Control of Selectivity in Palladium(II)-Catalyzed Oxidative

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Cite This: Acc. Chem. Res. 2018, 51, 1520−1531

Control of Selectivity in Palladium(II)-Catalyzed Oxidative Transformations of Allenes Bin Yang, Youai Qiu,* and Jan-E. Bac̈ kvall*

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Department of Organic Chemistry, Arrhenius Laboratory, Stockholm University, SE-106 91 Stockholm, Sweden

CONSPECTUS: Oxidation reactions play a central role in organic synthesis, and it is highly desirable that these reactions are mild and occur under catalytic conditions. In Nature, oxidation reactions occur under mild conditions via cascade processes, and furthermore, they often occur in an enantioselective manner with many of them involving molecular oxygen or hydrogen peroxide as the terminal oxidant. Inspired by the reactions in Nature, we have developed a number of Pd(II)-catalyzed cascade reactions under mild oxidative conditions. These reactions have an intrinsic advantage of step economy and rely on selectivity control in each step. In this Account, we will discuss the control of chemo-, regio-, and diastereoselectivity in Pd(II)-catalyzed dehydrogenative cascade coupling reactions. The enantioselective version of this methodology has also been addressed, and new chiral centers have been introduced using a catalytic amount of a chiral phosphoric acid (CPA). Research on this topic has provided access to important compounds attractive for synthetic and pharmaceutical chemists. These compounds include carbocyclic, heterocyclic, and polycyclic systems, as well as polyunsaturated open-chain structures. Reactions leading to these compounds are initiated by coordination of an allene and an unsaturated π-bond moiety, such as olefin, alkyne, or another allene, to the Pd(II) center, followed by allene attack involving a C(sp3)−H cleavage under mild reaction conditions. Recent progress within our research group has shown that weakly coordinating groups (e.g., hydroxyl, alkoxide, or ketone) could also initiate the allene attack on Pd(II), which is essential for the oxidative carbocyclization. Furthermore, a highly selective palladium-catalyzed allenic C(sp3)−H bond oxidation of allenes in the absence of an assisting group was developed, which provides a novel and straightforward synthesis of [3]dendralene derivatives. For the oxidative systems, benzoquinone (BQ) and its derivatives are commonly used as oxidants or catalytic co-oxidants (electron transfer mediators, ETMs) together with molecular oxygen. A variety of transformations including carbocyclization, acetoxylation, arylation, carbonylation, borylation, βhydride elimination, alkynylation, alkoxylation, and olefination have been demonstrated to be compatible with this Pd(II)-based catalytic oxidative system. Recently, several challenging synthetic targets, such as cyclobutenes, seven-membered ring carbocycles, spirocyclic derivatives, functional cyclohexenes, and chiral cyclopentenone derivatives were obtained with high selectivity using these methods. The mechanisms of the reactions were mainly studied by kinetic isotope effects (KIEs) or DFT computations, which showed that in most cases the C(sp3)−H cleavage is the rate-determining step (RDS) or partially RDS. This Account will describe our efforts toward the development of highly selective and atom-economic palladium(II)-catalyzed oxidative transformation of allenes (including enallenes, dienallenes, bisallenes, allenynes, simple allenes, and allenols) with a focus on overcoming the selectivity problem during the reactions.

1. INTRODUCTION

many important reactions. For example, the Wacker oxidation was one of the first homogeneous palladium-catalyzed processes applied to an industrial scale production.2 However, some challenges in the development of selective oxidative

Although palladium(0) catalysis under non-oxidative conditions is still dominating C−C bond-formation reactions, studies on Pd(II) and Pd(IV) catalysis under oxidative conditions for these reactions are attracting increasing attention.1 In a broader sense, palladium-catalyzed oxidation reactions have been studied for five decades and have led to the development of © 2018 American Chemical Society

Received: March 27, 2018 Published: May 24, 2018 1520

DOI: 10.1021/acs.accounts.8b00138 Acc. Chem. Res. 2018, 51, 1520−1531

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Accounts of Chemical Research

skeleton of the final products. The third and fourth parts describe important extensions of this methodology, namely, palladium-catalyzed enantioselective oxidative carbocyclization and aerobic oxidation of allenes.

