Heterocyclic Hydroperoxides in Selective Oxidations

Hydroxylation and ring-opening mechanism of an unusual flavoprotein monooxygenase, 2-methyl-3-hydroxypyridine-5-carboxylic acid oxygenase: a theoretical study

Hybrid meta-GGA density functional theory (the MPWB1K functional) was used to study the hydroxylation and ring-opening mechanism of 2-methyl-3-hydroxypyridine-5-carboxylic acid oxygenase (MHPCO). This enzyme catalyses the conversion of 2-methyl-3-hydroxypyridine-5-carboxylic acid (MHPC) to alpha-(N-acetylaminomethylene)succinic acid (AAMS), which is the essential ring-opening step in the bacterial degradation of vitamin B(6). MHPCO belongs to the flavin-containing aromatic hydroxylases family. However, MHPCO is capable of catalysing a subsequent aromatic ring-cleavage reaction to give acyclic products rather than hydroxylated aromatic ones. Our calculations show that the re-aromatisation of the hydroxylated intermediate occurs spontaneously in aqueous solution; this implies that the ring-opening process occurs inside the enzyme's active site, in which limited water is available. The instability of the hydroxylated intermediate of MHPCO is the main reason why acyclic products are formed. Previously proposed mechanisms for the ring-opening step were studied, and were shown to be less likely to occur (DeltaDeltaG(not equal298)>35 kcal mol(-1)). Two new pathways with reasonable barrier heights (DeltaDeltaG(not equal 298)<15 kcal mol(-1)) are reported herein, which are in accordance with all experimental information present to date.

Catalytic effect of alloxazinium and isoalloxazinium salts on oxidation of sulfides with hydrogen peroxide in micellar media

Three novel amphiphilic alloxazinium salts were prepared: 3-dodecyl-5-ethyl-7,8,10-trimethylisoalloxazinium perchlorate (1c), 1-dodecyl-5-ethyl-3-methylalloxazinium perchlorate (2b), and 3-dodecyl-5-ethyl-1-methylalloxazinium perchlorate (2c). Their catalytic activity in thioanisole (3) oxidation with hydrogen peroxide was investigated in micelles of sodium dodecylsulfate (SDS), hexadecyltrimethylammonium nitrate (CTANO3) and Brij 35. Reaction rates were strongly dependent on the catalyst structure, on the type of micelles, and on pH value. Alloxazinium salts 2 were more effective catalysts than isoalloxazinium salts 1. Due to the contribution of micellar catalysis, the vcat/v0 ratio of the catalyzed and non-catalyzed reaction rates was almost 80 with salt 2b solubilized in CTANO3 micelles. Nevertheless, the highest acceleration was observed with non-amphiphilic 5-ethyl-1,3-dimethylalloxazinium perchlorate (2a) in CTANO3 micelles (vcat/v0 = 134). In this case, salt 2a presumably acts as a phase-transfer catalyst bringing hydrogen peroxide from the aqueous phase into the micelle interior. Synthetic applicability of the investigated catalytic systems was verified on semi-preparative scale.

Synthesis of rigidified flavin-guanidinium ion conjugates and investigation of their photocatalytic properties

Flavin chromophores can mediate redox reactions upon irradiation by blue light. In an attempt to increase their catalytic efficacy, flavin derivatives bearing a guanidinium ion as oxoanion binding site were prepared. Chromophore and substrate binding site are linked by a rigid Kemp's acid structure. The molecular structure of the new flavins was confirmed by an X-ray structure analysis and their photocatalytic activity was investigated in benzyl ester cleavage, nitroarene reduction and a Diels-Alder reaction. The modified flavins photocatalyze the reactions, but the introduced substrate binding site does not enhance their performance.

Selective decomposition of alkyl hydroperoxides on H-type zeolite via a concerted approach

Catalyzed by H-type zeolite, the selective decomposition of alkyl hydroperoxides (HP), including 1-phenylethyl hydroperoxide (PEHP), phenylmethyl hydroperoxide (PMHP), and methyl hydroperoxide (MHP), to their corresponding carbonyl compounds and H2O have been investigated by DFT calculations. In this calculation, a 19-atom cluster containing four tetrahedral atoms (denoted as 4T) was used to model the H-type zeolite catalyst, and two competitive reaction channels were evaluated. Calculations predict that the concerted reaction channel is favorable for selective decomposition of alkyl hydroperoxides on the H-type zeolite from both kinetics and thermodynamics points of view. We found that the rate-determining step of the stepwise reaction channel proceeds via a one-hydrogen-bond transition state followed by dehydration. In the concerted reaction channel, whose transition state is an eight-membered ring with two hydrogen bonds, the decomposition of hydroperoxides occurs synchronously to form carbonyl compounds and H2O, involving the proton exchange between hydroperoxides and the zeolite.