Zoltán Szeverényi

Formation of manganate(V) in oxidations by permanganate ion in strongly alkaline solutions

Abstract The oxidation of tartronate and mesoxalate ion by permanganate has been studied in 0.1–2.0 M aqueous NaOH. Manganate(VI) ion is formed in the first rapid phase, which is kinetically first order with respect to both MnO 4 − and the substrate. The second-order rate constant depends on the OH − ion concentration, which implies higher reactivity of the species with deprotonated 2-hydroxy groups. The proposed mechanism consists of outer sphere electron transfer to the MnO 4 − ion, generating an anion-radical and manganate(VI). The latter reacts with both substrates in the second, slower phase, affording manganate(V) as a relatively stable product in strong alkali. The anion-radical is oxidized by MnO 4 − in a rapid step.

Kinetics of the oxidation of 2-aminophenol by dioxygen in the presence of tetrakis(3,5-di-t-butyl-4-hydroxyphenyl)-dodecachlorophthalocyaninatocobalt(II)

Abstract The kinetics and mechanism of the O2 oxidation of 2-aminophenol (H2AP) to 2-aminophenoxazin-3-one (APX) under ambient conditions, catalyzed by the recently synthesized tetrakis(3,5-di-t-butyl-4-hydroxyphenyl)-dodecachlorophthalocyaninatocobalt(II), (R4PcCo), have been studied by spectrophotometry. The rate of APX formation is first-order in [R4PcCo] and obeys Michaelis—Menten type kinetics with respect to [H2AP]. The suggested mechanism involves rate-determining inner-sphere electron transfer from coordinated H2AP to coordinated O2 in the superoxo complex.

Kinetics and mechanism of the activation of molecular hydrogen by bis(dimethylglyoximato)cobalt(II) derivatives

The reaction of H2 with bis(dimethylglyoximato)cobalt(II), [Co(Hdmg)2], and its 1 : 1 pyridine (py) adduct yields 3-aminobutan-2-one oxime via hydrogenation of the co-ordinated Hdmg–. Excess of Hdmg– is hydrogenated catalytically. The initial rate of H2 uptake is first order with respect to H2 pressure and second order with respect to overall cobalt concentration. Axial co-ordination of py, NEt2H, and PPh3 enhances the rate, the five-co-ordinate adducts being more reactive than the parent cobaloxime(II). The spectrophotometric stability constants for the 1 : 1 and 1 : 2 py adducts are 186 and 0.75 dm3 mol–1, respectively, at 20 °C. The rate-determining step of H2 uptake is a bimolecular reaction of two hydridocobaloxime(III) species. In the absence of py, the overall rate coefficient is 3.1 and 9.4 dm6 mol–2 s–1 in MeOH and 50% v/v methanol–water, respectively, at 20 °C. For the reactions involving the 1 : 1 adduct with py, the corresponding values are 34.7 and 323 dm6 mol–2 s–1, respectively. Hydrogenation of the intermediate, containing one CH–NHOH moiety, is interpreted in terms of N–O bond cleavage by cobaloxime(II) followed by reduction of the resulting cobaloxime(III) by hydridocobaloxime(III).

Metal ion promoted oxidation of a methyl group of 2,3-dihydro-2,2,4-trimethyl-1H-1,5-benzodiazepine

Abstract Metal ions such as Co2+, Mn2+, Ni2+, Zn2+ and Ca2+ have been found to catalyze the condensation of o-phenyleriediamine (1) with acetone, affording 2,3-dihydro-2,2,4-trimethyl-1H-1,5-benzodiazepine (2). In the presence of MnCl2, 2 is oxidized catalytically by O2 to the hitherto unknown 1,2-dihydro-2,2,4-trimethyl-3H-1,5-benzodiazepin-3-one, 5, (1,3,4,5-tetrahydro-4,4-dimethyl-2H-1,5-benzodiazepin-2-ylidene)methyl 2,3-dihydro-2,2-di-methyl-1H-1,5-benzodiazepin-4-yl ketone, 4 and to 5-formyl-1,3,4,5-tetrahydro-4,4-dimethyl-2H-1,5-benzodiazepin-2-one, 3. According to mechanistic studies, the reaction sequence leading to 4 involves an aldol condensation of 2 with 4-formyl-2,3-dihydro-2,2-dimethyl-1H-1,5-benzodiazepine, 6. In this reaction the expected product is not obtained, because instead of water elimination, the intermediate undergoes tautomerization to (1,3, 4,5-tetrahydro-4,4-dimethyl-2H-1,5-benzodiazepin-2-ylidine)methyl 2,3,4,5-tetrahydro-2, 2-dimethyl-1H-1,5-benzodiazepin-4-yl ketone, 10, which is followed by dehydrogenation with the Mn2+/O2 system, thus yielding 4.

