Reactions of alpha-amino acid-N-carboxyanhydrides (NCAs) with organometallic palladium(0) and platinum(0) compounds: structure of a metallated NCA product and its role in polypeptide synthesis

Pt(COD)(2) and two equivalents of PEt3 were reacted with L-tert-leucine N-c arboxyanhydride to yield the N-H oxidative addition product, H(PEt2)(2)PtNC (C(CH3)(3))HC(O)OC(O) (1). The structure and identity of 1 were determined by X-ray crystallography. Pt(COD)(2) and two equivalents of PEt3 were also reacted with N-alpha-carbobenzyloxy-L-phenylalanine NCA to yield the O-C-5 oxidative addition product, (PEt3)(2)PtC(O)C(CH2C6H5)HN(C(O)OBn)C(O)O (2). When 1 was reacted with excess gamma-benzyl-L-glutamate-N-carboxyanhydride (Glu NCA) in THF at room temperature, poly(gamma-benzyl-L-glutamate) was fo rmed in good yield, but with no control over polymer molecular weight. NMR studies showed that the N-carboxyanhydride ligand of 1 was responsible for polymerization initiation, which confirmed that metallated N-carboxyanhydri des, or :activated monomers' are active, albeit inefficient, initiating spe cies in NCA polymerizations. (C) 1999 Elsevier Science S.A. All rights rese rved.