Anionic pathways

Where does the hydrogen atom in the product of hydro-de-diazoniation, 2-chloro-nitrobenzene (8.66), come from in CH3OD It was found (Bunnett and Takayama, 1968 b Broxton and Bunnett, 1979) that in the reaction of Scheme 8-47 the deuterium content of 2-chloronitrobenzene was 79%, a figure which is not close to either zero or 100%. For other substituted benzenediazonium ions a very wide range of D incorporation was observed. This range is consistent with hydro-de-diazoniation by both homolytic and a competitive anionic mechanism. The anionic pathway is favored by an increase in methoxide ion concentration. 

Polymerisation of vinyl monomers with electron-donating substituents can proceed by a cationic mechanism, while monomers with electron-withdrawing group can polymerise by an anionic pathway. 

The radical anion pathway (e-c-P-d-p Scheme 2) requires a rate-determining protonation after cyclization, i.e., a slow protonation of a hard oxyanion. However, such proton transfer rates are usually diffusion controlled, so this seems unlikely [32,33], On the other hand, the carbanion closure (e-P-d-c-p) portrayed in Scheme 4 requires a very reasonable suggestion that the ratedetermining step corresponds to protonation of the soft, weakly basic radical anion 42, prior to cyclization [32-35] this is the preferred mechanism. One must use caution, however, realizing that these conclusions are drawn for the particular set of substrates which were examined. In some cases, radical anion cyclization remains a viable option. 

The chemistry of radical sites adjacent to phosphatoxy centers elicited interest because of the involvement of such species in DNA degradation processes. These species can give rise to rearrangement, elimination, and substitution products, and for some time concerted eliminations and migrations as well as heterolysis to a radical cation and a phosphate anion were considered to be involved (Scheme 2). Recently, experimental studies of the l,2-dibenzyl-2-(diphenylphosphatoxy)-2-phenylethyl radical and complementary theoretical studies of l,l-dimethyl-2-(dimethylphosphatoxy)ethyl radical have been interpreted as indicating that a radical cation/anion pathway with initial formation of 49 is favored. ... 

If the Cd is adsorbed on the substrate (either directly or indirectly through a hydroxide linkage) or on previously deposited CdS, then the same reaction would occur. If the CdS so formed remained bound to the substrate (it is assumed that CdS generated on previously deposited CdS would remain bound), the result would be film growth by an ion-by-ion, complex-decomposition mechanism. As with the cluster mechanism, it is difficult to distinguish experimentally between the complex-decomposition mechanism and the free-anion pathway. 

Original Anionic Pathway to New PA(PO)a Star-Shaped Block Polymers Based on Polyvinyl or Polydiene Hydrocarbons and Polyoxirane... 

The Neber rearrangement of oxime sulfonates has been considered to proceed via a nitrene pathway or an anion pathway. If the latter mechanism is operative, the use of a certain chiral base might result in the discrimination of two enantiotropic a-protons to furnish optically active a-amino ketones. Verification of this hypothesis was provided by realizing the asymmetric Neber rearrangement of simple oxime sulfonate 83,... 

Scheme 2 Photocycloaddition via radical/cation and radical/anion pathway.

In the diaryl amine series 191 (Scheme 47), additional, synthetically valuable, anionic pathways provide routes to anthranilates 190, oxindoles 192, and dibenzazepinones 194. Although not explored in terms of its scope [75c], the lateral metalation-cydization, 191 -> 192, is extensively precedented [69] but harbors intriguing potential for subsequent DreM chemistry. The rearrangement 191 —> 190 is an N —> ortho C anionic Fries equivalent of the aryl O-carbamate migration (Scheme 3E) [75c] and after N-methylation, 190 may be transformed into 1,2,3-trisubstituted systems 193 [76]. In another appealing manifestation of CIPE, the efficient conversion of 191 into dibenzazepinone 194 has been applied in an effective synthesis of the antiepileptic drug oxcarbazepine (Trileptal ) 195 [77] and may also be... 

Scheme 47. Anionic pathways to oxindoles, anthranilates, and dibenzazepinones.

Zhou, D.-L., Carrero, H. and Rusling, J.F. (1996) Radical vs anionic pathway in mediated electrochemical reduction of benzyl bromide in a bicontinuous microemulsion. Langmuir 12, 3067-3074. 

The behavior of 170c and 170d therefore supported the cyclopropyl anion pathway A in favor of the ET mechanism B. Furthermore, as far as the proposed rearrangement of the proposed radical anions 175 176 is concerned, it is shown later in this section that in... 

In the much faster ET-catalyzed reaction a new stereoisomer, r-phenyl-c-2,t-3-dimethylcyclopropane (cis,trims-184) shows up in equilibrium with the isomers cis,cis- and trans, trims-184. This excludes the possibility that the cyclopropyl anion pathway occurs exclusively. It also excludes the belief that under base-catalyzed conditions ET-catalyzed isomerizations must play a significant role. This result also supports the conclusion that in the base-catalyzed reaction of 170a the proposed ET mechanismis not valid (see... 

The first step of the mechanism is the deprotonation of the O-acylated ketoxime at its a-position, which gives rise to the corresponding enolate. This enolate then can react via two possible pathways 1) a concerted anionic pathway in which the leaving group is directly displaced to give the isolable 2H-azirine or 2) a nitrene pathway that leads to the same 2/-/-azirine intermediate via nitrene Insertion. The nitrene pathway has not been disproved experimentally. 

Ooi, T., Takahashi, M., Doda, K., Maruoka, K. Asymmetric Induction in the Neber Rearrangement of Simple Ketoxime Sulfonates under Phase-Transfer Conditions Experimental Evidence for the Participation of an Anionic Pathway. J. Am. Chem. Soc. 2002, 124, 7640-7641. 

Other authors concluded later 66) that the reaction of 93 a with t-BuOK in DMSO or HMPT (25 °C, 24 h), or with dimsylpotassium in DMSO (70 °C, 24 h) appears to proceed by a radical anion pathway. It was proposed that an initial electron transfer from base to 93 a affords the radical anion 97a which opens to 98 a. The latter was envisaged to rearrange to 99.a which loses an electron, possibly to 93 a, to give 96a (pathway B). 

Synthesis of organized mesoporous alumina - anionic pathway... 

Fig. 9. Transmission electron micrographs of organized mesoporous alumina synthesized using the anionic" pathway and calcined at 420 (A) and 800 °C (B) [78]...

Oral oseltamivir phosphate is absorbed rapidly and cleaved by esterases in the GI tract and liver to the active carboxylate. Low levels of the phosphate are detectable, but exposure is only 3 to 5% of that of the metabolite. The bioavailability of the carboxylate is estimated to be approximately 80%. The time to maximum plasma concentrations of the carboxylate is about 2.5 to 5 hours. Food does not decrease bioavailability but produces the risk of GI intolerance. After 75-mg doses, peak plasma concentrations average 0.07 pg/mL for oseltamivir phosphate and 0.35 pg/mL for the carboxylate. The carboxylate has a volume of disttibution similar to extracellular water. Broncho-alveolar lavage levels in animals and middle-ear fluid and sinus concentrations in humans are comparable with plasma levels. Following oral administtation, the plasma half-life of oseltamivir phosphate is 1 to 3 hours and that of the carboxylate ranges from 6 to 10 hours. Both the prodrug and active metabolite are eliminated primarily unchanged through the kidney. Probenecid doubles the plasma half-life of the carboxylate, which indicates tubular secretion by the anionic pathway. 

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