Recent Advances in Four-Coordinated Planar Cobalt Catalysis in Organic Synthesis

In the field of transition metal-catalyzed organic transformations, four-coordinated planar cobalt complexes represent an active area of research due to their low cost, abundance on earth, and unique catalytic activity based on the three oxidation states of cobalt. These complexes can behave as various reactive intermediates, such as Co(II) metalloradicals, Co(III) hydrides, Co(I) bases, and Co(I) nucleophiles, according to the ligand field theory. For instance, planar Co(II) species have a d(7) electronic configuration and act as metalloradical catalysts, which react with diazo and azide compounds to form reactive intermediates such as Co(III) carbene-radicals and Co(III) nitrene-radicals, respectively. These intermediates enable efficient cyclopropanation and aziridination of alkenes and C(sp(3))-H bond functionalization. Furthermore, the hydrogen bonding interaction between amide groups of planar ligands and polar substituents of the carbon radical on the Co(III) carbene-radicals not only creates robust chiral cavities but also contributes to the stabilization of the Co(III) carbene-radical and the transition state, consequently resulting in a dramatic improvement in reaction efficiency. The Co(III)-hydrides provide a useful method for the generation of carbon radicals through hydrocobaltation of alkenes followed by homolytic cleavage of the Co-C bond. The carbon radical formation can participate in various hydrofunctionalizations and alkene-isomerization. The d(8) planar Co(I) complexes act as a base because they have an electronically filled d(z2) orbital as the highest occupied molecular orbital (HOMO). This basicity enables various aromatic C-H functionalizations without a stoichiometric amount of oxidant. In contrast, the same planar Co(I) also acts as a superior nucleophilic catalyst, which can react with carbon electrophiles such as epoxides and organic (pseudo)halides to produce the corresponding alkyl-Co(III) species. The generated alkyl-Co(III) can be converted into the carbon radical or undergo transalkylation reaction with other transition metal catalysts. The strategies provide a new method for organic transformation with carbon electrophiles. This minireview covers recent developments in the field of four-coordinated planar cobalt-catalyzed organic transformations, paying particular attention to the relationship between their catalytic activities and oxidation states. The reactions have been categorized into those involving planar Co(II) complexes, Co(III) hydrides, and Co(I) complexes, with representative examples and insightful mechanistic discussions.