protocols still remain; for example, C−C bond-formation reactions would solve many problems associated with the preparation of bioactive molecules and design of new pharmaceutical agents. Our own research in this area has led to the development of novel and useful oxidative protocols. Among these transformations, a number of efficient catalytic cascade reactions involving palladium-catalyzed oxidative transformation, in particular carbocyclization of allenes, are synthetically interesting.3 In this Account, we discuss recent developments of this type of methodology with some focus on mechanistic aspects. Herein, we summarize our observed results on chemo-, regio-, and stereoselectivity in these transformations, which provide a number of useful structures and more importantly reveal completely new synthetic disconnections. Allenes, a class of readily accessible, air- and water-stable compounds, have been shown to be interesting and useful building blocks for the construction of complicated skeletons.4 Allenes have been found to possess unique reactivity toward a number of transition metals, such as palladium,5 which is one of the metals most frequently used in oxidative transformations. According to the reactivity−selectivity principle, the main obstacles for palladium(II)-catalyzed oxidation of allenes would be the selectivity control in these transformations. Taking simple allenes as examples, there are three possible reaction sites where palladium(II) can promote formation of new bonds: the central position and the two terminal positions. For instance, the nucleophilic attack on the terminal or central carbons could lead to a vinylpalladium complex, M1, or a (πallyl)palladium complex, M2, respectively (Scheme 1a).6 A

2. SELECTIVE FORMATION OF THE KEY INITIAL VINYLPALLADIUM(II) COMPLEX The Pd(II)-catalyzed cyclizations of allenes bearing a nucleophilic functionality are initiated by coordination of one carbon−carbon double bond of the allene to the Pd(II) center (Scheme 2a). A subsequent intramolecular nucleopalladation Scheme 2. Nucleophilic Cyclization vs Oxidative Carbocyclization of Allenes

Scheme 1. Some Mechanistic Pathways in Pd-Promoted Reactions of Allenes leads to a vinylpalladium intermediate, which is trapped by other reagents in the reaction mixture to give cyclic products (Scheme 2a).9 Our research group has been particularly interested in the palladium-catalyzed oxidative carbocyclizations3 of allenes bearing an unsaturated moiety, such as an olefin,10 alkyne,11 or allene12 (Scheme 2b).13 Simultaneous coordination of the π-bond moiety (such as the pending olefin) and the allene unit of substrate A to the Pd(II) center triggers an allene attack on Pd(II) via a C−H bond cleavage to produce a vinylpalladium intermediate. Afterward, cascade processes, which may include carbocyclization, carbonylation, arylation, borylation, acetoxylation, alkynylation, and alkoxylation of the palladium intermediate can occur. A directing group can greatly promote the “allene attacking” step on palladium (cf. compound 1 to Int-1, Scheme 3). We

migratory insertion of an allene into an R′−Pd(II) bond could also occur giving vinyl-type complex M3 and π-allyl-type complex M4 (Scheme 1b).5 In addition, an oxidative cyclometalation could occur resulting in palladacycle M5 (Scheme 1c).7 Finally, the vinyl-Pd complex M1 or M3 from one allene molecule could react with another allene via an insertion reaction to give a new (π-allyl)palladium complex, M6 (Scheme 1b).8 For the methodology developed within our research group, the allene substrates usually contain more unsaturated moieties such as an olefin, alkyne, allene, ester, or ketone. In this way, there are more possible sites that can coordinate to Pd(II) in the substrates. Herein, we discuss the selectivity control of the palladium(II)-catalyzed oxidation of allenes. The first part of this Account deals with mechanistic details concerning the initial stage of the oxidation of allenes. The second part focuses on the selectivity control in the formation of the carbon

Scheme 3. Pd(II)-Catalyzed Oxidative Carbocyclization− Arylation of Enallenes

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terminal alkynes were converted into their corresponding trienynes smoothly. From a mechanistic point of view, oxidative carbocyclizations of allenynes are of particular interest.11,16 In these reactions, besides the formation of vinylpalladium intermediate Int-4 involving allenic C−H bond cleavage (path a, Scheme 6), we

have observed that the directing effect from olefins, alkynes, or oxygen-containing moieties is indispensable for most transformations discussed in this Account. Palladium-catalyzed carbocyclization−arylation of enallenes 1 under mild oxidative conditions afforded bicyclic compounds 2 with aryl functionalization as the single diastereomer in good to excellent yields (Scheme 3).10j This efficient transformation showed a broad substrate scope and good tolerance for allenes with various functional groups. In addition, a wide range of arylboronic acids with electrondonating or electron-withdrawing groups were compatible with this methodology. Pd(II)-catalyzed oxidative arylation of enallenes without carbocyclization provided direct evidence that a directing group is necessary for the activation of the “allene attacking” step (Scheme 4).14 Thus, with a shorter carbon chain between the

