Synthesis and reactivity of N-heterocycle-B(C6F5)3 complexes.: 3.: Generation of N-methylpyrrol-2-yl and N-methylindol-2-yl borate zwitterions with acidic sp3 carbons

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)boranel-3H-1-methylindole (4), in which a C(alpha)-B bond and an acidic sp(3) methylene carbon are formed in the heterocyclic part of the molecule. Both derivatives present a restricted rotation around the C(a)-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 H-1 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 x 10(-5) and 6 x 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 Ind(2)ZrMe(2) for the polymerization of ethylene.