Pyridinium cations oxidation

The decarboxylation of carboxylate anions is carried out chemically by a variety of one-electron oxidants such as lead tetraacetate, uranyl nitrate, peroxides, quinones, pyridinium cations, etc.199 Importantly, the carboxylate anion (as... 

Fig. 8.14. Stepwise activation of dihydrotrigonelline-based chemical delivery systems, first by oxidation to a pyridinium cation (Reaction a), and then by hydrolysis to trigonelline and the drug ROH (Reaction b). Direct hydrolysis (Reaction c) is slow in comparison to the Reactions...

The rates of H-D exchange at the a-positions for a series of A-substituted pyridinium cations and pyridine 1-oxide derivatives in D20 at 75°C (Scheme 36) (70JA7547) correlate well with the Taft inductive parameter a, (p, = 15). A positively charged nitrogen in a ring is estimated to activate the a-position toward deprotonation and ylide formation by a factor of 1015. 

Pyridinium cations are reduced electrochemically or by metals to neutral radicals of considerable stability, especially when merostabilization by an a- or y-substituent occurs thus (381) has been isolated. Bispyridinium compounds are particularly readily reduced to radical cations, such as (383). Radical (383 R = Me) is the active species of the herbicide paraquat. Pyridyl radicals without such stabilization dimerize and form bispyridinium compounds by oxidation. 

Although the presence of pyridinium cations makes PDC slightly acidic, very acid sensitive functionalities are able to withstand the action of PDC. Some sodium acetate can be added as a buffer for a completely acid-free oxidation.124... 

Pyridine (111) oxidation can occur by four different routes (Scheme 39) paths a and c are direct electron transfers from 111 and the pyridinium cation, respectively. The other two pathways represent corresponding indirect... 

In summary, the triplet (do po) excited states of the d -d metal dimers [Ir(p-pz)(C0D)]2 and Pt2(pop)4 " undergo a variety of photochemical reactions. Electron transfer to one-electron quenchers such as pyridinium cations or halocarbons readily occurs with acceptors that have reduction potentials as negative as -2.0 V. With the latter reagents, net two-electron, photoinduced electron transfer yields d -d oxidative addition products. Additionally, the triplet (da pa) excited state of Pt2(pop)4 apparently is able to react by extracting a hydrogen atom from a C-H bond of an organic substrate. 

Under dioxygen-deficient conditions, pyridinium cations can compete with dioxygen in capturing alkyl radicals (Scheme 3). The resulting alkylpyridinium radical cations rapidly rearomatize by means of Fe(III)-induced oxidation to afford alkylpyridines and replenish the pool of Fe(II) sites. Other potential traps of alkyl radicals are Fe(III)—Cl moieties, which are known to effect oxidative chlorination (Eq. 8). 

Pyridine is relatively stable to ozonolysis under nonradical conditions, giving the N-oxide as almost the only product (Andreozzi et al., 1991). The pyridinium cation is practically inert to ozonation. At higher pHs many products, including ring cleavage compounds, ammonia, and nitrate are produced (Equation 5.43). 

Zhao et al. have synthesized novel ionic liquids 307 and 308, which contain (3-chloro-2-hydroxypropyl)-functionalized pyridinium cations, by the ultrasound-assisted reaction of pyridine with acid (HCl and HBF4) and 3-chloro-propylene oxide at room temperature, in excellent yields and purity, in whieh the acid provided the eorresponding anionic component of the ionic Uquids. Furthermore, they examined the appheation of new ionic liquids as solvents in the MBH reaction and found that IL-BF4 308 showed a better effect in rate enhanee-ment and an improved yield than IL-Cl 307 in some cases (Scheme 1.121). ... 

It was suggested that in the photochemically triggered oxidation of phenol by Ru(bpy)3, the proton charge was delocalized over a primary shell of water molecules firmly bound to the pyridinium cation.The tetra-2-oxypyridinate ruthenium(II, III) dimer [Ru2(hp)4Cl] and related diruthenium adducts were effective catalyst for the oxidation of allylic and benzylic C—H bonds. The bis(homoallylic) sulfamate esters were converted to the corresponding allylic amines using PhI(02C Bu)2 in CH2CI2. Both computational and experimental studies favoured a stepwise mechanism involving H atom abstraction/radical recombination and the intermediacy of a discrete short-lived diradical species. ... 

In the oxidized form of NAD, the pyridinium cation of nicotinamide is bound by an N-glycosidic linkage to Cl of D-ribose. This nicotinamide riboside moiety is linked to adenosine via a pyrophosphate group. NAD therefore has the structure of a dinucleotide (Fig. 1). Af, of the oxidized form (NAD ) = 663.4. 

The hydrogen reversibly carried by NAD is attached to the nicotinamide residue (Hg.2). In view of the positive charge on the coordinated pentavalent nitrogen of the pyridinium ion, oxidized NAD is represented as NAD , and in accordance with the guidelines of the International Commission on No-mendature, the reduced form is represented as NADH. Wten 2[H] and a pair of electrons are transferred from a substrate, the pyridinium cation loses its aromatidty and becomes reduced, and a proton is released Substrate-H2 + NAD Substrate + NADH + H . 

In the oxidation of a substrate, the pyridinium cation in the oxidized form of the coenzyme accepts two electrons and one proton to be reduced to the corresponding 1,4-dihydropyridine which reduces the substrate in the reverse reaction. The stoichiometry of the redox reaction is depicted in Scheme 1, where SH2 and S indicate the... 

Fig. S. The 400-nm absorption band is characteristic of the pyridinyl radical. The reaction involves the reduction of the pyridinium cation to the pyridinyl radical, while the 24a is oxidized to the mixed-valence species [Cu(I)Cu(I)Cu(II)] . ...

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