Transition-metal-mediated activation of arylisocyanates in supercritical carbon dioxide

• Published in: Journal of organometallic chemistry 2001-04, Vol.626 (1), p.227-232
• Main Authors: Montilla, Francisco, Clara, Elisabete, Avilés, Teresa, Casimiro, Teresa, Aguiar Ricardo, Ana, Nunes da Ponte, Manuel
• Format: Article
• Language: English
• Subjects: Arylisocyanates; Co complexes; Cyclotrimerisation; Ni complexes; Supercritical CO 2
• ISSN: 0022-328X; 1872-8561
• DOI: 10.1016/S0022-328X(01)00698-2

The reactivity of arylisocyanates in supercritical carbon dioxide (scCO 2) was studied using the easily available complexes CpCo(CO) 2, CpCoPPh 3Me 2 and Ni(cod) 2 as catalysts. A study of the solubility of the catalysts in scCO 2 was undertaken in all cases. The complex CpCo(CO) 2 is very soluble, 1.7×10 −1 mol kg −1, while CpCoPPh 3Me 2 has a lower solubility, 7.2×10 −3 mol kg −1, and Ni(cod) 2 is insoluble in scCO 2. For comparison purposes, the reactions were performed in parallel in scCO 2, using toluene as a solvent and just with the neat liquid arylisocyanate. Reactions in scCO 2 either do not take place at all, when CpCo(CO) 2 is used as catalyst, or occur with low yields affording the trimer of the corresponding arylisocyanate when CpCoPPh 3Me 2 or Ni(cod) 2 act as catalysts. No incorporation of CO 2 into the organic substrate was observed. Better conversions to triarylisocyanate were obtained when the reactions were performed by direct mixture of the liquid arylisocyanate ArNCO (Ar=Ph, p-CH 3C 6H 4, p-CH 3OC 6H 4) and the catalyst. Using toluene as a solvent, the yields of the trimers were lower than those obtained in neat arylisocyanate, and in some cases they were not formed at all. For instance in the reaction of CpCo(CO) 2 and tolylisocyanate either under stoichiometric or catalytic conditions the trimer is not obtained, instead the compound H 2R 3N 3C 2O 2 (R=CH 3C 6H 4), was isolated in low yield. In the reaction of Ni(cod) 2/PPh 3 with phenylisocyanate, the trimer was formed but in low yield. The lower yields of the trimers observed when the reactions were performed in scCO 2 or in toluene, compared to that observed in neat arylisocyanates, indicates that the decrease in reactivity is due to a decrease in concentration.