Scheme 6. Chemoselectivity-Controlled Pd(II)-Catalyzed Oxidative Carbocyclization−Arylation of Allenynes

Scheme 4. Activation of Allene Attack by Assisting Coordination observed an unexpected propargylic C−H bond cleavage pathway (path b, Scheme 6). The latter pathway leads to vinylallene product 8 via an allenylpalladium intermediate Int-6. Then, Int-7 is generated by the subsequent allene insertion into the allenyl−Pd bond. The mechanistic investigation of the oxidative arylating carbocyclizations using deuterium-labeled allenyne substrates provided evidence for selectivity being determined in the first irreversible step. Allenic C−H bond cleavage leads to intermediate Int-4, while propargylic C−H bond cleavage gives intermediate Int-6 with substoichiometric amounts of BF3·Et2O.16 In light of the Pd(II)-catalyzed oxidative carbocyclization of allenes assisted by a π-bond moiety, we were interested in palladium-catalyzed selective carbocyclization of enallenes bearing a hydroxyl moiety as a possible directing group. An important question was whether a weakly coordinating group, such as a hydroxyl group, would be able to trigger the allene attack on palladium resulting in intermediate Int-8 (Scheme 7).17 Gratifyingly, a highly regio- and diastereoselective palladium-catalyzed oxidative carbocyclization of enallenes 9 and 9′ having a weakly coordinating oxygen-containing group (hydroxyl, alkoxide, ketone) occurred affording functional cyclohexene derivatives.17 The reaction proceeds via a ligand exchange of the weakly coordinating group with a distant olefin group, and this assisting effect was demonstrated to be indispensable. The high diastereoselectivity of the hydroxyldirected reaction (9 → 10) was rationalized by a face selective coordination of the distant olefin. In addition to our studies on allenic C−H activation assisted by weakly coordinating groups, we were also interested in palladium-catalyzed selective allenic C−H bond oxidation of allenes in the absence of assisting groups (Scheme 8).18 It is likely that π-allylpalladium(II) intermediate Int-12 is generated through selective allenic C−H bond cleavage if the pKa value of the C−H hydrogen is low enough. This (π-allyl)palladium(II) intermediate would then be rearranged to vinylpalladium intermediate Int-13. Subsequent intermolecular olefin insertion into the vinyl−Pd bond of intermediate Int-13 would give highly valuable [3]dendralene derivatives. This selective palladium-catalyzed allenic C−H bond oxidation provided a novel and straightforward synthesis of [3]dendralene derivatives from allenes with good efficiency and high stereoselectivity.18

allene and the pending olefin moiety as in 3a, carbocyclization does not occur. Instead, a coupling between the arylboronic acid and the allene occurs to give 4a. It was demonstrated that the olefin was the key moiety for initiating the allene attack on Pd(II) as shown by the formation of 4a (Scheme 4).14 Substituted dienes were produced in good to excellent yields with exclusive stereoselectivity. By comparing different substituents on the allene for the Pd(II)-catalyzed oxidative arylation, evidence was obtained that an olefin/alkyne unit (3a/ 3e) is an indispensable element for triggering the allene attack for the formation of Int-3. This observation has important mechanistic implications for our previously described oxidative carbocyclizations. In subsequent work, olefin-assisted Pd(II)-catalyzed regioand stereoselective oxidative alkynylation of enallenes was also realized (Scheme 5).15 In this way, trienynes were obtained in good to excellent yields with a broad substrate scope, tolerating free alcohol, ester, and alkyl groups. In addition, a wide range of Scheme 5. Pd(II)-Catalyzed Oxidative Alkynylation of Enallenes

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Accounts of Chemical Research Scheme 7. Oxygen-Assisted Oxidative Carbocyclization of Allenes

Scheme 9. Oxidative Acetoxylative Carbocyclization of Arylallenes via C−H Activation

intermediate Int-15. The latter intermediate assists ortho C−H bond activation resulting in carbocycle 15 as the final product. In this section, we introduced several ways to form the initial vinyl-Pd intermediates. Synthetic methods were also presented to highlight the application of these species. The discussion should promote the rational design of new Pd-catalyzed synthetic transformations.

