2‐Chloro‐Azaborolyl Anion: A Source of 1,2‐Azaborole Isosteric to Cyclopentadienylidene

Abstract

Azaborolyl anions, the five-membered BN heterocycles, have attracted a considerable attention due to their aromaticity and isoelectronic relationship with ubiquitous cyclopentadienyl ligands. Besides their syntheses and applications in the preparation of metal complexes, the other aspects of their chemistry have been virtually unexplored. Reduction of the azabutadienyl chelate boron dichloride [ArNC(R)CHC(R)]BCl2 (2, Ar=2,6-Me2 C6 H3 , R=tBu) with two equivalents of potassium yielded the novel 2-chloro-azaborolyl anion [ArNC(R)CHC(R)BCl]K(thf) (3) as a stable product in good yield. Reaction of 3 with 1,3,4,5-tetramethylimidazol-2-ylidene (NHC) yielded the first NHC-azaborole adduct with the elimination of KCl. The salt elimination reaction was also observed in the reactions with H2 O and the organic azide ArN3 , leading to the formation of an oxo-bridged 3H-1,2-diazaborole and an intramolecular donor-stabilized iminoborane, demonstrating that 3 is a source of the unexplored 1,2-azaborole isosteric to cyclopentadienylidene.

Reactivity of the 2‐Chloroazaborolyl Anion

“…The product was identified as iminoborane 2 by 1 H, 13 C, and 11 B NMR spectroscopy, EI mass spectrometry, and elemental analysis. The 1 H and 13 C NMR spectra show peaks at δ = 6.47 and 189.6 ppm for the C=CH proton and the C=N carbon atom in the BC 3 N five‐membered ring, respectively, similar to those reported for the donor‐stabilized iminoborane . However, the 11 B NMR spectrum of 2 exhibits a resonance at δ = 10.3 ppm, which is significantly shifted upfield, relative to the value of δ = 23.5 ppm reported for the iminoborane, probably due to the conjugation between the B=N and N=C bonds.…”

“…In previous studies, we found that the reaction of 1 with the organic azide ArN 3 offered the intramolecular donor‐stabilized iminoborane by elimination of KCl . This result prompted us to investigate its reaction with diazomethanes, with the expectation of synthesizing the corresponding methyleneboranes.…”

“…Chemical shifts are referenced against external Me 4 Si ( 1 H, 13 C) and BF 3 · Et 2 O ( 11 B). The compound [LBCl]K(THF) { 1 ; L = [ArN=C(R)CH=C(R)] – ; Ar = 2,6‐Me 2 C 6 H 3 , R = t Bu} was prepared according to a literature method …”

“…Recently, we reported the synthesis and borylene‐like reactivity of the 2‐chloroazaborolyl anion (LBCl) – K + (THF) { 1 ; L = [ArNC(R)CHC(R)] – ; Ar = 2,6‐Me 2 C 6 H 3 , R = t Bu}; a preliminary study on the reactivity of 1 disclosed that it readily undergoes salt elimination with some organic substrates to afford novel types of azaborole derivatives. The reactivity of this anion resembles cyclopentadienylidene . It is envisioned that the elimination of KCl is very likely to generate 1,2‐azaborole [ArNC(R)CHC(R)B] (Scheme b) as an intermediate, which is isoelectronic with cyclopentadienylidene.…”

Isolable Boron Persulfide: Activation of Elemental Sulfur with a 2‐Chloro‐Azaborolyl Anion

“…We have recently reportedt he synthesis and borylene-like reactivity of the 2-chloro-azaborolyl anion (LBCl) À K + (THF) (1, L = [ArNC(R)CHC(R)] À ;A r = 2,6-Me 2 C 6 H 3 ,R = tBu, Scheme 2). [12] This promptedu st oexaminet he reactions of 1 with elemental chalcogens for the furtheru nderstanding of the chemical [ properties of this unique masked heterocylic borylene. Herein, we report the reaction of 1 with elemental sulfur,l eading to the isolation of the first boron persulfide LB(h 2 -S 2 )( 2).…”