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

Abstract

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.

CHNO – Formylnitrene, Cyanic, Isocyanic, Fulminic, and Isofulminic Acids and their Interrelationships at DFT and CASPT2 Levels of Theory

“…However, it is more favorable still for the CSS nitrene to isomerize to HNCO via TS6 at 27.5 (33.3) kcal/mol. An analogous mechanism was determined for the rearrangement of benzonitrile oxide to phenyl isocyanate …”

“…An analogous mechanism was determined for the rearrangement of benzonitrile oxide to phenyl isocyanate. 49 Photolysis of matrix-isolated HCNO with a Xe arc lamp causes isomerization to HOCN (minor) and HNCO (major) on irradiation, whereby the reaction HCNO → HOCN was found to be faster that the reaction HNCO → HOCN. 20 It was therefore concluded that in the photolysis, HCNO isomerizes directly to HOCN, not via HNCO.…”

“…Ground-state geometries and energies were determined at the M06-2X/6-311G­(d,p), B3LYP/6-311G­(d,p), dispersion-corrected B3LYP­(D3BJ)/6-311++G­(d,p), and CASPT2­(10,10)/6-311G­(d,p) levels of theory. The MO6-2X method was chosen on the basis of thermochemical benchmark studies and ability to produce accurate reaction energies, as determined in previous studies. , B3LYP and was chosen for comparison because of its widespread use and in order to provide continuity with our previous computational studies on CHNO and PhCNO isomers. , The dispersion-corrected B3LYP­(D3BJ) method provides an improved description of short- and medium-range interatomic interactions . Unrestricted DFT calculations were employed throughout.…”

“…63,64 B3LYP and was chosen for comparison because of its widespread use and in order to provide continuity with our previous computational studies on CHNO and PhCNO isomers. 42,49 The dispersion-corrected B3LYP(D3BJ) method provides an improved description of short-and medium-range interatomic interactions. 65 Unrestricted DFT calculations were employed throughout.…”

Formation of (Aza)fulvenallene, Cyanocyclopentadiene, and (Aza)fluorenes in the Thermal Rearrangements of Indazoles, Azaindazoles, and Homoquinolinic Anhydride

“…The nitrile imine 40 exists in the nonplanar, propargylic form 40a with a short (1.16 Å) CN bond and an N−N bond with a high degree of double-bond character (1.26 Å) in the singlet ground state (Figure 7B). The triplet states of nitrilium betaines are of much higher energies 21 and will not, therefore, be considered here. Due to the high degree of double-bond character, the cleavage of the N−N bond 22 does not occur under thermal conditions.…”