Anion delocalized

The hydrogeh atom bound to the amide nitrogen in 15 is rather acidic and it can be easily removed as a proton in the presence of some competent base. Naturally, such an event would afford a delocalized anion, a nucleophilic species, which could attack the proximal epoxide at position 16 in an intramolecular fashion to give the desired azabicyclo[3.2.1]octanol framework. In the event, when a solution of 15 in benzene is treated with sodium hydride at 100 °C, the processes just outlined do in fact take place and intermediate 14 is obtained after hydrolytic cleavage of the trifluoroacetyl group with potassium hydroxide. The formation of azabi-cyclo[3.2.1]octanol 14 in an overall yield of 43% from enone 16 underscores the efficiency of Overman s route to this heavily functionalized bicycle. 

More recently considered candidates are large molecular anions with delocalized anionic charge, which offer low lattice energies, relatively small ion-ion interaction, and hence sufficient solubility and relatively large conductivity. Delocalization of the charge is achieved by electron-with drawing substituents such as -F or - CF3. Furthermore, these anions show a good electrochemical stability to oxidation. In contrast to Lewis acid-based salts they are chemically more stable with various solvents and often also show excellent thermal stability. 

The X-ray structure of lithium l-(dimethylamido)boratabenzene, reported in 1993, provided the first crystallographic characterization of a transition metal-free boratabenzene (Scheme 13).18a The observed bond lengths are consistent with a delocalized anion and with significant B—N double-bond character. In a separate study, the B—N rotational barrier of [C5H5B—NMeBnjLi has been determined to be 10.1 kcal/mol, and it has been shown that TT-complexation to a transition metal can increase this barrier (e.g., 17.5 kcal/mol for (C5H5B-N(i-Pr)2)Mn(CO)3).24... 

Two main structural types have been identified for allyl alkali metal species solvated ions in the form of CIPs where a delocalized anion with metal coordination is perpendicular to the ligand plane,130-134 or unsolvated allylic lithium compounds displaying localized ligand systems with NMR spectra closely resembling those of alkenes.135-138... 

The transition states are composed of loose ion pairs in so far as they involve a charge-delocalized anion, thereby enhancing polarity compared with the ground states (in which the ion pairs are tighter), because of an increase in anionic dissociation as the more bulky product anion is formed. As a consequence, specific micro-wave effects, directly connected to polarity enhancement, should depend on the structure of reactive ion pairs in the GS ... 

Anionic SN2 Reactions Involving Charge-delocalized Anions... 

The reaction is thermodynamically controlled and was postulated to involve an achiral, delocalized anion 15, which cyclizes to a somewhat strained 7-membered ring complex 16 capable of existing in isomeric forms. The more stable form could be hydrolyzed to the predominant ( + )-(S)-allenic alcohol 17 (Scheme 1)... 

From these investigations it is clear that the Li chemical shift gives a clear indication of the lithium cation position when there are direct effects from ring currents in delocalized anions. However, as shown for the quinuclidine CIP and THF SSIP fluorenyllithium complexes, the correct assignment cannot be reached solely based on the chemical shifts. Furthermore, there is no clear-cut information about solvation to be gained from the chemical shifts. As we discuss in the following Section, the quadrupolar interaction is much more sensitive to these effects. In order to obtain maximal structural information from Li NMR spectroscopy, the chemical shift should be determined and used in combination with the quadrupolar coupling constant. 

Other delocalized anions have been investigated as well, such as complexes of indenyl and fluorenyllithium. These data are also included in Table 8. The sole investigated indenyllithium system was the TMEDA complex. It is known from X-ray crystallography that the lithium cation is located above the five-membered ring and that the TMEDA binds in a bidentate fashion . The x value is somewhat larger than for the corresponding cyclopentadienyllithium complexes (entry 9). 

There may be two proton transfers in the carbonic anhydrase II-catalyzed mechanism of CO2 hydration that are important in catalysis, and both of these transfers are affected by the active-site zinc ion. The first (intramolecular) proton transfer may actually be a tautomerization between the intermediate and product forms of the bicarbonate anion (Fig. 28). This is believed to be a necessary step in the carbonic anhydrase II mechanism, due to a consideration of the reverse reaction. The cou-lombic attraction between bicarbonate and zinc is optimal when both oxygens of the delocalized anion face zinc, that is, when the bicarbonate anion is oriented with syn stereochemistry toward zinc (this is analogous to a syn-oriented carboxylate-zinc interaction see Fig. 28a). This energetically favorable interaction probably dominates the initial recognition of bicarbonate, but the tautomerization of zinc-bound bicarbonate is subsequently required for turnover in the reverse reaction (Fig. 28b). 

A second and related consequence in aliphatic nitro compounds is the acidification of the directly bonded CH unit through the attendant stabilization of the derived conjugate bases (5,6). As with all delocalized anions, reprotonation gives rise to tautomers, the original C-nitro compound (I) and the oci-nitro or isonitro form (II), Eq. 2.1. The aci-nitro tautomers are typically present in very minor concentrations, with equilibrium constants (A eq) between 10 and 10 (7). Alkylation of the delocalized anion leads to both a-substituted nitro compounds and the regioisomeric nitronic esters (nitronates). Nitronates were described as early as 1894 (8), however, the first isolated nitronic ester was obtained several years later upon the addition of diazomethane to phenylazonitromethane (1), Eq. 2.2 (9). 

Influence of the Anionic Ends Structures In adding different monomer units at the end of monocarban-ionic polystyrenes, we obtain a set of carbanionic structures which habe been deactivated In the same way. The results (Table VI) show that the terminal unit, which allows the more delocalized anion or radical charge, and presents the more sterlc hindrance, gives the lower coupling ratio, and the best functionality. 

The structure of the anion may be described as a hybrid of five energetically equivalent structures, 34a through 34e. The unshared electron pair therefore is delocalized over five carbon atoms, and the resulting delocalized anion is much more stable than expected for any one of the equivalent localized structures ... 

In a broader view one may conjecture that there is here a whole new field to be developed, having to do with large delocalized anionic entities obtained by injecting electrons into extended organic Jt systems. Cryptatium species represent one class of such compounds having a central positive potential. Many others of either spheroidal or non-spheroidal shape may be envisaged [2.84c]. 

Pyridine is attacked by alkyllithium reagents at its most electropositive atom (C2) to give the charge delocalized anion 87112). In a similar manner the heavier heterobenzenes are attacked at their most electropositive atoms (the heteroatoms) to give heterocyclohexadienides 88 113). 

Decarboxylation leading to a delocalized anion (which could be an anion, an enol, or an enamine) could be treated by a model with three dimensions bond change, geometry change at carboxylate, and geometry change at the enolate carbon. Some neutral carboxylic acids might react by... 

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