Mechanism borderline

Thus far, we have discussed typical vinylic Sn2 and SnI reactions. The former involve reactions of good nucleophiles with simple 1-alkenyliodonium salts (S Ycr) and unactivated j8-halo derivatives (SnVtt), while the latter reactions occur under poor nucleophilic conditions (solvolysis) if the substrates can give stabilized vinylic cations of secondary or bridged structure. In this section, reactions which have characteristics of both categories are presented. 

REACTIONS OF iS,iS-DIALKYLVINYL lODONIUM SALTS WITH HALIDE IONS 

Obtained at [Bu4NX ] = 0.1 mol dm and 50°C. Tetrabutyl ammonium halide, BU4NX. 

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SECTION 5.3. DETAILED MECHANISTIC DESCRIPTION AND BORDERLINE MECHANISMS... 

The importance of solvent participation in the borderline mechanisms should be noted. Nucleophilic participation is minimized by high electronegativity, which reduces the Lewis basicity and polarizability of the solvent molecules. Trifluoroacetic acid and perfiuoro alcohols are among the least nucleophilic of the solvents used in solvolysis studies. These solvents are used to define the characteristics of reactions proceeding without nucleophilic solvent participation. Solvent nucleophilicity increases with the electron-donating capacity of the molecule. The order trifluoroacetic acid < trifluoroetha-nol <acetic acid < water < ethanol gives a qualitative indication of the trend in solvent nucleophilicity. More will be said about solvent nucleophilicity in Section 5.5. 

Studies of the stereochemical course of rmcleophilic substitution reactions are a powerful tool for investigation of the mechanisms of these reactions. Bimolecular direct displacement reactions by the limSj.j2 meohanism are expected to result in 100% inversion of configuration. The stereochemical outcome of the lirnSj l ionization mechanism is less predictable because it depends on whether reaction occurs via one of the ion-pair intermediates or through a completely dissociated ion. Borderline mechanisms may also show variable stereochemistry, depending upon the lifetime of the intermediates and the extent of internal return. It is important to dissect the overall stereochemical outcome into the various steps of such reactions. 

There are alternatives to the addition-elimination mechanism for nucleophilic substitution of acyl chlorides. Certain acyl chlorides are known to react with alcohols by a dissociative mechanism in which acylium ions are intermediates. This mechanism is observed with aroyl halides having electron-releasing substituents. Other acyl halides show reactivity indicative of mixed or borderline mechanisms. The existence of the SnI-like dissociative mechanism reflects the relative stability of acylium ions. 

Several proposals have been made to fit the borderline reactions into a well-defined mechanistic scheme. Most of these adopt one of two viewpoints either (1) borderline substrates undergo concurrent SnI and Sn2 processes, with the particular system determining which mechanism, if either, predominates or (2) all Sn reactions are related by essentially the same mechanism, which differs from case to case in the detailed disposition of electrons in the transition state. In this view pure SnI and Sn2 processes are merely the extreme limiting forms of a single mechanism, and the borderline mechanism is a merged process having some features of both. 

As a result of the inductive and hyperconjugative effects it is to be expected that tertiary carbonium ions will be more stable than secondary carbonium ions, which in turn will be more stable than primary ions. The stabilization of the corresponding transition states for ionization should be in the same order, since the transition state will somewhat resemble the ion. Thus the first order rate constant for the solvolysis of tert-buty bromide in alkaline 80% aqueous ethanol at 55° is about 4000 times that of isopropyl bromide, while for ethyl and methyl bromides the first order contribution to the hydrolysis rate is imperceptible against the contribution from the bimolecular hydrolysis.217 Formic acid is such a good ionizing solvent that even primary alkyl bromides hydrolyze at a rate nearly independent of water concentration. The relative rates at 100° are tertiary butyl, 108 isopropyl, 44.7 ethyl, 1.71 and methyl, 1.00.218>212 One a-phenyl substituent is about as effective in accelerating the ionization as two a-alkyl groups.212 Thus the reactions of benzyl compounds, like those of secondary alkyl compounds, are of borderline mechanism, while benzhydryl compounds react by the unimolecular ionization mechanism. 

The perchloric acid-catalyzed methanolysis of 2-methyloxetane gives a mixture of 4-methoxy-2-butanol and 3-methoxy-l-butanol, with the former somewhat predominating (equation 21). The effect of solvent on the product distribution and the reaction rates indicated that protonated 2-methyloxetane was reacting by a borderline n1-Sn2 mechanism (67MI51302). Similar studies with the acid-catalyzed methanolysis of oxetane itself indicated that methanol reacted with protonated oxetanium ion by the N2 process. The same type of studies with a series of 2-aryloxetanes indicated that methanolysis of these compounds involved the borderline mechanism for the protonated oxetanium ions (69MI5101, 72MI5102, 73MI5100). 

Nu — 1—T Nu (Sn2) in acid, nucleophile bonds to more substituted carbon with inversion (borderline mechanism)... 

Some nucleophilic substitution reactions that seem to involve a borderline mechanism actually do not. Thus, one of the principal indications that a borderline mechanism is taking place has been the finding of partial racemization and partial inversion. However, Weiner and Sneen have demonstrated that this type of stereochemical behavior is quite consistent with a strictly Sn2 process. These workers studied the reaction of optically active 2-octyl brosylate in 75% aqueous dioxane, under which conditions inverted 2-octanol was obtained in 77% optical purity. When... 

The literature (31b, 31c) on aliphatic nucleophilic substitution has drifted from pure SN1 and SN2 to a consideration of borderline mechanisms. Interestingly equation 7 contains within itself the elements needed to cover this borderline region. 

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