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. 2014 Feb 12;136(6):2555-63.
doi: 10.1021/ja411912p. Epub 2014 Jan 30.

Copper-catalyzed intermolecular amidation and imidation of unactivated alkanes

Affiliations

"V体育官网" Copper-catalyzed intermolecular amidation and imidation of unactivated alkanes

Ba L Tran et al. J Am Chem Soc. .

Abstract

We report a set of rare copper-catalyzed reactions of alkanes with simple amides, sulfonamides, and imides (i. e. , benzamides, tosylamides, carbamates, and phthalimide) to form the corresponding N-alkyl products. The reactions lead to functionalization at secondary C-H bonds over tertiary C-H bonds and even occur at primary C-H bonds. [(phen)Cu(phth)] (1-phth) and [(phen)Cu(phth)2] (1-phth2), which are potential intermediates in the reaction, have been isolated and fully characterized. The stoichiometric reactions of 1-phth and 1-phth2 with alkanes, alkyl radicals, and radical probes were investigated to elucidate the mechanism of the amidation. The catalytic and stoichiometric reactions require both copper and tBuOOtBu for the generation of N-alkyl product. Neither 1-phth nor 1-phth2 reacted with excess cyclohexane at 100 °C without tBuOOtBu. However, the reactions of 1-phth and 1-phth2 with tBuOOtBu afforded N-cyclohexylphthalimide (Cy-phth), N-methylphthalimide, and tert-butoxycyclohexane (Cy-OtBu) in approximate ratios of 70:20:30, respectively. Reactions with radical traps support the intermediacy of a tert-butoxy radical, which forms an alkyl radical intermediate. The intermediacy of an alkyl radical was evidenced by the catalytic reaction of cyclohexane with benzamide in the presence of CBr4, which formed exclusively bromocyclohexane. Furthermore, stoichiometric reactions of [(phen)Cu(phth)2] with tBuOOtBu and (Ph(Me)2CO)2 at 100 °C without cyclohexane afforded N-methylphthalimide (Me-phth) from β-Me scission of the alkoxy radicals to form a methyl radical. Separate reactions of cyclohexane and d12-cyclohexane with benzamide showed that the turnover-limiting step in the catalytic reaction is the C-H cleavage of cyclohexane by a tert-butoxy radical. These mechanistic data imply that the tert-butoxy radical reacts with the C-H bonds of alkanes, and the subsequent alkyl radical combines with 1-phth2 to form the corresponding N-alkyl imide product. VSports手机版.

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Figures

Scheme 1
Scheme 1. Conversion of Alkanes to N-Alkyl Products
Scheme 2
Scheme 2. Syntheses of Copper(I) and Copper(II) Amidate and Imidate Complexes
Figure 1
Figure 1
Molecular structures of [(phen)Cu(phth)2] (1-phth2), [(phen)2Cu] [Cu(NHSO2Ph)2] (4), [(L1)Cu(phth)] (6), and [(L1)Cu(NHSO2Ph)] (7) shown with 50% thermal ellipsoid. Selected bond lengths (Å) and angles (°) of 1-phth2: Cu1–N1 = 2.0480(14); Cu1–N2 = 2.0363(14); Cu1–N3 = 1.9667(14); N1–Cu1–N3 = 170.29(5); N1–Cu1–N2 = 93.45(5); N1–Cu1–O1 = 92.5(3). Selected bond lengths (Å) and angles (o) of 4: Cu1–N1 = 1.9920(15); Cu1–N2 = 2.1079(16); Cu1–N3 = 1.8553(17); N1–Cu1–N2 = 81.75(6); N3–Cu1–N3′ = 178.92(10). Selected bond lengths (Å) and angles (o) of 6: Cu1–N1 = 1.938(7); Cu1–N2 = 2.045(7); Cu1–N3 = 1.958(7); Cu1–O1 = 1.919(6); N1–Cu1–N3 = 175.2(3); N1–Cu1–N2 = 83.4(3); N1–Cu1–O1 = 92.5(3). Selected bond lengths (Å) and angles (o) of 7: Cu1–N1 = 1.956(6); Cu1–N2 = 2.104(6); Cu1–N3 = 1.972(6); Cu1–O1 = 1.993(5); N1–Cu1–N3 = 170.0(2); N1–Cu1–N2 = 83.7(2); N1–Cu1–O1 = 91.2(2).
Scheme 3
Scheme 3. Catalytic C–H Amidation of Cyclohexane with 1-phth2 and 1-phth
Scheme 4
Scheme 4. Isolation of 1-phth2 from 1-phth
Scheme 5
Scheme 5. Stoichiometric Reactions of 1-phth2 and 1-phth with Cyclohexane
Scheme 6
Scheme 6. Trapping of tert-Butoxy Radical
Scheme 7
Scheme 7. Trapping of the Putative Alkyl Radical
Scheme 8
Scheme 8. Trapping of the Putative Alkyl Radical with 1-phth2
Scheme 9
Scheme 9. Proposed Mechanism of Copper-Catalyzed Intermolecular Amidation of Cyclohexane

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