Kinetics And Mechanisms Of The Oxidation Of Organic Substrates By Oxo Complexes Of Ruthenium And Manganese

This work is divided into two parts. Part I is concerned with the kinetics and mechanisms of the oxidations of NADH analogues by trans-dioxoruthemium(VI) complexes. Part II describes the Lewis acid-activated oxidation of alcohols by permanganate.In Part I, the kinetics and mechanisms of the C-H activation of common NADH analogues, AcrH₂ and BNAH by trans-[Ru^VI(N₂O₂)(O)₂]²⁺ and trans-[Ru^VI(tmc)(O)₂]²⁺ in CH3CN were studied.The reactions occur in two distinct phases, corresponding to Ru^VI±RuIV and RuIV±RuII, which can be detected by ESI/MS. At 298K, the pseudo-first-order rate constants depend linearly on [AcrH₂] or [BNAH] but are independent of [Ru^VI]. For the reaction with AcrH₂, the second-order rate constants, k^f_AcrH2 and k^s_AcrH2, were found to be (1.17±0.03)±103 M^-1 s^-1 and (3.46±0.09) M^-1 s^-1 respectively for Ru^VI(N₂O₂)(O)₂²⁺; (1.51±0.02)±102 M^-1 s^-1 and (3.66±0.15)±10^-1 M^-1 s^-1 respectively for Ru^VI(tmc)(O)₂²⁺. The kinetic isotope effects k^f_AcrH2/k^f_AcrD2 and k^s_AcrH2/k^s_AcrD2 were found to be 11.5±0.6 and 1.2±0.2 for the reaction with Ru^VI(N₂O₂)(O)₂²⁺, k^f_AcrH2/k^f_AcrD2 = 5.8±0.4 and k^s_AcrH2/k^s_AcrD2 = 1.2±0.2 for the reaction with Ru^VI(tmc)(O)₂²⁺; For the reaction with BNAH, the second-order rate constants, kfBNAH and ksBNAH, were found to be (2.89±0.04)±105 M^-1 s^-1 and (2.59±0.05)±104 M^-1 s^-1 respectively for Ru^VI(N₂O₂)(O)₂²⁺ and (1.41±0.05)±105 M^-1 s^-1 and (1.52±0.04)±104 M^-1 s^-1 respectively for Ru^VI(tmc)(O)₂²⁺. The kinetic isotope effects kfBNAH/kfd2-BNAH = (4.0±0.4) for Ru^VI(N₂O₂)(O)₂²⁺ and kfBNAH/kfd2-BNAH = (3.7±0.2) for Ru^VI(tmc)(O)₂²⁺. However, no kinetic isotope effects were observed for the second phase of the reactions.In the C-H activation of AcrH₂, the major product in both cases is AcrH+, while trace amount of AcrO was detected for Ru^VI(N₂O₂)(O)₂²⁺, but no AcrO was found in the case of Ru^VI(tmc)(O)₂²⁺. Altogether 1 mol of AcrH+ was found to produce per mol of Ru^VI. For the C-H activation of BHAH by Ru^VI(N₂O₂)(O)₂²⁺ and Ru^VI(tmc)(O)₂²⁺, BNA+ is the only product. An excellent linear correlation between log(rate constant) for the first phase for reaction of AcrH₂ and BNAH by Ru^VI(N₂O₂)(O)₂²⁺ and the C-H BDEs of AcrH₂, BNAH and other alkylaromatic compounds is obtained, which strongly supports a HAT mechanism.In Part II, the oxidation of alcohols by KMnO₄ in CH₃CN is greatly accelerated by various Lewis acids, including BF₃, Sc(CF₃SO₃)₃, Zn(CF₃SO₃)₂, Ca(CF₃SO₃)₂ and Ba(CF₃SO₃)₂. These Lewis acids accelerate the oxidation of CH₃OH by KMnO₄ by 3-7 orders of magnitude. Spectrophotometric and kinetics studies show that BF₃·CH₃CN forms an adduct with KMnO4 in CH₃CN, [BF₃·MnO₄] -, which are the active species responsible for the oxidation of alcohols. The pseudo-first-order rate constant, kobs, increases linearly with [BF₃], and increases with [CH₃OH], but reaches saturation at high [CH₃OH] with the plot of 1/kobs vs 1/[CH₃OH] is linear. The rate law is:At 298 K, KCH₃OH and kKBF₃ are found to be (7.06±0.31)×10² M^-1 and (2.67±0.07)×10² M^-1 s^-1 respectively.The effects of other Lewis acids on the oxidation of CH3OH by KMnO4 have also been investigated. The rate law is: Rate = k₂[MnO₄][CH₃OH] for these Lewis acids, and no saturation kinetics are observed. kobs, increases linearly with [CH₃OH].Iodometric tritration and organic product analysis of cyclohexanol and benzyl alcohol oxidation showed that MnO4—was reduced to be Mn²⁺ rather than MnO2. In the absence of BF₃, brown precipitate of MnO₂ was observed at the end of the reaction. Apparently the Lewis acids can also activate the intermediate MnO2 to carry out further oxidation. In the oxidation of cyclobutanol by BF₃/MnO₄^-, cyclobutanone is formed exclusively, suggesting a two-electron hydride abstraction mechanism.DFT calculations have been performed on the oxidation of CH₃OH and PhCH(OH)CH₃ by MnO₄^-,BF₃·MnO₄^- and 2BF₃·MnO₄^- in order to gain more insight into the mechanism. The reaction barriers are substantially lowered in the presence of BF₃, consistent with the observed accelerating effects.