The Elimination-Addition Routes

Simpson and Burt have studied the same reactions in the presence of various amounts of ethanol and have plotted graphs of phosphonate (81 R = Ph) and phenyl acetylene produced against moles of alcohol added. Acetylene in the product reached a maximum (around 60%) when two moles of ethanol were added and stayed fairly constant beyond this, which suggests that the attack-on-halogen contribution to the mechanism is approximately 60%. The rest of the reaction presumably follows some other mechanism and the authors suggest the addition-elimination route (79) in view of the isolation of the phosphonate (83) from the reaction of tri(isopropyl) phosphite with the bromoacetylene (84). 

Equation lb, b is the defining equation for the addition-elimination route for one-electron transfer between X" and Y. It is important to note that although X-Y is a radical and the overall reaction results in the transfer of a single electron, in the actual electron transfer step an electron pair is shifted rather than a single electron [5]. This means that electron transfer is the consequence of a heterolysis reaction in which the electron pair joining X and Y ends up at... 

The operation of the addition-elimination route and its details are inferred from the use of several criteria which will be discussed in the order below (a) The element effect, i.e. comparison of the substitution rates of compounds which differ only in the leaving group, extended also to study of the competition of two leaving groups attached to the same or to different carbon atoms, of the same molecule, (b) The stereochemistry of the substitution, (c) The reactivity of various systems as a function of the structural parameters. The hydrogen-exchange criterion will be discussed in connection with the elimination-addition route. 

Since the bond cleavage is not important in the addition-elimination route, the element effect could be used for differentiation between it and the elimination-addition route, provided that a closely related model to the system studied is available. Such models for calibration of the element effect for the a-arylsulphonyl-/8-haIoethylenes (11) and the a-arylsulphonyl-j3-halopropenes (12) are the a-methyl analogues (13) for which the a,/3-elimination-addition route is impossible. Table 1 shows that for either cis- or trans- 13), the Br/ ci ratios are near unity for highly... 

The stereochemical course of substitution by the addition-elimination route is related to the structure of the transition state and to the timing of the bond-forming and bond-breaking processes. The terms retention , inversion and racemization will be used respectively to denote the processes in which the geometrical arrangement of the groups in the substitution product remains the same (equation 3), or changes... 

Table 5 summarizes the stereochemical information available for reactions which are assumed to follow the addition-elimination route. Unfortunately, the reliability of the data is not the same for all the systems. Earlier work, where minor products were neglected and proper control experiments were not performed, is subject to some uncertainties. For example, when 100% of one isomer was reported, this generally means that only one product was isolated. Recent data, obtained with more sensitive and less destructive methods, such as NMR, are much more reliable. 

It is noteworthy that the use of these strong nucleophiles results in the addition-elimination route for the unactivated halo-olefins. 

Owing to the emphasis in our treatment on criteria rather than on individual reactions, the various arguments that a specific reaction series followed the addition-elimination route were spread among the different sections. It is worthwhile to summarize that the use of stereochemical, isotope exchange, kinetics and element effects show that the a-arylsulphonyl-j8-haloethylenes (Modena et al.), the j3-halo-a-nitro-styrenes (Modena et al.,) the a-aroyl-/ -haloethylenes (Montanari et al.) and the /3-halocrotonic esters and nitriles (Theron, 1967) systems react with thioanions via this route. Use of some of these criteria together show its operation for other reaction systems. 

Various criteria have been used to deduce the operation of this mechanism. Since some of them are dependent on comparison with the addition-elimination route, the competition between these two routes will be also discussed. 

Several of the above criteria are usually used together in order to ascertain the reaction mechanism. The examples below will demonstrate their use, as well as the close relationship between the elimination-addition and the addition-elimination routes. 

Obviously, tetrachloroethylene (146) which is sufficiently active owing to the four chlorine atoms, could react only via the addition-elimination route, and the fraras-disubstituted product (149) is formed by two such consecutive steps (Truce and Kassinger, 1958b Truce et al., 1965). It is interesting that the monothioaryl derivative (147) reacts only in the presence of base. Since elimination-addition is impossible, this was taken as indication that the base is required for the formation of the carbanion (148) which should be the reaction intermediate in this case. The tetrasubstituted product is obtained under drastic conditions only. 

Kinetic Parameters for Comparison of the Elimination-Addition and the Addition-Elimination Routes with MeO- ion in Methanol... 

In the addition-elimination routes, either via a carbanionic intermediate (I) or via a neutral adduct (II), the anionic nucleophile Nu or the neutral nucleophile NuH attacks the /3-carbon with the expulsion of X. In the a,/8-route (IV), the /9,/3-route (VI) and the /8, y- elimination-addition routes (VII), HX is eliminated in the initial step, and the nucleophile and hydrogen are then added to the intermediates. Substitution occurs also by heterolytic C—X bond cleavage in an SN1 process (X). Initial prototropy followed by substitution can also give vinylic substitution products (XII, XIV), as well as two consecutive Sn2 reactions (XV) where the leaving group leaves from an allylic position. 

The addition-elimination route is the most studied one in Scheme 1. Since it involves a reaction of the nucleophile with the vinylic carbon atom, it is also the one which in actual fact is most correctly described as a nucleophilic vinylic substitution . We will therefore deal with it in the greatest detail. 

---