Ion pairs return from

There is also relatively direct evidence for internal ion pair return from 1 sO-scrambling studies [eqn (3)] during solvolyses of simple... 

Ion pair return from (66) results in partial loss of enantiomeric purity85. The amount lost starting with (67) was determined from the krac/keq ratio and the amount lost starting with (65) was determined from the enantiomeric purity of (67) produced in the solvolysis of optically active (65). Within experimental error the stereochemistry of return is the same starting from either (65) or (67), e.g. 65% racemization in 90% aqueous acetone. About one third of the return involves rebond-... 

Ion pair return from (354), X=OTs, results in racemization of threo- (351) which is about 4 times faster than acetolysis i.e. only 20% of the products come from optically active threo- (351) and 80% come from racemic tosylate. These proportions are in quantitative agreement with those determined from kinetic studies293. Ion pair... 

Interestingly, the order of the fractions of ion pairs returned from the 17 X ion pairs (0.47, 0.38, and 0.24 for X = Br , Cl , and OMs , respectively), which reflects the intramolecular nucleophilicity in the ion pair, is parallel to the a values (74). The order of return from ion-paired X or from external X-seems therefore to be governed by similar factors. 

S-4 participation has also been implicated in the racemization of (46a) upon heating in methyl ethyl ketone. Since the deuterated derivative (46b) produced a mixture of (46b) and (47b) under the same conditions, the reaction may involve either ion-pair return from (48) or the four-centered transition state (49). 

Stabilization of a carbocation intermediate by benzylic conjugation, as in the 1-phenylethyl system shown in entry 8, leads to substitution with diminished stereosped-ficity. A thorough analysis of stereochemical, kinetic, and isotope effect data on solvolysis reactions of 1-phenylethyl chloride has been carried out. The system has been analyzed in terms of the fate of the intimate ion-pair and solvent-separated ion-pair intermediates. From this analysis, it has been estimated that for every 100 molecules of 1-phenylethyl chloride that undergo ionization to an intimate ion pair (in trifluoroethanol), 80 return to starting material of retained configuration, 7 return to inverted starting material, and 13 go on to the solvent-separated ion pair. 

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... 

In certain favorable cases of rearranging systems, the occurrence of internal return distinct from external ion-pair return can be demonstrated without recourse to optical rotation or isotopic labeling experiments. See S. Winstein and A. H. Fainberg, J. Amer. Chem. Soc., 80, 459 (1958) S. Winstein, P. E. Klinedinst, Jr., and E. Clippinger, J. Amer. Chem. Soc., 83, 4986 (1961). 

Other evidence for ion pair return is less relevant to solvolyses of simple secondary substrates. It is known that during solvolyses of certain optically active substrates racemization occurs more rapidly than solvolysis, but it is not known whether the ion-pair intermediates undergoing racemization are the same as those undergoing solvolysis (Hammett, 1970a). Such behaviour is usually observed in solvolyses where neighbouring group participation occurs and the intermediates are probably more stable than those from simple secondary solvolyses. As the stabilities of the intermediates increase, there appears to be a general trend towards formation of more dissociated species,4 and thus the relevance of these results is questionable. 

The remarkable agreement in the above results surprised Bunton and Del Pesco (1969) because, as they pointed out, no allowance was made for the temperatures of the two processes, and for the presence of solvent and ion-pair return processes. From extensive studies of structurally related methyl norbornyl chlorides (e.g. 19), they concluded... 

Typical product-forming behavior for solvolyses of cyclopropylidenemethyl derivatives is that below. The l-bromo-2-methylcyclobutene results from ion-pair return. 

Even for reactions in which the Sij2 contribution to ionization is negligible, one does not have a means of estimating from solvolysis rates, the solvent activity coefficients for the transition state corresponding to ionization of RX. Although Vrx easily found from Henry s Law constants, and equation (16) does produce an activity coefficient for some transition state, this may not be a simple transition state corresponding to ionization of RX. Solvolysis rates may be smaller than ionization rates of many compounds, in certain solvents, because of ion-pair return, a phenomenon which has been firmly established by the investigations of Winstein et al. (1965). No matter whether Ag is a titrimetric rate constant 7... 

External (unimolecular) ion-pair return should be distinguished from external (bimolecular) ion return, the (reversible) process whereby dissociated ions are converted into loose ion pairs ... 

