure. On the other hand, MALDI-TOF analyses indicated that a major fraction of polymer pressure. On the other hand, MALDI-TOF analyses indicated that a major fraction of polymer chains chains formed under pressure were initiated by water molecules. The experimental results indicated formed under pressure were initiated by water molecules. The experimental results indicated that that the reaction mechanism in the presence of wet β-CD, similar to that for lipase-catalyzed ROP of the reaction mechanism in the presence of wet β-CD, similar to that for lipase-catalyzed ROP of cyclic cyclicMolecules esters, 2017, 22is, 1475operative, i.e., the secondary hydroxyl group of β-CD could mimic the role 8of of the16 esters, is operative, i.e., the secondary hydroxyl group of β-CD could mimic the role of the hydroxyl hydroxyl group of serine 105. The hydroxyl groups of β-CD brought a nucleophilic attack on the groupcarbonyl of serine of the 105.cyclic The ester hydroxyl resulting groups in the of formatβ-CDion brought of a linear a nucleophilic acyl complex, attack while on the the carbonyl inner water of the cyclicin the ester cavity resulting of β-CD in theacted formation as the role of a of linear water acyl molecules complex, bound while theto the inner hydration water in shell the cavity of the of βenzyme,-CD acted resulting as the rolein the of formation water molecules of an α bound, ω-hydroxy to the acid. hydration shell of the enzyme, resulting in the formation of an α, ω-hydroxy acid. 2.4. Conventional Substitution Reaction 2.4. Conventional Substitution Reaction Substitution reaction is a chemical reaction where one functional group in a molecule is replacedSubstitution by another reaction functional is a chemical group. reaction where one functional group in a molecule is replaced by anotherKiasat functionalet al. [42] constructed group. the β-cyclodextrin-polyurethane, polymer-coated, Fe3O4 magnetic nanoparticleKiasat et as al. a [42 novel] constructed class of thecatalystβ-cyclodextrin-polyurethane, for the substitution reaction polymer-coated, of benzyl halides Fe3O 4inmagnetic water nanoparticle(Scheme 16a). as As a novel a solid–liquid class of catalyst phase-transfer for the substitutioncatalyst, it reactionshowed ofability benzyl as halidesa nucleophilic in water (Schemesubstitution 16a). reaction As a solid–liquid of benzyl phase-transfer halides with azide, catalyst, cyanide, it showed thiocyanate, ability as aand nucleophilic acetate anions. substitution The reactionpossible of mechanism benzyl halides of this with reaction azide, cyanide, was proposed thiocyanate, (Scheme and acetate16b). It anions.was more The interesting possible mechanism that no ofevidence this reaction for the was formation proposed of (Schemeby-products 16b). was It was observ moreed interestingand the targeted that no products evidence were for the obtained formation in ofhigh by-products purity without was observed further purification. and the targeted In addi productstion, the were nanomagnetic obtained in polymer high purity brush without catalyst further was puriﬁcation.readily removed In addition, from the the solution nanomagnetic through polymer the application brush catalyst of a magnetic was readily field, removed thus allowing from the solutionstraightforward through reuse. the application of a magnetic ﬁeld, thus allowing straightforward reuse.

X 2

OCONH(CH2)6NHCO OCH2 CH CH2OCONH(CH2)6NHCO Fe3O4 -X CH2Y G OCONH(CH2)6NHCO OCH2 CH2OCONH(CH2)6NHCO Fe O 3 4 n G

n β-CDPU-MNPs (a) (b)

SchemeScheme 16.16.( a()a) The The substitution substitution reaction reaction of benzyl of benzyl halides halides catalyzed catalyzed by β-CDPU-MNPs; by β-CDPU-MNPs; (b) the proposed(b) the mechanismproposed mechanism of the substitution of the substitu reactiontion of benzylreaction halides. of benzyl halides.

KiasatKiasat etet al.al. [[43]43] developeddeveloped the substitution re reactionaction of of phenacyl phenacyl derivatives derivatives in in water water using using β-cyclodextrin immobilized on Dowex resin as an efficient solid-liquid phase transfer catalyst β-cyclodextrin immobilized on Dowex resin as an efﬁcient solid-liquid phase transfer catalyst (Scheme 17a). The nucleophilic substitution reactions were carried out under a mild reaction (Scheme 17a). The nucleophilic substitution reactions were carried out under a mild reaction condition condition and given the products in good yields (80–95%). Also, the plausible reaction mechanism and given the products in good yields (80–95%). Also, the plausible reaction mechanism of alkyl of alkyl halides catalyzed by Dower-β-CD was proposed (Scheme 17b). Furthermore, the catalyst could be recycled and reused by convenient separation with little loss of activity. y p y Br H O Nu O

H SO3

SO3H

(a) (b)

Scheme 17. (a) The substitution reaction of alkyl halides catalyzed by Dower-β-CD; (b) the proposed mechanism of the substitution reaction of alkyl halides.

Molecules 2017, 22, 1475 8 of 16 carbonyl of the cyclic ester resulting in the formation of a linear acyl complex, while the inner water in the cavity of β-CD acted as the role of water molecules bound to the hydration shell of the enzyme, resulting in the formation of an α, ω-hydroxy acid.

