Asymmetric Catalytic Halofunctionalization of Î±,Î²-Unsaturated

Perspective Cite This: J. Org. Chem. 2019, 84, 1−13

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Asymmetric Catalytic Halofunctionalization of α,β-Unsaturated Carbonyl Compounds Yunfei Cai,†,‡ Xiaohua Liu,† Pengfei Zhou,† and Xiaoming Feng*,† †

J. Org. Chem. 2019.84:1-13. Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/09/19. For personal use only.

Key Laboratory of Green Chemistry & Technology, Ministry of Education, College of Chemistry, Sichuan University, Chengdu 610064, China ‡ School of Chemistry and Chemical Engineering, Chongqing University, 174 Shazheng Street, Chongqing 400030, China ABSTRACT: Halofunctionalization methods enable the vicinal difunctionalization of alkenes with heteroatom nucleophiles and halogen moieties. As a fundamental transformation in organic synthesis, the catalytic asymmetric variants have only recently been reported. In sharp contrast to the asymmetric halocyclization of simple alkenes which involves a nucleophile-assisted alkene activation process, the asymmetric halofunctionalization of enones developed by our laboratory features an electrophile-assisted 1,4-addition pathway. Our work in this area has resulted in the development of several different types of regio-, diastereo-, and enantioselective processes, including inter- and intramolecular haloaminations, haloetherifications, and haloazidations. The scope, updated mechanism, limitations, and future perspective of these reactions are discussed.

1. INTRODUCTION The halofunctionalization of the carbon−carbon double bond (Scheme 1, eqs 1 and 2) is a useful and widely studied reaction

addition mechanism was also provided to elucidate the observation that reaction rates were accelerated by proximity of the nucleophilic component to the alkene during the past few decades by many groups (Shilov and Staninets, Williams, Cambie, and others).6 Very recently, Jackson, Borhan, and Denmark reaffirmed this concerted AdE-3-type mechanism where electron donation from the nucleophilic addition partner activates the alkene for electrophilic attack, namely a nucleophile-assisted alkene activation (NAAA) process.7 Although alkene halofunctionalization has been extensively studied for over 100 years,8 the catalytic enantioselective variants only recently began to appear in the literature,9 and significant advances in the field of catalytic asymmetric halocyclization reactions (Scheme 1, eq 2) have also been achieved since 2010 as evidenced by various reviews.5,9 The most successful strategy is based on chiral Lewis base activation via the strong interaction between chiral Lewis base and halogenating agent.10 In many cases, hydrogen bonding to O- or N-nucleophiles or other functional groups embellished in the olefins is also involved. Several chiral Lewis base catalysts containing basic N-, O-, S-, and Se-heteroatoms have been developed and applied to the enantioselective halofunctionalization of styrene derivatives. Representative examples have been listed in Figure 1, including BINOLderived phosphine or phosphoramide (or thiophosphoramide, selenophosphoramide),11 diaminocyclohexane-derived urea,12 cinchonine- or prolinol-derived thiocarbamate,13,14 mannitol-

Scheme 1. Halofunctionalization of Alkenes

in organic chemistry.1 The process enables concomitant formation of stereocontrolled vicinal halo−carbon and heteroatom−carbon bonds. The resulting halogenated compounds are versatile motifs of many biologically active compounds and also key intermediates in organic synthesis as the halo group can be readily transformed into other functional groups.2 Mechanistically, early in 1937, Kimball3 proposed a stepwise pathway involving initial formation of a bridged halonium ion instead of cationic intermediate and subsequent nucleophilic bond formation by its anti opening of a nucleophile to rationalize the observed exclusive formation of anti products. This overgeneralized textbook scheme of stepwise mechanism was also widely used and invoked in many recent publications.4,5 However, an alternative concerted © 2018 American Chemical Society

Received: July 30, 2018 Published: October 19, 2018 1

DOI: 10.1021/acs.joc.8b01951 J. Org. Chem. 2019, 84, 1−13

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The Journal of Organic Chemistry

Scheme 2. Asymmetric Halocyclization of Alkenes

Figure 1. Representative chiral organocatalysts.

derived cyclic sulfide or selenide,15 (DHQD)2PHAL,16−20 phosphate,21,22 and trisimidazolinylbenzene,23 etc. For instance, Borhan and co-workers discovered the first example of catalytic asymmetric chlorolactonization to yield chlorolactones (type I) with biscinchonine alkaloid (DHQD)2PHAL catalyst (Scheme 2, eq 1).16a Two reactive complexes were proposed: a hydrogen bonding of the chlorine source with the protonated catalyst or tight ion pair between the chlorinated catalyst and the monochloro anion of the chlorine source. Later on, this catalyst system was expanded to chlorocyclization of unsaturated amides, bromolactonization of alkynes, haloetherification, and haloesterfication of allyl amides, dichlorination of allylic alcohols (type VI), and even fluorocyclization and others.16−20 Another distinct ion-pair mode using a chiral phase-transfer catalyst was demonstrated by Toste and co-workers.22 Enantioselective fluorocyclization of olefins (type II) proceeded through the chiral anion directed catalysis: a chiral anionic phosphate catalyst brings a cationic fluorinating agent into solution (Scheme 2, eq 2). In addition to the chiral organocatalyst-mediated approach, actually, application of chiral Lewis acids in enantioselective iodolactonization reaction emerged early in 1992.24 Later on, highly efficient asymmetric iodocarbocyclization (type V) could be accomplished by the use of chiral titanium taddolate catalyst and a base (Figure 2).25 Significant breakthroughs have been accomplished recently using Co(II)/Cr(III)−salen,26,27 Ni(II)−pybidine,28 Zn(II)−imidazolidine,29 Cu(II)−iminoamino,30 Sc(III)−phosphine,31 and Sc(III)−phosphine oxide32 complexes and others (Figure 2). These catalysts could activate the nucleophile and/or the halogen electrophile. As shown in Scheme 2, eq 3, chiral Cr(III)−salen complex could control the enantioselectivity in the iodocyclization to form tetrahydrofurans. The slow release of ICl from NCS and iodine anion

