The Photochemistry of Saccharins

M.L. CRISTIANO, A. ISMAEL, L. DUARTE, A. BORBA, A. GOMÉZ ZAVAGLIA, I. REVA, R.FAUSTO

Praga

Congreso; European Symposium of Organic Chemistry (ESOR). República Checa. 1-6/9'/2013.; 2013

Saccharin and saccharinates are widely applied in food chemistry, agriculture and medicine, acting as cheap and versatile starting materials in synthesis, as ligands in catalysis and as anions in saccharin-based ionic liquids. This plethora of applications and the potential for new uses often rely on the thermal and photochemical stability of the saccharyl system. Early studies revealed that some saccharyl derivatives undergo photocleavage of the saccharyl core when exposed to ultraviolet irradiation, in solution1 but the photostability of monomeric saccharins had not been addressed. Thus, we undertook an investigation of the photochemistry of saccharin and selected saccharinates in inert media and confined environments. The photochemistry of saccharin, isolated in solid argon was studied by means of infrared spectroscopy and DFT calculations. Among several methods used, the O3LYP/6-311++G(3df,3pd) level gave the best match with the experimental spectra. Irradiation of matrix-isolated saccharin, with a narrow-band source (290 nm), generated a so far unknown isomer that we called iso-saccharin.2 Observed photoisomerization begins with an nπ* excitation of the carbonyl moiety of the saccharyl system and involves homolysis of the S−N bond, generating a delocalized biradicaloid, which after internal rotation and after ring closure produces the rearranged product. This photoisomerization of saccharin may have implications in its uses, in particular when deprotonation is facilitated. The structures of the conjugate bases derived from saccharin and iso-saccharin were computed theoretically.2 Their free energies and dipole moments suggest that both anions may be relevant in systems where saccharin participates, as is the case of the recently proposed saccharin-based ionic liquids. The photochemistry of selected tetrazole-saccharinates of interest as bidentate nitrogen ligands was investigated using the same approach. Photolysis of 2-[1-(1H-tetrazol-5-yl)ethyl]-1,2-benzisothiazol-3(2H)-one 1,1-dioxide led to cleavage of the saccharyl core. 3 Pathways will to be proposed and discussed.