A study on the synthesis of organic carbonate with by steepslope9876

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									A study on the synthesis of organic carbonate with carbon
dioxide: the role of the catalyst and reaction conditions.
Comparison between soluble and grafted catalysts.
Michele Aresta, Angela Dibenedetto, Francesco Nocito and Carlo Pastore

Department of Chemistry and Interuniversity Consortium on Chemical Reactivity and Catalysis-
CIRCC, University of Bari, Campus Universitario, Via Orabona, 4, 70126 Bari
e-mail: nocito@chimica.uniba.it

Organic carbonates characterized by a low toxicity, good biodegradability and high boiling point
are useful solvents and chemical intermediates. In particular, dimethyl carbonate finds several
applications in different sectors such as polymers, fuels and organic synthesis. The conventional
production involves the use of toxic phosgene or carbon monoxide. The synthesis of organic
carbonates from alcohols and CO2 2 is a hot theme today as the exploitation of such synthetic
methodology would supplant the use of phosgene in the chemical industry with great benefit from
the point of view of risk reduction.



Several catalytic systems have been used so far, either homogeneous or heterogeneous. Also,
organic promoters (such as carbodiimides)3 have been found to be very active.



Soluble tin metallorganic species have been used as catalysts4 in reaction 1. They show an
acceptable kinetics for the first cycle, after which the rate sensibly decreases. Species
Bun2Sn(OMe)2 is formed from Bun2SnCl2. The following step is the insertion of CO2 into one of the
Sn-OMe bonds, with subsequent formation of the tin emi-carbonate. An intra- or inter-molecular
methyl transfer affords the organic carbonate5 and residuates a metallorganic tin-oxide that has an
oligomeric structure.




The latter has an activity that is much lower than that of the starting complex. The reduction of
activity is a serious problem that negatively affects the usability of the catalyst. An approach to
avoid the formation of oligomeric tin-oxo-complexes would be the use of tin complexes designed in
such a way that tin centres are separated. Such structural constraint might push a different reaction
mechanism, preventing the formation of inactive oligomers. To this end we have grafted the
metallorganic tin species on a polystyrene-PS matrix. Here we report on the synthesis of the first
example of a grafted tin-complex [PS-C6H4-CH2CH2-SnBun(OMe)2] that is an active catalyst in the
synthesis of DMC.
The PS-Sn(OMe)2 grafted complex exposed to CO2 rapidly inserts a CO2 molecule to afford PS-
(OMe)(OCOOMe) as shown by FTIR measurements. Used in a catalytic run in methanol
(methanol/tin ratio=1300) at 423 K and under 4 MPa of CO2, after 1.5 h causes the conversion of
methanol with a ratio DMC/tin higher than 1.5.

Aknowledgements:
The authors wish to thank the University of Bari and the EU Project TOPCOMBI for financial
support.

References
1
  M. Aresta, A. Dibenedetto, F. Nocito, C. Pastore, Inorg. Chim. ACTA, accepted
2
  M. Aresta, A. Dibenedetto e C. Pastore, Inorg. Chem., 2003, 3256-3261
3
  M. Aresta, A. Dibenedetto, E. Fracchiolla, P. Giannoccaro, C. Pastore, I. Papai, G. Schubert, J. Org. Chem., 2006, 70,
6177-6186
4
  D. Ballivet-Tkatchenko, O. Douteau, S. Stutzmann, Organometallics, 2000, 19, 4563-4567
5
  D. Ballivet-Tkatchenko, R. Burgat, S. Chambrey, L. Plasseraud, J. of Organom. Chem, 2006, 691, 1498 - 1504

								
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