The catalytic decarboxylation of cyanoacetic acid: Anionic tungsten carboxylates as homogeneous catalysts

Article abstract of DOI:10.1021/ja00064a031

Cyanoacetic acid was observed to catalytically decompose to its component parts, CO₂ and CH₃CN, in the presence of soluble tungsten(0) carboxylates. In the case in which W(CO)₅O₂CCH₂CN^- was used as the catalyst, the reaction was inhibited by the substrate via an equilibrium process in which the free acid displaced the carboxylate ligand and bound to the metal through the nitrogen atom. The equilibrium constant for this process was measured and determined to be 0.413 at 50°C, where k_f = 6.11 × 10^-3 M^-1 s^-1 and k_r = 1.48 × 10^-2 M^-1 s^-1. Activation parameters for the decarboxylation process were determined and yielded ΔH* = 21.0 ± 0.7 kcal·mol^-1 and ΔS* = -3.4 ± 1.9 eu. The rate limiting step is proposed to be loss of cis CO from the metal with concomitant formation of cis-W(CO)₄(O₂CCH₂CN)(NCCH₂COOH) ^-, since the free energy of activation is quite similar to that for cis carbonyl loss, 23.0 ± 0.9 kcal·mol^-1. Subsequent proton transfer and CO₂ loss are fast relative to cis CO displacement. The carboxylate ligand acts as an intramolecular Lewis base, mediating the proton-transfer steps. This was demonstrated by the use of W(CO)PPh₂(CH₂)_nX^- (n = 1 or 2, X = base) as catalysts for the same decarboxylation. Detailed kinetic studies and a proposed mechanism are presented.