Unappreciated Redox Reactivity of Palladium
We have discovered fundamentally unappreciated redox reactivity of palladium. Historically, for over 40 years, Pd(II)/Pd(IV) catalysis cycles have been proposed for many oxidative transformations catalyzed by palladium. We have presented detailed studies of Pd(OAc)2-catalyzed aromatic C–H functionalization reactions and proposed for the first time dinuclear Pd(III) complexes as intermediates. We have identified bridged dinuclear Pd(II) complexes, which self-assemble during catalysis, as the catalyst resting state, and shown their relevance in a multitude of palladium-catalyzed oxidation reactions. In situ monitoring of catalysis has revealed that turnover-limiting oxidation of a dinuclear resting state is energetically more favorable than the corresponding monometallic pathway. Informed by reaction kinetics analysis, relevant dinuclear Pd(III) complexes have been prepared and observed to undergo selective reductive elimination reactions. These transformations were the first appreciated reactions from dinuclear Pd(III) complexes reported, and provided the framework to better appreciate and predict oxidative palladium-catalyzed reactions, as well as enabled the development of previously inaccessible transformations. Based on the newly discovered reactivity, we could also prepare unprecedented infinite, one-dimensional palladium wires that displayed desirable optical and electronic properties. Because our results have substantial bearings on many Pd-catalyzed oxidations reactions, as well as on redox catalysis in general, our findings have in the meantime found their way into textbooks.
D. P. Powers, T. Ritter “Bimetallic Pd(III) Complexes in Palladium-Catalyzed Carbon–Heteroatom Bond Formation” Nature Chem. 2009, 1, 302–309.
D. C. Powers, M. A. L. Geibel, J. E. M. N. Klein, T. Ritter “Bimetallic Palladium Catalysis: Direct Observation of Pd(III)–Pd(III) Intermediates” J. Am. Chem. Soc. 2009, 131, 17050–17051.
D. C. Powers, D. Benitez, E. Tkatchouk, W. A. Goddard, III, T. Ritter “Bimetallic Reductive Elimination from Dinuclear Pd(III) Complexes” J. Am. Chem. Soc. 2010, 132, 14092–14103.
D. C. Powers, D. Y. Xiao, M. A. L. Geibel, T. Ritter “On the Mechanism of Palladium-Catalyzed Aromatic C–H Oxidation” J. Am. Chem. Soc. 2010, 132, 14530–14536.
G. J. Chuang, W. Wang, E. Lee, T. Ritter “A Dinuclear Palladium Catalyst for α-Hydroxylation of Carbonyls with O2” J. Am. Chem. Soc. 2011, 133, 1760–1762.
M. G. Campbell, D. C. Powers, J. Raynaud, M. J. Graham, P. Xie, E. Lee, T. Ritter “Synthesis and structure of solution-stable one-dimensional palladium wires” Nature Chem. 2011, 3, 949–953.