It has been reported that β -MnO 2 has photocatalytic activity for the oxidative-coupling of 2-naphthols into 1,1′-bi-2-naphthols. Nevertheless, it is hard to exclude the possibility that the oxidative-coupling of 2-naphthols is initiated by β -MnO 2 catalysis in dark due to the insufficient investigations in the related reports. In the present work, the oxidative-coupling of 2-naphthols into 1,1′-bi-2-naphthols with different phases MnO 2 catalysis in dark and under visible-light irradiation were systematically investigated. The results revealed that the oxidative-coupling of 2-naphthols is jointly initiated by MnO 2 catalysis and O 2 -oxidation, not by MnO 2 photocatalysis. Among the α -MnO 2 , β -MnO 2 , γ -MnO 2 and δ -MnO 2 catalysis, β -MnO 2 catalysis has the optimal performance, its selectivity for the oxidative-coupling of 2-naphthols into 1,1′-bi-2-naphthols is close to 100%, and its catalytic capacity could be well retained after multiple using. Our findings provide comprehensive and accurate understanding the catalytic role of MnO 2 for the oxidative-coupling of 2-naphthols into 1,1′-bi-2-naphthols. Graphic In the present work, the oxidative-coupling of 2-naphthols into 1,1′-bi-2-naphthols was proved to be jointly initiated by MnO 2 catalysis and O 2 -oxidation, not by MnO 2 photocatalysis. β -MnO 2 has the optimal catalytic activity for the oxidative-coupling of 2-naphthols relative to α -MnO 2 , γ -MnO 2 and δ -MnO 2 .
Pd-catalyzed oxidative coupling reaction was of great importance in the aromatic C H activation and the formation of new C O and C C bonds. Sanford has pioneered practical, directed C H activation reactions employing Pd(OAc) 2 as catalyst since 2004. However, until now, the speculated reactive Pd(IV) transient intermediates in these reactions have not been isolated or directly detected from reaction solution. Electrospray ionization tandem mass spectrometry (ESI-MS/MS) was used to intercept and characterize the reactive Pd(IV) transient intermediates in the solutions of Pd(OAc) 2 -catalyzed oxidative coupling reactions. In this study, the Pd(IV) transient intermediates were detected from the solution of Pd(OAc) 2 -catalyzed oxidative coupling reactions by ESI-MS and the MS/MS of the intercepted Pd(IV) transient intermediate in reaction system was the same with the synthesized authentic Pd(IV) complex. Our ESI-MS(/MS) studies confirmed the presence of Pd(IV) reaction transient intermediates. Most interestingly, the MS/MS of Pd(IV) transient intermediates showed the reductive elimination reactivity to Pd(II) complexes with new C O bond formation into product in gas phase, which was consistent with the proposed reactivity of the Pd(IV) transient intermediates in solution.
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