Electroremovable Traceless Hydrazides for Cobalt-Catalyzed Electro

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Cite This: J. Am. Chem. Soc. 2018, 140, 7913−7921

Electroremovable Traceless Hydrazides for Cobalt-Catalyzed ElectroOxidative C−H/N−H Activation with Internal Alkynes Ruhuai Mei,† Nicolas Sauermann,† Joaõ C. A. Oliveira,† and Lutz Ackermann*,†,‡,§ †

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Institut für Organische und Biomolekulare Chemie, Georg-August-Universität Göttingen, Tammannstraße 2, 37077, Göttingen, Germany ‡ Department of Chemistry, University of Pavia, Viale Taramelli, 10, 27100 Pavia, Italy § International Center for Advanced Studies of Energy Conversion (ICASEC), Georg-August-Universität Göttingen, Tammannstraße 6, 37077, Göttingen, Germany S Supporting Information *

ABSTRACT: Electrochemical oxidative C−H/N−H activations have been accomplished with a versatile cobalt catalyst in terms of [4 + 2] annulations of internal alkynes. The electrooxidative C−H activation manifold proved viable with an undivided cell setup under exceedingly mild reaction conditions at room temperature using earth-abundant cobalt catalysts. The electrochemical cobalt catalysis prevents the use of transition metal oxidants in C−H activation catalysis, generating H2 as the sole byproduct. Detailed mechanistic studies provided strong support for a facile C−H cobaltation by an initially formed cobalt(III) catalyst. The subsequent alkyne migratory insertion was interrogated by mass spectrometry and DFT calculations, providing strong support for a facile C−H activation and the formation of a key seven-membered cobalta(III) cycle in a regioselective fashion. Key to success for the unprecedented use of internal alkynes in electrochemical C−H/N−H activations was represented by the use of N2-pyridylhydrazides, for which we developed a traceless electrocleavage strategy by electroreductive samarium catalysis at room temperature.

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and Xu, among others.45−59 Alternatively, the power of precious palladium catalysts has been unraveled at elevated temperatures (70−90 °C), as elegantly developed by Kakiuchi, and Mei, among others.60−69 In contrast, we have introduced in 2017 earth-abundant 3d transition metals for electrooxidative70−73 C−H activation catalysis.74 Thus, cost-effective base metal75−81 cobalt(II) catalysts82−94 enabled sustainable C−H oxygenations in the absence of any chemical oxidants.74 On the basis of key mechanistic insights into the working mode of these C−H alkoxylations, we unraveled C−H/N−H activations for the step-economical [4 + 2] annulation of alkynes.95 In spite of major progress in the field,95,96 the cobaltcatalyzed alkyne annulation regime continues to be severely restricted to exclusively include terminal alkynes. Thus, synthetically meaningful internal acetylenes were thus far completely inert (Scheme 1). On the one hand this represents a major limitation in the synthesis of diversely substituted heterocycles by electrocatalysis. On the other hand, these findings feature important mechanistic implications in that two viable reaction pathways could not unambiguously be discriminated. Thus, the two

INTRODUCTION C−H activation has been recognized as a powerful tool in molecular synthesis,1−16 with enabling applications to material sciences,17 natural product synthesis,18 and pharmaceutical industries,19,20 among others. Particularly, oxidative C−H transformations have proven instrumental for the development of step-economical organic syntheses.21−24 Despite considerable advances, these oxidative C−H functionalizations largely rely on stoichiometric amounts of costly and/or toxic transition metal oxidants that generate undesired metal-containing byproducts, thereby compromising the atom-economical nature of the C−H activation approach.25 In the meantime, electrosynthesis has emerged as an increasingly viable platform for molecular synthesis.26−37 Specifically, electrocatalysis holds great potential for avoiding the use of cost-intensive chemical reagents in redox-processes, thereby reducing the footprint of undesirable byproducts.28,29,35,38−42 Thus, in recent years, electrosynthesis was illustrated to significantly improve the sustainability of molecular transformations.43,44 A considerable recent momentum was gained by merging organometallic electrosynthesis and catalyzed C−H activation.28,29,39−42 In this context, major advances were accomplished by innate reactivity guidance under metal-free conditions, as reported by Barran, Waldvogel, © 2018 American Chemical Society

