Tight ion-pair

Attack occurs from tiie side opposite the departing leaving group 

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Dicarbocyanine and trie arbo cyanine laser dyes such as stmcture (1) (n = 2 and n = 3, X = oxygen) and stmcture (34) (n = 3) are photoexcited in ethanol solution to produce relatively long-Hved photoisomers (lO " -10 s), and the absorption spectra are shifted to longer wavelength by several tens of nanometers (41,42). In polar media like ethanol, the excited state relaxation times for trie arbo cyanine (34) (n = 3) are independent of the anion, but in less polar solvent (dichloroethane) significant dependence on the anion occurs (43). The carbocyanine from stmcture (34) (n = 1) exists as a tight ion pair with borate anions, represented RB(CgH5 )g, in benzene solution photoexcitation of this dye—anion pair yields a new, transient species, presumably due to intra-ion pair electron transfer from the borate to yield the neutral dye radical (ie, the reduced state of the dye) (44). 

In the APh-2-MV2+ system, a tight ion pair can be formed because the motional freedom of the Phen+ residue and a free access of MV + to the Phen + site allow the ion pair to realize an optimal distance and orientation, thus giving rise to a shorter-lived geminate ion pair. This explains why the back ET in the... 

The reaction (Eq. (5)) in THF yields labile THF adducts which are converted into the more stable HMPA adducts by addition of HMPA. The various equilibria existing between Na2Fe(CO)4 and several donor solvents are described in a detailed paper by Collman in HMPA, the solvent-separated supernucleophilic ion pair [Na+x HMPA x Fe(CO)4 ] is the kinetically dominant species, with no kinetic contribution from free [Fe(CO)J2 . In THF, Na2Fe(CO)4 is much less dissociated, with tight-ion paired [NaFe(CO)4] as the kinetically important species [96],... 

They represent tight ion-pairs solvated externally by some segments of their own chains. This additional binding of the cation to the polymer accounts for the very low dissociation constants of such ion-pairs (Kdiss < 10-9 M). A similar interpretation was evoked in rationalization of the low dissociation constants of the salts of living polyvinyl pyridine 40). 

Although the rule the reactivity of tight ion-pairs increases with increasing interionic distance still holds, the above results show that the ionic radius is not the only factor that determines the reactivity of tight ion-pairs. 

In the previous Sections, the properties of acids and bases in macrocycles and other concave structures have been compared. A number of factors have been recognized which influence the acidity or basicity of an acid or base (i) hydrogen bonds, (ii) hindered solvation (exclusion of solvent), (iii) formation of tight ion pairs (high microacidity but low overall acidity), and (iv) Coulomb forces when poly anions are formed. A fifth influence, (v) steric hindrance, still has to be discussed. 

Halides are ubiquitous co-ligands for cobalt(III), and are met throughout this review. Anation of (solvent)cobalt(III) complexes by halide has been examined from time to time. An example is substitution of coordinated acetonitrile in [Co(L)(MeCN)2]3+ (L = tetraaza-macrocycle) by Cl-and Br-.1096 A mechanism involving interchange from within tight ion pairs was proposed. Halo-bridged polymeric complexes are well known with both classical and organometallic complexes. 

Lithium 2-diisopropylamino-2-boratanaphthalene (cf. 28) with Me2-NCH2CH2NMe2 (TMEDA) forms a crystalline derivative Li(TMEDA) (C9H7BNPr 2) which has been characterized by X-ray structure determination (102). The lithium cation is situated above the center of the boratabenzene moiety of the anion and, in addition, is chelated by one TMEDA ligand. Thus, the crystal consists of discrete tight ion-pair molecules. 

If tight ion pairs (between two hard ions) are involved in the reaction the microwave-accelerating effect then becomes more important, because of enhancement of ionic dissociation during the course of the reaction as tight ion pairs (GS) are transformed into more polar loose ion pairs (TS). 

In this case, the anion being hard and with a high charge density, the reactions are concerned with tight ion pairs. During the course of the reaction, ionic dissociation is increased and hence polarity is enhanced from the GS towards the TS. Specific microwave effects should be expected. 

These observations are consistent with the reactive species being constituted from tight ion pairs between cations and the alkoxide anions resulting from abstraction of hydrogen atoms in A, B and C (Scheme 3.11). 

The reactive species under these conditions consist of tight ion pairs involving the alkoxide anion from the carbohydrate (charge localized anion). The less reactive long chain methyl laurate leads to a later TS along the reaction coordinates and the magnitude of the microwave effect is therefore increased. 

In the radical anions of the norbornane-linked naphthalenes [37] mentioned earlier (Gerson et al, 1990) no counterion effects were detected for [37a], which has a small spatial separation, but the esr/ENDOR spectra of [37b]- and [37c]- indicate that the electron-spin transfer between the naphthalene moieties is determined by the rate of synchronous counterion migration (Gerson et al., 1990). For tight ion pairs the electron is localized, while for loose ion-pair conditions, e.g. by using solvents of high cation-solvating power, the transfer becomes fast on the hyperfine timescale (k > 107 Hz). 

Figure 2. The tight ion pair 17 in the asymmetric alkylation of the cyclic ketones.

Scheme 5. An intermediacy of a tight ion pair 17 fixed by an electrostatic effect and hydrogen bonding as well as ji-ji stacking interactions was proposed to account for the result (Figure 2).1161...

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