Electrophilic addition solvent effects

Numerous p-values for various electrophilic additions to styrene itself are available (Schmid and Garratt, 1977). Strictly speaking, the reaction constants measure only the sensitivity of the reaction to substituent effects they depend at the same time on the solvent, on the position of the transition state on the reaction coordinate (charge magnitude) and on the way in which substituent effects are transmitted (charge location). In particular, the observed trend of p-values for the chlorination ( — 3.22 Yates and Leung, 1980), bromination (—5.7 Ruasse et al, 1978) and sulfenylation ( — 2.41 ... 

The very small p- and m-values observed for the fast bromination of a-methoxystyrenes deserve comment since they are the smallest found for this electrophilic addition. The rates, almost but not quite diffusion-controlled, are amongst the highest. The sensitivity to polar effects of ring substituents is very attenuated but still significant that to resonance is nil. These unusually low p-values for a reaction leading to a benzylic carbocation are accompanied by a very small sensitivity to the solvent. All these data support a very early transition state for this olefin series. Accordingly, for the still more reactive acetophenone enols, the bromination of which is diffusion-controlled, the usual sensitivity to substituents is annulled. 

The second series of data on protic solvent effects in bromination that are related to transition states comprises the m-values of solvent-reactivity correlations. First, it is important to underline that 7-parameters, the solvent ionizing powers, established from solvolytic displacements, work fairly well in this electrophilic addition. This is expected since bromination, like SN1 reactions, leads to a cation-anion pair by heterolytic dissociation of the bromine-olefin CTC, a process similar to the ionization of halogenated or ether derivatives (Scheme 14). 

In fact, the analogy between the mechanisms of heterolytic nucleophilic substitutions and electrophilic bromine additions, shown by the similarity of kinetic substituent and solvent effects (Ruasse and Motallebi, 1991), tends to support Brown s conclusion. If cationic intermediates are formed reversibly in solvolysis, analogous bromocations obtained from bromine and an ethylenic compound could also be formed reversibly. Nevertheless, return is a priori less favourable in bromination than in solvolysis because of the charge distribution in the bromocations. Return in bromination implies that the counter-ion, a bromide ion in protic solvents, attacks the bromine atom of the bromonium ion rather than a carbon atom (see [27]). Now, it is known (Galland et al, 1990) that the charge on this bromine atom is very small in bridged intermediates and obviously nil in /f-bromocarbocations [28]. 

Macroscopic solvent effects can be described by the dielectric constant of a medium, whereas the effects of polarization, induced dipoles, and specific solvation are examples of microscopic solvent effects. Carbenium ions are very strong electrophiles that interact reversibly with several components of the reaction mixture in addition to undergoing initiation, propagation, transfer, and termination. These interactions may be relatively weak as in dispersive interactions, which last less than it takes for a bond vibration (<10 14 sec), and are thus considered to involve "sticky collisions. Stronger interactions lead to long-lived intermediates and/or complex formation, often with a change of hybridization. For example, onium ions are formed with -donors. Even stable trityl ions react very rapidly with amines to form ammonium ions [41], and with water, alcohol, ethers, and esters to form oxonium ions. Onium ion formation is reversible, with the equilibrium constant depending on the nucleophile, cation, solvent, and temperature (cf., Section IV.C.3). 

If concentrations of carbenium ions are too low to be observed directly, they must be detected indirectly in kinetic studies of the racemiza-tion of optically active dormant species, ligand exchange and/or detailed studies of the effect of substituents, solvent and salts. Some of the most convincing and elegant work in this area was presented in Chapter 2 using primarily benzhydryl derivatives. As discussed in the next section, correlations between ionization rates and equilibrium constants, rates of solvolysis and rate constants of electrophilic addition can be interpolated and in some cases extrapolated to cationic polymerizations of alkenes to evaluate the reactivities of various active species and the dynamics of their isomerization. 

Transfer processes can be caused by monomer, counterion, and other components of the reaction mixture (additives, solvents, impurities). The latter reactions are sometimes called spontaneous because they are zero order in monomer. However, the spontaneous elimination of /3-protons is very unlikely, and proton elimination must be assisted by some basic reagent. The ratio of the rate constants of /8-proton elimination to that of electrophilic addition depends on several factors. The relative rate of transfer decreases with temperature, and therefore polymers with higher molecular weights are formed at sufficiently low temperatures. The effect of solvent and counterion is not yet sufficiently understood. 

It has been shown that in the case of bromine addition to 1-pentene in solvents of different polarity, the overall rate constant varies by a factor of 10 ( ) [81]. This dramatic solvent effect has been taken - together with other findings - as strong evidence for the so-called AdnCl-mechanism, which involves considerable charge separation in the activation step. It has also been demonstrated that protic solvents enhance this addition by a specific electrophilic solvation of the anionic part of the activated complex... 

Recent observations bearing on reactivity have usually been scattered and of uneven quality. We can add very few kinetic data on additions (equation 1) to those of a previous review on the other hand, kinetic data for substitutions (equation 2) are available. Studies of substituent, steric and solvent effects, which influence nucleo-philicity and electrophilicity orders as well as stereoselectivity, are limited and usually qualitative. For these reasons, we shall treat some of the large issues in this section and pick others up later in the context of specific nucleophiles. 

Reaction of 2,4-dinitrobenzenesulfenyl chloride with norbornadiene, used to increase the effective electrophilicity of weak electrophiles in electrophilic addition to 1,3-dienes, gave a mixture of chloronorbornenyl and chlorotricyclo[2.2.1.0 ]heptyl sulfides 20 and 21 with regioselec-tivity and stereochemistry depending on the solvent and temperature of the addition.Thus in acetic acid at 25°C two acetoxy sulfides trans- and cis-22 were also formed. 

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