Anions and aromaticity

Sodium azide also adds to olefins of this t3rpe to give w-triazoles in fairly good yields. A mechanism involving nucleophilic displacement of the substituent X by azide, followed by cyclization of the vinyl azide in the presence of azide ions, has been suggested. An alternative mechanism involves conjugate addition of azide to the double bond, cyclization of the resulting anion, and aromatization. 

For a given anion and aromatic, the larger the cation the larger the A/A. 

The various steps in the overall sequence will here be considered individually, but only briefly, and no attempt will be made to indicate the scope of the WEH procedure which, as has already been indicated, has been widely reviewed. The aldol condensation which leads to the ions 154 is considered to be essentially reversible, a feature which has been observed in the reactions between diethyl (prop-2-enyl)phosphonate anion and aromatic aldehy-des Reversibility has also been demonstrated in a variety of other reactions that include crossover experiments, based on the system from benzaldehyde and 153 (Z = CN or COOMe) into which a more electrophilic aldehyde is added this results in the incorporation of the latter into products in such a way that the dissociation of the phospho-nate-benzaldehyde adduct must have occurred The addition of an aldehyde to a deuterium-labelled adduct in the presence of NaOEt-EtOH affords a mixture of labelled and unlabelled alkenes in the ratio of ca 1 1. The product (158) from the interaction of HO (Na2C03 in Et0H-H20) and a dialkyl (a-cyanoethenyl)phosphonate decomposes into the expected alkene, but also dissociates into a carbonyl compound together with a carbanion the latter can then be trapped by the addition of a different aldehyde or ketone (Scheme 30) ... 

In most of the recent work, cleaning baths were found to be sufficient to ensure a satisfactory reaction. A series of aryl and hetero-aryl radical anions and aromatic dianions were prepared. scheme describes the experimental set-up. In the presence of N,N -tetramethylethane- or N,N -tetramethyl-propanediamine, non-ethereal solvents can be used.77/78... 

The Sj j. 1 mechanism, involving the formation of arsine radical anions, adequately explains the formation of scrambled products by competition between ET and C—As bond fragmentation (via ET j ) (Scheme 10.11) [59]. In this system, the a MO of the C—As bonds have similar energy to the low-lying antibonding Jt MO of the aromatic system, and the orbital crossing can occur. In this case, the extra electron in [ArAsPh ] " could occupy the a MO of any C—As bonds, and the radical anion decomposes into different arsine anions and aromatic a radicals (Ar and Ph ). 

In general, divalent cations form stronger complexes with the monofunctional acid anions than with the difunctional acid anions and aromatic compounds. Typical speciation calculation results are shown in Figs. 2, 3, and 4. In all cases, the divalent ion is the predominant species at low... 

The selectivity relationship merely expresses the proportionality between intermolecular and intramolecular selectivities in electrophilic substitution, and it is not surprising that these quantities should be related. There are examples of related reactions in which connections between selectivity and reactivity have been demonstrated. For example, the ratio of the rates of reaction with the azide anion and water of the triphenylmethyl, diphenylmethyl and tert-butyl carbonium ions were 2-8x10 , 2-4x10 and 3-9 respectively the selectivities of the ions decrease as the reactivities increase. The existence, under very restricted and closely related conditions, of a relationship between reactivity and selectivity in the reactions mentioned above, does not permit the assumption that a similar relationship holds over the wide range of different electrophilic aromatic substitutions. In these substitution reactions a difficulty arises in defining the concept of reactivity it is not sufficient to assume that the reactivity of an electrophile is related... 

Aromatic Radical Anions. Many aromatic hydrocarbons react with alkaU metals in polar aprotic solvents to form stable solutions of the corresponding radical anions as shown in equation 8 (3,20). These solutions can be analyzed by uv-visible spectroscopy and stored for further use. The unpaired electron is added to the lowest unoccupied molecular orbital of the aromatic hydrocarbon and a... 

Stibine Oxides and Related Compounds. Both aUphatic and aromatic stibine oxides, R SbO, or their hydrates, R3Sb(OH)2, are known. Thus both dihydroxotrimethylantimony [19727-41-4], C3H2202Sb, and trimethyl stibine oxide [19727-40-3], C H OSb, have been prepared. The former maybe readily obtained by passing an aqueous solution of dichi orotrimethyl antimony [13059-67-1], C3H2Cl2 > through an anionic-exchange resin (151). 

The azo coupling reaction proceeds by the electrophilic aromatic substitution mechanism. In the case of 4-chlorobenzenediazonium compound with l-naphthol-4-sulfonic acid [84-87-7] the reaction is not base-catalyzed, but that with l-naphthol-3-sulfonic acid and 2-naphthol-8-sulfonic acid [92-40-0] is moderately and strongly base-catalyzed, respectively. The different rates of reaction agree with kinetic studies of hydrogen isotope effects in coupling components. The magnitude of the isotope effect increases with increased steric hindrance at the coupler reaction site. The addition of bases, even if pH is not changed, can affect the reaction rate. In polar aprotic media, reaction rate is different with alkyl-ammonium ions. Cationic, anionic, and nonionic surfactants can also influence the reaction rate (27). 

The pyrazole ring is particularly difficult to cleave and, amongst the azoles, pyrazoles together with the 1,2,4-triazoles are the most stable and easiest to work with. This qualitative description of pyrazole stability covers the neutral, anionic and cationic aromatic species. On the other hand, the saturated or partially saturated derivatives can be considered as hydrazine derivatives their ring opening reactions usually involve cleavage of the N—C bond and seldom cleavage of the N—N bond. It should be noted, however, that upon irradiation or electron impact the N—N bond of pyrazoles can be broken. 

In this paper the electtode anodic reactions of a number of dihydropyridine (DHP) derivatives, quantum-chemical calculations of reactions between DHP cation-radicals and electrochemiluminescers anion-radicals (aromatic compounds) and DHP indirect ECL assay were investigated. The actuality of this work and its analytical value follow from the fact that objects of investigation - DHP derivatives - have pronounced importance due to its phaiTnacology properties as high effective hypertensive medical product. 

In the presence of a proton source, the radical anion is protonated and further reduction occurs (the Birch reduction Part B, Section 5.5.1). In general, when no proton source is present, it is relatively difficult to add a second electron. Solutions of the radical anions of aromatic hydrocarbons can be maintained for relatively long periods in the absence of oxygen or protons. 

The CC bond distances in cyclopentadienyl anion, C5H5, are all equal, because the anion is aromatic (see Chapter 12, Problem 10). Electrophiles that interact electrostaticaUy with the anion, such as Na", interact equally with all five carbons, and do not disturb the anion s aromatic character. On the other hand, electrophiles that make covalent bonds, such as H", might interact more strongly with one particular carbon and destroy the aromaticity of the ring. 

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