Anions formation

It looks as though all that is needed is to prepare the acetylenic anion then alkylate it with methyl iodide (Section 9 6) There is a complication however The carbonyl group m the starting alkyne will neither tolerate the strongly basic conditions required for anion formation nor survive m a solution containing carbanions Acetyhde ions add to carbonyl... 

Side chain reactivity is also enhanced and is typified by the difference in reactivity of 2-methylpyrazine and 2-methylpyrazine 1,4-dioxide towards anion formation and subsequent condensation reactions. 2-Methylpyrazine undergoes condensation with benzal-dehyde at 180 °C, with zinc chloride catalysis, to yield the styrylpyrazine (58), whereas the corresponding reaction of 2-methylpyrazine 1,4-dioxide proceeds at 25 °C under base catalysis (67KGS419). 

The high chemical stability of pterins towards aqueous base is due to anion formation suppressing nucleophilic attack at a ring carbon atom by electrostatic repulsion. Substitution... 

Alkylazoles reactions involving essentially complete anion formation... 

Anions of small heterocyclics are little known. They seem to be involved in some elimination reactions of oxetan-2-ones (80JA3620). Anions of large heterocycles often resemble their acyclic counterparts. However, anion formation can adjust the number of electrons in suitable systems so as to make a system conform to the Hiickel rule, and render it aromatic if flat geometry can be attained. Examples are found in Chapter 5.20. Anion formation in selected large heterocycles can also initiate transannular reactions (see also Section 5.02.7 below). 

Covalent hydration has been demonstrated in the following families of compounds 1,6-naphthyridines, quinazolines, quinazoline. 3-oxides, four families of l,3,x-triazanapththalenes, both l,4,x-triazanaphthalenes, pteridines and some other tetraazanaphthalenes, and 8-azapurines these compounds are discussed in that order. In general, for any particular compound (e.g. 6-hydroxypteridine) the highest ratio of the hydrated to the anhydrous species follows the order cation > neutral species > anion. In some cases, however, anion formation is possible only when the species are hydrated, e.g. pteridine cf. 21 and N-methyl-hydroxypteridines (Section III, E, 1, d). Table V in ref. 10 should be consulted for the extent of hydration in the substances discussed here. 

For reasons discussed in Section VI, a survey of the purine series (29) is being made in this Department, but so far no example (including 2-hydroxy- and 8-trifluoromethyl-2-hydroxy-purine) of covalent hydration has come to light. An examination of ionization constants disclosed no apparent anomalies, although the interpretation is made more difficult by the ease of anion formation in the 9-position, which often competes with that from other anionic substituents. The only abnormal spectrum seems to be that of the anion of 2-mercaptopurine which is being further examined. 

Finally, we can see that, neutral meolecules, either in nitro-type or in aci-nitro-type, are more stable than acid-dissociated anions the anion formation is a high endothermic reaction. The energy difference between neutral molecules and acid-dissociated anions calculated at the MP2/6311+-I-G level is 1539 kJ/mol for nitro-type species, and 1683 kJ/mol for aci-nitro-type species. It is clear that, in these conditions, the acid dissociation of the neutral molecules can hardly occur.in pure nitromethane solutions. It provides another theoretieal support for nitromethane as an ideal model of aprotic solvents. 

Whereas phosphonium ylides normally react with carbonyl compounds to give alkenes, dimethylsulfonium methylide and dimethylsulfoxonium methylide yield epoxides. Instead of a four-center elimination, the adducts from the sulfur ylides undergo intramolecular displacement of the sulfur substituent by oxygen. In this reaction, the sulfur substituent serves both to promote anion formation and as the leaving group. 

Itoh, M. Mukaihata, H. Nakagawa, T. Kohtani, S. Picosecond and two-step LIF studies of the excited-state proton transfer in 3-hydroxyxanthone and 7-hydroxyflavone methanol solutions reinvestigation of tautomer and anion formations. J. Am. Chem. Soc. 1984, 116, 10612-10618. 

J. Vasquez-Vivar, N. Hogg, K.A. Pritchard, and B. Kalyanaraman, Superoxide anion formation from ludgenin an electron spin resonance spin-trapping study. FEBS Lett. 403, 127—130 (1997). 

A major concern of this review is the tailoring of the redox behaviour of organic compounds, i.e. the optimization of such systems for electron storage and electron hopping. While the emphasis is on reduction and thus on anion formation, it has been shown on many occasions that oxidative cation formation leads to analogous conclusions (Meerholz and Heinze, 1990 Lewis and Singer, 1965). The structure of this text is thus obvious. 

Chemiluminescence also occurs during electrolysis of mixtures of DPACI2 99 and rubrene or perylene In the case of rubrene the chemiluminescence matches the fluorescence of the latter at the reduction potential of rubrene radical anion formation ( — 1.4 V) at —1.9 V, the reduction potential of DPA radical anion, a mixed emission is observed consisting of rubrene and DPA fluorescence. Similar results were obtained with the dibromide 100 and DPA and/or rubrene. An energy-transfer mechanism from excited DPA to rubrene could not be detected under the reaction conditions (see also 154>). There seems to be no explanation yet as to why, in mixtures of halides like DPACI2 and aromatic hydrocarbons, electrogenerated chemiluminescence always stems from that hydrocarbon which is most easily reduced. A great number of aryl and alkyl halides is reported to exhibit this type of rather efficient chemiluminescence 155>. 

Some key reactions relevant to this section were not covered in CHEC-II(1996) <1996CHEC-II(4)179>, and hence some references to important work prior to the appearance of CHEC-IK1996) are included. Nonetheless, CHEC-11(1996) does contain many pertinent reactions of ring carbon substituents, such as important information on Curtius rearrangements and a-anion formation and reactivity. 

A rapid synthesis of trisubstituted l,2,4-triazolo[4,3-b]pyridazines has been devised to give selective variation of the three substituents through combinations of silicon-directed anion formation, palladium-catalyzed couplings and SnAt displacements <00TL781>. The synthesis of new l,2,4-triazolo[l,5-a]pyrimidines <00M1435>, 1,2,3-triazolo[4,5-(f]... 

Hine has shown that the relative ability of substituent halogens to enhance trihalo anion formation is... 

Shallow acceptor levels lie close to the valence band and take up electrons from it to create holes in the valence band and produce p-type semiconductors. Interstitial nonmetal atoms often generate shallow acceptor levels because anion formation involves taking up extra electrons. Acceptor levels are said to be ionized when they take electrons from the valence band, creating holes in the process. The energy of a neutral acceptor atom is different to that of an ionized acceptor. The electrons on the ionized anions are often trapped and do not contribute to the conductivity. 

The pKa of the protonation of the nitrogen in position 8 has been reported as 6.02 and the pKa for the carboxylate anion formulation has been reported as -0.94. These were determined by Staroscik and Sulkowska by a spectrophotometric method.(14) Further study by the same workers on the partition equilibria of nalidixic acid between water and various organic solvents led to calculations of the pKa values of 5.99 + 0.03 for N-protonation and -0.86 4- 0.07 for carboxylate anion formation.(12) Takasugi and co-workers reported the apparent pKa of nalidixic acid to be 5.9 at 28° by a spectrophotometric method.(13)... 

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