Oxidative Alkene Functionalizations via Selenium-Ï-Acid Catalysis

Perspective pubs.acs.org/acscatalysis

Oxidative Alkene Functionalizations via Selenium-π-Acid Catalysis Stefan Ortgies and Alexander Breder* Institut für Organische und Biomolekulare Chemie, Universität Göttingen, Tammannstrasse 2, 37077 Göttingen, Germany ABSTRACT: Catalytic oxidative functionalizations of simple, nonpolarized alkenes represent one of the lynchpin technologies in the realm of modern methodological chemical research. In this context, Lewis-acidic selenium species have experienced a steadily increasing scope of applications in catalytic oxidations of simple alkenes throughout recent years. In analogy to their metallic counterparts, such as cationic gold and platinum complexes, selenenium ions (i.e., RSe+) display an exceptional reactivity toward π-bonds, which allows for the highly chemoselective electrophilic functionalization of alkenes. This distinct reactivity profile enabled the development of a diverse array of catalytic bond-forming processes, such as allylic and vinylic aminations, inter- and intramolecular esterifications, halogenations, and etherifications. Remarkable features associated with such protocols are the high regiocontrol, the commonly mild reaction conditions, the operational simplicity by which selenium-catalyzed alkene oxidations can be conducted, and the exquisite functional group tolerance. These aspects make selenium-π-acid catalysis very attractive for late-stage oxidations of polyfunctionalized molecules, an asset that still remains to be fully explored. In this Perspective, the latest contributions to the field of selenium-π-acid catalysis are delineated and placed into context with indicatory insights gained from previous methodological, mechanistic, and theoretical studies. KEYWORDS: π-acid catalysis, selenium, alkene, oxidation, cooperativity

1. INTRODUCTION The use of structurally simple, nonpolarized alkenes as inexpensive building blocks for the target-oriented synthesis of complex molecular architectures represents a privileged strategy in both chemical industry as well as concurrent methodological chemical sciences. Among the numerous routine operations conducted with olefins on a daily basis, oxidation reactions arguably constitute one of the most paramount classes of transformations. Even though the majority of contemporary catalytic methods for the oxidation of alkenes are reliant on metal catalysts,1 there has also been an alternative branch of research centered on the design and use of nonmetallic catalysts as a powerful complement to the methodological repertoire. In this context, a steadily growing number of investigations focusing on the use of organic sulfur and selenium species as potential catalysts for the oxidation of alkenes has evolved into a discrete subdomain of frontier chemical research throughout the last six decades.2 1.1. Electrophilic Chalcogen-Catalysis: The Activation Principles. In the realm of sulfur- and selenium-catalyzed alkene oxidations, two orthogonal activation principles can be distinguished. The first of which enables the Lewis-basecatalyzed conversion of simple alkenes with electrophiles. A key feature of this activation principle is the Lewis-base-mediated reactivity enhancement of electrophilic reaction partners that would otherwise be inert toward alkenes (Figure 1a).3 By contrast, the second activation principle relies on the Lewisacid-catalyzed reaction of alkenes with nucleophiles. In this scenario, the π-bond of the olefinic reaction partner is directly © XXXX American Chemical Society

activated through the interaction with an electrophilic sulfur or selenium catalyst (Figure 1b). This latter concept is to some degree reminiscent of electrophilic π-bond interactions that are distinctive of carbophilic late transition metals (e.g., gold and platinum complexes).4 In the case of transition metals, the πbond activation dominantly proceeds through the dative interaction between the HOMO of the alkene and an empty d- or σ*-orbital of the metal center.4,5 Additionally, a backdonation from a filled d-orbital of the metal center into the π*orbital of the alkene is taking place (Figure 1c). A conceptually analogous transition-state interaction between the olefinic πorbital and the σ*-orbital of the chalcogen catalyst as well as back-donation from the chalcogen lone pairs to the olefinic π*orbital has been suggested to be decisive for the initial coordination of chalcogenenium ions (R3Ch+, Figure 1b) onto olefinic π-bonds.6 Consequently, these interactions eventually culminate in the formation of a highly electrophilic chalcogeniranium intermediate I, which can be rapidly trapped by given nucleophiles (Figure 1b).6 Accordingly, reaction regimes that operate through the second activation principle are subsumed under the category of chalcogen-π-acid catalysis in order to cleanly distinguish between these electrophilic processes and those proceeding through electrophilic Lewisbase catalysis. As will be exemplified further below, the frontier orbital considerations delineated in Figure 1b aid to rationalize Received: April 14, 2017 Revised: July 15, 2017

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DOI: 10.1021/acscatal.7b01216 ACS Catal. 2017, 7, 5828−5840

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Scheme 1. Mechanistic Proposal for the PhSeCl-Mediated Selenolactonization of Alkenoic Acids 110

PhSeCl) are, at least in certain cases, kinetically favored even in comparison to intramolecularly competing nucleophiles, such as a carboxylate group. As will be discussed further below, the finding that endogenous nucloephiles are decisive factors for the course of chalcogen-catalyzed alkene functionalizations is of superordinate importance for both the design of new Lewisbase catalysis applications and the development of catalytic selenium-π-acid protocols. 1.3. Configurational Integrity of Chalcogeniranium Ions. Another important aspect, particularly concerning the design of enantioselective catalytic processes involving intermediates of type I, is their configurational stability (Scheme 2). In preceding studies it was shown that the initial

Figure 1. Activation principles in electrophilic chalcogen-catalysis for alkene functionalizations via (a) chalcogen Lewis-bases and (b) chalcogen π-acids in conceptual comparison with (c) transition metal π-acids. Ch = S or Se; M = transition metal; LB = Lewis-base; TS = transition state; L = ligand.