Figure 2. Representative Lewis acid catalysts.

is critical, which performs as a zwitterion-like species for the enantioselective iodocyclization after interaction with the metal complex. In the halofunctionalization of carbon−carbon double bond of alkenes bearing an electron-withdrawing carbonyl group, the halo agent (Figure 3), the nucleophile, and the electronic

Figure 3. Structures of relevant halogen sources. 2


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Scheme 3. Diastereoselective Aminochlorination of α,βUnsaturated Carbonyl Compounds

nature of the substituent on the alkenes and the catalyst as well have a remarkable effect on the reactivity, regio-, diastereo-, and enantioselectivity. In comparison with the halocyclization of simple alkenes, the halofunctionalization of enones proceeded successfully even in an intermolecular threecomponent manner. Our laboratory has mainly developed asymmetric halofunctionalization of enones by the use of Lewis acid catalysts based on chiral N,N′-dioxide ligands (Figure 4).33 In this Perspective, we primarily focus on these

Scheme 4. Regioselective Bromoamination of α,βUnsaturated Carbonyl Compounds

which delivered the corresponding β-amino products. Likewise, the reversal of regioselectivity in the reaction of α,βunsaturated carbonyl compounds with TsNH2/NBS was also found by the use of the catalyst, such as hypervalent iodine, copper, aluminum or elemental silicon powder.41 To rationalize the observed regiochemistry, both bromonium- and aziridinium-based mechanisms were proposed. However, it is contentious since there is no unambiguous evidence to support these hypotheses. An alternative concerted AdE-3 mechanism as reaffirmed by Borhan and Denmark based on their recent studies might be more possible.7 In terms of asymmetric halofunctionalization of α,βunsaturated carbonyl compounds, very limited examples existed before 2009, which used substrate- and reagentcontrolled methods. As early as 1977, a chiral substrateinduced asymmetric bromolactonization of (S)-N-(α,β-unsaturated) carbonylprolines readily prepared from α,β-unsaturated acids and (S)-proline was first reported by Terashima.42a Using NBS as bromine source, the halolactonization reaction proceeded almost stereospecifically, giving a mixture of the diastereomeric halolactones with up to 94.5:4.5 dr (Scheme 5).

Figure 4. Structures of N,N′-dioxide ligands and crystal structures of relevant metal complexes.

developments from our laboratory, including haloamination, haloetherification, and haloazidation. Related reports from others are briefly introduced. The discussion includes the research discovery, scope, mechanism, and limitation of these reactions. The background references are summarized, which highlights the difficulties shown in the halofunctionalization of enones, involving substrate or Lewis acid directed regioselectivity, and the stereogenicity forged in the first addition step. Our efforts on the development of chiral Lewis acid catalysis led to highly diastereo- and enantioselective generation of βnucleophilic addition α-halo-substituted ketone derivatives.

2. ASYMMETRIC CATALYTIC HALOAMINATIONS OF ENONES State of the Field ca. 2009. The haloamination (or aminohalogenation) of olefins with the simultaneous formation of C−N and C−X (X = Cl, Br, I) bonds is one of the most powerful methods for the preparation of the vicinal haloamines.34 This reaction has long been recognized; the addition of N,N-dihalobenzenesulfonamide to cyclohexene, for example, was recorded in 1967.35 In regard to haloaminations of electron-deficient alkenes, Li and co-workers in 1999 described copper(I) triflate (CuOTf) catalyzed highly diastereoselective aminochlorination of α,β-unsaturated esters and chalcones providing anti-substituted α-amino-β-halo esters and ketones (Scheme 3).36−38 Aliphatic substrates were found to favor formation of α-halo-β-amino adducts.39 In the presence of CuI, MnSO4, or V2O5, the regioselectivity of the reaction among α,β-unsaturated carbonyl compounds, N-bromosuccinimide (NBS) and p-toluenesulfonamide (TsNH2) was found to depend on the electronic properties of the substituent on the β-aryl group, as reported by Sudalai and co-workers in 2003 (Scheme 4).40 Generally, the amine functionality is introduced at the α-position to the carbonyl group except for the aromatic substituent with a p-OMe group,

Scheme 5. Substrate-Controlled Asymmetric Bromolactonization of α,β-Unsaturated Amide

