Organometallic Half‐Sandwich Complexes Promote the Formation of Linear Oligopeptides from Amino Acid Esters

Abstract

AbstractOrganometallic dipeptide ester complexes of the general formula [(L)M(Cl)(K2‐NH2CH2CONCH2CO2R)] (1: LCp*, MRh, 2: LCp*, MIr, 3: Lη6‐C6Me6, MRu) react smoothly with various α‐L‐amino acid esters in the presence of NEt3 to yield the tripeptide ester complexes [(L)M(Cl)(K2‐NH2CHR'CONCH2CONHCH2CO2R)] (5–7). In the same fashion chloro K2‐tetrapeptide ester complexes 10 and 11 are obtained either from tripeptide ester complexes or by subsequent addition of two equivalents of amino acid ester to a dipeptide ester complex. When the strong base NaOMe is used in the reaction of the diglycine ester compounds with amino acid esters. K3‐tripeptide ester complexes 12 and 13 are produced, in which one of the two coordinated peptide nitrogen atoms is pyramidal. The hexamethylbenzene ruthenium complexes 13 with tripeptide ligands are formed with very high diastereoselectivity. A plausible reaction mechanism for the metal‐promoted peptide synthesis is presented. Synthesis and isolation of the peptide esters proceeds without racemization.