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

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

EUROPEAN JOURNAL OF ORGANIC CHEMISTRY

WILEY-V C H VERLAG GMBH

Lugar: Weinheim; Año: 2021

1434-193X

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 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 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.