In 2004, Li and co-workers developed a chiral substratecontrolled asymmetric aminochlorination of optically active α,β-unsaturated N-acyl-4-phenyloxazolidinones with TsNCl2 as both chlorine and nitrogen sources.42b α-Amino-β-chlorosubstituted products were obtained in good yield and moderate diastereoselectivity (Scheme 6). Work in the Field since 2009. When our group realized the powerful coordination ability of chiral N,N′-dioxides with metal salts,33 we became interested in chiral Lewis acid 3


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Scheme 8. Asymmetric Bromoamination of α,β-Unsaturated Ketones

Scheme 6. Chiral Auxiliary Group Induced Diastereoselective Aminochlorination

promoted enantioselective haloamination of enones.43 Initially, the bromoamination reaction of chalcone with TsNH2 and NBS was tested by the use of the metal complexes of chiral ligand L-PiPh (Scheme 7). Interesting information was Scheme 7. Influence of Metal Salts on the Reaction of Chalcone with TsNH2 and NBS

derived from this early study. These reactions take place to provide racemic anti-α-amino-β-bromo ketone adduct with low to moderate yield by the use of a series of metal salts, including CuOTf, Cu(OTf)2, Zn(OTf)2, Fe(OAc)2, Fe(OTf)3, Ni(ClO4)2, Co(ClO4)2, Mg(OTf)2, InBr3, CrCl3, Cd(OAc)2· 2H2O, and SnCl2. Nevertheless, study using Sc(OTf)3 as the metal salt revealed a different regioselectivity, yielding an antiα-bromo-β-amino ketone adduct with good ee value. Although N,N′-dioxides act as tetradentate ligands that securely bind a metal ion forming similar crystal structures of distorted octahedral complexes, the malleability inherent in these metal complexes strongly impacts the reaction.33 During investigation of the effect of metal ions on the reaction, other L-PiPh−metal complexes except L-PiPh−Sc(III) complex exhibited no reactivity on the regioselective bromoamination reaction to afford α-Br product, and the observation of low to moderate yield of β-Br adduct might be due to the background reaction of chalcone with TsNBr2 generated from the Lewis acid catalyzed reaction of NBS and TsNH2. Given this set of circumstances, we next explored the chiral N,N′-dioxide−Sc(OTf)3 complex catalyzed asymmetric bromoamination reaction. After further optimization of reaction parameters including chiral ligand (L-PiEPh), additive (4 Å MS), and reaction temperature (0 °C), excellent yield with high enantioselectivity and diastereoselectivity was achieved. A wide range of substrates with various β-aryl or benzoyl groups was examined, and selected substituent variation is shown in Scheme 8. The electronic nature of the substituents had a little effect on the regio-, diastereo-, and enantioselectivity of the reaction. It was noteworthy that only 0.05 mol % of catalyst loading is enough for the reaction. Besides using TsNH2 as the nucleophile, other aryl-substituted sulfonamides and methanesulfonamides were also suitable nitrogen sources for this reaction, and the corresponding anti-α-bromo-β-amino ketones were obtained with excellent enantioselectivities and diastereoselectivities. Furthermore, this reaction was readily

scalable, and the vicinal bromoamine product was readily transformed into chiral aziridine. Actually, α-bromo-β-amino substituted ketones were observed as the sole adducts in our studies by the use of chiral N,N′-dioxide−Sc(III) catalyst, regardless of the electronic nature of the substituent on the substrates or the halo/ nitrogen sources. Explanations for the regio- and stereoselective lability of the catalytic bromoamination reaction of enone are difficult. Initially, a bromonium-based mechanism was proposed mainly based on the observed regioselectivity of the reaction and previous literature reports.40,41 Our controlled experimentation also showed that primary sulfonamide was a weak nucleophile and 1,4-addition product was not observed in the absence of halogen source under the optimal reaction conditions. However, our recent studies on other halofunctionalization reactions of enones (see sections 3 and 4) strongly support the synergistic Michael addition/halogenation pathway. The β-Re-face was blocked by the amide unit of N,N′dioxide; thus, the β-Si-face attack of sulfonamide anion generated from the reaction of NBS and TsNH2 to the C C bond of enone followed by α-bromination of the resulting corresponding enolate intermediate from the opposite face afforded the (1R,2R)-configuration product (Scheme 8). The process depicted in Scheme 9 represents our later encouraging progress on a catalytic iodoamination of α,βunsaturated carbonyl compounds.44 The reactions were carried out under strictly anhydrous conditions with freshly dried molecular sieves (4 Å MS) in the dark using 0.5 mol % of LPiEPh−Sc(OTf)3 complex as the catalyst. N-Iodosuccinimide (NIS) and TsNH2 could be used as the iodo and nitrogen sources, respectively. The substituent at the β-position could be aryl, ester, methyl, trifluoromethyl, or hydrogen, and the outcomes remained excellent. The existence of I2 can strongly switch the diastereoselectivity of the reaction, which enabled the transformation of anti-α-iodo-β-amino ketone into syn-αiodo-β-amino ketone due to the epimerization and crystal4


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The next major advance came after we identified the use of the TsNCl2/TsNH2 combination as active reaction reagents. For the catalytic asymmetric chloroamination of 3-alkylideneand 3-arylideneindolin-2-ones, we found that the L-PiEPh− Fe(acac)3 complex showed a markedly higher enantioselectivity than other transition metals, including previously privileged Sc(OTf)3 (Scheme 11).46 The use of iron(III) is undoubtedly

