Special-salt effect

The special salt effect is a constant feature of the activation of substrates in cages subsequent to ET from electron-reservoir complexes. In the present case, the salt effect inhibits the C-H activation process [59], but in other cases, the result of the special effect can be favorable. For instance, when the reduction of a substrate is expected, one wishes to avoid the cage reaction with the sandwich. An example is the reduction of alkynes and of aldehydes or ketones [60], These reductions follow a pathway which is comparable to the one observed in the reaction with 02. In the absence of Na + PFg, coupling of the substrate with the sandwich is observed. Thus one equiv. Na+PFg is used to avoid this cage coupling and, in the presence of ethanol as a proton donor, hydrogenation is obtained (Scheme VII). 

There is also the special salt effect of LiC104, mentioned on page 399. In addition to these effects, SnI rates are also greatly accelerated when there are ions present that... 

The special salt effect. The addition of LiC104 or LiBr in the acetolysis of certain tosylates produced an initial steep rate acceleration that then decreased to the normal linear acceleration (caused by the ordinary salt effect).45 This is interpreted as follows the CIO., ... 

Scheme 5 depicts Winstein s complete solvolysis mechanism.29 Ion-pair return can be from the intimate ion pair (ion-pair return or internal return), from the external ion pair (external ion-pair return), or from the free ions (external ion return). The term external return refers to the sum of external ion-pair return and external ion return. The special salt effect operates by diversion of the external.ion pair, probably through the mechanism shown in Equation 519, so that it can no longer... 

Aroyl esters of anthracene-9-methanol are photolysed in methanol to give products consistent with the anthracene-9-methyl cation as an intermediate.41 Rate constants for the solvolyses of secondary alkyl tosylates in fluorinated solvents were analysed in terms of the possible involvement of very short-lived carbocation-tosylate ion pair intermediates.42 The effect of added electrolytes on the rate of solvolysis of cumyl chloride and its -methyl derivative was studied in 90% aqueous acetone and 80% aqueous DMSO, with the results revealing a combination of a special salt effect and a mass law effect.43 Kinetic parameters obtained for the solvolysis of (8) (R1 = R2 = Me and R1 = Ar, R2 = H) show that there is substantial n, n participation in the transition state [e.g. (9). 44... 

A normal, a special, and a negative special salt effect have been detected in the SnI reaction between benzhydryl chloride and LiC104 in y-butyrolactone.71... 

Fig. 2. Dependence of the transient spectra on added lithium perchlorate 100 ps after flash photolytic excitation of fluorenone (5) in the presence of tetraethoxyethene (5) in acetonitrile. Special salt effect [32].

In the investigations of the systems so far mentioned, only the kinetics of the long-wavelength absorbing radical anions could be monitored spectroscopically. Simultaneous analysis of both transient species was performed using p -chloranil (9) as the acceptor and 2-methoxy-l,l-diphenylethene (10) as the donor [33, 36]. Both the radical anion 9 and the radical cation 10+ decay within the same halflife of ca. 1 ps, as expected for back electron transfer as the major process (Fig. 6). Utilizing the special salt effect (addition of lithium perchlorate) increases the lifetime of both intermediates by a factor of ten. 

Ion pair separation can also be facilitated by utilizing salt effects [2,6,16]. The basic principle is exemplary illustrated in Sch. 4 for a special salt effect induced by the addition of e.g., lithium perchlorate (generally in a polar solvent like acetonitrile). Applying this procedure, the primarily formed radical ion pairs (either as contact or solvent separate ion pairs) are subsequently replaced by the formation of a new and tight ion pair between the acceptor radical anion (A -) and lithium cation (Li+). PET reactions often proceed solely under these condition, e.g., when using ketones as PET-sensitizers [16]. 

For example, anchimerically assisted solvolyses may show the special salt effect , implicating solvent-separated ion pairs, and solvolyses of highly stabilized substrates (e.g. diphenylmethyl compounds) show the common ion effect , implicating kinetically free cations (Raber et al., 1974). Also, Hammett (1970a) has drawn attention to an inconsistency in the widely accepted interpretation of the special salt effect. 

Three approaches have been used to decrease the molecular weight distributions. One approach suppresses dissociation of ion pairs to free ions by adding salts with common counteranions however, this may cause a special salt effect [274]. Addition of a common ion salt shifts the equilibrium between ions and ion pairs toward the latter by mass law. (In spite of speculation to the contrary [275], the common ions can not influence the equilibrium between covalent species and ion pairs.) The kinetics of association is also affected because ion pair formation is a bimolecular reaction whose rate increases with increasing anion concentration, This decreases the lifetime of free ions. In such systems, kdeact in Eq. (68) should be replaced with the product of /cdeacl and deactivator [D] = [A ], in which the deactivator is a counteranion (DP ./DP = 1 + [l]0 p/ ([D] deacl)). 

Salts with less nucleophilic ions accelerate the polymerization of vinyl ethers initiated by HI adducts alone. This can be again ascribed to a special salt effect and exchange of counterions [176]. 

In some systems it is necessary to add a large amount of salts to obtain polymers with low polydispersities. This happens when salts participate in ligand/anion exchange (special salt effect) and when they enhance ionization of covalent compounds through the increase of ionic strength. The special salt effect may either reduce or enhance ionization. Strong rate increases observed in the polymerization of isobutyl vinyl ether initiated by an alkyl iodide in the presence of tetrabutylammonium perchlorate or triflate can be explained by the special salt effect [109]. The reduction in polymerization rate of cyclohexyl vinyl ether initiated by its HI adduct in the presence of ammonium bromide and chloride can be also ascribed to the special salt effect [33]. The breadth of MWD depends on the relative rate of conversion of ion pairs to covalent species and is affected by the structure of the counterions. 

The observation that the rate of loss of optical activity during the solvolysis of certain chiral substrates R — X exceeded the rate of acid production and the occurrence of a special salt effect led to the postulation of two distinct ion-pair intermediates [161, 162]. The basic Winstein solvolysis scheme is given by Eq. (2-20). 

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