Electro-Removable Traceless Hydrazides for Cobalt-Catalyzed Electro

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Electro-Removable Traceless Hydrazides for Cobalt-Catalyzed Electro-Oxidative C–H/N–H Activation with Internal Alkynes Ruhuai Mei, Nicolas Sauermann, João C. A. Oliveira, and Lutz Ackermann J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b03521 • Publication Date (Web): 29 May 2018 Downloaded from http://pubs.acs.org on May 29, 2018

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Journal of the American Chemical Society

Electro-Removable Traceless Hydrazides for Cobalt-Catalyzed Electro-Oxidative C–H/N–H Activation with Internal Alkynes Ruhuai Mei,[a] Nicolas Sauermann,[a] João C. A. Oliveira,[a] and Lutz Ackermann*[a,b,c] [a] Institut für Organische und Biomolekulare Chemie, Georg-August-Universität Göttingen, Tammannstraße 2, 37077, Göttingen (Germany). [b] Department of Chemistry, University of Pavia, Viale Taramelli, 10, 27100 Pavia, Italy [c] International Center for Advanced Studies of Energy Conversion (ICASEC), Georg-August-Universität Göttingen

Supporting Information Placeholder

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 electro-oxidative C–H activation manifold proved viable with an undivided cell set-up 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 regio-selective fashion. Key to success for the unprecedented use of internal alkynes in electrochemical C–H/N–H activations was represented by the use of N-2pyridylhydrazides, for which we developed a traceless electro-cleavage strategy by electro-reductive samarium catalysis at room temperature.

70-73

Introduction C–H activation has been recognized as a powerful tool in 1-16 molecular synthesis, with enabling applications to material 17 18 sciences, natural product synthesis, and pharmaceutical 19-20 industries, among others. Particularly, oxidative C–H transformations have proven instrumental for the develop21-24 ment of step-economical organic syntheses. Despite of 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 atom25 economical nature of the C–H activation approach. In the meantime, electrosynthesis has emerged as an increas26-37 ingly viable platform for molecular synthesis. Specifically, electrocatalysis holds great potential for avoiding the use of cost-intensive chemical reagents in redox-processes, thereby 28-29, 35, 38-42 reducing the footprint of undesirable byproducts. Thus, in recent years, electrosynthesis was illustrated to significantly improve the sustainability of molecular trans43-44 formations. A considerable recent momentum was gained by merging organometallic electrosynthesis and cata28-29, 39-42 lyzed C–H activation. In this context, major advances were accomplished by innate reactivity guidance under metal-free conditions, as reported by Barran, Waldvogel, and Xu, 45-59 among others. Alternatively, the power of precious palladium catalysts has been unraveled at elevated temperatures (70-90 °C), as elegantly developed by Kakiuchi, and Mei, 60-69 among others. In contrast, we have introduced in 2017

earth-abundant 3d transition metals for electro-oxidative 74 75-81 C–H activation catalysis. Thus, cost-effective base metal 82-94 cobalt(II) catalysts enabled sustainable C–H oxygenations 74 in the absence of any chemical oxidants. Based on key mechanistic insights into the working mode of these C–H alkoxylations, we unraveled C–H/N–H activations for the 95 step-economical [4+2] annulation of alkynes. In spite of 95-96 major progress in the field, the cobalt-catalyzed 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).

Scheme 1. [4+2] Annulation with Terminal Alkynes.

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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 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 97-98 continued interest in sustainable C–H activation, we have now devised conditions for the first base metal-catalyzed electro-chemical C–H activation with internal alkynes, on 99 which we report herein. Thus, pyridyl-benzhydrazides 1 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 100-102 pharmaceutical industries. Herein, we report the development of cobalt-catalyzed electro-oxidative C–H/N–H activation with internal alkynes, employing electricity as the sole oxidant, which avoid the use of stoichiometric metal salts as sacrificial oxidants (Figure 1). Salient features of our strategy include (i) electrochemical C–H activation with internal 76-77, 103-104 alkynes, (ii) earth-abundant cobalt catalysis, (iii) high levels of position- and chemo-selectivity, (iv) an userfriendly undivided cell set-up, and (v) exceedingly mild room temperature conditions. It is also noteworthy that we have established reaction conditions for the electro-reductive removal of the benzhydrazide directing groups by samarium catalysis, setting the stage for the first electro-cleavable 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.

