Mechanistic studies of Ce(IV)-mediated oxidation of beta-dicarbonyls: solvent-dependent behavior of radical cation intermediates

Construction of 3-Oxazolin-5-one via Visible Light-Induced Nondecarboxylative Coupling and Sequential Reactions

3-Oxazolin-5-ones are precursors of nitrile ylides and represent valuable 1,3-dipoles for constructing cyclic imines or pyrrole compounds. Harnessing the power of photocatalysis, we accomplished a straightforward synthesis of 3-oxazolin-5-ones from redox-active esters and secondary nitro compounds. Visible light-induced nondecarboxylative coupling of a redox-active ester, nitro aldol condensation, and subsequent visible light-induced N-oxide deoxygenation were accomplished within 2 h. The reaction mechanism was supported by experimental data and DFT calculations.

Cerium Ammonium Nitrate-Mediated Access to Biaryl Lactones: Substrate Scopes and Mechanism Studies

Herein we described an access to biaryl lactones from ortho-aryl benzoic acids via intramolecular O-H/C-H oxidative coupling with the commonly used cerium ammonium nitrate (CAN) as the one-electron oxidant under a thermal condition. The radical interrupting experiment suggested a radical process, while the kinetic isotope effect (KIE) showed that the C-H cleavage likely was not involved in the rate-determining step. Competitive reactions, especially the strikingly different ρ values of Hammett equations, indicated that the reaction rate was more sensitive to the electronic properties on the aryl moiety rather than the carboxylic moiety, which corresponded to the first single electron transfer (SET) step. In addition, the quite negative ρ values (-4.7) of the aryl moiety unveiled the remarkable electrophilic nature of the second intramolecular radical addition process, which was also consistent with product yields and regioselectivity. Moreover, control experiments disclosed that the single electron in the third step was also transferred to Ce^IV instead of molecular oxygen. Besides, the possible role of co-solvents trifluoroethanol (TFE) and its influences on the Ce^IV species were discussed. This work elucidated the possible mechanism by proposing the step that had more effects on the total reaction rate and the species that was responsible for the last single electron transfer.

Homo- and Heteropolymetallic Complexes of the Hybrid, Ambidentate N-Heterocyclic Carbene Ligand IMes-acac

The anionic 5-acetylimidazol-2-ylidene-4-olate 1 ^-, named as "IMes-acac", is composed of fused diaminocarbene and acetylacetonato units in the same IMes-based imidazolyl ring. The bifunctional compound 1 ^- is shown to act as an effective, ditopic bridging ligand for transition metal centers. Several new complexes supported by this ligand were prepared, including the complex [RuCl(p-Cym)(κ² O,O-1·H)](BF₄) (2), which can be regarded as a metallated imidazolium salt, the homobimetallic complex [((COD)Rh)(RhCl(COD))(μ-1κ² O,O:2κ¹ C-1)] (4), the heterobimetallic complexes [((p-Cym)ClRu)(RhCl(COD))(μ-1κ² O,O:2κ¹ C-1)] (3), [((p-Cym)ClRu)(RhCl(CO)₂)(μ-1κ² O,O:2κ¹ C-1)] (5), [((p-Cym)ClRu)(Cu(IPr))(μ-1κ² O,O:2κ¹ C-1)] (9), the anionic homoleptic Cu(I) complexes [Cu(κ¹ C-1)₂]K ([10]K) and [Cu(κ¹ C-1)₂](NEt₄) ([10](NEt₄)), and the heterotrimetallic complex [((p-Cym)RuCl)₂(Cu)(μ-1κ² O,O:3κ¹ C-1)(μ-2κ² O,O:3κ¹ C-1)](PF₆) (11). Preliminary studies on the possible preparation of supramolecular metallopolymers and electrochemical studies on the series of complexes are also reported.