3. SELECTIVE FORMATION OF CARBON SKELETONS Early studies on palladium-catalyzed oxidative carbocyclization showed that the enallenes (e.g., compound 1) formed fivemembered ring carbocycles as the major product. Taking oxidative borylation of enallene as an example (Schemes 10 and 11a),10i with enallene 1 as starting material, borylated carbocycle 16 was obtained in good yields with excellent selectivity. Scheme 10. Oxidative Carbocyclization−Borylation of Enallenes

Scheme 8. Oxidative Olefination of Allenes

Scheme 11. Chemoselective Pd(II)-Catalyzed Oxidative Transformations of Enallenes

Another possibility for generating a vinylpalladium intermediate from an allene is via an external nucleophilic attack. We have reported a novel and efficient Pd(II)-catalyzed oxidative tandem acetoxylation/carbocyclization of arylallenes via this pathway that provides synthetically important functionalized indene skeletons (Scheme 9).19 Int-14 is expected by initial allene activation, followed by an acetate attack on the coordinated allene leading to the formation of vinyl Pd(II)-

The studies on Pd(II)-catalyzed oxidative transformations were extended to substrates 3. We were pleased to find that substrates 3 gave several new intermediates, which may participate in a wide range of transformations to provide diverse interesting skeletons (Scheme 11b). The oxidative carbocyclization was extended to the synthesis of a highly strained four-membered carbocycle (Scheme 12).20 1523


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Accounts of Chemical Research Scheme 12. Chemoselective Oxidative Transformations of Enallenes

Scheme 14. Oxidative Carbocyclization of Enallenes to SixMembered Carbocycles

During these studies, we discovered that carbocyclization to cyclobutene, in fact, can occur in high yield under certain conditions. The formation of Int-16 had been considered to be disfavored, but the formation of the four-membered ring occurs in the presence of Et3N and H2O using MeOH as the solvent. This result is synthetically interesting as cyclobutenes are key structural elements in biologically relevant compounds and natural products. In addition, they participate in a number of useful synthetic transformations.21 Importantly, the chemoselectivity can be totally reversed. Thus, oxidative borylation of enallenes 3 with AcOH as solvent leads to exclusive formation of the open-chain derivatives 18. Based on the above-mentioned formation of cyclobutenes, we envisioned that the olefin insertion of intermediate Int-17 could produce intermediate Int-18, which might lead to a second carbocyclization to form spirocyclic intermediate Int-19 (Scheme 13). However, Int-17 underwent ligand exchange and olefin insertion to give six-membered carbocyclic intermediate Int-21, which is quenched by external coupling partners giving a six-membered ring as the product (Scheme 14). It was

demonstrated that the assisting olefin moiety is indispensable for the formation of six-membered ring intermediate Int-21.22 In the absence of the assisting olefin, no carbocyclization occurred (Scheme 13b). It is worth noting that several external coupling reagents are compatible with the reaction conditions giving a number of carbocycles in good to excellent yields. Taking the borylation reaction as an example, we observed a broad substrate scope and good tolerance toward various functional groups, including carboxylic acid ester, free hydroxyl, imide, and alkyl groups.22 Interestingly, the substrate 19a (R = −(CH2)3CHCH2) gave the corresponding product 20a as the only product in 84% yield, suggesting that the pendant olefin between the distant olefin and the allene moiety is the key for the reactivity. Interestingly, both arylation and carbonylation are efficient, permitting the formation of carbocycles with different functionalities.22 These oxidation methodologies were extended to bisallenes. Palladium-catalyzed oxidative carbocyclization−arylation cascade reaction of bisallenes with arylboronic acids gives a wide range of functionalized cyclohexadiene derivatives 24 in good to excellent yields (Scheme 15a).12a The reason for the selective formation of six-membered rings 24 over 24′ is that the carbocyclization to give Int-23 via Int-22 is geometrically favored due to perpendicular π-systems of the allene. An extension of the work on oxidative carbocyclization of bisallenes was carried out with B2pin2 as the coupling partner giving cyclohexadiene boronates 25 in good yields (Scheme 15b).12b Interestingly, in the latter reaction it was possible to obtain a useful cascade reaction if an aldehyde was present in the reaction mixture from the beginning. Continuous trapping

