Solvolysis of benzylic

The specific rates of solvolysis of benzyl p-toluenesulfonate and nine benzylic-ring-substituted derivatives (324) have been satisfactorily correlated using Aij and Tots scales within the extended Grunwald-Winstein equation. The reactions of Z-phenylethyl X-benzenesulfonates (325) with Y-pyridines (326) in acetonitrile at 60 °C have been studied at high pressures. The results indicated that the mechanism of the reaction moves from a dissociative 5)vr2 to an early-type concerted 5)vr2 with increasing pressure. 

The competing processes were sufficiently slower than the reaction of interest only in the case of the nickel complex, [Ni(NiL2)2]Cl2. Attempts to determine the rate of reaction of [Pd(NiL2)2]Cl2 with benzyl bromide revealed a very slow process occurring at a rate comparable to, but slightly slower than, the solvolysis of benzyl bromide. It is concluded that the rate of reaction of this complex with benzyl bromide is too slow for accurate rate study in methanol. 

A pronounced rate-retardation of 1.65 x 104-fold by an a-McsSi group relative to Me in the solvolysis of benzylic p-toluenesulfonates is due to a steric effect66. The rate increment in ethanol decreases with increasing steric size of the a-silyl group attached to the benzylic position and follow the order listed in entry 54 of Table 1. 

Fig. 9.6 Demonstration of a trappable intermediate in the solvolysis of benzyl azoxytosylate.

A situation of this type was encountered in the investigation of the solvolysis of benzyl azoxytosylate, 14 in Fig. 9.6, in aqueous trifluoroethanol [23]. In the absence of added nucleophilic reagents the products were benzyl alcohol and benzyl trifluoroethyl ether. Added excess sodium thiocyanate had minimal effect upon the rates, but the reaction then yielded substantial amounts of benzyl thiocyanate. The plot of the inverse mole fraction of benzyl thiocyanate against 1/ [NaSCN], shown in Fig. 9.6, is linear with a non-zero intercept, consistent with the intermediacy of a species trappable by thiocyanate, and a competing pathway for the formation of ethereal and alcoholic products other than via this species. 

While we can make substitutions on the aromatic ring of the nucleophile or the leaving group for methyl transfers, it is clearly impossible to do the same for the transferring group. The solvolysis of benzyl halides (Bennett and Jones, 1935) gives curved Hammett plots. The interaction of the aromatic ring with the reaction centre makes this a more complicated reaction. Hence we will not consider these reactions in detail. Thus the simplest system we can consider is... 

Figure 51. Dependence of volume parameters for the solvolysis of benzyl chloride on mole fraction of alcohol in ethyl alcohol + water mixtures at 323 K (Mackinnon et al., 1970).

The rate constant for the hydrolysis of t-butyl chloride at 298 K decreases as x2 increases in DMSO + water mixtures (Heinonen and Tommila, 1965). A clear-cut contrast between TA and TNAN mixtures is shown by the volumes of activation and related parameters for the solvolysis of benzyl chloride in acetone + water (TA) and DMSO + water mixtures (Fig. 57). Thus, in the latter system, the curves show no marked extrema but there is a shallow minimum in AV near x2 = 0 4. Extrema in Sm AH and T. 5m AS for the hydrolysis of benzyl chloride are also smoothed out when the co-solvent is changed from acetone to DMSO (Tommila, 1966). A similar trend is observed in the kinetic parameters for the hydrolysis of chloromethyl and methyl trifluoroacetates (Cleve, 1972a). For example, in the case of the chloro derivative, 6mACp decreases gradually over the range 0 < x2 < 0-2 for DMSO + water mixtures, whereas a minimum is observed in this range for acetone + water mixtures. 

Figure 5 7. Comparison of the volumes of activation for solvolysis of benzyl chloride and related quantities as a function of co-solvent mole fraction at 323 K for acetone + water mixtures (full line) and dimethyl sulphoxide + water mixtures (Macdonald and Hyne, 1970a).