Scheme 9. Asymmetric Iodoamination of Chalcones and 4Aryl-4-oxobut-2-enoates

Scheme 11. Iron(III)-Catalyzed Asymmetric Haloamination

lization-induced resolution. As a result, both cis- and transaziridine derivatives are available. However, with N-chlorosuccinimide (NCS) and TsNH2 as chlorine and nitrogen sources, chalcone was chloroaminated with very low conversion ( NIS > NCS. 5


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mediate from the opposite face afforded the major (1R,2R)configuration product. The observation of low diastereoselectivity in some cases might be due to the poor selectivity in the second step of halogenations (Scheme 13). Apart from the presented examples of enantioselective intermolecular haloamination reactions, the haloaminocyclization reaction with internal nitrogen sources is also useful because cyclic sulfamates are intriguing targets in medicinal chemistry.49 Unlike the haloamination of enones using an external nitrogen source, the diastereoselectivity in chiral Lewis acid promoted haloaminocyclization of enone derivatives bearing a primary sulfamate ester exhibit an obvious dependence on the halo reagent and the ligand (Scheme 14).50 The

Scheme 12. Catalyst-Free Aminohalogenation of Chalcones with TsNX2/TsNH2 Combinations

electron-deficient chalcones, the β-Br products were observed as the major products. A similar tendency was also observed in the chloroamination reaction by the use of the ArSO2NH2/ ArSO2NCl2 combination. These results reveal that the reaction has an electrophilic addition feature and might undergo a concerted AdE-3 reaction7 like styrene halofunctionalization in the absence of Lewis acid. The reason for the observed low enantioselectivity in the chloro- and bromoamination reaction of 3-(4-methoxyphenyl)-1-phenylprop-2-enone, bearing a relatively electron-rich carbon−carbon double bond, might ascribe to the strong background reaction. As experienced in the above catalytic three-component haloamination reaction, the halo/nitrogen sources came from NBS/TsNH2 and TsNX2/TsNH2. Additionally, TsNX2 and NCX themselves can act as the halo/nitrogen sources. Prior to our study, Masson realized a syn-selective bromoamination of ene carbamates with NBS as both the bromine source and the amide source.47 Indeed, our investigation48 revealed that the addition between NBS and chalcone yielded the trans-αbromo-β-N-pyrrolidine-2,5-dione-substituted products in the presence of chiral L-RaPr2−Sc(NTf2)3 catalyst and 3 Å molecular sieves as an additive (Scheme 13). In comparison

Scheme 14. Diastereoselective Switchable Asymmetric Haloaminocyclization of Enones

Scheme 13. Asymmetric Haloamination of Chalcones with NXS

anti/syn ratio of the bromoaminocyclization product increased gradually following the sequence of NBS, NBS/BsNH2, and BsNBr2 with the same catalyst. Typically, the use of NXS (X = Br, Cl, I) as the halo source and chiral L-PiEPh−Sc(OTf)3 as the catalyst was favored in the enantioselective formation of the syn-adduct as the majors, while the use of TsNCl2 or TsNBr2 and chiral L-RaPr2−Lu(OTf)3 catalyst led to the generation of anti-adducts in high enantioselectivity. Similar to intermolecular haloamination of chalcone, both the catalyst and halogen source have a marked impact on the diastereoselectivity, supporting the synergistic Michael addition/halogenation pathway.

3. CATALYTIC ASYMMETRIC HALOETHERIFICATION OF ENONES Besides nitrogen nucleophiles, oxygen-containing nucleophile was also proven effective in the enantioselective halofunctionalizations.51a By judicious selection of aryl sulfonamide or succinimide derived halogen sources and chiral N,N′-dioxide/ metal complex catalysts, we realized intramolecular haloetherification of enones. This type of reaction enables the diastereoand enantioselective synthesis of oxaheterocycles, including sixmembered and seven-membered ones (Scheme 15). Three types of halogen atoms could be easily installed into the αposition of the ketones in good to excellent yields and stereoselectivity. The use of iron(III) salt as the catalyst precursor could promote the formation of tetrahydropyran

with the initial bromoamination reaction in which TsNH2 was involved, the two-component bromoamination was sluggish with higher catalyst loading. The diastereo- and enantioselectivity dropped a little. NCS and NIS as the bifunctional species were also tolerable, albeit only moderate stereoselectivity was achieved. Generally, trans-selectivity held dominance over these cases. Similar to the case in the haloamination of chalcones with TsNH2/NXS or ArSO2NH2/ ArSO2NX2 combinations, the β-Si-face attack of succinimide or the corresponding anion to the CC bond of chalcone followed by α-halogenation of the resulting enolate inter6


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The Journal of Organic Chemistry Scheme 15. Asymmetric Intramolecular Haloetherification of Enones

