Nucleophilic scale toward vinylic carbon

The final chapter of this section is by Rappoport and is concerned with nucleophilic reactions at vinylic carbon. Two reaction types are considered, those of neutral vinyl derivatives and those of vinyl cations. Correlation of rates for these reactions with both Ritchie and Swain-Scott equations was attempted without success. Rappoport concludes that these reactions are subject to a complex blend of polar, steric, and symbiotic effects and that a quantitative nucleophilicity scale toward vinylic carbon cannot be constructed . This conclusion is reminiscent of the earlier observation of Pearson (see the introduction to the section on the Brpnsted equation) and the later observation of Ritchie (Chapter 11) regarding the difficulty of correlating nucleophilic reactivity with a single equation. Rappoport finds another familiar situation when he explores the relationship between reactivity and selectivity for the vinyl substrates sometimes the RSP is obeyed and sometimes it is not. 

The probes and methods for determining the relative nucleophilicities of nucleophiles toward electrophilic olefins and toward vinyl cations were examined. Literature data were used in an attempt to construct a substrate-independent nucleophilicity scale toward vinylic carbon. 

Taking into account these reservations (as much as possible), we searched the literature to answer the following questions (1) Is there a substrate-independent nucleophilic scale toward vinylic carbon (2) If not, are different scales applicable for addition and substitution (3) What is the role of electronic and steric effects and hard-soft interactions on the nucleophilic order Because of the lack of space, we will mention only briefly the solvent dependence of the nucleophilicity. 

Table III gives kN3 /kpiperidine rati°s- The values for the three (5-chlo-rovinyl ketones (35) demonstrate two important features. First, they increase in the dipolar aprotic solvent, by relative increase in the reactivity of the anionic nucleophile. Second, and less expected, the order of the ratios changes from N3 < piperidine for E-PhCOC(Ph) = CHCl to N3 > piperidine for the cyclic P-haloketone. Consequently, difficulties are expected in attempts to construct even a qualitative nucleophilicity scale toward vinylic carbon.

From this discussion, clearly, a quantitative nucleophilicity scale toward vinylic carbon cannot be constructed. Neither Ritchies nor Swain-Scott s correlations are applicable. Different blends of contribution of polar, steric, and symbiotic effect can change the reactivity order. Whether a qualitative order prevails could be inferred by comparing the three substitution reactions of chloro olefins, which are the only processes for which a relatively extensive change in the type of nucleophile was conducted (Table X). 

The nucleophilicities are found to be dependent on electronic, steric, and symbiotic effects, and limited series obeyed a constant selectivity , a reactivity-selectivity or a dual-parameter linear free-energy relationship. The conclusion made was that because of different blends of the effects, the construction of a substrate-independent nucleophilicity scale was impossible at present, but an approximate scale was presented. In nucleophilic reactions on relatively long lived vinyl cations, the steric effects predominate, but at constant steric effects, reactivity-selectivity relationships were found for very short series of substrates. Additional data are required for constructing more reliable nucleophilicity scales toward neutral and positively charged vinylic carbons. 

An interesting conclusion is that in spite of the fact that the chloro-substituted vinylic carbon is relatively soft, several of the reactive nucleophiles of the Swain-Scott scale such as N3 , Br, SCN-, and S032- show relatively low nucleophilicity in Table XI. When a more quantitative scale will be available, comparing it with the nucleophilicity order toward activated aromatic carbon would be constructive. 

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