Selective, Intermolecular Alkylarylation of Alkenes via Photoredox

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Cite This: Org. Lett. XXXX, XXX, XXX−XXX

Selective, Intermolecular Alkylarylation of Alkenes via Photoredox/ Nickel Dual Catalysis Lei Guo,† Hai-Yong Tu,† Shengqing Zhu, and Lingling Chu* State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, Center for Advanced Low-Dimension Materials, College of Chemistry, Chemical Engineering and Biotechnology, Donghua University, Shanghai 201620, China

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S Supporting Information *

ABSTRACT: A regioselective, intermolecular 1,2-alkylarylation of alkenes with aryl halides and alkyl oxalates has been developed via photoredox/nickel dual catalysis. This dual-catalytic protocol involves a radical relay process, where radical addition is followed by a nickelassisted coupling, forging two consecutive C−C bonds in a single operation. The mild and redox-neutral conditions allow for good compatibility in the scope of olefins, (hetero)aryl halides, as well as alkyl oxalates.

T

Table 1. Reaction Optimizationa

ransition-metal-catalyzed cross-coupling reactions have become an indipensible platform for the construction of

entry

variations from standard conditions

yieldb (%)

1 2 3 4 5 6 7

none w/o Ir-1 w/o NiCl2·DME w/o dtbbpy w/o light DMAc, DMPU, NMP CH3CN, DME

82 0 0 trace 0 5−12 0

a Reaction conditions: Ir-1 (3 mol %), NiCl2·DME (20 mol %), dtbbpy (20 mol %), alkene 2 (0.1 mmol), bromide 3 (2.0 equiv), oxalate 4 (2.0 equiv), DMSO (0.05 M), 35 °C, 90 W blue LED. b Yields were determined by 1H NMR analysis of the crude reaction mixtures. dtbbpy = 4,4′-di-tert-butyl-2,2′-dipyridyl.

Figure 1. Photoredox/nickel-catalyzed intermolecular 1,2-alkylarylation of alkenes with aryl halides and alkyl oxalates.

that nickel catalyst could undergo a single-electron transfer pathway by engaging with radical species. Significant advances have been made in the field of Ni-catalyzed cross-coupling reactions, notably those involving the coupling of C(sp3) fragments.2c−f More recently, the merger of nickel catalysis with visible-light photoredox catalysis,3 a powerful activation

C−C bonds, a fundamental research topic in organic synthesis and chemical industry.1 During the past decades, ongoing effort has been devoted to the development of catalytic protocols utilizing nickel as a catalyst because of its relatively low cost and more importantly novel reactivity profile compared with palladium.2 For example, it has been revealed © XXXX American Chemical Society

Received: May 9, 2019

A

DOI: 10.1021/acs.orglett.9b01658 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters Scheme 1. Scope of Alkenesa

Reaction conditions: Ir-1 (3 mol %), NiCl2·DME (20 mol %), dtbbpy (20 mol %), alkene (0.1 mmol), 4-bromopyridine (2.0 equiv), oxalate (2.0 equiv), DMSO (0.05 M), 35 °C, 90 W blue LED. All yields are isolated yields. dtbbpy = 4,4′-di-tert-butyl-2,2′-dipyridyl.

a

intermolecular dicarbofunctionalization of alkenes that are challenging through the classical transition-metal catalysis.9 Furthermore, this radical/nickel approach provides a valuable platform to install C(sp3)-components onto alkenes, exemplified by the recent work from the groups of Baran,9a Nevado,9c Zhang,9b Giri,9d Chu,9e and Wang9f (Figure 1a). Despite significant advances, alkyl precursors explored in these reactions are mainly restricted to alkyl halides,9b−f with the exception of Baran’s system using derivatives of carboxylic acids.9a Inspired by the remarkable progress in photoredox catalysis where an array of feedstocks including alcohols could function as efficient C(sp3) partners,3 our group recently described a metallaphotoredox-catalyzed syn-selective alkylarylation of alkynes with aryl halides and derivatives of alcohols.6b We questioned whether simple alkenes, readily available and abundant chemical feedstocks, could be employed as coupling partners under this synergistic catalytic system. Herein, we demonstrate a selective, intermolecular 1,2alkylarylation of olefins with alkyl oxalates through synergistic photoredox/nickel catalysis (Figure 1b). This dual-catalytic manifold provides efficient access to synthetically challenging 1,2-dicarbofunctionalized alkane scaffolds from simple starting materials under redox-neutral and mild conditions. We initially investigated the possibility of this transformation with allyl acetate, 4-bromopyridine, and tertiary alkyl oxalate11,9h as the coupling partners (Table 1). Under the irridation of blue LED, we were pleased to find that an optimal yield of the desired alkylarylation product could be obtained