2.4. Conventional Substitution Reaction Substitution reaction is a chemical reaction where one functional group in a molecule is replaced by another functional group. Kiasat et al. [42] constructed the β-cyclodextrin-polyurethane, polymer-coated, Fe3O4 magnetic nanoparticle as a novel class of catalyst for the substitution reaction of benzyl halides in water (Scheme 16a). As a solid–liquid phase-transfer catalyst, it showed ability as a nucleophilic substitution reaction of benzyl halides with azide, cyanide, thiocyanate, and acetate anions. The possible mechanism of this reaction was proposed (Scheme 16b). It was more interesting that no evidence for the formation of by-products was observed and the targeted products were obtained in high purity without further purification. In addition, the nanomagnetic polymer brush catalyst was readily removed from the solution through the application of a magnetic field, thus allowing straightforward reuse.

X 2

OCONH(CH2)6NHCO OCH2 CH CH2OCONH(CH2)6NHCO Fe3O4 -X CH2Y G OCONH(CH2)6NHCO OCH2 CH2OCONH(CH2)6NHCO Fe O 3 4 n G

n β-CDPU-MNPs (a) (b)

Scheme 16. (a) The substitution reaction of benzyl halides catalyzed by β-CDPU-MNPs; (b) the proposed mechanism of the substitution reaction of benzyl halides.

Kiasat et al. [43] developed the substitution reaction of phenacyl derivatives in water using β-cyclodextrin immobilized on Dowex resin as an efficient solid-liquid phase transfer catalyst Molecules(Scheme2017 17a)., 22, 1475The nucleophilic substitution reactions were carried out under a mild reaction9 of 17 condition and given the products in good yields (80–95%). Also, the plausible reaction mechanism halidesof alkyl catalyzedhalides catalyzed by Dower- byβ -CDDower- wasβ-CD proposed was proposed (Scheme 17(Schemeb). Furthermore, 17b). Furthermore, the catalyst the could catalyst be recycledcould be andrecycled reused and by reused convenient by convenient separation separation with little with loss oflittle activity. loss of activity. y p y Br H O Nu O

H SO3

SO3H

(a) (b)

Scheme 17. (a) The substitution reaction of alkyl halides catalyzed by Dower-ββ-CD; (b) the proposed mechanism of thethe substitutionsubstitution reactionreaction ofof alkylalkyl halides.halides.

2.5. Conventional Hydrolysis Reaction

Cyclodextrins are widely applied to the hydrolysis reaction in organic chemistry. Zhou et al. [44] constructed the zinc (II) inclusion complexes using β-CD as a host molecule, in which the guest molecule with the adamantane pendant was inserted into the cavity of β-CD. As the model of metallohydrolase (Scheme 18a), the pH behavior of the rate constant of bis (4-nitrophenyl) phosphate hydrolysis exhibited an obvious increase with the second-order rate constant (2.68 × 10−3 M−1 s−1) assigned to the di-hydroxo species, which was almost an order of magnitude higher than that of the reported mono-Zn (II)-hydroxo species. The enhanced reactivity probably was ascribed to the hydroxyl-rich microenvironment provided by β-CD, which might have an effect on the stability of the labile zinc-hydroxo species or the catalytic transition state (Scheme 18b).

Scheme 18. (a) The hydrolysis reaction of bis(4-nitrophenyl)phosphate catalyzed by β-cyclodextrin; (b) the mechanism of the hydrolysis reaction of bis(4-nitrophenyl)phosphate.

On the other hand, Andres et al. [45] studied the hydrolysis of phthalate ester in water in the presence of hydroxypropyl-β-cyclodextrin. The observed rate constant for the hydrolysis decreased in a non-linear fashion when the HP-β-CD concentration increased (Scheme 19a). The kinetic result was interpreted in terms of the formation of an inclusion complex of phthalate ester with HP-β-CD. In this complex, the carboxylate group formed hydrogen bonds with the hydroxyl groups at the rim of HP-β-CD, and the ionized group of HP-β-CD was far away from the reaction center due to electrostatic repulsion (Scheme 19b). Molecules 2017, 22, 1475 9 of 16

2.5. Conventional Hydrolysis Reaction Cyclodextrins are widely applied to the hydrolysis reaction in organic chemistry. Zhou et al. [44] constructed the zinc (II) inclusion complexes using β-CD as a host molecule, in which the guest molecule with the adamantane pendant was inserted into the cavity of β-CD. As the model of metallohydrolase (Scheme 18a), the pH behavior of the rate constant of bis (4-nitrophenyl) phosphate hydrolysis exhibited an obvious increase with the second-order rate constant (2.68 × 10−3 M−1 s−1) assigned to the di-hydroxo species, which was almost an order of magnitude higher than that of the reported mono-Zn (II)-hydroxo species. The enhanced reactivity probably was ascribed to the hydroxyl-rich microenvironment provided by β-CD, which might have an effect on the stability of the labile zinc-hydroxo species or the catalytic transition state (Scheme 18b).

O O O O O O O O Catalyst P P P P + HO NO O2N O OH 2 O2N O O NO2 O2N O O NO2 O2N O O NO2 aqueous solutions Cl Cl HO OH2 HO OH H2O OH2 Zn Zn Cl Cl Zn Zn N N N N N N N N Zn N N N N N Catalyst: N N pKa pKa1 2 H2O

(a) (b)

Scheme 18. (a) The hydrolysis reaction of bis(4-nitrophenyl)phosphate catalyzed by β-cyclodextrin; (b) the mechanism of the hydrolysis reaction of bis(4-nitrophenyl)phosphate.

