A SAFE AND MILD SYNTHESIS OF ORGANIC CARBONATES Monica Orsini, Laura Palombi, Leucio Rossi, Mirella Verdecchia

Dipartimento di Chimica, Ingegneria Chimica e Materiali. Università degli Studi. Monteluco di Roio; I-67040 L’Aquila, ITALY.

I dialchil carbonati sono dei composti di notevole interesse date le loro numerose e varie applicazioni industriali e nel campo della sintesi. I metodi di sintesi più importanti richiedono l’uso diretto o indiretto del fosgene un reattivo altamente tossico e corrosivo. Altri metodi impiegano CO, condizioni di reazione drastiche e/o catalizzatori metallici il cui impatto ambientale non può essere trascurato. Recentemente è stata proposta, da numerosi autori, la sostituzione di fosgene e CO con la più innocua anidride carbonica. In particolare in nostro gruppo di ricerca ha studiato due reagenti derivati dalla CO₂(carbonato e idrogeno carbonato di tetraetilammonio TEAC e TEAHC) in grado di carbossilare alcoli ed ammine per dare, rispettivamente carbonati e carbammati lineari e ciclici.[1] In questo lavoro vengono presentati due nuovi reagenti: tetrabutilammonio carbonato di metile ed etile (rispettivamente TBAMC e TBAEC) ottenuti semplicemente per reazione della CO₂ con i corrispondenti alcossidi di tetrabutilammonio. Tali composti trattati con alogenuri e tosilati alchilici forniscono i corrispondenti carbonati, in alte rese, secondo lo schema riportato. Questo metodo evita l’uso dei classici reagenti tossici e nocivi a permette di operare in condizioni di reazione molto blande.

Dialkyl carbonates are useful substrates for a variety of industrial and synthetic applications. The most important route to organic carbonates involve the direct or indirect use of phosgene, a very toxic and corrosive reagent. Other available methods use CO, drastic reaction conditions and/or metal catalysts, whose potential environmental impact should not be underestimated. Recently, the replacement of phosgene and CO with carbon dioxide has been proposed by several authors. Our research group found in tetraethylammonium carbonate (TEAC) and tetraethylammonium hydrogen carbonate (TEAHC) two valuable carboxylating reagents derived from CO₂ able to allows the obtaining of linear and cyclic carbonates and carbamates from, respectively alcohols and amines.[1] Here we report that methyl and ethyl tetrabutylammonium carbonates (TBAMC and TBAEC respectively) simply obtained by reaction of CO₂ with the corresponding tetrabutylammonium alkoxides, react with alkyl halides and tosylates yielding the corresponding carbonates in high yields according to the scheme reported. This method avoids the use of classical toxic and harmful chemicals and works under very mild reaction conditions.

[1] A. Inesi, V. Mucciante, L. Rossi, J. Org. Chem. 1998, 63, 1337; M. A. Casadei, A. Inesi, L. Rossi,

Tetrahedron Letters 1997, 3565.

Con controllo del pH prossimo alla neutralità, nel caso di chetoni aciclici o aril alchil chetoni (così come con chetoni a grande o medio anello), la perdita di chetone é trascurabile. Quindi, con diossirani generati in situ, il processo risulta essere un trasferimento di ossigeno da un perossido inorganico a un substrato organico (S, in Figura 1) catalizzato da chetone.

Il sistema appare adatto a ossidazioni selettive tipiche della chimica “verde” (e.g., epossidazioni) dirette alla sintesi di intermedi di interesse nell’ambito della chimica fine. Infatti, la maggior parte di chetoni semplici (e.g., acetone) sono sostanzialmente tollerabili dall’ambiente, e possono essere impiegati in mezzo acquoso; tra l’altro non sono apprezzabilmente consumati giacché agiscono solamente da catalizzatori. Per substrati che non sono apprezzabilmente solubili in mezzo acquoso, possono essere messi a punto sistemi a trasferimento di fase (PTC) (sebbene l’uso di acetone in eccesso come cosolvente potrebbe essere un ovvio rimedio al problema della solubilità in mezzo acquoso).1,2 _{In effetti, opportuni chetoni semplici che presentano solo una limitata solubilità in acqua} (e.g., metil etilchetone, MEK) potrebbero essere utilizzati come fase organica nelle ossidazioni con il sistema caroato/chetone in condizioni PTC. Perciò, il solo superabile inconveniente (limitatamente a processi su larga scala) sembra consistere nella formazione di notevoli quantità di sali solfato (prodotto di riduzione del caroato)

Riferimenti

1. (a) Curci, R.; Fiorentino, M.; Troisi, L.; Edwards, J. O.; Pater, R. H. J. Org. Chem. 1980, 45, 4758. (b) Cicala, G.; Curci, R.; Fiorentino, M.; Laricchiuta, O. J. Org. Chem. 1982, 47, 2670. (c) Curci, R.; Fiorentino, M.; Serio, M. R. J. Chem. Soc., Chem. Comm. 1984, 155

2. For reviews, see: (a) Curci, R.; Dinoi, A.; Rubino, M. F. Pure Appl. Chem. 1995, 67, 811. See references therein. (b) Adam. W.; Hadjiarapoglou, L. P.; Curci, R.; Mello, R. in Organic Peroxides; Ando, W., Ed.; Wiley: New York, 1992; Chapter 4, pp 195-219. (c) Curci, R. in Advances in Oxygenated Processes; Baumstark, A. L., Ed.; JAI: Greenwich, CT, 1990; Vol 2, Chapter I, 1-59. (d) Adam, W.; Curci, R.; Edwards, J. O. Acc. Chem. Res. 1989, 22, 205. (e) Murray, R. W. Chem. Rev. 1989, 89, 1187.

