Reversible Aromaticity Transfer in a Bora-Cycle: Boron–Ligand Cooperation

Aromaticity is a
central concept in chemistry. Reaction pathways
involving reversible ligand dearomatization sequences emerged as a
powerful tool for bond activation by metal complexes. Exploring this
concept with a metal-free system, we have synthesized a pyridine-coordinated
aminoborane which undergoes a temperature-induced formal dearomatization
of the pyridine ring. NMR studies and DFT calculations revealed that
this formal dearomatization sequence led to an aromaticity switch
and the formation of a six-π-electron boron-containing heteroaromatic
system. Disrupting this aromatic system by coordination of an amine
or a carboxylic acid to the boron center enabled N–H activation
and O–H cleavage, leading to an unprecedented reversal aromaticity
switch.