Olefin metal-mediated enantioselective

Metal alkoxides undergo alkoxide exchange with alcoholic compounds such as alcohols, hydro-xamic acids, and alkyl hydroperoxides. Alkyl hydroperoxides themselves do not epoxidize olefins. However, hydroperoxides coordinated to a metal ion are activated by coordination of the distal oxygen (O2) and undergo epoxidation (Scheme 1). When the olefin is an allylic alcohol, both hydroperoxide and olefin are coordinated to the metal ion and the epoxidation occurs swiftly in an intramolecular manner.22 Thus, the epoxidation of an allylic alcohol proceeds selectively in the presence of an isolated olefin.23,24 In this metal-mediated epoxidation of allylic alcohols, some alkoxide(s) (—OR) do not participate in the epoxidation. Therefore, if such bystander alkoxide(s) are replaced with optically active ones, the epoxidation is expected to be enantioselective. Indeed, Yamada et al.25 and Sharp less et al.26 independently reported the epoxidation of allylic alcohols using Mo02(acac)2 modified with V-methyl-ephedrine and VO (acac)2 modified with an optically active hydroxamic acid as the catalyst, respectively, albeit with modest enantioselectivity. 

In this chapter, metal-mediated asymmetric oxygen-, nitrogen-, and carbon-atom transfer reactions were described. The substrates were limited to olefins. Owing to limited space, examples of the reactions were collected with the priority given to enantioselectivity, except for some germinal... 

The discussion about the possible formation of metalla-2-oxetanes in transition metal-mediated oxidation reactions began with the ground breaking work of Sharpless in the field of enantioselective dihydroxylation of olefins with osmium tetraoxide using cinchona alkaloids as ligands [6]. The transfer of the stereochemical information of the chiral ligand to the substrate was explained by Sharpless with a two-step mechanism for the addition reaction, which should occur rather than a concerted [3+2] addition as originally proposed [110] (Fig. 15). 

The oxidative functionalization of olefins mediated by transition metal oxides leads to a variety of products including epoxides, 1,2-diols, 1,2-aminoalcohols, and 1,2-diamines [1]. Also the formation of tetrahydrofurans (THF) from 1,5-dienes has been observed, and enantioselective versions of the different reactions have been developed. Although a lot of experimental data has been available, the reaction mechanisms have been a subject of controversial discussion. Especially, osmium (VIII) complexes play an important role there, as the proposal of a stepwise mechanism [2] for the dihydroxylation (DH) of olefins by osmium tetroxide (OSO4) had started an intense discussion about the mechanism [2—11],... 

Iridium has been found to be a very robust late transition metal which can mediate or catalyze C—H bond activation reactions very efficiently. However, the highly enantioselective Ir-catalyzed C—H bond functionalization via a transient C—Ir species for the construction of C—C or C—X bonds only emerged recently. Mechanistically, the catalytic cycle starts with oxidative addition of the Ii catalyst to the inert C—H bonds (such as aromatic, olefinic, or aliphatic C—H bonds), which are usually assisted with an ortho directing group. Subsequently, the formed C—Ir species inserts into an unsaturated functionality such as alkene, alkyne, or imine, delivering a new C—Ir speeies. Finally, the reductive elimination releases the products and regenerates the Ir catalyst. 

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