Solvolysis structural effects

A unimolecular ionization was shown to be the mechanism of solvolysis by means of rate studies, solvent effects, salt effects, and structural effects (179,180). The products of reaction consist of benzo [bjthiophen derivatives 209 or nucleophilic substitution products 210, depending upon the solvent system employed. By means of a series of elegant studies, Modena and co-workers have shown that the intermediate ion 208 can have either the open vinyl cation structure 208a or the cyclic thiirenium ion 208b, depending... 

K, J. Morgan, and F. J. Chloupek Structural effects in solvolytic reactions. I. The role of equilibrating cations in carbonium ion chemistry. Nature of the intermediates involved in the solvolysis of symmetrically substituted 3-phcnylethyl derivatives. J. Amer. chem. Soc. 87, 2137 (1965). 

A quantitative scale for the structural effect of various silyl groups is established, as shown in entry 57 of Table 1, by the rates of solvolysis of 40 triorganosilyl chlorides in aqueous dioxane under neutral conditions69. The structural effect involves the steric effect and, in some examples, the electronic effect. Because little difference exists in the electronic effect among alkyl groups, their steric effect at silicon follows the order primary < secondary < tertiary substituents. 

Brown, H. C. Rao, C. G. Ravindranathan, M. Structural effects in solvolytic reactions. 23. New cr+ constants for activating substituents. The solvolysis of 1-aryl-l-cyclopropyl 3,5-dini-trobenzoates containing activating substituents in the aryl group. The tool of increasing electron demand and I-strain./. Am. Chem. Soc. 1977, 99, 7663-7667. 

The nucleophilic substitution reactions of anilines with ero-2-norbomyl arenesulfonates, 2, present an interesting example of the preassociation mechanism55 (Scheme 1). The rate is faster with 2-exo (k2 = 15.9 x 10 4 and 3.24 x 10-5 M 1 s 1 when X = Z = H in MeOH and MeCN at 60.0 °C, respectively) than with 2-endo (k2 = 0.552 x 10 5 M 1 s 1 with X = Z = H in MeOH at 60.0 °C). These reactions are characterized by a large pz (1.8 and 1.2 for 2-exo and 2-endo) coupled with a small magnitude of px (—0.21 and —0.15 for 2-exo and 2-endo). The pz values for the aniline reactions are even larger than those for the SY 1 solvolysis in MeOH (pz =1.5 and 1.0 for solvolysis of 2-exo and 2-endo). Thus the abnormal substituent effect in the anilinolysis of 2 can only be accounted for by the preassociation mechanism of Scheme 1. The upper route is the normal S/v 1 pathway, and the lower route is the preassociation pathway. The preassociation step, Xass, and association of the Nu to the ion pairs, kn. occur in a diffusion limited or fast process and k is the rate-limiting step. This mechanism leads to second-order kinetics and therefore is an SY/2 process, but structural effects on rates are very similar to those of S l reactions, since the R+ Z pair consists essentially of the two free ions. 

Solvent Dependence of Reactivity. Solvolysis reactions were investigated to obtain structure-reactivity relationships, but these studies were complicated by the solvent dependence of relative rates (Table I). These results show a 1010 variation in relative rates of solvolyses of methyl and 2-adamantyl tosylates (2-AdOTs) in trifluoroacetic acid (TFA) compared with those of ethanolysis. Even for two secondary systems, relative rates for 2-AdOTs-(CH3)2CHOTs vary from 36 in trifluoroacetic acid to 0.0011 in ethanol (4). Hence, separate intrinsic structural effects must be separated from solvent-induced effects. 

It is not apparent whether the initial abstractions in oxidation involve hydrogen atoms or hydride ions. Simplicity favors a radical mechanism, with similar abstractions for the initial and final hydrogen removals. The good correlation with reactivities in known radical reactions is also suggestive. However, initial carbonium ion formation would also involve the same qualitative dependence on structure, although structural effects are usually more pronounced for carbonium ions than for radicals. Qualitatively, the oxidation results correlate well with carbonium ion reactivities found in the solvolysis of substituted aUyl halides (142), again with internal olefins appearing slow in oxidation. 

A great part of the data on the structural effects of the substituents at the C atom is concerned with carbonium ion rearrangements in the reactions of solvolysis and... 