Received: April 7, 2018 Published: May 29, 2018 7913

DOI: 10.1021/jacs.8b03521 J. Am. Chem. Soc. 2018, 140, 7913−7921

Article

Journal of the American Chemical Society Scheme 1. [4 + 2] Annulation with Terminal Alkynes

Table 1. Electro-Oxidative C−H/N−H Activation with Internal Alkyne 2aa

scenarios involving (a) a migratory insertion/reductive elimination manifold or (b) a C−H alkynylation/hydroamidation regime continue to be viable options for the electrochemical [4 + 2] alkyne annulation. With our continued interest in sustainable C−H activation,97,98 we have now devised conditions for the first base metal-catalyzed electrochemical C−H activation with internal alkynes, on which we report herein. Thus, pyridylbenzhydrazides 199 underwent effective electrochemical C−H/N−H activations with internal alkynes to provide expedient access to diversely decorated isoquinolones, key structural motifs of relevance to natural product chemistry, medicinal chemistry, and pharmaceutical industries.100−102 Herein, we report the development of cobaltcatalyzed electro-oxidative C−H/N−H activation with internal alkynes, employing electricity as the sole oxidant, which avoids the use of stoichiometric metal salts as sacrificial oxidants (Figure 1). Salient features of our strategy include (i)

entry

solvent

additive

T [°C]

yield [%]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

MeOH EtOH H2O TFE TFE TFE TFE TFE TFE TFE TFE TFE TFE TFE TFE TFE

NaOPiv NaOPiv NaOPiv NaOPiv NaOPiv NaOPiv Na2CO3 K3PO4 NaOAc n-Bu4NPF6 PivOH − − PivOH PivOH PivOH

23 23 23 23 60 80 23 23 23 23 23 23 23 23 23 23

trace trace − 48 34 39 trace − 51 63 79 37 −b 71b −c −d

a

Reaction conditions: Undivided cell, 1a (0.5 mmol), 2a (0.6 mmol), Co(OAc)2 (10 mol %), additive (2.0 equiv), solvent (3.0 mL), 5.0 mA, 16 h, RVC anode (1.0 × 1.5 cm), Pt-plate cathode (1.0 × 1.5 cm), under N2. bCoCl2 (10 mol %). cWithout cobalt. dWithout electricity.

Scheme 2. Effect of N-Substituents on Electrochemical C− H/N−H Activation

Figure 1. Electro-oxidative cobalt-catalyzed C−H/N−H activation featuring electroreductive hydrazide cleavage.

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RESULTS AND DISCUSSION Optimization Studies. We initiated our studies by exploring reaction conditions for the envisioned electrooxidative [4 + 2] annulation of internal alkyne 2a by 2pyridylbenzhydrazide 1a (Table 1 and Table S-1 in the Supporting Information).105 After considerable preliminary experimentation, we observed that the desired electrochemical C−H activation with internal alkyne 2a was best accomplished with TFE as the solvent and NaOPiv as the key additive in a most user-friendly undivided cell (entries 1−4). The catalytic efficacy of the electro-oxidation was reflected by efficient C−H/ N−H activation occurring at an ambient temperature of 23 °C (entries 4−6). The catalytic performance could be further improved with pivalic acid as the additive (entries 7−11), which can be rationalized by improved solubility and chemoselectivity as well as a more effective cathodic reduction. Control experiments confirmed the essential role of the cobalt catalyst,

electrochemical C−H activation with internal alkynes, (ii) earth-abundant cobalt76,77,103,104 catalysis, (iii) high levels of position selectivity and chemoselectivity, (iv) a user-friendly undivided cell setup, and (v) exceedingly mild room temperature conditions. It is also noteworthy that we have established reaction conditions for the electroreductive removal of the benzhydrazide directing groups by samarium catalysis, setting the stage for the first electrocleavable directing group strategy in C−H activation catalysis. We, furthermore, disclose detailed mechanistic insights on the C−H/N−H activation, including experimental spectrometric and computational DFT studies on the selectivity control in cobalt-catalyzed electro-oxidation catalysis. 7914


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Journal of the American Chemical Society Scheme 3. Electrochemical C−H/N−H Activation: [4 + 2] Annulation of Internal Alkynes 2

Scheme 4. Electrochemical C−H/N−H Activation: Terminal Alkynes 2

the carboxylate additive (entries 13−15), and the electricity (entry 16). Thereafter, we probed the effect exerted by the N-substituent of the benzamide motif in substrates 1 (Scheme 2). Interestingly, none of the frequently used N-directing groups, such as the omnipresent 8-quinolinyl106 or the previously employed74,95 2-N-oxypyridyl substituents, allowed for the transformation of the internal alkyne 2a. In sharp contrast, solely the 2-pyridylhydrazide 1a proved amenable to the desired C−H/N−H activation.

