Epoxides reaction with imidazole

The observation that addition of imidazoles and carboxylic acids significantly improved the epoxidation reaction resulted in the development of Mn-porphyrin complexes containing these groups covalently linked to the porphyrin platform as attached pendant arms (11) [63]. When these catalysts were employed in the epoxidation of simple olefins with hydrogen peroxide, enhanced oxidation rates were obtained in combination with perfect product selectivity (Table 6.6, Entry 3). In contrast with epoxidations catalyzed by other metals, the Mn-porphyrin system yields products with scrambled stereochemistry the epoxidation of cis-stilbene with Mn(TPP)Cl (TPP = tetraphenylporphyrin) and iodosylbenzene, for example, generated cis- and trans-stilbene oxide in a ratio of 35 65. The low stereospecificity was improved by use of heterocyclic additives such as pyridines or imidazoles. The epoxidation system, with hydrogen peroxide as terminal oxidant, was reported to be stereospecific for ris-olefins, whereas trans-olefins are poor substrates with these catalysts. 

A rather complex microwave-assisted ring-opening of chiral difluorinated epoxy-cyclooctenones has been studied by Percy and coworkers (Scheme 6.131) [265]. The epoxide resisted conventional hydrolysis, but reacted smoothly in basic aqueous media (ammonia or N-methylimidazole) under microwave irradiation at 100 °C for 10 min to afford unique hemiacetals and hemiaminals in good yields. Other nitrogen nucleophiles, such as sodium azide or imidazole, failed to trigger the reaction. The reaction with sodium hydroxide led to much poorer conversion of the starting material. 

Among the earliest examples for the successful application of the epoxide method is a publication from Glas et al. [169] in which they introduce the reaction of imidazole with cyclohexene oxide, subsequent quartemisation with methyl iodide and the derivatisation of the hydroxyl functional group to an ester (see Figure 3.59). 

Using optically active epoxides, chiral N-alkylated pyrazole and imidazole derivative ligands have been prepared by high pressure reactions with 103 or 104 their catalytic efficiency as chiral ligands in the enantioselective addition of diethylzinc to benzaldehyde has been tested. 

Benzimidazolethiones are cyclized in a mild reaction with either an arylazo-chloroacetyl chloride [2366] or with chloroacetic acid and a benzaldehyde [2454]. Imidazole-2-thione adds on to DM AD also without the need for external heating [2270] the epoxide of arylidenemalononitrile cyclizes 2-benzimidazol-ethione very efficiently to the thiazolone analogue [3933]. 

The dicationic complex [Ru(py-NHC)(terpy)(OH2)] (terpy=2,2 6, 2 -terpyridine) containing the bidentate pyridyl-NHC ligand 3-meth)d-l-(pyridine-2-yl)imidazol-2-ylidene catalyzed the epoxidation of terminal alkenes vrith Phi (OAc)2 in CH2CI2 at room temperature (Table 12.7). In an effort to make the system reusable, a solvent system comprising a 1.2 0.8 mixture of CH2Q2 and the ionic liquid [bmim] [PFg] was employed. Tests with cyclooctene showed that this allowed 10 consecutive epoxidation reactions to be carried out without any drop in catalyst performance [107]. 

Imidazole is a well-known initiator for the anionic ring-opening polymerization of epoxides. The photogeneration of imidazole and its subsequent use as an initiator for the polymerization of multifunctional epoxy-novolak resins have been reported by Nishikubo et Photolysis of the nitro-benzyl derivative 119 as shown in Scheme 36 generates imidazole, 120, which forms zwitterionic intermediate, 121, by reaction with the epoxide. The latter species is a rather weak nucleophile and the application of heat (120 °C) was required to drive the polymerization to completion. 

Berkessel designed a chiral dihydrosalen Hgand with a covalently attached imidazole group. With this new salen complex (14) (Figure 10.5), 1,2-dihydronaphthalene was converted into the corresponding epoxide in 72% yield and with moderate ee (up to 64% Table 10.2) using a dilute (1%) aqueous solution of H2O2 as oxidant. An important feature of this system is that epoxidation reactions can be performed without the need for further additives [41 bj. 

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. 

These compounds can initiate anionic polymerisation of epoxides, and when R, = H the secondary amine can react by addition to an epoxide group. Farkas and Strohm 64> have studied the reaction of 2-ethyl-4-methyl imidazole with phenyl glycidyl ether and BADGE resin using chemical analysis and proton NMR spectroscopy. They found that the imidazole readily forms adducts with epoxide of 1 1 and 1 2 molecular ratio ... 

In the wake of this report, many chiral iron(III)- and Mn(III)-porphyrin complexes have been synthesized and applied to the epoxidation of styrene derivatives [20]. Because these asymmetric epoxidations are discussed in the first edition of this book [21], the discussion on metalloporphyrin-catalyzed epoxidation here is limited to some recent examples. Most chiral metallopor-phyrins bear chiral auxi Maries such as the one derived from a-amino acid or binapthol. Differing from these complexes is complex 6, which has no chiral auxiliary but is endowed with facial chirality by introducing a strap and has been reported by Inoue et al. [20f]. Epoxidation of styrene by using only 6 as the catalyst shows low enantioselectivity, but the selectivity is remarkably enhanced when the reaction is performed in the presence of imidazole (Scheme 6B.11). This result can be explained by assuming that imidazole coordinates to the unhindered face of the complex and the reaction occur on the strapped face [20f. ... 

This reaction is quite sensitive to effects of ligands, solvents, etc for example, H2O2 reacts with Mn(lfl)-tetrakis(2,6-dichlorophenyl)porphyrin to form only a Mn(V)=0 species in the presence of an axial imidazole ligand (107). These Fe(V) and Mn(V) oxo species can then convert an olefin into an epoxide, an alkane into an alcohol, etc. 

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