Development of Salt-free Reduction Methods for High-oxidation State Transition Metal Complexes

摘要

In this review, we report a new reduction protocol for early transition metal complexes – a salt-free reduction. In this salt-free reduction process, electron-rich organosilicon compounds, such as 1,4-bis(trimethylsilyl)-2,5-cyclohexadiene (1a), 1-methyl-3,6-bis(trimethylsilyl)-1,4-cyclohexadiene (1b), 1,4-bis(trimethylsilyl)-1,4-diaza-2,5-cyclohexadiene (1c), 2,5-dimethyl-1,4-bis(trimethylsilyl)-1,4-diaza-2,5-cyclohexadiene (1d), 2,3,5,6-tetramethyl-1,4-bis(trimethylsilyl)-1,4-diaza-2,5-cyclohexadiene (1e), and 1,1’-bis(trimethylsilyl)-1,1’-dihydro-4,4’-bipyridine (1f), serve as versatile reducing reagents for group 4—6 metal chloride complexes, where easily removable ClSiMe_(3) and the corresponding aromatic compounds, e.g., benzene, toluene, pyrazine, and 4,4’-bipyridyl, are generated as the reduction byproducts. This reductant-derived metal waste-free system enables us to directly observe catalytic intermediates by spectroscopic methods due to the homogeneity of the reaction mixture. In addition, simplicity of removing the reduction byproducts by evacuation or washing allowed us to isolate low-valent metal complexes that could be applied for direct complexation to redox-active ligands. Furthermore, 1e, containing methyl substituents around the nitrogen atom of the reductant is applicable as a stoichiometric reductant for a low-valent titanocene-catalyzed Reformatsky reaction of aldehydes and alkyl 2-bromoalkanoates without forming any reductant-derived metal waste.