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 cation s [Fe(CO)(3)(2-XC(6)H(6))](+) (Ia, X = MeO; m, X = H) to give first th e 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)(k 2) [Fe(CO)(3){XC(6)H(6).NHCH(R)(CO(2)Et)] (III). Results of H-1 NMR sp ectral studies confirm that the addition of the amino acid esters to t he dienyl rings of (I) is exo. With the chiral substrate Ia, moderate chiral discrimination occurs in the attack of (R)-amino acid eaters, p roviding a new route to optically active Ia. Kinetic studies of the re actions reveal the general rate law, Rate = k[Fe][RNH(2)]; this observ ation may be rationalized in terms of a mechanism involving the steady -state formation of the intermediate II. Since k(2)[RNH(2)] can be exp ected to be much greater than k(-1), (and k(-2) negligible), the secon d-order rate constants k equate with k(1) for the initial ring additio n step. The large negative entropies of activation support such a bimo lecular process. The data permit amino acid esters to be placed in a q uantitative order of nucleophility with some 40 other nucleophiles tow ards cation Ib.