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

Abstract

Cis-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 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.