Scheme 2. Mechanistic Pathways for the Stereochemical Erosion of Chalcogeniranium Intermediates I11−13

the distinct carbophilicity of positively polarized or cationic chalcogen centers as well as the high chemoselectivity of their reactions. These assets bear significant implications for the design of highly chemoselective alkene functionalizations through selenium-π-acid catalysis. 1.2. Role of Oxidants. Although noncatalytic methods involving sulfenium and selenenium species date back to the pioneering reports first by Turner et al.7 and later by Kharasch et al.8 as well as Jenny and Hölzle,9 a systematic, spectroscopybased analysis of these cationic intermediates in the context of catalytic applications remained elusive until 2006. In a series of highly indicatory mechanistic investigations on PhSeCl and PhSeBr-mediated selenolactonizations of alkenoic acids, Denmark et al. were the first to draw a lucid picture of the various elementary steps involved in this transformation.10 These efforts set the rational basis for the development of Lewis-base catalyzed variants of sulfeno- and selenolactonizations as well as mechanistically related processes. A key finding from these studies is the circumstance that the formation of βselenolactones 3 via the conversion of PhSeCl with alkenoic acids 1 generally and reversibly occurs through the intermediacy of the β-chloro selenide adduct 4a (Markovnikov product, Scheme 1). Compound 4a, in turn, is derived from the nucleophilic attack of a chloride ion onto seleniranium intermediate 2. The intermediacy of adduct 4a provided a first indication that nucleophiles stemming from the oxidant (hereafter termed endogenous nucleophiles, e.g.; Cl− from

attack of a chalcogen electrophile onto an olefinic π-bond proceeds stereospecifically11 and, in the case of chiral nonracemic selenium electrophiles, diastereoselectively.12 However, the configurational stability of chalcogeniranium ions I was found to be highly dependent on multiple factors, such as the nature of the chalcogen atom (S vs Se), the substituents directly bound to the chalcogen atom (aliphatic vs aromatic), and the presence or absence of Lewis-basic coordinating groups in proximity of the chalcogenonium center.13 Due to the 5829


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importance when intermolecular processes are considered. In the majority of respective intermolecular methods reported so far, the terminal oxidant has a dual function. On the one hand, it serves as an acceptor of electrons indirectly derived from the alkene and on the other hand it serves as the source of the nucleophilic reaction partner that will be incorporated into the carbon scaffold of the former olefin (Scheme 3). As a

stereospecificity of both the electrophilic attack of chalcogenenium moieties (RCh+; Ch = S, Se) onto olefinic π-bonds and the subsequent attack of resultant chalcogeniranium ion I by nucleophiles, the formation of species I becomes the stereodetermining step. In this regard, two major pathways were identified by which enantioenriched analogues of intermediates I may erode in terms of optical purity: a) by reversible and nonstereoselective interolefinic chalcogenenium migration and b) by the transfer of the chalcogenenium residue from intermediates I onto heteroatomic nucleophiles, which results in the formation of achiral reaction intermediates (Scheme 2).13 1 H NMR spectroscopic evidence for the interolefinic migration of both sulfenium and selenenium ions (pathway a) was first reported by Denmark et al.,11 which experimentally corroborated computational predictions made by Borodkin et al.14 and Radom et al. on cognate transfer processes.15 Wirth et al. and others had previously shown that the stereochemical integrity of seleniranium ions, and consequently, the diastereoselectivity of selenofunctionalizations of certain alkenes can be significantly increased by utilizing chiral selenium electrophiles that possess Lewis-basic side chains.12,16 The authors proposed that the diastereomeric seleniranium ions resulting from re- or si-face attack onto the olefin would be unequal in energy, thus leading to the interconversion of the less favored into the thermodynamically favored diastereomer. Denmark et al. later showed that even achiral selenium electrophiles may lead to configurationally stable, enantioenriched seleniranium ions if certain criteria were fulfilled.13 If electron-withdrawing groups or coordinating entities that additionally exert an electron-withdrawing effectsuch as an o-NO2 groupwere present in the aromatic ring of ArSe-electrophiles, the resulting seleniranium species were found to exhibit significantly improved configurational stability compared with electronneutral ArSe-electrophiles. It was speculated that the migratory aptitude of the selenenium ion (ArSe+) was markedly reduced by coordination of the Lewis-basic side chain (e.g.; oxygen atom of an o-NO2 group) onto the cationic selenium center. This interaction blocks the requisite coordination site at the selenium atom which would otherwise be responsible for the stereochemically uncontrolled transfer of the selenenium entity onto other alkenes or nucleophiles. In addition, it was surmised that the presence of electron-withdrawing groups would aid in polarizing the endocyclic Se−C bonds. This effect was believed to lead to a preferred stereospecific attack of a given nucleophile at these carbon atoms rather than an attack at the selenium center. Consequently, the formation of achiral reaction intermediates (Scheme 2, pathway b) is avoided. 1.4. Strategic Considerations for the Design of Intermolecular Selenium-π-Acid Catalysis Protocols. Although the sum of mechanistic investigations on the reactivity and configurational stability of chalcogeniranium ions in general and seleniranium ions in particular have either been made in the context of noncatalytic or Lewis-base catalytic studies, the body of findings derived from these investigations has a profound impactas will be shown further belowon the development of novel procedures that rely on the concept of selenium-π-acid catalysis. As was already indicated above, the characteristics of the employed oxidant and consequently of the resulting endogenous nucleophiles (i.e., nucleophilic species stemming from the oxidant or chalcogen electrophile) have a significant influence on the scope of carbon−heteroatom bonds (heteroatom = nitrogen, oxygen, halogen) that can be formed through selenium-π-acid catalysis. This aspect is of particular

Scheme 3. Generalized Mechanistic Scheme for Oxidative Alkene Derivatizations through Selenium-π-Acid Catalysis

consequence, the identification of appropriate oxidants that are capable to (a) chemoselectively augment the nucleofugacity of the selenium moiety (R3Se) within structure V and (b) transfer the requisite endogenous nucleophile becomes a matter of minute analysis and reaction design. Therefore, in addition to the general discussion on the application of selenium-π-acid catalysis in the realm of oxidative alkene functionalizations, a particular emphasis will be placed on the different strategies that have been developed so far to address the challenging task of realizing intermolecular processes chemo- and regioseletively. Furthermore, the discussion will include recent efforts to expand the scope of carbon-heteratom bonds that are accessible via selenium-π-acid catalysis through the use of exogenous nucleophiles (i.e., nucleophiles that are not derived from the oxidant). Each of these concepts will be discussed in the context of the individual bond motifs (C−N, C−Hal, and C− O) for which recent selenium-π-acid catalysis protocols are reported in the literature.

