Crystal Clear Structural Evidence for Electron Delocalization in 1,2-Dihydro-1,2-azaborines

Abstract

The first examples of "pre-aromatic" 1,2-dihydro-1,2-azaborine heterocycles have been structurally characterized, enabling the direct comparison of delocalized bonds of 1,2-dihydro-1,2-azaborines to their corresponding formal double and single bonds in nonaromatic systems. The crystallographic data provide an unprecedented look into the structural changes that occur in six-membered BN-heterocycles on their road to aromaticity, and they establish with little ambiguity that 1,2-dihydro-1,2-azaborines possess delocalized structures consistent with aromaticity.

B2N2-Dibenzo[a,e]pentalenes: Effect of the BN Orientation Pattern on Antiaromaticity and Optoelectronic Properties

“…The B−N bond lengths in BNNB-DBP are in the range of 1.40 -1.44 Å, indicative of localized BN double bond character. 19 The N−N bond length is around 1.42 Å, a typical value for N−N single bond (1.40 Å in compound 2). These structural features indicate that the bond length alteration in BNNB-DBP is similar to that in DBP.…”

Metall‐vermittelte Synthese von 1,4‐Di‐tert‐butyl‐1,4‐azaborin

“…Die Ausbeute lässt darauf schließen, dass 2 als Katalysator mit Umsatzzahlen von 3-6 wirkt. Das 11 B-NMR-Signal von 3 erscheint bei d = 48 ppm als Singulett und ist im Vergleich zu den Signalen der anderen Isomere (1-tert-Butyl-2-diphenylamin-1,2-azaborin [22] (4): Erstaunlicherweise lieferte die Reaktion nicht das 1,2-, sondern das 1,4-Isomer, das nur unter Spaltung der B-NDreifachbindung gebildet werden kann. Ein solcher Bindungsbruch ist präzedenzlos und schien wegen der B-N-Bindungsenergie der Stammverbindung (HB NH) von 176.9 kcal mol À1 , [23] die nur geringfügig kleiner als die der C-C-Dreifachbindung in Acetylen (193.8 kcal mol À1 ) ist, [24] unwahrscheinlich zu sein.…”

Nucleophilic Aromatic Substitution Reactions of 1,2‐Dihydro‐1,2‐Azaborine

“…Among the four commonly investigated criteria for aromaticity (structure, magnetism, energy, and reactivity), we have determined that 1,2-azaborines exhibit delocalized bonding, [7] have appropriate predicted NICS (À7.27 ppm; nucleus-independent chemical shift) values, [8] and have an experimentally determined resonance stabilization energy of 16.6 kcal mol À1 , [9] which is consistent with significant aromatic character. With regard to the reactivity criterion, Ashe and co-workers have demonstrated that substituted 1,2-azaborines undergo electrophilic aromatic substitution reactions.…”