Tables 6 and 7 present some examples of structural effects of substituents at the C" atom. Analysis of these data shows that the substitution of the methyl group at C by an ethyl one usually results in a somewhat increased rate of 1,2-shifts in the solvolysis of primary alkyl sulphonates. A similar effect is observed for the 1,2-shifts of different groups in the dehydration of P-disubstituted 1,3-propandiols. As the chain of -alkyl groups at the C" atom grows longer the rate of 1,2-shifts tends to rise this agrees with the increase in the electron-donating ability of these groups.

A variety of kinetic data permit the assignment of relative reactivities toward solvolysis of a series of systems related to the norbornane skeleton. Offer a general discussion of the structural effects that are responsible for the observed relative reactivity data. 

The more extensive problem of correlating substituent effects in electrophilic substitution by a two-parameter equation has been examined by Brown and his co-workers. In order to define a new set of substituent constants. Brown chose as a model reaction the solvolysis of substituted dimethylphenylcarbinyl chlorides in 90% aq. acetone. In the case ofp-substituted compounds, the transition state, represented by the following resonance structures, is stabilized by direct resonance interaction between the substituent and the site of reaction. 

Probably the most important development of the past decade was the introduction by Brown and co-workers of a set of substituent constants,ct+, derived from the solvolysis of cumyl chlorides and presumably applicable to reaction series in which a delocalization of a positive charge from the reaction site into the aromatic nucleus is important in the transition state or, in other words, where the importance of resonance structures placing a positive charge on the substituent - -M effect) changes substantially between the initial and transition (or final) states. These ct+-values have found wide application, not only in the particular side-chain reactions for which they were designed, but equally in electrophilic nuclear substitution reactions. Although such a scale was first proposed by Pearson et al. under the label of and by Deno et Brown s systematic work made the scale definitive. 

An example of a reaction series in which large deviations are shown by — R para-substituents is provided by the rate constants for the solvolysis of substituted t-cumyl chlorides, ArCMe2Cl54. This reaction follows an SN1 mechanism, with intermediate formation of the cation ArCMe2 +. A —R para-substituent such as OMe may stabilize the activated complex, which resembles the carbocation-chloride ion pair, through delocalization involving structure 21. Such delocalization will clearly be more pronounced than in the species involved in the ionization of p-methoxybenzoic acid, which has a reaction center of feeble + R type (22). The effective a value for p-OMe in the solvolysis of t-cumyl chloride is thus — 0.78, compared with the value of — 0.27 based on the ionization of benzoic acids. 

What concerns us here are three topics addressing the fates of bromonium ions in solution and details of the mechanism for the addition reaction. In what follows, we will discuss the x-ray structure of the world s only known stable bromonium ion, that of adamantylideneadamantane, (Ad-Ad, 1) and show that it is capable of an extremely rapid degenerate transfer of Br+ in solution to an acceptor olefin. Second, we will discuss the use of secondary a-deuterium kinetic isotope effects (DKie) in mechanistic studies of the addition of Br2 to various deuterated cyclohexenes 2,2. Finally, we will explore the possibility of whether a bromonium ion, generated in solution from the solvolysis of traAU -2-bromo-l-[(trifluoromethanesulfonyl)oxy]cyclohexane 4, can be captured by Br on the Br+ of the bromonium ion, thereby generating olefin and Br2. This process would be... 

The key cyclization in Step B-2 was followed by a sequence of steps that effected a ring expansion via a carbene addition and cyclopropyl halide solvolysis. The products of Steps E and F are interesting in that the tricyclic structures are largely converted to tetracyclic derivatives by intramolecular aldol reactions. The extraneous bond was broken in Step G. First a diol was formed by NaBH4 reduction and this was converted via the lithium alkoxide to a monomesylate. The resulting (3-hydroxy mesylate is capable of a concerted fragmentation, which occurred on treatment with potassium f-butoxide. 

The effect of structure on relative reactivity may be seen particularly clearly when a halogen atom is located at the bridgehead of a bicyclic system. Thus the following rates were observed for solvolysis in 80% aqueous ethanol at 25° ... 

The kinetic isotope effect has its origin in force constant changes occurring at an isotopically substituted position as the react2mt is converted into an activated complex. Hence it provides information about the transition state in the solvolysis reaction, but not necessarily about the stmcture of possible intermediates. This limits the utility of information drawn from isotope studies in resolving the structure of ions under stabilizing conditions. 

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