Synthesis of Phenylenediamines via (4 + 1 + 1) Photocycloaddition of 1,3-Dienes and Isocyanides Enabled by a Gallium(I)/(III) Redox: The Key Role of a Phenalenyl-Based Ligand

A gallacyclopentene complex is generated via (4 + 1) cycloaddition of a Ga(I) complex bearing a phenalenyl-based ligand with a 1,3-diene. Upon visible-light irradiation, the Ga(III) complex undergoes homolytic cleavage of a Ga-C bond, facilitating the subsequent mono- and double-insertion reactions with isocyanides. The double insertion complex undergoes C-C bond formation to afford 1,2-phenylenediamine, achieving a (4 + 1 + 1) cycloaddition of 1,3-dienes and isocyanides via a stoichiometric redox process and insertion on gallium. This study demonstrates a proof-of-concept for the formal two-electron redox processes of group 13 elements for organic synthesis enabled by the involvement of radical intermediates.