Cobalt-Catalyzed Cross-Dehydrogenative Coupling Reaction

Letter pubs.acs.org/OrgLett

Cobalt-Catalyzed Cross-Dehydrogenative Coupling Reaction between Unactivated C(sp2)−H and C(sp3)−H Bonds Qun Li,† Weipeng Hu,† Renjian Hu,† Hongjian Lu,*,† and Guigen Li*,†,‡ †

Institute of Chemistry & BioMedical Sciences, Nanjing University, Nanjing, 210023, China Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409-1061, United States

‡

S Supporting Information *

ABSTRACT: Catalytic oxidative cross-dehydrogenative coupling between unactivated C(sp2)−H and C(sp3)−H bonds is achieved by the cobalt-catalyzed o-alkylation reaction of aromatic carboxamides containing (pyridin-2-yl)isopropyl amine (PIP−NH2) as a N,N-bidentate directing group. Many different C(sp3)−H bonds in alkanes, toluene derivatives and even in the α-position of ethers and thioethers can be used as coupling partners. This method has a broad substrate scope and the tolerance of various functional groups.

T

such reactions to promote the generation of the reactive alkylcobalt species which, assisted by the extra ligand initiates C−H bond activation. Despite the breakthrough in the primary and the more challenging secondary alkylation of (hetero)arenes, the use of prefunctionalized coupling partners reduces the efficiency and generality of this transformation. Herein we report a first example of high-valent-cobalt-catalyzed10e−h,11 CDC reaction between unactivated C(sp2)−H and C(sp3)−H bonds,13 with the assistance of a removable N,N-bidentate directing group.14,15 In this reaction, C(sp3)−H bonds in alkanes and toluene derivatives and also in the α-position of ethers and thioethers are used as coupling partners, providing various osubstituted aromatic carboxamides in good to excellent yield. Inspired by our recent work on cobalt-catalyzed C−H bond functionalizations,16 our investigation commenced with the direct coupling of benzamide (1a) with cyclohexane in the present of di-tert-butyl peroxide (DTBP) and different commercially available cobalt complexes as catalyst (Table 1, entries 1−6). The anion bound to cobalt plays a crucial role in the catalysis. Cobalt compounds, including CoBr2, CoCl2, CoF2, and Co(OAc)2 all fail to give the desired product (3a) but Co(acac)2 (acac = acetylacetone) gives the desired result in 82% yield, determined by NMR spectroscopy. Co(acac)3 also affords 3a in 42% yield under oxidative conditions, suggesting that the C(sp2)−H cleavage may be initiated by a cobalt(III) species. The product 3a is not formed in the absence of a cobalt catalyst (entry 7). Replacement of DTBP with other peroxides, such as tert-Butyl peroxybenzoate (TBPB), tert-Butyl hydroperoxide (TBHP), Benzoyl peroxide (BPO) and Dicumyl peroxide (DCP) fails to give better results (entries 8−11) and it was found that air can significantly influence the result (entry 12). When the reaction is conducted with reduced catalyst loading, or a reduced amount of peroxide, a decrease in the yield of 3a is observed (entries 13−14).

he transition metal-catalyzed C−H bond functionalization reaction for selective C−C bond formation has emerged as a versatile, efficient and straightforward approach,1 in which various functionalized compounds such as organometallic reagents, olefins and halides are used as coupling partners. Recently, catalytic cross-dehydrogenative coupling (CDC) of two different C−H bonds has become more efficient and an ideal strategy for the C−C bond formation because it has no requirement for functionalized compounds.2,3 Following the pioneering work reported by Stuart and Fagnou,4 remarkable progress has been made in the oxidative cross coupling reactions of C(sp2)−H/C(sp2)−H bonds.2,5 The coupling reactions are another type of CDC reaction involving C(sp2)−H/C(sp3)−H bonds and have attracted much attention recently.3,6 However, most of these examples typically require electron-rich aryl or acidic C(sp2)−H bonds, and/or relatively reactive C(sp3)−H bonds, such as C(sp3)−H bonds adjacent to nitro or carbonyl groups. The CDC reaction between unactivated C(sp2)−H and C(sp3)−H bonds represents the next goal. In fact, Li et al.7 and more recently Shi and Zhao8 reported ruthenium-catalyzed cross-coupling reactions of arenes containing pyridine as an unremovable directing group with simple cycloalkanes or toluene derivatives. In addition, Chatani et al. reported the nickelcatalyzed CDC reaction of benzamides containing an 8aminoquinoline moiety as a directing group with toluene derivatives.9 The catalytic CDC reaction between unactivated C(sp2)−H bonds and versatile C(sp3)−H bonds, however, remains a challenging issue. On the other hand, cobalt catalysis has recently exhibited its great potential in C−H bond functionalization reactions due to its low-cost, environmentally benign and unique catalytic mode.10,11 Concerning these transformations, a number of cobalt-catalyzed direct C−H alkylations of (hetero)arenes have been achieved.12 The groups of Nakamura,12a,b Yoshikai12c−h and Ackermann12i have demonstrated low-valent-cobalt-catalyzed direct C−H alkylation of (hetero)arenes with alkenes or alkyl halides. Excess Grignard reagents were indispensable in © 2017 American Chemical Society