2. OXIDATIVE C−N BOND FORMATIONS 2.1. Allylic Aminations. Given the large number of nitrogenated organic compounds associated with biological, pharmaceutical, or material scientific applications, it stands to reason that there is a considerable demand for the design of efficient and economic catalysis protocols to form carbon− nitrogen bonds. In this context, the use of simple alkenes as readily available and inexpensive building blocks for the regiocontrolled oxidative incorporation of nitrogen substituents has proven to be a fruitful tactic. Despite the fact that the main part of advancements achieved thus far has been realized by means of transition metal catalysis, organoselenium compounds were very recently shown for the first time to serve as complementary and highly potent catalysts for the oxidative allylic amination of unactivated alkenes. On the basis of 5830


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ACS Catalysis previous work on Se-catalyzed allylic halogenations first reported by Sharpless et al.17 and later by Tunge et al.,18 Breder and co-workers established an unprecedented selenocatalytic reaction regime that permitted the conceptually analogous allylic amination of alkenes.19 Key to the success of their strategy was the judicious choice of the oxidant. Traditional methods for cognate alkene halofunctionalizations predominantly involve the use of N-haloimides and -amides as both the terminal oxidant and halogen source. These reagents are known to convert diaryldiselanes into the corresponding arylselenyl halides (cf. Scheme 3, structure VII, Y = halide, R3 = aryl).17 The halide anion serves as an endogenous nucleophile that facilitates the conversion of seleniranium intermediate IV into adduct V. The imide or amide counterion on the other hand acts as an intrinsic base that, upon reaction of intermediate V with the halogenating reagent, initiates the dehydrodeselenenylation of intermediate VI. As a result of these events, allylic and vinylic halides of types VIII and IX are eventually furnished.18 The mechanistic rationale for the design of a selenium-π-acid catalyzed allylic amination was to manipulate the reactivity profile of N-heteroatomic imides in such a way that the nitrogen group would serve as the endogenous nucleophile (Scheme 3, Y = NR2) and the Nheteroatomic group (X) would serve as the intrinsic base. According to this design plan, Breder et al. were able to identify N-fluorobenzenesulfonimide (NFSI) as a reagent that fully satisfied this mechanistic criterion. Consequently, the use of only 5 mol % of (PhSe)2 in the presence of 1 equiv of NFSI in THF at ambient temperature was sufficient to facilitate the allylic imidation of alkenes 5 in reasonable to very good yields (Scheme 4, 49−89%). The title procedure was highly tolerant

omethanesulfonate (FP-OTf, 1.2 equiv) was employed as the terminal oxidant in the presence of 10 mol % of (PhSe)2 and sodium fluoride as a base (Scheme 5). These title conditions Scheme 5. Synthesis of Saturated N-Heterocycles through Selenium-π-Acid Catalysis20

Scheme 4. Representative Scope of the Selenium-π-Acid Catalyzed Allylic Amination of Alkenes 519 allowed for the selenocatalytic conversion of δ,ε- and ε,ζunsaturated N-tosylamides 7 into various pyrrolidines 8 and a piperidine derivative 9, respectively, in yields ranging from 54 to 93%. In this context, Zhao et al. made the interesting observation that substrates 7 could also be selectively converted into the respective tetrahydroazepines 10 if the amount of base was lowered to 0.5 equiv. The authors rationalized this outcome with a Brønsted-acid catalyzed isomerization of pyrrolidines 8 into tetrahydroazepines 10 by residual HF. On the basis of NMR-aided mechanistic studies, Zhao et al. proposed a catalytic scenario analogous to the generalized scheme discussed above (Scheme 3). Accordingly, (PhSe)2 is initially oxidized by FP-OTf to afford either Se(II) or Se(IV) species, which electrophilically attack the olefinic π-bond to form seleniranium intermediate 11 (Scheme 5, bottom). Upon intramolecular nucleophilic attack of the nitrogen substituent, the corresponding Markovnikov selenoamination products 12 are transiently formed. Species 12 is believed to readily react with FP-OTf followed by dehydrodeselenenylation to afford target compounds 8 and 9. 2.2. Intermolecular Vinylic Aminations. In the course of their endeavors toward intermolecular oxidative aminations of alkenes via selenium-π-acid catalysis (cf. Scheme 4),19 Breder et al. discovered that 5- and 6-membered cyclic olefins 14 selectively furnished the corresponding enimides 15 when the former were exposed to 1 equiv of NFSI and 5 mol % of (PhSe)2 using 1,4-dioxane as the reaction medium.22 This unprecedented selenocatalytic aza-Wacker functionalization

of various functional groups, such as esters, nitriles, imides, ketones, sulfones, and phosphonates and exhibited an exquisite selectivity for the allylic C−N bond motif. In 2016, Zhao et al. expanded the scope of the seleniumcatalyzed oxidative allylic aminations to intramolecular processes.20 Again, the nature of the oxidant was crucial for the success of the method development. Conventional oxidants frequently used in selenium-catalyzed alkene derivatizations, such as PhI(OAc)2, PhI(OCOCF3)2, and (NH4)2S2O8, turned out to be completely ineffective in the analogous intramolecular allylic cycloetherification.21 N-Fluorinated oxidants, on the other hand, proved to be suitable not only for the cycloetherification but also for the cognate allylic amination. The best results were obtained when N-fluoropyridinium trifluor5831


