Anion traps

A relative of the latter class of compounds are the macrotricyclic quaternary ammonium salts which have been reported by Schmidtchen. The bridges may contain either methylenes or ethyleneoxy units and the nitrogens are quaternarized. The underlying principle is to provide a cavity suitable for solvating or at least trapping anions. Schmidtchen presents evidence which suggests the formation of halide inclusion complexes. The synthesis of these molecules is accomplished along more or less traditional lines Such a species is illustrated above as compound 19. 

Some of the earliest work using guanidiniums for trapping anionic guests dealt with binding inorganic phosphate. Lehn and coworkers synthesized the three... 

Table I. Monomers, structures of the growing species and trapped anions, according to (5).

In the inhomogeneous coulombic field generated by the trapped cations and anions formed by irradiation, positive holes will migrate in the direction of trapped anions. When, as a result of this process, a matrix (RjH) radical cation becomes adjacent to a solute (RiiH) molecule, positive-hole transfer to the solute may occur. 

The second glow peak is reduced when the number of trapped anions is reduced. Therefore, the second glow peak must be ascribed at least partially to the diffusion of biphenyl cations and anions and their mutual neutralization ... 

Photo-excitation of gas-phase ions may result in the photodetachment of an electron rather than photo-fragmentation. Coulombic considerations dictate that this process is more prevalent for anions than for cations. Electron photodetachment action spectroscopy of trapped anions has proved also to be a valuable source of molecular information. In some systems, electron photodetachment and PD compete. The mechanisms for these two processes in large molecules are yet to be understood fully consequently, their branching ratios in specific experimental conditions cannot be predicted as yet. One exciting possibility is the idea of using frequency and phase-shaped pulses to promote selected photochemical pathways. 

The lamellar structure of LDH, based on a stacking of positive layers trapping anionic species in the interlayer domains, is highly favorable to anion diffusion, and LDH is one of the principal classes of inorganic ion exchangers (Clearfield 1988). This property has been mainly used in order to prepare new LDH phases by anionic exchange reactions. The reaction can be described by the following equilibrium (3) ... 

Addition ofGrignard reagents to 1,1-difluoroethylene yields an acetylide anion which can be subsequently trapped with electrophiles. 

In these cydizations, the reaction can be terminated in other ways than elimination of /3-hydrogen. Typically the reaction ends by an anion capture process[154]. The following anion transfer agents are known H, OAc , CN, S02Ph, CH(C02R)2, NHRj, CO/ROH, and RM [M = Sn(IV), B(lll), Zn(II)]. Trapping with an amine after alkene insertion to give 189 and 190 is an example. A-Acetyl protection is important in this reaction[155]. 

In an efficient diastereo-differentiative assembly of three components of norbornene, tv. v-alkenyl iodide, and KCN, the isomerization of the cis to the trans double bond takes place to give the coupled product 224. The isomerization is explained by the formation of the cyclopropane 222. its rearrangement to give a irans double bond in 223, and trapping with CN anion to give 224[168],... 

The intramolecular insertion of a conjugated diene into 7r-allylpalladium, initially formed in 789, generates another rr-allyl complex 790, which is trapped with acetate anion to give a new allylic acetate 791. No further reaction of the allylic acetate with alkene takes place[489]. 

Certain neutral technetium complexes can be used to image cerebral perfusion (Fig. 4). Those in Figure 4a and 4b have been approved for clinical use. Two other complexes (Fig. 4c and 4d) were tested in early clinical trials, but were not developed further. An effective cerebral perfusion agent must first cross the blood brain barrier and then be retained for the period necessary for image acquisition. Tc-bicisate is retained owing to a stereospecific hydrolysis in brain tissue of one of the ester groups to form the anionic complex TcO(ECD) , which does not cross the barrier. This mechanism of retention is termed metaboHc trapping. 

Azidopyrazoles exist as such (459), but their anions (460) are in equilibrium with tetrazole anions (461) which can be trapped as (462). 

However, when the molecules are converted into their anions, the cyclic pyrazolo[l,5- fjtetrazole (471) form predominates. It is possible to trap the cyclized anions such as (471) by methylation and to obtain stable azapentalenes (472) and (473). 

Unsubstituted 3-alkyl- or 3-aryl-isoxazoles undergo ring cleavage reactions under more vigorous conditions. In these substrates the deprotonation of the H-5 proton is concurrent with fission of the N—O and C(3)—-C(4) bonds, giving a nitrile and an ethynolate anion. The latter is usually hydrolyzed on work-up to a carboxylic acid, but can be trapped at low temperature. As shown by Scheme 33, such reactions could provide useful syntheses of ketenes and /3-lactones (79LA219). 

The stereochemistry observed in hydrogen-exchange reactions of carbanions is very dependent on the conditions under which the anion is formed and trapped by proton... 

Hexafluoro-2-butyne readily undergoes anionic oligomerization in the presence of fluoride ion 24 ], but the intermediate vinylic carbamon can be trapped by highly electrophilic fluormated heterocycles [249] (equation 52)... 

The Na is 555 pm from the nearest N and 516 pm from the nearest O, indicating that it is a separate entity in the structure. Potas-sides, rubidides and caesides have similarly been prepared. The same technique has been used to prepare solutions and even crystals of electrides, in which trapped electrons can play the role of anion. Typical examples are [K(cryptand)]+e and [Cs(18-crown-... 

Tliis conclusion is supported by the data on the azide-tetrazole equilibrium for s-triazolo[2,3-d]tetrazoles (128) [79JCS(P1)2886]. Methylation of neutral azide resulted in the 1-methyl derivatives of 3-azidotriazole 128A only, whereas on methylation of the anion 128A, the tetrazoles 129 and 130 were also trapped in 25 and 10% yields, respeetively. Tire predominanee of 128T over 128T" was attributed to these two bieyelie anions but no ealeu-lations on relative energies have been performed. Tire azide-tetrazole equilibrium eonstants were measured for 128A7T in DMSO-dg 0.45 at 27°C (R = H), and 0.78 at 23°C and 1.80 at 80°C (R = Ph). 

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