The osmium-catalyzed dihydroxylation of various olefins using molecular oxygen or air as the
stoichiometric oxidant is reported. Aromatic olefins yield the corresponding diols in good to excellent
chemoselectivities under optimized pH conditions (pH = 10.4−12.0). Air can be used under moderate pressures
(3−9 bar) instead of dioxygen as the reoxidant. By increasing the oxygen content of the solution, it is possible
to achieve highly efficient conversion at low catalyst amount (catalyst/substrate = 1:4000). Tri- and
tetrasubstituted olefins are oxidized at pH > 11 to give the corresponding 1,2-diols in good to very good
yields without requiring the addition of sulfonamides or other hydrolysis agents. Studies of the dihydroxylation
of functionalized olefins demonstrate that the reaction conditions tolerate a variety of functional groups. In the
presence of dihydroquinine or dihydroquinidine derivatives (Sharpless ligands), asymmetric dihydroxylations
occur with lower enantioselectivities than tose of the classical K3[Fe(CN)6] reoxidation system.
Asymmetric epoxidation of olefins with 30 % H2O2 in the presence of [Ru(pybox)(pydic)] 1 and [Ru(pyboxazine)(pydic)] 2 has been studied in detail (pybox = pyridine-2,6-bisoxazoline, pyboxazine = pyridine-2,6-bisoxazine, pydic = 2,6-pyridinedicarboxylate). 35 Ruthenium complexes with sterically and electronically different substituents have been tested in environmentally benign epoxidation reactions. Mono-, 1,1-di-, cis- and trans-1,2-di-, tri-, and tetra-substituted aromatic olefins with versatile functional groups can be epoxidized with this type of catalyst in good to excellent yields (up to 100 %) with moderate to good enantioselectivies (up to 84 % ee). Additive and solvent effects as well as the relative rate of reaction with different catalysts have been established. It is shown that the presence of weak organic acids or an electron-withdrawing group on the catalyst increases the reactivity. New insights on the reaction intermediates and reaction pathway of the ruthenium-catalyzed epoxidation are proposed on the basis of density functional theory calculation and experiments.
A small ligand library of chiral tridentate N,N,N-pyridinebisimidazolines have been synthesized for the first time. This new class of ligands can be easily tuned and synthesized on multi g-scale. The usefulness of the ligands is shown in the ruthenium-catalyzed asymmetric epoxidation with hydrogen peroxide as oxidant. Excellent yields (>99%) and good enantioselectivities (up to 71% ee) have been obtained for the epoxidation of aromatic olefins. [reaction: see text]
Novel pyboxazines and the known pybox ligands are used in the ruthenium‐catalyzed asymmetric epoxidation of olefins with H2O2 (see scheme). This new catalytic system is successful in the conversion of differently substituted aromatic olefins and gives ee values of up to 84 %.
[reaction: see text] The complex [Ru(tpy)(pydic)] (1a) is an active catalyst for epoxidation of alkenes by aqueous 30% hydrogen peroxide in tertiary alcohols. The protocol is simple to operate and gives the corresponding epoxides in good to excellent yields. Chiral enantiopure [Ru(tpy)(pydic)] complexes have been synthesized and successfully applied in this procedure.
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