On the other hand, Andres et al. [45] studied the hydrolysis of phthalate ester in water in the presence of hydroxypropyl-β-cyclodextrin. The observed rate constant for the hydrolysis decreased in a non-linear fashion when the HP-β-CD concentration increased (Scheme 19a). The kinetic result was interpreted in terms of the formation of an inclusion complex of phthalate ester with HP-β-CD. In this complex, the carboxylate group formed hydrogen bonds with the hydroxyl groups at the rim of HP-β-CD, and the ionized group of HP-β-CD was far away from the reaction center due to Molecules 22 electrostatic2017, repulsion, 1475 (Scheme 19b). 10 of 17

O O O O O O O O O O O O O O O OH

k-n kb ke + OH kn

(a) (b)

Scheme 19.19. ((aa)) TheThe hydrolysishydrolysis reaction reaction of of phthalate phthalate ester ester catalyzed catalyzed by by HP- HP-β-CD;β-CD; (b )(b the) the mechanism mechanism of theof the hydrolysis hydrolysis reaction reaction of phthalateof phthalate ester. ester.

3. Application of Cyclodextrin in Asymmetric Reaction 3. Application of Cyclodextrin in Asymmetric Reaction

3.1. Asymmetric Oxidation Reaction Cyclodextrins have have been been used used as as chiral chiral reaction reaction containers containers for forthe theasymmetric asymmetric oxidation oxidation of aryl of arylalkyl alkylsulfides sulfides in moderate in moderate to poor to poorenantiomet enantiometricric excesses excesses by Drabowicz, by Drabowicz, Mikolajczyk Mikolajczyk [46], [and46], andCzarnik Czarnik [47]. [47]. InIn recentrecent years,years, ShenShen etet al.al. [[48]48] designeddesigned andand synthesizedsynthesized aa serialserial ofof aminoamino alcohol-modifiedalcohol-modified β-cyclodextrins for asymmetric oxidation in water (Scheme(Scheme 2020a).a). Using the asymmetric oxidation of thioanisolethioanisole as model reaction, the moderatemoderate ee valuevalue was achieved, which might be ascribed to the twotwo differentdifferent bindingbinding sitessites ofof modifiedmodified ββ-cyclodextrin-cyclodextrin withwith thioanisole.thioanisole. The intramolecular catalysiscatalysis produced (S)-methyl phenyl sulfoxide, while the intermolecular catalysis formed (R,S)-methyl producedMolecules 2017 (S, )-methyl22, 1475 phenyl sulfoxide, while the intermolecular catalysis formed (R,S)-methyl phenyl10 of 16 sulfoxide.phenyl sulfoxide. The delicate The delicate cooperation cooperation between be thetween substituent the substituent groups groups and the and parent the βparent-CD played β-CD anplayed important an important role in inducingrole in inducing enantioselectivity enantioselectivity in the modified in the modifiedβ-CDs (Scheme β-CDs 20(Schemeb). Inaddition, 20b). In theaddition, enantioselectivity the enantioselectivity in the asymmetric in the asymmetric oxidation ofoxidation thioanisole of thioanisole catalyzed by catalyzed modified byβ -CDsmodified was illustratedβ-CDs was through illustrated quantum through calculation, quantum which calculation, provided which a practical provided method a thatpractical explained method the origin that ofexplained the enantioselectivity the origin of the in theenantioselectivity asymmetric reaction. in the asymmetric reaction. p y jy[ ] ik [47]. HN OH H O NaMoO4 O O Mo O HN Ⅰ O CH3 S S CH3

H H O CH3 O CH3 O Ⅱ O S S O O CH3 Ⅱ O Mo O S O Mo O O CH HN NaMoO4 HN 3 O CH S 3 S O (S)- (R,S)- O CH OH S 3 (S)- HN (R,S)- S CH3 Ⅳ Ⅳ

H H O O O O O O Ⅲ O O H O Mo Mo 2 2 Ⅲ HN O HN O H2O2 O O O O CH S 3

S CH3 or O O O Mo HO O O O N H

S (a) H3 C (b)

Scheme 20. (a) The The asymmetric asymmetric oxidation of thioanisole catalyzed by modiﬁedmodified β-cyclodextrin; (b) the proposed mechanism of the asymmetric oxidationoxidation ofof thioanisole.thioanisole.

Mojr et al. [49,50] synthesized two β-cyclodextrin-flavin conjugates as supramolecular catalysts for the sulfoxidation of methyl phenyl sulfides with H2O2 in water (Scheme 21a). The catalytic system, based on the β-cyclodextrin-alloxazine conjugate, had high enantioselectivity up to 80% ee. In addition, it was remarkable that the reaction was performed in water with very low loadings of the catalyst (low to 0.2 mol %) with a turnover number up to 395. The mechanism of the asymmetric oxidation of methyl phenyl sulfides through the inclusion complex with β-cyclodextrin-flavin conjugate was proposed (Scheme 21b).

O Ar CH3 H2O2 O S 2.5 equiv, H2O2 S Ar Me FLCD(cat.) S S Ar Me Ar CH3 Ar CH H2O, [pH 7.5] 3 e Ⅰ Ⅱ e e M M M O N O O N N O H O O O N H H N N N N N Me N N N N N FLCD: N O Pol: beta-cyclodextrin N O N O N N O H N Pol

O (a) (b)

Scheme 21. (a) The asymmetric oxidation of methyl phenyl sulfides catalyzed by β-cyclodextrin-flavin conjugates; (b) the proposed mechanism of the asymmetric oxidation of methyl phenyl sulfides.