Scheme 13. Olefin-Assisted Oxidative Carbocyclization of Enallenes

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preparation of medium-sized rings is challenging.24 The bisallene substrate without the “helping” olefin failed to give the corresponding cyclization product, suggesting that in this case the intermediate corresponding to Int-25 is not formed. The reaction in Scheme 16 showed excellent substrate scope and functional group compatibility on the allene moiety, including carboxylic acid ester, free hydroxyl, imide, and alkyl groups. After the formation of the initial vinylpalladium intermediate (e.g., Int-3, Int-4, Int-8, Int-13, Int-15, Int-17, or Int-24), CO insertion into the C−Pd bond can occur instead of carbocyclization or ligand exchange. For example, insertion of CO into Int-3 could generate Int-27, which can cyclize to 5membered ring intermediate Int-28. Subsequent CO insertion and reaction with an acetylene gives the product 28 (Scheme 17).25a This cascade reaction is highly efficient, and in total,

Scheme 15. Oxidative Carbocyclization of Bisallenes

Scheme 17. Pd(II)-Catalyzed Oxidative Carbonylation− Carbocyclization−Carbonylation−Alkynylation of Enallenes

of the generated allylboron compound by the aldehyde led to the stereoselective formation of alcohols (see the section on aerobic oxidations for a detailed discussion). The ligand-exchange strategy (cf. Int-17 to Int-21) mentioned above was also applied in the oxidative carbocyclization of bisallenes 26 (Scheme 16).23 In this study, highly chemo- and regioselective formation of seven-membered ring skeleton 27 was developed, involving ligand exchange via Int24 to Int-25. This method is synthetically useful since the

four C−C bonds are formed with an exclusive chemoselectivity. It is worth noting that with a phenyl substituent on the olefin moiety (R2 = Ph) the corresponding product was obtained as a single diastereoisomer. We also studied the formation of spirocarbocycles with an all-carbon quaternary carbon center using this carbonylative oxidation strategy.26 We envisioned that Int-30 generated from 29 bearing an extra olefin chain may undergo cascade insertions CO−olefin−olefin−CO via intermediates Int-31, Int-32, and Int-33 to give spirocarbocyclic products 30. Alternatively, the cyclobutene palladium intermediate Int-34 may be generated; then, a subsequent olefin insertion could give a spirocarbocyclic intermediate Int-35 (Scheme 18). We were pleased to find that both pathways can be realized. Thus, in dichloroethane as solvent spiro[4.4]nonene derivatives 30 were obtained selectively via cascade CO−olefin−olefin− CO insertion reactions involving the formation of overall five new C−C bonds. The substrate scope study shows that the method tolerates different functionalities on the acetylene (Scheme 19).26 In addition, substrates with two methyl groups, cyclopentylidene, cyclohexylidene, or cyclooctylidene on the allene moiety all performed well, affording the corresponding products in good to excellent yields with dr values from 91/9 to 95/5.

Scheme 16. Oxidative Carbocyclization− Alkoxycarbonylation of Bisallenes to Seven-Membered Carbocycles

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Accounts of Chemical Research Scheme 18. Carbocyclization-Cascade Reactions toward Spirocycles

Scheme 20. Oxidative Carbocyclization-Cascade Reaction toward Spirocyclobutenes

Scheme 19. Oxidative Carbocyclization-Cascade Reaction toward Spirocyclopentenones

Scheme 21. Oxidative Carbonylation−Carbocyclization− Carbonylation−Alkoxycarbonylation of Enallenols to Spirocycles

By changing the reaction conditions (using a more polar solvent and increasing the reaction temperature), the chemoselectivity was switched and an exclusive formation of spirocyclobutenes (spiro[3,4]octenes) through palladium-catalyzed carbocyclization−carbocyclization−carbonylation−alkynylation was obtained. In this case, four new C−C bonds were formed in the cascade reaction. The reaction shows a broad substrate scope and good tolerance for various functional groups on the allene moiety, including carboxylic acid esters, acetoxy, and alkyl groups. Also, a wide range of terminal alkynes with electron-donating and electron-withdrawing aryl, heteroaryl, alkyl, and trimethylsilyl groups are tolerated (Scheme 20).26 In a subsequent study, spirolactones were synthesized via the above-mentioned Pd(II)-catalyzed carbonylative spirocyclization strategy (Scheme 21).27 In this transformation, simultaneous coordination of the olefin and allene units of substrate 32 to the Pd(II) center triggers an allene attack on Pd(II) via C− H bond cleavage to produce Int-36. Intermediate Int-36 then undergoes a cascade CO and olefin insertion to give Int-37. The latter intermediate undergoes an additional CO insertion followed by a lactonization, which provides the desired spirolactone 33 bearing an all-carbon quaternary stereocenter.27 The reaction shows excellent chemoselectivity, a broad substrate scope, and good tolerance for various functional groups on the allene moiety, including carboxylic acid esters, free hydroxyl, and benzyl groups. This section involved the formation of the carbon skeletons from the initial vinyl-Pd species and described mechanistic insights into the selectivity control. The resulting carbocycles

should be interesting for synthetic and pharmaceutical chemists.

