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

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.

Synthesis of neutral and zwitterionic phosphinomethylpyrrolato complexes of nickel

“…The 13 C NMR signal which correlates with these protons (d 76.4) also did not couple to phosphorus. These observations are consistent with the presence of acidic sp 3 protons in 5-membered N-heterocyclic compounds [18,19]. The 1 H NMR signals for the diastereotopic CH 2 P group were found as two multiplets centred at d 2.51 and 2.74; the corresponding 13 C signal appears as a doublet (J C-P = 26.6 Hz) and its coupling pattern is, as expected, altered in the 1 H{ 31 P} NMR spectrum (these 1 H NMR signals do not in fact become simple doublets of doublets as a small coupling exists between this ABX system and the AB methylene system situated on the 3-position of the pyrrole ring, as shown by 2D COSY NMR).…”

“…However, with B(C 6 F 5 ) 3 they give the catalytically active zwitterionic products 5a (R = Ph) and 5c (R = Cy), respectively. Some time ago Erker et al reported the abstraction of a pyrrolate ligand from Cp 2 ZrMe(NC 4 H 4 ) by B(C 6 F 5 ) 3 to give the [(1-pyrrolyl)B(C 6 F 5 ) 3 ] À anion [16] and synthesized 5H-pyrrole-B(C 6 F 5 ) 3 complex by acidification of (Et 2 O) 2 Li[(1-pyrrolyl)B(C 6 F 5 ) 3 ] [17], while Resconi and co-workers undertook a detailed study of the reaction of B(C 6 F 5 ) 3 with pyrroles, indoles, imidazoles and pyrazoles [18][19][20]. In all these cases, with the exception of N-alkyl pyrroles, the boron adds to the nitrogen atom.…”

“…NMR spectra were recorded using a Bruker DPX-300 spectrometer with a 5 mm BBO probe. Chemical shifts are reported in ppm and referenced to residual solvent resonances ( 1 H, 13 C), 19 Cl (120 cm 3 ). The mixture was agitated for 15 min and left to separate.…”

Catalytic Electrophilic C−H Borylation Using NHC⋅Boranes and Iodine Forms C2‐, not C3‐, Borylated Indoles

“…8 Precedence for this hypothesis is provided by the reaction of B(C6F5)3 with N-Me-indole which over 10 days affords the C2-borylated arenium species (Figure 1 middle), 9 indicating that migration of boron from C3 to C2 can occur in indoles in the absence of an effective exogenous Brønsted base.…”

A Boronium Ion with Exceptional Electrophilicity

“…[28] Borylation of N-methylindole afforded exclusively the 3-substituted regioisomer, in sharp contrast to the previously reported reaction with B(C 6 F 5 ) 3 , which produces the 2-borylated N-methylindole. [29] Pyrrole 15c gave a mixture of mono-borylated regioisomers along with some of the diborylated 16c using one equivalent of 8a (ca. 80% conversion of 15c ), but two equivalents of the borylating agent cleanly produced the diborylated pyrrole 16c .…”