Scheme 17. Transformation of the Products

substitution of the anti-α-Br ketone (eq 1). Removal of the bromo substituent gave a β-methoxyl ketone derivative. Reduction and intramolecular substitution afforded epoxide (eq 2). The excellent performance of chiral L-PiPr2−Fe(acac)3 was also highlighted in the kinetic resolution of racemic secondary alcohol via the enantioselective bromoetherification (eq 3 and Scheme 17). The synthetic utility of this bromoetherification reaction was further demonstrated by the synthesis of (−)-centrolobine. When the identified chiral catalysts were used for either intermolecular or intramolecular oxa-Michael addition reaction in the absence of the halo reagents, the reactions were sluggish, and only the tetrahydropyran-type of Michael adduct was isolated in moderate yield with extremely low enantioselectivity. None of the oxa-Michael addition products underwent αbromination in the catalytic reaction conditions. The observation of low diastereoselectivity in some examples and the low reactivity of the substrate with electron-donating substituent on benzoyl group support a stepwise Michael addition/α-halogenation process. Our recent research51b showed that methanol could occupy one position of the N,N′-dioxide complexed Lewis acid; thus, we thought that chiral iron catalyst could coordinated methanol or the pendent hydroxyl group and promote the 1,4-addition from the dominant face of enones (Scheme 18).

derivatives. The chiral Ce(III) complex was efficient for the generation of oxepane derivatives; nevertheless, the diastereoselectivity was poor, which was in sharp contrast to the bromoamination reaction and the formation of six-membered oxa-heterocyclic adducts. High enantioselectivity was observed after the two diastereomers were transformed into the debromination adducts, which indicated that the oxygenjointed carbon center had the same configuration. Moreover, for an intermolecular version, only methanol can serve as the alcoholic nucleophile (Scheme 16) since the electrophilic Scheme 16. Asymmetric Intermolecular Haloetherification of Enones

Scheme 18. Working Mechanistic Models in Asymmetric Haloetherification Reactionsa

bromine reagent has strong enough oxidation ability to oxidize alcohol into the corresponding aldehyde in the current catalytic system. The anti-isomers were available in good enantioselectivity using a chiral Sc(III) complex catalyst. In addition, the resulted chiral α-Br ketone products can be readily transformed into various optically enriched derivatives (Scheme 17). For example, syn-α-azideketone could be obtained with maintained enantioselectivity via an SN2

a

Note: the catalytic model is based on the X-ray structure of L-RaPr2Sc(OTf)3. 7


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4. CATALYTIC ASYMMETRIC HALOAZIDATION OF α,β-UNSATURATED KETONES An advance related to enantioselective haloazidation of α,βunsaturated ketones was realized by the exploration of the LRaPr2−Fe(OTf)2 catalyst system. Azide52 as the nucleophile and halo reagents similar to those used in haloetherification were used for this purpose.53 Although the related azide 1,4addition to α,β-unsaturated compounds has been achieved by several groups, the substrates are still limited to α,βunsaturated imides/ketones and nitro alkenes, and the enantiomeric excesses are not satisfactory in many examples.54 In this haloazidation reaction, a broad range of enones is identified, including both aryl- and alkyl-substituted ones (Scheme 19). Bromoazidation performed better than chloro-

Scheme 20. Proposed Mechanism of Bromoazidation of Enone

Scheme 19. Catalytic Asymmetric Haloazidation of α,βUnsaturated Ketones

Scheme 21. Halofunctionalization of α,β-Unsaturated Aldehyde

and iodo-azidation in terms of diastereoselectivity. It is noteworthy that the introduction of the halo-reagent accelerated the nucleophilic addition of azide due to the fact that no conjugate addition adduct of azide was detected in the absence of halogen source under an optimal catalyst system. We further conducted some kinetic studies with the help of in situ IR to gain a better understanding of the reaction process. A clear first-order dependence on chiral catalyst, enone substrate A, and TMSN3 but zero order on bromine reagent were presented. This rate equation strongly supports the 1,4-addition/bromination pathway (Scheme 20). We proposed that the azide ion coordinate with iron ion based on a recent publication55 and the coordinated azide ion attack the β-position of enone from Si-face since the Re-face was shielded by amine moiety.

(E)-3-phenylbut-2-enal. The hydro-chlorination process provided higher diastereoselectivity than hydro-fluorination in the presence of one chiral imidazolidinone catalyst. More importantly, the diastereoselectivity of the hydro-fluorination could be improved and modulated by merging two different amine catalysts and judicious selection of the imidazolidinone enantiomer involved in the second catalytic cycle (Scheme 22). Merging amine-based relay iminium and enamine activation catalyst has also been demonstrated by Córdova and coworkers in aminosulfenylation of α,β-unsaturated aldehydes.57 Although neither the separate conjugate transformation nor αsulfenylation reaction of aldehydes is productive, the domino process involving the two catalytic cycles pushed the equilibrium toward bifunctionalization products (Scheme 23). N-Benzylthiosuccinimide acts as the electrophile and a masked nucleophilic component, and the β-amino-α-mercaptoaldehydes were available in good yield and enantioselectivity in the presence of the protected chiral diarylprolinol organocatalyst. The low diastereoselectivity observed in this

