Pekka Pietikäinen

CONVENIENT ASYMMETRIC (SALEN)MN(III)-CATALYZED EPOXIDATION OF UNFUNCTIONALIZED ALKENES WITH HYDROGEN PEROXIDE USING CARBOXYLATE SALT COCATALYSTS

Abstract Asymmetric epoxidation of unfunctionalized alkenes is reported using chiral (salen)Mn(III) complexes 1–5 together with a carboxylate salt cocatalyst in the presence of either aqueous H 2 O 2 or anhydrous urea-H 2 O 2 adduct as oxidant. Several simple soluble salts (acetates, formates, benzoates) were studied all giving good yields of epoxides with moderate to excellent enantioselectivity. For example, 1,1-diphenyl-1-propene was converted into the corresponding epoxide of 96 % ee in 84 % yield. Generally, this epoxidation method gave better results than a previously described system using nitrogen heterocycles as cocatalysts.

Catalytic and asymmetric epoxidation of unfunctionalized alkenes with hydrogen peroxide and (Salen)Mn(III) complexes

Abstract Catalytic epoxidation of two unfunctionalized alkenes, 1,2-dihydronaphthalene and trans-β-methylstyrene, is reported using achiral and chiral (Salen)Mn(III) complexes 1–3 together with a nitrogen heterocycle as axial ligand in the presence of 30 % aqueous hydrogen peroxide as oxidant. Typically, the reaction system consisted of the substrate, oxidant, ligand, and salen in a ratio of 1: 2–3.5: 0.4: 0.025–0.05. The best ligands were imidazole and N-methyl imidazole. The highest ee-values obtained were 60 % for 1,2-dihydronaphthalene oxide and 47 % for trans-β-methylstyrene oxide.

Asymmetric epoxidation of unfunctionalized alkenes with periodates catalyzed by chiral (salen)Mn(III) complexes

Sodium and tetra-n-butylammonium periodates were used as oxidants in the asymmetric epoxidation of unfunctionalized alkenes with chiral (salen)Mn(III) complexes 1–2 together with imidazole as donor ligand. Reaction system used consisted of the alkene, oxidant, donor ligand, and salen in ratios of 1: 2.5: 0.75: 0.06. Both cis- and trans-alkenes were epoxidized with reasonable yield and enantioselectivity (up to 85% ee for 2,2-dimethylchromene).

Asymmetric Mn(III)-salen catalyzed epoxidation of unfunctionalized alkenes with in situ generated peroxycarboxylic acids

Abstract Unfunctionalized aromatic alkenes were enantioselectively epoxidized with peroxycarboxylic acids prepared in situ from urea-H2O2 (and other anhydrous adducts of H2O2) and carboxylic acid anhydrides (maleic, phthalic, and acetic anhydride) using chiral Mn(III)-salen complexes as catalysts and N-methylmorpholine N-oxide (NMO) as an additive. Experimental results were compared with those reported earlier that employed aqueous hydrogen peroxide as the primary oxidant and the method presented here was found to offer both higher enantioselectivities and shorter reaction times. This novel epoxidation system was also compared with the Jacobsen’s MCPBA/NMO system, and some differences in reactivity and selectivity were observed. These differences could possibly be explained assuming the presence of alternative mechanistic pathways during the catalytic cycle of the asymmetric epoxidation.

Salen Complexes with Bulky Substituents as Useful Tools for Biomimetic Phenol Oxidation Research

The catalytic properties of bulky water-soluble Co-, Cu-, Fe- and Mn-salen complexes in the oxidation of phenolic lignin model compounds have been studied in aqueous water--dioxane solutions (pH 3--10). Mn catalysts were found to oxidize coniferyl alcohol in a same reaction time as horseradish peroxidase (HRP) enzyme and Mn and Co catalysts showed different regioselectivity suggesting a different substrate to catalyst interaction in the oxidative coupling. When the oxidation of material more relevant to plant polyphenolics was studied, the results indicated that the complexes catalyze one- and two-electron oxidations depending on the bulk of the substrate.

Synthesis and catalytic activity of new chiral unsymmetrical Mn(III)-Schiff-base complexes containing salicylaldehyde and 1-(2-hydroxyphenyl)ketone units

Abstract Two new chiral unsymmetrical (non-C 2 -symmetric) Schiff-base ligands containing salicylaldehyde and 1-(2-hydroxyphenyl)ketone units were synthesized from ( R , R )-1,2-diphenylethylenediamine as the chiral diamine using a stepwise approach. The Mn(III) complexes of the ligands were subsequently used as catalysts in asymmetric epoxidation of unfunctionalized alkenes and the results were compared with those obtained using typical C 2 -symmetric Mn(III)–salen complexes. Possible reasons for the differences in reactivity and selectivity between the two types of catalysts are briefly discussed.

Asymmetric Mn(III)-salen catalyzed epoxidation of unfunctionalized alkenes with tetrabutylammonium monopersulfate

Abstract Asymmetric Mn(III)-salen catalyzed epoxidation of simple cis -disubstituted and trisubstituted alkenes in mild conditions was performed using tetrabutylammonium monopersulfate (Bu 4 NHSO 5 ) as the oxidant together with N -methylmorpholine N -oxide as an additive. Particularly high yields of epoxides (up to 97 %) and good enantiomeric excesses (ee up to 93 %) were obtained in the epoxidation of 2,2-dialkylchromenes and trisubstituted alkenes.

Synthesis and characterization of bulky salen-type complexes of Co, Cu, Fe, Mn and Ni with amphiphilic solubility properties

Several new bulky salen-type Schiff base ligands and their complexes with first-row transition metals Co, Cu, Fe, Mn and Ni have been synthesized and characterized. The ligands contain tert-butyl and methyl(triphenylphosphonium chloride) substituents in aromatic rings providing flexible solubility properties. Crystal structures of some of the complexes were determined.

Asymmetric Epoxidation of Unfunctionalized Alkenes with Ammonium and Phosphonium Monopersulfates Catalyzed by Chiral Mn(III)–Salen Complexes

Abstract Simple cis-disubstituted and trisubstituted alkenes were enantioselectively epoxidized in mild conditions using various Mn(III)–salen complexes as catalysts and quaternary ammonium and phosphonium monopersulfates (Bu4NHSO5, Ph4PHSO5) as oxidants together with amine N-oxides as additives. The effect of the catalyst structure on the stereochemical outcome of the epoxidation reactions was studied. Generally, the 1,2-diphenylethylenediamine-derived complexes were found to give higher asymmetric induction compared to their 1,2-diaminocyclohexane-derived counterparts. Particularly high yields of epoxides (up to 98%) and good enantiomeric excesses (ee up to 93%) were obtained in the epoxidation of 2,2-dialkylchromenes and trisubstituted alkenes.