Versatility. With the optimized cobalt-catalyzed electrooxidative C−H activation in hand, we probed its robustness with a variety of internal alkynes 2 (Scheme 3a). Thus, disubstituted acetylenes 2b−f bearing electron-donating or electron-withdrawing aryl, alkyl, and even ester groups were smoothly converted within the [4 + 2] alkyne annulation regime. Generally, the electrochemical C−H/N−H activation occurred efficiently at room temperature. Detailed NMR spectroscopic studies revealed that the annulation of unsymmetrically substituted alkynes 2g−i proceeded with synthetically useful levels of regiocontrol (Scheme 3b), generally placing the alkyl-group distal and the π-containing motif proximal to nitrogen (vide infra). As to unsymmetrical diarylalkynes 2j−1, the cobalt-catalyzed C−H/N−H activation placed the electron-deficient oligofluoroaryl moiety preferentially distal to nitrogen, as was elaborated by NMR spectroscopic analysis and single crystal X-ray diffraction.105 This observation is indicative of attractive π−π dispersion interactions controlling the key migratory insertion step. Moreover, the versatile cobalt catalyst was applicable to the site-selective functionalization of arenes 1b−m displaying para-, meta- and ortho-substituents (Scheme 3c). Thereby, syntheti7915


Article

Journal of the American Chemical Society Scheme 5. Summary of Key Mechanistic Findings

Scheme 6. Mass Spectrometric Analysis

Figure 2. Cyclic voltammogram at 100 mV/s in MeOH. n-Bu4NPF6 (0.1 M in MeOH), alkyne = 7a, [Co] = Co(OAc)2.105

substituents, which should prove invaluable for the further latestage diversification of the thus-obtained products 3jc−mc. The broadly applicable electro-oxidative C−H/N−H activation manifold was not limited to internal alkynes 2. Indeed, various terminal alkynes 7 proved to be viable substrates as well (Scheme 4). Hence, a wealth of aryl-decorated alkynes 7 was found to be suitable as well. Likewise, the aryl bromide and heteroaryl alkyne 7h and 7l, respectively, were efficiently

cally meaningful electrophilic functional groups were fully tolerated, including ester, chloro, bromo, iodo, and cyano 7916


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Journal of the American Chemical Society

Figure 4. Computed Gibbs free energies (ΔG298.15) in kcal mol−1 for the regioselectivity of the migratory insertion of alkyne 2k. [Co] = Co(OPiv). In the computed transition-state structures nonparticipating H atoms were omitted for clarity.105

confirmed the formation of molecular hydrogen as the sole byproduct (Scheme 5d and the Supporting Information). Thereafter, we interrogated viable key intermediates of the electrochemical C−H activation for the first time by means of mass spectrometry (Scheme 6). Thus, careful analysis by electrospray ionization (ESI) analysis provided strong support for the formation of the putative seven-membered cobalta(III) cycle88,115,116 9 as the key intermediate. As to the catalyst’s electrochemical mode of action, detailed cyclovoltammetric studies unraveled the formation of a cobalt(III) species by anodic oxidation (Figure 2). Furthermore, the alkyne 7a (blue) did not show significant oxidation up to a potential of 1.55 VSCE, while substrate 1a was oxidized within several waves with an onset potential of 0.80 VSCE, and the most notable peak at 1.61 VSCE (red). A mixture of Co(OAc)2 and NaOPiv in MeOH (pink) featured an oxidation potential of 1.31 VSCE for the oxidation of cobalt(II) to cobalt(III). However, a mixture of Co(OAc)2, NaOPiv, and 1a in MeOH (green) highlighted a new broad oxidation wave with an onset potential of 0.52 VSCE and a current maximum at 0.99 VSCE. These observations are indicative of an oxidation of cobalt(II) to cobalt(III) in the presence of the substrate at significantly lower potential. The addition of the alkyne 7a to this mixture (blue) did not lead to a significant quenching.105 Computational Mechanistic Studies. Thereafter, we explored the mechanism of the electrochemical cobalt-catalyzed C−H activation by computational studies at the PW6B95D3BJ/def2-TZVP-SMD(TFE)//PBE0-D3BJ/def2-SVP level of theory.117−125 Thus, the regiocontrol of the key migratory insertion was found to place the aryl group proximal to the heteroatom, thus reflecting the experimentally observed selectivities. Notably, the regioselectivity126 was largely governed by secondary attractive dispersion π−π interactions between the (hetero)aryl motifs of the pyridine and the arylacetylene moieties (Figure 3). Moreover, computation provided further support for the observed (Scheme 3) preferential π−π interaction between the

