Reactivity Studies of Catalytically Relevant Palladium Model Complexes.

Abstract

Recently high oxidation state palladium chemistry has emerged as efficient tool for direct C−H bond functionalization that does not require prefunctionalized starting materials. Investigation of transition metal model complexes is an efficient approach for optimization of known reactions and development of new methodologies. This dissertation describes reactivity studies of Pd(IV) model systems with an aim to gain better understanding of Pd(IV) mediated C−H bond functionalization reactions. In addition, investigation of Pd(II) model complexes to assess the feasibility of hypothetical Pd(0)/(II) catalyzed trifluoromethylation reaction with trifluoroacetic anhydride as a CF3 source is also described. In order to obtain detailed information about processes occurring at Pd(IV) during catalysis, relatively stable yet reactive model systems had to be identified. First, we designed and synthesized a number of Pd(IV) complexes supported by facial tridentate NNN and NCN ligands and investigated their reactivity. Next, we identified appropriate model system for detailed investigation of C−H bond cleavage at Pd(IV) centers. Mechanistic information about this transformation was obtained through the following: (i) extensive one- and two-dimensional NMR analysis, (ii) reactivity studies of a series of substituted analogues, and (iii) isotope effect studies. These experiments suggest that C−H activation at [(Py3CH)Pd(IV)(biphenyl)Cl2]+ model system occurs via a multistep process involving chloride-to-acetate ligand exchange followed by conformational and configurational isomerization and then C−H cleavage. The data also suggest that C−H cleavage proceeds via an acetate-assisted mechanism with the carboxylate likely serving as an intramolecular base. In a unrelated project we propose a catalytic cycle for nickel(0/II) or palladium(0/II) catalyzed decarbonylative trifluoromethylation using trifluoroacetic esters as CF3 sources. The catalytic cycle consists of four elementary steps: (1) oxidative addition of a trifluoroacetic ester to M(0) center, (2) CO deinsertion from the resulting trifluoroacyl M(II) complex, (3) transmetallation of a zinc-aryl to M(II), and (4) aryl–CF3 bond-forming reductive elimination. We demonstrated that the use of RuPhos as the supporting ligand for palladium enables each of these steps to proceed under mild conditions. These studies set the stage for the development of catalytic arene trifluoromethylation and perfluoroalkylation reactions using inexpensive trifluoroacetic acid-derived CF3 sources.