Synthesis and molecular structure of [RuCl{C(CHPh)OC(O)CH2CH3}(CO)(PPh3)2]: a real intermediate in ruthenium complex-catalyzed selective synthesis of a (Z)-enol ester

Sterically encumbered monomeric transition-metal complexes were examined as epoxidation catalysts. Bulky ligands such as tetradentate triamidoamines of the type [(RNCH2CH2)3N]3− and tridentate tris(3-t-butylpyrazolyl)borate were used to diminish the deactivation of the catalysts via μ-oxo dimerization. Several monomeric manganese and iron complexes, including M[(RNCH2CH2)3N] [R=trimethylsilyl: M=Mn3+ (1); R=C6F5: M=Fe3+ (2), Mn3+ (3)], {η3-HB(3-tBuPz)3}MCl [M=Fe2+ (4), Mn2+ (6)] and {η3-HB(3-tBuPz)3}FeOTf (5), were synthesized and examined as possible catalysts for the selective epoxidation of norbornylene and styrene. Oxygen atom sources were PhIO or molecular O2–isobutyraldehyde. With PhIO as the oxidant, complexes 1, 3, and 6 oxidized norbornylene and styrene catalytically to the corresponding epoxides in high selectivity, while complexes 2 and 4 exhibited low epoxidation activity. By using O2 with isobutyraldehyde as a co-reductant, complexes 1, 3, 4, and 6 showed efficient catalytic activity with high selectivity and reaction rate for epoxidation, in comparison to control experiments without a catalyst. Complex 4 with O2 and isobutyraldehyde also catalyzed the epoxidation of stilbenes to epoxides. Both cis- and trans-stilbenes are converted mainly to the trans-stilbene oxide. The possible role of the metal complexes as the active epoxidation catalysts is discussed.