Recent Advances in Homogeneous Metal-Catalyzed Aerobic C–H Oxidation of Benzylic Compounds

Abstract

Csp 3 -H oxidation of benzylic methylene compounds is an established strategy for the synthesis of aromatic ketones, esters, and amides. The need for more sustainable oxidizers has encouraged researchers to explore the use of molecular oxygen. In particular, homogeneous metal-catalyzed aerobic oxidation of benzylic methylenes has attracted much attention. This account summarizes the development of this oxidative strategy in the last two decades, examining key factors such as reaction yields, substrate:catalyst ratio, substrate scope, selectivity over other oxidation byproducts, and reaction conditions including solvents and temperature. Finally, several mechanistic proposals to explain the observed results will be discussed. relevant parameters such as reaction yields, substrate:catalyst ratio, selectivity towards target carbonyl compounds, reaction media, and other experimental conditions. Considering that metal complexes and combination of metal salts and ligands have been described as catalysts or precatalysts in this field, this review is organized according to the metal core involved in the reaction. Although mechanism for this transformation is far from being completely elucidated, several proposals will be discussed. Catalysts 2018, 8, x FOR PEER REVIEW 2 of 21Following the pioneering works by Ishii [63-66], cobalt(II) salts combined with N-hydroxyphtalimide (NHPI) derivatives have been amply used for the aerobic C-H oxidation of methylene compounds at atmospheric pressure. An in situ-generated phthalimide-N-oxyl radical (PINO) is proposed to play a key catalytic role as hydrogen atom abstracting species in the reaction, and other N-hydroxylated ligand/additives prone to form related radical species have also been evaluated and compared with NHPI. Among other transition metals, cobalt salts have been defined as promoters Catalysts 2018, 8, 640 3 of 21for the formation of PINO. Nolte and coworkers published the use of 0.5 mol% of Co(OAc) 2 ·4H 2 O combined with NHPI (10 mol%) in acetic acid to perform benzylic oxidation of 2-(hetero)arylacetic esters to the corresponding α-ketoesters at relatively low temperatures (40-80 • C). Lower yields (<30%) were obtained from some o-alkoxylated and brominated substrates, as well as from all the heteroaromatic esters. Finally, with regard to the oxidation of ethylbenzene to acetophenone, several N-hydroxy-phthalimides were evaluated as co-catalysts, achieving good conversion rates (except for nitro-substituted NHPIs) and relatively good acetophenone:1-phenylethanol ratios (59-88:2-5) [94].The solvent-free oxidation of several alkylbenzenes to the corresponding ketones using di-n-decyl-di-methyl ammonium bromide (DDAB) and other phase transfer agents was explored by Patil et al. A 0.5 mol% of the latter quaternary ammonium salt was required along with NHPI (3 mol%) and CoCl 2 ·6H 2 O (1 mol%) to perform the oxidation under oxygen gas flow rate of 110 mL/min at 65 • C, although other metal sources (MnCl 2 (H 2 O) 4 and CrCl 3 (H 2 O) 6 ) and ammonium phase transfer catal...

Selective Degradation of Styrene‐Related Plastics Catalyzed by Iron under Visible Light**

“…Earth-abundant metals were found to transform alkanes to alcohols and ketones under oxygen atmosphere. [24][25][26][27][28][29][30][31][32][33] In addition, CÀ C bond scission of alcohols and ketones through alkoxyl radical intermediates applying inexpensive catalysts were reported recently. [34][35][36][37][38][39][40][41][42][43] Despite limitations, these results illustrate the possibility of developing an economic system for oxidative CÀ C bond cleavage in an environmentally friendly way (Figure 1b).…”

Light-mediated aerobic oxidation of C(sp3)–H bonds by a Ce(iv) hexachloride complex

“…[9][10][11][12][13][14][15][16] The abundance of molecular oxygen in the Earth's atmosphere renders catalytic C-H oxygenation using O 2 as the terminal oxidant particularly attractive. [11][12][13][15][16][17][18][19][20] Additionally, low-cost metal catalysts are needed for practical applications. The development of new methods utilizing molecular oxygen and earth-abundant catalysts to achieve the C-H oxygenation of hydrocarbons, including light alkanes, is, therefore, a high priority.…”

Clean Oxidation of (Hetero)benzylic Csp3–H Bonds with Molecular Oxygen

“…(Hetero)­aromatic ketones are widely present in natural products, biologically active compounds, and artificial drugs. − They also act as useful building blocks and versatile synthetic precursors in organic synthesis and material sciences. − As a consequence, much effort and energy have been dedicated to developing various methodologies for the preparation of such motifs. Among the known protocols, the benzylic C sp3 –H bond oxidation reaction is one of the most efficient methods, given the inexpensiveness, abundance, and accessibility of alkyl aromatics as industrial materials. , Traditionally, (super)­stoichiometric amounts of hazardous oxidants (e.g., high-valent metal salts, halides, and peroxides) were employed for these chemical transformations, generally generating large amounts of chemical waste and unwanted byproducts (Scheme a). − Therefore, developing an eco-friendly and general protocol for the oxidation of alkyl (hetero)­aromatics using safe and environment friendly oxidants is highly appealing.…”

“…Among the known protocols, the benzylic C sp3 −H bond oxidation reaction is one of the most efficient methods, given the inexpensiveness, abundance, and accessibility of alkyl aromatics as industrial materials. 9,10 Traditionally, (super)stoichiometric amounts of hazardous oxidants (e.g., high-valent metal salts, halides, and peroxides) were employed for these chemical transformations, generally generating large amounts of chemical waste and unwanted byproducts (Scheme 1a). 11−17 Therefore, developing an ecofriendly and general protocol for the oxidation of alkyl (hetero)aromatics using safe and environment friendly oxidants is highly appealing.…”