Epoxides with imidazole

The introduction of chlorinated porphyrins (10) allowed for hydrogen peroxide to be used as terminal oxidant [62], These catalysts, discovered by Mansuy and coworkers, were demonstrated to resist decomposition, and efficient epoxidations of olefins were achieved when they were used together with imidazole or imidazo-lium carboxylates as additives, (Table 6.6, Entries 1 and 2). 

Complex (17) of Class 3 has no chiral auxiliary, but is endowed with facial chirality by the presence of a bridging strap (Figure 4).65 Treatment of (17) with oxidant generates metal oxo bonds, preferentially on the sterically less hindered (nonbridged) side of the complex, and epoxidation with (17) is low in enantioselectivity (Scheme 10). However, the enantioselectivity is considerably improved by the addition of imidazole. The imidazole has been considered to coordinate the metal center from the nonbridged side and to force the formation of metal oxo bonds on the bridged (chiral) side, thus enhancing enantioselectivity. 

Without additives, radical formation is the main reaction in the manganese-catalyzed oxidation of alkenes and epoxide yields are poor. The heterolytic peroxide-bond-cleavage and therefore epoxide formation can be favored by using nitrogen heterocycles as cocatalysts (imidazoles, pyridines , tertiary amine Af-oxides ) acting as bases or as axial ligands on the metal catalyst. With the Mn-salen complex Mn-[AI,AI -ethylenebis(5,5 -dinitrosalicylideneaminato)], and in the presence of imidazole as cocatalyst and TBHP as oxidant, various alkenes could be epoxidized with yields between 6% and 90% (in some cases ionol was employed as additive), whereby the yields based on the amount of TBHP consumed were low (10-15%). Sterically hindered additives like 2,6-di-f-butylpyridine did not promote the epoxidation. 

Another immobilized system was recently reported by Wei and Liu [64] using amphiphilic coblock polymers consistent of ethylene glycol and methyl acrylate monomers that had been functionalized with imidazole groups. In combination with oxone (KHS05) as the oxidant in an ethyl acetate/water mixture, this system yields the 1,2-diol product in 62% yield, which is probably formed by ring opening of the epoxide by the acidic oxone. 

One paper from the group of Zhang reports [75] on the testing of various oils with MT0/H202 with CH3CN as solvent and imidazole as the nitrogen base. A base/MTO/H202/double bond ratio of 50 1 250 100 is used, and excellent results are obtained after 2 h at room temperature. Fatty acid methyl ester, sunflower oil, rapeseed oil, cottonseed oil, and palm oil can all be epoxidized with >99% selectivity and >98% yield with this system. 

Under ideal conditions, epoxidations with homogeneous or anchored Mn and Fe porphyrin catalysts are characterized by high numbers of turnovers and high stereospecificities (104-106,116,117,130,131). For example, 93% cz s-epoxide was obtained from dx-stilbene, with H2O2 and Fe-tetraphenyl-porphyrin on imidazole-modified SiCU (106). Various oxidants, including NaOCl, can be used (117) ... 

Reaction of 1-chloro-1,2-epoxides with formamide yields 4,5-disubstituted imidazoles.73... 

In spite of the great importance of polyaddition (polyetherification ) of epoxy groups in many formulations, a rigorous branching theory is still to be developed. Polyetherification is often released by a primary reaction of epoxide with amines, carboxyl groups, dicyandiamide, or is initiated by BF3-amine complexes, imidazoles, etc. The polyetherification reaction is to be regarded as an initiated reaction and the initiator is either added at the beginning of the reaction of formed in the first reaction... 

D. Mohajer, S. Tangestaninejad, Efficient olefin epoxidation with tetrabutylammonium periodate catalyzed by manganese porphyrin in the presence of imidazole. Tetrahedron Lett. 35 (1994) 945. 

A series of polymer-anchored epoxidation catalysts was obtained by modifying Merrifield resin with imidazole [61], diphosphines [62], or piperazine [63] followed by treatment with UV-activated Mo(CO)6. High activities in the epoxidation of cyclic (cyclooctene, cyclohexene, indene, and a-pinene) as well as linear alkenes (styrene, a-methylstyrene, 1-heptene, 1-dodecene, cis- and frans-stilbene) were observed using TBHP as oxidant. The catalysts were recovered and reused up to 10 times in the epoxidation of cyclooctene without loss of activity. 

Whereas the silica gel-catalysed reactions of epoxides with indoles 101 are more efficient than the high-pressure activated reactions for pyrrole 102, pyrazoles 103 and imidazoles 104, the best results were obtained with reactions carried out under high pressure. The reaction between epoxides 100 and indoles 101 occurred regioselectively and only 3-substituted indoles were produced (Scheme 7.26). 

Mo [24], W [24-25], Mn [26-32], Fe [26-28] porphyrins have been reported. Mn and Fe porphyrins, in the presence of imidazole or other donor axial ligands, catalyse epoxidation with high yields and stereoselectivity (retention of... 

In the olefin epoxidation, the mechanistic scheme commonly proposed for the oxygen-transfer reaction consists of a two-step catalytic cycle (Pig. 21) 123). In the first step, an oxygen atom is transferred from the primary oxidant to the Mn -salen catalyst, which in the second step carries the activated oxygen to the olefinic double bond. The main problem in Mn-salen catalyzed epoxidation with H2O2 was the formation of HO radicals by the homolytic cleavage of the weak 0-0 bond, leading to indiscriminate oxidation 124). Addition of Lewis bases, such as imidazole, pyridine, or... 

Oxidation of cyclic enol ether 35 with dimethyl dioxirane (DMDO) followed by in situ reduction of the intermediate epoxide with DIBALH gave secondary alcohol as a 10 1 mixture of diastereom-ers. Oxidation of these alcohols with TPAP/NMO afforded a 10 1 mixture of ketone 37 and its C16 epimer. The isomers were separated and the minor isomer was recycled to a 4 1 mixture of isomers by treatment with imidazole. Subsequent construction of the D-ring was performed by radical reduction of mixed thioacetal in the same way as that adopted by the Sasaki group, leading to octacycle 38. Stereoselective installation of the triene side chain was then carried out via (Z)-vinyl iodide 39... 

A chiral [Mn (saZen)X] complex containing chloromethyl groups was anchored onto SBA-15, MCM-41, MCM 8 and amorphous silica previously modified with imidazole groups, Pigure 11.7. All the materials were veiy active in the epoxidation of styrene, a-methylstyrene and indene, with... 

Active imidazoles are present throughout the course of curing, and overall curing rates (formation of polyethers) vary with imidazole structure, even though all imidazoles form adducts rapidly. Riccardi et al. [ 102] explain this discrepancy as the instability of adducts of imidazoles with epoxides. Parallel to the polymerization, there occur cleavage of N—C bonds and the deprotonation of the carbon at the 2-position of imidazole, and thus the imidazoles are regenerated. 

For the mechanism of the curing of the epoxides with the imidazoles, as seen from previous studies and the experimental data obtained by Jisova [103], the reaction scheme shown in Schemes 6-9 is suggested. The initiation occurs with an induction period. The 1 1 (resp., 1 2) adducts of the imidazole and the epoxide are formed quickly (Scheme 6). In the presence of the excess epoxides, growth centers appear after the dissociation or the rearrangement of the formed adduct (Scheme 7). The addition of a further molecule of epoxide to the growth center starts the propagation of polymer (Scheme 8). 

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