Stereochemistry of the nucleophilic cleavage of cobalt–carbon bonds in organocobalt(IV) compounds

Ph(i-PrO)SiH2: An Exceptional Reductant for Metal-Catalyzed Hydrogen Atom Transfers

“…However, like many previous HAT-initiated reactions mediated by iron and manganese, alcohol was required as solvent. Obviously, this solvent restriction can be problematic for cationic ,, and anionic ,,, radical-polar crossover reactions, where nucleophilic and protic solvents lead to charge quenching. Here we demonstrate that isopropoxy­(phenyl)­silane ( 6 , Ph­( i -PrO)­SiH 2 ) is a uniquely efficient reductant that permits the use of multiple solvents in manganese and iron-catalyzed reactions.…”

Catalytic Radical–Polar Crossover Reactions of Allylic Alcohols

“…Halpern and co-workers previously demonstrated that relevant cobalt­(III) complexes undergo oxidation to the corresponding cationic species . The resulting so-called alkylcobalt­(IV) intermediates , were found to undergo facile displacement with various nucleophiles, which could proceed with a high degree of inversion of the stereochemical configuration. − Formation of alkylcobalt­(III) complexes via hydrogen atom transfer (HAT) , followed by the radical pair collapse was previously implicated in hydroformylations and more recently in cobalt-catalyzed isomerizations and hydroarylations of alkenes, where participation of alkylcobalt­(IV) complexes in the transmetalation event was also suggested . We propose that the radical–polar crossover hydrofunctionalizations of the tertiary allylic alcohols with catalysts 4 and 7 involve alkylcobalt­(IV) complexes as electrophilic intermediates .…”

Branch-Selective Hydroarylation: Iodoarene–Olefin Cross-Coupling

“…But since radical collapse ( k 2 ) appears to terminate our isomerization of terminal alkenes at room temperature, either these conditions decrease the rate of collapse and allow cage escape or cleavage of the collapsed organocobalt is rapid, possibly via oxidation by 5 . An alternative polar mechanism for interception of arylnickel is possible, especially in light of recent work by Shigehisa and Hiroya. , Suffice it to say, full elucidation of the mechanism will require some work; the sketch in Figure is likely to be a gross oversimplification …”