5. OTHER MISCELLANEOUS DIFUNCTIONALIZATIONS OF α,β-UNSATURATED COMPOUNDS In the case of the difunctionalization reaction of α,βunsaturated aldehydes, relay iminium and enamine activation catalysis played an important role.56 Catalytic asymmetric cascade difunctionalization by the use of electron-rich arene nucleophiles (such as indole, furan and thiophene, butenolides, and tertiary amino lactone equivalents) and halogen electrophiles has been well-designed by MacMillan and co-workers (Scheme 21). It was noteworthy that syn-diastereoselectivity in all cases was directed by the chiral amine catalyst rather than the stereogenicity forged in the first nucleophilic addition step. This organo-cascade catalysis strategy is also presented in transfer hydrogenation in conjunction with halogenation of 8


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electrophilic halogenation step controlled by chiral Lewis acid catalyst accelerates the β-nucleophilic addition of these weak heteroatom nucleophiles. An alternative and also traditional method to this vicinal halofunctionalized products is through stepwise asymmetric conjugate addition of α,β-unsaturated carbonyl compounds to generate chiral β-nucleophilic addition adducts, followed by α-halogenation reaction. However, the direct asymmetric conjugate addition of these heteroatom nucleophiles to electron-deficient olefins has remained a significant challenge in organic synthesis owing to its low reactivity and the reversibility of the reaction. Despite recent advances in this field of three-component halofunctionalization of α,β-unsaturated carbonyl compounds, many challenges remain. (1) The current substrate scope is extremely limited to enones. A wide range of other electrondeficient α,β-unsaturated compounds such as nitro alkenes, α,β-unsaturated esters and amides need to be explored further. (2) The compatibility between nucleophile and electrophile is another issue due to the possible direct reaction of strong nucleophiles (widely used in conjugate additions such as enolates, phenols, indoles) with halogen electrophiles. (3) F+based electrophile is still not tolerated, and judicious selection of a combination of F+ electrophile and other stronger nucleophiles might pave the way to address this issue in the future.

Scheme 22. Reversal of Diastereoselectivity in Hydrofluorination of α,β-Unsaturated Aldehyde

Scheme 23. Aminosulfenylation of α,β-Unsaturated Aldehyde

transformation was attributed to the epimerization of the αcarbon of the product by the chiral amine catalyst. Chiral metal complex promoted trifluoromethylthiolation of acyclic enones was recently realized by Wang and co-workers. The tandem 1,4-addition of diethylzinc and electrophilic trifluoromethylthiolation enabled the formation of α-SCF3-βsubstituted carbonyl compounds in moderate to good yield and stereoselection (Scheme 24).58 Unlike the amine-cascade

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Xiaohua Liu: 0000-0001-9555-0555 Xiaoming Feng: 0000-0003-4507-0478

Scheme 24. Tandem 1,4-Addition/ Trifluoromethylthiolation of Chalcone

Notes

The authors declare no competing financial interest. Biography

process, the induction of the second step in this case was closely related to the stereogenicity forged in the first catalytic step as evidenced from the racemic product from the reaction of 1-phenylpropenone.

6. CONCLUSION AND OUTLOOK Halofunctionalization of α,β-unsaturated carbonyl compounds is a powerful approach to the synthesis of chiral β-nucleophile substituted α-halo carbonyl compounds in organic chemistry. As summarized in sections 2−4, we have developed Lewis acid catalyzed regio-, diastereo-, and enantioselective haloamination, haloetherification, and haloazidation of a broad range of enones with nitrogen, oxygen, and azide heteroatom nucleophiles, respectively. In sharp contrast to simple olefin halogenations involving a nucleophile-assisted alkene activation (NAAA) process, these reactions feature a halogen electrophile assisted 1,4-addition pathway. The synergic α-

Xiaoming Feng was born in 1964. He received his B.S. in 1985 and M.S. in 1988 from Lanzhou University. He then worked at Southwest Normal University from 1988 to 1993, becoming an associate professor in 1991. In 1996, he received his Ph.D. from the Chinese Academy of Sciences (CAS) under the supervision of Professors Zhitang Huang and Yaozhong Jiang. He went to the Chengdu Institute of Organic Chemistry, CAS, from 1996 to 2000 and was appointed a professor in 1997. He did postdoctoral research at Colorado State University in 1998−1999 with Professor Yian Shi. In 2000, he moved to Sichuan University as a professor. He was selected as an Academician of the Chinese Academy of Sciences in 2013. He 9


