Redox-induced carbon-carbon bond formation by using noninnocent ligands

The control of radical reactions to afford selective carbon-carbon bond formation is a significant synthetic challenge with applications ranging from small-molecule activation to natural product synthesis. Oxidation of (IP-) ₂Al(CH₃) (1, IP = iminopyridine) with TrBPh₄ (Tr = trityl) afforded the C-C coupled product [(IP)(Tr-IP)Al(CH ₃)][BPh₄](2) in which the trityl radical and the IP-radical have undergone C-C bond formation. In contrast, oxidation of 1 with TrBArF₂₄ {BArF₂₄ = tetrakis[(3,5-trifluoromethyl)phenyl] borate} or TrB(C₆F₅)₄ affords cationic [(IP)(IP-)Al(CH₃)][BArF₂₄](3a) or [(IP)(IP-)-Al(CH ₃)][B(C₆F₅)₄](3b), respectively. The different reaction outcomes provided by the different counteranions of Tr+ imply that a difference in stability of the products or of the intermediate mixed-valent [(IP)(IP-)Al(CH₃)]+ state exists. We speculate that the most likely factor is the difference in solubility afforded by the different anions of the products that are formed. We also show that the formation of stable, cationic biradical complexes is possible and that these complexes do not undergo C-C radical coupling at the IP ligand. Cationic [(IP-) ₂Al(OEt₂)][BArF₂₄](4) was obtained by protonolysis of 1 with H(OEt₂)₂BArF₂₄, and two-electron oxidation of [(IP^2-)_2- Al]-(5) afforded [(IP-)₂Al(thf)][BArF₂₄](6).