The development of efficient methods for preparing chiral epoxides with high optical purity is a topic of great importance in organic synthesis. [1][2][3] As part of our studies on titanium-catalyzed asymmetric oxidation reactions using hydrogen peroxide as an oxidant, [4][5][6] we recently reported that the readily available salan compound 1 serves as an effective ligand for the titanium-catalyzed asymmetric epoxidation of unfunctionalized olefins (Scheme 1 a). [7] On the other hand, the corresponding titanium(salen) complexes (in which double bonds replace the single bonds between the benzylic carbon atoms and the nitrogen atoms of the ligand) have been found to react with hydrogen peroxide to form a peroxo complex that is responsible for sulfide oxidation but is inert to olefin epoxidation. On the basis of these findings, we proposed that the titanium(salan) complexes provide more reactive peroxo species, which arise from a hydrogen bond between one of the peroxo oxygen atoms and the amino proton (Scheme 1 b). Although the participation of the hydrogen-bonded peroxo species would explain the sharp contrast in reactivities, the detailed reaction mechanism was unclear. Herein, we report the synthesis of an isolable m-oxom-h 2 :h 2 -peroxo titanium complex that plays an important role as a reservoir of catalytically active species for asymmetric epoxidation.We have reported that the premade di-m-oxo titanium-(salan) catalyst with ligand ent-1 a (the enantiomer of 1 a) and the catalyst prepared in situ from Ti(OiPr) 4 and salan ent-1 a gave comparable yields and enantioselectivities in the asymmetric epoxidation of olefins, and the X-ray structure analysis of the di-m-oxo complex with ent-1 a revealed its homochiral structure, in which each cis-b titanium(salan) unit (in which one oxygen atom of the four coordinating atoms occupies an axial position) has the same chirality with respect to the titanium center (anti-L,R,R,S N ,S N -L,R,R,S N ,S N ). [7a, 8] We have also found that the catalyst prepared in situ with salan 1 b showed a higher catalytic performance. Thus, we decided to evaluate the asymmetric catalysis of its di-m-oxo titanium complex. Since ligand ent-1 a resulted only in the di-m-oxo complex with the (anti-L,R,R,S N ,S N -L,R,R,S N ,S N ) configuration, we expected the production of a single isomer. [9] Contrary to our expectations, however, salan 1 b resulted in three di-m-oxo complexes (2, 3, and 4; Scheme 2), although complex 2 was the major product. Fortunately, each structure was unambiguously characterized by X-ray crystallographic analysis.[10] While complex 2 has a homochiral (anti-D,S,S,R N ,R N -D,S,S,R N ,R N ) configuration identical to the di-moxo complex with 1 a, complexes 3 and 4 are pseudoheterochiral, as the titanium centers in the two units have opposite chirality (anti-L,S,S,S N ,R N -D,S,S,R N ,R N for 3; anti-L,S,S,S N ,S N -D,S,S,R N ,R N for 4; see Figure 1 and Figures S1 and S2 in the Supporting Information). Complexes 3 and 4 differ in the stereochemistry of the chiral nitr...

Figure 4. Bridging core structure of complex 5. N blue, O red, Ti purple. Selected bond lengths [] and angles [8]: Ti1-O1 1.848(2), Ti1-O2 2.051(2), Ti1-O3 2.017(2), Ti2-O1 1.876(2), Ti2-O2 2.030(2), Ti2-O3 2.046(2), O2-O3 1.449(3); Ti1-O1-Ti2 98.09(11), Ti1-O2-Ti2 87.15(9), Ti1-O3-Ti2 87.61(9).Scheme 3. Asymmetric epoxidation with complex 5.Scheme 4. A possible reaction pathway. Scheme 5. Asymmetric epoxidation with Ti(OiPr) 4 and salalen 6.

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The X-ray structures and electrochemical properties of the dinuclear cobaltadithiolene complexes, μ-(1,4-disubstituted 1,3-butadiene-1,2,3,4-tetrathiolato)-bis[(η5-cyclopentadienyl)cobalt(III)], have been investigated. In the diphenyl derivative, two benzene rings in the same molecule are located face to face with the distance of 3.5 Å, while in the dimethyl derivative, two methyl groups are in remote positions. However, in the electrically neutral dinuclear complexes, there is no evidence for the intramolecular interaction between two aryl groups in solutions. Electrochemical and spectrochemical investigations revealed that in electrically neutral state, the interaction between two cobaltadithiolene moieties is small, while in monoanioic state it becomes greater. The Hammett’s plot for ΔEr1/2 (ΔEr1/2 = Er1/2(red.1) − Er1/2(red.2)) shows that the electron-attracting substituents can stabilize the mixed-valence state, [Co(II)–Co(III)]−. The smaller ΔEr1/2 values for the complexes having aryl substituents are due to the smaller interactions between the anioic and neutral moieties, because of the steric hindrance of aryl groups which inhibit the conjugation of the two cobaltadithiolene rings.

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