Manganese-Catalyzed ortho-C-H Amidation of Weakly Coordinating

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Manganese-Catalyzed ortho-C‑H Amidation of Weakly Coordinating Aromatic Ketones Xianqiang Kong and Bo Xu* College of Chemistry, Chemical Engineering and Biotechnology, Donghua University, 2999 North Renmin Lu, Shanghai 201620, China

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S Supporting Information *

ABSTRACT: An efficient manganese-catalyzed ortho-C-H amidation of weakly coordinating aromatic ketones using the readily available sulfonyl azide as the amination reagent is developed. The key step is the ketone directed aromatic metalation using the in situ generated reactive Mn intermediate, MnMe(CO)5. This method offers excellent chemical yields, high regioselectivities, and good functional group tolerance.

A

Scheme 1. Selected Examples of Mn-Catalyzed C−H Amidations

romatic ketones are among the most important targets in materials, pharmaceuticals, and other fine chemicals. Aromatic ketones are also commonly used as building blocks in organic synthesis. Transitional metals, especially late transition metals such as Rh,1 Pd,2 Ir,3 and Ru4 catalyzed C−H functionalizations, have evolved as atom- and step-economic synthetic tools. Along this line, the ketone directed C−H functionalization of aromatic ketones offers a straightforward and efficient protocol for synthesis of highly functionalized ketones. Indeed, since Murai and co-workers’ pioneering works on the Ru-catalyzed aromatic C−H alkylation,5 the ketone directed C−H activations have attracted much attention.6 However, most of these reactions have relied heavily on precious late transition metals such as Ru, Rh, Pd, and Ir.6 More specifically, due to the importance of aromatic amines in materials and pharmaceuticals,7 direct ketone-directed C−H amidations have attracted much attention (Scheme 1).8 For example, Liu and co-workers reported a palladium-catalyzed ketone directed C−H amidation of aromatic ketones (Scheme 1, eq 1).9 Using organo azides as amination reagents,10 Sahoo and co-workers reported a Ru(II)-catalyzed amidation of aromatic ketones (Scheme 1, eq 2).11 Chang and co-workers also reported iridium-catalyzed (Scheme 1, eq 3)3c C−H amidations and ruthenium-catalyzed (Scheme 1, eq 4)12 C−H amidation. Very recently, Rasheed and co-workers reported a Ru-catalyzed C−H amination of aromatic aldehyde (Scheme 1, eq 5).13 Because of the weak coordinating power of ketone groups and the lower catalytic reactivity of base metals (such as iron,14 cobalt,15 copper,2e,16 and manganese17) compared with the late transition metals, the use of base metal catalysts in the ketonedirected C−H activations is challenging. To the best of our knowledge, base metal catalyzed C−H amidation of weakly coordinating aromatic ketones has not been achieved. Recently, Wang and co-workers reported a manganese-catalyzed aromatic C−H addition of aromatic ketones to imines using the in situ generated reactive Mn specie, MnMe(CO)5.18 We envisioned that a similar strategy could be used in base metal catalyzed ketone-directed C−H amidation. Herein, we are glad to report a © XXXX American Chemical Society

highly efficient manganese-catalyzed ortho-C-H amidation of weakly coordinating aromatic ketones by using the readily available sulfonyl azides as amidation reagents. We used the amidation of acetophenone 1a as our model reaction (Table 1). Using MnBr(CO)5 as catalyst, the standard condition used by Wang and co-workers (MnBr(CO)5/ZnBr2/ Me2Zn)18 did not give any product (Table 1, entry 1). We thought the Lewis acid used, ZnBr2, may not be strong enough to activate the relatively weak electrophile−sulfonyl azide 2a. Thus, we screened many Lewis acidic activators, especially Lewis acids, which were known to be good activators for organoazides (e.g., silver and copper Lewis acids)19 (Table 1, entries 2−7). Received: June 9, 2018

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DOI: 10.1021/acs.orglett.8b01770 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters Table 1. Optimization of Conditionsa

Scheme 2. Scope for Manganese-Catalyzed Amidation of Aromatic Ketonesa

a Conditions: 1a (0.2 mmol), 2a (0.3 mmol), MnBr(CO)5 (5 mol %), Lewis acid, additive, solvent (1 mL), 6 h.

