Contribution to the Reactivity of N,N′‐Diaryl‐1,4‐diazabutadienes Aryl–N=CH–CH=N–Aryl (Aryl = 2,6‐Dimethylphenyl; 2,4,6‐Trimethylphenyl) Towards Boron Trichloride

Abstract

The reaction of Aryl–N=CH–CH=N–Aryl (3a: Aryl = 2,6‐Me2C6H3; 3b: Aryl = 2,4,6‐Me3C6H2) with 2 equiv. BCl3 in a toluene/hexane mixture at –50 °C led to an inseparable mixture of borolium salts [Aryl–Na=CH–CH=N(Aryl)BCl2(Na–B)]+BCl4– (4a: Aryl = 2,6‐Me2C6H3; 9a: Aryl = 2,4,6‐Me3C6H2) and [Aryl–Na=CH–C(Cl)=N(Aryl)BCl2(Na–B)]+BCl4– (4b: Aryl = 2,6‐Me2C6H3; 9b: Aryl = 2,4,6‐Me3C6H2) and the bicyclic species [HCa=N(Aryl)BCl2N(Aryl)–Cb=(Ca–Cb)]2 (5: Aryl = 2,6‐Me2C6H3; 10: Aryl = 2,4,6‐Me3C6H2). Sodium amalgam reduction of borolium salts 4a,b and 9a,b afforded inseparable mixtures of the diazaboroles 2,6‐Me2C6H3NaCH=CR–Nb(2,6‐Me2C6H3)BCl(Na–B) (1: R = H; 6: R = Cl) and 2,4,6‐Me3C6H2NaCH=CR–Nb(2,4,6‐Me3C6H2)BCl(Na–B) (2: R = H; 11: R = Cl). In order to obtain pure 1 and 2, diazabutadienes 3a and 3b were slowly combined with 2 equiv. BCl3 in the same solvents at –78 °C. The acyclic adducts Cl3BN(Aryl)=CH–CH=N(Aryl)BCl3 (7a: Aryl = 2,6‐Me2C6H3; 7b: Aryl = 2,4,6‐Me3C6H2) were reduced with sodium amalgam to furnish the aminoboranes Cl2BN(Aryl)CH=CH–N(Aryl)BCl2 (8: Aryl = 2,6‐Me2C6H3; 12: Aryl = 2,4,6‐Me3C6H2). Stirring solutions of 8 and 12 in the presence of CaH2 cleanly gave the diazaboroles 1 and 2, respectively. The novel compounds were characterized by elemental analyses and 1H‐, 11B‐, 13C NMR and mass spectra. The molecular structures of 1, 4a, 5 and 8 were elucidated by single X‐ray diffraction analyses. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Synthesis and reactivity of N,N′-1,4-diazabutadiene derived borocations

“…Additionally, the 11 B spectrum shows a single sharp resonance at δ = 10.6 ppm, indicating a 4-coordinate boron species, in keeping with boronium formation. 18 Upon addition of AlCl 3 to this initial product, no notable change was observed in the 1 H NMR spectrum, however the 11 B NMR spectrum showed reformation of BCl 3 , in addition to the previous resonance (δ = 10.6 ppm) which was consistent with the spectrum prior to AlCl 3 addition. The 27 Al spectrum showed a resonance at 103.98 ppm, indicating AlCl 4 -formation, and that a pseudo-counterion exchange process had occurred to yield the borocation tetrachloroaluminate species.…”

Recent developments in the chemistry of 1,3,2-diazaborolines-(2,3-dihydro-1H-1,3,2-diazaboroles)

“…From the concentrated mother liquor bicyclic compound 36 was isolated as dark green crystals in 14% yield. Sodium amalgam reduction of salts 34 and 35 afforded an inseparable mixture of diazaborolines 37 and 38 (Scheme 12) [22].…”

“…of BCl 3 in hexane. The green borolium salt 42 (63% yield) was reduced with sodium amalgam to quantitatively yield the thermolabile diazaboroline 43 (Scheme 13) [22].…”

“…Thermolabile 2-chloro-1,3,2-diazaboroline 43 was in situ converted into stable 85 by treatment with AgCN during 20 h at room temperature (yield 41%) (Scheme 24) [22].…”

“…of BCl 3 at −78 • C. Adduct 31 precipitated as a red-brown solid, the reduction of which with sodium amalgam furnished the colorless crystalline E-N,N-bis-amino ethene 32 in 70% yield. Cyclization of the latter species to give pure 30 was accomplished by treatment with solid CaH 2 in hexane (Scheme 12) [22].…”