3.2. Asymmetric Reduction Reaction In the aspect of asymmetric reduction, Park et al. [51] studied the asymmetric reduction of various prochiral ketones with sodium borohydride using β-cyclodextrin and its derivatives as a chiral template (Scheme 22a) and found that the enantioselectivity in the asymmetric reduction of

Molecules 2017, 22, 1475 10 of 16 played an important role in inducing enantioselectivity in the modified β-CDs (Scheme 20b). In addition, the enantioselectivity in the asymmetric oxidation of thioanisole catalyzed by modified β-CDs was illustrated through quantum calculation, which provided a practical method that explained the origin of the enantioselectivity in the asymmetric reaction. p y jy[ ] ik [47]. HN OH H O NaMoO4 O O Mo O HN Ⅰ O CH3 S S CH3

H H O CH3 O CH3 O Ⅱ O S S O O CH3 Ⅱ O Mo O S O Mo O O CH HN NaMoO4 HN 3 O CH S 3 S O (S)- (R,S)- O CH OH S 3 (S)- HN (R,S)- S CH3 Ⅳ Ⅳ

H H O O O O O O Ⅲ O O H O Mo Mo 2 2 Ⅲ HN O HN O H2O2 O O O O CH S 3

S CH3 or O O O Mo HO O O O N H

S (a) H3 C (b)

Scheme 20. (a) The asymmetric oxidation of thioanisole catalyzed by modified β-cyclodextrin; (b) the Molecules 2017, 22, 1475 11 of 17 proposed mechanism of the asymmetric oxidation of thioanisole.

Mojr et al. [[49,50]49,50] synthesized two β-cyclodextrin-flavin-cyclodextrin-flavin conjugates as supramolecular catalysts for the sulfoxidation of methyl phenyl sulfides with H2O2 in water (Scheme 21a). The catalytic for the sulfoxidation of methyl phenyl sulfides with H2O2 in water (Scheme 21a). The catalytic system, system,based on based the β -cyclodextrin-alloxazineon the β-cyclodextrin-alloxazine conjugate, conjugate, had high enantioselectivityhad high enantioselectivity up to 80% ee.up In to addition, 80% ee. Init wasaddition, remarkable it was thatremarkable the reaction that the was reaction performed was in performed water with in verywater low with loadings very low of theloadings catalyst of (lowthe catalyst to 0.2 mol(low %) to with0.2 mol a turnover %) with numbera turnover up nu tomber 395. Theup to mechanism 395. The mechanism of the asymmetric of the asymmetric oxidation oxidationof methyl phenylof methyl sulfides phenyl through sulfides the inclusionthrough the complex inclusion with complexβ-cyclodextrin-flavin with β-cyclodextrin-flavin conjugate was proposedconjugate (Schemewas proposed 21b). (Scheme 21b).

O (a) (b)

Scheme 21. ((aa)) The The asymmetric asymmetric oxidation oxidation of of methyl methyl phenyl phenyl sulfides sulfides catalyzed by β-cyclodextrin-flavin-cyclodextrin-flavin conjugates; ( (bb)) the the proposed proposed mechanism mechanism of of the the asymmetric asymmetric oxidation oxidation of of methyl methyl phenyl phenyl sulfides. sulfides.

3.2. Asymmetric Reduction Reaction In the aspect of asymmetric reduction, Park et al. [[51]51] studied the asymmetric reduction of various prochiral ketones with sodium borohydride using β-cyclodextrin and and its derivatives as a Molecules 2017, 22, 1475 11 of 16 chiral template (Scheme 2222a)a) and found that the enantioselectivityenantioselectivity in the asymmetric reduction of ketonesketones toto secondarysecondary alcoholsalcohols waswas dependentdependent on thethe structuresstructures ofof hostshosts andand ketones,ketones, asas wellwell asas thethe reactionreaction temperature.temperature. In In addition, addition, the the results results indicated indicated that thethat absolute the absolute conﬁguration configuration of the products of the dependedproducts depended on the conformational on the conformational structure of structure the guest-host of the complexes,guest-host complexes, and the decrease and the of thedecrease degrees of ofthe freedom degrees of of the freedom guest in of the the complexes guest in wasthe complexes a major factor was for a themajor improvement factor for the of enantioselectivity improvement of (Schemeenantioselectivity 22b). (Scheme 22b).

NaBH4 H OH

O CH3 O H CH3

R R

(a) (b) r SchemeScheme 22.22. ((aa)) TheThe reductioneduction reactionreaction ofof ketonesketones catalyzedcatalyzed bybyβ β-CD;-CD; ((bb)) thethe proposedproposed mechanismmechanism ofof thethe reductionreduction reactionreaction ofof ketones.ketones.

Deratani et al. [52] reported the effect of substituent groups on the asymmetric reduction induced Deratani et al. [52] reported the effect of substituent groups on the asymmetric reduction induced by β-cyclodextrin in the reduction of a series of o-, m-, and p-substituted acetophenones (X = H, Br, C1, by β-cyclodextrin in the reduction of a series of o-, m-, and p-substituted acetophenones (X = H, CH3, NO2, OCH3) with aqueous NaBH4 (Scheme 23a). The substitutions resulted in higher Br, C1, CH , NO , OCH ) with aqueous NaBH (Scheme 23a). The substitutions resulted in higher enantioselectivities3 2 than that3 obtained from acetophenone.4 On the other hand, acetophenone and its m- enantioselectivities than that obtained from acetophenone. On the other hand, acetophenone and and p-derivatives produced mainly (−)-alcohol, while the o-derivatives gave the (+)-alcohol enantiomer. its m- and p-derivatives produced mainly (−)-alcohol, while the o-derivatives gave the (+)-alcohol Enantioselectivity was mainly controlled by the shape and size of the β-cyclodextrin cavity in the case of enantiomer. Enantioselectivity was mainly controlled by the shape and size of the β-cyclodextrin cavity the o-substituted acetophenones and governed by hydrophobic interactions in the case of the in the case of the o-substituted acetophenones and governed by hydrophobic interactions in the case of m-derivatives, while the asymmetic reduction of p-derivatives was complicated (Scheme 23b). the m-derivatives, while the asymmetic reduction of p-derivatives was complicated (Scheme 23b).

