A Ru(I) Metalloradical That Catalyzes Nitrene Coupling to Azoarenes from Arylazides

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• 作者：Ayumi Takaoka ; Marc-Etienne Moret ; Jonas C. Peters
• 刊名：The Journal of the American Chemical Society
• 出版年：2012
• 出版时间：April 18, 2012
• 年：2012
• 卷：134
• 期：15
• 页码：6695-6706
• 全文大小：648K
• 年卷期：v.134,no.15(April 18, 2012)
• ISSN：1520-5126

文摘

Unusual N鈥揘 coupling of aryl azides to yield azoarenes is demonstrated by the Ru(I) metalloradical, [SiP^iPr₃]Ru(N₂) (4) ([SiP^iPr₃] = (2-iPr₂PC₆H₄)₃Si^{鈥?/sup>). The yield of the azoarene is dependent on the substituent on the aryl azide, and the reaction is catalytic for p-methoxy and p-ethoxy phenyl azides, while no azoarene is observed for p-trifluoromethylphenyl azide. Studies aimed at probing the viability of a bimolecular coupling mechanism of metal imide species, as shown in the related [SiPiPr₃]Fe system, have led to the isolation of several structurally unusual complexes including the ruthenium(IV) imide, 7-OMe, as well as the Ru(II) azide adduct 8-OMe. One electron reduction of 7-OMe complex led to the isolation of the formally Ru(III) imide complex, [SiPiPr₃]Ru(NAr) (Ar = p-MeOC₆H₄, 5-OMe). EPR spectroscopy on 5-OMe suggests that the complex is electronically similar to the previously reported imide complex, [SiPiPr₃]Ru(NAr) (Ar = p-CF₃C₆H_4,5-CF₃), and features radical character on the NAr moiety, but to a greater degree. The stability of 5-OMe establishes that bimolecular coupling of 5-OMe is kinetically inconsistent with the reaction. Further studies rule out mechanisms in which 5-OMe reacts directly with free aryl azide or a transient Ru(I) azide adduct. Together, these studies show that 5-OMe is likely uninvolved in the catalytic cycle and demonstrates the influence of the metal center on the mechanism of reaction. Instead, we favor a mechanism in which free aryl nitrene is released during the catalytic cycle and combines with itself or with free aryl azide to yield the azoarene.}