Kinetic and mechanistic studies of reactive intermediates in photochemical and transition-metal assisted oxidation, decarboxylation, and alkyl transfer reactions

Reactive species like high-valent metal-oxo complexes and carbon and oxygen centered radicals are important intermediates in enzymatic systems, atmospheric chemistry, and industrial processes. Understanding the pathways by which these intermediates form, their relative reactivity, and their fate after reactions is of the utmost importance. Herein are described the mechanistic detail for the generation of several reactive intermediates, synthesis of precursors, characterization of precursors, and methods to direct the chemistry to more desirable outcomes yielding `greener\u27 sources of commodity chemicals and fuels. High-valent Chromium from Hydroperoxido-Chromium(III) The decomposition of pentaaquahydroperoxido chromium(III) ion (hereafter CraqOOH2+) in acidic aqueous solutions is kinetically complex and generates mixtures of products (Craq3+, HCrO4-, H2O2, and O2). The yield of high-valent chromium products (known carcinogens) increased from a few percent at pH 1 to 70 % at pH 5.5 (near biological pH). Yields of H2O2 increased with acid concentration. The reproducibility of the kinetic data was poor, but became simplified in the presence of H2O2 or 2,2′-azinobis(3-ethylbenzothiazoline-6-sulfonate) dianion (ABTS2-). Both are capable of scavenging strongly oxidizing intermediates). The observed rate constants (pH 1, [O2] ≤ 0.03 mM) in the presence of these scavengers are independent of [scavenger] and within the error are the same (k,ABTS2- = (4.9 +/- 0.2) Ã? 10-4 s-1 and kH2O2 = (5.3 +/- 0.7) Ã? 10-4 s-1); indicating involvement of the scavengers in post-rate determining steps. In the presence of either scavenger, decomposition of CrOOH2+ obeyed a two-term rate law, kobs / s-1 = (6.7 +/- 0.7) Ã? 10-4 + (7.6 +/- 1.1) Ã? 10-4 [H+]. Effect of [H+] on the kinetics and the product distribution, cleaner kinetics in the presence of scavengers, and independence of kobs on [scavenger] suggest a dual-pathway mechanism for the decay of CraqOOH2+. The H+-catalyzed path leads to the dissociation of H2O2 from Cr(III), while in the H+-independent reaction, CraqOOH2+ is transformed to Cr(V). Both scavengers rapidly remove Cr(V) and simplify both the kinetics and products by impeding formation of Cr(IV, V, VI). Syntheses, Reactivity, and Thermodynamic Considerations LRhR2+ Macrocyclic rhodium(II) complexes LRh(H2O)2+ (L = L1= cyclam and L2 = meso-Me6-cyclam) react with alkyl hydroperoxides R(CH3)2COOH to generate the corresponding rhodium(III) alkyls LRh(H2O)R2+ (R = CH3, C2H5, PhCH2). Methyl and benzyl complexes can also be prepared by bimolecular group transfer from alkyl cobaloximes (dmgX)2(H2O)CoR (where R = CH3, CH2Ph and dmgX is either dimethylglyoxime or a BF2-capped derivative of dmg) to LRh(H2O)2+. When R = C2H5, C3H7 or C4H9, the mechanism changes from group transfer to hydrogen atom abstraction from the coordinated alkyl and produces LRh(H2O)H2+ and an a-olefin. The new LRh(H2O)R2+ complexes were characterized by solution NMR and by crystal structure analysis. They exhibit great stability in aqueous solution at room temperature, but undergo efficient Rh-C bond cleavage upon photolysis. `Green\u27 Model for Decarboxylation of Biomass Derived Acids via Photolysis of in situ formed Metal-Carboxylate Complexes Photolysis of aqueous solutions containing propionic acid and Fe in the absence of oxygen generates a mixture of hydrocarbons (ethane, ethylene and butane), carbon dioxide, and Fe2+. Photolysis in the presence of O2 yields catalytic amounts of hydrocarbon products. When halide ions are present during photolysis; nearly quantitative yields of ethyl halides are produced via extraction of a halide atom from FeX2+ by ethyl radical. The rate constants for ethyl radical reactions with FeCl2+ (k = 4.0 (± 0.5) Ã? 106 M-1s-1) and with FeBr2+ (k = 3.0 (± 0.5) Ã? 107 M-1s-1) were determined via competition reactions. Irradiation of solutions containing aqueous Cu2+ salts and linear carboxylic acids yield a-olefins selectively. This process is made catalytic by the introduction of O2. Photochemical decarboxylation of propionic acid in the presence of Cu2+ generates ethylene and Cu+. Longer-chain acids also yield alpha olefins as exclusive products. In the absence of continued purging with O2 to aid removal of olefin, Cu+(olefin) complexes accumulate and catalytic activity slows dramatically due to depletion of Cu2+. The results underscore the profound effect that the choice of metal ions, the medium, and reaction conditions exert on the photochemistry of carboxylic acids. Free Oxygen Atom in Solution from 4-Benzoylpyridine N-Oxide Excited Singlet Photolysis of 4-benzoylpyridine N-oxide (BPyO) in the presence of quenchers of the triplet excited state produces up to 41% O(3P) (as determined by generation of ethylene upon scavenging with cyclopentene). In the absence of 3BPyO* quenchers a maximum of 13% O(3P) relative to consumed BPyO is obtained. The remaining products are hydroxylated-4-benzoylpyridine and 4-benzoylpyridine. Additionally, the rate of BPyO consumption (as determined by UV-vis) decreases in the presence of 3BPyO* quenching agents. Second order rate constants for 3BPyO* quenching were determined. A mechanism for photochemical deoxygenation of BPyO is proposed on the basis of kinetic data and product distribution under various conditions. Additionally, comparisons are made between the observed intermediates and similar triplet excited states and radical anions