The role of copper(1) in homogeneously catalyzed decarboxylation reactions of carboxylic acids

Abstract

Heterobimetallics of the form (L2)CuCo(CO)4 were found to be catalyst precursors for the decarboxylation of NCCH2CO2H. The rate of decarboxylation is affected by the donating ability of the L2 ligand, i.e., more basic ligands enhance the rate. The concentration of the catalytically active species is increased by addition of [PPN][O2CCH2CN], and thus the rate of decarboxylation is dependent upon the concentration of added carboxylate. The X-ray crystallographic characterization of (mebpy)CuCo(CO)4 revealed the presence of a linear semi-bridging carbonyl between the two metal centers, similar to that observed in the tmed and bpy derivatives. Complex 1, [Cu(phen)2]2[Cu(O2CCH3)2]*(O2CCH3)2H was prepared and characterized by X-ray diffraction. Complex 1 homogeneously catalyzes the decarboxylation of cyanoacetic and 9-fluorenecarboxylic acid to produce CO2 and the corresponding hydrocarbons, CH3CN and fluorene. The rate of decarboxylation for this system was observed to be approximately five times faster than the rates observed in the (L2)CuCo(CO)4 systems. The activation parameters calculated for the decarboxylation of NCCH2CO2H in the presence of 1 over the temperature range 25° - 40° C are ΔH = 25.05 [plus or minus] 1.47 kcal and ΔS = -0.28 [plus or minus] 4.85 e.u. An in situ prepared catalyst from Cu(I) acetate or Cu(I) cyanoacetate and a suitable L2 ligand is equally effective for the decarboxylation process. Several phosphine derivatives of Cu(I) carboxylates were prepared and characterized by X-ray diffraction. [(Ph3P)2CuO2CCH2CN]2 was crystallographically characterized and catalyzes the decarboxylation of NCCH2CO2H. The rate of decarboxylation was determined to be independent of the acid concentration and first-order dependent on the catalyst concentration. The first-order rate constant for the decarboxylation at 40°C was 5.41 x 10^-2 min^-1. The reversible decarboxylation of the complex was established by 13C-NMR and infrared spectroscopy where the incorporation of 13CO2 was observed. The rate of CO2 exchange at 40°C (4.12 x 10^-2 min^-1) was found to be similar to the rate of decarboxylation.