Fig. 7 The Y-T plot of solvolysis of benzyl tosylates [21] in 80% aqueous acetone at 25°C r = 1.29. For interpretation of symbols, see Fig. 1. Reproduced with permission from Fujio et al. (1990b). Copyright 1990 Chemical Society of Japan.

Recently, Pettit and co-workers (134) have shown that the relative rate of solvolysis of benzyl chloride-chromium tricarbonyl is about a million times faster than for uncomplexed benzyl chloride under analogous aSatI conditions. The area of carbonium ion formation and stabilization in arene-metal systems thus represents another fertile field for future investigation. 

It is generally agreed that the mechanism of the solvolysis of benzyl halides lies near the region which marks the transition from Sjfl to Sjf2 solvolysis. Considerations of the kinetic chlorine isotope effect have recently led to the conclusion that even 4-nitrobenzyl chloride undergoes Sul solvolysis (Hhl and Pry, 1962) but an examination of the data suggests that this effect does not represent a sensitive test of solvolytic mechanism (Kohnstam, 1967). On the other hand, the values oi AC, AC I AS, and AS show that the solvolysis of the parent compoimd has the characteristic featvues of an 8 2 reaction (Tables 5, 6), and other evidence also supports this conclusion (see Bensley and Kohnstam, 1957). 

The solvolysis of isopropyl halides is generally considered to occur near the mechanistic border-line which marks the beginning of entirely Sul reaction, like the solvolysis of benzyl halides. Results relevant to the present discussion are available only for reaction with water and have been included with those for other Su2 processes in Table 6 since AC is less negative than would have been expected for entirely Sul solvolysis no complicating features which might reverse this conclusion have been envisaged. 

The kinetic effect of introducing a 4-nitro-substituent suggests that the transition state in the solvolysis of benzyl j)-toluenesulphonate is subject to a greater contribution from structure (8), relative to (9), than when the substrate is the chloride (see Table 5). Nevertheless, the values of AC jAS do not indicate that the mechanism is altered when the solvent changes from 60% to 85% acetone (see Table 6). 

Another important form of bromine in MC oxidation is benzylic bromides. In the process of oxidation, inorganic bromides are rapidly converted to benzylic bromides in proportions ranging from 10% to 90% until the oxidation has been completed [8, 26, 50]. Partenheimer has reported that benzylic bromides are catalytically inactive and that their accumulation is responsible for decelerating the oxidation rate. Solvolysis of benzylic bromides liberates catalytically active inorganic bromides, thus establishing the observed equilibrium between the two forms of bromine [26]. However, it was later uncovered that benzylic bromides can be easily oxidized in the presence of oxygen and cobalt(II), and this oxidation appears to be the main route in which inorganic bromides are restored [30, 31]. 

Worthy of brief mention are the inverse effects of y-deutera-tion on solvolysis of n-propyl derivatives (Leffek et al., 1960b), as well as the effect of deuterated alkyl substituents (Lewis and Coppinger, 1954 Shiner and Verbanic, 1957) or direct ring deu-teration (Klein and Streitwieser, 1961) on solvolysis of benzyl- or benzhydryl-type compounds. The reader is referred to the original articles for discussions of the origins of these effects. 

Solvolysis of benzyl arenesulphenates proceeds via S—O bond cleavage and is a second-order process, while benzyl trichloromethanesulphenates undergo first-order C—O bond cleavage on solvolysis. Formation of a sulphoxide from a benzyl trichloromethanesulphenate is one of two reaction paths resulting from ionization in polar media, the alternative to this ionization-recombination process being further dissociation into dichloro-sulphine (ClgCSO) and a benzyl chloride, ... 

TABLE 9.4 Relative Rates of Solvolysis of Benzylic Halides Substrate MeCI PhCH CI Ph CHCI Ph3CCI... 

Kinetic studies of the n-butylaminolysis of p-nitrophenyl triphenyl-methanesulphenate reveal second-cHrder characteristics, while reaction with benzamidine is first-order, as it is with carboxylic esters. S—O versus C—O fission has been contemplated for solvolysis of benzyl trichloromethanesulphenates, which provide the first example of C—O fission, to give Cl2C=S=0. Ethyl benzenesulphenate reacts with phosphites to give... 

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