Primary, Secondary, and Tertiary γ-C(sp3)–H Vinylation of Amides via

Letter Cite This: Org. Lett. 2018, 20, 6255−6259

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Primary, Secondary, and Tertiary γ‑C(sp3)−H Vinylation of Amides via Organic Photoredox-Catalyzed Hydrogen Atom Transfer Hui Chen, Liangliang Guo, and Shouyun Yu* State Key Laboratory of Analytical Chemistry for Life Science, Jiangsu Key Laboratory of Advanced Organic Materials, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, China

Org. Lett. 2018.20:6255-6259. Downloaded from pubs.acs.org by UNIV OF SUNDERLAND on 10/05/18. For personal use only.

S Supporting Information *

ABSTRACT: An efficient strategy for primary, secondary and tertiary aliphatic γ-C(sp3)−H vinylation of amides with alkenylboronic acids is reported. These reactions are catalyzed by visible-light organic photoredox agents. Regioselective γC(sp3)−H vinylation of amides is controlled by a 1,5-hydrogen atom transfer of an amidyl radical generated in situ.

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to date. Herein, we report our transition-metal-free solution to this synthetic challenge (Figure 1b). With their weak N−O bond, hydroxylamine derivatives have recently been exploited as nitrogen-centered radical (Nradical) precursors in visible-light photocatalysis.12 Their synthetic potential can be controlled by tuning substituents on the nitrogen and oxygen atoms. Based on this knowledge, we believe that hydroxamic acid derivatives (1, Figure 1b) could serve as the precursors to N-radicals and subsequently as platforms for the remote C(sp3)−H functionalization under photoredox catalysis. Mechanistically, the hydroxamic acid derivative (1), upon single-electron transfer (SET) reduction by the excited photocatalyst eosin Y*,13 forms an amidyl radical (A) and a carboxylate anion (ArCO2−). A 1,5-HAT from the γ-C−H bond to the amidyl radical (A) provides a carbon-centered radical, the C-radical (B), which is intermolecularly trapped by a vinylboronic acid (2), giving the corresponding C-radical (C). This radical (C) is then oxidized by eosin Y+•, delivering a cationic intermediate (D) and regenerating the photocatalyst, eosin Y. After deboronation,14 the remote C(sp3)−H vinylation product (3) is ultimately produced (Figure 1c). Since the substituents on the nitrogen atom can affect the stability and reactivity of the N-radical, a series of hydroxamic acid derivatives (1a−f) with different functional groups on the

he development of selective strategies to convert inert C(sp3)−H bonds to C−C bonds, which enables direct access to molecules that would otherwise be inaccessible in a single step, remains a formidable challenge in organic chemistry.1 In a variety of functional groups capable of directing C−H activation, carboxycarbonyls are attractive because carboxylic acids and their derivatives are among the most common organic molecules found in nature and represent ideal starting materials with which to generate new connections.2 Direct functionalizations of carboxylic acid derivatives at the α- or β-positions, through enolate chemistry3 or transition-metal-catalyzed directing C−H activation,4 respectively, are well established, but derivatization at the γposition has been much less frequently reported. Transitionmetal-catalyzed arylation, olefination, and chalcogenation reactions of the γ-C(sp3)−H of amides have been recently developed by Chen,5 Yu,6 and Maiti7 (Figure 1a, top). These reactions proceed through a sterically controlled and less favorable six-membered metallacycle, and only primary γC(sp3)−H moieties are amenable to these reactions.6b,7 Another elegant strategy, reported by Yu,8 Rovis,9 and Leonori10 employs an intramolecular 1,5-hydrogen atom transfer (1,5-HAT) process11 by amidyl radicals (Figure 1a, bottom). Functionalization of tertiary and secondary γC(sp3)−H units has been achieved using this method, but unactivated primary C(sp3)−H bonds are not suited to this reaction. There is a lack of an efficient and unified strategy for the intermolecular functionalization of primary, secondary, and tertiary inert γ-C(sp3)−H bonds of carboxylic acid derivatives © 2018 American Chemical Society

