For epoxidation of alkenes

Based on these calculations, one can conclude that TSs for epoxidation of alkenes and allylic alcohols with peroxy acids, dioxiranes, and Re-peroxo complexes share a spiro geometry in which the plane of the attacking peroxo... 

Alkyl- and aryl-hydrazones of aldehydes and ketones readily peroxidise in solution and rearrange to azo hydroperoxides [1], some of which are explosively unstable [2], Dry samples of the p-bromo- and p-fluoro-hydroperoxybenzylazobenzenes, prepared by oxygenation of benzene solutions of the phenylhydrazones, exploded while on filter paper in the dark, initiated by vibration of the table or tapping the paper. Samples were later stored moist with benzene at —60°C to prevent explosion [3], A series of a-phenylazo hydroperoxides derived from the phenyl-or p-bromophcnyl-hydrazones of acetone, acetophenone or cyclohexanone, and useful for epoxidation of alkenes, are all explosive [4], The stability of several substituted phenylazo hydroperoxides was found to be strongly controlled by novel substituent effects [5],... 

Alkoxides and imido are used as anionic ligands in zirconium and titanium catalysts for the polymerisation of alkenes, sometimes as the only anions, but often in combination with cyclopentadienyl ligands. Imides linked to cyclopentadienyl groups form part of the single-site catalyst developed by Dow (Chapter 10) (Figure 1.9, 1). In very different titanium catalysts, namely those used for epoxidation of alkenes, also alkoxide ligands are used (Chapter 14). 

SCHEME 45. Hypothetical mechanism for epoxidation of alkenes by H2O2 in phenol... 

Synthesis and Modification of Ti-containing Catalysts for Epoxidation of Alkene... 

For epoxidation of alkenes catalyzed by metal complexes an oxo-metal species have become widely accepted as the active intermediate (ref. 3). With an Ag-O surface species such as 4, and with an orbital picture as shown in Figure 2d, and the the ji and n frontier orbitals of ethylene, two types of interaction between oxygen in 4 and ethylene are possible. The asymmetric approach 8 corresponds to an interaction of the O Ag-O orbital with the n of ethylene, whereas the symmetric approach, 9, is an interaction of the k Ag-O with the k orbital of ethylene. Besides the attractive interaction between the HOMOs of the oxygen and the LUMO of ethylene in 9, repulsive interactions between the Ag-O o orbitals and the HOMO of ethylene are also observed. 

A) Nucleophilic attack of the alkene on the electrophilic oxygen atom covalently bound to the metal, which is reminiscent of Bartlett s butterfly mechanism for epoxidation of alkenes by percarboxylic acids.229... 

The molybdenum complex 227 is an effective catalyst for epoxidation of alkenes and has allowed the development of the polystyrene-supported peptide-linked epoxidation catalyst 228351. 

Epoxidation. The two reagents (premixed in CHC13) can serve as a catalyst for epoxidation of alkenes in H20 by H202 (60% yield). In some cases (hexene, o-pinene) addition of trimethylamine is essential for high yields, 57-87%. Surprisingly, addition of a phase-transfer catalyst depresses the yield. 

Traylor (38) has also shown that biomimetic iron N-alkylporphyrins themselves are competent catalysts for epoxidation of alkenes with a rate constant of about 104 M-1 s-1. On the basis of these observations and rearrangement reactions of specific alkenes, Traylor has proposed the reaction sequence outlined in Scheme 3 as representative of the oxidation and N-alkylation reactions of the P-450 model systems. In this scheme, the epoxide and the N-alkylated heme are derived from a common, electron-transfer intermediate (caged ferrylporphyrin-alkene cation radical). Collman and co-workers (28, 29) prefer a concerted mechanism (or a short-lived, acyclic intermediate) for epoxidation and N-alkylation reactions. Both authors note that the reactions catalyzed by cytochrome P-450 (and biomimetic reactions) probably can not be ascribed to any single mechanism. 

