Ion-pairing mechanism

Kinetic studies of the addition of hydrogen chloride to styrene support the conclusion that an ion-pair mechanism operates because aromatic conjugation is involved. The reaction is first-order in hydrogen chloride, indicating that only one molecule of hydrogen chloride participates in the rate-determining step. ... 

In certain cases, e.g. with Z = tert-butyl, the experimental findings may better be rationalized by an ion-pair mechanism rather than a radical-pair mechanism. A heterolytic cleavage of the N-R bond will lead to the ion-pair 4b, held together in a solvent cage ... 

The ion pair mechanism initially suggested by Darwish and McLaren28 (equation 2) has received further support from related studies conducted by several other investigators38-42. For example, Fava and coworkers38 have reported that during isomerization in acetic acid, optically active benzhydryl p-toluenesulfmate loses optical activity at a rate which is about two and a half times faster than the rate of sulfone formation, thus indicating that return from an ion-pair species is occurring (equation 3). 

The evidence presented so far excludes the formation of dissociated ions as the principal precursor to sulfone, since such a mechanism would yield a mixture of two isomeric sulfones. Similarly, in the case of optically active ester a racemic product should be formed. The observed data are consistent with either an ion-pair mechanism or a more concerted cyclic intramolecular mechanism involving little change between the polarity of the ground state and transition state. Support for the second alternative was found from measurements of the substituent and solvent effects on the rate of reaction. 

An interesting sulfone to sulfinate rearrangement has been observed by Schank and Schmitt100 in the thermal fragmentation of a-alkoxysulfones, and is believed to involve an ion-pair mechanism (equation 32). 

Recently, a study of this rearrangement has been repeated and extended in order to determine the influence of a- and y-substitution on the position of the 68-69 equilibrium in the presence of silica, and the utility of this reaction for a novel and convenient synthesis of highly substituted a, / -unsaturated ketones, by subsequent treatment with CuCl2 in methanol-water136. An ion-pair mechanism can also be suggested for the facile rearrangement of sulfone 70 to 71, a key intermediate in the Hoffmann-La Roche Sulfone Route to Vitamin A137. 

The equilibrium between propargyl- and allenyl-tin compounds is not spontaneous, but it occurs in the presence of Lewis acids or coordinating solvents, and an ion-pair mechanism has been proposed (159). Substitution by iodine, or addition to chloral, occurs with propargyl/al-lenyl rearrangement (160, 161), analogous to the allylic rearrangement already mentioned. 

The preparation and chemistry of nickel trithiocarbonate complexes have been studied in detail, and both [NKCSs) ] and [Ni(CS4)2 have been isolated (379). Shul man and co-workers (379) reprepared the known [Ni(NH3)3(CS3)]and [Ni(en)3]CS3, and the kinetics of the reaction of Ni(II) with the trithiocarbonate ion in methanol was studied the results confirmed the previously proposed ion-pair mechanism (380). 

Sneen et al. formulated an intermediate-mechanism theory. The formulation is in fact very broad and applies not only to borderline behavior but to all nucleophilic substitutions at a saturated carbon. According to Sneen, all SnI and Sn2 reactions can be accommodated by one basic mechanism (the ion-pair mechanism). The substrate first ionizes to an intermediate ion pair that is then converted to products ... 

These data have also been explained as being in accord with the ion-pair mechanism Sneen, R.A. Larsen, J.W.. Am. Chem. Soc., 1969, 91, 6031. 

The mechanism as we have pictured it can lead only to an ortho product. However, a small amount of para product has been obtained in some cases. A mechanism in which there is a dissociation of the ArC—N bond (similar to the ion-pair mechanism of the Stevens rearrangement, p. 1419) has been invoked to explain the para products that are observed. 

The El reactions can involve ion pairs, just as is true for S l reactions (p. 398), This effect is naturally greatest for nondissociating solvents it is least in water, greater in ethanol, and greater still in acetic acid. It has been proposed that the ion-pair mechanism (p. 400) extends to elimination reactions too, and that the S l, Sn2, El, and E2 mechanisms possess in common an ion-pair intermediate, at least occasionally. ... 