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to excellent yields (63−95%) with exclusive trans-selectivity. It is noteworthy that this title method nicely complements the portfolio of Pd-catalyzed aza-Wacker type reactions, as in these cases Michael acceptors are most commonly required to provide access to anti-Markovnikov amination products.24 An additional synthetically useful feature of this method is the fact that in the absence of base allylic alcohols 16 were directly converted into the corresponding α,β-enals through in situ hydrolysis of transiently formed imides 17. Another interesting example of intermolecular vinylic C−N bond formations was reported by Zhao et al. in early 2017 in the form of a diselane-catalyzed pyridination of olefinic πbonds.25 This work was based on a recent study by Denmark and co-workers on Se-catalyzed syn-specific dichlorinations, in which N-fluoropyridinium salts were introduced as a novel oxidant class in the realm of electrophilic selenium catalysis.26 Thus, when N-fluoropyridinium trifluoromethanesulfonate (FP-OTf) was employed as the oxidant and source of the endogenous nitrogen nucleophile, Zhao et al. succeeded in the regioselective pyridination of various alkenes and oligoenes 18 in yields ranging between 40 and 96% (Scheme 8, top).

could be generalized onto a variety of cycloalkenes 14 (Scheme 6). Both dihydronaphthalenes and indenes displayed an Scheme 6. Exemplary Scope of Selenium-π-Acid Catalyzed Aza-Wacker Reactions of Cycloalkenes 1419

Scheme 8. Selenium-π-Acid Catalyzed Pyridination of Alkenes and Oligoenes 1825

exclusive selectivity for the Markovnikov amination products irrespective of the electronic nature of the arene substituents. Interestingly, β-methylstyrene (14e) was also converted into vinylic imide 15e with absolute Markovnikov selectivity. With regard to cycloalkenes, two potential reasons for the switch in selectivity (allylic vs vinylic) were considered: (a) the double bond geometry (E vs Z) and (b) topology of the substrates (acyclic vs cyclic). The authors were able to rule out hypothesis (a) through the synthesis of a Z-configured yet acyclic substrate, which selectively furnished an allylic imidation product. From this observation, Breder et al. concluded that the molecular topology of the olefinic substrates (alkenes 5 vs cycloalkenes 14) was the decisive factor that dictates the direction of the dehydrodeselenenylation step. Zhao et al. showed in 2015 that the hydroxyl function of allylic alcohols can serve as a directing group that assists in the selective formation of aza-Wacker amination products 17 with a high degree of anti-Markovnikov selectivity (Scheme 7).23 Among the various oxidants tested during the optimization studies only NFSI led to satisfactory results. Optimal reaction conditions included 5 mol % of (PhSe)2 and a mixture of bases consisting of NaF/pyridine (1.2:1), which allowed for the conversion of allylic alcohols 16 into vinylic imides 17 in good Scheme 7. Selenium-π-Acid Catalyzed Vinylic Aminations of Allylic Alcohols 1623

Remarkably, 1,3-dienes were exclusively pyridinated at the 2positions (Markovnikov-selectivity). Analogous processes catalyzed by transition metal complexes were reported to display high levels of anti-Markovnikov selectivity, incorporating the pyridine group at the terminal position of the 1,3-diene unit.27 These examples nicely underscore the complementarity of selenium-π-acid catalysis in contrast to certain akin transition metal-catalyzed processes. During the optimization, the authors observed that no product was formed when N-fluoro-2,4,6-trimethylpyridinium tetrafluoroborate ([TMPyF][BF4]) was used as the oxidant. 5832


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to styrene derivatives but also tolerates ortho-vinylated aminopyridines as readily available substrates for the synthesis of aza-indoles. By means of a series of control experiments involving both diselane and arylselenenyl halide catalysts, Ortgies and Breder were able to propose a mechanistic scenario for the selenium-πacid catalyzed (aza-)indole synthesis (Scheme 10). Accordingly,

The suggested reason was the steric shielding of the nitrogen lone pair by the adjacent methyl groups, which was surmised to strongly hamper the nucleophilicity of the pyridine. The suppressed nucleophilicity of the TMPy group was successfully exploited for the incorporation of substituted yet less sterically hindered pyridines as exogenous nucleophiles (Scheme 8, bottom). 2.3. Intramolecular Vinylic Aminations. In an effort to further expand the scope of catalytic aza-Wacker type oxidations, first Ortgies and Breder28 and shortly thereafter Zhao and co-workers29 disclosed the first intramolecular oxidative vinylic aminations of alkenes facilitated by seleniumπ-acid catalysis. Both research groups could demonstrate that a broad series of o-tosylamidostyrenes 20 was readily cyclized to furnish the corresponding indole derivatives 21 using 1.05 equiv of NFSI as the terminal oxidant (Scheme 9). When the reaction was performed at elevated temperatures (≥100 °C), the loading of (PhSe)2 could even be reduced to 2.5 mol %. In either protocol the isolated yields were generally good to excellent (31−99%). Notably, Ortgies and Breder could furthermore show that the title transformation is not limited

Scheme 10. Mechanistic Hypothesis on the Selenium-π-Acid Catalyzed Synthesis of (Aza-)Indoles28

Scheme 9. Selenocatalytic Synthesis of Indoles and AzaIndoles via Intramolecular Vinylic Alkene Amination28,29

it was proposed that upon initial oxidative cleavage of the Se− Se bond of the diselane catalyst and formation of seleniranium ion 23, selenenylated intermediate 24 is formed. The authors hypothesized that a second PhSe-group is transferred onto the selenium atom of intermediate 24 in order to regenerate the Se−Se bond and activate the selenium moiety toward elimination, upon which product 21 and (PhSe)2 would be liberated and the catalytic cycle closed. An alternative pathway for the activation of the PhSe-group, not explicitly discussed by the authors, is based on the direct oxidation of the Se atom of intermediate 24 by NFSI followed by elimination.