mode for generating reactive open-shell radical species under mild conditions, has been pioneered by the groups of Molander, MacMillan, and Doyle4 and been extensively exploited by themselves and others.5 This synergistic strategy enables diverse and efficient cross-couplings with simple, abundant feedstocks, providing vast opportunities for the invention of a wide array of challenging and useful transformations.5 Nevertheless, most of these transformations proceed in two-component mode, forging one single C−C or C−heteroatom bond. The development of three-component cross-coupling, which would allow for more efficient buildup of molecular complexity from simple starting materials, via photoredox/nickel catalysis remain underdeveloped.6 Three-component dicarbofunctionalization of alkenes is a straightforward strategy in organic synthesis that allows for the construction of two consecutive C−C bonds in a single step and building-up of molecular complexity from simple starting materials.7 One of the most efficient and powerful platforms for this type of transformation is transition-metal-catalyzed conjugative cross-coupling of alkenes with a C-nucleophile and a C-electrophile.8 Nevertheless, this strategy is largely challenged by a competitive two-component reaction and βH elimination, particularly for the cases of unactivated alkenes. In this context, radical-based approach offers an elegant alternative to address these issues.9,10 Specifically, the synergistic action of radical chemistry with nickel catalysis, involving a radical addition to an alkene followed by a nickelassisted coupling, has enabled a number of selective, B

DOI: 10.1021/acs.orglett.9b01658 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters Scheme 2. Scope of Aryl Halides and Oxalatesa

Reaction conditions: Ir-1 (3 mol %), NiCl2·DME (20 mol %), dtbbpy (20 mol %), alkene (0.1 mmol), aryl halide (2.0 equiv), oxalate (2.0 equiv), DMSO (0.05 M), 35 °C, 90 W blue LED. All yields are isolated yields. dtbbpy = 4,4′-di-tert-butyl-2,2′-dipyridyl. a

Unactivated alkenes embedded with esters, carbomates, and carbonates all underwent the three-component difunctionalization smoothly, delivering the alkylarylated products with moderate to good efficiency (products 5−11, 53%−82% yields). Notably, 2-phenyl-1-butene turned out to be a suitable substrate, going through the desired 1,2-alkylarylation with excellent selectivity and synthetically useful yield (product 12, 50% yield). Moreover, electron-deficient alkenes, exemplified as common acrylates, worked well under the optimal conditions and yielded the difunctionalized products with moderate efficiency (products 13 and 14, 52% and 60% yield, respectively). Pleasingly, this metallaphotoredox catalytic protocol was applicable to electron-rich alkenes as well. For example, a series of vinyl esters, vinyl aryl ethers, vinyl alkyl ethers, and vinyl thioethers were viable alkene substrates under the photocatalytic, redox-neutral conditions, generating the corresponding α-arylated ethers and α-arylated thioethers with moderate to excellent yields (products 15−21, 42%−85% yields). Moreover, pharmaceutical-derived complex olefins were well tolerated in this photocatalytic system, providing potential applications in the fields of organic synthesis and medicinal chemistry (products 22 and 23, 70% yields).

