Synthesis and Reactivity of (C₆F₅)₃B−N-Heterocycle Complexes. 1. Generation of Highly Acidic sp³Carbons in Pyrroles and Indoles

Abstract

The reaction of pyrroles and indoles with B(C(6)F(5))(3) and BCl(3) produces 1:1 B-N complexes containing highly acidic sp(3) carbons, for example, N-[tris(pentafluorophenyl)borane]-5H-pyrrole (1) and N-[tris(pentafluorophenyl)borane]-3H-indole (2), that are formed by a new formal N-to-C hydrogen shift, the mechanism of which is discussed. With some derivatives, restricted rotation around the B-N bond and/or the B-C bonds was observed by NMR techniques, and some rotational barriers were calculated from experimental data. The acidity of the sp(3) carbons in these complexes is shown by their ability to protonate NEt(3), with formation of pyrrolyl- and indolyl-borate ammonium salts. The driving force for this reaction is given by the restoration of the aromaticity of the heterocycle.

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.…”

Hydrogen and Amine Activation by a Frustrated Lewis Pair of a Bulky N‐Heterocyclic Carbene and B(C6F5)3

“…[30] The ammonia, aniline, and diphenylamine adducts of B(C 6 F 5 ) 3 [31] are deprotonated by ItBu to generate ionic complexes 3-5, respectively, which is analogous to reaction with dihydrogen (Scheme 3).…”

Synthesis and Reactivity of (C6F5)3BN‐heterocycle Complexes, 2

“…of B(C 6 F 5 ) 3 gave quantitative and instantaneous conversion to the 5 H ‐pyrrole · B(C 6 F 5 ) 3 complexes and 3 H ‐indole · B(C 6 F 5 ) 3 complexes (Scheme ). Most of these complexes were formed quantitatively and instantaneously, irrespective of the solvent or the order of addition 14c…”

“…The presence of the imminic double bond combined with the presence of the strong electron‐withdrawing B(C 6 F 5 ) 3 Lewis acid generates quite a strong acidity of the proton(s) on the sp 3 C(3) or C(5). For example, 1 and 2 react with NEt 3 to give quantitatively the [B(1‐pyrrolyl)(C 6 F 5 ) 3 ][HNEt 3 ] salt ( 1a ) and the [B(1‐indolyl)(C 6 F 5 ) 3 ][HNEt 3 ] salt ( 2a ) respectively (Scheme ) 14b,c. In addition to 1a and 2a , we obtained the salts [B(2‐methyl‐indolyl)(C 6 F 5 ) 3 ][HNEt 3 ] ( 3a ), [B(3‐methyl‐indolyl)(C 6 F 5 ) 3 ][HNEt 3 ] ( 4a ), [B(5‐methoxy‐indolyl)(C 6 F 5 ) 3 ][HNEt 3 ] ( 6a ), [B(2,5‐dimethyl‐1‐pyrrolyl)(C 6 F 5 ) 3 ][HNEt 3 ] ( 9a ), [B(2‐ethyl‐1‐pyrrolyl)(C 6 F 5 ) 3 ][HNEt 3 ] ( 10a ) and [B(imidazolyl)(C 6 F 5 ) 3 ][HNEt 3 ].…”