A new organocobaloxime with an acetoxy substituted tertiary carbon to cobalt bond, derived from enolpyruvic acid

Abstract The reaction of pyridinecobaloxime(II) with methyl 2-acetoxyacrylate under H 2 atmosphere leads to a novel organocobaloxime containing an acetoxy substituted tertiary α-carbon. The reaction occurs via addition of the hydridocobaloxime across the double bond.

Synthesis, characterization and decomposition of 2-aryl-2-hydroxyethylcobaloximes

Abstract 2-Phenyl-2-hydroxyethyl(aquo)cobaloxime(Ia) and the p -methyl (Ib) and p -cyano (Ic) derivatives have been synthesized and characterized. The parent compound and the p -methyl derivative decompose spontaneously both in the solid state and in methanol to give mixtures of the appropriate styrenes and acetophenones. Kinetics and product ratios of the decomposition of Ia and Ib in methanol have been studied as a function of temperature. Evidence is presented that both reaction pathways are ionic in nature and that an intermediate (presumed to be a phenethylcobaloxime carbonium ion) is involved in the styrene forming pathway. Acetophenones are apparently formed via a 1,2-hydride shift mechanism with solvent acting as a general base. Both reaction pathways show a large substituent effect with electron donating substituents increasing reactivity, Ic proved to be extraordinarily stable in methanol but decomposed readily in aqueous sulfuric acid to produce primarily p -cyanoacetophenone. Kinetic evidence for formation of a cationic intermediate is presented.

Kinetics and mechanism of alkali-induced decomposition of 2-alkoxyethylcobaloximes

Abstract Kinetics of the base-induced decomposition of five 2-alkoxyethyl(aquo)cobaloximes, ROCH 2 CH 2 - Co(D 2 H 2 )OH 2 (R = C 6 H 5 , CF 3 CH 2 , CH 3 , CH 3 CH 2 , (CH 3 ) 2 CH), have been studied manometrically in aqueous base, ionic strength 1.0 M (KC1) at 25.0± 0.1 °C under an argon atmosphere. For the complexes with good leaving group alkoxide substituents (R = C 6 H 5 and CF 3 CH 2 ) the reactions are first- order in cobaloxime and first-order in hydroxide ion and produce stoichiometric amounts of ethylene and leaving group alcohol (ROH). NMR observation of decomposing solutions and workup of cobalt chelate products show that the reaction is initiated by hydroxide ion attack on an equatorial quaternary carbon leading to formation of an altered cobal- oxime product in which one of the Schiffs base linkages has become hydrated. For the remainer of the complexes the yield of ethylene is less than stoichiometric and pH-dependent, and the ethylene evolving reaction is second-order in hydroxide ion activity. The yield-limiting side reaction is shown to be base-catalyzed formation of a base-stable but photolabile alkoxyethylcobaloxime analog in which a Schiffs base linkage of the chelate has become hydrated, β-Elimination to form alkyl vinyl ethers was not observed for any of the alkoxyethylcobal- oximes. The second-order dependence of ethylene formation on hydroxide ion activity for R = CH 3 , CH 2 CH 3 , and CH(CH 3 ) 2 is discussed at some length, but is not well understood at present.

Oxidation of a methyl to a formyl group in 2,3-dihydro-2,2,4-trimethyl-1H-1,5-benzodiazepine by O2 in the presence of metal ions

Abstract The mechanistic details of the metal ion catalyzed O 2 oxidation of 2, 3-dihydro-2, 2, 4-trimethyl-1H-1, 5-benzodiazepine are discussed.

Cobalt phthalocyanine catalysis in autoxidation of 2-aminophenol by O2

Abstract The kinetics and mechanism of the O2-oxidation of 2-aminophenol (H2AP) to 2-aminophenoxazine-3-one (APX) under ambient conditions, catalyzed by the recently synthesized tetrakis(3, 5-di-tert-butyl-4-hydroxyphenyl)dodecachlorophthalocyaninatocobalt(II). (R4PcCo), have been studied by spectrophotometry. The rate of APX formation is first-order in [R4PcCo] and obeys a Michaelis-Menten type kinetics with respect to [H2AP]. The suggested mechanism involves rate-determining metal ion mediated electron transfer from coordinated H2AP to coordinated o2 in the superoxo complex.