3. OXIDATIVE C−CL BOND FORMATION 3.1. 1,2-syn-Dichlorinations. The carbon−chlorine bond is a molecular motif ubiquitously found in various subdomains of chemical research, such as material sciences, natural products, pharmaceuticals, and agrochemicals. Against this background, considerable efforts have been made to devise new synthetic and in part catalytic techniques for the incorporation of chlorine atoms into carbon skeletons. Among these, the oxidative dichlorination of olefinic double bonds constitutes a focal point of modern methodological investigations.30 Despite the great achievements in terms of reagent design for an increased manageability and safety of electrophilic chlorine sources, a long-lasting stereochemical challenge, namely the syn-specific catalytic dichlorination of alkenes, had remained elusive until very recently.30,31 In a landmark publication, Denmark et al. delineated a selenocatalytic solution to this particular challenge.26 Their conceptual approach was based on the unification of the literature-known, stereospecific chloroselenenylation of olefins by ArSeCl332 and the SN2-displacement of Se(IV) entities33 into a single catalytic reaction manifold. The second step is of particular importance, since in related chlorinations of alkenes catalyzed by organoselenium 5833


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Scheme 11. Se-Catalyzed syn-Specific 1,2-Dichlorination of Alkenes 26Exemplary Scope and Mechanistic Proposal26

species the dehydrodeselenenylation is the predominant reaction channel by which the Se-residue departs from the carbon skeleton of the former olefin. As a net result of this scenario, allylic chlorination products are most commonly obtained as reaction products.17,18 A crucial role for the design of a catalytic syn-specific dichlorination reaction played the nature of the terminal oxidant. On the one hand, the oxidant had to be mild enough to not oxidize the alkene or chloride nucleophile directly but only the selenium catalyst, and on the other hand, it had to not release any endogenous nucleophiles that could possibly outcompete the chloride ion. With these preconditions in mind, the authors found that a combination of N-fluoropyridinium tetrafluoroborate (oxidant) and a mixture of n-Bu4NCl and Me3SiCl (exogenous chloride source) was suitable for the title transformation. Additionally, Me3SiCl served as a fluoride ion scavenger in order to suppress any undesired interference of F− with the catalytic turnover. Thus, under optimal conditions a broad series of alkenes 26 was converted into their respective vicinal syn-dichlorides 27 in isolated yields ranging from 37 to 91% using 5 mol % of (PhSe)2 as the precatalyst (Scheme 11). The title method displays a marked tolerance toward functional groups, such as (silyl)ethers, esters, acetals, imides, carbamates, and unprotected alcohols, and exhibits a very high diastereoselectivity in favor of the syn-products irrespective of the double bond configuration (i.e., E vs Z olefins). Certain alkene classes were found to be problematic under this reaction manifold, such as conjugated or 1,1-disubstituted alkenes, alkenes with competitive intramolecular nucleophiles or electron-withdrawing substituents, and alkenes with steric encumbrance in allylic position. In particular, the last influence, namely steric hindrance in allylic position, has been reported to be a commonly limiting factor in selenium-catalyzed alkene functionalizations.34 The catalytic cycle proposed by Denmark et al. (Scheme 11) is initiated by the oxidation of (PhSe)2 to give the high-valent Se(IV) chloride 28 in the presence of an excess of chloride ions. The authors hypothesized that a potential reversible loss of a chloride ligand may increase the electrophilicity of the resulting selenonium species 29 and may eventually lead to the formation of seleniranium ion 30 upon attack of a π-bond. Nucleophilic ring-opening of seleniranium ion 30 furnishes known chloroselenenylated intermediate 31,33 which in turn undergoes chlorodeselenylation34 to yield product 27 and Se(II) species 32. The catalytic cycle is closed with the final reoxidation of PhSeCl (32) to give PhSeCl3 (28). The role of the 2,6-dimethylpyridine-N-oxide additive in the catalytic cycle and the origin of its rate accelerating effect remain unclear at this point. However, a possible function was believed to be the stabilization of cationic intermediates 29 and 30 (cf. Scheme 11) via a Lewis base adduct. Similar stabilization modes had been proposed by Denmark and co-workers for Lewis-base catalyzed seleno- and thiofunctionalizations of alkenes35 and are known to be operative in sulfenium cations.36

co-workers39 documented the use of persulfate salts as terminal oxidants. However, persulfates were reported to possess limited solubility in common organic solvents and would frequently lead to moderate turnover numbers40a circumstance that initiated a search for more efficient substitutes. On the basis of a report by Tiecco et al. on the stoichiometric generation of selenium electrophiles from diselanes using hypervalent iodine reagents as oxidants,41 Wirth and co-workers conjectured that the combination of iodine(III) compounds with substoichiometric quantities of a diselane catalyst may render the catalytic functionalization of alkenes a feasible process.40 To test this hypothesis, the authors screened a series of iodine(III) and iodine(V) reagents in the oxidative lactonization of 3-alkenoic acids. In the course of their investigations, iodobenzene I,Ibis(trifluoroacetate) (PIFA) was found to provide the best results. Accordingly, the combination of 3-alkenoic acids 33 with 1.05 equiv of PIFA and 5 mol % of (PhSe)2 in acetonitrile

4. OXIDATIVE C−O BOND FORMATIONS 4.1. Iodine(III)-Based Oxidants. The bond motif that has arguably spurred the most prolific activities in the realm of oxidative selenium-catalyzed alkene functionalizations is the carbon−oxygen bond. Seminal examples date back to 1981, when Torii et al. reported on the electrochemical oxygenation of alkenes using a selenium catalyst.37 In the early 1990′s Iwaoka and Tomoda38 and soon thereafter Tiecco, Santi, and 5834


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was much faster compared with the electrophilic activation by species 36.40 In subsequent studies, Wirth and co-workers were able to expand the scope of the I(III)-reliant, Se-catalyzed lactonization protocol to the preparation of isocoumarins43a and δvalerolactones.43b The conditions applied for these reactions were nearly identical to those of the selenocatalytic butenolide synthesis (Scheme 12) except for an increase in catalyst loading to 10 mol % to secure satisfactory yields (Scheme 13).40,43 In

provided access to butenolides 34 in 49−96% isolated yield (Scheme 12). The authors also tested chiral, nonracemic Scheme 12. Selenium-Catalyzed Oxidative Lactonization of 3-Alkenoic Acids 3340

