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Nickel-Catalyzed Remote and Proximal Wacker-Type Oxidation

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Nickel-Catalyzed Remote and Proximal Wacker-Type Oxidation ARTICLE https://doi.org/10.1038/s42004-018-0107-y OPEN Nickel-catalyzed remote and proximal Wacker- type oxidation Binbin Liu1, Penghui Hu1, Fangning Xu1, Lu Cheng1, Mingxi Tan1 & Wei Han 1 1234567890():,; Wacker oxidation chemistry is widely applied to oxidation of olefins to carbonyls in the synthesis of pharmaceuticals, natural products, and commodity chemicals. However, in this chemistry efficient oxidation of internal olefins and highly selective oxidation of unbiased internal olefins without reliance upon suitable coordinating groups have remained significant challenges. Here we report a nickel-catalyzed remote Wacker-type oxidation where reactions occur at remote and less-reactive sp3 C–H sites in the presence of a priori more reactive ones through a chain-walking mechanism with excellent regio- and chemo- selectivity. This transformation has attractive features including the use of ambient air as the sole oxidant, naturally-abundant nickel as the catalyst, and polymethylhydrosiloxane as the hydride source at room temperature, allowing for effective oxidation of challenging olefins. Notably, this approach enables direct access to a broad array of complex, medicinally relevant molecules from structurally complex substrates and chemical feedstocks. 1 Jiangsu Collaborative Innovation Center of Biomedical Functional Materials, Jiangsu Key Laboratory of Biofunctional Materials, Key Laboratory of Applied Photochemistry, School of Chemistry and Materials Science, Nanjing Normal University, Wenyuan Road No.1, 210023 Nanjing, China. These authors contributed equally: Binbin Liu, Penghui Hu. Correspondence and requests for materials should be addressed to W.H. (email: [email protected]) COMMUNICATIONS CHEMISTRY | (2019) 2:5 | https://doi.org/10.1038/s42004-018-0107-y | www.nature.com/commschem 1 ARTICLE COMMUNICATIONS CHEMISTRY | https://doi.org/10.1038/s42004-018-0107-y acker oxidation, that is, the reaction of Pd-catalyzed hydride and the unstable organometallic intermediate are com- oxidation of alkenes into high value-added and syn- monly sensitive to oxidative conditions. Moreover, alkenes are W π thetically versatile carbonyls, is a central transforma- more reactive than alkanes owing to the exposed -bonding tion in chemistry1–4 (Fig. 1a). This classical transformation is well electrons, leading to oxidation of carbon–carbon double bonds in established for oxidation of terminal olefins. However, in this preference to the remote sp3 C–H. Indeed, there were few clues in reaction internal olefins are relatively unactivated without reli- the literature implying the exceptional difficulty for the devel- ance upon suitable coordinating groups5–9. Moreover, the inter- opment of this protocol22–33. For example, isomerization of ole- nal olefins, particularly unbiased internal olefins, commonly finic alcohols enabled by transition-metal hydride species leads to provide inseparable regioisomers with comparable yields5–9. the ketone products, which requires migration of the olefinic Actually, the regioselectivity issue of oxidation of internal unsaturations toward the tethered alcohols22–29. Alternatively, olefins represents another longstanding challenge in Wacker internal olefins isomerization/hydroborations or isomerization/ chemistry1–9. Consequently, these challenges inherently under- hydrosilylations followed by oxidations can realize remote oxi- mine the utility of Wacker chemistry because the vast majority of dations30–34. While these methods are efficient, they all require alkenes are unbiased internal olefins readily accessible from pet- prefunctionalized starting materials. Apparently, straightforward roleum and renewable resources such as seed oils10 and through oxidation of olefins’ remote sp3 C–H to valuable ketones serves as well-established synthetic routes such as carbonyl olefination11 a more practical approach (For an example of Pd-catalyzed tan- and olefin metathesis12. dem isomerization–Wacker oxidation of allyl arenes, see ref. 35.). Remote functionalization that allows direct bond formation at We previously identified an iron catalysis system enabling a distal and specific position other than the initial reactive site is a highly efficient Wacker-type oxidation36. This oxidation trans- significant challenge13–21, particularly in remote Wacker-type formation involves iron hydride in situ generated from iron(II) oxidation. This is because the remote Wacker-type oxidation and hydrosilane to react with an alkene producing the adduct would involve transition-metal hydride addition of an alkene into alkyl iron intermediate that gives carbon-centered radical via the organometallic intermediate transition-metal alkyl, followed single-electron