LászlóI. Simándi

Kinetics and mechanism of the cobaloxime(II) catalyzed oxidative dehydrogenation and double bond cleavage of 3,3′,5,5′-tetra-tert-butyl-4,4′-dihydroxystilbene by O2

In the presence of the cobaloxime(II) catalyst [Co(Hdmg) 2 (Ph 3 P) 2 ], at room temperature and 1 atm O 2 or air, the dihydroxystilbene derivative referred to in the title (H 2 StQ) undergoes oxidative dehydrogenation to the corresponding stilbenequinone (StQ), and parallel oxidative cleavage at the C=C double bond to afford 2,6-di-tert-butyl-4-hydroxybe-nzaldehyde (Ald). The two products are formed via a common anion radical intermediate stable enough for detection by ESR spectroscopy. A detailed kinetic analysis has been carried out by volumetric, spectrophotometric and HPLC techniques. The observed kinetic behavior is consistent with the formation of a superoxocobaloxime(III) intermediate, which generates the free radical intermediate from H 2 StQ. In the steady state, the catalyst is present in a hydroxo-cobaloxime(III) precursor state, from which it is released via reduction to cobaloxime(II) by the anion radical.

Oxidations with unstable manganese (VI) in acidic solution

Abstract Despite its instability in acidic aqueous solution, acidified Mn(VI) is capable of oxidizing some alcohols and aldehydes. The product composition of the manganese species depends on the nature and the concentration of the substrate and seems to be governed by the relative rates of disproportionation and oxidation reactions involving both Mn(VI) and Mn(V). The high reactivity of Mn(V) and Mn(VI) towards OH-containing substrates indicates the existence of alternative routes to the long-sought hypomanganate and manganate esters which have played a central role as hypothetical intermediates in oxidations by permanganate ion.

RELATIVE REACTIVITIES OF HYDROXY COMPOUNDS WITH SHORT-LIVED MANGANESE(V)

Abstract Short-lived manganese(V) was generated by reduction of the permanganate ion with As(III) in acidic solution. It is unstable against disproportionation, but this can be suppressed by oxidizable substrates such as alcohols, formic acid, phosphorus acid and Mn(II), reacting rapidly with transient Mn(V). The relative reactivities of Mn(V) toward various substrates and manganese(II) were characterized by competition experiments. The possible mechanisms of these fast reactions are discussed.

Addition of radicals to quinones. ESR study of some radical reactions of 3,3′,5,5′-tetra-tert-butylstilbene-4,4′-quinone

Abstract Reactions of 3,3′,5,5′-tetra- tert -butylstilbene-4,4′-quinone with free radicals from the decomposition of azo compounds and peroxides (R . ) have been studied by ESR spectroscopy. Relatively stable phenoxyl-type free radicals are formed via addition of R . across the exo CC bond. Their coupling constants with the methine proton reflect steric effects. Further addition products lead to sterically crowded molecules which spontaneously dissociate into radicals via C(4)C(7) bond cleavage.

Oxidation of propane-1,2-diol by acidic manganese(V) and manganese(VI)

Abstract Propane-1,2-diol (PD) reacts rapidly and selectively with unstable manganese(V) and manganese(VI) in acidic aqueous solution, thereby suppressing the known disproportionation of these species. Manganese(V) was generated by reducing the MnO 4 ion with As(III), whereas manganese(VI) was produced by rapid acidification of a stable alkaline solution in the presence of PD, both in a stopped-flow device. The organic and Mn products were analyzed spectrometrically. The oxidation of propene by MnO 4 yields the same product as PD with Mn(V), which is evidence for a common intermediate and a related mechanism.