Solvent separated ion pair, formation

The formulation of two types of ion-pair is an attractive hypothesis which has been used for other systems [130] to explain differences in reactivity. The polymerization of styrene-type monomers in ether solvents, all of which solvate small cations efficiently, seems to be a particularly favourable case for the formation of thermodynamically distinct species. Situations can be visualized, however, in which two distinct species do not exist but only a more gradual change in properties of the ion-pair occurs as the solvent properties are changed. These possibilities, together with the factors influencing solvent-separated ion-pair formation, are discussed elsewhere [131, 132]. In the present case some of the temperature variation of rate coefficient could be explained in terms of better solvation of the transition state by the more basic ethers, a factor which will increase at lower temperatures [111]. This could produce a decrease in activation energy, particularly at low temperatures. It would, however, be difficult to explain the whole of the fep versus 1/T curve in tetrahydrofuran with its double inflection by this hypothesis and the independent spectroscopic and conductimetric evidence lends confidence to the whole scheme. 

When a double bond is added to an acyclic co-methoxy arenesulfonate competition between n and methoxy participation is possible. The acetolysis of cis-5-methoxypent-3-en-l-yl p-bromobenzenesulfonate (42a) is anchimeri-cally assisted and yields compounds (43)-(46) as well as the corresponding methyl sulfonate ester unless lithium perchlorate is present.It was shown that lithium perchlorate also prevented (by eliminating return from solvent separated ion pairs) formation of the allylic toluene-p-sulfonate (47), thus products (45) and (46) must be formed from allylic cation (48). 

The ion-pair return phenomenon can also be demonstrated by comparing the rate of loss of enantiomeric purity of reactant with the rate of product formation. For a number of systems, including 1-aiylethyl tosylates, ftie rate of decrease of optical rotation is greater than the rate of product formation. This indicates the existence of an intermediate that can re-form racemic reactant. The solvent-separated ion pair is the most likely intermediate in the Winstein scheme to pl this role. 

According to Eigen and Tamm [87,88], ion-pair formation proceeds stepwise, starting from separated solvated ions which form a solvent-separated ion pair [C+SSA ]°, followed by a solvent-shared ion pair [C+SA ]° and finally a contact ion pair, [C+A ]° [Eqs. (4)-(6)]. All these species are solvated. The types of ion pair formed depend on the relative strength of the interaction of the involved species. 

Alkenyllithium derivatives, carrying carbanion-stabilizing substituents, which facilitate the formation of solvent-separated ion pairs, can also exhibit preparatively useful configurational stability in respect to the double bond of the precursor. 

We emphasize that the critical ion pair stilbene+, CA in the two photoactivation methodologies (i.e., charge-transfer activation as well as chloranil activation) is the same, and the different multiplicities of the ion pairs control only the timescale of reaction sequences.14 Moreover, based on the detailed kinetic analysis of the time-resolved absorption spectra and the effect of solvent polarity (and added salt) on photochemical efficiencies for the oxetane formation, it is readily concluded that the initially formed ion pair undergoes a slow coupling (kc - 108 s-1). Thus competition to form solvent-separated ion pairs as well as back electron transfer limits the quantum yields of oxetane production. Such ion-pair dynamics are readily modulated by choosing a solvent of low polarity for the efficient production of oxetane. Also note that a similar electron-transfer mechanism was demonstrated for the cycloaddition of a variety of diarylacetylenes with a quinone via the [D, A] complex56 (Scheme 12). 

With this mechanistic scheme, the chemoselectivity of the addition and the formation of rearranged chlorides (but not acetates) have been chosen as criteria to differentiate the ion pair mechanism from the purely ionic one and, on the basis of both criteria, the authors suggest the involvement of a tight ion pair for the addition of ArSCl in AcOH to diene 62 and of solvent separated ion pairs to triene 108. The effects related to the presence of added electrolytes, which favor the formation of rearranged acetates, have been considered in this work127 as evidence that even a larger separation of ions, which should lead to more electrophilic species, is possible. 

Contact- and solvent-separated ion pairs form whenever solvolysis proceeds to the free carbocation. However, these intermediates are generally only thought of as significant when their formation can be detected by experiment. We have focused on several different reactions of ion pairs that leave detectable signatures. 

The ethylene bromonium and 1-bromoethyl cations and their neutral and anionic counterparts have been the subject of a tandem mass spectrometric study of dissociation and gas-phase redox reactions. IR and Raman studies of the bioactive bromonium cation (19), as its hydrogensulfate salt, agree with the results of an X-ray structure determination, and theoretical calculations are also in agreement, except for the details of the NO2 groups. The azaallenium ion (22) is an intermediate in the photolysis of (20) (21) and (22) could both be seen. Flash photolysis of (23) leads to (24), (25), and (26), all of which could be trapped by nucleophiles (27) was not an intermediate. NMR lineshape analysis of the spectmm of (28) leads to reaction rate constants of formation for both the intimate ion pair (29) and the solvent-separated ion pair (30). ... 