4. PALLADIUM(II)-CATALYZED ASYMMETRIC OXIDATIVE CARBOCYCLIZATION OF ENALLENES In previous sections, quick assembly of diverse carbo- and heterocycles from allenes enabled by Pd(II) under mild oxidative conditions was presented. In the carbocyclization step (e.g., Scheme 10), a stereogenic center can be created, which provides an opportunity to induce enantioselectivity via asymmetric catalysis. However, there are some obstacles in asymmetric oxidative carbocyclization: (1) some commonly used chiral ligands for Pd(II) (e.g., Lewis-basic phosphine ligands) might be unstable under oxidative conditions; (2) the weak coordination of substrate to Pd(II) is prohibited by addition of phosphine and strong multidentate coordinating ligands; (3) coordination of chiral ligands could change the electron density on Pd dramatically, which may inhibit the desired reactivity and lead to a different reaction outcome. In 2012, pioneering work on asymmetric ring expansion to generate spirocyclic products under oxidative conditions was reported by Chai and Rainey via a Pd(II)/chiral phosphoric acid (CPA) cocatalyzed strategy.28 Encouraged by this report, we set out to study the Pd(II)-catalyzed asymmetric oxidative carbocyclization−borylation of enallenes. 1526


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during an irreversible migratory insertion that generates the cyclopentene ring. The enantio-determining step to give (S)-16 or (R)-16 is the transformation of Int-41 to cyclized intermediates Int-42S (via TS-2S) and Int-42R (via TS-2R), respectively. The DFT calculations predict that (S)-16 should be the predominant enantiomer, which was also observed from the X-ray study. In addition, the calculated free energy difference between TS-2S and TS-2R is 1.2 kcal/mol, which matches the experimental enantioselectivity of 85% ee (corresponding to a 1.4 kcal/mol difference). Preliminary attempts to develop an enantioselective carbocyclization−borylation of enallenes revealed that a reasonably good ee (64%) was observed in the presence of catalytic amounts of Pd(OAc)2 and biphenol-type phosphoric acid B (Scheme 24).22

After extensive screening on CPAs and reaction conditions, we found that biphenol-based CPA B is a good ligand. Reaction at 13 °C using m-xylene as solvent gave the highest ee value of the product (Scheme 22).29 Enallenes with cyclopentylidene, Scheme 22. Pd(II)/CPA-Catalyzed Enantioselective Oxidative Carbocyclization−Borylation of Enallenes

Scheme 24. Enantioselective Oxidative Carbocyclization− Borylation of Enallenes to Six-Membered Carbocycles

cyclohexylidene, and cycloheptylidene moieties gave better ee values (92−93% ee) than enallene with dimethyl groups (84% ee). An internal substituent (Me) was introduced on the alkene, which resulted in a lower enantioselectivity (75% ee). It is interesting to note that an all-carbon substituted quaternary stereogenic carbon center was obtained in the latter case. An aza-enallene was found to be incompatible with the Pd(II)/ CPA cocatalysis system, and acid-catalyzed decomposition of the substrate was observed. The absolute configuration of the products obtained in Scheme 22 has been determined by X-ray diffractometry in a follow-up study (Scheme 23).30 With this information, the

Other attempts were made to advance the reaction cascade carbonylation/carbocyclization into an asymmetric version (Scheme 25). Pd(OAc)2 was used as palladium source for Scheme 25. Pd(II)/CPA-Catalyzed Enantioselective Oxidative Carbonylation−Carbocyclization− Carbonylation−Alkynylation of Enallenes

Scheme 23. Mechanism of Enantioselective Pd(II)-Catalyzed Oxidative Carbocyclization−Borylation of Enallenes

this cascade process.31 The screening of CPAs with different scaffolds was carried out, and CPA C was found to be superior. Enallenes with cyclopentylidene moiety perform excellently giving 82% yield with 90% ee. Lower yields and ee values were observed for enallenes with dimethyl moiety. Enallenes with cyclopentylidene moiety perform well, and their corresponding products were obtained in 81% yield with 83% ee. Enallenes possessing cycloheptylidene also performed well. Substituents at the olefin moiety lowered the yield and ee value. Sulfonyl ester was found to be compatible with this reaction cascade giving 28 in good enantioselectivities and yields. After realization of the selective construction of spirolactones via the palladium-catalyzed cascade carbonylative spirolactonization of enallenols (shown in Scheme 21), we next carried out

analysis of the stereochemical course of the reaction and the elucidation of the origin of enantioselectivity was investigated via detailed DFT calculations.30 The key steps of the catalytic cycle responsible for the formation of the new stereogenic center are shown in Scheme 23. The calculations show that the final stereochemical outcome of the process is determined 1527