Received: July 27, 2017 Published: August 22, 2017 4676

DOI: 10.1021/acs.orglett.7b02316 Org. Lett. 2017, 19, 4676−4679

Letter

Organic Letters

a mixture of mono- and dialkylated products (3i/i′−l/l′), the latter being preferred. Both the mono- and dialkylated products can be isolated separately. The substrates are not limited to benzamides, as polyaromatic carboxamide (1-naphthamide, 3m) and heteroaromatic carboxamides (benzo[b]thiophene-2-carboxamide and thiophene-2-carboxamide, 3n−o) are also tolerated in this reaction. Next, the scope of the alkanes was examined (Scheme 2). Several cycloalkanes with different ring size including cyclo-

Table 1. Optimization of Reaction Conditions

entry

[Co] (mol %)

peroxide

yield (%)a

1 2 3 4 5 6 7 8 9 10 11 12c 13d 14e

CoF2 CoCl2 CoBr2 Co(OAc)2 Co(acac)2 Co(acac)3 − Co(acac)2 Co(acac)2 Co(acac)2 Co(acac)2 Co(acac)2 Co(acac)2 Co(acac)2

DTBP DTBP DTBP DTBP DTBP DTBP DTBP DCP TBPB BPO TBHP DTBP DTBP DTBP

0 0 0 0 82(78b) 42 0 68 trace 0 0 34 43 47

Scheme 2. Scope of Alkyl Coupling Partnersa

a Crude 1H NMR yields determined by using dibromomethane as an internal standard. bIsolated yield. cUnder air. d2.0 equiv of DTBP was used. e10 mol % Co(acac)2 was used.

Under the optimized reaction conditions, the scope of benzamides was then investigated. As shown in Scheme 1, Scheme 1. Scope of Aromatic Carboxamidesa

a

Isolated yield. bDetermined by 1H NMR analysis. cUsing 2 (4.0 mmol)/benzene (1.0 mL) instead of 2 (1.0 mL). dThe isomers were isolated separately.

a

pentane (3p), cyclohexane (3a), cycloheptane (3q) and cyclooctane (3r) show good reaction efficiency under the standard conditions. Bridged compounds such as norbornane also couple with 1a to form two regioisomers (3s) in a ratio of 1:6. The reaction of 1a with a linear alkane, such as n-hexane results in a mixture of three regioisomers (3t) with a ratio of 1:6:2, and the less sterically hindered secondary C(sp3)−H bond is highly preferred. To broaden the application of this method, we then extended our strategy to the cross coupling of aromatic carboxamides with toluene derivatives. The direct coupling of toluene with 1a proceed smoothly to afford the desired product in good yield (4a). Substitution in different position of aromatic ring leads to products (4b−d) with no effect on the efficiency of the reactions. Halogen-substituted toluenes also give good yields of the desired products (4e−g). Toluenes containing both electron-donating groups and electron-withdrawing groups react smoothly with 1a under the standard conditions to give the desired products (4b− d, 4h). In addition, 2-methylthiophene serves well in this reaction producing the product 4i.

Isolated yield.

benzamides containing either electron-donating groups such as −Me and −OMe, or electron-withdrawing groups including −CF3, −CN, −F, −Cl and −Br, react smoothly with cyclohexane under the standard conditions, giving products 3a−l. Reactions with o-substituted benzamides produce monoalkylated products in moderate to high yield (3a−3e). Substituted groups in the mposition can affect both the electron density and the accessibility of the reaction site, which in turn, could control the reactivity and selectivity of C−H bonds.17 When there is an electronwithdrawing group in the m-position, mono C−H alkylation takes place predominately at the less-hindered position giving for example, the products 3f/f′. When the m-substituent of the benzamide is a methyl or methoxyl group, a mixture of monoand dialkylated products (3g/g′ and 3h/h′) is obtained. Reaction with benzamide or p-substituted benzamides provides 4677

