Flash Vacuum Pyrolysis (FVP) of cis‐N ‐phenyl‐hexahydro‐2 H ‐benzo[ d ][1,3]oxazin‐2‐imine and Thiazin‐2‐imine Derivatives

Cis ‐ N ‐phenyl‐hexahydro‐2 H ‐benzo[ d ][1,3]oxazin‐2‐imines and cis‐ N ‐phenyl‐hexahydro‐2 H ‐benzo[ d ][1,3]thiazin‐2‐imines were prepared and subjected to Flash Vacuum Pyrolysis (FVP). NH− and NCH 3 − oxazines reacted at lower temperatures than the thio‐ analogs, showing lower thermal stability, while NCH 2 Ph− compounds presented the opposite behavior. Oxazines and thiazines yielded different products through homolytic fragmentations of the heterocycle moiety, the stability of which increased with the N− substitution. Based on the analysis of the reaction temperatures and the unraveling of the obtained products, it is proposed that any heterocyclic ring‐opening reactions (initiated by X3−C4, C8a−N1, or C2−X3 bond rupture) require less energy than the retro‐Diels–Alder process (RDA) that yields butadiene. This concerted reaction path is only achieved at temperatures higher than 470 °C. Kinetic measurements reveal negative entropies of activation, which suggest a partially concerted way of reaction or slightly asynchronous fragmentation processes, with higher energy of activation (Ea) values for the N‐substituted derivatives compared to the unsubstituted ones. Experimental results agree with calculations at the DFT level (B3LYP/6‐311+G(d,p)) and M06‐2X (6‐311+G(d,p)) from Gaussian09 software.