Two-step oxidation-reduction

Because Walton started with pure W and observed no WO3 in the product, he concluded that his method likely follows the direct route. More likely, however, is a two-step oxidation-reduction process that generates volatile W02(0H)2 as an intermediate, such as... 

Ai85,86 is discussed on p. 114. Agarwal et al.102 as well as Sharma et al.103 studied this reaction using silica-supported V2Os-alkali metal sulphate catalysts. A two-step oxidation-reduction mechanism gave the best description of the process. The activity increased with increasing atomic number of the added alkali metal for which no interpretation was offered. In an electron microscopic study of these catalysts Sharma et al.103 showed that K2 S04 and V2 05 are present as separate phases but that the sulphate causes the presence of a larger amount of V2 05 in the form of needle-like crystals which appear to be more active for the methanol oxidation. A similar result was obtained by these authors for catalytic oxidation of toluene over these catalysts.104... 

Attempts to invert the configuration at C(l) were fruitless, owing to severe steric hindrance. So a two-step oxidation-reduction sequence was used to obtain the C(l)-C(2) syn product. Reduction of the ketone 98 with NaBtU in the presence of CeCl3 afforded the best result (99 97 = 85 15). The... 

Simple two-step oxidation/reduction mechanisms are often used to explain industrial reactions. The oxidation of a molecule X can proceed by two mechanisms (Scheme 5-3). 

In order to avoid the two-step oxidation-reduction method for conversion of a mixture of 2- and 3-cephem isomers to a 3-cephem, Pfaendler et al. (1976) discovered that a mixture of isomeric benzhydryl ethers... 

Two-Step Oxidation—Reduction Strategies of Hydroxyl Functions... 

An alternative route to sulphones utilizes the reaction of the appropriate activated halide with sodium dithionite or sodium hydroxymethanesulphinite [6], This procedure is limited to the preparation of symmetrical dialkyl sulphones and, although as a one-step reaction from the alkyl halide it is superior to the two-step oxidative route from the dialkyl sulphides, the overall yields tend to be moderately low (the best yield of 62% for dibenzyl sulphoxide using sodium dithionite is obtained after 20 hours at 120°C). The mechanism proposed for the reaction of sodium hydroxymethanesulphinite is shown in Scheme 4.20. The reaction is promoted by the addition of base and the best yield is obtained using Aliquat in the presence of potassium carbonate. It is noteworthy, however, that a comparable yield can be obtained in the absence of the catalyst. The reaction of phenacyl halides with sodium hydroxy-methane sulphinite leads to reductive dehalogenation [7]. 

Proposed Mechanisms for Reduction Reactions. Any mechanism proposed for the reduction of niobium(V) halides with pyridine must incorporate the necessary two-electron oxidation-reduction step required for the oxidation of pyridine to l-(4-pyridyl) pyridinium ion. In view of the known acid properties of the niobium(V) halides and the rapid reaction of the tantalum (V) halides to give 1 to 1 pyridine adducts, the mechanism must also include the initial coordination of pyridine to the niobium(V) halide. The reduction might then proceed through the steps shown opposite. 

The results obtained corresponding to the consumption of paraxylene and the formation of PTN (see Figs. 3 and 4) are interpreted by a redox mechanism which implies a stationary state whith equality between the rates of the two steps of reduction of the catalyst by paraxylene and its oxidation by nitrogen monoxide (ref. 3). Table 3 gives the values of the rate constants of the two steps and shows that Kq and calculated for the consumption of paraxylene or the formation of paratolunitrile, the main product, are very similar in agreement with the redox mechanism. 

Heterolytic oxidations generally require activation of the substrate by coordination to the metal and are generally highly selective and steroespecific. They do not involve free radical intermediates and use second- and third-row transition metals which conserve their oxidation state or change it by a two-electron oxidation-reduction step. 

Although the term hydroboration is most commonly employed2-3 16 to denote the addition of a boron-hydrogen linkage to carbon-carbon multiple bonds, it has also been used for the two-step oxidative process to distinguish it from the process of reduction involving H-B addition and protonolysis. 18... 

The conversion of carboxylic acids to aldehydes is normally conducted in two steps by reduction of the acids or their derivatives to the corresponding alcohols followed by mild oxidation. 

