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

Article abstract of DOI:10.1016/0022-328X(94)87275-9

Amino acid esters add rapidly and reversibly to the dienyl iron cations ⁺ (Ia, X=MeO; Ib, X=H) to give first the cation adducts II and then the neutral adducts III, as follows: .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; this observation may be rationalized in terms of a mechanism involving the steady-state formation of the intermediate II.Since k₂ can be expected to be >>k_-1 (and k_-2 negligible), the second-order rate constants k equate with k₁ 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. Key words: Iron; Carbonyl; Amino acid; Kinetics