Scheme 13. Selenium-Catalyzed Oxidative Lactonization of 3-Alkenoic Acids 4243

the case of the isocoumarin synthesis, several 2-stilbene carboxylic acids 42 could be converted into cyclization products 43 with excellent yields (81−99%).43a In this context, the authors also tested the prospective catalytic activity of disulfides. While (PhS)2 was compatible with PIFA under stoichiometric conditions, irreversible oxidative degradation was recorded when the disulfide was exposed to excess quantities of PIFA, thus indicating that a catalytic employment of disulfides could not be implemented for this lactonization strategy. In case of γ,δ-unsaturated carboxylic acids, the corresponding lactones 44 were obtained in good yields (51−87%) and excellent Markovnikov selectivity.43b Numerous 1,1-disubstituted alkenes were tolerated under the reaction conditions. However, when styrene derivatives were used, the yield dropped significantly (Scheme 13, 44b), and in the case of terminal alkenes, the reaction shut down completely. 4.2. N-Fluorinated Oxidants. In contrast to the classical Markovnikov selectivity in the cyclization of alkenoic acids observed by Wirth et al.,40,43 Breder and co-workers disclosed an entirely complementary and until then unprecedented regioand chemoselectivity when they oxidatively cyclized orthocinnamylated benzoic acids 45 in the presence of 1 equiv of NFSI and 10 mol % of (PhSe)2 at ambient temperature.44 Under these conditions, not the expected dihydroisocoumarin products were formed, but isobenzofuranones 46 − a product class that formally derives from a C(sp3)−H acyloxylation of the allylic methylene group (Scheme 14). This structural motif is an integral core unit of biologically active phthalide natural products.45 The isolated yields ranged between 39 and 81%, but

analogues of (PhSe)2 as potential catalysts for an asymmetric variant of the title procedure. However, the lactone products were isolated in only moderate yields and very low enantiomeric ratios of 53:47−61:39 under catalytic conditions.42 Subsequent mechanistic studies revealed that the oxidative lactonization proceeds through the intermediacy of phenylselenenyl trifluoroacetate 36, which was suggested to be the active selenium catalyst within the mechanistic cycle (Scheme 12).40,43 Following oxidation of the diselane precatalyst by PIFA and reductive elimination of PhI (40), electrophilic selenium species 36 is generated. Formation of seleniranium ion 37 is then followed by nucleophilic attack of the intramolecular carboxylate group, thereby furnishing βselenenylated lactone 38. Oxidation of the selenium center leads to the formation of species 39, which, upon elimination, liberates product 34 and regenerates catalyst 35. The electrophilic activation of the PhSe-group in intermediate 38 by PhSeO2CCF3 (36) was ruled out by the authors, because control experiments revealed that the direct oxidation by PIFA 5835


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ACS Catalysis Scheme 14. Selenium-π-Acid Catalyzed C(sp3)−H Acyloxylation of ortho-Cinnamylated Benzoic Acids 4544

Scheme 15. Enantioselective Selenium-Catalyzed Oxidative Lactonizations of Alkenoic Acids 5047

were very sensitive toward the substitution pattern of the substrate and the electronic properties of the substituents. While a methoxy group in the 7-position of the benzofuranone moiety was well tolerated (cf. Scheme 14, 46b), the corresponding 6-methoxylated analogue 46c could only be obtained with a drastically decreased yield of 39%. Also, strongly electron-withdrawing substituents at the distal aryl moiety were not well tolerated and resulted in low yields, at best.44 From a series of mechanistic control experiments, the authors postulated the following catalytic scenario (Scheme 14, bottom): after initial formation of the seleniranium ion 47, an elimination is taking place instead of the more commonly observed attack of an endogenous or exogenous nucleophile. The oxidation of the resulting intermediate 48 then furnishes species 49, which liberates product 46 after SN2′ displacement of the selenium moiety. The formation of allylic selanes of type 48 from alkenes and selenium electrophiles has been previously described in the literature.46 Interestingly, when the distal substituent on the alkene was an alkyl residue instead of an arene ring, the expected 6-exo-trig cyclization was observed, leading to the respective dihydroisocoumarin product.44 In a groundbreaking study, Maruoka et al. reported on the enantioselective cyclization of alkenoic acids 50.47 In their endeavors toward a reliable and highly selective chiral diselane catalyst, the authors identified Indane-derived precatalyst (S)52 as a suitable candidate for an efficient stereoinduction in the title reaction (Scheme 15, bottom).48 Most of the chiral diselane catalysts reported so far are reliant upon noncovalent interactions of Lewis-basic side chains with the selenium atoman effect that limits the conformational flexibility of the chiral backbone of the selenium catalyst.49 Maruoka et al.

surmised that such an interaction may still not be sufficient to create a firmly rigidified carbon skeleton that would configurationally stabilize any transient seleniranium intermediates (cf. Scheme 2). Consequently, the authors believed that this classical catalyst design would presumably fail to induce high levels of enantioselectivity in the asymmetric lactonization at ambient temperature.47 Therefore, the authors devised a sixstep synthesis for precatalyst (S)-52 starting from commercial 6-methoxy-1-indanone. A screening for oxidants that are compatible with selane (S)-52 revealed that NFSI19 displayed by far the best performance in terms of yield compared with iodine(III) and persulfate-based reagents. Eventually, the combination of 10 mol % of precatalyst (S)-52 with 1.1 equiv of NFSI and 3 equiv of CaCO3 in toluene allowed for the conversion of a broad variety of aliphatic and alicyclic 3alkenoic acids 50 into their respective γ-butyrolactones 51 in excellent yields (60−99%) and enantioselectivities (93−96% ee, Scheme 15). In the case of alkenoic acids possessing an aryl group at the distal position of the alkene moiety, TMSOCOCF3 had to be used instead of CaCO3 in order to obtain optimal enantioselectivities. The authors speculated that in the presence of TMSOCOCF3 a catalyst would be formed that carries a trifluoroacetate counterion, which was believed to be responsible for improved selectivity in this substrate subclass. The remarkable potency in terms of stereoinduction was further demonstrated when substrate 50d was subjected to the title conditions. While its conversion with precatalyst (S)52 gave access to product 51d in 97% yield and 98:2 dr, the same reaction performed with enantiomeric precatalyst (R)-52 led to the diastereomer of product 51d in 81% yield with a dr of 97:3. These results clearly showcase that the architecture of selenium-precatalyst (S)-52 very efficiently suppresses any of the stereochemically erosive pathways delineated in Scheme 2. 5836