DOI: 10.1021/acs.orglett.7b02316 Org. Lett. 2017, 19, 4676−4679

Letter

Organic Letters Since the C−H bond adjacent to an oxygen atom has a bond dissociation energy similar to that of a C−H bond at the benzylic position, the ether may be a good coupling partner although it has not previously been utilized in CDC reactions between unactivated C(sp2)−H and C(sp3)−H bonds.7−9 Accordingly, a wide scope of ethers was investigated, saturated cyclic ethers, such as tetrahydrofuran, tetrahydropyran, 1,4-dioxane, 1,3dioxolane and their derivatives are compatible with this reaction (5a−f). In the competition between the C−H bonds adjacent to oxygen and to the aryl group, the former C−H bond is highly preferred and gives a single product (5e) which can be isolated in good yield. Linear ethers, such as diethyl ether and ethoxybenzene also give the desired products (5g, 5i) in good yield. In addition to the secondary C(sp3)−H bond, the compound with a primary C−H bond adjacent to oxygen is also a good substrate in this reaction (5h). Ethers such as 1,2diethoxyethane and 1,2-dimethoxyethane, with two different active sites adjacent to the oxgen atom, produce a mixture of two isomers, both of which can be isolated separately (5j/j′ and 5k/ k′). Although thioethers are very useful moieties in natural products and bioactive compounds and are common in pharmaceutically relevant architectures,18 thioethers have not been utilized in the CDC reactions. The absence of the metal catalyzed reactions of thioethers is presumably because the sulfur atom can easily bind to metals, leading to deactivation of the metal catalyst or they could be oxidized to sulfenyl and sulfonyl groups under the oxidative conditions. To broaden the application of this methodology, various thioethers were investigated. Both dialkyl sufides and alkyl phenylsulfides are compatible with this reaction (6a−d). The C−H bond adjacent to sulfur is highly preferred and a single product can be isolated in good yield. The primary C(sp3)−H bond adjacent to sulfur is also a good coupling partner in this reaction, giving products 6d− e in good yield. In addition, the cyclic thioethers are well tolerated under the standard conditions to afford good yields of the desired products (6f−g). Use of ethers, thioethers and toluene derivatives as the solvent, is not required (see note c in Scheme 2) and the reactions take place exclusively at relatively active C(sp3)−H bonds, which are adjacent to oxygen, sulfur or an aryl group (4−6). To clarify the mechanism of the reaction, several control experiments were carried out (Scheme S1 in Supporting Information (SI)). Upon the addition of a radical inhibitor 2,2,6,6-tetramethylpiperidinyloxy (TEMPO, 4.0 equiv), the CDC reaction of 1a with cyclohexane (c-C6H12) is completely suppressed, and the cyclohexyl radical trapping product (7) was isolated in 72% yield based on TEMPO (eq S1). A primary kinetic isotope effect in parallel KIE experiments (cyclohexane vs D12-cyclohexane) was observed (kH/kD = 2.1, eq S2), suggesting that the cleavage of C(sp3)−H bond might be involved in the turnover-limiting step.19 The electronic effect on the phenyl ring was then investigated through competition experiments (1f vs 1g), and the electron-deficient benzamide (1f) was found to exhibit higher reactivity (eq S3). When the deuterated benzamide (D5-1i) was used as a substrate under the standard reaction conditions, but only for 2 h, 65% of D5-1i was recovered without any H/D exchange being observed (eq S4). Finally, no kinetic isotope effects in parallel KIE experiments (1i vs D5-1i) were observed (eq S5). On the basis of the above results and previous reports,6a,11,15 a possible catalytic cycle was proposed and is shown in Figure 1. Initially, an intermolecular single electron transfer (SET) process

Figure 1. Possible catalytic cycles.

between the Co(II) precatalysts and DTBP provides a Co(III) species and alkoxy radical t-BuO·. Coordination of 1a to a Co(III) species by a ligand exchange process generates intermediate A, and the subsequent C(sp2)−H cleavage via concerted metalation-deprotonation (CMD) forms intermediate B. Meanwhile the alkoxy radical that was formed abstracts a hydrogen atom from the alkane to generate an alkyl radical, which then is oxidatively added into the cobalt center of intermediate B, providing intermediate C. Finally, reductive elimination of intermediate C, followed by protonation gives product 3a and Co(II) species, which can be reoxidized to continue the catalytic cycle. In summary, we have reported a unique strategy that enables a catalytic CDC reaction of unactivated C(sp2)−H bonds with a wide range of C(sp3)−H bonds via a cobalt-catalyzed reaction of secondary aromatic carboxamides with hydrocarbons. The unique features of this method include the tolerance of various functional groups and structural variety of the aromatic carboxamides and hydrocarbon substrates, which include alkanes, toluene derivatives, ethers and thioethers. Preliminary mechanistic studies by radical trapping experiments, hydrogen/ deuterium exchange experiments and kinetic isotope effects suggest that the C(sp2)−H bond is cleaved by the cobalt catalyst and C(sp3)−H bond cleavage is achieved by a radical hydrogen atom abstraction step. We expect that the strategy developed here may lead to the design of new CDC reactions.

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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.7b02316. Experimental details and characterization data for all new compounds (PDF)

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AUTHOR INFORMATION

Corresponding Authors

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

Hongjian Lu: 0000-0001-7132-3905 Guigen Li: 0000-0002-9312-412X Notes

The authors declare no competing financial interest. 4678

DOI: 10.1021/acs.orglett.7b02316 Org. Lett. 2017, 19, 4676−4679

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Organic Letters

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ACKNOWLEDGMENTS We would like to acknowledge the financial support from the National Natural Science Foundation of China (No. 21332005, 21472085, 21672100), the Fundamental Research Funds for the Central Universities (No. 020514380114) and Robert A. Welch Foundation (D-1361, USA).

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DOI: 10.1021/acs.orglett.7b02316 Org. Lett. 2017, 19, 4676−4679