with the previously employed combination of catalysts: Ir[dF(CF3)ppy]2(dtbbpy)PF6 1 and NiCl2·DME, 4,4′-di-tertbutyl-2,2′-dipyridyl (dtbbpy) (Table 1, entry 1).6b Only a single regioisomer (as shown in Table 1) was observed under this photocatalytic system. As expected, control experiments suggested that photocatalyst, nickel catalyst, ligand, and light were all essential for this three-component difunctionalization, as no product was observed in the absence of either of them (entries 2−5). Further investigations showed that the solvent system played a critical role in the reaction efficiency (entries 6 and 7). Switching DMSO for other polar aprotic solvents (e.g., DMAc, DMPU, and NMP) led to a dramatic decrease in yield of the desired products, while employing less polar solvents (CH3CN and DME) resulted in no products at all (entries 6 and 7) (see the Supporting Information for more details). Notably, this visible-light-induced system was redox-neutral, with no requirement of exogenous reductants or oxidants. With the optimal conditions in hand, we next turned our attention to evaluate the scope of alkenes using this synergistically catalytic protocol. As shown in Scheme 1, a diverse array of alkenes could be successfully employed in this three-component, selective alkylarylation reaction, constructing two consecutive C−C bonds in one single operation. C

DOI: 10.1021/acs.orglett.9b01658 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters Scheme 3. Mechanistic Studiesa

case of 33, exclusively delivering the 4-pyridylated product in 71% yield. Pleasingly, electron-rich and electron-neutral aryl iodides also worked in this three-component system, furnishing the difunctionalized products with slightly diminished efficiency (products 40−42, 45−60% yields). Furthermore, this photocatalytic protocol could be extended to chlorobenzothiozoles, allowing the installation of five-membered heterocycles with moderate efficiency (products 37−39, 53−68% yields). Finally, we turned our attention to investigating the scope with respect to alkyl oxalates, which are easily prepared from alcohols (Scheme 2).11 An array of cyclic and linear tertiary alkyl oxalates underwent the desired 1,2-alkylarylation with high efficiency under redox-neutral conditions (products 43− 55, 34−85% yields). The ring size of cyclic oxalates was found to affect the reaction efficiency, and six-membered oxalates afforded higher yields compared with four- to seven-membered systems (products 50−52, 34−51% yields). It should be noted that the use of tertiary alkyl substrates was critical to the success of this photocatalytic, three-component dicarbofunctionalization of olefins. We presumed that the steric hindrance of tertiary alkyl oxalates could compress the competitive, undesired two-component coupling between oxalates and aryl halides.4 To gain insight into the possible reaction pathway, a set of preliminary mechanistic studies have been performed (Scheme 3). The standard reaction in the presence of a stoichiometric amount of TEMPO was completely inhibited, with the detection of the corresponding TEMPO-methoxylate adduct 56 (Scheme 3A). A radical clock experiment with αcyclopropyl-substituted styrene 57 gave the rearranged coupling product 58 in 55% yield, consistent with a radical relay process (Scheme 3B). Furthermore, a stoichiometric experiment with the preisolated aryl−Ni(II)−Br complex12 59 in the presence of stoichiometric or catalytic photocatalyst 1 failed to yield the desired 1,2-alkylarylation product (Scheme 3C). On the basis of these experimental results as well as previous literature,13 a plausible catalytic cycle is depicted in Scheme 4. Photoexcited catalyst A would oxidize oxalate B to generate the tertiary alkyl radical D, which then adds to alkene to produce the secondary alkyl radical E. Concurrently, Ni(0) F would be intercepted by the nucleophilic alkyl radical E to afford (alkyl)NiI species G, with subsequent oxidative addition of which with aryl bromide H to generate the crucial (aryl)(alkyl)NiIII species I. The high-valent NiIII species I would undergo facile reductive elimination to furnish the final 1,2-alkylarylated product J and NiI species K. Single-electron reduction of K by the reducing IrII C would regenerate the ground-state photocatalyst Ir-1 as well as Ni(0) species F to close two catalytic cycles. The stoichiometric experiment with well-defined NiII complex (Scheme 3c) might not support another nickel pathway, which could proceed via an oxidative addition of aryl halide with Ni(0) to give (aryl)NiII species L and subsequent an alkyl radical trapping.4,13 Nevertheless, we could not preclude this alternative pathway at this stage. In summary, we have developed a catalytic, selective, threecomponent alkylarylation of alkenes with aryl halides and alkyl oxalates. This protocol takes advantage of photoredox and nickel dual catalysis, enabling efficient assembly of two consecutive C−C bonds in one single operation under mild and redox-neutral conditions. This methodology demonstrates excellent compatibility of functional groups and substrate

a

Key: (A) radical-trapping reaction; (B) radical clock reaction; (C) stoichiometric experiment with Ni(II) complex.

