Metal-free, one-pot, sequential protocol for transforming α,β-epoxy ketones to β-hydroxy ketones and α-methylene ketones

Selective Oxidative Cleavage of the C-C Bond in α,β-Epoxy Ketone into Carbonyl Compounds

This method afforded aromatic carbonyl compounds under TBHP via selective oxidative cleavage of the C-C bond in α,β-epoxy ketones. Aromatic acid came from the aroyl section, and aromatic aldehyde came from the other aromatic group. TBHP acted as a free radical initiator and oxidant. The reaction within the solvent went through a ring-opening addition, cleavage of the C-C bond in the ethylene oxide section, and oxidation, affording the target compounds in moderate to good yields. The HPLC yield of aromatic aldehyde was up to 91%. The HPLC yield of aromatic acid was up to 99%. The reaction under solvent-free conditions gave two kinds of aromatic acids coming from different moieties of α,β-epoxy ketone via the further oxidation of aromatic aldehyde. The substituent effect was discussed, and the reaction mechanism was proposed. This method allowed the reaction to occur in a simple system metal-free.

Imidazolidine Hydride Donors in Palladium-Catalyzed Alkyne Hydroarylation

Aldehyde-derived imidazolidines participate as hydride donors in intramolecular reductive Heck-type reactions. N,N′-Diphenylimidazolidines prepared from ortho-alkynyl benzaldehydes underwent regio- and stereoselective palladium-catalyzed hydroarylation followed by formal 1,5-hydride transfer and reductive elimination to afford substituted alkenes and imidazolium moieties, the latter conveniently converted in situ to ring-opened benzanilides to simplify product isolation. Internal alkynes were converted to trisubstituted alkenes via a syn hydroarylation process, while a terminal alkyne was converted to a cis alkene via a formal trans hydroarylation reaction. Benzanilide products could be converted to carboxylic acid derivatives under basic conditions, resulting in the net conversion of alkynyl aldehydes to alkenyl carboxylic acids. A styrene derivative with an attached N,N′-dimethylbenzimidazoline hydride donor was also found to undergo an analogous hydroarylation/benzimidazoline oxidation to give a diarylethane product.

Copper(I) Bis(diimine) Complexes with High Photooxidation Power: Reductive Quenching of the Excited State with a Benzimidazoline Sacrificial Donor

The reductive quenching of photoexcited photosensitizers is a very efficient way to achieve challenging reduction reactions. In this process, the excited photosensitizer is reduced by a sacrificial electron donor. This mechanism is rarely observed with copper(I) bis(diimine) complexes, which are nevertheless acknowledged as very promising photosensitizers. This is due to the fact that they are very poor photooxidants and prove unable to react with common donors once promoted in their excited state. In this article, we evidence the rare reductive quenching cycle with two specially designed copper(I) complexes. These complexes exhibit improved photooxidation power thanks to an optimized coordination sphere made of strongly π-accepting ligands. Reductive quenching of the excited state of the latter complexes with a classical benzimidazoline sacrificial donor is monitored, and reduced complexes are accumulated during prolonged photolysis. Trials to utilize the photogenerated reductive power are presented.

Interplay between Structure and Mechanism in Reductive Dissociative Electron Transfers to α,β -Epoxyketones

The electrochemical reduction of several α,β -epoxyketones was studied using cyclic (linear sweep) voltammetry, convolution voltammetry, and homogeneous redox catalysis. The results were reconciled to pertinent theories of electron transfer. α,β -Epoxyketones undergo dissociative electron-transfer reactions with C-O bond cleavage, via both stepwise and concerted mechanisms, depending on their structure. For aliphatic ketones, the preferred mechanism of reduction is consistent with the "sticky" concerted model for dissociative electron transfer. Bond cleavage occurs simultaneously with electron transfer, and there is a residual, electrostatic interaction in the ring-opened (distonic) radical anion. In contrast, for aromatic ketones, because the ring-closed radical anions are resonance-stabilized and exist at energy minima, a stepwise mechanism operates (electron transfer and bond cleavage occur in discrete steps). The rate constants for ring opening are on the order of 10⁸  s^-1 , and not significantly affected by substituents on the 3-membered ring (consistent with C-O bond cleavage). These results and conclusions were fully supported and augmented by molecular orbital calculations.

Chemoselective α-Methylenation of Aromatic Ketones Using the NaAuCl4/Selectfluor/DMSO System

Gold-catalyzed chemoselective α-methylenation of aromatic ketones was developed through the use of Selectfluor as a methylenating agent. A variety of useful 1,2-disubstituted propenone derivatives can be prepared in good yields via the present protocol. This reaction features a simple operation, good functional group tolerance, and broad scope of substrates.

2-Aryl-1,3-dimethylbenzimidazolines as Effective Electron and Hydrogen Donors in Photoinduced Electron-Transfer Reactions

2-Aryl-1,3-dimethylbenzimidazolines (DMBIHs) have been applied to photoinduced electron-transfer reductions of various organic substrates. Either direct or indirect electron transfer between the substrates and DMBIHs is utilized to promote the desired transformations. Photoexcitation of the substrates using light above 280 nm was carried out in the former protocol whereas a photosensitization method using materials such as substituted pyrenes, ruthenium and iridium complexes that absorb longer-wavelength light was employed in the latter. In these reactions, DMBIHs undergo initial electron transfer and subsequent proton or hydrogen atom transfer.