Figure 3. Computed Gibbs free energies (ΔG298.15) in kcal mol−1 for the regioselectivity of the migratory alkyne insertion within the electrochemcial cobalt-catalyzed C−H/N−H activation. [Co] = Co(OPiv). In the computed transition-state structures nonparticipating H atoms were omitted for clarity.105

converted, leaving the oxidation-sensitive thiophene moiety untouched. Generally, the hydrogen was placed distal to nitrogen with excellent levels of regiocontrol. The sterically congested tert-butyl alkyne 7q was also a competent substrate, while the alkyl nitrile 7r was fully tolerated without any signs for a nucleophilic attack. The electro-oxidative C−H/N−H activation further enabled the late-stage diversification to deliver erlotinib derivative 8at in a step-economical manner. Mechanistic Studies by Experiment. Intrigued by the unique performance of the electro-oxidative C−H activation with internal alkynes 2, we became attracted to probing its modus operandi. To this end, intermolecular competition experiments revealed electron-rich arenes 1 to be inherently superior (Scheme 5a). This observation is not in good agreement with competition experiments for a concerted metalation-deprotonation manifold107−110 but can better be rationalized in terms of a base-assisted internal electrophilictype substitution (BIES).111−114 Reactions conducted with isotopically labeled D2O as the cosolvent did not lead to any H/D scrambling (Scheme 5b). In good agreement with this finding, kinetic studies provided support for a facile C−H cleavage with a minor kinetic isotope effect (KIE) of only kH/ kD ≈ 1.1 (Scheme 5c). Gas-chromatographic headspace analysis 7917


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Figure 5. Computed Gibbs free energies (ΔG298.15) in kcal mol−1 for the reaction profile for the cobalt-catalyzed C−H/N−H [4 + 2] alkyne annulation. All values include dispersion corrections. In the computed transition-state structures nonparticipating H atoms were omitted for clarity.105

Table 2. Electroreductive Hydrazide Removala

Scheme 7. Plausible Catalytic Cycle

entry

anode

cathode

1 2 3 4 5 6

Al Al Al Al Al Mg

Ni Ni Ni Pt Pt Pt

electrolyte n-Bu4NPF6, n-Bu4NPF6, n-Bu4NPF6, n-Bu4NPF6, n-Bu4NI n-Bu4NPF6,

NaI NaI NaI n-Bu4NI NaI

yield (%) 37b 44c 50 48 37 90

a

Reaction conditions: Undivided cell, 3cc (0.15 mmol), overall electrolyte (0.20 M), DMF (3.0 mL), 5.0 mA, 5 h, anode (1.0 × 1.5 cm), cathode (1.0 × 1.5 cm), 23 °C, 5.0 h, under N2. bTHF as the solvent. cCH3CN as the solvent.

