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

PINILLA PEÑA, DIANA C.; RUIZ PEREYRA, ELBA NAHIR; FIRPO, GUADALUPE RUIZ; CEBALLOS, NOELIA MARCELA; FÜLÖP, FERENC; SZAKONYI, ZSOLT; IRIARTE, ANA GRACIELA; PELÁEZ, WALTER JOSÉ

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 theobtained 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 470ºC. Kinetic measurements reveal negative entropies of activation, which suggest a partially concerted way of r