H OH

O CH3 O H CH3

NaBH4

R R

(a) (b)

Scheme 23. (a) The reduction reaction of substituted acetophenones catalyzed by β-CD; (b) the reduction mechanism of substituted acetophenones catalyzed by β-CD.

Similarly, the asymmetric reductions of prochiral ketones were also reported in different enantiometric excesses using modified cyclodextrins as supramolecular catalysts [53,54]. In the metal catalysts containing cyclodextrins, Jouffroy et al. [55] constructed a serial of phosphane-phosphite chelators comprising α-cyclodextrin, which were applied in Rh-catalysed asymmetric hydrogenation and hydroformylation. The results indicated that the highly mobile chelate complexes gave poor enantioselectivity, while the more rigid ones gave significant ee values up to 92%.

3.3. Asymmetric Addition Reaction Cyclodextrin was also used in the asymmetric addition reaction. Pitchumani et al. [56] studied the asymmetric Michael addition of nitromethane and aliphatic thiols in aqueous media using per-6-amino-β-cyclodextrin (per-6-ABCD) as a chiral base catalyst (Scheme 24a). A better enantiomeric excess (60.6%) was observed in water at room temperature with good yield. The catalyst could be recovered and reused with little loss in its activity. The mechanism of the asymmetric addition of nitromethane and aliphatic thiols through the inclusion complex with per-6-ABCD was proposed (Scheme 24b). Using the same catalyst (per-6-ABCD), Pitchumani et al. [57] reported the asymmetric synthesis of 2-aryl-2,3-dihydro-4-quinolones with high yield (up to 99%) and enantiomeric excess (up to 99%).

Molecules 2017, 22, 1475 11 of 16

ketones to secondary alcohols was dependent on the structures of hosts and ketones, as well as the reaction temperature. In addition, the results indicated that the absolute configuration of the products depended on the conformational structure of the guest-host complexes, and the decrease of the degrees of freedom of the guest in the complexes was a major factor for the improvement of enantioselectivity (Scheme 22b).

NaBH4 H OH

O CH3 O H CH3

R R

(a) (b)

Scheme 22. (a) The reduction reaction of ketones catalyzed by β-CD; (b) the proposed mechanism of the reduction reaction of ketones.

Deratani et al. [52] reported the effect of substituent groups on the asymmetric reduction induced by β-cyclodextrin in the reduction of a series of o-, m-, and p-substituted acetophenones (X = H, Br, C1, CH3, NO2, OCH3) with aqueous NaBH4 (Scheme 23a). The substitutions resulted in higher enantioselectivities than that obtained from acetophenone. On the other hand, acetophenone and its m- and p-derivatives produced mainly (−)-alcohol, while the o-derivatives gave the (+)-alcohol enantiomer. Enantioselectivity was mainly controlled by the shape and size of the β-cyclodextrin cavity in the case of Moleculesthe o-substituted2017, 22, 1475 acetophenones and governed by hydrophobic interactions in the case 12of ofthe 17 m-derivatives, while the asymmetic reduction of p-derivatives was complicated (Scheme 23b).

H OH

O CH3 O H CH3

NaBH4

R R

(a) (b)

SchemeScheme 23.23.( a()a The) The reduction reduction reaction reaction of substituted of substituted acetophenones acetophenones catalyzed catalyzed by β-CD; by (b )β the-CD; reduction (b) the mechanismreduction mechanism of substituted of substitu acetophenonested acetophenones catalyzed by catalyzedβ-CD. by β-CD.

Similarly, the asymmetric reductions of prochiral ketones were also reported in different Similarly, the asymmetric reductions of prochiral ketones were also reported in different enantiometric excesses using modified cyclodextrins as supramolecular catalysts [53,54]. enantiometric excesses using modiﬁed cyclodextrins as supramolecular catalysts [53,54]. In the metal catalysts containing cyclodextrins, Jouffroy et al. [55] constructed a serial of In the metal catalysts containing cyclodextrins, Jouffroy et al. [55] constructed a serial of phosphane-phosphite chelators comprising α-cyclodextrin, which were applied in Rh-catalysed phosphane-phosphite chelators comprising α-cyclodextrin, which were applied in Rh-catalysed asymmetric hydrogenation and hydroformylation. The results indicated that the highly mobile chelate asymmetric hydrogenation and hydroformylation. The results indicated that the highly mobile complexes gave poor enantioselectivity, while the more rigid ones gave significant ee values up to 92%. chelate complexes gave poor enantioselectivity, while the more rigid ones gave signiﬁcant ee values up3.3. to Asymmetric 92%. Addition Reaction 3.3. AsymmetricCyclodextrin Addition was also Reaction used in the asymmetric addition reaction. Pitchumani et al. [56] studied the asymmetricCyclodextrin Michael was also addition used in theof nitromethane asymmetric addition and aliphatic reaction. thiols Pitchumani in aqueous et al. media [56] studied using theper-6-amino- asymmetricβ-cyclodextrin Michael addition (per-6-ABCD) of nitromethane as a chiral and aliphaticbase catalyst thiols (Scheme in aqueous 24a). media A usingbetter per-6-amino-enantiomericβ -cyclodextrinexcess (60.6%) (per-6-ABCD) was observed as ain chiral water base at catalyst room temperature (Scheme 24a). with A better good enantiomeric yield. The excesscatalyst (60.6%) could wasbe recovered observed inand water reused at room with temperature little loss in with its goodactivity. yield. The The mechanism catalyst could of the be recoveredasymmetric and addition reused withof nitromethane little loss in itsand activity. aliphatic The thiols mechanism through of the the inclusion asymmetric complex addition with of nitromethaneper-6-ABCD was and proposed aliphatic thiols(Scheme through 24b). Using the inclusion the same complex catalyst with (per-6-ABCD), per-6-ABCD Pitchumani was proposed et al. (Scheme[57] reported 24b). Usingthe asymmetric the same catalystsynthesis (per-6-ABCD), of 2-aryl-2,3-dihydro-4-quinolones Pitchumani et al. [57] reported with high the yield asymmetric (up to synthesis99%) and ofenantiomeric 2-aryl-2,3-dihydro-4-quinolones excess (up to 99%). with high yield (up to 99%) and enantiomeric excess (up toMolecules 99%). 2017, 22, 1475 12 of 16

