“New heteropolyacids as catalysts for the selective oxidation of sulfides to sulfoxides or sulfones with hydrogen peroxide”

G. ROMANELLI, P. VÁZQUEZ, P. TUNDO

Nogoya, Japon

Conferencia; IUPAC- 15 International Conference on Organic Synthesis; 2004

IUPAC

The catalytic transformation of sulfides to sulfoxides or sulfones is a very important target both in industry and in academic research. The selective catalytic of sulfides to sulfoxides has been a challenge for many years, owing to the importance of sulfoxides as intermediates in organic synthesis. In the other hand sulfones are important compounds employed in the preparation and functionalization of a wide variety of products by stabilizing a-radicals, a anions and acting as cationic synthons. During the last years very useful procedures involving hydrogen peroxide (H2O2) as oxidant and a catalyst have been developed to promote oxidations of organic substrates, due to the effective oxygen-content, low cost, safety in storage and operation and environmentally friendly character of hydrogen peroxide. The design of practical catalysts at atomic or molecular level has long been pursued, since precisely designed multifunctional catalysts are obviously desirable. However, catalyst design at this level is still a distant goal in many instances. Currently there is considerable interest in exploiting both the structure of catalyst precursor and the multicenter active sites to facilite catalysis by heteropolyacids (HPA). The present work investigates the influence of pyridine salts, with Keggin structure for the selective oxidation of sulfides to sulfoxides or sulfones, it varing to reaction conditions, with hydrogen peroxide. The pyridine (Py) salts of the acids H4PMo11VO40, H5PMo10V2O40, H9PMo6V6O40 obtained by V substitution in the structure of H3PMo12O40 were synthesized. They were characterized by FT-IR, and the variation of their acid properties and that of their original acids was determined by titration with n-butylamine. Vibrational spectrum of primary structure Keggin was modified when Mo+6 atoms were substituted by V+5atoms. The pyridine acts as a reductant to lower the average oxidation states of vanadium, if this is present in the structure of heteropolyanion. The better reaction conditions for selective oxidation of methyl phenyl sulfide to sulfoxides 2, with hydrogen peroxide, are: room temperature, molar sustrate to hydrogen peroxide ratio: 1:2, acetonitrile as solvent and 1% (in mmol) of catalysts. For these conditions, the PM11VO40H3Py is more selective catalyst (99% to sulfoxides 2) with 98% of convertion by reactive, at 30 min of reaction time. On the other hand, the better reaction conditions for selective oxidation of methyl phenyl sulfide to sulfone 3 are similar to the previous reaction, but hydrogen peroxide is in excess. For this reaction, are used PMo10V2O40(Py)5 and PMo6V6O40(Py)9 catalysts with high selective, near to 100%, in both cases of pyridine salts. In order to establish the general applicability of the method, various funcionalized sulfides were subjected to the oxidation protocol.