On Neutral Unsaturated Ouroboric Borylenes

The search is on for stable isolated borylenes. Potential
roles
in modern synthetic chemistry for boron analogues of carbenes continue
to motivate interest in locating them. Using density functional and
ab initio methods, we posit and examine the thermochemistry, and chemical
bonding, including aromaticity, of several classes of 5- and 6-membered
borylenic rings. In these systems, cyclization relies on dative bonding
(ouroboric coordination) and π donation to a monovalent boron
center from an adjacent O center. Certain neutral five-membered rings
(heterocyclic cyclopentadienyl analogues) in particular are found
to exhibit exceptionally strong preferences for the singlet multiplicity,
each with singlet–triplet (S–T) gaps in excess of 40
kcal·mol–1. The singlet five-membered rings
with the largest S–T gaps and some of the six-membered rings
show evidence of weak aromaticity. Relationships of the form N = A·r–b, in line with Gordy’s and other functions
linking bond order, N, and covalent bond length,
are identified for dative B←O contacts, r,
reinforced in rings by π-delocalization.