Element Effects and the Carbanionic Theory

Element Effects and the Carbanionic Theory 1. The element effect 

The element efFect is usually studied for the easily available chloro-and bromo-olefins, but larger differences are expected on comparing them to the fluoro-olefin. Only two systems have been investigated in this respect. In the reaction of piperidine with PhCO.CMe=CHX (X = F, Cl) at 30° in ethanol and in dimethylformamide the kF/ka values are 204 and 263, respectively (Beltrame et al., 1968). In the reaction of 1,1-diphenylvinyl halide with ethoxide ion in ethanol, 

1-Diphenylvinyl ethyl ether was not always the sole product 100, 91 and 40% of it are formed when X = F, Cl and Br, respectively. Diphenylacetylene formed in an elimination-rearrangement process 

The kRrlka ratios for the elimination-rearrangement process at 120° are 20,46 and 41 for R = H, Me and MeO, respectively. Bond-breaking is therefore important, and slow elimination of halide ion from vinyl carbanions, which are formed in pre-equilibrium, seems plausible (Jones and Damico, 1963). 

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 

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The element effects are best explained by assuming a two-step process. Recent discussions on the stereochemistry therefore dismiss the one-step mechanism and explain the stereochemical results with the aid of intermediate carbanions. We will first discuss the theory of the substitution in general form and then evaluate the experimental data according to it. The discussion is partially along the lines of those of Miller and Yonan (1957), Jones et al. (1960) and Rappoport et al. (1963). 

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