THE CATALYTIC DECARBOXYLATION OF CYANOACETIC ACID - ANIONIC TUNGSTEN CARBOXYLATES AS HOMOGENEOUS CATALYSTS

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-degrees-C, where k(f) = 6.11 x 10(-3) M-1 s-1 and k(r) = 1.48 x 10 2 M-1 s-1. Activation parameters for the decarboxylation process were determined and yielded DELTAH* = 21.0 +/- 0.7 kcal.mol-1 and DELTAS* = -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.