However, the E2C mechanism has been criticized, and it has been contended that all the experimental results can be explained by the normal E2 mechanism. McLennan suggested that the transition state is that shown as 18. An ion-pair mechanism has also been proposed. Although the actual mechanisms involved may be a matter of controversy, there is no doubt that a class of elimination reactions exists that is characterized by second-order attack by weak bases. " These reactions also have the following general characteristics (1) they are favored by good leaving groups (2) they are favored by polar aprotic solvents (3) the reactivity order is tertiary > secondary > primary, the opposite of the normal E2 order (p. 1319) (4) the elimination is always anti (syn elimination is not found), but in cyclohexyl systems, a diequatorial anti elimination is about as favorable as a diaxial anti elimination (unlike the normal E2 reaction, p. 1302) (5) they follow Zaitsev s rule (see below), where this does not conflict with the requirement for anti elimination. 

A recent investigation by Macomber (100) of the system where Rj = CH3, 126 (R2 and all other R = H), sheds additional light on the mechanism of homoallenic participation in solvolysis. In this case, the value of the integrated rate constant increased significantly throughout acetolysis, implying rearrangement to a system with comparable reactivity. The results were interpreted by means of an ion-pair mechanism shown in Scheme VII. 

It has also been reported" " that rearrangements of both a- and y-propylallyl triflinates on heating in acetonitrile yield the same sulfone y-propylallyl triflone. Although the possibility of an ion-pair mechanism may be responsible for the lack of allylic rearrangement in one case as a result of the better leaving group ability of the triflinate anion as compared with the arenesulfmate anion, it is just as likely a consequence of the unbuffered conditions in which these reactions were performed. In this respect this is reminiscent of the results observed by Cope and coworkers" mentioned above which were also performed under nonbuffered conditions, and which could be simply corrected by the addition of 2,6-lutidine . 

Reaction of 3-ketoester 2-97 and acrolein 2-98 in presence of stoichiometric amounts of 2-103 led to the desired product 2-100 in 45 % yield. A transition-state model 2-99 may be postulated assuming an ion-pairing mechanism as reported for similar asymmetric transformations [37]. The diastereomeric mixture of 2-100 was transformed into 2-101 by mesylation and subsequent elimination. Despite the moderate 64% ee determined for 2-101, it was possible to obtain optically pure 2-101 by recrystallization from hexane. 

The low activity of water in these feed solutions ensures that activity of proton is high in acidic solutions. In addition to the problem this creates with regard to the corrosiveness, there is a tendency for even the very weakly basic pyridine nitrogen in CLX50 (7) to become protonated by aqueous feeds with high acidity allowing extraction of Fein by an ion-pairing mechanism,... 

Quantitative extraction of PtIV by ra-octylaniline affords a method of separation from Fe111, Co, Ni11, and Cu11 in hydrochloric acid.306 A similar ion-pairing mechanism accounts for extraction by TOPO305 310 and Alamine 304304 (see Tables 5 and 6 for reagent structures) ... 

In the critical area of (1-mannoside synthesis [317-321], the evidence strongly suggests that a-mannosyl triflate serves as a reservoir for a transient contact ion pair (CIP), which is the glycosylating species (Scheme 4.37), although the possibility of an SN2-like mechanism with an exploded transition state cannot be completely excluded [135]. In view of the probable operation of the contact ion-pair mechanism... 

Reactions of 3 in alcoholic and aqueous solvents result in a normal solvolysis product 8 (and cyclohexanone 9) as well as the recombination product 6 (eq 4).5 This is again rationalized by an ion-pair mechanism. 2-Methylcyclohex-l-enyl(phenyl)iodonium tetrafluoroborate (10) undergoes solvolysis about 250 times as fast as 3, and gives some rearranged product 12 in accord with the SnI solvolysis mechanism (Scheme 1). 

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. 

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