Received: August 27, 2018 Published: September 24, 2018 6255

DOI: 10.1021/acs.orglett.8b02737 Org. Lett. 2018, 20, 6255−6259

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Figure 2. Investigation of the protecting groups on the nitrogen atom. Reaction conditions: 1 (0.2 mmol, 1.0 equiv), 2a (0.6 mmol, 3.0 equiv), Na2CO3 (0.4 mmol, 2.0 equiv), eosin Y (2 mol %), DMSO (2.0 mL), rt, 90 W white LEDs. Yields are for the isolated products. N. D. = not detected.

Table 1. Variation of Reaction Parametersa

entry

variation of reaction conditions

yieldb (%)

1 2 3 4 5 6 7 8 9

none without light or eosin Y or Na2CO3 air instead of N2 Ir(ppy)3 instead of eosin Y Ru(bpy)3Cl2 instead of eosin Y DIPEA instead of Na2CO3 Na2HPO4 instead of Na2CO3 CH3CN instead of DMSO MeOH instead of DMSO

86 (85c) trace 33 11 22 21 44 19 21

a

Figure 1. γ-C(sp3)−H functionalization of amides.

Reaction conditions: 1f (0.1 mmol, 1.0 equiv), 2a (0.3 mmol, 3.0 equiv), base (0.2 mmol, 2.0 equiv), photocatalyst (2 mol %), solvent (1.0 mL), rt, 90 W white LEDs. bThe yield was determined by GC in the presence of tetradecane as an internal standard. cIsolated yield on a 0.2 mmol scale.

nitrogen atom were prepared. Adjustments of the substituents on the nitrogen atom provide an opportunity to control the reactivity of the N-radicals and subsequently address remote functionalization of secondary and even primary C(sp3)−H bonds, which is still synthetically challenging. Hydroxamic acid derivatives (1) were coupled with (E)-styrenylboronic acid (2a) in the presence of eosin Y under visible-light irradiation (Figure 2). Hydroxamic acids with a free nitrogen or an Nphenyl group failed to give any of the desired products. It is well documented that the higher electrophilic character of the amidyl radicals the more likely they are to undergo hydrogen atom abstraction.9,15 Polar effects of radicals also play a significant role in the HAT process.16 Accordingly, electron deficient group substituted hydroxamic acids were investigated. The desired products (3ca and 3da) could be isolated, but with unsatisfactory yields of 37% and 27%, respectively. Finally, it was found that N-alkyl groups gave better results. With tert-butyl (t-Bu) as the substituent,17 3fa was obtained in 85% isolated yield as the sole E-isomer (E/Z > 20/1). The reaction parameters were established using a hydroxamic acid derivative (1f) and (E)-styrylboronic acid (2a) as the reactants, and the results highlighted in Table 1 were obtained. For the comprehensive optimization of the reaction conditions, see the Supporting Information (SI). A mixture of 1f (1 equiv) and 2a (3 equiv) in DMSO was

irradiated with 90 W white LEDs at room temperature in the presence of 2 mol % of eosin Y as the photocatalyst and Na2CO3 as the base. The desired γ-C(sp3)−H vinylation product 3fa was obtained in 86% yield based on GC analysis (85% isolated yield) (entry 1). Control experiments confirmed that the photocatalyst, base, light, and nitrogen atomosphere are all essential for this reaction (entries 2 and 3). Eosin Y, Na2CO3, and DMSO were established as the optimal combination among the photocatalysts (entries 4 and 5), bases (entries 6 and 7), and solvents (entries 8 and 9), respectively. With the optimized conditions in hand, we evaluated the generality of this photoredox-catalyzed γ-C(sp3)−H vinylation of amides. Amides with different types of inert C(sp3)−H bonds were first examined (Figure 3a−c). As functionalization of unactivated secondary and primary C(sp3)−H bonds through a radical pathway remains a synthetic challenge, we decided to investigate vinylation of secondary inert C(sp3)−H bonds and even more challenging primary inert C(sp3)−H bonds. Consistent with our preliminary results, which were obtained with product 3fa, other secondary C(sp3)−H bonds reacted smoothly with boronic acid (2a) under our reaction 6256

