Iodosyl benzenes, olefin epoxidation

Asymmetric epoxidation of olefins with ruthenium catalysts based either on chiral porphyrins or on pyridine-2,6-bisoxazoline (pybox) ligands has been reported (Scheme 6.21). Berkessel et al. reported that catalysts 27 and 28 were efficient catalysts for the enantioselective epoxidation of aryl-substituted olefins (Table 6.10) [139]. Enantioselectivities of up to 83% were obtained in the epoxidation of 1,2-dihydronaphthalene with catalyst 28 and 2,6-DCPNO. Simple olefins such as oct-l-ene reacted poorly and gave epoxides with low enantioselectivity. The use of pybox ligands in ruthenium-catalyzed asymmetric epoxidations was first reported by Nishiyama et al., who used catalyst 30 in combination with iodosyl benzene, bisacetoxyiodo benzene [PhI(OAc)2], or TBHP for the oxidation of trons-stilbene [140], In their best result, with PhI(OAc)2 as oxidant, they obtained trons-stilbene oxide in 80% yield and with 63% ee. More recently, Beller and coworkers have reexamined this catalytic system, finding that asymmetric epoxidations could be perfonned with ruthenium catalysts 29 and 30 and 30% aqueous hydrogen peroxide (Table 6.11) [141]. Development of the pybox ligand provided ruthenium complex 31, which turned out to be the most efficient catalyst for asymmetric... 

The mechanisms proposed for manganese-porphyrin-catalyzed epoxidation of olefins are similar in several respects to that proposed for salen systems (365, 400), and this was utilized initially to provide insight into the Mn-salen systems. In the case of porphyrins, a Mnv=0 moiety is also formed by reaction with the secondary oxidant, for example iodosyl benzene, and the olefin is then believed to approach in a side-on manner. Various intermediates have been proposed for the epoxidation of the olefin, including ones in which the alkene is polarized to give positive and radical ends, and those shown earlier for the salen systems. The concept of the metallaoxe-tane (371, 372) was employed to explain isomerization of some of the olefinic substrates during conversion to the epoxide products, but now... 

Because of this catalyst degradation, organometallic catalysts are currently the best synthetic reagents for enantioselective epoxidation of olefins. Chiral Mn(III)-salen complexes yield up to 99% ee for cw-disubstituted, tri- and tetra-substituted alkenes [62], but the best results require less desirable oxidants - iodosyl benzene or hypochlorite. Other catalysts accept a more limited substrate range the Sharpless-Katsuki titanium-tartrate ester [65] for allylic alcohols and the JuUa-Colonna epoxidation for a,P-unsaturated ketones [66]. 

Complexation of [H2Pc (CF3)8] with FeCU affords the Fe complex containing two coordinated pyridine molecules [Py2FePc (CF3)8] [67], It is oxidized in an acidic medium to Fe" complex isolated as p-oxo dimeric species p-0[FePc (CF3)8]2 Its X-Ray crystal structure (see Fig. 6) evidence that severe saddle-type distortion of the macrocycles due to interaction of a-CF3 groups leads to almost linear r-(FeOFe) bridge [68], The p-oxo species exhibit catalytic activity in epoxidation of olefins with iodosyl benzene to stabilize the Fe" =0 (ferryl) intermediate by electron-accepting CF3 groups [67],... 

---