Electrophilic insertion and ring growth in 1,2,5-azadiborolidines: theoretical evidence for boron-driven expansion

This study computationally investigates the reactivity of 1,2,5-azadiborolidine derivatives toward carbon monoxide (CO), extending ring-expansion mechanisms from borole chemistry. Establishing structural and electronic analogies through isosterism, we propose that endocyclic CO insertion operates in this new class of boron heterocycles. 1,2,5-Tri-tert-butyl-1,2,5-azadiborolidine emerged as the optimal candidate, exhibiting a favourable three-step pathway: CO insertion, dimerization of the ring-expanded intermediate, and subsequent double [1,2]-migration. Thermochemical analysis confirms viability under mild conditions, with solvents (DCM, n-pentane, THF) providing similar barriers and reaction Gibbs energies. Key reactivity indicators include B–C(O) and C–O bond lengths, while correlations between kinetic barriers and σ → π*(CO) backdonation energies provide predictive insight. This work broadens boron-based reactivity by proposing a fourth class of CO-inserting boracycles and highlights 1,2,5-azadiborolidines as versatile, metal-free frameworks for small-molecule activation.