H H H H O O O O O O O O

Cage Escape NO 2 NO2 CH2NO2 CHNO2 O H NO2 H N H N 2 2 H NH2 H2N (a) (b)

SchemeScheme 24.24.( a(a)) The The asymmetric asymmetric MichaelMichael additionaddition of nitromethane and and thiols thiols to to chalcones chalcones catalyzed catalyzed byby Per-6-ABCD;Per-6-ABCD; (b) the proposed proposed mechanism mechanism for for the the asymmetric asymmetric Michael Michael addition addition catalyzed catalyzed by byper-6-ABCD. per-6-ABCD.

Similarly, the asymmetric Michael addition of cyclohexanone and 2-nitro-β-nitrostyrene was β reportedSimilarly, by theZhu asymmetric et al. [58]. Michael Using addition (S)-2-aminomethylpyrrolidine-modified of cyclohexanone and 2-nitro- -nitrostyrene β-CD as the was reportedsupramolecular by Zhu etcatalyst al. [58]. (Scheme Using (S )-2-aminomethylpyrrolidine-modiﬁed25a), a good enantiomeric excess of β71%-CD aswas the observed supramolecular in an catalystaqueous (Scheme buffer solution.25a), a good The enantiomericmechanism of excess the asy ofmmetric 71% was Michael observed addition in an aqueous of cyclohexanone buffer solution. and The2-nitro- mechanismβ-nitrostyrene of the asymmetricwas proposed Michael (Scheme addition 25b). of cyclohexanone and 2-nitro-β-nitrostyrene was proposed (Scheme 25b).

O HN HN HN

O R NH N N NO2

NO2 NO2 O2N

R R R (a) (b)

Scheme 25. (a) The asymmetric Michael addition of cyclohexanone and 2-nitro-β-nitrostyrene catalyzed by (S)-2-aminomethylpyrrolidine-modified β-CD; (b) the proposed mechanism for the asymmetric Michael addition of cyclohexanone and 2-nitro-β-nitrostyrene.

In the direct asymmetric aldol reactions of aldehydes and acetone using cyclodextrins as the supramolecular catalysts, Liu et al. [59] synthesized a new β-cyclodextrin-proline conjugated through a urea linker. Using the proline derived β-CD as the supramolecular catalyst (Scheme 26), a more excellent enantiomeric excess of 99% was observed in the moderate yield compared to that reported in the references [60,61]. In addition, the good recyclability and reusability of the catalyst was confirmed.

O O O O OH O H3C CH3 R1 H H2O R1 CH 3 1 R CH3 (a) (b)

Scheme 26. (a) The asymmetric aldol reaction of aldehydes and acetone catalyzed by β-CD; (b) the plausible mechanism of the asymmetric aldol reaction of aldehydes and acetone.

In the application of the asymmetric addition reaction based on cyclodextrins, Ni et al. [62] developed the asymmetric synthesis of (+)-2-methoxy-2-methylchroman-7-ol in solid-state using β-cyclodextrin as the chiral catalyst (Scheme 27). The chiral product was obtained from β-CD/1, 3-dihydroxylbenzene, and methyl vinyl ketone/β-CD in 82.8% yield and 78.4% ee.

HO OH O

Complex A Complex B (a) (b)

Scheme 27. The asymmetric synthesis of (+)-2-methoxy-2-methylchroman-7-ol catalyzed by β-CD.

Molecules 2017, 22, 1475 12 of 16

H H H H O O O O O O O O

Cage Escape NO 2 NO2 CH2NO2 CHNO2 O H NO2 H N H N 2 2 H NH2 H2N

Molecules 2017 , 22(a, )1475 (b) 12 of 16 SchemeMolecules 24. 2017 (a), 22The, 1475 asymmetric Michael addition of nitromethane and thiols to chalcones catalyzed12 of 16