Cuprous acetate monomer, complexed with the quinoline solvent, is in rapid equilibrium with dimer. The equilibrium constant is such that dimer formation is incomplete. Activation of the hydrogen occurs by a slow reaction between dimer complex and dissolved molecular hydrogen. Following activation of the hydrogen, the substrate quickly reacts with the hydrogen. Reaction (11) is believed rate controlling. Weller and Mills (5) attempted to establish whether the reaction went through a two-step oxidation and reduction of the Cu1 catalyst however, the conclusion was that the reaction depicted above best fits the observed facts. 

When 125 was subjected to a two-step sequence, reductive cleavage of the 0-0 bond and subsequent oxidative dehydration, by treatment with zinc in AcOH, the allylic diol 135 and the 1,2-oxazocine 126, as minor product, were isolated. Oxidation of the diol 135 under several different reaction conditions (pyridinium chlorochromate (PCC), 2-iodoxybenzoic acid (IBX)/EtOH, IBX/DMSO, Pyr/SOj) gave 126 in high yield (68-92%), while with tetrapropylammonium perruthenate/Wmethylmorphaline Ar-oxidc (TPAP/NMO) as oxidizing agent 126 was the minor product and the a,/3-unsaturated 7-butyrolactone 136 was obtained in 68% yield (Scheme 28) <2005JOC6995>. 

The vast majority of flavoenzymes catalyze oxidation-reduction reactions in which one substrate becomes oxidized and a second substrate becomes reduced and the isoalloxazine ring of the flavin prosthetic group (Figure 1) serves as a temporary repository for the substrate-derived electrons. The catalytic reaction can be broken conveniently into two steps, a reductive half reaction (from the viewpoint of the flavin) and an oxidative half reaction. The flavin ring has great utility as a redox cofactor since it has the ability to exist as a stable semiquinone radical. Thus, a flavoenzyme can oxidize an organic substrate such as lactate by removal of two electrons and transfer them as a pair to a 2-electron acceptor such as molecular oxygen, or individually to a 1-electron acceptor such as a cytochrome. 

Both 1,4- [340-342] and 1,5-benzodiazepines [343] are polarographically reducible the latter is somewhat unstable in aqueous solution, which complicates the investigations. 1,4-Benzodiazepines like 7-chloro-2-methylamino-5-phenyl-3i7-l,4-diazepine-4-oxide are reducible in acid solution in three steps. The first two steps, the reduction of the A -oxide and the saturation of the benzophenone imine, are straightforward. The mechanism of the third step, a reductive ring contraction to 6-chloro-2-methyl-4-phenyl-3,4-dihydroquina-zoline, is more controversial [51, 157]. 

In general, the breaking of the carbon—oxygen bond appears difficult to achieve and needs activation by strongly polar groups. For these reasons, alcohol deprotection is carried out cathodically under the form of benzylic[91] and allylic [92] ethers as well as tosylates [91]. Thus, Torii [93] used the 4-nitrobenzyl group to protect alcohols. The deprotection was carried out in two steps (1) reduction of the nitrogroup of the amine and (2) oxidation of the amine at a platinum electrode (yield up to 93%). 

The complex (p-tithb)[Ru(aeac)2]2) 8 with the noninnocent 3,3, 4,4 -tetra-imino-3,3, 4,4 -tetrahydrobiphenyl (tithb) ligand shows a clean double two-step oxidation and reduction behaviour (Figure 3.4)." One-eleetron oxidation to 8 produces a weak NIR band at 1570nm (a = 800M cm ) while a more conspicuous NIR absorption around 2160 nm (a = 4000 cm ) occurs when 8 is... 

The relatively efficient cyclisation of monocyclofamesic acid with boron trifluoride etherate to methyl bicyclofarnesate (46) has led to a successful synthesis of drimenin (48) via acid-catalysed lactonisation of the allylic alcohol (47), a product of singlet oxygen addition to (46). It has also been shown that hydride reduction of drimenin to the allylic diol (49) followed by a two-step oxidation procedure yields cinnamolide (39). A Ci antifungal mould metabolite has... 

Simple transient state experiments in laboratory plug flow reactors can qualitatively tell whether the surface processes mentioned above are instantaneous or not. For example, a step composition change can lead to two types of results. Either the outlet composition follows what is expected from tlie steady state rate equations or not. In the fonner case, one may assume quasi steady state. In the latter case, one is facing a slow surface step (oxidation/reduction of rhodiiun, oxygen storage/release, transient deactivation by SO2, etc.) that affects the main reaction mechanism. 

EE-protected lactaldehyde 606 is readily available from lactate 310 in two steps by reduction of the ester with lithium aluminum hydride to give (5)-2-ethoxyethyl-1,2-propanediol (311) followed by Swem oxidation to the aldehyde [189]. 

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