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 °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.