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ACS Catalysis

which a non-nucleophilic counterion would be generated. Accordingly, in the presence of an alkene the corresponding seleniranium intermediate would not suffer from direct nucleophilic attack of the counterion anymore (cf. intermediate IV, Scheme 3) but would be available for the conversion with any competent exogenous nucleophile. To probe the feasibility of the dual selenium-π-acid/ photoredox catalysis, the authors chose the intermolecular oxidative esterification of alkenes53 with acetic acid as the test reaction (Scheme 17).50 Best results were obtained when a 3:2

In the context of their investigations on the oxidative intramolecular allylic amination of unsaturated tosylamides 7 (Scheme 5), Zhao et al. also studied the oxidative cyclization of alkenols 53 to furnish tetrahydrofuran and -pyran derivatives 54.20 In contrast to the catalyst loading of 10 mol % during the C−N bond formations, only 5 mol % of (PhSe)2 and 1.2 equiv of FP-OTf in the presence of sodium fluoride as a proton scavenger proved sufficient to convert a series of alcohols 53 possessing 1,2-di- and trisubstituted olefin units into the respective cyclic ethers 54 with exquisite Markovnikov selectivity and, with the exception of the disubstituted substrates 53d and 53f, exclusive formation of E-configured products (Scheme 16). This methodological advancement is all

Scheme 17. Oxidative Aerobic Esterification of Alkenes via a Dual Selenium-π-Acid/Photoredox Catalysis Concept50

Scheme 16. Selenium-Catalyzed Oxidative Etherification of Alkenols 5320

the more of superordinate importance to the field of seleniumπ-acid catalysis because it demonstrates that less acidic nucleophiles, such as simple alcohols, can serve as adequate reactants for the construction of carbon-heteroatom bonds. 4.3. Molecular Oxygen as a Terminal Oxidant. As was already indicated earlier, a key factor in the design of new selenium-π-acid catalyzed alkene oxidations is the nature of the oxidant, as it must neither directly react with the alkene nor with the nucleophile that is meant to be coupled to the carbon framework of the former olefin. An additional criterion that needs to be met by the oxidant comes into play when intermolecular transformations are considered. In the majority of recent reports on the intermolecular functionalization of alkenes by selenium-π-acid catalysis the corresponding nucleophiles were derived from the respective oxidant (i.e., endogenous nucleophiles). However, in the case of any reaction manifold, in which the desired nucleophile is not released from the terminal oxidant but is additionally administered to the reaction medium, a potential chemoselectivity problem may arise from a competition between the desired exogenous and any undesired endogenous nucleophiles. In their pursuit for a generalized solution to this inherently difficile problem, Breder and co-workers recently disclosed an unprecedented abiotic oxidase catalysis concept that is based on the well-adjusted interplay between a photoredox- and a selenium-π-acid catalyst.50 This dual catalysis protocol allowed for the use of O2 as the terminal oxidant without any unwanted incorporation of either of its two oxygen atoms. The underlying design plan that had led to this discovery was based on the assumption that cationic selenium electrophiles may be generated through a single-electron-transfer process (SET)51,52 in the course of

ratio of acetonitrile and acetic acid (56a) was used as the reaction medium in the presence of benzyl 3-hexenoate (55a), 10 mol % of (PhSe)2, and 5 mol % of catalyst 5754 under an atmosphere of air and irradiation with LED light (465 nm).50 Through a series of control experiments the necessity of all reaction parameters was unambiguously confirmed. No product formation was observed in the absence of either of the two catalysts, O2, or light. These results clearly indicate that oxygen serves as the final oxidant and that photocatalyst 57 is needed to mediate between the redox chemistry of oxygen and that of the selenium catalyst. The scope of the title method was quite general and, most importantly, ambient air could be used as a free-of-charge reagent for most of the substrates (Scheme 17). Various functional groups, such as esters, nitriles, phosphonates, amides, and sulfones, were tolerated and gave the products in moderate to good yields (31−89%). Furthermore, completely unfunctionalized alkenes were applicable in this transformation with excellent results, irrespective of the double bond configuration (cf. comp. 58eE/58eZ). Besides acetic acid, other carboxylic acids could be used as nucleophiles, too. In general, a correlation between the steric demand of the respective acid (e.g., formic vs pivalic acid) and the product 5837


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ACS Catalysis yield (81 vs 26%) was observed. Variation of the pKa value of the carboxylic acids did not negatively interfere with the yield. Besides acetic acid (pKa = 4.75), methoxyacetic acid (pKa = 3.53)55 and bromoacetic acid (pKa = 2.86)55 gave the corresponding products in comparable yields. Even trifluoroacetic acid (pKa = 0.23)55 resulted in the formation of 63% of product 58f. Importantly, an amino acid could also be used as the nucleophile, leading to 68% of the phthaloyl-protected leucine derivative 58g. In this case, the amount of acid could be reduced to 5 equiv with respect to the alkene,50 which clearly underlines the utility of this functionalization protocol. Based on a series of mechanistic investigations the authors proposed a catalytic cycle that commences with a reversible SET process between (PhSe)2 and photocatalyst 57 (Scheme 18). The resulting diselane radical cation 5951,56 subsequently

Scheme 19. Application of the Selenium-π-Acid Catalysis in the Total Synthesis of (+)−Greek Tobacco Lactone (66)57

