Synthetic and kinetic studies of the reaction of amino acid esters with tricarbonyl(dienyl)iron cations

Abstract

Amino acid esters add rapidly and reversibly to the dienyl iron cations [Fe(CO) 3(2-XC 6H 6)] + ( Ia, X  MeO; Ib, X  H) to give first the cation adducts II and then the neutral adducts III, as follows: [Fe(CO) 3(XC 6H 6)] + ( I) + H 2NCH(R)CO 2,Et K1 K −1[Fe(CO) 3XC 6H 6·NH 2CH( R)CO 2Et)] + ( II) K1 K− [Fe(CO) 3XC 6H 6· NHCH(R)(CO 2Et)] ( III). Results of 1H 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 esters, 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 >> k −1 (and k −2 negligible), the second-order rate constants k equate with k 1for 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.