Rearrangements of Aromatic Nitrile Oxides and Nitrile Ylides: Potential Ring Expansion to Cycloheptatetraene Derivatives Mimicking Arylcarbenes

Nitrile imines, nitrile oxides and nitrile ylides are widely used in 1,3‐dipolar cycloaddition reactions. They also undergo thermal and photochemical rearrangements to carbodiimides, isocyanates, and ketenimines, respectively. Calculations at DFT and CASPT2 levels of theory reveal novel, potential rearrangements, in which the aromatic 1,3‐dipoles mimic phenylcarbene and undergo ring expansion to cycloheptatetraene derivatives. These rearrangements can potentially take place in both the singlet ground states and the triplet excited states, and they are accelerated by m,m’‐bis(dimethylamino) substitution on the phenyl moieties. The new rearrangement becomes the energetically preferred path for m,m’‐bis(dimethylamino)benzonitrile oxide in the triplet state. In the m,m’‐bis(dimethylamino)benzo nitrile ylide, the cyclization to the 2‐phenyl‐1‐azirine is favored over the ring expansion to a cycloheptatetraene by ca. 5 kcal mol−1 in the singlet state. In the bent triplet states, 1,3‐hydrogen shifts interconverting nitrile ylides are potentially possible. Nitrile oxide and nitrile ylide rearrangements: New rearrangements of nitrile oxides 10 (X=O) and nitrile ylides 16 (X=CH2) to cycloheptatetraene derivatives 14 and 21 have been examined at DFT and CASPT2 levels of theory. The ring expansion becomes the preferred path for the triplet state of m,m’‐bis(dimethylamino)benzonitrile oxide.