The most widely accepted method for epoxidation of alkenes remains oxidation with organic peracids. The early work (up to 1970) in this field shows that a large number of dienes and polyenes were oxidized in this manner . The most commonly used peracids are peracetic, monoperphthalic and perbenzoic acids which are most dominant in industrial applications. On the other hand, in laboratory procedures m-chloroperbenzoic acid, MCPBA, is often used, with trifluoroperacetic acid cited in more difficult transformations. Recently, the transportation of m-chloroperbenzoic acid has been restricted and the use of other peroxygen agents has been gaining acceptance as a general alternative. Among the substrate types epoxidized it would be especially worthy to point out polyunsaturated... 

Oxidations. Aromatic aldehydes are obtained by oxidation catalyzed with a Mn(IV) complex. A procedure for oxidation of alcohols which is organic solvent free and halide free employs 30% H Oj, Na2W04 dihydrate, and a quaternary ammonium hydrogensulfate. Under such conditions secondary alcohols are oxidized 4-5 times faster than primary alcohols. In toluene the conversion of benzylic alcohols to aldehydes or acids (depending on quantities of HjOj) is accomplished. A similar system is also effective for epoxidation of alkenes. Terminal epoxides are obtained in reactions mediated by a Mn(II) complex or Mg-Al-O-t-Bu hydrotalcite. The last catalyst is capable of inducing epoxide formation from other alkenes and enones. 

E. M. Thorsteinson, Carbonate-supported catalytic system for epoxidation of alkenes, Canadian Patent CP 1,282,772, April 9,1991, To the Union Carbide Corporation. 

M. Benaglia, T. Danelli, G. Pozzi, Synthesis of poly(ethylene glycol)-supported manganese porphyrins Efficient, recoverable and recyclable catalysts for epoxidation of alkenes, Org. Biomol. Chem. 1 (2003) 454. 

Epoxidations. Combination of Oxone and the iminium salt derived from pyrrolidine and o-trlfluoromethylbenzaldehyde is effective for epoxidation of alkenes. In the case of an active alkene (e.g., trisubstituted alkene), pyrrolidine is an adequate catalyst. Other types of mediators include a-functionalized ketones (e.g., ot-acetaminoacetone) and the A, A -dialkylalloxans 1. ... 

Thallium(lll) salts have been used for epoxidation of alkenes. Oct-1-ene with TICI3 in THF-H2O gave only octan-2-one (60% selectivity) whereas Tl(02CEt)3 in THF-H20-EtC02H showed selectivities to formation of oxiran and of ketone of 80 and 13%, respectively. 

Whilst [MnCl(l)] complexes are effective in the epoxidation of Z-alkenes [38, 107], the [CrCl(l)] complex is also very suitable for epoxidation of alkenes. In order to identify the paramagnetic intermediates involved in the [CrCl(l)] epoxidation reaction, Bryliakov et al. [115, 116] studied two structurally related complexes, [CrCl(l)] and racew c-A,A -bis(3,4,5,6-fefra-deutero-salicylidene)-... 

Neumann, R., and G. Mohammad, Highly Active Manganese-Containing Polyoxomet-alate as Catalyst for Epoxidation of Alkenes with Hydrogen Peroxide, J. Am. Chem. Soc. 116 5509-5510 (1994). 

Complexes of the type [Ru(CO)L] (L = Frechet-functionalized weso-tetraphenylporphyrins) have been shown to be highly seleetive eatalysts for epoxidation of alkenes with chemo- and diastereoseleetivity inereasing with the generation of the dendron. ... 

The reaction of TiCU with such a silsesquioxane cluster connects Ti through oxygen atoms with four silsesquioxane clusters (Fig. 8.3, bottom left) . The result is a very flexible gel. The titanium atoms become part of a rather loose network of silsesquioxane clusters connected through Ti atoms. This system appears to be catalytically active for epoxidation of alkenes by hydroperoxide. When contacted with the Ti center, the reaction sequence shown in Fig. 8.4 occurs in Ti-silicalite and in the gel. 

Moghadam M, Mohammadpoor-Baltork 1, Tangestaninejad S, Mirkhani V, Kargar H, Zeini-Isfahani N. Manganese(lll) porphyrin supported on multi-wall carbon nanotubes a highly efficient and reusable biomimetic catalyst for epoxidation of alkenes with sodium periodate. Polyhedron 2009 28 3816-22. 

These are used for epoxidations of alkenes in good yields (Scheme 3). 

---