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

The reactivity of arylisocyanates in supercritical carbon dioxide (scCO2) was studied using the easily available complexes CpCo(CO)2, CpCoPPh3Me2 and Ni(cod)2 as catalysts. A study of the solubility of the catalysts in scCO2 was undertaken in all cases. The complex CpCo(CO)2 is very soluble, 1.7×10−1 mol kg−1, while CpCoPPh3Me2 has a lower solubility, 7.2×10−3 mol kg−1, and Ni(cod)2 is insoluble in scCO2. For comparison purposes, the reactions were performed in parallel in scCO2, using toluene as a solvent and just with the neat liquid arylisocyanate. Reactions in scCO2 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 CpCoPPh3Me2 or Ni(cod)2 act as catalysts. No incorporation of CO2 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-CH3C6H4, p-CH3OC6H4) 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 H2R3N3C2O2 (R=CH3C6H4), was isolated in low yield. In the reaction of Ni(cod)2/PPh3 with phenylisocyanate, the trimer was formed but in low yield. The lower yields of the trimers observed when the reactions were performed in scCO2 or in toluene, compared to that observed in neat arylisocyanates, indicates that the decrease in reactivity is due to a decrease in concentration.