METAL-COMPLEXES OF BIOLOGICALLY IMPORTANT LIGANDS .88. ORGANOMETALLICHALF-SANDWICH COMPLEXES PROMOTE THE FORMATION OF LINEAR OLIGOPEPTIDESFROM AMINO-ACID ESTERS

Organometallic dipeptide ester complexes of the general formula [(L)M( Cl)(kappa(2)-NH(2)CH(2)CONCH(2)CO(2)R)] (1: L = Cp, M = Rh, 2: L = Cp , M = Ir, 3: L = eta(6)-C(6)Me(6), M = Ru) react smoothly with variou s alpha-L-amino acid esters in the presence of NEt(3) to yield the tri peptide ester complexes )M(Cl)(kappa(2)-NH(2)CHR'CONCH(2)CONHCH(2)CO(2 )R)] (5-7). In the same fashion chloro kappa(2)-tetrapeptide ester com plexes 10 and 11 are obtained either from tripeptide ester complexes o r by subsequent addition of two equivalents of amino acid ester to a d ipeptide ester complex. When the strong base NaOMe is used in the reac tion of the diglycine ester compounds with amino acid esters, kappa(3) -tripeptide ester complexes 12 and 13 are produced, in which one of th e two coordinated peptide nitrogen atoms is pyramidal. The hexamethylb enzene 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 isolatio n of the peptide esters proceeds without racemization.