Monoanion

In stereoselective antitheses of chiral open-chain molecules transformations into cyclic precursors should be tried. The erythro-configurated acetylenic alcohol given below, for example, is disconnected into an acetylene monoanion and a symmetrical oxirane (M. A. Adams, 1979). Since nucleophilic substitution occurs with inversion of configuration this oxirane must be trens-conilgurated its precursor is commercially available trans-2-butene. 

Significant variations in the properties of polypyrrole [30604-81-0] ate controlled by the electrolyte used in the polymerization. Monoanionic, multianionic, and polyelectrolyte dopants have been studied extensively (61—67). Properties can also be controlled by polymerization of substituted pyrrole monomers, with substitution being at either the 3 position (5) (68—71) or on the nitrogen (6) (72—75). An interesting approach has been to substitute the monomer with a group terminated by an ion, which can then act as the dopant in the oxidized form of the polymer forming a so-called self-doped system such as the one shown in (7) (76—80). 

The five possible azole monoanions are shown (one canonical form only) in Scheme 2 all heteroatoms are now nitrogens. 

The pH-rate profile (see Fig. 8.6) indicates that of the species that are available, the monoanion of the acetal is the most reactive. The reaction is fastest in the intermediate pH range, where the concentration of this species is at a maximum. The concentration of the neutral molecule decreases with increasing pH the converse is true of the concentration of the dianion. 

The transition state for the rapid hydrolysis of the monoanion has been depicted as involving an intramolecular general acid catalysis by the carboxylic acid group, with participation by the anionic carboxylate group, which becomes bound at the developing electrophilic center... 

On treatment with potassium metal, cij-bicyclo[6.1.0]nona-2,4,6-triene gives a mono-cyclic dianion. The trams isomer under similar conditions gives only a bicyclic monoanion (radical anion). Explain how the stereochemistry of the ring junction can control the course of these reductions. 

An intriguing class of persistent radicals are those formed by the one-electron oxidation of the hexagonal prismatic clusters Li2[E(N Bu)3] 2 (3.21, E = S, Se). The air oxidation of 3.21 produces deep blue (E = S) or green (E = Se) solutions in toluene. The EPR spectra of these solutions consist of a septet (1 3 6 7 6 3 1) of decets (Eig. 3.5). This pattern results from interaction of the unpaired electron with three equivalent 7=1 nuclei, i.e., and three equivalent I = 3/2 nuclei, i.e., Ei. It has been proposed that the one-electron oxidation of 3.21 is accompanied by the removal of an Ei" cation from the cluster to give the neutral radical 3.22 in which the dianion [S(N Bu)3] and the radical monoanion [S(N Bu)3] are bridged by three Ei" cations. 

Paradoxically, although they are electron-rich, S-N compounds are good electron acceptors because the lowest unoccupied molecular orbitals (LUMOs) are low-lying relative to those in the analogous carbon systems. For example, the ten r-electron [SsNs] anion undergoes a two-electron electrochemical reduction to form the trianion [SsNs] whereas benzene, the aromatic hydrocarbon analogue of [SsNs], forms the monoanion radical [CeHg] upon reduction. ... 

Complexes of the monoanion [S2N3] can be obtained by the reactions of [PPh4][Pd2Cl6] with or, preferably, Me3SiNSNSNSNSiMe3. ... 

Figure 6-14 shows FhjS. Fhs, and Fs plotted against pH for an acid with pF, = 5.0 and pA"2 = 10.0. Evidently with such widely spaced dissociation constants the solution contains, at any one pH, significant fractions of only two species. The fraction of monoanion rises essentially to unity at one point. The pH at which the monoanion fraction achieves its maximum value is calculated by differentiating Eq. (6-74) and setting the result equal to zero this gives... 

Now suppose that only the monoanionic form of the dibasic acid H2S undergoes reaction and that neither the hydronium nor the hydroxide ion is directly involved. The kinetic scheme is, therefore. 

Van Leusen and co-workers also demonstrated the utility of dilithio-tosylmethyl isocyanide (dilithio-TosMIC) to extend the scope of the application. Dilithio-TosMIC is readily formed from TosMIC and two equivalents of n-butyllithium (BuLi) in THF at -70"C. Dilithio-TosMIC converts ethyl benzoate to oxazole 14 in 70% yield whereas TosMIC monoanion does not react. In addition, unsaturated, conjugated esters (15) react with dilithio-TosMIC exclusively through the ester carbonyl to provide oxazoles (16). On the other hand, use of the softer TosMIC-monoanion provides pyrroles through reaction of the carbon-carbon double bond in the Michael acceptor. 

Early investigators adduced various kinds of chemical evidence in support of a monohydroxy-dioxo structure for barbituric acid (112) (a) reaction with diazomethane afforded a mono-O-methyl deriva- iye,i59,i6o barbituric acid and its 5-alkyl derivatives are much stronger acids than the 5,5-dialkyl derivatives, and (c) the 5-bromo and 5,5-dibromo derivatives have different chemical properties. - The early physical evidence also appeared to substantiate the monoenol structure, this formulation having been suggested for barbituric acid in 1926 on the basis of its ultraviolet spectrum and again in 1934, In the 1940 s, ultraviolet spectroscopic studies led to the suggestion of other monohydroxy and dihydroxy structures for barbituric acid, whereas its monoanion was assigned structure 113 (a clear distinction between ionization and tautomerism was not made in these papers). 

Sheinker and Pomerantsev one of the protons remains bonded to a hetero nitrogen atom in the monoanion of 127. 2,5-Dihydroxypyra-zines could exist in a dioxo form (128) or in a zwitterion form (129), and, although structure 128 was assigned to the 5-butyl derivative in 1927, the evidence for structure 128 as opposed to 129 is inconclusive. 

The ultraviolet and infrared spectra and the crystal structure (as determined by X-rays) of cyanuric acid have been interpreted on the basis of the trioxo structure 157 however, other ultraviolet data have been reported from which it was concluded that approximately 5% of this compound exists in a hydroxy form in solu-tion. The monoanion of cyanuric acid exists as 158. Ultraviolet spectral data suggest that ammelide (159) and ammeline (160) exist as shown (available infrared spectra offer less conclusive evi-... 

Fully unsaturated azocines are 7r-equivalent heterocyclic analogs of cy-clooctatetraene. Addition of two electrons to the completely unsaturated azocine (34) can lead to a dianion 35 and removal of a proton from a dihy-droazocine (36) to the monoanion 37. Both the mono- and the dianions are lOTT-electron systems, corresponding to 7r-equivalent and 7r-excess analogs of cyclooctatetraenide [84CHEC-I(7)653], Aromatic dianions related to 35 have been fully characterized by and NMR (87TL2517). 

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