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Neighbouring Bromine in the Bromination and Iodination of Olefins. J. Chem. Soc. B 1970, 1275−1278. (d) Cambie, R. C.; Hayward, R. C.; Roberts, J. L.; Rutledge, P. S. Iodolactonizations Using Thallium(I) Carboxylates. J. Chem. Soc., Perkin Trans. 1 1974, 1, 1864−1867. (e) Doi, J. T.; Luehr, G. W.; Delcarmen, D.; Lippsmeyer, B. C. Iodo Enol Lactone Formation and Hydrolysis. J. Org. Chem. 1989, 54, 2764−2767. (f) Snider, B. B.; Johnston, M. I. Regioselectivity of the Halolactonization of γ,δ-Unsaturated acids. Tetrahedron Lett. 1985, 26, 5497−5500. (7) (a) Ashtekar, K. D.; Vetticatt, M.; Yousefi, R.; Jackson, J. E.; Borhan, B. Nucleophile-Assisted Alkene Activation: Olefins Alone Are Often Incompeten. J. Am. Chem. Soc. 2016, 138, 8114−8119. (b) Denmark, S. E.; Ryabchuk, P.; Burk, M. T.; Gilbert, B. B. Toward Catalytic, Enantioselective Chlorolactonization of 1,2Disubstituted Styrenyl Carboxylic Acids. J. Org. Chem. 2016, 81, 10411−10423. (c) Salehi Marzijarani, N.; Yousefi, R.; Jaganathan, A.; Ashtekar, K. D.; Jackson, J. E.; Borhan, B. Absolute and Relative Facial Selectivities in Organocatalytic Asymmetric Chlorocyclization Reactions. Chem. Sci. 2018, 9, 2898−2908. (8) (a) Reynolds, J. W. Quart. On ‘‘Propylene,” a New Hydrocarbon of the Series CnHn. J., Chem. Soc., London 1851, 3, 111−120. (b) Wislicenus, J.; Moldenhauer, W. Ueber das Cholesterindibromür. Justus Liebigs Ann. Chem. 1868, 146, 175−180. (9) For selected recent examples on halocyclizations, see: (a) Woerly, E. M.; Banik, S. M.; Jacobsen, E. N. Enantioselective, Catalytic Fluorolactonization Reactions with a Nucleophilic Fluoride Source. J. Am. Chem. Soc. 2016, 138, 13858−13861. (b) Xia, Z.; Hu, J.; Shen, Z.; Wan, X.; Yao, Q.; Lai, Y.; Gao, J.-M.; Xie, W.-Q. Enantioselective Bromo-oxycyclization of Silanol. Org. Lett. 2016, 18, 80−83. (c) Pan, H.; Huang, H.; Liu, W.; Tian, H.; Shi, Y. Phosphine Oxide−Sc(OTf)3 Catalyzed Highly Regio- and Enantioselective Bromoaminocyclization of (E)-Cinnamyl Tosylcarbamates. An Approach to a Class of Synthetically Versatile Functionalized Molecules. Org. Lett. 2016, 18, 896−899. (d) Kawato, Y.; Ono, H.; Kubota, A.; Nagao, Y.; Morita, N.; Egami, H.; Hamashima, Y. Highly Enantioselective Bromocyclization of Allylic Amides with a P/P = O Double-Site Lewis Base Catalyst. Chem. - Eur. J. 2016, 22, 2127−2133. (e) Jiang, H.-J.; Liu, K.; Yu, J.; Zhang, L.; Gong, L.-Z. Switchable Stereoselectivity in Bromoaminocyclization of Olefins: Using Brønsted Acids of Anionic Chiral Cobalt(III) Complexes. Angew. Chem., Int. Ed. 2017, 56, 11931−11935. (f) Yu, Y.-M.; Huang, Y.-N.; Deng, J. Catalytic Asymmetric Chlorocyclization of 2-Vinylphenylcarbamates for Synthesis of 1,4-Dihydro-2H-3,1-benzoxazin-2-one Derivatives. Org. Lett. 2017, 19, 1224−1227. For selected recent examples on asymmetric dihalogenation, see: (g) Soltanzadeh, B.; Jaganathan, A.; Yi, Y.; Yi, H.; Staples, R. J.; Borhan, B. Highly Regio- and Enantioselective Vicinal Dihalogenation of Allyl Amides. J. Am. Chem. Soc. 2017, 139, 2132− 2135. (h) Landry, M. L.; Hu, D. X.; McKenna, G. M.; Burns, N. Z. Catalytic Enantioselective Dihalogenation and the Selective Synthesis of (−)-Deschloromytilipin A and (−)-Danicalipin A. J. Am. Chem. Soc. 2016, 138, 5150−5158. For selected examples on asymmetric halogenations/rearrangement reactions: (i) Chen, Z.-M.; Zhang, Q.W.; Chen, Z.-H.; Li, H.; Tu, Y.-Q.; Zhang, F.-M.; Tian, J.-M. Organocatalytic Asymmetric Halogenation/Semipinacol Rearrangement: Highly Efficient Synthesis of Chiral r-Oxa-Quaternary βHaloketones. J. Am. Chem. Soc. 2011, 133, 8818−8821. (j) RomanovMichailidis, F.; Pupier, M.; Guénée, L.; Alexakis, A. Enantioselective Halogenative Semi-pinacol Rearrangement: a Stereodivergent Reaction on a Racemic Mixture. Chem. Commun. 2014, 50, 13461− 13464. (k) Yin, Q.; You, S.-L. Asymmetric Chlorination/Ring Expansion for the Synthesis of α-Quaternary Cycloalkanones. Org. Lett. 2014, 16, 1810−1813. (l) Muller, C. H.; Wilking, M.; Ruhlmann, A.; Wibbeling, B.; Hennecke, U. Catalytic, Asymmetric, BromineInduced Semipinacol Rearrangements at Unactivated Double Bonds. Synlett 2011, 2011, 2043−2047. (m) Chen, Z.-M.; Yang, B.-M.; Chen, Z.-H.; Zhang, Q.-W.; Wang, M.; Tu, Y.-Q. Organocatalytic Asymmetric Fluorination/Semipinacol Rearrangement: An Efficient Approach to Chiral beta-Fluoroketones. Chem. - Eur. J. 2012, 18, 12950−12954. (n) Romanov-Michailidis, F.; Guénée, L.; Alexakis, A.

focuses on the design of chiral catalysts, development of new synthetic methods, and synthesis of bioactive compounds.

