Amine oxides transfer

Organic Reagents. Amine oxides are used ia synthetic organic chemistry ia the preparation of olefins, or phase-transfer catalysts (47), ia alkoxylation reactions (48), ia polymerization, and as oxidizing agents (49,50). 

Enby 6 is an example of a stereospecific elimination reaction of an alkyl halide in which the transition state requires die proton and bromide ion that are lost to be in an anti orientation with respect to each odier. The diastereomeric threo- and e/ytAra-l-bromo-1,2-diphenyl-propanes undergo )3-elimination to produce stereoisomeric products. Enby 7 is an example of a pyrolytic elimination requiring a syn orientation of die proton that is removed and the nitrogen atom of the amine oxide group. The elimination proceeds through a cyclic transition state in which the proton is transferred to die oxygen of die amine oxide group. 

Similarly the active oxygen of oxaziranes can be transferred to triphenylphosphine with the formation of ]ihosphine oxide and to tertiary amines yielding amine oxides. ... 

Few studies have systematically examined how chemical characteristics of organic reductants influence rates of reductive dissolution. Oxidation of aliphatic alcohols and amines by iron, cobalt, and nickel oxide-coated electrodes was examined by Fleischman et al. (38). Experiments revealed that reductant molecules adsorb to the oxide surface, and that electron transfer within the surface complex is the rate-limiting step. It was also found that (i) amines are oxidized more quickly than corresponding alcohols, (ii) primary alcohols and amines are oxidized more quickly than secondary and tertiary analogs, and (iii) increased chain length and branching inhibit the reaction (38). The three different transition metal oxide surfaces exhibited different behavior as well. Rates of amine oxidation by the oxides considered decreased in the order Ni > Co >... 

The mechanism of this oxygen transfer involves attack of the amine oxide on a coordinated carbonyl to yield the indicated products, with the resulting vacant coordination site again being filled by a carbonyl. 

Carbazole, like most aromatic amines, oxidizes readily via electron transfer. We recognized early that electron transfer may be an important initiation process for polymerizing the N-vinyl derivative. Some years ago we showed (29) that cycloheptatrienyl cation could act as an efficient one-electron transfer reagent, producing the appropriate cation radicals from reactive amines such as phenothiazine and tetramethyl-p-phenylene-diamine. It was also suggested that the product of the reaction between cycloheptatrientyl cation and carbazole itself was the carbazole cation radical. However, our recent work (21) has demonstrated that one-electron oxidation of carbazole leads directly to the 3,3-dicarbazoyl cation radical (VII). 

The mechanism by which amines are oxidised by flavoproteins has been an issue of considerable debate in recent years. The debate has been particularly heated in the case of the enzyme monoamine oxidase (Silver-man, 1995). Through the use of a variety of mechanism-based inhibitors and based on studies of nonenzymic mechanisms of amine oxidation, a mechanism for monoamine oxidase in which substrate is initially oxidized by single electron transfer to the enzyme flavin to give an aminium cation... 

Oxidation of the <5-dibenzylamino-a,/J-unsaturated esters 4, prepared in enantiomerically pure form from the corresponding amino acids, with 3-chloroperbenzoic acid gave the hydroxyl-amines 6 in 70-80% yields63. The optical purity of the a-hydroxy esters 7, obtained from 6 by reduction with H2/Pd(OH)2, was determined to be higher than 95% by the Mosher procedure. Thus, the sigmatropic rearrangement of the intermediate amine oxides 5 proceeds with essentially complete transfer of chirality. 

Mechanistic studies of the chemical oxidation of aliphatic amines have been reviewed extensively by Chow et al. [22]. Many studies of the mechanism of oxidation of amines have been performed with chlorine dioxide or ferricyanide as oxidants, because they have absorption bands with maxima at 357 and 420 nm, respectively. Changes in the absorbance at these wavelengths for the respective oxidants can be conveniently used to follow the kinetics of the reactions. On the basis of these studies, the electron-transfer mechanism shown in Scheme 1 has been proposed for amine oxidation. 

Cytochrome P-450 and hemoproteins are known to be involved in the oxidation of a broad variety of drugs, pesticides, carcinogens, steroids and fat soluble vitamins [18, 184, 185]. Amine oxidation by P-450 has been proposed to proceed either via electron/proton transfer mechanism or via hydrogen-atom abstraction mechanism (Scheme 23). 

The a-aminoalkyl radicals as well as iminium ions generated as intermediates in electron-transfer reactions of amines can be used for bringing about synthetically useful transformations of amines. The synthetic applications of amine oxidation reactions brought about by thermal, electrochemical and photochemical methods as discussed below. 

Monovalent metal cations also influence the redox reactivity of tryptophan tryp-tophylquinone (TTQ) coenzyme in the amine oxidation and the subsequent electron transfer to the biological electron acceptor proteins such as amicyanin [246 251]. 

Aromatic amine oxides have a iarge photochemicai iiterature and a major reaction of this functionality is transfer of the oxygen from nitrogen to a ring position. The -oxlde Iiterature includes many contributions from Albini s group and this year they have reported the results of a study of the photochemistry of simple substituted pyridine N-oxIdes. The products formed from pyridine... 

There are not many examples of the direct transformation of an allylic amino to a rearranged oxygen functionality. One example is found in the Meisenheimer rearrangement of amine oxides, which occurs with almost complete chirality transfer, as seen in equation (23). ... 

Chiao, W.-B., Saunders, W. H., Jr. Mechanisms of elimination reactions. 29. Deuterium kinetic isotope effects in eliminations from amine oxides. The consequences of nonlinear proton transfer. J. Am. Chem. Soc. 1978,100, 2802-2805. 

A controversy currently abroad in the field seems a particularly likely candidate for investigations using model reactions. A number of bacterial alcohol dehydrogenases make use of free-standing PQQ (see Fig. 4.3 for the structure) as a cofactor. Figure 4.8 shows two possible mechanisms for a critical step in the mechanism the question is whether the reaction follows a proton-transfer route or a hydride-transfer route [69, 70]. The question is essentially limited to the alcohol and sugar dehydrogenases, while the enzymes that catalyze amine oxidations tend... 

Although H-transfer in flavoprotein amine dehydrogenases has been shown to occur by tunneling, the mechanisms of amine oxidation by flavoproteins remain controversial. Over the years mechanisms involving the following have been con-... 

Rate constants in Eq. (2) are 10-5 dm3 mol-1 s-1 for monoamines 9 and 10 and 10-2 dm3 mol-1 s l for PD 11 [50]. At a proper substitution, the amount of amines oxidized according to Eq. (2) is negligible in comparison with the process in Eq. (1). A considerable charge separation was observed in the transition state of Eq. (1) [51], Charge-transfer-complexes (CTC) have been envisaged for aromatic monoamines (40) and diamines (41) as the primary step preceding formation of the aminyl ... 

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