Collman James P, Zeng Li, Brauman John I
Department of Chemistry, Stanford University, Stanford, California 94305, USA.
Inorg Chem. 2004 Apr 19;43(8):2672-9. doi: 10.1021/ic035360c.
The stereoselectivity of olefin epoxidation catalyzed by Mn(III)(salen)X (1a, X = Cl(-); 1b, X = BF(4)(-)) complexes is examined in the presence of neutral donor ligands, employing various iodosylarenes (ArIO: PhIO, C(6)F(5)IO, and MesIO) as the oxygen atom source. The cis/trans ratios of stilbene oxides and the enantiomeric excesses of styrene oxide and 1,2-dihydronaphthalene oxide are found to be strongly dependent on the nature of the iodosylarenes under certain conditions. In other cases, olefin epoxidation is shown to proceed with essentially identical diastereoselectivities or enantioselectivities, regardless of the oxygen atom source used. We propose that a Mn(V)(salen)-oxo intermediate and a complex between the catalyst and the terminal oxidant competitively effect the epoxidation when the stereoselectivities are markedly dependent on the oxygen atom source. A single Mn(V)(salen)-oxo species is considered to be the sole oxygenating intermediate when the terminal oxidants do not exert a notable influence on the product selectivities. Our results clearly demonstrate the existence of multiple oxidizing species and the conditions in which only a single oxygenating intermediate is involved. The axial donor ligands (both anionic ligands and neutral ligands) are shown to strongly influence both the identity and the reactivity of the oxygenating species.
在中性供体配体存在的情况下，使用各种芳基碘酰（ArIO：PhIO、C₆F₅IO和MesIO）作为氧原子源，研究了Mn(III)(salen)X（1a，X = Cl⁻；1b，X = BF₄⁻）配合物催化烯烃环氧化反应的立体选择性。在某些条件下，发现氧化苯乙烯和顺式/反式氧化二苯乙烯的对映体过量以及氧化萘的对映体过量强烈依赖于芳基碘酰的性质。在其他情况下，无论使用何种氧原子源，烯烃环氧化反应都显示出基本相同的非对映选择性或对映选择性。我们提出，当立体选择性明显依赖于氧原子源时，Mn(V)(salen)-氧代中间体以及催化剂与末端氧化剂之间的配合物会竞争性地影响环氧化反应。当末端氧化剂对产物选择性没有显著影响时，单一的Mn(V)(salen)-氧代物种被认为是唯一的氧化中间体。我们的结果清楚地证明了多种氧化物种的存在以及仅涉及单一氧化中间体的条件。轴向供体配体（阴离子配体和中性配体）均显示出对氧化物种的特性和反应性有强烈影响。
