C–H Activation by Iron(III), Manganese(II) and Rhoda(III)electro Catalysis

Abstract

Therefore, the selective C-H functionalization can serve as an elegant tool to diminish these problems, [24] combining the broad practicability of cross-couplings with the nature of green chemistry-atom economy and environmentally friendly methods (Scheme 1.1).Moreover, two-fold C-H dehydrogenative activation also contribute to the formation of C-C bonds while external oxidants are required in the dehydrative step. [25] Scheme 1.1. Comparison of traditional cross-coupling vs. C-H activation.The last thirty years have seen many examples of C-H activation at different metal centers, usually with good regio-and chemoselectivity and under mild conditions. The selective transformation of ubiquitous but inert C-H bonds to other functional groups has farreaching practical implications, ranging from more efficient strategies for fine chemical synthesis to the replacement of current petrochemical feedstocks by less expensive and more readily available alkanes. [26] All the potential practical applications have inspired chemists to study how these organometallic reactions occur, and what their inherent advantages and limitations for practical alkane conversion and late-stage functionalization are. As the transition metal-facilitated cleavage of the C-H bonds is the common key step in the above-mentioned C-H functionalization strategies, it has been heavily examined.Excluding outer-sphere mechanisms, such as carbene/nitrene insertions [27] or radical reactions [28] , the bond dissociation proceeds generally via five different pathways, depending on the nature of the metal, the ligands and oxidation states. [29] These methods (Scheme 1.2) are oxidative addition, electrophilic substitution, -bond metathesis, 1,2addition and base-assisted metalation. Electron-rich complexes of late transition metals 1 Introduction 4 are prone to cleave the inert C-H bonds by oxidative addition, while this mode of action is unfavorable for early transition metals. [29b] Most late transition metals in higher oxidation states often act as a Lewis acid to cleave C-H bond by an electrophilic substitution mode.The -bond metathesis is observed for early transition metals which cannot undergo oxidative addition. Metals containing an unsaturated M=X bond tend to undergo C-H activation via 1,2-addition. This fashion can be found in early transition metals. Scheme 1.2. Mechanistic pathways for the C-H activation. Besides the mechanistic scenarios, many examples proceed via the base-assisted C-H metalation events (Scheme 1.3). Further research on this base-assisted C-H activation led to the proposal of several transition states. The base-assisted deprotonation takes place via a six-or five-membered transition state respectively in the presence of 1 Introduction 5carboxylate or a secondary phosphine oxide. [29a, 30] This C-H cleavage mode was classified as Concerted Metalation-Deprotonation (CMD) [30b] and ambiphillic metal-ligand activation (AMLA). [31] An additional mechanism is the base assisted intramolecular electrophilic substitution (BIES)...