DOI: 10.1021/acs.orglett.8b02737 Org. Lett. 2018, 20, 6255−6259

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Figure 3. Scope of the remote vinylation of amides. Reaction conditions: 1 (0.2 mmol), 2 (0.6 mmol), Na2CO3 (0.4 mmol), eosin Y (2 mol %), DMSO (2.0 mL), rt, 90 W white LEDs. Yields for the isolated products. Ratios of stereoisomers were determined by 1H NMR analysis. (a) 6 mmol scale. (b) 5 mmol scale.

conditions to give the desired products 3fa−oa in 53−92% yields (Figure 3a). Functional groups, such as ether (3ga and 3ja), thioether (3oa), amide (3ha and 3ka), and ester (3na), were all tolerated. With hydroxamic acid derivatives containing two or more potential hydrogen abstraction sites, only γvinylation products were observed in all cases (3ia−na). This remarkable regioselectivity indicates that 1,5-HAT has a strong preference over other HAT pathways in this reaction. The trans-configuration of C4 and C5 in 3ia was established by Xray crystallography. Remarkably, this method can also be applied to the vinylation of primary inert C(sp3)−H bonds, as exemplified by the high-yield synthesis of products 3pa−ta (52−84%), albeit with slightly lower E/Z stereoselectivity (Figure 3b). Tertiary inert C−H bonds, such as in substrates 1u−x, were converted to 3ua−xa in good yields with excellent E/Z stereoselectivities (Figure 3c). Amino acid derived hydroxamic acids also undergo this remote vinylation, giving 3oa, 3sa, and 3za in good yields. Notably, the reaction can perform at gram scale as demonstrated by the synthesis of 3fa

and 3ua, which were isolated with 83% and 78% yields, respectively. Next, we examined the vinylboronic acid partners. Styrylboronic acids with electron-donating groups such as Me, t-Bu, and OMe or electron-withdrawing groups such as F, CF3, and CO2Me, at the para position, gave good yields of the respective products (Figure 3d). Substituents at different positions on the phenyl ring were well suited to this transformation, affording satisfactory yields of the corresponding products (3fi, 3fl, and 3uj−uk). Notably, all of the products were isolated with a greater than 20/1 E/Z ratio. The N-tert-butyl amide (3fa) could be hydrolyzed in hot aqueous HCl solution. The acid (4) was obtained in 73% yield and can support further transformations (eq 1).

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For a comprehensive understanding of this protocol, we further studied the selectivity of 1,5-HAT when simultaneously presented with two inert C(sp3)−H bonds. With the hydroxamic acid derivative 1a′, the HAT occurred at the tertiary C(sp3)−H bond (eq 2). When substrate 1b′ was

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b02737. Experimental details, NMR spectra, and details of experiments (PDF) Accession Codes

CCDC 1860263 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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subjected to the standard vinylation conditions, the HAT was completely regioselective, and only vinylation of secondary C(sp3)−H bond was observed (eq 3). In summary, we have developed an efficient approach for primary, secondary, and tertiary γ-C(sp3)−H vinylation of amides with alkenylboronic acids. The salient features of this transformation include absence of transition metals, redox neutrality, mild reaction conditions, and broad substrate scope, which make the method highly attractive. Use of this strategy will enable further discovery of remote C(sp3)−H functionalization and is currently in progress in our laboratory.

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Letter

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Shouyun Yu: 0000-0003-4292-4714 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (21732003), the National Key Research and Development Program of China (2018YFC0310900), and the Fundamental Research Funds for the Central Universities (020514380131 and 020814380092) is acknowledged. 6258

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