Scheme 18. Mechanistic Proposal for the Dual Selenium-πAcid/Photoredox Catalysis Reaction50

linalool (64) using 5 mol % of VO(acac)2 and 1.5 equiv of tertbutyl hydroperoxide (TBHP) in refluxing benzene for 24 h to afford the corresponding epoxide in 30% as a single diastereomer. Exposure of this epoxide to 2.5 equiv of KCN in the presence of p-TsOH·H2O resulted in the ring opening at the distal carbon atom, affording nitrile 67 in 78%. Next, the lactone ring was formed through a sequence consisting of basic nitrile hydrolysis (aq NaOH) and Brønsted-acid mediated lactonization (75%, one-pot). The final task in this endeavor, namely, the oxidative construction of the C7−O8 bond, initially created significant difficulties, since all attempts to forge this bond through well-established Pd-catalyzed Wacker-type methods60 ended up with the exclusive formation of undesired byproducts. Gratifyingly, when lactone 65 was treated with 10 mol % of (PhSe)2 either in the presence of 1.05 equiv of NFSI and 3 equiv of CaCO3 (conditions A) or in combination with photocatalyst 57 (5 mol %) and Na2HPO4 (0.8 equiv) under air and irradiation with blue LED light (465 nm, conditions B), target compound 66 could be isolated in 60 and 83% (dr = 84:16), respectively.50,57 With this final operation, the total synthesis of (+)−Greek tobacco lactone (66) could be completed in only 4 steps and a total yield of 15% from commercial (R)-linalool (64).

decomposes to give the requisite selenium-π-acid catalyst 60. Upon formation of the seleniranium cation 61 and nucleophilic attack of the carboxylate group, β-selenoacyloxylation intermediate 62 is transiently generated. Species 62 undergoes a second SET step which furnishes radical cation 63. This, in turn, decomposes to regenerate the selenium catalyst and liberates product 58. The respective pathways by which the two selanyl radical cations 59 and 63 fragment remain unclear at this point. Also, the exact structure of the π-acidic selenium species 60 remains not fully elucidated so far. In a collaborative effort, the research groups of Stark, Breder, and Christmann successfully implemented the dual selenium-πacid/photoredox catalysis concept in the total synthesis of (+)− Greek tobacco lactone (66).57 Retrosynthetic analysis of target structure 66 revealed that the bicyclic ring system could be readily simplified to commercially available (R)-linalool (64) (Scheme 19). The total synthesis commenced with the vanadium-catalyzed,58 diastereoablative59 epoxidation of (R)-

5. CONCLUSIONS The chemoselective activation of olefinic π-bonds by Lewisacidic catalysts for the regiocontrolled construction of carbon− carbon and carbon−heteroatom bonds represents a powerful strategy in modern organic synthesis. In this context, electrophilic organoselenium compounds were found to display a distinct carbophilicity toward simple, nonpolarized alkenes. 5838


ACS Catalysis This particular asset of selenium-π-acids has been methodologically harnessed in the catalytic, oxidative derivatization of various olefins. On the basis of thorough mechanistic investigations, which have led to the identification of the iranium ion as the central element of the π-bond activation principle by which selenium-π-acids operate, three main categories of oxidative alkene manipulations have been developednamely, allylic, vinylic, and 1,2-difunctionalizations. As has been delineated throughout the course of the foregoing chapters, the selenium-π-acid catalysis concept provides a compelling approach toward the expeditious construction of diverse bond motifs, such as C(sp2)−N/O and C(sp3)−Het (Het = N, O, Cl). The exceptionally high functional group tolerance and the usually very mild reaction conditions of selenium-π-acid-catalyzed reactions further underscore their enormous synthetic utility. Moreover, the unique πbond activation principle associated with this catalysis concept provides access to functional units such as anti-Markovnikov aza-Wacker or regioselective allylic oxidation products of 1,2disubstituted alkenes that are difficult to reach by cognate transition metal-catalyzed techniques. Irrespective of the great achievements that have been made thus far in the realm of selenium-π-acid catalysis, the overall methodology still resides in its infancy, particularly with regard to asymmetric catalytic processes. Two key challenges that need to be overcome in order to reach synthetically meaningful levels of stereoinduction are (a) to gain stereocontrol in the initial attack of the selenium-electrophile onto the π-bond and (b) to configurationally stabilize the resulting chiral, nonracemic seleniranium intermediates. A prime example for a successful catalyst design that fulfills these criteria has been recently disclosed by Maruoka et al. in the context of oxidative lactonizations of alkenoic acids. In order to extent this seminal example to a broader array of enantioselective bond formations, future research will need to focus on the development of novel selenium-π-acid catalysts that possess chiral backbones that allow for high levels of stereoinduction. Furthermore, other substrate classes, such as alkynes and allenes, as well as alternative nucleophilic reaction partners (e.g.; carbon, phosphorus, fluorine, bromine, and iodine-based nucleophiles) will need to be investigated in more detail to fully exploit the potential of this methodology. Finally, the identification of suitable oxidants will play a paramount role in future research campaigns. Since the oxidant must not interfere with any of the reaction constituents but the selenium catalyst, the prospective success in the utilization of new substrate classes will be tightly bound to the nature of the respective oxidants. A seminal report by Breder et al. on the use of air as a terminal oxidant has already indicated that O2 may be a potential candidate for such purposes. Therefore, the generalization of the use of air as an environmentally nonhazardous and free-of-charge oxidant for alkene functionalization reactions will be one of the key future goals within the realm of selenium-π-acid catalysis.

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ACKNOWLEDGMENTS

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REFERENCES

This work was financially supported by the University of Göttingen and the Deutsche Forschungsgemeinschaft (DFG, Emmy Noether Fellowship to A.B. [BR 4907/1-1]). S.O. thanks the Fonds der Chemischen Industrie (FCI) for his Chemiefonds Fellowship.

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*E-mail: [email protected]. ORCID

Alexander Breder: 0000-0003-4899-5919 Notes

The authors declare no competing financial interest. 5839


Perspective

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