ROLE OF THE METAL CENTER IN THE HOMOGENEOUS CATALYTIC DECARBOXYLATIONOF SELECT CARBOXYLIC-ACIDS - COPPER(I) AND ZINC(II) DERIVATIVES

The mechanism by which copper(I) influences the decarboxylation of cya noacetic acid has been studied comprehensively by means of structural and kinetic investigations. The copper(I) complexes, [(R(3)P)(2)CuO2CC H2CN](1,2), have been synthesized from the reaction of copper(I) n-but yrate with 1 equiv of cyanoacetic acid and 2 equiv of phosphine. In th e 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 b y two cyanoacetate groups that are bound to copper through both the ca rboxylate functionality and the nitrogen. On the other hand, for the s terically encumbered phosphine (R = Cy), the complex (3) is found by X -ray crystallography to be monomeric and to contain a monodentate carb oxylate group. The monodentate nature of the cyanoacetate binding was demonstrated to be a function of the electron-withdrawing ability of t he cyanoacetate ligand as revealed by an examination of the solid-stat e structure of the (Cy(3)P)(2)Cu(butyrate) (4) analog, where the more basic butyrate ligand was shown to be bound in a bidentate manner. Bot h phosphine derivatives of copper(I) cyanoacetate were observed to rea dily undergo reversible decarboxylation/carboxylation processes as evi denced by their exchange reactions with (CO2)-C-13. A similar, much sl ower, exchange reaction with C-13-labeled CO2 was noted for the [PPN][ O2CCH2CN] and eta(3)-HB(3-PhPz)(3)Zn(O2CCH2CN) (5) salts. These (CO2)- C-13 exchange processes were found to be first-order in the respective substrate, with the Cy(3)P derivative undergoing more rapid exchange than the Ph(3)P complex. Furthermore, the phosphine derivatives of cop per(I) cyanoacetate were efficient catalysts for the decarboxylation o f cyanoacetic acid to afford CH3CN and CO2 at rates quite similar to t he CO2 exchange process. These reactions were first-order in copper(I) complexes and zero-order in cyanoacetic acid concentrations below 0.0 5 M. At higher acid concentrations the reaction was inhibited by cyano acetic acid due to its complexation with copper(I). Both eta(3)-HB(3-P hPz)(3)Zn(O2CCH2CN) and [([12]ane(3))Zn(O2CCH3)][Ph(4)B] are effective catalysts as well for the decarboxylation of cyanoacetic acid, with t he latter cationic derivative being more active. This difference in ca talytic behavior is attributed to the weaker Zn-O bond in the cationic derivative as determined by X-ray crystallography, 1.941 vs 1.912 Ang strom. A mechanism for decarboxylation is proposed which involves CO2 elimination from a cyanoacetic ligand that is nitrile bound to the met al center, i.e., electrophilic catalysis. Crystal data for 3: monoclin ic space group P2(1)/n, a = 10.619(2) Angstrom, b = 20.628(3) Angstrom , c = 18.146(3) Angstrom, beta = 93.89(1)degrees, Z = 2, R = 6.40%. Cr ystal data for 4: triclinic space group P1, a = 9.706(2) Angstrom, b = 10.442(2) Angstrom, c = 22.423(4) Angstrom, alpha = 97.51(2)degrees, beta 92.30(2)degrees, gamma = 116.22(1)degrees, Z = 2, R = 4.91%. Crys tal data for 5: triclinic space group P1, a = 13.197(2) Angstrom, b = 14.657(2) Angstrom, c = 16.049(3) Angstrom, alpha = 103.44(1)degrees, beta = 107.10(1)degrees, gamma = 92.19(1)degrees, Z = 2, R = 4.72%.