Manganese‐Catalyzed CH Oxygenation Reactions

Transition Metal-Catalyzed Direct Stereoselective Oxygenations of C(sp3)–H Groups

“…The mechanistic studies revealed the rate-limiting C–H abstraction by electrophilic high-valent, presumably triplet ( S = 1) Mn V O species (Scheme a). ,,− Therefore, the oxidation selectivity depends on the innate substrate reactivity, which in turn correlates with the nucleophilicity of a particular C–H site. For example, benzylic and etherial C–H groups are more reactive than aliphatic C–H groups, and 3 °C–H groups are more reactive than 2° ones. ,,− In a similar manner, the C­(OH)–H functionalities of the initially formed sec -alcohols, “activated” by the presence of an adjacent OH group, are more reactive than those of the original CH 2 groups. In effect, the yields of the desired productschiral alcoholsdid not exceed 5–6%, owing to pronounced overoxidation of sec -alcohols to the ketones …”

“…For complex molecules, having many different C–H sites, the oxidation regioselectivity is determined by that of the H abstraction step, accomplished by the electrophilic metal–oxygen species. Considering only electronic effects, more electron-rich 3° carbons are oxidized more readily than 2° sites . Overriding this “natural substrate bias” can be achieved by either applying steric or chirality-induced constraints over the catalyst’s architecture or introducing substrate recognition elements into the ligand scaffold. , …”

“…Considering only electronic effects, more electron-rich 3°carbons are oxidized more readily than 2°s ites. 45 Overriding this "natural substrate bias" can be achieved by either applying steric or chirality-induced constraints over the catalyst's architecture or introducing substrate recognition elements into the ligand scaffold. 32,46 In the next section, the early examples of transition metalcatalyzed enantioselective C−H oxygenations, mostly reported before 2000, are surveyed.…”

Ni‐ and Pd‐based homogeneous catalyst systems for direct oxygenation of C(sp3)‐H groups

“…[19][20][21][22][23][24][25][26][27][28][29][30][31][32] Designing catalysts for such processes is quite challenging, since organic synthesis relies on the transformations of functional groups or structural patterns with sufficient chemical reactivity, while C-H groups are thought of as "nonfunctional" groups due to the lack of reactivity in heterolytic reactions. [33,34] Therefore, securing high selectivity in such reactions has been a difficult task: On the one hand, the use of powerful oxidants that is necessary for the oxygenation of weakly reactive C-H groups often leads to undesired overoxidation. On the other hand, the very little difference in reactivity between various C-H bonds in complex substrates makes it difficult to achieve an acceptable level of regioselectivity.…”

A synthetically useful catalytic system for aliphatic C–H oxidation with a nonheme cobalt complex and m-CPBA

“…3,4 Inspired by the structural characteristics of the catalytic metal center and the reaction mechanisms that underline their enzymatic function as catalysts for aerobic oxidative transformations, numerous iron and manganese complexes bearing porphyrinic or non-porphyrinic ligands have been developed for the selective oxidation of aliphatic C(sp 3 )-H bonds via well-established mechanisms of hydrogen atom transfer (HAT) followed by oxygen rebound. [5][6][7][8][9][10][11][12][13][14][15] As a result, significant advances have been made in the selective oxidation of alkanes based on intrinsic factors, such as bond strength, electronic effect, steric effect, stereoelectronic effect, directing group, chirality, and more recently, medium effect and non-covalent interactions in the secondary coordination sphere. 5 Moreover, since the identification of the first two iron(IV)-oxo complexes in enzymatic and biomimetic studies in 2003, 16,17 mechanistic studies have also advanced greatly through synthesis, spectroscopic characterization and kinetic studies of high-valent metal-oxo intermediates.…”