2- imidazole pyridines, oxidative

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. 

Sulfotransferase activity is not restricted to minoxidil. The ability of other pyrimidine-, as well as pyridine-, triazine- and imidazole N-oxides to serve as substrates was investigated using soluble liver preparation and PAPS. The variety of structures studied indicated that heteroaromatic N-oxides are generally metabolized by sulfotransferases183. Presumably, all of the heterocycles tested were conjugated via their N-oxide oxygens. 

The aromatic imidazole 1-oxides 228 discussed in Section 3.1 are derived from imidazoles 248 by appending an oxygen atom to the pyridine type ring nitrogen atom of the imidazole nucleus. The second nitrogen atom of the imidazole ring can be attached to an alkyl or aryl group or to a... 

From these structural and chemical features considered necessary to mimic oxyhemoglobin or oxymyoglobin, there are at least two minimum requirements that any realistic models must satisfy (1) formation of a five-coordinate heme precursor having a proximal base (imidazole, pyridine and so on) and (2) limitation of pathways that lead to irreversible oxidation. 

The chemistry of azole X-oxides is relatively under developed compared, for example, with that of pyridine X-oxides, largely because of difficulty in their preparation from the azoles themselves. Some ring synthetic methods can be used, for example the reaction of 1,2-dicarbonyl mono-oximes with imines as shown. 1-Substituted imidazole 3-oxides can be converted into nitriles with loss of the oxygen using trimethylsilyl cyanide, careful choice of solvent minimising a tendency for isomeric mixtures to be formed. ... 

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... 

Necessary conditions for synthetic models to mimic oxyhemoglobins are formation of a five-coordinate heme precursor with a proximal base (e.g., imidazole, pyridine), prevention of p-peroxo/p-oxo (M-02-M/M-0-M) dimer formation upon oxygenation (i.e., reversible oxygenation), and exclusion of protons and nucleophiles. Model systems should be easily modified and allow probing of distal and proximal influences. Tetraphenyl-porphyrins (TPP) serve as the basis of numerous model complexes, because they are less susceptible to oxidation... 

Scheme 42 Pd-catalyzed cross-dehydrogenative heteroarylation of xanthines, (benz)-1/V-imidazoles, pyridine W-oxides, and quinoline N-oxides.

Electrophilic Trapping of Lithium Species. The deprotonation of arene or heteroarene species followed by quench with electrophiles is a general method for the introduction of functional groups in organic molecules. The arene moiety needs to be electron poor (i.e., thiazoles, imidazoles, pyridine (V-oxides, etc.). Possible electrophiles include sources of halogens, carbonyls, or sulfur. Phenylacetylenes can also be deprotonated with LiO/Bu and quenched with various aldehydes. ... 

Resorcinol or hydroquinone production from m- or -diisopropylben2ene [100-18-5] is realized in two steps, air oxidation and cleavage, as shown above. Air oxidation to obtain the dihydroperoxide (DHP) coproduces the corresponding hydroxyhydroperoxide (HHP) and dicarbinol (DC). This formation of alcohols is inherent to the autooxidation process itself and the amounts increase as DIPB conversion increases. Generally, this oxidation is carried out at 90—100°C in aqueous sodium hydroxide with eventually, in addition, organic bases (pyridine, imidazole, citrate, or oxalate) (8) as well as cobalt or copper salts (9). 

Isoxazole compounds can be converted into the corresponding isothiazoles by successive catalytic hydrogenation, sulfuration with phosphorus pentasulfide and oxidation with chloranil (72AHC(14)l, 75SST(3)541). 2,1-Benzisoxazoles give the 2,1-benzisothiazoles directly, by the action of phosphorus pentasulfide in either pyridine or molten imidazole (73SST(2)556, 77SST(4)339). (See also Chapter 4.16 for further discussion of these topics.)... 

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. 

The redox potentials of zinc-substituted phthalocyanines are shown to be linearly dependent on the total Hammett substituent constant.837 In 1987, Stillman and co-workers used the absorption and magnetic circular dichroism spectra of the zinc phthalocyanine and its 7r-cation-radical species to assign the observed bands on the basis of theoretical calculations. The neutral and oxidized zinc phthalocyanine complexes with cyanide, imidazole, and pyridine were used with the key factor in these studies the stability of the 7r-cation-radical species.838 The structure of zinc chloro(phthalocyaninato) has been determined and conductivity investigated.839... 

The synthesis of the furan-imidazole derivatives, shown in Scheme 2, were also described by Wang et al. [34]. Reaction of 4-(dimethylamino)benzalde-hyde (20) with trimethylsilylcyanide (TMS)-CN in the presence of Znl2 produced the TMS cyanohydrin 21. Compound 21 was treated with LDA followed by the addition of 3,4,5-trimethoxybenzaldehyde to give the benzoin intermediate 22. Oxidation with CUSO4 in aqueous pyridine, followed by reaction with 3-furaldehyde in acetic acid, produced the substituted imidazole 23. 

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