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Thiocarbonyl-enabled ferrocene C–H nitrogenation by cobalt(III) catalysis: thermal and mechanochemical

  1. ‡ ,
  2. ‡ ,
  3. and
  4. § ORCID Logo
Institut für Organische und Biomolekulare Chemie, Georg-August-Universität Göttingen, Tammannstraße 2, 37077 Göttingen, Germany
  1. Corresponding author email
  2. ‡ Equal contributors
§ http://www.org.chemie.uni-goettingen.de/ackermann/
This article is part of the thematic issue "Cobalt catalysis".
Guest Editor: S. Matsunaga
Beilstein J. Org. Chem. 2018, 14, 1546–1553. https://doi.org/10.3762/bjoc.14.131
Received 14 May 2018, Accepted 15 Jun 2018, Published 25 Jun 2018
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Abstract

Versatile C–H amidations of synthetically useful ferrocenes were accomplished by weakly-coordinating thiocarbonyl-assisted cobalt catalysis. Thus, carboxylates enabled ferrocene C–H nitrogenations with dioxazolones, featuring ample substrate scope and robust functional group tolerance. Mechanistic studies provided strong support for a facile organometallic C–H activation manifold.

Keywords: amidation; C–H activation; cobalt; ferrocene; mechanochemistry

Introduction

C–H activation has surfaced as a transformative tool in molecular sciences [1-9]. While major advances have been accomplished with precious 4d transition metals, recent focus has shifted towards more sustainable base metals [10-17], with considerable progress by earth-abundant cobalt catalysts [18-22]. In this context, well-defined cyclopentadienyl-derived cobalt(III) complexes have proven instrumental for enabling a wealth of C–H transformations [23-41], prominently featuring transformative C–H nitrogenations [42,43] in an atom- and step-economical fashion [44-59]. Within our program on cobalt-catalyzed C–H activation [60-68], we have now devised C–H nitrogenations assisted by weakly-coordinating [69] thiocarbonyls [70,71], allowing the direct C–H activation on substituted ferrocenes [72-93] – key structural motifs of powerful transition metal catalyst ligands and organocatalysts (Figure 1) [94-97]. During the preparation of this article, the use of strongly-coordinating, difficult to remove directing groups has been reported [70,71]. In sharp contrast, notable features of our approach include (i) cobalt-catalyzed C–H amidations of thiocarbonylferrocenes by weak coordination, (ii) thermal and mechanochemical [98-100] cobalt-catalyzed ferrocene C–H nitrogenations, (iii) versatile access to synthetically useful aminoketones, and (iv) key mechanistic insights on facile C–H cobaltation.

[1860-5397-14-131-1]

Figure 1: Selected ferrocene-based ligands and organocatalysts.

Results and Discussion

We initiated our studies by probing various reaction conditions for the envisioned C–H amidation of ferrocene 1a (Table 1). Among a variety of ligands, N-heterocyclic carbenes and phosphines provided unsatisfactory results (Table 1, entries 1–3), while the product 3aa was formed when using amino acid derivatives, albeit as of yet in a racemic fashion (Table 1, entries 4–7). Yet, optimal catalytic performance was realized with 1-AdCO2H (Table 1, entries 8 and 9) [101-104], particularly when using DCE as the solvent (Table 1, entries 9–12). A control experiment verified the essential nature of the cobalt catalyst (Table 1, entry 13). In contrast to the thiocarbonyl-assisted C–H amidation, the corresponding ketone failed thus far to deliver the desired product, under otherwise identical reaction conditions.

