Spectroscopic methods as tool for investigating mechanisms of reactions: The case ofpseudosacharyl ethers'rearrangement

KACZOR A; GÓMEZ-ZAVAGLIA A; ALMEIDA R; MOBILI P; MARIA TM; EUSÉBIO ME; CRISTIANO ML; RUI FAUSTO

Santiago de Compostela, España

Congreso; 10th Iberian Joint Meeting on Atomic and Molecular Physics; 2009

Real Sociedad Española de Física, Real Sociedad Española de Química y Sociedade Portuguesa de Física

Pseudosaccharyl O-ethers possess quite unusual structure due to the CR-O-CA linkage (R=heteroaromatic ring, A=aliphatic/aryl group) with unexpectedly short and long CR-O and CA-O bonds, respectively. This geometry is a molecular basis for thermal isomerization of pseudosaccharyl O-ethers to their N-isomers, the reaction that we address in this study. The simplest member of the family, 3-(methoxy)-1,2-benzisothiazole 1,1-dioxide (MBID), was shown to undergo thermal rearrangement not only in the melt, but also in the solid state at temperatures well below its melting point.1 A multidisciplinary approach including DSC, polarized light microscopy, X-ray diffraction, temperature dependent IR spectroscopy and computational modeling was applied to investigate the mechanism of this rearrangement, indicating that the Chapman-type [1,3’] “quasi-simultaneous” intermolecular mechanism is energetically stabilized over intramolecular ones.1,2 Contrarily to MBID, the 3-(allyloxy)-substituted O-ether isomerizes thermally only above its melting point. The reaction was shown to be first order with an activation energy of ca. 92 kJ mol-1, which suggests a [3,3’] sigmatropic mechanism of rearrangement.3 The reasons for the substituent-dependent mechanism of pseudosacharyl ethers´ rearrangement are proposed and discussed.