Organo-photocatalytic C-H bond oxidation: an operationally simple and scalable method to prepare ketones with ambient air

Oxidative C-H functionalization with O₂ is a sustainable strategy to convert feedstock-like chemicals into valuable products. Nevertheless, eco-friendly O₂-utilizing chemical processes, which are scalable yet operationally simple, are challenging to develop. Here, we report our efforts, via organo-photocatalysis, in devising such protocols for catalytic C-H bond oxidation of alcohols and alkylbenzenes to ketones using ambient air as the oxidant. The protocols employed tetrabutylammonium anthraquinone-2-sulfonate as the organic photocatalyst which is readily available from a scalable ion exchange of inexpensive salts and is easy to separate from neutral organic products. Cobalt(ii) acetylacetonate was found to be greatly instrumental to oxidation of alcohols and therefore was included as an additive in evaluating the alcohol scope. The protocols employed a nontoxic solvent, could accommodate a variety of functional groups, and were readily scaled to 500 mmol scale in a simple batch setting using round-bottom flasks and ambient air. A preliminary mechanistic study of C-H bond oxidation of alcohols supported the validity of one possible mechanistic pathway, nested in a more complex network of potential pathways, in which the anthraquinone form - the oxidized form - of the photocatalyst activates alcohols and the anthrahydroquinone form - the relevant reduced form of the photocatalyst - activates O₂. A detailed mechanism, which reflected such a pathway and was consistent with previously accepted mechanisms, was proposed to account for formation of ketones from aerobic C-H bond oxidation of both alcohols and alkylbenzenes.

Mesomeric Betaines Based on Adamantylated 1,2,4-Triazolo[4,3-a]pyrimidin-5-ones: Synthesis, Structure and Conversion into Anionic N-Heterocyclic Carbenes

The C-N coupling of 1,2,4-triazolo[1,5-a]pyrimidin-7-ones with 1-adamantanol/1-bromoadamantane leads to 1,2,4-triazolo[4,3-a]pyrimidinium-5-olates, which are represented as mesomeric betaines (MBs). The formation of MBs involves not only N-alkylation of heterocyclic framework but also the rearrangement leading to a change in the type of fusion between pyrimidine and 1,2,4-triazole fragments. The structures of the obtained products were confirmed by the X-ray analysis and measurements of ¹³ C-¹³ C (J_CC ) coupling constants in the 1D ¹³ C NMR spectra of selectively ¹³ C-labeled samples. Treatment of the betaines with lithium bis(trimethylsilyl)amide (LiHMDS) gave anionic carbenes, which were detected by ¹³ C NMR spectroscopy and were trapped by reactions with phenyl isothiocyanate and sulfur. Density functional theory (DFT) and the quantum theory of atoms in molecules (QTAIM) analyses allowed for an insight into the electronic structure of the obtained betaines and N-heterocyclic carbene derivatives.

Rh(I)-catalysed imine-directed C-H functionalization via the oxidative [3 + 2] cycloaddition of benzylamine derivatives with maleimides

The Rh(I)-catalysed imine-directed oxidative [3 + 2] cycloaddition of benzylamines with maleimides is reported. A wide range of both benzylamines and maleimides is applicable to the reaction. A one-pot three component strategy using benzylamines, 2-pyridinecarboxaldehyde, and maleimides is successfully achieved. Mechanistic studies including deuterium labelling experiments suggest that a zwitterionic intermediate is formed and is a key intermediate through the Rh-catalysed activation of a benzylic C(sp³)-H bond of the imine.

Visible-light photocatalytic selective oxidation of C(sp3)–H bonds by anion–cation dual-metal-site nanoscale localized carbon nitride

Anion–cation dual-metal-site nanoscale localized carbon nitride exhibits a significantly enhanced photocatalytic activity for the oxidation of alkanes and alcohols with a high activity and a wide functional group tolerance.

N-Heterocyclic Carbene Catalyzed Synthesis of Trisubstituted Epoxides via Tandem Amidation/Epoxidation Sequence

A tandem amidation/epoxidation sequence between various substituted chalcones and N,N-dimethylformamide (DMF) for the synthesis of trisubstituted epoxides employing N-heterocyclic carbene catalysis was developed. This reaction was performed under metal-free conditions in the presence of tert-butyl hydroperoxide (TBHP) as the oxidant. Trisubstituted epoxides bearing a ketone and an amide functionality (N,N-dimethyl formyl group) were synthesized starting from a wide range of chalcones in moderate to good yields with excellent diastereoselectivity.

Selective Oxidation of Benzylic C-H Bonds Catalyzed by Cu(II)/{PMo12}

Precise catalytic regulation of carbon radical generation by a highly active oxygen radical to abstract the H atom in a C-H bond is an effective method for the selective activation of C-H synthetic chemistry. Herein, we report a facile catalyst system with commercially available copper(II)/{PMo₁₂} to form a tert-butanol radical intermediate for the selective oxidation of benzylic C-H bonds. The reaction shows a broad range of substrates (benzyl methylene, benzyl alcohols) with good functional group tolerance and chemical selectivity. The corresponding carbonyl compounds were synthesized with good yields under mild conditions. DFT calculations and experimental analysis further demonstrated a reasonable carbon radical mechanism for this type of organic transformation reaction.

Aerobic conversion of benzylic sp3 C–H in diphenylmethanes and benzyl ethers to CO bonds under catalyst-, additive- and light-free conditions

Catalyst-, additive- and light-free aerobic conversion of benzylic C–H to CO bonds is, for the first time, reported.

The cumene/O2 system: a very simple tool for the radical chain oxidation of some functional groups

Due to the relative stability of the cumyl radical, cumenes and α-methyl-styrenes are ideally structured to directly harvest the oxidizing reactivity of O2 and initiate radical chain reactions in catalyst-free conditions. In the absence of additional substrates, these processes can lead to acetophenones. In the presence of substrates, the cumene oxidation process can be intercepted in various chain reactions, affording very simple protocols for functional group oxidation.