Efficient, recoverable, copper-catalyzed aerobic oxidation of alcohols under FBS and thermomorphic mode

Abstract

A series of fluorinated bipyridine derivatives, (4,4′-bis(R fCH 2OCH 2)-2,2′-bpy) {R f = n-C 8F 17 ( 1a), n-C 9F 19 ( 1b), n-C 10F 21 ( 1c), n-C 11F 23 ( 1d)} have been successfully synthesized using 4,4′-bis(bromomethylene)-2,2′-bpy and fluorinated alkoxides. Bpy 1a– d have been characterized by multi-nuclei ( 1H, 19F, and 13C) NMR, GC/MS and FTIR. The Cu complexes 2a– d could be generated in situ by stirring ligands 1a– d with CuBr·Me 2S at room temperature, respectively. The 3-component systems 3c– d, CuBr·Me 2S/Bpy ( 1c– d)/2,2,6,6-Tetramethylpiperidine 1-oxyl (TEMPO), were successfully used to the aerobic oxidation of alcohols under the fluorous biphasic system (FBS). The resulting products from FBS could be easily recovered by two phase separation with high yields up to 8 runs (>90%). In order to avoid using the expensive fluorous solvents, systems 3a– d, CuBr·Me 2S/Bpy ( 1a– d)/TEMPO, were also successfully shown to catalyze the aerobic alcohol oxidation under the thermomorphic condition (in C 6H 5Cl), and the yields of oxidation of 4-nitrobenzyl alcohol were close to 100% even after 8 runs. In particular, 3a was most effective under the thermomorphic mode in the chemoselectivity of aerobic oxidation of aliphatic primary alcohols to aldehydes without any overoxidized acids.