Synthesis and Reactivity of <i>N</i>-Heterocycle-B(C₆F₅)₃ Complexes. 3. Generation of <i>N</i>-Methylpyrrol-2-yl and <i>N</i>-Methylindol-2-yl Borate Zwitterions with Acidic sp³ Carbons

Abstract

The stoichiometric reactions of N-methylpyrrole and N-methylindole with B(C6F5)3 produce the zwitterionic species 2-[tris(pentafluorophenyl)borane]-5H-1-methylpyrrole (3) and 2-[tris(pentafluorophenyl)borane]-3H-1-methylindole (4), in which a C(α)−B bond and an acidic sp3 methylene carbon are formed in the heterocyclic part of the molecule. Both derivatives present a restricted rotation around the C(α)−B and/or B−C6F5 bonds, and their rotational barriers (13.8 and 14.8 kcal mol-1 for 3 and 4, respectively) were calculated from 1H NMR experimental data. A kinetic study of the reaction, carried out by following the conversion by NMR, gave rate constant values (at 298 K in dichloromethane) of 3 × 10-5 and 6 × 10-5 M-1 s-1 for 3 and 4, respectively. Complexes 3 and 4 react quantitatively with triethylamine to give the corresponding triethylammonium salts 5 and 6. Both the zwitterionic complexes and their ammonium salts are efficient activators of Ind2ZrMe2 for the polymerization of ethylene.