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

PINILLA PEÑA, DIANA C.; RUIZ PEREYRA, ELBA N.; FIRPO, GUADALUPE; CEBALLOS, NOELIA M.; FÜLÖP, FERENC; SZAKONYI, ZSOLT; IRIARTE, ANA G.; PELÁEZ, WALTER J.

EUROPEAN JOURNAL OF ORGANIC CHEMISTRY

WILEY-V C H VERLAG GMBH

Año: 2021

1434-193X

is‐N‐phenyl‐hexahydro‐2H‐benzo[d][1,3]oxazin‐2‐imines and cis‐N‐phenyl‐hexahydro‐2H‐benzo[d][1,3]thiazin‐2‐imines were prepared and subjected to Flash Vacuum Pyrolysis (FVP). NH− and NCH3− oxazines reacted at lower temperatures than the thio‐ analogs, showing lower thermal stability, while NCH2Ph− 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 tem