Olefin complexes with achiral catalysts

Catalytic enantioselective alkene metathesis has recently been developed as a powerful method for the synthesis of complex natural products [86]. The availability of various chiral catalysts for olefin metathesis provides more flexible and concise means to construct efficiently highly functionalized and enantiomerically pure frameworks than using achiral catalytic complexes with chiral nonracemic substrates. 

Despite the tremendous success of hydroxyl-directed asymmetric epoxidation of allylic alcohols and homoallylic alcohols, the development of efficient asymmetric epoxidation methods for unfunctionalized olefins is still of great importance. In the mid-1980s, Kochi and co-workers studied achiral epoxidation of unfunctionalized olefins using achiral Cr(III)-salen and Mn(lll)-salen comlexes as catalysts, and they proposed that a high valent metal oxo (such as 0=Cr(V)-salen and 0=Mn(V)-salen species) as the reactive intermediate was responsible for the epoxidation of olefins. Such a reactive intermediate was formed in a catalytic cycle by oxidation of the catalyst (such as Cr(III)-salen or Mn(lll)-salen complex) with the stoichiometric oxidant (such as PhlO or NaOCl). ... 

In 2002, Hoveyda et al. reported the synthesis, structure and reactivity of a chiral bidentate Ru-based catalyst 65, bearing a binaphthyl moiety, for olefin metathesis [33]. Preference for a bidentate chiral imidazolinylidene was based on the hypothesis that such a ligand would induce chirality more efficiently. This catalyst was designed by analogy with similar achiral complexes 66 that... 

Many efforts have been made to develop salen catalysts for the epoxidation of unfunctionalized olefins, and such work has been well documented.93 Very recently, Ito and Katsuki94 proposed that the ligand of the oxo salen species is not planar, but folded as shown in Figure 4-7 (R/ / H, R2 = H, L = achiral axial ligand). This folded chiral structure amplifies asymmetric induction by the Mn-salen complex. This transition state proposed by Ito and Katsuki is not compatible with the proposal by Palucki et al.95 that the salen ligands of oxo species are planar. 

Complexes of rhodium compounds with enantiomerically pure phosphines facilitate enantiose-lective hydroboration of prostereogenic olefins with 1,3,2-benzodioxaborole (catecholboranc), an achiral hydroborating agent. The reactions are carried out using a 1 1.1 -1.2 molar ratio of the olefin and catecholboranc in the presence of 1 mol % of the catalyst which is generated in situ111"113. The following catalysts are used ... 

The titanium catalysts have been widely used in the catalysis of polymerization of olefins [335], the details of their applications will not be discussed here. Chiral titanium complexes have also been employed as the catalysts for asymmetric polymerization of a variety of carbodiimides by Novak [336]. As shown in Scheme 14.149, the polymerization of achiral carbodiimides in the presence of a chiral titanium complex derived from 3,3 -Br2BINOL and Ti(O Bu)4, gave the well-defined regio- and stereoregular helical polyguanidines with a relatively... 

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