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ACKNOWLEDGMENTS We appreciate financial support from the National Natural Science Foundation of China (Nos. 21432006, 21290182, and 21625205).

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REFERENCES

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Perspective

The Journal of Organic Chemistry (52) For selected haloazidation examples, see: (a) Hassner, A.; Levy, L. A. Additions of Iodine Azide to Olefins. Stereospecific Introduction of Azide Functions. J. Am. Chem. Soc. 1965, 87, 4203−4204. (b) Kirschning, A.; Hashem, Md. A.; Monenschein, H.; Rose, L.; Schöning, K.-U. Preparation of Novel Haloazide Equivalents by Iodine(III)-Promoted Oxidation of Halide Anions. J. Org. Chem. 1999, 64, 6522−6526. (c) Hajra, S.; Bhowmick, M.; Sinha, D. Highly Regio- and Stereoselective Asymmetric Bromoazidation of Chiral α,βUnsaturated Carboxylic Acid Derivatives: Scope and Limitations. J. Org. Chem. 2006, 71, 9237−9240. (d) Valiulin, R. A.; Mamidyala, S.; Finn, M. G. Taming Chlorine Azide: Access to 1,2-Azidochlorides from Alkenes. J. Org. Chem. 2015, 80, 2740−2755. (e) Chen, L.; Xing, H.; Zhang, H.; Jiang, Z.-X.; Yang, Z. Copper-Catalyzed Intermolecular Chloroazidation of α, β-Unsaturated Amides. Org. Biomol. Chem. 2016, 14, 7463−7467. (53) Zhou, P. F.; Lin, L. L.; Chen, L.; Zhong, X.; Liu, X. H.; Feng, X. M. Iron-Catalyzed Asymmetric Haloazidation of α,β-Unsaturated Ketones: Construction of Organic Azides with Two Vicinal Stereocenters. J. Am. Chem. Soc. 2017, 139, 13414−13419. (54) Myers, J. K.; Jacobsen, E. N. Asymmetric Synthesis of β-Amino Acid Derivatives via Catalytic Conjugate Addition of Hydrazoic Acid to Unsaturated Imides. J. Am. Chem. Soc. 1999, 121, 8959−8960. (b) Taylor, M. S.; Zalatan, D. N.; Lerchner, A. M.; Jacobsen, E. N. Highly Enantioselective Conjugate Additions to α,β-Unsaturated Ketones Catalyzed by a (Salen)Al Complex. J. Am. Chem. Soc. 2005, 127, 1313−1317. (c) Horstmann, T. E.; Guerin, D. J.; Miller, S. J. Asymmetric Conjugate Addition of Azide to αβ-Unsaturated Carbonyl Compounds Catalyzed by Simple Peptides. Angew. Chem., Int. Ed. 2000, 39, 3635−3638. (d) Guerin, D. J.; Miller, S. J. Asymmetric Azidation−Cycloaddition with Open-Chain PeptideBased Catalysts. A Sequential Enantioselective Route to Triazoles. J. Am. Chem. Soc. 2002, 124, 2134−2136. (e) Nielsen, M.; Zhuang, W.; Jørgensen, K. A. Asymmetric Conjugate Addition of Azide to α,βUnsaturated Nitro Compounds Catalyzed by Cinchona Alkaloids. Tetrahedron 2007, 63, 5849−5854. (f) Bellavista, T.; Meninno, S.; Lattanzi, A.; Sala, G. D. Asymmetric Hydroazidation of Nitroalkenes Promoted by a Secondary Amine-Thiourea Catalyst. Adv. Synth. Catal. 2015, 357, 3365−3373. (g) Xue, Z.-K.; Fu, N.-K.; Luo, S.-Z. Asymmetric Hydroazidation of α-Substituted Vinyl Ketones Catalyzed by Chiral Primary Amine. Chin. Chem. Lett. 2017, 28, 1083− 1086. (55) Zhu, C.-L.; Wang, C.; Qin, Q.-X.; Yruegas, S.; Martin, C. D.; Xu, H. Iron-Catalyzed Olefin Azido-Trifluoromethylation for Vicinal Trifluoromethyl Amine Synthesis. ACS Catal. 2018, 8, 5032−5037. (56) Huang, Y.; Walji, A. M.; Larsen, C. H.; MacMillan, D. W. C. Enantioselective Organo-Cascade Catalysis. J. Am. Chem. Soc. 2005, 127, 15051−15053. (57) Zhao, G.-L.; Rios, R.; Vesely, J.; Eriksson, L.; Córdova, A. Organocatalytic Enantioselective Aminosulfenylation of α,β-Unsaturated Aldehydes. Angew. Chem., Int. Ed. 2008, 47, 8468−8472. (58) Jin, M. Y.; Li, J.; Huang, R.; Zhou, Y.; Chung, L. W.; Wang, J. Catalytic Asymmetric Trifluoromethylthiolation of Carbonyl Compounds via a Diastereo and Enantioselective Cu-Catalyzed Tandem Reaction. Chem. Commun. 2018, 54, 4581−4584.

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Asymmetric Catalytic Halofunctionalization of Î±,Î²-Unsaturated

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