Solvolysis mechanism

The following considerations and observations are in accord with the metaphosphorimidate solvolysis mechanism 192-+195->196- 197 ... 

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). 

Rates and product distributions obtained during the solvolysis of secondary 2md tertiary cationic precursors provide the data for the norbomyl controversy. One of the reasons the debate has not been ended is the complexity of the solvolysis mechanism. Rate measurements should provide information on the energy difference between the ground and transition states of the reaction. Their analysis, however, is often clouded by a series of questions. 

Thornton, E. R. (1964). Solvolysis Mechanisms , pp. 197-206. Ronald Press, New York... 

It is, however, not always possible to draw conclusions on the solvolysis mechanism from the configuration of the products, since the latter may not necessarily be the initially formed solvolysis products. Schlosser has shown for example, that the same product mixture, consisting out of the 2-fluoropropene derivatives VIII-X is obtained from the fluorochlorocyclopropanes shown, regardless whether the ( )- or the (Z)-isomer is solvolyzed. To rationalize this observation it has been proposed that the tertiary alcohol IX is formed first, and that this... 

Unimolecular substitutions are discussed in detail in the following sources (a) C. A. Bunton, Nucleophilic Substitution at a Saturated Carbon Atom, Elsevier, Amsterdam, 1963 (b) C. K. Ingold, Structure and Mechanism in Organic Chemistry, 2nd ed., Cornell University Press, Ithaca, N.Y., 1969 (c) A. Streitwieser, Jr., Solvolytic Displacement Reactions, McGraw-Hill, New York, 1962 (d) E. R. Thornton, Solvolysis Mechanisms, Ronald Press, New York, 1964. 

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... 

Returning, then, to the two alternatives for solvolysis mechanisms with which we began this section, it appears that it is indeed possible to construct systems that solvolyze without nucleophilic assistance from solvent. For solvent-assisted reactions, the two alternatives are essentially equivalent we can therefore choose the first alternative as being more consistent with current information. 

The solvolysis mechanisms of 2,2-dimethyl-3-pentyl- and l-(l-adamantyl)-propyl sulfonates appeal- to involve partial reversible ionization to the ultimate ion pair followed by competing elimination and solvent separation, substitution products being formed from the separated ions.27 The lifetimes of simple tertiary carbocations may be some 100 times shorter than previously thought several 3-(4-methoxyphenyl)-l,l-dimethylpropyl species hydrolyse in 50% aqueous TFE with rate constants estimated at some 3.5 x 1012 s-1.28 Much elimination was also observed.28 Two studies concerning proposed carbocation intermediates in enzymatic processes are reported.29,30... 

The overall kinetic effect of the disilanyl group is, however, small due to the different solvolysis mechanisms. 224 solvolyses only 3.48 times faster than 226 in TFE at 100 °C. For the k process Shimizu and coworkers calculated the relative rates for 224 225 226 to be 1.60 1.0 10.7. They concluded that these small silicon effects reflect the concerted nature of the k solvolysis involving transition states with a highly delocalized charge distribution88. 

Reaction according to equation (22) is also distinguishable from that involving vinyl cations but it must be carefully taken into account before reaching a definite conclusion on the solvolysis mechanism (Hanack, 1970). In answer to a paper by Schubert and Barfnecht (1970) which is very critical of the current mechanistic interpretation of some solvolytic reactions of vinyl compounds, Rappoport et al., (1970) gave 20 distinctive criteria for mechanisms of equations (21) and (22), in particular the strong rate dependence upon proton concentration in the reaction media. 

The relative stabilities of substituted benzyl cations [21C ] are correlated by equation (27) with a high resonance demand parameter Tq = 1.29 (Mishima et al., 1987, 1995). The linear correlation for the whole range of substituents down to the 3,5-(Cp3)2 group (Fig. 27), contrasts with the concave Y-T plot (Fig. 7) of the solvolytic reactivities of [21]. Note that the Tq value for the gas-phase stabilities of [21C ] is identical with the r value assigned for the SnI solvolysis of [21] tosylates hence, the r value of 1.29 must be an intrinsic index inherent in [21C ], rather than a correlational artifact of a non-linear relationship for the complex solvolysis mechanism. 

Solvolysis mechanisms covering the entire spectrum from SN1 to SN2 substitutions. 

Thus, steric factv>rs affect the value only for the endo series the observed similarity of the q values for the exo and endo series is an incidental result of simultaneous influence of steric and electronic factors. It is wrong to conclude from this similarity of values that the solvolysis mechanisms in either series are similar as well. ... 

It is very instructive to follow the change in the views of Goering et al. on the solvolysis mechanism of l,2-dimethyl-2-exo-benzonorbornenyl derivatives. Initially the authors showed the solvolysis of optically active l,2-dimethyl-2-exo-benzonor-bornenyl chloride 275 or -p-nitrobenzoate 276 to form optically active products ... 

The solvolysis mechanism of tertiary systems is not concerted, at least in the absence of powerful electron-releasing substituents in the migrant — a P-phenyl group The transition state in these cases is assumed to be similar to a ic-bridged ion (weak participation) rather than to a phenonium ion (strong participation), cf. Ref. I74,i85-i i>... 

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