Flash Vacuum Pyrolysis (FVP) of cis-fused cyclohexen-1,3-Oxazine and 1,3-Thiazine derivatives.

DIANA CAROLINA PINILLA PEÑA

EUROPEAN JOURNAL OF ORGANIC CHEMISTRY

WILEY-V C H VERLAG GMBH

Lugar: Weinheim; Año: 2021

1434-193X

bstract: Cyclohexen-fused 1,3-oxazines and 1,3-thiazines 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, which stability increased with the N-substitution. From the analysis of the reaction temperatures and unraveling the obtained products, it is proposed that any of heterocyclic ring-opening reactions (initiated by X3-C4, C8a-N1 or C2-X3 bond rupture) require lower energy than the retro Diels-Alder process (RDA) that affords 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 reaction or slightly asynchronous fragmentation processes, with higher energy of