Epoxidation of Cyclic and Linear Alkenes

Epoxidation reactions are important because epoxides are useful intermediates -ring opening allows subsequent reactions at one or both of the carbon atoms, and this is particularly attractive if asymmetric epoxidations can be accomplished (3.1.1.3). This is an increasingly important area for Ru-catalysed oxidations, and a review [2] emphasises in particular mechanistic aspects of such processes. There are also earlier brief reviews of the topic [4, 7,18], in particular by porphyrin-based systems [17-22], 

Few epoxidations have been accomplished with RuO since the reagent is a prime alkene cleavage agent (but see Fig. 3.1 below). Because of the intrinsic interest in, and need for, epoxidations many Ru complexes have been studied for the purpose, but generally only those which are effective (i.e. give good yields and selectivities), are catalytic and do not require forcing conditions are included. Some of those omitted here are mentioned in Ch. 1 and listed in 3.1.1.4 below. 

The first Ru-catalysed epoxidation was reported in 1983 by James et al. using RuBr(PPh3)(OEP)/PhlO/CH2Cl2 with styrene, norbomene and aT-stilbene in low yields cf. mech. Ch. 1 [23]. Eater work showed that fran -Ru(0)2(TMP)/02/CgHg catalysed aerobic alkene epoxidation of cyclo-octene, cis- and trans- -methylstyrenes and norbomene (Fig. 1.26) [24], 

Much exploratory work has been done on model substrates. Epoxidations of cyclo-octene and styrene give relatively clean products, and several studies have also been carried out on styrene, norbomene and 1-octene. 

For cyclo-octene and -hexene, styrene, stilbenes, norbomene and 1-octene, cf. 3.1.1.1  

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Jimenez et al. have developed tridentate cyclopentadienyl-silesquioxanate titanium complexes for the epoxidation of cyclic and linear alkenes with aqueous hydrogen peroxide under mild reaction conditions with excellent reactivity and selectivity. The authors extended the studies of these complexes as catalysts for the oxidation of sulfides to sulfoxides or sulfones under mild reaction conditions. The catalysts showed high chemoselec-tivity and proved to be very stable as no loss of activity or selectivity was observed after 14 cycles. 

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