Cobalt-Complexed Propargyl Cations: Generation under Neutral Conditions and Spontaneous, High-Temperature Conversion to Propargyl Radicals

A novel method for the generation of Co-2(CO)(6)-complexed propargyl cations under neutral conditions is developed. The optimized experimental protocol involves treatment of the respective Co-2(CO)(6)-complexed propargyl methyl ethers 10 and 16-19 with equimolar quantities of triflic anhydride (2) or trifluoroacetic anhydride (13) at 83 degrees C for 3-6 min. The conversion to propargyl triflates, such as 3, occurs in two steps, via the oxonium-triflate ionic pair 11. The transition of diamagnetic propargyl triflates to the respective propargyl radicals allegedly takes place via a cluster-to-cluster reduction, followed by a cluster-to-ligand single-electron transfer. Radical dimeric products, polysubstituted 3,4-diaryl-1,5-alkadiynes 6 and 20-23, are formed in high yields (73-82%) and excellent d,l-diastereoselectivity (89-99%). Decomplexation with ceric ammonium nitrate affords topologically diverse d,l-3,4-diaryl-1,5-alkadiynes 26-30 in good to high yields (54-90%). The generation of Co-2(CO)(6)-complexed propargyl cations under neutral conditions substantially expands the scope of both ionic and radical reactions, allowing for involvement of substrates with acid-sensitive peripheral functionalities.