Oxidative carbonylation of phenols catalyzed by homogeneous and heterogeneous Pd precursors

Homogeneous and heterogeneous Pd-based catalysts (in the presence of Co(acac)3/ligand/BQ/TBAB and molecular sieve 4A), have been compared in the oxidative carbonylation of phenols. The best yields have been observed in the presence of bulky bidentate N-N ligands such as dmphen and triind. The comparison between homogeneous and heterogeneous systems shows similar behavior in terms of yield and selectivity in DPC, as in 4 h of reaction only small differences of activity have been observed. On the contrary, in 20 h of reaction only the systems Pd(OAc)2/Co(acac)3/dmphen/BQ/TBAB and Pd(OAc)2/Co(acac)3/triind/BQ/TBAB double the yield in DPC, while with the other systems the conversion is blocked. The beneficial effect of the ligands and of TBAB might be due to their capacity of inhibiting the growing of Pd nanoparticles that are likely to form, thus easing their reoxidation to Pd(II) species. The initial rate of phenol or methyl substituted phenols in CH2Cl2 as a solvent are only slightly influenced, except in the case of bulkier 2,6-dimethylphenol. The conversion vs. time profile evidences that fast catalyst deactivation occurs in any case.