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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 chemo-selectivity as well as a more effective cathodic reduction. Control experiments confirmed the essential role of the cobalt catalyst, the carboxylate additive (entries 13-15), and the electricity (entry 16).

Table 1. Electro-Oxidative C–H/N–H Activation with Internal Alkyne 2a.[a]

Figure 1. Electro-Oxidative Cobalt-Catalyzed C–H/N–H Activation Featuring Electro-Reductive Hydrazide Cleavage.

Entry

Solvent

Additive

T [°C]

Yield [%]

1

MeOH

NaOPiv

23

trace

2

EtOH

NaOPiv

23

trace

3

H2O

NaOPiv

23

---

4

TFE

NaOPiv

23

48

5

TFE

NaOPiv

60

34

6

TFE

NaOPiv

80

39

7

TFE

Na2CO3

23

trace

8

TFE

K3PO4

23

---

9

TFE

NaOAc

23

51

10

TFE

n-Bu4NPF6

23

63

11

TFE

PivOH

23

79

12

TFE

---

23

37

13

TFE

---

23

---[b]

14

TFE

PivOH

23

71[b]

15

TFE

PivOH

23

---[c]

16

TFE

PivOH

23

---[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. [b] CoCl2 (10 mol %). [c] Without cobalt. [d] Without electricity.

RESULTS AND DISCUSSION Optimization Studies We initiated our studies by exploring reaction conditions for the envisioned electro-oxidative [4+2] annulation of internal alkyne 2a by 2-pyridyl-benzhydrazide 1a (Table 1 and Table 105 S-1 in the Supporting Information). After considerable preliminary experimentation, we observed that the desired 2


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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 106 as the omnipresent 8-quinolinyl or the previously em74, 95 ployed 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.

Scheme 3. Electrochemical C–H/N–H [4+2] Annulation of Internal Alkynes 2.

Activation:

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

Versatility With the optimized cobalt-catalyzed electro-oxidative C–H activation in hand, we probed its robustness with a variety of internal alkynes 2 (Scheme 3a). Thus, disubstituted acetylenes 2b-2f bearing electron-donating or electronwithdrawing 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-2i proceeded with synthetically useful levels of regio-control (Scheme 3b), generally placing the alkyl-group distal, and the -containing motif proximal to nitrogen (vide infra). As to unsymmetrical diarylalkynes 2j-21, 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 105 and single crystal X-ray diffraction. 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-1m displaying para-, meta- and orthosubstituents (Scheme 3c). Thereby, synthetically meaningful electrophilic functional groups were fully tolerated, including ester, chloro, bromo, iodo and, cyano substituents, which should prove invaluable for the further late-stage diversification of the thus-obtained products 3jc-3mc.

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 converted, leaving the oxidation-sensitive thiophene moiety untouched. Generally, the hydrogen was placed distal to 3



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ducted with isotopically labeled D2O as the co-solvent 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 confirmed the formation of molecular hydrogen as the sole byproduct (Scheme 5d and the Supporting Information).

nitrogen with excellent levels of regio-control. 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. Scheme 4. Electrochemical C–H/N–H Activation: Terminal Alkynes 2.

Scheme 5. Summary of Key Mechanistic Findings.

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 co88, 115-116 balta(III)cycle 9 as the key intermediate. 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 con107-110 certed metalation-deprotonation manifold, but can better be rationalized in terms of a base-assisted internal 111-114 electrophilic-type substitution (BIES). Reactions con4


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Scheme 6. Mass Spectrometric Analysis.

Computational Mechanistic Studies Thereafter, we explored the mechanism of the electrochemical cobalt-catalyzed C–H activation by computational studies at the PW6B95-D3BJ/def2-TZVP-SMD(TFE)//PBE0117-125 D3BJ/def2-SVP level of theory. Thus, the regio-control of the key migratory insertion was found to place the aryl group proximal to the heteroatom, thus reflecting the experimen126 tally observed selectivities. Notably, the regio-selectivity was largely governed by secondary attractive dispersion π-π interactions between the (hetero)aryl motifs of the pyridine and the arylacetylene moieties (Figure 3).

