Solvent effects on carbocation - Nucleophile combination reactions: A comparison of π-nucleophilicity in aqueous and organic solvents

The following selectivities are reported for partitioning of the α- (N,N-dimethylthiocarbamoyl)-4-methoxybenzyl carbocation 1⁺ between nucleophilic addition of π-nucleophiles and a solvent of 50:50 (v:v) acetonitrile/water at 25 °C (nucleophile, k(Nu)/k(s)): pyrrole, 23 M^-1; 2- methoxythiophene, ≃2 M^-1. No nucleophile adducts (<2% of total products) were detected from the reaction of this carbocation in the same solvent containing the following carbon nucleophiles: thiophene (0.10 M), furan (0.10 M), ethyl 1-propenyl ether (0.10 M), amd 1-cyclohexenyl trimethylsilyl ether (0.01 M). These results show that only π-nucleophiles with Mayr nucleophilicity parameters greater than N ≃ 6 are sufficiently reactive to compete with nucleophilic aqueous solvents for addition to 1⁺, and they provide support for a simple relationship between the Ritchie N₊ scale of nucleophilicity in water and the Mayr N scale of nucleophilicity in weakly polar nonnucleophilic solvents. Our data require a large effective molarity for the intramolecular addition of a trisubstituted alkene to an allylic carbocation.