Scheme 4. Proposed Mechanism

Unfortunately, neither 1,1-disubstituted or 1,2-disubstituted alkenes worked under the standard conditions. Next, we began to explore the applicability of aryl halides in this three-component alkylarylation protocol. As shown in Scheme 2, aryl bromides incorporated with an electronwithdrawing group, including esters, ketones, aldehydes, sulfones, and amides, are viable coupling partners under the optimal conditions, giving the desired alkylarylated product in moderate to good yields (products 24−28, 65−82% yields). The mild conditions allow for the good compatibility of these common functionalities, as well as sertraline-derived complex molecule (product 36, 68% yield). Furthermore, a wide range of pyridines, an important catalog of heterocycles found in the pharmaceutics, agrochemicals, and natural products, could be efficiently introduced with simple bromopyridines as coupling partners via this catalytic platform (products 29−35, 61−90% yields). In particular, chlorine atom remained untouched in the D

DOI: 10.1021/acs.orglett.9b01658 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters

(4) For seminal work, see: (a) Tellis, J. C.; Primer, D. N.; Molander, G. A. Single-electron transmetalation in organoboron cross-coupling by photoredox/nickel dual catalysis. Science 2014, 345 (6195), 433− 436. (b) Zuo, Z.; Ahneman, D. T.; Chu, L.; Terrett, J. A.; Doyle, A. G.; MacMillan, D. W. C. Merging photoredox with nickel catalysis: Coupling of α-carboxyl sp3-carbons with aryl halides. Science 2014, 345 (6195), 437−440. (5) For representative reviews, see: (a) Cavalcanti, L. N.; Molander, G. A. Photoredox Catalysis in Nickel-Catalyzed Cross-Coupling. Top. Curr. Chem. 2016, 374 (4), 39. (b) Gui, Y.-Y.; Sun, L.; Lu, Z.-P.; Yu, D.-G. Photoredox sheds new light on nickel catalysis: from carboncarbon to carbon-heteroatom bond formation. Org. Chem. Front. 2016, 3 (4), 522−526. (c) Twilton, J.; Le, C.; Zhang, P.; Shaw, M. H.; Evans, R. W.; MacMillan, D. W. C. The merger of transition metal and photocatalysis. Nat. Rev. Chem. 2017, 1 (7), 0052. (d) Milligan, J. A.; Phelan, J. P.; Badir, S. O.; Molander, G. A. Alkyl Carbon−Carbon Bond Formation by Nickel/Photoredox Cross-Coupling. Angew. Chem., Int. Ed. 2019, 58 (19), 6152−6163. (6) (a) Li, J.; Luo, Y.; Cheo, H. W.; Lan, Y.; Wu, J. PhotoredoxCatalysis-Modulated, Nickel-Catalyzed Divergent Difunctionalization of Ethylene. Chem. 2019, 5 (1), 192−203. (b) Guo, L.; Song, F.; Zhu, S.; Li, H.; Chu, L. syn-Selective alkylarylation of terminal alkynes via the combination of photoredox and nickel catalysis. Nat. Commun. 2018, 9 (1), 4543. (7) For selected reviews, see: (a) Giri, R.; Kc, S. Strategies toward Dicarbofunctionalization of Unactivated Olefins by Combined Heck Carbometalation and Cross-Coupling. J. Org. Chem. 2018, 83 (6), 3013−3022. (b) Dhungana, R. K.; Kc, S.; Basnet, P.; Giri, R. Transition Metal-Catalyzed Dicarbofunctionalization of Unactivated Olefins. Chem. Rec. 2018, 18 (9), 1314−1340. (c) Derosa, J.; Tran, V. T.; van der Puyl, V. A.; Engle, K. M. Carbon−Carbon π-Bonds as Conjunctive Reagents in Cross-Coupling. Aldrichimica Acta 2018, 51, 21−32. (8) For recent examples of three-component dicarbofunctionalization of alkenes via TM-catalyzed conjugate couplings, see: (a) Terao, J.; Bando, F.; Kambe, N. Ni-catalyzed regioselective three-component coupling of alkyl halides, arylalkynes, or enynes with R-M (M = MgX, ZnX). Chem. Commun. 