Table 1: Thiocarbonyl-assisted C−H nitrogenation of ferrocene 1a.a

[Graphic 1]
Entry Solvent Ligand Yield (%)
1 DCE – –
2 DCE IMes·HCl –
3 DCE PPh3 –
4 DCE Boc-Leu-OH 40
5 DCE Boc-Val-OH 55
6 DCE Boc-Pro-OH 30
7 DCE Boc-Ala-OH 62
8 DCE MesCO2H 80
9 DCE 1-AdCO2H 84
10 1,4-dioxane 1-AdCO2H 75
11 toluene 1-AdCO2H 79
12 GVL 1-AdCO2H 35
13 DCE 1-AdCO2H –b

aReaction conditions: 1a (0.13 mmol), 2a (0.15 mmol), ligand (30 mol %), [Co] (5.0 mol %), solvent (1.0 mL). bReaction performed in the absence of [Cp*Co(CH3CN)3][SbF6]2. Yields of isolated product.

With the optimized reaction conditions in hand, we explored the robustness of the cobalt-catalyzed ferrocene C–H amidation with a variety of 1,4,2-dioxazol-5-ones 2 (Scheme 1). Hence, the chemoselectivity of the cobalt catalyst was reflected by fully tolerating sensitive electrophilic functional groups, including amido, chloro, bromo and nitro substituents in the para-, meta- and even the more congested ortho-position.

[1860-5397-14-131-i1]

Scheme 1: Scope of substituted dioxazolones 2.

The versatile cobalt-catalyzed C–H amidation was not limited to mono-substituted ferrocenes 1 (Scheme 2). Indeed, the arylated ferrocenes 1b–d were identified as viable substrates likewise.

[1860-5397-14-131-i2]

Scheme 2: C–H Amidation of arylated ferrocenes 1.

Moreover, differently substituted thiocarbonyls 1 were found to be amenable within the cobalt-catalyzed C–H amidation manifold by weak-coordination (Scheme 3).

[1860-5397-14-131-i3]

Scheme 3: Thiocarbonyl-assisted C–H amidation.

Given the versatility of the cobalt-catalyzed C–H nitrogenation, we became intrigued to delineating its mode of action. To this end, C–H amidations in the presence of isotopically labelled co-solvents led to a significant H/D scrambling in proximity to the thiocarbonyl group. These findings are indicative of a reversible, thus facile organometallic C–H cobaltation regime (Scheme 4).

[1860-5397-14-131-i4]

Scheme 4: H/D Exchange reactions.

Next, intermolecular competition experiments revealed that electron-rich arylated thiocarbonylferrocene 1 reacted preferentially, which can be rationalized with a base-assisted internal electrophilic substitution (BIES) [24,105] C–H cobaltation mechanism. In addition, the electron-rich amidating reagent 2c was found to be inherently more reactive (Scheme 5).

[1860-5397-14-131-i5]

Scheme 5: Intermolecular competition experiments.

As to further late-stage manipulation of the thus-obtained products, the amidated thiocarbonylferrocene 3aa could be easily transformed into the corresponding synthetically useful aminoketone 4aa (Scheme 6), illustrating the unique synthetic utility of our strategy.

[1860-5397-14-131-i6]

Scheme 6: Synthesis of aminoketone 4aa.

Mechanochemical molecular synthesis has attracted recent renewed attention as an attractive alternative for facilitating sustainable organic syntheses [106]. Thus, we were delighted to observe that the mechanochemical C–H nitrogenations proved likewise viable by thiocarbonyl assistance in an effective manner (Scheme 7).

[1860-5397-14-131-i7]

Scheme 7: Mechanochemical ferrocene C–H nitrogenation.

Conclusion

In conclusion, we have reported on the unprecedented cobalt-catalyzed C–H nitrogenation of ferrocenes by weakly-coordinating thiocarbonyls. The carboxylate-assisted cobalt catalysis was characterized by high functional group tolerance and ample substrate scope. Mechanistic studies provided evidence for a facile C–H activation. The C–H amidation was achieved in a thermal fashion as well as by means of mechanochemistry, providing access to synthetically meaningful aminoketones.

Supporting Information

Supporting Information File 1: Experimental procedures, characterization data, and NMR spectra for new compounds.
Format: PDF Size: 2.8 MB Download

Acknowledgements

Generous support by the DFG (Gottfried Wilhelm Leibniz prize), and the CSC (fellowships to Z.S. and H.W.) is gratefully acknowledged.

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