Activation of metal-oxo and nitrido species by Lewis acids
Student thesis: Doctoral Thesis
The effects of Lewis acids on the stoichiometric and catalytic oxidation of organic substrates by metal oxo and nitrido species have been investigated at room temperature in various solvents. The metal complexes studied were KMnO4, (Bu4N)[MnO4], (Bu4N)2[Cr2O7], PPh4[RuO4], (Bu4N)[ReO4], (Bu4N)[OsO3N], and (Bu4N)[M(N)Cl4] (M=Ru, Os). The Lewis acid used included BF3, AlCl3, FeCl3, Fe(CF3SO3)3, ZnCl2, LiC1, Cu(CF3SO3)2, Al(CF3SO3)3, Ag(CF3SO3). A solution of (Bu4N)2 [Cr2O7] in CH2C12 containing cyclohexane is stable at room temperature for over 10 h. However upon adding a few equivalents of BF3MeCO2H, cyclohexanone was formed within minutes in yields around 30%. The reaction was further enhanced by the addition of a small amount of acetic anhydride. Other Lewis acids gave similar yields but the rates were slower, while ZnCl2 and CF3SO3Ag were rather ineffective. Other oxo-chromium(VI) species were similarly activated by Lewis acids. Similar accelerating effects by BF, were also observed with KMnO4 and (Bu4N)[MnO4]. Other Lewis acids were less effective. Carbonyl compounds were predominantly formed from the oxidation of primary and secondary C-H bonds. PPh4[RuO4] is a mild oxidant and is stable in a solution containing cyclohexane. However the complex can again be activated by the addition of a few equivalents of Lewis acids such as FeCl3, ZnCl2, and BF3MeCO2H, and cyclohexane can be oxidized to cyclohexanone within minutes. The oxidation of cyclohexane can also be made catalytic by using PPh4[RuO4]/FeCl3 as the catalytic system and TBHP as the terminal oxidant. (Bu4N)[ReO4] was unable to oxidize alkanes even in the presence of Lewis acids. However (Bu4N)[ReO4]/Lewis acid functioned as a catalytic system for the oxidation of alkanes by TBHP. In the absence of Lewis acid, the catalytic activity was negligible. (Bu4N)[OsO3N] is indefinitely stable in an alkane solution at room temperature. However, oxidation of alkanes occurred within minutes with high yields when a few equivalents of Lewis acids were added. The [OsO3N]‾/FeC13 system was also able to catalyse the oxidation of alkanes by TBHP. (Bu4N)[M(N)Cl4] (M= Ru, Os) was able to function as stoichiometric oxidant but could catalyse the oxidation of alkanes by TBHP. The similarities in product distributions (ketones are formed predominantly over alcohols) and kinetic isotope effects between reactions activated by Lewis acids and by protons suggest that the mechanisms are similar in the two cases. The Lewis acid (E) most likely acts by bonding to an 0x0 (or nitrido ligard): M=O + E → M = O → E The accelerating effects are thus due to electron withdrawal by the Lewis acid, which would make the complex more strongly oxidizing. Product distributions were different between Lewis acid activated stoichiometric and catalytic oxidations. It is proposed that for stoichiometric oxidations the active intermediate was an oxo-metal species while in catalytic oxidations using TBHP the active intermediate was an alkylperoxy species.
- Lewis acids