1,n′-Disubstituted ferrocenoyl amino acids and dipeptides: Conformational analysis by CD spectroscopy, X-ray crystallography, and DFT calculations

An experimental and computational study on the conformational preference of 1,n′-disubstituted ferrocenoyl amino acids and dipeptides is presented. Only l-amino acids were used for the synthesis of Fe[C5H4-CO-Met-Met-OMe]2 (4), but according to the X-ray structure a 4:1 mixture of l,d,M,d,l and l,d,M,l,l isomers is obtained (l describes amino acid chirality and M the helical chirality of the ferrocene core). This result is in agreement with IR and CD solution phase data and can be explained with a racemization by 1 M NaOH during the synthesis. In order to determine the relative stabilities of the different conformations, DFT calculations on model compounds Fe[C5H4-CO-Gly-NH2]2 (5) and Fe[C5H4-CO-Ala-OMe]2 (6) were performed using the B3LYP/LanL2DZ method with ECPs on the heavy atoms. Conformers 5A–5C with different hydrogen bond patterns have significantly different stabilities with a stabilization by about 30 kJ mol−1 per hydrogen bond. The “Herrick conformation” 5A with two hydrogen bonds is the most stable in the gas phase, in accordance with the solution and solid phase data. In contrast, only small energetic differences (less than 10 kJ mol−1) were calculated for conformers l,P,l-6A, l,P,d-6A and d,P,d-6A, which differ only in amino acid chirality.