SOLVENT EFFECTS ON CARBOCATION-NUCLEOPHILE COMBINATION REACTIONS - A COMPARISON OF PI-NUCLEOPHILICITY IN AQUEOUS AND ORGANIC-SOLVENTS

The following selectivities are reported for partitioning of the alpha -(N,N-dimethylthiocarbamoyl)-4-methoxybenzyl carbocation 1(+) between nucleophilic addition of pi-nucleophiles and a solvent of 50:50 (v:v) acetonitrile/water at 25 degrees C (nucleophile, k(Nu)/k(s)): pyrrole, 23 M-1; 2-methoxythiophene, approximate to 2 M-1. No nucleophile addu cts (<2% of total products) were detected from the reaction of this ca rbocation in the same solvent containing the following carbon nucleoph iles: thiophene (0.10 M), furan (0.10 M), ethyl l-propenyl ether (0.10 M), amd 1-cyclohexenyl trimethylsilyl ether (0.01 M). These results s how that only pi-nucleophiles with Mayr nucleophilicity parameters gre ater than N approximate to 6 are sufficiently reactive to compete with nucleophilic aqueous solvents for addition to 1+, and they provide su pport for a simple relationship between the Ritchie N+ scale of nucleo philicity in water and the Mayr N scale of nucleophilicity in weakly p olar nonnucleophilic solvents.(2,3) Our data require a large effective molarity for the intramolecular addition of a trisubstituted alkene t o an allylic carbocation.