Scope and Mechanism of the Redox-Active 1,2-Benzoquinone Enabled Ruthenium-Catalyzed Deaminative α‑Alkylation of Ketones with Amines
The catalytic system formed in situ from the reaction of a cationic Ru–H complex with 3,4,5,6-tetrachloro-1,2-benzoquinone was found to mediate a regioselective deaminative coupling reaction of ketones with amines to form the α-alkylated ketone products. Both benzylic and aliphatic primary amines we...
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Veröffentlicht in: | ACS catalysis 2021-11, Vol.11 (22), p.13962-13972 |
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Sprache: | eng |
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Zusammenfassung: | The catalytic system formed in situ from the reaction of a cationic Ru–H complex with 3,4,5,6-tetrachloro-1,2-benzoquinone was found to mediate a regioselective deaminative coupling reaction of ketones with amines to form the α-alkylated ketone products. Both benzylic and aliphatic primary amines were found to be suitable substrates for the coupling reaction with ketones in forming the α-alkylated ketone products. The coupling reaction of PhCOCD3 with 4-methoxybenzylamine showed an extensive H/D exchange on both α-CH2 (41% D) and β-CH2 (21%) positions on the alkylation product. The Hammett plot obtained from the reaction of acetophenone with para-substituted benzylamines p-X-C6H4CH2NH2 (X = OMe, Me, H, F, Cl, CF3) showed a strong promotional effect by the amine substrates with electron-releasing groups (ρ = −0.49 ± 0.1). The most significant carbon isotope effect was observed on the α-carbon of the alkylation product (C α = 1.020) from the coupling reaction of acetophenone with 4-methoxybenzylamine. The kinetics of the alkylation reaction from an isolated imine substrate led to the empirical rate law: rate = k[Ru][imine]. A catalytically active Ru–catecholate complex was synthesized from the reaction of the cationic Ru–H complex with 3,5-di-tert-butyl-1,2-benzoquinone and PCy3. The DFT computational study was performed on the alkylation reaction, which revealed a stepwise mechanism of the [1,3]-carbon migration step via the formation of a Ru(IV)-alkyl species with a moderate energy of activation (ΔG ‡ = 32–42 kcal/mol). A plausible mechanism of the catalytic alkylation reaction via an intramolecular [1,3]-alkyl migration of an Ru-enamine intermediate has been compiled on the basis of these experimental and computational data. |
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ISSN: | 2155-5435 2155-5435 |
DOI: | 10.1021/acscatal.1c04732 |