Benzylic compounds have—compared to the corresponding methyl derivatives—a higher thermodynamic acidity by 10 to 15 pATa units . Mesomeric stabilization requires a considerable flattening of the carbanionic centre towards sp hybridization (the sum of bond angles is 360° for sp and 328° for sp ). However, we should be aware that even if the carbanionic framework would be completely planar, the ion pair 209 is a planar-chiral species. For epimerization, the cation has to migrate from one face to the other one (equation 48). Due to a more facile flipping of the carbanionic centre and an easier formation of solvent-separated ion pairs, most of chiral benzyUithium compounds 208/ewi-208 racemize with great ease. 

The same type of addition—as shown by X-ray analysis—occurs in the cationic polymerization of alkenyl ethers R—CH=CH—OR and of 8-chlorovinyl ethers (395). However, NMR analysis showed the presence of some configurational disorder (396). The stereochemistry of acrylate polymerization, determined by the use of deuterated monomers, was found to be strongly dependent on the reaction environment and, in particular, on the solvation of the growing-chain-catalyst system at both the a and jS carbon atoms (390, 397-399). Non-solvated contact ion pairs such as those existing in the presence of lithium catalysts in toluene at low temperature, are responsible for the formation of threo isotactic sequences from cis monomers and, therefore, involve a trans addition in contrast, solvent separated ion pairs (fluorenyllithium in THF) give rise to a predominantly syndiotactic polymer. Finally, in mixed ether-hydrocarbon solvents where there are probably peripherally solvated ion pairs, a predominantly isotactic polymer with nonconstant stereochemistry in the jS position is obtained. It seems evident fiom this complexity of situations that the micro-tacticity of anionic poly(methyl methacrylate) cannot be interpreted by a simple Bernoulli distribution, as has already been discussed in Sect. III-A. 

Bimolecular photoinduced electron transfer between an electron donor and an electron acceptor in a polar solvent may result in the formation of free ions (FI). Weller and coworkers [1] have invoked several types of intermediates for describing this process (Fig.la) exciplex or contact ion pair (CIP), loose ion pair (LIP), also called solvent separated ion pair. The knowledge of the structures of these intermediates is fundamental for understanding the details of bimolecular reactions in solution. However, up to now, no spectroscopic technique has been able to differentiate them. The UV-Vis absorption spectra of the ion pairs and the free ions are very similar [2], Furthermore, previous time resolved resonant Raman investigations [3] have shown that these species exhibit essentially the same high frequency vibrational spectrum. 

The proposition that locally excited triplet states can be formed from back electron transfer within a doublet-doublet radical ion pair has firm theoretical (88) and experimental support. For example, with time-resolved Resonance Raman spectroscopy, one can directly monitor the chemical fate of the exciplex, solvent separated ion pair, and doublet free radical ion pairs formed between stilbene and amines. As might be expected from the above discussion, adduct formation is observed from the exciplex or contact ion pairs, whereas enhanced intersystem crossing ensues from the solvent separated ion pairs, producing spectroscopically observable stilbene triplets. This back electron transfer process, eq. 30 (89),... 

Finally, the effect of reaction conditions and solvents on chemoselectivity should also be considered.18 Empirically, polar, more basic solvents, e.g. HMPA, DMF, serve to minimize counterion effects by formation of solvent-separated ion pairs and promote electron-transfer processes which are conducive for... 

In solvent-separated ion pairs, the solvation shells of the cation and the anion touch each other in solvent-bridged ion pairs, the ions share solvent molecules. In contact ion pairs, the cation and the anion are bound directly to each other and are surrounded by a common solvation shell. In penetrated ion pairs, an empty space between edge groups in one ion of a salt is occupied to a certain degree by a counterion. The two latter types of ion pair may have quite a different electronic distribution than the corresponding naked ions. The following examples show the influence of ion-pair formation. 

Time profiles of the formation of fullerene radical anions in polar solvents as well as the decay of 3C o obey pseudo first-order kinetics due to high concentrations of the donor molecule [120,125,127,146,159], By changing to nonpolar solvents the rise kinetics of Go changes to second-order as well as the decay kinetics for 3C o [120,125,133,148], The analysis of the decay kinetics of the fullerene radical anions confirm this suggestion as well. In the case of polar solvents, the decay of the radical ion absorptions obey second-order kinetics, while changing to nonpolar solvents the decay obey first-order kinetics [120,125,127,133,147]. This can be explained by radical ion pairs of the C o and the donor radical cation in less polar and nonpolar solvents, which do not dissociate. The back-electron transfer takes place within the ion pair. This is also the reason for the fast back-electron transfer in comparison to the slower back-electron transfer in polar solvents, where the radical ions are solvated as free ions or solvent-separated ion pairs [120,125,147]. However, back-electron transfer is suppressed when using mixtures of fullerene and borates as donors in o-dichlorobenzene (less polar solvent), since the borate radicals immediately dissociate into Ph3B and Bu /Ph" [Eq. (2)][156],... 

The exchange of alkyl groups between anions MR4 and M R4 does not proceed readily, but reaction between M(CH3)3 and M(CH3)4- has been observed in several instances (110, 112, 153). On the basis of their studies of the Al(CH3)3-LiAl(CH3)4 systems, Williams and Brown suggested that the rate-determining step is the formation of solvent-separated ion pairs,... 

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