A MORE INTIMATE EXAMINATION OF THE ROLE OF COPPER(I) IN THE DECARBOXYLATION OF DERIVATIVES OF MALONIC-ACID - COMPARISONS WITH ZINC(II) ANALOGS

The mechanism by which Cu(I) and Zn(II) influence the decarboxylation of malonic acid derivatives has been investigated comprehensively by m eans of structural and kinetic studies. The air-sensitive bis(phosphin e)copper(I) complexes of phenylmalonic acid and its benzyl hemiester h ave been synthesized from 1 equiv of the acid and cuprous butyrate in the presence of phosphine ligands. As revealed by structural studies t he copper(I) center is three-coordinate containing two phosphine ligan ds and a monodentately bound carboxylate group. On the other hand, sol uble zinc(II) phenylmalonic acid and ester derivatives were prepared f rom (HB(3-Phpz)(3))ZnO2CCH3, HB(3-Phpz)(3) = tris(3-phenylpyrazolyl)hy droborate, and the corresponding malonic acid or hemiester. The Zn(II) complexes were all shown by X-ray crystallography to be four-coordina te with monodentately bound carboxylates. Kinetic evidence is presente d herein which demonstrates that decarboxylation of malonic acid or he miester derivatives in the presence of bis(phosphine)copper(I) or eta( 3)-HB(3-Phpz)(3)Zn-II carboxylate salts occurs via a predissociation s tep involving metal-carboxylate bond rupture. Consistent with this mec hanistic proposal, the rates of decarboxylation are greatly enhanced u pon sequestering the metal cations with chelating nitrogen bases or up on replacing the Cu(I) or Zn(II) cations with a noninteracting counter ion. Furthermore, because of the stronger Zn-O carboxylate bonds as co mpared with their Cu(I)-O analogs, the zinc carboxylates are more stab le toward decarboxylation or (CO2)-C-13 exchange processes. Indeed, th e rates of these processes parallel the Zn-O bond lengths with the pyr azolylhydroborato complex, which has a shorter Zn-O bond distance than its triazacyclododecane analog, undergoing decarboxylation at a much slower rate. Crystal data for (Ph(3)P)(3)CuO2CC(C2H5)(C6H5)CO2H (1): t riclinic space group <P(1)over bar>, a = 11.631(5) Angstrom, b = 12.20 6(4) Angstrom, c = 20.000(6) Angstrom, alpha = 85.91(3)degrees, beta = 89.61(3)degrees, gamma = 71.24(3)degrees, Z = 2, R = 4.20%. Crystal d ata for eta(3)-HB(3-Phpz)(3)ZnO2CCH2COOH (2): triclinic space group <P (1)over bar>, a = 11.935(3) Angstrom, b = 12.227(4) Angstrom, c = 12.6 43(5) Angstrom, alpha = 77.56(3)degrees, beta = 72.18(3)degrees, gamma = 73.21(2)degrees, Z = 2, R = 7.00%. Crystal data for eta(3)-HB(3-Php z)(3)ZnO(2)CCH(2)Ph: triclinic space group <P(1)over bar>, a = 9.884(( 2) Angstrom, b = 12.189(3) Angstrom, c = 15.482(2) Angstrom, alpha = 1 05.80(1)degrees, beta = 92.46(1)degrees, gamma = 90.94(2)degrees, Z = 2, R = 8.21%.