comparison of calculations with and without Grimme’s D3 correction. Proposed Catalytic Cycle for Electro-Oxidative C−H/ N−H Activation. On the basis of our detailed experimental and computational mechanistic studies, we propose a plausible catalytic cycle to be initiated by the electro-oxidative formation of the catalytically competent cobalt(III) carboxylate species 10 by means of anodic oxidation (Scheme 7). Thereafter, facile carboxylate-assisted C−H activation provides intermediate 11, while subsequent migratory insertion gives rise to the sevenmembered cobalta(III) cycle 9. Then, reductive elimination delivers the desired product 3 and forms the cobalt(I) species 12. Finally, the catalytically active cobalt(III) carboxylate complex 10 is regenerated by anodic oxidation, overall avoiding

electron-deficient pyridyl group and the more electron-rich aryl motif in C−H/N−H activation with unsymmetrical alkyne 2k (Figure 4). Thereby, the migratory insertion delivers the sevenmembered cobalta(III) cycle 9 (Figure 5), which we observed by electrospray mass spectrometry (vide supra). Thereafter, facile reductive elimination occurs on the triplet surface without any indication for spin-crossover behavior. Hence, the cobalt(I) species is formed which sets the stage for the anodic oxidation to regenerate the catalytically competent species. Weak dispersion127 interactions were found to stabilize the crucial transition states,105 as was primarily deduced from the 7918


Article

Journal of the American Chemical Society Scheme 8. Electroreductive Amine Removala

catalyst enabled C−H activations on benzhydrazides with high levels of chemo-, position- and regio-control. The electrooxidative cobalt catalysis was conducted in an operationally simple undivided cell setup under exceedingly mild conditions at room temperature.129 Detailed mechanistic studies by competition experiments, isotopic labeling, headspace analysis, mass spectrometry, cyclovoltametry, and DFT calculations provided strong support for a facile C−H activation by a catalytically competent cobalt(III) species. The use of electrical energy overall prevents stoichiometric amounts of toxic and costly transition metals as oxidants in C−H activations, being enabled by an earth-abundant, water-stable cobalt catalyst under ambient conditions. It also noteworthy that we have provided the proof of concept for electroreductive directing group removal. Hence, the electrochemical samarium-catalyzed amino removal was chemoselectively accomplished in an undivided cell at room temperature.

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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/jacs.8b03521. Experimental details, detailed mechanistic and cyclovoltammogrammic experiments, and compound characterization data (1H/13C NMR, IR, mass) (PDF)

a

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Ratio of regioisomers in parentheses.

AUTHOR INFORMATION

Corresponding Author

*[email protected] the use of toxic and expensive metals as sacrificial stoichiometric oxidants, instead generating H2 as the sole byproduct. Electroreductive Catalytic Removal of Hydrazides. Finally, we envisioned the, to the best of our knowledge, first electrocatalytic removal3,128 of a directing group in C−H activation chemistry. Specifically, we tested the cleavage of the hydrazide motif in isoquinolone 3cc by means of samariumcatalyzed cathodic electroreduction, again with a user-friendly undivided cell setup. First, we explored various electrolytes and solvents for the electroreductive hydrazide removal on isoquinolone 3cc at ambient temperature (Table 2), with DMF emerging as the solvent of choice (entries 1−3). Among various electrolytes, an equimolar combination of n-Bu4NPF6 and NaI proved optimal, particularly when employing a sacrificial magnesium anode (entries 4−6). Thereby, the effective use of catalytic quantities SmI2 proved viable, delivering the desired product 13cc in 90% yield in a sustainable manner at a room temperature of 23 °C (entry 6). With the optimized conditions for the electrocatalytic N−N cleavage in hand, we explored its scope for a variety of isoquinolones 3/8 (Scheme 8). Hence, the electroreductive hydrazide removal was found to be generally applicable, highlighting the power of electrosynthesis by dialing in an appropriate potential. Thereby, N-aminoisoquinolones 3 and 8 derived from both internal and terminal alkyl and aryl alkynes were efficiently converted into the desired products 13 and 14, respectively, by means of samarium electrocatalysis.

ORCID

Lutz Ackermann: 0000-0001-7034-8772 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Generous support by the CaSuS Ph.D. Program (scholarship to N.S.), the CSC (scholarship to R.M.), and the DFG (GottfriedWilhelm-Leibniz Award) is most gratefully acknowledged. We thank Regione Lombardia and Fondazione Cariplo − Sottomisura A. We also thank Lucas A. Paul and Prof. Dr. Inke Siewert (Göttingen University) for assistance with the headspace GC analysis.

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CONCLUSIONS In summary, we have reported on the first electrochemical cobalt-catalyzed C−H/N−H activation for [4 + 2] annulations that is applicable to internal alkynes. Thus, a versatile cobalt 7919


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