3. (a) Mello, R.; Fiorentino, M.; Fusco, C.; Curci, R. J. Am. Chem. Soc. 1989, 111, 6749. (b) Adam, W.; Curci, R.; González Nuñez, M. E.; Mello, R. J. Am. Chem. Soc. 1991, 113, 7654. (c) Curci, R.; D’Accolti, L.; Fusco, C. Tetrahedron Lett. 2001, 42, 7087-7090. See also references therein.

P49

OXIDATIVE DESTRUCTION OF CHLOROBENZENE ON Pt/ZSM5 CATALYSTS Salvatore Scirè, Simona Minicò, Carmelo Crisafulli, Vito Giuffrida

Dip. Scienze Chimiche, Univ. Catania, Viale A.Doria, 6 – 95125 Catania (Italy).

The removal of chlorinated aromatics from waste gases is an important goal due to the high toxicity and persistence in the environment of these compounds. Metal oxides or supported noble metals are the most studied catalysts for the destruction of chlorinated aromatics [1-2]. On oxides, volatile metaloxychlorides formation is the main problem, causing catalyst deactivation. Noble metals are more active and stable, but lead to high amounts of polychlorinated compounds, more toxic than the starting material [2]. Recently zeolites have been successfully used in the oxidation of halogenated hydrocarbons [3]. We now report a study on the use of Pt/ZSM5 catalysts for the combustion of chlorobenzene (PhCl), taken as a model of chlorinated aromatics. Pt (0.5 wt%) catalysts were prepared by impregnation of four H-ZSM5 and one g-Al₂O₃ with H₂PtCl₆ aqueous solutions. Samples were treated at 450°C in H₂ for 1h and then in air for 3h. Pt dispersion of catalysts, measured by H₂chemisorption, was 15-20%. Activity tests were carried out at atmospheric pressure in a fixed-bed reactor using 10% O₂and 0.2% PhCl diluted with He as reactant (GHSV=18600 h-1_). The reaction products were analyzed with two on-line gas chromatographs, one for the CO/CO₂ analysis and the other for that of polychlorinated benzenes (PhCl_x).

On Pt/Al₂O₃ PhCl combustion started at 300°C reaching 50% PhCl conversion at 340°C. PhCl_x formation showed a maximum (2.1% yield) at 500°C, temperature at which PhCl reached 100% conversion. PhCl₂(p-PhCl₂ ª45%, m-PhCl₂ª45%, o-PhCl₂ª10%) and PhCl₃(mainly 1,2,4-PhCl₃), formed in comparable amounts, were the main PhCl_x. The activity of Pt/ZSM5 samples was higher than that of Pt/Al₂O₃, decreasing on increasing the SiO₂/Al₂O₃ratio, suggesting that acid sites are involved in the PhCl oxidation [3]. On Pt/ZSM5 samples the formation of PhCl_xwas sensibly lower (maximum yield of 0.6% at 300°C) than that observed on the Pt/Al₂O₃ sample. On Pt/ZSM5 catalysts p-PhCl₂was the main isomer (ª60% of total PhCl₂), very low amounts of PhCl₃or higher PhCl_xwere always detected. No substantial deactivation was observed during 10h of reaction. On supports alone the oxidation of PhCl occurred at much higher temperatures (light-off > 400°C) without PhCl_xformation.

FT-IR spectra of adsorbed PhCl indicated that PhCl enters easily the ZSM5 channels, interacting with OH groups of the zeolite and Pt sites. PhCl_xare formed by chlorination of the adsorbed PhCl, probably involving Pt oxychloride species [2]. It was suggested that the zeolite structure induces a product shape selectivity effect, which hinders the chlorination of PhCl to PhCl_x and affects the PhCl_xdistribution, favouring the formation of isomers with lower steric hindrance.

On the basis of the reported results it can be concluded that Pt/ZSM5 is a promising system for the catalytic combustion of chlorobenzene, exhibiting good activity, low deactivation rate and very low formation of polychlorinated benzenes.

References

1. G.H. Hutchings, C.S. Heneghan, I.D. Hudson, S.H. Taylor, Nature 384 (1996) 341. 2. R.W. van den Brink, R. Louw, P. Mulder, Appl. Catal. B 16 (1998) 219.

3. J.R. Gonzalez-Velasco, R.Lopez-Fonseca, A.Aranzabal, J.I.Gutierrez-Ortiz, P.Steltenpohl, Appl. Catal. B 24 (2000) 233.

ARSENIC PROJECT IN MAREMMA (TUSCANY)