Conformational behavior of symmetrical and unsymmetrical mono(alkynylpeptide)-tungsten complexes

Abstract

A series of N-protected amino acid alkynylesters were prepared by reaction of the amino acid carboxylate group with either propargyl alcohol (to yield the asymmetric esters 2a–c) or with 1, 4-but-2-yne diol (to yield the symmetric esters 3a–d). The alkynylesters were reacted with W(CO)3(dmtc)2 to yield monoalkyne complexes having the general formula W(CO)(dmtc)2(alkynyl ester). The monoalkyne complexes 6a–f were unstable in the presence of oxygen and had to be kept under an inert atmosphere. Analysis of the NMR spectra of the monoalkyne complexes showed that two diastereomers were formed in the synthesis, and that there was rapid rotation of the alkyne about the tungsten center at 23 °C and above with both diastereomers. At lower temperatures alkyne rotation is significantly slowed. Symmetric alkynylesters of a dipeptide (4) and tripeptide (5) were also prepared and reacted with W(CO)3(dmtc)2 to yield monoalkyne complexes. The resulting complexes (6g and 6h) also formed two diastereomers and displayed rapid rotation of the alkyne about the tungsten center at 23 °C and above, and slow rotation at lower temperatures. The amide NH protons in 6g and 6h were probed by DMSO titration to see if they were involved in intramolecular hydrogen bonds; they were not, which indicates that the peptide portions of 6g and 6h adopt an extended conformation in solution.