Divergent C–H Oxidative Radical Functionalization of Olefins to Install

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

pubs.acs.org/OrgLett

Divergent C−H Oxidative Radical Functionalization of Olefins to Install Tertiary Alkyl Motifs Enabled by Copper Catalysis Ming-Qing Tian,† Cong Wang,† Xu-Hong Hu,*,† and Teck-Peng Loh*,†,‡ †

Institute of Advanced Synthesis, School of Chemistry and Molecular Engineering, Jiangsu National Synergetic Innovation Center for Advanced Materials, Nanjing Tech University, Nanjing 211816, China ‡ Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371, Singapore

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S Supporting Information *

ABSTRACT: An efficient tertiary alkylation reaction of olefins with 1,3-dicarbonyl compounds was developed by virtue of copper catalyst without the use of expensive ligands or additives. In contrast to alkyl Heck-type reaction, alkyl halide is not required. Notably, by varying the nitrogen and air atmosphere, the reaction selectively produces alkylation and alkylation−oxygenation products, respectively. Initial investigations revealed that an α-carbonyl alkyl radical species might be involved in the process.

O

xidative coupling reactions of olefin feedstocks,1 with respect to atom- and step-economical considerations, have been well recognized as a powerful tool for the construction of diversely and uniquely functionalized olefins of pharmaceutical importance. A particularly attractive protocol is the efficient installation of synthetically useful tertiary alkyl motifs into olefins to build molecular complexity along with the formation of new quarternary carbon centers.2 The most reliable strategy for such sterically hindered alkylation relies largely on the tertiary alkyl radical species, which are often generated from activated precursors, such as the corresponding halides,3 alcohols,4 carboxylic acids,5 and others,6 as represented by alkyl Heck-type reactions (Scheme 1a).7 Therefore, the direct utilization of simple tertiary hydrocarbons via a single-electron-transfer process8 for C−H alkylation of olefins would be highly appealing and desirable. 1,3-Dicarbonyl motifs are prevalent in a number of pharmaceuticals and natural products, and they have the ability to be synthetically modified.9 In particular, since the seminal work on manganese-mediated radical cyclization of olefins by Snider and co-workers,10 tremendous effort in this field has led to the dramatic development of C−H alkylation of olefins by combining transition-metal catalysis with the carbonyl reactivity beyond the classical nucleophilic substitution.11 During the past few decades, typical transformation involving α-carbonyl alkyl radical species addition into olefins mainly resulted in reductive addition12 or difunctionalization adducts,3e,f,i,l,13 whereas the elimination protocol giving rise to α-carbonyl olefins has been limitedly explored so far.3b,d,g,h,j Nishikata and co-workers recently disclosed an elegant copper-/triamine-cocatalyzed tertiary alkylation of styrenes with tertiary alkyl halide (Scheme 1b).3d,h,i On the basis of the literature precedent on C−H oxidative radical functionalization of olefins, the installation of sterically hindered alkyl fragments © XXXX American Chemical Society

Scheme 1. Tertiary C−H Alkylation via a Radical Pathway

into styrenes for the construction of α-alkyl olefins could be envisioned through direct oxidation of tertiary hydrocarbon adjacent to the carbonyl group. In the course of our studies on C−H oxidative radical functionalization of olefins,14 we have recently reported a practical approach to generate tertiary alkyl radicals with 1,3-dicarbonyls in the coupling of α-alkylated styrenes.14d Herein we disclose an efficient tertiary C−H alkylation of olefins with 1,3-dicarbonyls enabled by copper catalysis without the need for expensive ancillary ligands or additives, which highlights opportunities to enable the Received: January 13, 2019

A

DOI: 10.1021/acs.orglett.9b00142 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters incorporation of tertiary functionalized alkyl motifs into olefins. Notably, this catalytic protocol provides a facile and efficient route to both elimination and oxygenation products by the judicious tuning of the atmosphere (Scheme 1c). We commenced our studies by investigating the oxdative reaction of p-methylstyrene (1a) with diethyl methylmalonate (2a) in DMSO medium in the presence of copper catalyst under a nitrogen atmosphere (Table 1). Initially, a series of

