Metal-stabilized Radical Research Articles

Metalloradical complex Co-C˙Ph3 catalyzes the CO2 reduction in gas phase: a theoretical study.

Metal-stabilized radicals have been increasingly exploited in modern organic synthesis. Here, we theoretically designed a metalloradical complex Co-C˙Ph3 with the triplet characters through the transition metal cobalt (Co0) coordinating a triphenylmethyl radical. The potential catalytic role of this novel metalloradical in the CO2 reduction with H2/CH4 in the gas phase was explored via density functional theory (DFT) calculations. For the CO2 reduction reaction with H2, there are two possible pathways: one (path A) is the activation of CO2 by Co-C˙Ph3, followed by the hydrogenation of CO2. The other (path B) starts from the splitting of the H-H bond by Co-C˙Ph3, leading to the transition-metal hydride complex CoH-H, which can reduce CO2. DFT computations show that path B is more favorable than path A as their rate-determining free energy barriers are 18.3 and 27.2 kcal mol-1, respectively. However, for the reduction of CO2 by CH4 two different products, CH3COOH and HCOOCH3, can be generated following different reaction routes. Both routes begin with one CH4 molecule approaching the metalloradical Co-C˙Ph3 to form the intermediate CoH-CH3. This intermediate can evolve following two different pathways, depending on whether the H bonded to Co is transferred to the O (pathway PO) or the C (pathway PC) of CO2. Comparing their rate-determining steps, we identified that the PO route is more favorable for the reduction of CO2 by CH4 to CH3COOH with the reaction barrier 24.5 kcal mol-1. Thus, the present Co0-based metalloradical system represents a viable catalytic protocol that can contribute to the effective utilization of small molecules (H2 and CH4) to reduce CO2, and provides an alternative strategy for the exploration of CO2 conversion.

Reduction of Carbonyl Groups by Uranium(III) and Formation of a Stable Amide Radical Anion.

Methyl benzoate, N,N-dimethylbenzamide, and benzophenone were reduced by UIII [N(SiMe3 )2 ]3 resulting in uranium(IV) products. Reduction of benzophenone lead to UIV [OC⋅Ph2 )][N(SiMe3 )2 ]3 , (1.1) which forms the dinuclear complex, [N(SiMe3 )2 ]3 UIV (OCPhPh-CPh2 O)UIV [N(SiMe3 )2 ]3 (1.2), through coupling of the ketyl radical species upon crystallization. Reaction of N,N-dimethylbenzamide with UIII [N(SiMe3 )2 ]3 resulted in UIV [OC⋅(Ph)(NMe2 )][N(SiMe3 )2 ]3 (2), a uranium(IV) compound and the first example of a charge-separated amide radical. In the case of methyl benzoate, the reduction resulted in UIV (OMe)[N(SiMe3 )2 ]3 (3) and benzaldehyde as the reduced organic fragment. Compound 2 showed the ability to act as a uranium(III) synthon in its reactivity with trimethylsilyl azide, a reaction that yielded UV (=NSiMe3 )[N(SiMe3 )2 ]3 . Additionally, 2 was reduced with potassium graphite resulting in [U(μ-O)[O=C(NMe2 )(Ph)][N(SiMe3 )2 ]2 ]2 (4), a dinuclear uranium compound bridged by oxo ligands. Reduction of 2 in the presence of 15-crown-5 afforded isolation of the mono-oxo compound, [(15-crown-5)2 K][UO[N(SiMe3 )2 ]3 ] (5). The results expand the reduction capabilities of UIII complexes and demonstrate a strategy for isolating novel metal-stabilized radicals.

Stereoselective Hydrodehalogenation via a Radical‐Based Mechanism Involving T‐Shaped Chiral Nickel(I) Pincer Complexes

We herein report the catalytic enantioselective hydrodehalogenation based on the interplay of a chiral molecular nickel(I)/nickel(II)hydride system. Prochiral geminal dihalogenides are dehalogenated via a secondary configurationally unstable, potentially metal-stabilized radical intermediate. In a subsequent step, the liberated radical is then trapped by the nickel(II) hydrido complex, present in a large excess under the catalytic conditions, which in turn induces the enantioselectivity during the hydrogen atom transfer onto the radical intermediate. These new chiral nickel(I) complexes were found to catalyze the asymmetric hydrodehalogenation of geminal dihalogenides with moderate to good enantiomeric excess values using LiEt3 BH as reductant. The main side product generally observed is the dehalogenated alkene, whereas the hydrodehalogenation of the chiral monohalogen compound occurred much more slowly despite the large excess of reductant.

Free radicals of biological interest as studied by electron spin resonance

Electron spin resonance has become a major tool for investigating biological one-electron or radical group transfer. The scope and limitations of this method are considered and emphasis is placed on the first of the following two questions that govern the field: (1) What is the structure, stability and potential biological function of radicals that might occur as biological intermediates? (2) Which radicals have been demonstrated up to now as ( a ) occurring in biological reactions, ( b ) being essential biological intermediates? The two questions deserve independent consideration and supplement each other. However, the second question can hardly be decided before the first one, though there may be a severe temptation to claim the occurrence or even stabilization of a certain radical without any structural evidence. Among the free radicals considered here are phenoxyls, mercaptyls, semidiones, (aza)semi-quinones (from flavins and pteridines), and metal-stabilized radicals.