In contrast to (74), optically active l,2-dimethyl-2-norbornyl p-nitrobenzoate (79) gives active products the SN1 product (80) is formed with ca. 9% and the El product (81) with ca. 63% retention of configuration96. Interconversion of enantiomeric 1,2-dimethylnorbornyl cations apparently competes with the product forming steps. The different optical purities of (80) and (81) show that they are derived from different intermediates. The authors suggest that most, or all, of the El product is formed from an intimate ion pair and that the SN1 product is formed from a solvent-separated ion pair or a dissociated carbocation. Solvolysis is accompanied by ion pair return which results in racemization of (79) and equilibration of 180-labeled (79). The rate of racemization exceeds that of scrambling of 180 by a factor of ca. 2. Substrate re-formed by ion pair return must be at least as optically active as the El product (81). Therefore, and keq correspond to upper limits of 37% and 20% of the total return, respectively. Scrambling of 180 detects only a small fraction of the total ion pair return in the solvolysis of (79). 

Compounds (5J)—(55)" 102) represent some more substrates which afford rearranged ion pair return products in an apparently stereospecific manner, with inversion at the origin of the 1,2 alkyl shift. Formation of endo-2-norbornyl p-nitroben-zoate from (87) is noteworthy because ion pair return occurs contrary to the natural" exo preference of the norbornyl cation101 The stereochemistry of return may be explained in terms of bridged carbocations. Alternatively the anion may rebond to the same side of the carbon framework from which it departed. Although return to the opposite side was clearly established with allyl cations (Section 4.2.1), reorien-... 

Ion pair return to positions more remote than the adjacent carbon is quite common with homoallyl-cyclopropylcarbinyl cations, cf. compounds (92)-(94)l05 lol Even the vinyl tosylate (96) is produced from either of the allenic isomers (95) and (97)108 In contrast, 5-norbornen-2-yl brosylate (98) does not yield any nortricyclyl brosylate (99) although scrambling of a deuterium label, (98a) (98b), is ca. 12 times faster than solvolysis109. ... 

The tricyclo [3.2.0.02-7] hept-3-yl p-nitrobenzoates (100) and (101) provide interesting stereochemical aspects 110. Hydrolysis of either epimer proceeded with 9-11% ion pair return to unreactive esters. Whereas the hydrolysis product distribution was essentially the same from either epimer, the ion pair return product distributions were different. (100) and (101) apparently ionize to separate ion pairs, but return is much less stereospecific than with the Wagner-Meerwein related pairs discussed above. 

Ion pair return to products resulting from a 1,3 hydrogen shift was observed concurrently with acetolysis of 7-chloro-2-norbornyl tosylates11. The syn-exo tosy-late (102) is converted into the anti-exo isomer (103) but the reverse reaction could not be detected. 

An ambident anion is one with different kinds of atoms which may become attached to a carbocation. Isomerization of the anionic portion permits detection of ion pair return in close analogy to the equilibration of 180 labeled benzoate or sulfonate esters. The isomerization of thiocyanates (112) to isothiocyanates (113) is the most studied of these ambident rearrangements 5, "6 In some respects these reactions differ from the return processes of carboxylate or sulfonate ion pairs ... 

The isomerization of thioncarboxylates to thiolcarboxylates has been less extensively studied, but also served to demonstrate the formation of a symmetrical (or rapidly equilibrating) intermediate from the norbornyl derivative (122). Acetolysis of optically active (122) gave ka = kt, i.e., there is no ion pair return to (122). The products were racemic and a deuterium label at C—2 of (122) was equally distributed among C-l and C—2 of the products122. ... 

Table 18). The solvolyses of (73) are accompanied by ion pair return which leads to interconversion of the enantiomers of the unsolvolyzed ester555 and to 180 scrambling within the sulfonate group87 (Section 4.2.2). Evidently (73) and (481) give rise to the same carbocation (816) by the a and ir routes, respectively. The minor amount of (277) and (278) in the product mixture is thought to result from isomer-... 

The ionization rate constants (fcj) for the 5 1 reactions of various diarylmethyl chlorides whose Ef values are known have been measured in the presence of piperidine, pyridine, or PPhj, which suppresses ion-pair return, in several dipolar aprotic solvents. The Hammett p values range from -4.0 to -4.3 in the different dipolar aprotic solvents, proving that the reactions occur by an 5 1 mechanism. The nucleofu-gality parameters Nf and Sf from the log k = Sf Ef + Nf) equation) for the diarylmethyl... 

There is need for caution in accepting the above conclusion. The guidelines for the interpretation of data produced from the probes used by Grob et al. in arriving at their conclusion are essentially based on traditional Sj l and S 2 mechanisms and not on the existance of hidden ion-pair return. If hidden ion-pair formation is as common as envisioned by some," the interpretations of m and p values may be based, at least in part, on the reaction of ion pairs and not neutral substrates. The direction of the curvature of the log k versus plot for (83a)-(83d) (if the curvature is real) could be taken as evidence for the ion-pair mechanism. Hence all that can be said until the question of the general involvement of ion pairs is resolved is that nitrogen participation is demonstrated. 

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