Apoptosis induced by 2-aryl benzothiazoles-mediated photodynamic therapy in melanomas via mitochondrial dysfunction

A mild and efficient synthetic development of 2-arylbenzothiazoles 5 mediated by ceric ammonium nitrate (CAN) via intramolecular cyclization of N-phenyl-thiobenzamides 4 was achieved. Further compounds 5 were reduced to corresponding amines 6, and their photodynamic therapy (PDT) effect was evaluated on malignant human melanoma A375 cells. Amine 6l plus ultraviolet A (UVA) induced caspase-3 activity, poly(ADP-ribose)polymerase cleavage, M30 positive CytoDeath staining, and subsequent apoptotic cell death. Our data disclosed that treatment of A375 cells with 6l plus UVA resulted in a decrease in mitochondrial membrane potential (ΔΨmt), oxidative phosphorylation system (OXPHOS) subunits, and adenosine triphosphate (ATP) but an increase in mitochondrial DNA 4977-bp deletion via reactive oxygen species (ROS) generation. Transmission electron microscopy (TEM) observations also showed major ultrastructural alterations of mitochondria. Additionally, 6l plus UVA was also shown to reduce murine melanoma size in a mouse model. The present study supports the hypothesis that 6l-PDT may serve as a potential ancillary modality for the treatment of melanoma.

Mechanistic studies on the CAN-mediated intramolecular cyclization of δ-aryl-β-dicarbonyl compounds

The synthesis of 2-tetralones through the cyclization of δ-aryl-β-dicarbonyl substrates by using CAN is described. Appropriately functionalized aromatic substrates undergo intramolecular cyclizations generating 2-tetralone derivatives in moderate to good yields. DFT computational studies indicate that successful formation of 2-tetralones from δ-aryl-β-dicarbonyl radicals is dependent on the stability of the subsequent cyclohexadienyl radical intermediates. Furthermore, DFT computational studies were used to rationalize the observed site selectivity in the 2-tetralone products.

Corrigendum to Solvent-dependent oxidative coupling of 1-aryl-1,3-dicarbonyls and styrene

This report describes the scope and mechanism of the solvent-dependent, chemoselective oxidative coupling of 1-aryl-1,3-dicarbonyls with styrene using Ce(IV) reagents. Dihydrofuran derivatives are obtained when reactions are performed in methanol whereas nitrate esters can be selectively synthesized in acetonitrile and methylene chloride. Mechanistic studies are consistent with the rate of solvent-assisted deprotonation of a radical cation intermediate playing an integral role in the selective formation of products.

Solvent-dependent oxidative coupling of 1-aryl-1,3-dicarbonyls and styrene

This report describes the scope and mechanism of the solvent-dependent, chemoselective oxidative coupling of 1-aryl-1,3-dicarbonyls with styrene using Ce(IV) reagents. Dihydrofuran derivatives are obtained when reactions are performed in methanol whereas alpha-tetralones can be selectively synthesized in acetonitrile and methylene chloride. Mechanistic studies are consistent with the rate of solvent-assisted deprotonation of a radical cation intermediate playing an integral role in the selective formation of products.

Mechanisms of Mn(OAc)(3)-based oxidative free-radical additions and cyclizations

The mechanistic details of Mn(OAc)(3)-based oxidative free-radical additions and cyclizations are reviewed. The mechanisms of electron transfer to generate radicals, electron transfer to convert the radicals to oxidized products, and further oxidation of the products are covered.

Synthesis of gamma-halogenated ketones via the Ce(IV)-mediated oxidative coupling of cyclobutanols and inorganic halides

A straightforward method for the synthesis of gamma-halo-substituted ketones formed via the CAN-initiated oxidative addition of halides to 1-substituted cyclobutanols has been developed. This method has short reaction times, and provides access to a range of bromo and iodo gamma-substituted ketones in good to excellent yields.

Facile Route to Tetrasubstituted Pyrazoles Utilizing Ceric Ammonium Nitrate

A convenient approach for the synthesis of tetrasubstituted pyrazoles is described. The method involves the treatment of 1,3-diketones and allyltrimethylsilane with CAN followed by cerium-catalyzed addition of substituted hydrazines to construct pyrazoles in good yields.