H H H H O O by Per-6-ABCD; (b) the proposed mechanism for the asymmetricO O O MichaelO additionO O catalyzed by H H H H O O O O O O O CageO per-6-ABCD. Escape NO 2 NO2 CH2NO2 CHNO2 Cage O Escape H NO NO2 2 NO2 CH2NO2 CHNO2 H N H N H N O 2 2 H NH2 2 Similarly, the asymmetric Michael addition of cyclohexanoneH and 2-nitro-β-nitrostyreneNO2 was H N H N reported by Zhu et (aal.) [58]. Using ( S )-2-aminomethylpyrrolidine-modified 2 2 H(bNH) 2 H2N β-CD as the (a) (b) supramolecularScheme catalyst 24. (a) The (Scheme asymmetric 25a), Michael a goodaddition en of antiomericnitromethane andexcess thiols toof chalcones 71% was catalyzed observed in an aqueous bufferbyScheme Per-6-ABCD; solution. 24. (a) The The(b )asymmetric the mechanism proposed Michael mechanism of addition the asyfor of mmetricthe nitromethane asymmetric Michael and Michael thiols addition toaddition chalcones catalyzedof catalyzedcyclohexanone by and per-6-ABCD.by Per-6-ABCD; (b) the proposed mechanism for the asymmetric Michael addition catalyzed by 2-nitro-β-nitrostyrene was proposed (Scheme 25b). per-6-ABCD. Similarly, the asymmetric Michael addition of cyclohexanone and 2-nitro-β-nitrostyrene was O reportedSimilarly, by Zhuthe asymmetricet al. [58]. Michael Using a ddition(S)-2-aminomethylpyrrolidine-modifiedHN of cyclohexanoneHN and 2-nitro-βHN-nitrostyrene β-CD as wasthe reported by Zhu et al. [58]. Using (S)-2-aminomethylpyrrolidine-modified β-CD as the supramolecular catalyst (Scheme 25a), a good enantiomeric excess of 71% was observed inO an R NH N N aqueoussupramolecular buffer solution. catalyst The(Scheme mechanism 25a), aof good the asy enmmetricantiomeric Michael excess addition of 71% of was cyclohexanone observed in and an NO2

NO2 2-nitro-aqueousβ -nitrostyrenebuffer solution. was The proposed mechanism (Scheme of the 25b). asy mmetricNO 2 Michael addition of cyclohexanone and Molecules 2017, 22, 1475 O2N 13 of 17 2-nitro-β-nitrostyrene was proposed (Scheme 25b). R R R O HN HN HN (a) O (b) HN HN HN O R NH N N NO2 O R NO2 NO2 N N NH NO Scheme 25. (a) The asymmetric Michael addition of cyclohexanone Oand2N 2-nitro-β-nitrostyrene2 NO2 NO2 R catalyzed by (S)-2-aminomethylpyrrolidine-modified β-CD;R (bR ) the proposedO2N mechanism for the

(a) R (b) R asymmetric Michael addition of cyclohexanone and 2-nitro-R β-nitrostyrene. (a) (b) Scheme 25. (a) The asymmetric Michael addition of cyclohexanone and 2-nitro-β-nitrostyrene In the directcatalyzedScheme asymmetric 25. 25.by( a ()aS The))-2-aminomethylpyrrolidine-modified The asymmetric asymmetric aldol Michael reactions Michael addition additionof of al cyclohexanonedehydes ofβ-CD; cyclohexanone (b and) andthe 2-nitro-acetoneproposed andβ -nitrostyrene2-nitro- mechanismusingβ-nitrostyrene cyclodextrins catalyzed for the as the supramolecularasymmetricbycatalyzed (S )-2-aminomethylpyrrolidine-modiﬁedcatalysts, by Michael (S)-2-aminomethylpyrrolidine-modified Liu addition et ofal. cyclohexanone [59] synthesizedβ-CD; and ( b2-nitro-) theβ-CD; proposedaβ-nitrostyrene. (newb) the mechanism βproposed-cyclodextrin-proline mechanism for the asymmetric for the conjugated β through a ureaMichaelasymmetric linker. addition Michael Using of cyclohexanone addition the proline of cyclohexanone and derived 2-nitro- and -nitrostyrene.β-CD 2-nitro- as βthe-nitrostyrene. supramolecular catalyst (Scheme 26), a In the direct asymmetric aldol reactions of aldehydes and acetone using cyclodextrins as the more excellent enantiomeric excess of 99% was observed in the moderate yield compared to that supramolecularInIn the directdirect catalysts, asymmetricasymmetric Liu aldolaldolet al. reactionsreactions [59] synthesized ofof aldehydesaldehydes a new andand acetoneacetoneβ-cyclodextrin-proline usingusing cyclodextrinscyclodextrins conjugated asas the reportedthroughsupramolecular in the areferences urea linker. catalysts,catalysts, [60,61].Using Liu Liu the et In al.et proline [addition,al.59 ][59] synthesized derived synthesized the β-CD agood new as a theβrecyclability -cyclodextrin-prolinenew supramolecular β-cyclodextrin-proline and catalyst reusability conjugated (Scheme conjugated throughof 26), the a catalyst was confirmed.moreathrough urea excellent linker. a urea Using enantiomericlinker. the Using proline excessthe derivedproline of 99% derivedβ-CD was as obseβ the-CDrved supramolecular as the in supramolecularthe moderate catalyst yield catalyst (Scheme compared (Scheme 26), ato more26), that a reportedexcellentmore excellent enantiomericin the enantiomericreferences excess [60,61]. ofexcess 99% In wasof addition, 99% observed was the obsein good therved moderaterecyclability in the moderate yield and compared reusability yield tocompared that of the reported catalyst to that in wasreported confirmed. in the references [60,61]. In addition, the good recyclability and reusability of the catalyst the references [60,61]. In addition, the good recyclability andO reusability ofO the catalyst was conﬁrmed. was confirmed. O O OH O H3C CH3 R1 O H O H2O 1 R CH3 O O R1 CH O O OH O 3 H3C CH3 R1 H H2O 1 O O R CH3 R1 OH O CH H3C CH3 3 R1 H H2O R1 CH 3 1 (a) (b) R CH3 (a) (b) Scheme 26. (a) The asymmetric(a) aldol reaction of aldehydes and acetone(b catalyzed) by β-CD; (b) the Scheme 26. (a) The asymmetric aldol reaction of aldehydes and acetone catalyzed by β-CD; (b) the plausibleScheme mechanism 26. (a) Theof the asymmetric asymmetric aldol aldol reaction reaction of aldehydes of aldehydes and acetone and catalyzed acetone. by β-CD; (b) the plausibleScheme 26. mechanism (a) The asymmetric of the asymmetric aldol reaction aldol reaction of aldehydes ofof aldehydesaldehydes and acetone andand acetone.acetone. catalyzed by β-CD; (b) the plausible mechanism of the asymmetric aldol reaction of aldehydes and acetone. In the applicationIn the application of the of theasymmetric asymmetric addition addition rereactionaction based based on cyclodextrins,on cyclodextrins, Ni et al. Ni [62] et al. [62] In the application of the asymmetric addition reaction based on cyclodextrins, Ni et al. [62] developedIn the theapplication asymmetric of the synthesis asymmetric of (+)-2-meth addition oxy-2-methylchroman-7-olreaction based on cyclodextrins, in solid-state Ni et al. using [62] developeddeveloped the asymmetric the asymmetric synthesis synthesis of of (+)-2-meth (+)-2-methoxy-2-methylchroman-7-oloxy-2-methylchroman-7-ol in solid-state in solid-state using using βdeveloped-cyclodextrin the asasymmetric the chiral synthesiscatalyst (Scheme of (+)-2-meth 27). Theoxy-2-methylchroman-7-ol chiral product was obtained in solid-state from β-CD/1, using β-cyclodextrinβ-cyclodextrin as the as chiral the chiral catalyst catalyst (Scheme (Scheme 27).27). TheThe chiral chiral product product was was obtained obtained from β -CD/1,from β-CD/1, 3-dihydroxylbenzene,β-cyclodextrin as the andchiral methyl catalyst vinyl (Scheme ketone/ 27).β-CD The in 82.8%chiral yieldproduct and was 78.4% obtained ee. from β-CD/1, β 3-dihydroxylbenzene,3-dihydroxylbenzene, and methyl andand methylmethyl vinyl vinylvinyl ketone/ ketone/ketone/ββ-CD-CD-CD in in 82.8% 82.8%82.8% yield yield yield and and and 78.4% 78.4% 78.4% ee. ee. ee.