Scheme 2. Cu-Catalyzed Tertiary C−H Alkylation of Olefinsa

Table 1. Optimization of Cu-Catalyzed Tertiary C−H Alkylation of 1aa

entry

Cu(II) salt (x mol %)

t (°C)

yield (%)b

1 2 3 4 5 6 7 8 9 10 11 12

Cu(OAc)2 (40) Cu(OTf)2 (40) Cu(acac)2 (40) Cu(tfacac)2 (40) CuF2 (40) CuCl2 (40) CuBr2 (40) CuBr2 (20) CuBr2 (10) CuBr2 (7.5) CuBr2 (10) CuBr2 (7.5)

120 120 120 120 120 120 120 120 120 120 130 130

3 19 51 81 80 75 71 81 78

a

The reaction was conducted with 1a (0.2 mmol), 2a (2.0 equiv), and copper salt (x mol %) in 2.0 mL of DMSO under a N2 atmosphere for 24 h. bYield of isolated product.

Cu(II) salts were screened for the coupling at 120 °C for 24 h (entries 1−7). We were delighted to observe that CuBr2 exhibited satisfactory catalytic activity with the desired product 3aa being formed in 81% yield (entry 7). It should be mentioned that DMSO acts as both a solvent and an oxidant in this catalytic system, as evidenced by the foul smell of dimethyl sulfide. Moreover, the product yield did not significantly diminish with reducing catalyst loading (entries 8−10). Additional surveying indicated that the yield of 3aa was well maintained by using 10 mol % of CuBr2 at 130 °C (entry 11). The established optimal reaction conditions for tertiary C− H alkylation reaction prompted us to explore the generality and limitation of this protocol (Scheme 2). An ortho-methyl substituent has minimal steric effect (3ab). The coupling of methylmalonate with styrenes bearing electron-donating substituents such as methyl and methoxyl proceeded smoothly to give the desired products 3aa−ae in moderate-to-good yields, while electon-withdrawing substituent was sluggish for the reaction (3af−ah). The phenomenon might signify that the electrophilic addition of α-carbonyl radical species into styrene occurred via a radical process. Moreover, diethyl αsubstituted malonate derivatives having benzyl, phenyl, nitrile, and cyclopropyl moieties also participated well in the coupling, giving the expected products 3ai−al in 69−83% yields. In addition, the reaction of naphthalenyl and heteroaryl ethylenes proved to be suitable to yield the coupling products 3am−ao. Notably, disubstituted olefins including 1,1-diphenylethylene (3ap), α-methylstyrene (3aq), indene (3ar), and dialin (3as) successfully underwent tertiary C−H alkylation under our new method. Late-stage modification of vinylestrone proceeded

a

The reaction was conducted with 1 (0.2 mmol), 2 (2.0 equiv), and CuBr2 (10 mol %) in 2.0 mL of DMSO under a N2 atmosphere at 130 °C for 24 h. Yield of isolated product. b1 mmol scale. c140 °C. d125 °C.

well to deliver the product 3at in 56% yield. Unfortunately, the reaction of aliphatic olefins with 2a, as exemplified with 4phenyl-1-butene, failed to afford the desired product. Furthermore, we serendipitously found that exposure of the substrates 1a and 2a with CuBr2 salt under an air atmosphere resulted in the formation of alkylation-oxygenation product 4aa in 15% yield along with a trace amount of 3aa. In fact, in contrast to the prosperous advances on olefin difunctionalization by introducing two heteroatom functionalities,15 carbooxygenation reactions via a radical pathway still remained challenging and have been relatively less reported.16 A short screening of copper(II) salts quickly determined that Cu(OAc)2 showed the highest efficiency for this aerobic oxidation reaction,17 providing 4aa in 70% yield (Scheme 3). A variety of substituted styrenes were examined to determine the electronic and steric influences on the reactivity (Scheme 4). To our delight, styrenes possessing both electronrich substituents, including methyl, tert-butyl, and methoxy and electron-deficient substituents such as fluoro, chloro, nitro, trifluoromethyl, and cyano, coupled well with 2a to generate 4aa−ma under the present catalytic system, irrespective of their steric properties. Besides styrenes, naphthalenyl, thioB

DOI: 10.1021/acs.orglett.9b00142 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters Scheme 3. Catalyst Screening of C−H AlkylationOxygenation of 1aa

Scheme 5. Cu-Catalyzed Tertiary C−H AlkylationOxygenation of 1aa

a

The reaction was conducted with 1a (0.2 mmol), 2a (2.0 equiv), and copper salt (40 mol %) in 2.0 mL of DMSO under an air atmosphere for 24 h. bYield of isolated product. c30 mol % of Cu(OAc)2 was employed.