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redox catalyst (SSRC)red, for example, a reduced form of a transition-metal catalyst. To circumvent this problem, the concept of electron transfer mediators (ETMs) was introduced.32a These ETMs facilitate the electron transfer to O2 from the reduced form of the catalyst (SSRC)red (Scheme 27b). Such biomimetic oxidations were developed within our research group and with Pd(II) as SSRC; ETM1 is often BQ, and ETM2 is typically Co(salophen) or iron(phthalocyanine).32a Direct reoxidation of Pd(0) by O2 has been realized in oxidation reactions, but often these reactions were carried out at high reaction temperatures, with high catalyst loading, or with the use of ancillary ligands that can stabilize the Pd(0) intermediate.34 There is an interesting feature of the oxidation reactions introduced in this Account, which is that many of them can be advanced into aerobic biomimetic oxidations. In the aerobic approach, a catalytic amount of BQ was used to realize the oxidation of allenes. When enallene 19a was treated with Pd(OAc)2 (5 mol %), cobalt(salophen) (5 mol %), and BQ (20 mol %), in the presence of B2pin2 (1.3 equiv) or ArB(OH)2 (1.3 equiv) under an atmospheric pressure of O2, the products 20a and 21a, respectively, were obtained in excellent yields (Scheme 28).22

preliminary studies on the enantioselective variant of this reaction (Scheme 26). To our delight, we found that when Scheme 26. Enantioselective Oxidative CarbocyclizationCascade Reaction of Enallenols

chiral benzoquinone (BQ) (S)-D was used in place of BQ, 33a was obtained with a relatively low ee (26%), which suggests that the quinone coordinates to Pd in the enantiodiscriminating step. Interestingly, a reasonably good ee (62%) was achieved when (R)-VAPOL phosphoric acid (10 mol %) was used as cocatalyst in toluene.27 A slightly higher ee (66%) was obtained with substrate 33b, which incorporated a cyclohexylidene enallenol moiety. To sum it up, for Pd(II)-catalyzed asymmetric oxidative carbocyclizations, CPAs were proven to be the superior cocatalyst, while other types of ligands failed to give any cyclized product or resulted in low enantioselectivity.

Scheme 28. Pd(II)-Catalyzed Biomimetic Oxidative Carbocyclization of Enallenes

5. AEROBIC OXIDATION OF ALLENES Oxidation reactions carried out in Nature usually occur in a cascade manner under mild reaction conditions with exclusive selectivity, and they are often enantioselective. In addition, molecular oxygen is used as the terminal oxidant in many oxidations occurring in Nature.32 It is highly desirable to mimic naturally occurring oxidation reactions; they meet the requirements of “green” catalytic chemistry.33 In a catalytic oxidation reaction, a substrate-selective redox catalyst (SSRC), often a transition metal, is oxidizing the substrate, and the reduced form of the catalyst is reoxidized by a stoichiometric oxidant (Scheme 27a). In modern oxidation methodology, it is desirable that this stoichiometric oxidant is either molecular oxygen or hydrogen peroxide. Often it is difficult to use these simple oxidants to directly reoxidize the reduced form of the

Similarly, the strategy of regeneration of BQ has been applied to the synthesis of open-chain products using allenes as starting materials.15 Compared to the protocols with a stoichiometric amount of BQ, the aerobic oxidation protocols give similar yields at ambient temperature (Scheme 29). Scheme 29. Biomimetic Oxidation of Allenes to Open-Chain Products

Scheme 27. Aerobic Biomimetic Oxidation

The aerobic oxidation strategy can be applied to more sophisticated systems. For example, a palladium-catalyzed aerobic oxidative carbocyclization−borylation−aldehyde trapping cascade reaction of bisallenes to give triene alcohols 34 with high diastereoselectivity was realized.12b F4-BQ was regenerated by cat. Co(salophen)/O2 oxidation (Scheme 30). It is interesting to note that the selectivity from aerobic oxidation was higher than that under conditions with stoichiometric amounts of F4-BQ (cf. compound 34a in 1528


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forming reactions and to apply these methodologies in target synthesis in the coming years.

