SYNTHETIC AND KINETIC-STUDIES OF THE REACTION OF AMINO-ACID ESTERS WITH TRICARBONYL(DIENYL)IRON CATIONS

Amino acid esters add rapidly and reversibly to the dienyl iron cations [Fe(CO)(3)(2-XC(6)H(6))](+) (Ia, X = MeO; m, X = H) to give first the cation adducts II and then the neutral adducts III, as follows: [Fe(CO)(3)(XC(6)H(6))](+) (I) + H2NCH(R)CO(2)Et reversible arrow(k-1)(k1) [Fe(CO)(3){XC(6)H(6).NH2CH(R)CO(2)Et)](+) (II) reversible arrow(k-2)(k2) [Fe(CO)(3){XC(6)H(6).NHCH(R)(CO(2)Et)] (III). Results of H-1 NMR spectral studies confirm that the addition of the amino acid esters to the dienyl rings of (I) is exo. With the chiral substrate Ia, moderate chiral discrimination occurs in the attack of (R)-amino acid eaters, providing a new route to optically active Ia. Kinetic studies of the reactions reveal the general rate law, Rate = k[Fe][RNH(2)]; this observation may be rationalized in terms of a mechanism involving the steady-state formation of the intermediate II. Since k(2)[RNH(2)] can be expected to be much greater than k(-1), (and k(-2) negligible), the second-order rate constants k equate with k(1) for the initial ring addition step. The large negative entropies of activation support such a bimolecular process. The data permit amino acid esters to be placed in a quantitative order of nucleophility with some 40 other nucleophiles towards cation Ib.