Scheme 4. Cu-Catalyzed Tertiary C−H AlkylationOxygenation of Olefinsa

a

The reaction was conducted with 1a (0.2 mmol), 2 (2.0 equiv), and Cu(OAc)2 (40 mol %) in 2.0 mL of DMSO under an air atmosphere at 120 °C for 24 h. Yield of isolated product. b50 mol % of Cu(OAc)2 was employed. c110 °C.

ing alkylation-oxygenation products (4ab−ak). Dimethyl αmethyl malonate was also found to be competent coupling partner (4al). Radical-trapping and -clock experiments were carried out to verify a radical pathway. Addition of a common radical scavenger, namely, 2,2,6,6-tetramethylpiperidine N-oxide (TEMPO), completely suppressed the formation of the alkylation products 3aa and 4aa. Instead, the TEMPOmalonate adduct was observed, which was detected by HRMS analysis. Additionally, treatment of radical-clock reagent for the alkylation-oxygenation gave rise to the rearranged product, which indicates that α-carbonyl alkyl radical species might be involved in the reaction (for details, see the Supporting Information). Based on our mechanistic study and literature precedence,17a,d,18 a plausible reaction mechanism for chemoselective copper-catalyzed alkylation is outlined in Scheme 6. The reaction commences with the addition of in situ generated αcarbonyl alkyl radical species A into styrene 1a to generate the adduct B. In the absence of oxygen (path I), the benzyl radical

a

The reaction was conducted with 1 (0.2 mmol), 2a (2.0 equiv), and Cu(OAc)2 (40 mol %) in 2.0 mL of DMSO under an air atmosphere at 120 °C for 24 h. Yield of isolated product. b1 mmol scale. c100 °C. d 50 mol % of Cu(OAc)2 was employed. e80 °C. f130 °C.

Scheme 6. Proposed Mechanisms phenyl, and indolyl substituted ethylenes also afforded the alkylation-oxygenation products 4na−pa in good yields. Dialin exhibited low reactivity toward this transformation (4qa). Similarly, vinylestrone reacted readily to give 4ra in 42% yield. To further demonstrate the potential of this coppercatalyzed aerobic oxidation of olefin, a wide range of αsubstituted malonate derivatives were tested. As shown in Scheme 5, diethyl malonates tethered with a diversity of functional groups (ethyl, isopropyl, benzyl, phenyl, cyano, ester, alkynyl, alkenyl, cyclopentyl, cyclopropyl) were expediently installed without difficulty to generate the correspondC

DOI: 10.1021/acs.orglett.9b00142 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters

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B is subsequently oxidized by Cu(II) salt to carbenium ion C, which undergoes β-H elimination to furnish the alkylation product 3aa. DMSO takes the responsibility for the oxidation of Cu(I) to Cu(II). For path II, the radical B is trapped by molecular oxygen to form the peroxide intermediate D, which upon the reduction by Cu(I) produces intermediate E. Finally, intermediate E undergoes an elimination of a water molecule to release the alkylation-oxygenation product 4aa. However, an alternative conjecture for the transformation of intermediate D into 4aa via metal-catalyzed fragmentation of peroxides19 analogous to Kornblum−DeLaMare rearrangement cannot be ruled out. In summary, we have developed a straightforward and facile copper-catalyzed coupling of olefins with diversely functionalized 1,3-dicarbonyls for the generation of tertiary alkylation products. The atmosphere played the crucial role in the chemoselectivity control. Importantly, this protocol represents an appealing alternative strategy to alkyl Heck-type reactions, which features the advantages of easily available starting materials, sustainable catalyst with ligand or additive free, broad substrate scope, and excellent functional group compatibility. Currently, we are exploring further applications of this method and deeper mechanistic understanding.

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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.9b00142. Experimental procedures and characterization data (PDF)

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AUTHOR INFORMATION

Corresponding Authors

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

Xu-Hong Hu: 0000-0001-6584-2901 Teck-Peng Loh: 0000-0002-2936-337X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Natural Science Foundation of China (21702106), the Natural Science Foundation of Jiangsu Province (BK20170967), and the Start-up Grant from Nanjing Tech University (39839101 and 39837101) for financial support. The SICAM Fellowship by Jiangsu National Synergetic Innovation Center for Advanced Materials is also acknowledged.

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DOI: 10.1021/acs.orglett.9b00142 Org. Lett. XXXX, XXX, XXX−XXX