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 0.8 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 105 this mixture (blue) did not lead to a significant quenching.

Figure 3. Computed Gibbs free energies (ΔG298.15) in kcal ˗1 mol for the regio-selectivity of the migratory alkyne insertion within the electro-chemcial cobalt-catalyzed C–H/N–H activation. [Co] = Co(OPiv). In the computed transition-state structures non-participating H atoms were omitted for clari105 ty. Moreover, computation provided further support for the observed (Scheme 3) preferential π-π interaction between the electron-deficient pyridyl group and the more electron-rich aryl motif in C–H/N–H activation with unsymmetrical alkyne 2k (Figure 4).

Figure 2. Cyclic voltammogram at 100 mV/s in MeOH. N105 Bu4NPF6 (0.1 M in MeOH), Alkyne = 7a, [Co] = Co(OAc)2.

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Proposed Catalytic Cycle for Electro-Oxidative C–H/N–H Activation Based on 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 the use of toxic and expensive metals as sacrificial stoichiometric oxidants, instead generating H2 as the sole byproduct.

Figure 4. Computed Gibbs free energies (ΔG298.15) in kcal ˗1 mol for the regio-selectivity of the migratory insertion of alkyne 2k. [Co] = Co(OPiv). In the computed transition-state structures non-participating H atoms were omitted for clari105 ty.

Scheme 7. Plausible Catalytic Cycle.

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 127 species. Weak dispersion interactions were found to stabi105 lize the crucial transition states, as was primarily deduced from the comparison of calculations with and without Grimme’s D3 correction.

Electro-Reductive Catalytic Removal of Hydrazides Finally, we envisioned the, to the best of our knowledge, first 3, 128 electro-catalytic removal of a directing group in C–H activation chemistry. Specifically, we tested the cleavage of the hydrazide motif in isoquinolone 3cc by means of samarium-catalyzed cathodic electro-reduction, again with a userfriendly undivided cell set-up. First, we explored various electrolytes and solvents for the electro-reductive 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 nBu4NPF6 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).

Figure 5. Computed Gibbs free energies (ΔG298.15) in kcal ˗1 mol 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 105 non-participating H atoms were omitted for clarity.

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Table 2. Electro-Reductive Hydrazide Removal[a]

entry

anode

cathode

electrolyte

Yield (%)

1

Al

Ni

n-Bu4NPF6, NaI

37[b]

2

Al

Ni

n-Bu4NPF6, NaI

44[c]

3

Al

Ni

n-Bu4NPF6, NaI

50

4

Al

Pt

n-Bu4NPF6, n-Bu4NI

48

5

Al

Pt

n-Bu4NI

37

6

Mg

Pt

n-Bu4NPF6, NaI

90

3. 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 catalyst enabled C–H activations on benzhydrazides with high levels of chemo-, position- and regio-control. The electro-oxidative cobalt catalysis was conducted in an operationally simple undivided cell set-up under exceedingly mild 129 conditions at room temperature. 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.

[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 [b] [c] N2. THF as the solvent. CH3CN as the solvent.

ASSOCIATED CONTENT With the optimized conditions for the electro-catalytic N–N cleavage in hand, we explored its scope for a variety of isoquinolones 3/8 (Scheme 8). Hence, the electro-reductive 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.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Experimental details, detailed mechanistic and cyclovoltammogrammic experiments and compound characterization 1 13 data ( H-/ C-NMR, IR, Mass) (PDF).

AUTHOR INFORMATION Corresponding Author

Scheme 8. Electro-Reductive Amine Removal. Ratio of Regio-isomers in parenthesis.

E-mail: [email protected]

Author Contributions +

These authors contributed equally.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT Generous support by the CaSuS PhD Program (Scholarship to N.S.) the CSC (Scholarship to R.M.) and the DFG (Gottfried-Wilhelm-Leibniz award) is most gratefully acknowledged. We also thank Mr. Lucas A. Paul and Prof. Dr. Inke Siewert (Göttingen University) for assistance with the headspace GC analysis.

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Electro-Removable Traceless Hydrazides for Cobalt-Catalyzed Electro

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