transfer37,38, followed by generation of the iron by a sequential β-hydride elimination/migratory-insertion itera- peroxide complex in the presence of oxygen, ultimately to release tion process reaching a specific remote sp3 C–H position where the desired carbonyl product36. This work led us to attempt oxidation occurs to liberate the desired single carbonyl remote Wacker-type oxidation with an analogous process. product13,14; unfortunately, both the reductive transition-metal Apparently, this new strategy would require the discovery of a a O Classical Remote Wacker-type Wacker-type O oxidation oxidation + Pd catalysis Ni catalysis O - Oxidation reaction at remote sp3 carbon - Oxidation reactions at original sp2 carbons - Single regioisomers - Inseparable regioisomers with comparable yields - Internal alkenes are reactive at room - Internal alkenes are unreactive temperature b O O O O NH COOH N F N nBu N Ph O Fenbufen Dyclonine Benperidol (treatment of pain and inflammation) (an oral anaesthetic) (treatment of schizophrenia) O Ph OH O N O O N OH HO OH OH Ph O N H OH Centpropazine Propafenone Phloretin (antidepressant) (treatment of irregular heart rhythm) (a natural product) c 1 [NiII] β-H IV H [Si–H] I hydrometallation II Ar elimination L[NiII]X L[Ni ]H [NiII]H Ar Ar [NiII]L [Si–X] Ar H II III [Si–H] Iteration process II L[Ni ]OH L[NiII] VI IX II O L[Ni ]OO O2 Ar O V Ar Ar L[NiII]OO [NiII]L VII O 2 Ar VIII VI Fig. 1 Nickel-catalyzed Wacker-type oxidation at remote sp3 C–H sites. a Classical Wacker oxidation vs. remote Wacker-type oxidation. b Examples of biologically active organic molecules that contain an aromatic ketone motif. c Mechanistic rationale for the nickel-catalyzed Wacker-type oxidation at remote sp3 C–H sites 2 COMMUNICATIONS CHEMISTRY | (2019) 2:5 | https://doi.org/10.1038/s42004-018-0107-y | www.nature.com/commschem COMMUNICATIONS CHEMISTRY | https://doi.org/10.1038/s42004-018-0107-y ARTICLE catalyst enabling migration of an olefinic unsaturation toward a provide alkyl nickel intermediate II40–43, followed by β-hydride specific position where oxidation occurs. Encouragement in this elimination to give a new metal-bonded hydride with a con- regard was found in the work of inexpensive nickel-catalyzed comitant migration of the double bond III. Subsequently, re- remote sp3 C–H functionalizations through a chain-walking addition of the metal hydride generates a new alkyl–Ni species mechanism via iterative β-hydride elimination/nickel-hydride IV. Iterative the β-hydride elimination/migratory insertion species migratory insertion yielding a nickel alkyl sequences achieve a chain-walking process, delivering a critical sequences31,32,39–44. Inspired by these investigations, we ques- benzyl nickel intermediate V39–43. Then the reaction of V with tioned whether we could utilize a nickel catalyst for the chal- oxygen would form a thermodynamically favored benzyl lenging remote oxidation of unactivated sp3 C–H sites in the radical VII, followed by generation of a nickel peroxide complex presence of a priori more reactive carbon–carbon double bonds. VIII46–50. Finally, VIII would undergo homolytic cleavage of On the basis of the above-mentioned results, we hypothesize O–O and C–H bond breaking to liberate the thermodynamically that the remote Wacker-type oxidation proceeds through a single more stable product aryl ketone 2, while providing the LNiII-OH electron transfer (SET) process. To achieve requisite selectivity species, which could regenerate the nickel hydride I catalyst by and avoid the formation of regioisomeric oxidation products, this hydride transfer in the presence of a hydrosilane50,51. remote Wacker-type oxidation process should preferably have To probe this potential, our initial studies tested the sufficient kinetic and thermodynamic favorability. In this context, isomerization–Wacker-type oxidation of 4-allylanisole (1a) with aryl-substituted olefins selected as substrates would be the opti- NiCl2/L1 (neocuproine) catalyst system using PhSiD3 as hydro- mal choice because they can generate stable benzyl radical silane and EtOH as solvent and was found to allow the formation intermediates and give the corresponding conjugated aryl ketone of desired aryl ketone 2a-d bearing similar degrees of D- products. Notably, aryl-substituted olefins that are widely present incorporation at all positions of its hydrocarbon chain (Fig. 2a), in readily available and renewable plants of the family Anacar- suggesting that the process proceeds via Ni-D hydrometalation diaceae and Ginkgo biloba fruits10,45 undergo such a transfor- and subsequent β-hydride elimination/migratory insertion mation to deliver aryl ketones, a privileged scaffold in medicinal sequences and is bidirectional39–43. Furthermore, when olefin chemistry and pharmaceutical agents (Fig. 1b). 1p was subjected to the reaction conditions
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