Role of the metal center in the homogeneous catalytic decarboxylation of select carboxylic acids. Copper(I) and zinc(II) derivatives of cyanoacetate

Article abstract of DOI:10.1021/ja00106a034

The mechanism by which copper(I) influences the decarboxylation of cyanoacetic acid has been studied comprehensively by means of structural and kinetic investigations. The copper(I) complexes, [(R₃P)₂CuO₂CCH₂-CN]_1,2, have been synthesized from the reaction of copper(I) n-butyrate with 1 equiv of cyanoacetic acid and 2 equiv of phosphine. In the case of R = Ph, the complex is shown to be a dimer, both in solution and in the solid state, consisting of two copper(I) centers bridged by two cyanoacetate groups that are bound to copper through both the carboxylate functionality and the nitrogen. On the other hand, for the sterically encumbered phosphine (R = Cy), the complex (3) is found by X-ray crystallography to be monomeric and to contain a monodentate carboxylate group. The monodentate nature of the cyanoacetate binding was demonstrated to be a function of the electronwithdrawing ability of the cyanoacetate ligand as revealed by an examination of the solid-state structure of the (Cy₃P)₂Cu(butyrate) (4) analog, where the more basic butyrate ligand was shown to be bound in a bidentate manner. Both phosphine derivatives of copper(I) cyanoacetate were observed to readily undergo reversible decarboxylation/ carboxylation processes as evidenced by their exchange reactions with ¹³CO₂. A similar, much slower, exchange reaction with ¹³C-labeled CO₂ was noted for the [PPN][O₂CCH₂CN] and η³-HB(3-PhPz)₃Zn(O₂CCH₂CN) (5) salts. These ¹³CO₂ exchange processes were found to be first-order in the respective substrate, with the Cy₃P derivative undergoing more rapid exchange than the Ph₃P complex. Furthermore, the phosphine derivatives of copper(I) cyanoacetate were efficient catalysts for the decarboxylation of cyanoacetic acid to afford CH₃CN and CO₂ at rates quite similar to the CO₂ exchange process. These reactions were first-order in copper(I) complexes and zero-order in cyanoacetic acid concentrations below 0.05 M. At higher acid concentrations the reaction was inhibited by cyanoacetic acid due to its complexation with copper(I). Both η³-HB(3-PhPz)₃Zn(O₂CCH₂CN) and [([12]ane₃)Zn(O₂-CCH₃)][Ph₄B] are effective catalysts as well for the decarboxylation of cyanoacetic acid, with the latter cationic derivative being more active. This difference in catalytic behavior is attributed to the weaker Zn-O bond in the cationic derivative as determined by X-ray crystallography, 1.941 vs 1.912 A?. A mechanism for decarboxylation is proposed which involves CO₂ elimination from a cyanoacetic ligand that is nitrile bound to the metal center, i.e., electrophilic catalysis. Crystal data for 3: monoclinic space group P2₁/n, a = 10.619(2) A?, b = 20.628(3) A?, c = 18.146(3) A?, β = 93.89(1)°, Z = 2, R = 6.40%. Crystal data for 4: triclinic space group P1, a = 9.706(2) A?, b = 10.442(2) A?, c = 22.423(4) A?, α = 97.51(2)°, β = 92.30(2)°, γ = 116.22(1)°, Z = 2, R = 4.91%. Crystal data for 5: triclinic space group P1, a = 13.197(2) A?, α = 14.657(2) A?, c = 16.049(3) A?, α = 103.44(1)°, β = 107.10(1)°, γ = 92.19(1)°, Z = 2, R = 4.72%.