Hydrogen peroxide with pyridine

The procedure is experimentally simple, and the workup involves only the destruction of the traces of hydrogen peroxide with manganese dioxide and evaporation of the hexamethyldisiloxane. Pyridine additives serve to buffer the highly acidic rhenium species and to shut down the detrimental acid-catalyzed epoxideopening pathways. The scope of this transformation is best appreciated through the examples presented in Table 12.2 [28],... 

Compounds 43 and 44 are not intermediates in the formation of pyrrole blacks. During oxidation with hydrogen peroxide in pyridine, small quantities of succinimide and maleimide were identified by paper chromatography.4 4... 

The two Schotten-Baumann methods of preparing diacyl peroxides suffer from the formation of difficult emulsions in the alkaline medium and loss in yield due to hydrolysis of the acid chloride. Silbert and Swern developed an improved method involving acylation of 50-65% hydrogen peroxide with the acid chloride in ethei pyridine solution and obtained highly pure diacyl peroxides in nearly quantitative yield. For example, a solution of myristoyl chloride in ether is cooled to 0°. hydrogen... 

Like hemin the parahematins react with hydrogen peroxide. For pyridine parahematin Haurowitz, Brdicka, and Kraus (12) have suggested that the following replacement reactions occur ... 

Peracids.—Acyl chlorides have been converted into peracids, in ca. 60% yields, by treatment with hydrogen peroxide and pyridine the precise amount of the latter used is crucial to the success of the method. Two reports have appealed on the preparation and uses of polymer supported peracids detailing their application in the epoxidation of olefins, and the oxidation of sulphides, notably penicillins to the corresponding sulphoxides. a-Hydroperoxy acids are available in high yield by addition of oxygen to lithium enolates at -100 °C. 

The condensation of thymine, uracil and cytosine with 30 went smoothly to give the corresponding 2 -phenylseleno nucleosides (58, 59 and 60) (Scheme 13). For the cytidine derivative 60, it was found that the use of A/ -acetyl cytosine gave the best results in regard to yield and ease of purification. These compounds underwent smooth conversion to the dideoxy analogues ddT (63), ddU (64) and ddC (67) on treatment with tributyltin hydride/triethylborane at room temperature, followed by deprotection, or to the unsaturated analogues d4T (70) and d4U (71) on treatment with hydrogen peroxide/cat. pyridine and desilylation. 

An alternative procedure for the preparation of furoic acid by oxidation of furfural with hydrogen peroxide in pyridine has been reported. 

Type C Syntheses [N—C—N—C—S]. From amidino-thiono-compounds. The general synthesis of 1,2,4-thiadiazoles by the oxidative cyclization of compounds incorporating the amidino-thiono-grouping [—C(=NH)NHCS—] has been extended by the synthesis of l-acyl(and -sulphonyl)-3-thioacyl-guanidines (41) from acyl- or sulphonyl-guanidines (40) with thioacid O-esters in the presence of sodium hydride, followed by their oxidation with hydrogen peroxide in pyridine. 

In order to study the oxidative steps more readily, Libowitzky and Fischer (87,88) used a symmetrically constituted iron porphin, iron copro-porphin I tetramethyl ester, in which all the C methene atoms were identical so that a better chance was given for the isolation of oxidation products. By treating this compound with hydrogen peroxide in pyridine, a green ferric compound with a band at 655 mu was isolated in which the methene... 

Acid-cataly2ed hydroxylation of naphthalene with 90% hydrogen peroxide gives either 1-naphthol or 2-naphthiol at a 98% yield, depending on the acidity of the system and the solvent used. In anhydrous hydrogen fluoride or 70% HF—30% pyridine solution at — 10 to + 20°C, 1-naphthol is the product formed in > 98% selectivity. In contrast, 2-naphthol is obtained in hydroxylation in super acid (HF—BF, HF—SbF, HF—TaF, FSO H—SbF ) solution at — 60 to — 78°C in > 98% selectivity (57). Of the three commercial methods of manufacture, the pressure hydrolysis of 1-naphthaleneamine with aqueous sulfuric acid at 180°C has been abandoned, at least in the United States. The caustic fusion of sodium 1-naphthalenesulfonate with 50 wt % aqueous sodium hydroxide at ca 290°C followed by the neutralization gives 1-naphthalenol in a ca 90% yield. 