To our delight, the Cu(OAc)2 system gave a small amount of product 3a (Table 1, entry 7). We investigated the solvent effect (Table 1, entries 8−13). The solvent had a big influence on the reaction: most solvents only gave a trace amount of product 3a and only dioxane offered moderate conversion (Table 1, entry 9). We then screened combinations of Cu(OAc)2 equivalents and Me2Zn equivalents (Table 1, entries 14−18). To our delight, when 1.5 equiv of Cu(OAc)2 and 2.0 equiv of Me2Zn were used, 73% yield of product 3a was obtained (Table 1, entry 17). In the last, we found that the chemical yield could be further improved by increasing the reaction temperature from 60 to 80 °C (Table 1, entry 19). However, further increasing reaction temperature to 90 °C had a deleterious effect (Table 1, entry 20). With the optimized conditions in hand, we explored the substrate scope and functional group tolerance for the manganese-catalyzed amidation of aromatic ketones. First, we evaluated the scope of aromatic ketones by reactions with phenylsulfonyl azide 2a (Scheme 2). The substitution pattern (meta, para) and electronic properties of substitution (electron deficient or rich) on the aromatic ring of ketone 2 played a small role; good yields were obtained regardless (3a−3s) (Scheme 2). Various functional groups such as halides (3b, 3c, 3g, 3k) (Scheme 2), nitro group (3f) (Scheme 2), ethers (3h, 3l) (Scheme 2), ester (3i) (Scheme 2), ketone (3r) (Scheme 2), alkyl groups (3d, 3j) (Scheme 2), and trifluoromethyl group (3e) (Scheme 2) were well tolerated. This reaction also worked equally well for a ketone with a long aliphatic chain (3m) (Scheme 2) and cyclic ketones (3n−3q) (Scheme 2). Our method also worked well for amidation of fused aromatic ketone such as 1-naphthalenyl ketone (3s) (Scheme 2). When diaryl ketone was used, higher a temperature (120 °C) and longer reaction time were needed (3t, 3u) (Scheme 2). For reaction of unsymmetrical diarylketone, a mixture of inseparable regioisomeric mixture was obtained (3u, 3u′) (Scheme 2). We also tested reactions of aromatic compounds such as benzaldehydes, benzamides, methyl benzoate, and azobenzol using standard conditions, and no amidation product was obtained.

a

Conditions: 1 (0.2 mmol), sulfonyl azide 2 (0.3 mmol), MnBr(CO)5 (5 mol %), Cu(AcO)2 (0.3 mmol), dioxane (1.0 mL), and Me2Zn (1.2 M in toluene, 0.4 mmol), 80 °C, 6 h. (b) 120 °C, 36 h.

Then we explored the amidation of acetophenone 1a with various aromatic and aliphatic sulfonyl azides 2 (Scheme 3). Good yields of amidation products were obtained in general. Various functional groups such as amine (3v) (Scheme 3), trifluoromethyl (3ad, 3af) (Scheme 3), ether (3z) (Scheme 3), halides (3w, 3ac, 3ae, 3ag, 3ah) (Scheme 3), and ester (3y) (Scheme 3) were well tolerated. This reaction also worked equally well for aliphatic sulfonyl azides 2 (3ai, 3aj, 3ak) (Scheme 3). It should be noted that no reaction took place when a highly bulky sulfonyl azide was used (3al) (Scheme 3). To elucidate the effect of electronic factor on the rate of manganese-catalyzed ortho-C-H amidation, we conducted a competition experiment (Scheme 4, eq 1). An electron-donating group (−OMe) enhanced the amination compared to electronwithdrawing group (−COOMe) but in a small margin (2:1). Also, our methodology can be used in larger scale synthesis without complications (Scheme 4, eq 2). The proposed mechanism is illustrated in Scheme 5 based on precedent reports.17,18,20 The catalytic cycle starts with the generation of MnMe(CO)5 from transmetalation of MnBr(CO)5 with Me2Zn by releasing methane gas.21 Then complexation of B

DOI: 10.1021/acs.orglett.8b01770 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters Scheme 3. Scope for Sulfonyl Azides 2a

MnMe(CO)5 with acetophenone 1a gives intermediate A. Then aromatic metalation of A gives manganacycle intermediate B.21 A reversible coordination of tosylazide to the cationic Mn center and then an amido insertion leads to Mn-intermediate C by releasing a nitrogen molecule. Because Cu(OAc)2 is a strong oxidant, it could be reduced by Et2Zn to give Cu(I) species22 (represented as [Cu]), which may be able to activate azides 2. In the last, the transmetalation of C with Me2Zn affords intermediate D, which undergoes a ligand exchange with 1a to reproduce intermediate A and also generates the copper containing intermediate E. At last, the workup of E gives the final product 3a. In summary, we have developed an efficient amidation of various aromatic ketones via C−H bond activation using readily available sulfonyl azides catalyzed by a base metal catalyst, MnBr(CO)5. At present, MnBr(CO)5 is still relatively expensive, use of cheaper manganese sources in C−H activations is currently being investigated in our laboratories.

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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.8b01770. Experimental details and copies of NMR spectra (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID a

Conditions: 1 (0.2 mmol), sulfonyl azide 2 (0.3 mmol), MnBr(CO)5 (5 mol %), Cu(AcO)2 (0.3 mmol), dioxane (1.0 mL), and Me2Zn (1.2 M in toluene, 0.4 mmol), 80 °C, 6 h.

Bo Xu: 0000-0001-8702-1872 Notes

The authors declare no competing financial interest.

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Scheme 4. Mechanistic Study and Larger Scale Synthesis

ACKNOWLEDGMENTS We are grateful to the National Science Foundation of China for financial support (NSFC-21472018 and NSFC-21672035). REFERENCES

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Scheme 5. Proposed Mechanism for C−H Amidation of 1a

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DOI: 10.1021/acs.orglett.8b01770 Org. Lett. XXXX, XXX, XXX−XXX