HO OH O

HO OH HOComplex A OH ComplexO B O (a) Complex A (b) Complex B (a) Complex A (b) Complex B Scheme 27. The asymmetric(a) synthesis of (+)-2-methoxy-2-methylchroman-7-ol(b) catalyzed by β-CD. Scheme 27. The asymmetric synthesis of (+)-2-methoxy-2-methylchroman-7-ol catalyzed by β-CD.

Scheme Scheme27. The 27. asymmetricThe asymmetric synthesis synthesis of of (+)-2-methoxy-2-methylchroman-7-ol (+)-2-methoxy-2-methylchroman-7-ol catalyzed catalyzed by β-CD. by β-CD.

4. Conclusions and Future Perspectives Nowadays, the principles of supramolecular chemistry have broadly permeated through organic chemistry sub-disciplines besides organic synthetic methodology. The indiscernible, non-bonded, and bonding supramolecular interactions play a signiﬁcant role in organic reactions and catalysis. Cyclodextrins belong to one of the most important and promising macrocyclic host molecules, since they are inexpensive, water-soluble, non-toxic, easily functionalized, and commercially available. In this mini-review, we focused on the cyclodextrin-based supramolecular systems for organic synthesis, and emphatically discussed the organic reaction mechanisms and their applications, including addition reactions, elimination reactions, substitution reactions, pericyclic reactions, rearrangement reactions, photochemical reactions, oxidation reactions, and reduction reactions. Among the above-mentioned organic reactions, the asymmetric synthesis was highlighted. The advantages of cyclodextrin-based supramolecular catalysts consisted in their facile preparation Molecules 2017, 22, 1475 14 of 17 from inexpensive and easily available starting materials, their simple isolation, and their possibility of reuse. In addition, easy performance of the reaction under mild aqueous conditions brings great beneﬁts to ecologically sustainable chemical processes and technologies. At present, considerable interest has been aroused in the functionization of cyclodextrins as enantioselective hosts for organic reactions, and cyclodextrin-based supramolecular chemistry will continually have an impact on organic synthetic methodology.

Acknowledgments: The authors gratefully acknowledge the financial support of the Natural Science Foundation of China (No. 21666031 and No. 21506104) and the key research & development program of Ningxia (No. 2016KJHM48). Author Contributions: Bai C.C. reviewed the literature, discussed the layout, did the artworks (Figures, Schemes and Tables), and finalized the paper. Tian B.R. reviewed the literature, discussed the layout, wrote the text, and finalized the paper. Zhao T. retrieved the relevant literature of Section2, discussed the layout, and finalized the paper. Huang Q. retrieved the relevant literature of Section3, discussed the layout, checked for accuracy and verified that the information was factual, and finalized the paper. Wang Z.Z. pointed out the background, meaning, and writing thoughts of the review, guided writing, checked for accuracy and verified that the information was factual, and finalized the paper. Conflicts of Interest: The authors declare no conflict of interest.

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