Vinylic carbon, quantitative nucleophilicity

Nucleophilic vinylic substitutions of 4//-pyran-4-onc and 2,6-dimethyl-4//-pyran-4-one with a hydroxide ion in aqueous solution were calculated by the density functional theory (B3LYP) and ab initio (MP2) methods using the 6-31+G(d) and 6-31G (d) basis sets. The aqueous solution was modelled by a supermolecular approach, where 11 water molecules were involved in the reaction system. The calculations confirmed a different addition-elimination mechanism of the reaction compared with that in the gas phase or non-polar solution. Addition of OH- at the C(2) vinylic carbon of the pyranone ring with an activation barrier of 10-11 kcalmol-1 (B3LYP) has been identified as the rate-determining step, in good quantitative and qualitative agreement with experimental kinetics. Solvent effects increase the activation barrier of the addition step and, conversely, decrease the barrier of the elimination step.138... 

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. 

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). 

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. 

Leitner and co-workers described Pd-catalyzed nucleophilic substitutions ofallylic substrates with different nucleophiles [27]. They used Pd2(dba)3 as the palladium source and phosphane 20 as perfluoro-tagged ligand [Eq. (5)]. Reaction between cinnamyl methyl carbonate (21) and various nucleophiles (Nu-H) were performed in a THF/C7FJ4 biphasic mixture. A decrease in conversion was observed only after the ninth run (with 5 mol% Pd complex). By reducing the amoimt of Pd complex to 1 mol%, five quantitative recyclings were possible. The standard protocol was also applied to the condensation of dimethyl malonate with allyl methyl carbonate, (2-vinyl)butyl carbonate, and cyclohex-2-enyl carbonate. In each case two recyclings were performed without any decrease in conversion. 

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