Bimetallic redox chemistry in carbon-heteroatom bond formation .

摘要

Polynuclear transition metal complexes, embedded in the active sites of metalloenzymes, are responsible for effecting a diverse array of oxidation reactions in Nature. The research described in this thesis has been motivated by the hypothesis that synergistic redox chemistry between two transition metal centers can lower the activation barriers of redox transformations relevant to catalysis. Redox transformations of dinuclear Pd complexes, both in stoichiometric organometallic reactions, as well as in oxidative Pd-catalyzed C--H functionalization reactions, have been studied to probe the role of cooperative bimetallic redox chemistry in C--heteroatom (C--X) bond forming reactions.;Stoichiometric organometallic studies of the oxidation of dinuclear Pd(II) complexes to dinuclear Pd(III) complexes and subsequent C--X reductive elimination from the resulting dinuclear Pd(III) complexes have confirmed the viability of C--X bond-forming reactions mediated by dinuclear Pd(III) complexes. Metal--metal bond formation, which proceeds concurrently with oxidation of dinuclear Pd(II) complexes, can lower the activation barrier for oxidation. Experimental and theoretical work, which suggests that C--X reductive elimination is also facilitated by redox cooperation of both metals during reductive elimination, will also be discussed. The effect of ligand modification on the structure and reactivity of dinuclear Pd(III) complexes will be presented in light of the impact that ligand structure can exert on the structure and reactivity of dinuclear Pd(III) complexes.;Historically, many oxidative Pd-catalyzed C--H functionalization reactions have been proposed to proceed via mononuclear Pd(IV) intermediates. Herein, I will propose that some Pd-catalyzed oxidation reactions proceed via dinuclear Pd(III) intermediates and will provide evidence that dinuclear Pd complexes may be involved in C--O, C--Cl, and C--C bond-forming reactions. Given the historical success of the Pd(II)/(IV) redox paradigm in catalysis for guiding new reaction development, I discuss whether appreciation of bimetallic Pd(III) redox chemistry in catalysis is of academic interest exclusively and how understanding the role bimetallic Pd(III) redox chemistry in catalysis may enable future reaction development. A new hydroxylation reaction, which was developed based on the hypothesis that bimetallic redox chemistry can provide access to facile redox catalysis, will be discussed.