Noble Gas Atoms as Electron Donors: Is the Stabilization of Strongly Electrophilic Borylnitrenes Feasible?

Computational Exploration of the Intersystem Crossing from the X̃3A2 to the ã1A1 State in Boryl Nitrenes upon Photoexcitation

The boryl nitrene CatBN (Cat = catecholato) turns highly reactive toward small inert molecules upon irradiation of its triplet ground state X̃³A₂ with light of wavelength λ > 550 nm. A computational study of a model boryl nitrene using complete active space self-consistent field (CASSCF) theory provides evidence for the population of the highly reactive electronic state ã¹A₁ upon irradiation. Potential energy scans connecting different critical points (minima, minimum energy crossing points, and conical intersections) reveal two possible pathways that could relax photoexcited boryl nitrene from the Franck-Condon region of óB₁ to the ã¹A₁ state minimum. Considering the energy barriers to relaxation from one electronic state to another and the magnitude of spin-orbit couplings, the energetically most favorable pathway involves photoexcitation to óB₂, followed by intersystem crossing to the open-shell singlet state (b̃¹A₂) and internal conversion to ã¹A₁. The relevant minimum energy crossing point is about 7-8 kcal mol^-1 higher in energy than the Franck-Condon region.

The Reaction of CO2 with a Borylnitrene: Formation of an 3-Oxaziridinone

The reaction of a borylnitrene with carbon dioxide is studied under cryogenic matrix isolation conditions. Photogenerated CatBN (Cat=catecholato) reacts with CO₂ under formation of the cycloaddition product CatBNCO₂, a 3‐oxaziridinone derivative, after photoexcitation (>550 nm). The product shows Fermi resonances between the CO stretching and ring deformation modes that cause unusual ¹³C and ¹⁸O isotopic shifts. A computational analysis of the 3‐oxaziridinone shows this cyclic carbamate to be less strained than an α‐lactone or an α‐lactame.

Isomerization and fragmentation pathways of 1,2-azaborine

The generation of 1,2-azaborine (4), the BN-analogue of ortho-benzyne, was recently achieved by elimination of tert-butyldimethylchlorosilane under the conditions of flash vacuum pyrolysis. The present investigation identifies by computational means pathways for the thermal isomerization and fragmentation of 1,2-azaborine. The computations were performed using single reference (hybrid/density functional, second order Møller-Plesset perturbation, and coupled cluster theories) as well as multiconfiguration methods (complete active space SCF based second order perturbation theory, multireference configuration interaction, and multiconfiguration coupled electron pair approximation) with basis sets up to polarized triple-ζ quality. The 1,2-azaborine is, despite the distortion of its molecular structure, the most stable C4H4BN isomer investigated. The formation of BN-endiyne isomers is highly unfavorable as the identified pathways involve barriers close to 80 kcal mol(-1). The concerted fragmentation to ethyne and 2-aza-3-bora-butadiyne even has a barrier close to 120 kcal mol(-1). The fragmentation of BN-enediynes has energetic requirements similar to enediynes.