Scheme 30. Aerobic Oxidative Carbocyclization−Borylation and Aldehyde Trapping

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Jan-E. Bäckvall: 0000-0001-8462-4176 Notes

The authors declare no competing financial interest. Biographies Bin Yang obtained his M.Sc. degree in chemistry from Uppsala University in 2012 and Ph.D. degree from Stockholm University in 2017, under the supervision of Prof. Jan-Erling Bäckvall. He performed postdoctoral research in the same group after he received his Ph.D. His research interests include organometallic chemistry and asymmetric catalysis. Youai Qiu received his B.S. degree in chemistry from China University of Mining and Technology (2007) and his Ph.D. degree from Zhejiang University (2015), under the supervision of Professor Shengming Ma. His research included the discovery of novel transition-metal-catalyzed intramolecular hydroarylation reactions of allenes or alkynes. Since September 2015, he has been performing postdoctoral research in the laboratory of Professor Jan-Erling Bäckvall at Stockholm University. His current research interests focus on palladium-catalyzed oxidative transformations.

Scheme 30). In this study, we also illustrated that a wide range of bisallenes bearing different functionalities and aldehydes with various functional groups were tolerated under biomimetic conditions. In short, molecular oxygen can be used as the terminal oxidant in Pd(II)-catalyzed oxidative transformations, which can increase the atom-economy of the reaction.

6. CONCLUSIONS AND PERSPECTIVES In this Account, we have summarized our recent studies on palladium-catalyzed oxidative transformation of allenes containing unsaturated moieties such as an olefin, alkyne, or another allene, which have enabled synthesis of a wide range of synthetically versatile monocyclic, bicyclic, fused polycyclic, and spirocyclic compounds. Most of these reactions proceed via the key vinylpalladium intermediates generated by an allenic C−H bond cleavage. The latter reaction is triggered by the coordination of the allene and another unsaturated moiety or a weakly coordinating oxygen function such as a hydroxyl group. Insertion of the unsaturated moiety into the vinylpalladium bond produces the carbocyclic palladium intermediate, which is subsequently quenched with various reagents to afford the final carbocyclization product. Mechanistic studies showed that the allenic C−H bond cleavage is the ratedetermining or partially rate-determining step and also the first irreversible step. Benzoquinone (BQ) is the common oxidant in the oxidation of allenes. The use of molecular oxygen as the terminal oxidant for the continuous reoxidation of hydroquinone to benzoquinone enabled the development of a biomimetic catalytic system for the aerobic oxidation of allenes. Selective oxidative carbocyclization, acetoxylation, arylation, carbonylation, borylation, alkoxylation, and cascade reactions have been developed. Asymmetric versions of borylative carbocyclization and cascade carbonylative carbocyclization were achieved and should be promising areas of future endeavors. The studies on the palladium-catalyzed oxidative asymmetric carbocyclizations of allenes have opened up new avenues for the creation of new carbocyclic products that are difficult to make by other methods. The achievements of enantioselective versions of these carbocyclization reactions are promising and provide new opportunities in asymmetric catalysis. We are continuing our efforts to develop new oxidative C−C bond

Jan-Erling Bäckvall was born in Malung, Sweden, in 1947. He received his Ph.D. from the Royal Institute of Technology, Stockholm, in 1975 with Prof. B. Åkermark. After postdoctoral work (1975−1976) with Prof. K. B. Sharpless at Massachusetts Institute of Technology, he joined the faculty at the Royal Institute of Technology. He was appointed Professor of Organic Chemistry at Uppsala University in 1986. In 1997, he moved to Stockholm University where he is currently Professor of Organic Chemistry. He is a member of the Royal Swedish Academy of Sciences, Finnish Academy of Science and Letters, and Academia Europaea. During 2008-2016 he was a member of the Nobel Committee for Chemistry. He is a member of a number of Editorial Boards of journals, and he is the Chairman of the Editorial Board of Chemistry−A European Journal. His current research interests include transition-metal-catalyzed organic transformations, biomimetic oxidations, and enzyme catalysis.

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ACKNOWLEDGMENTS We gratefully acknowledge all the co-workers that have participated in this project for their significant contributions to the projects described herein. Financial support from the European Research Council (ERC AdG 247014), The Swedish Research Council (621-2013-4653), the Berzelii Center EXSELENT, and the Knut and Alice Wallenberg Foundation is gratefully acknowledged.

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REFERENCES

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