Ethers of benzenepentol have been obtained by Dakin oxidation of the appropriately substituted acetophenone. Thus, the oxidation of 2-hydroxy-3,4,6-ttimethoxyacetophenone and 2-hydroxy-3,4,5-ttimethoxyacetophenone with hydrogen peroxide ia the presence of alkali gives l,2-dihydroxy-3,4,6-ttimethoxybenzene and l,2-dihydroxy-3,4,5-ttimethoxybenzene, respectively further methylation of these ethers yields the pentamethyl ether of benzenepentol (mp 58—59 degC) (253). The one-step aromatization of myoinositol to produce esters of pentahydroxybenzene is achieved by treatment with carboxylic acid anhydrides ia DMSO and ia the presence of pyridine (254) (see Vitamins). 6-Alkyl- or... 

Constitution. When coniine is distilled with zinc dust or heated with silver acetate/ a new base, coiiyrine, CgH N, differing from coniine by six atoms of hydrogen, is formed. This on oxidation yields pyridine-2-carboxylic acid and, since it is not identical with 2-isopropylpyridine, must be 2-propylpyridine (I). When coniine is heated with hydriodic acid at 300° it yields w-octane (II). These and other observations due mainly to A. W. Hofmann, made it clear by 1885 that coniine was probably a-propylpiperidine (III), and this has been amply confirmed by other reactions of the alkaloid and by syntheses. Thus, Wolffenstein showed that on oxidation with hydrogen peroxide, coniine is converted into amino-w-propylvaleraldehyde (IV) ... 

This chemoselectivity stands in contrast to that of 2,6-disubstituted pyridines. For example, 2,6-dimethylpyridine 35 was reacted with hydrogen peroxide and acetic anhydride to produce the expected acetoxy derivative 36. A second iteration of the previous reaction conditions did not afford an aldehyde, as in the previous example, but 2,6-bis-acetoxy derivative 37. 

The use of molybdenum catalysts in combination with hydrogen peroxide is not so common. Nevertheless, there are a number of systems in which molybdates have been employed for the activation of hydrogen peroxide. A catalytic amount of sodium molybdate in combination with monodentate ligands (e.g., hexaalkyl phosphorus triamides or pyridine-N-oxides), and sulfuric acid allowed the epoxidation of simple linear or cyclic olefins [46]. The selectivity obtained by this method was quite low, and significant amounts of diol were formed, even though highly concentrated hydrogen peroxide (>70%) was employed. 

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 second major discovery regarding the use of MTO as an epoxidation catalyst came in 1996, when Sharpless and coworkers reported on the use of substoichio-metric amounts of pyridine as a co-catalyst in the system [103]. A change of solvent from tert-butanol to dichloromethane and the introduction of 12 mol% of pyridine even allowed the synthesis of very sensitive epoxides with aqueous hydrogen peroxide as the terminal oxidant. A significant rate acceleration was also observed for the epoxidation reaction performed in the presence of pyridine. This discovery was the first example of an efficient MTO-based system for epoxidation under neutral to basic conditions. Under these conditions the detrimental acid-induced decomposition of the epoxide is effectively avoided. With this novel system, a variety of... 

Asymmetric epoxidation of olefins with ruthenium catalysts based either on chiral porphyrins or on pyridine-2,6-bisoxazoline (pybox) ligands has been reported (Scheme 6.21). Berkessel et al. reported that catalysts 27 and 28 were efficient catalysts for the enantioselective epoxidation of aryl-substituted olefins (Table 6.10) [139]. Enantioselectivities of up to 83% were obtained in the epoxidation of 1,2-dihydronaphthalene with catalyst 28 and 2,6-DCPNO. Simple olefins such as oct-l-ene reacted poorly and gave epoxides with low enantioselectivity. The use of pybox ligands in ruthenium-catalyzed asymmetric epoxidations was first reported by Nishiyama et al., who used catalyst 30 in combination with iodosyl benzene, bisacetoxyiodo benzene [PhI(OAc)2], or TBHP for the oxidation of trons-stilbene [140], In their best result, with PhI(OAc)2 as oxidant, they obtained trons-stilbene oxide in 80% yield and with 63% ee. More recently, Beller and coworkers have reexamined this catalytic system, finding that asymmetric epoxidations could be perfonned with ruthenium catalysts 29 and 30 and 30% aqueous hydrogen peroxide (Table 6.11) [141]. Development of the pybox ligand provided ruthenium complex 31, which turned out to be the most efficient catalyst for asymmetric... 

Cyclodextrin mediated oxidation ofprochiral sulphides by achiral oxidation reagents leads also to optically active sulphoxides (e.e. up to 30%). When oxidation was carried out in pyridine the highest optical purities were obtained294 with hydrogen peroxide, whereas in water the best results were observed with m-chloroperbenzoic acid295.. 

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