2009, No. 47, 7336−7338. (b) Liao, L.; Jana, R.; Urkalan, K. B.; Sigman, M. S. A Palladium-Catalyzed ThreeComponent Cross-Coupling of Conjugated Dienes or Terminal Alkenes with Vinyl Triflates and Boronic Acids. J. Am. Chem. Soc. 2011, 133 (15), 5784−5787. (c) McCammant, M. S.; Liao, L.; Sigman, M. S. Palladium-Catalyzed 1,4-Difunctionalization of Butadiene To Form Skipped Polyenes. J. Am. Chem. Soc. 2013, 135 (11), 4167−4170. (d) Zhang, L.; Lovinger, G. J.; Edelstein, E. K.; Szymaniak, A. A.; Chierchia, M. P.; Morken, J. P. Catalytic conjunctive cross-coupling enabled by metal-induced metallate rearrangement. Science 2016, 351 (6268), 70. (e) Shrestha, B.; Basnet, P.; Dhungana, R. K.; Kc, S.; Thapa, S.; Sears, J. M.; Giri, R. Ni-Catalyzed Regioselective 1,2-Dicarbofunctionalization of Olefins by Intercepting Heck Intermediates as Imine-Stabilized Transient Metallacycles. J. Am. Chem. Soc. 2017, 139 (31), 10653−10656. (f) Derosa, J.; Tran, V. T.; Boulous, M. N.; Chen, J. S.; Engle, K. M. Nickel-Catalyzed β,γ-Dicarbofunctionalization of Alkenyl Carbonyl Compounds via Conjunctive Cross-Coupling. J. Am. Chem. Soc. 2017, 139 (31), 10657−10660. (g) Derosa, J.; van der Puyl, V. A.; Tran, V. T.; Liu, M.; Engle, K. Directed nickel-catalyzed 1,2-dialkylation of alkenyl carbonyl compounds. Chem. Sci. 2018, 9 (23), 5278−5283. (h) Derosa, J.; Kleinmans, R.; Tran, V. T.; Karunananda, M. K.; Wisniewski, S. R.; Eastgate, M. D.; Engle, K. M. Nickel-Catalyzed 1,2Diarylation of Simple Alkenyl Amides. J. Am. Chem. Soc. 2018, 140 (51), 17878−17883. (i) Thapa, S.; Dhungana, R. K.; Magar, R. T.; Shrestha, B.; Kc, S.; Giri, R. Ni-catalysed regioselective 1,2-diarylation of unactivated olefins by stabilizing Heck intermediates as pyridylsilylcoordinated transient metallacycles. Chem. Sci. 2018, 9 (4), 904−909. (9) For recent examples of three-component dicarbofunctionalization of alkenes via nickel-catalyzed radical relay, see: (a) Qin, T.; Cornella, J.; Li, C.; Malins, L. R.; Edwards, J. T.; Kawamura, S.; Maxwell, B. D.; Eastgate, M. D.; Baran, P. S. A general alkyl-alkyl

scope in the alkenes, (hetero)aryl halides, as well as oxalate partners. Further efforts will focus on the development of an enantioselective version of this dicarbofunctionalization reaction.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.9b01658. Experimental procedures and compound characterization data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Lingling Chu: 0000-0001-7969-0531 Author Contributions †

L.G. and H.-Y.T. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Natural Science Foundation of China (21702029), the “Thousand Plan” Youth program, the Shanghai Sailing Program (17YF1400100), and the Fundamental Research Funds for the Central Universities for financial support.

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