Turn conformations in a metallacyclictripeptide and a metallacyclictetrapeptide induced by tungsten-alkyne coordination

Tungsten-alkyne coordination was employed for the preparation of a metallacyclictripeptide and a metallacyclictetrapeptide. First, the dialkynyltripeptide 7 and the dialkynyltetrapeptide 9 were prepared using solution phase peptide synthesis. In both 7 and 9 the two alkynes were attached to the peptide at the N- and C-termini. Reaction of 7 with W(CO)₃(dmtc)₂ yielded the metallacyclictripeptide 10, while reaction of 9 with W(CO)₃(dmtc)₂ yielded the metallacyclictetrapeptide 11. The identities of 10 and 11 were established using ¹H NMR spectroscopy and positive ion electrospray mass spectrometry. The ¹H NMR spectra of 10 and 11 show that these species exist as a complex mixture of isomers that differ in how the ligands are arranged around the tungsten center. Interconversion between these isomers is slow at 23 °C, but the rate of interconversion increases as the temperature is raised. With 10, all the alkyne hydrogen signals coalesce at 95 °C. However, with 11 the alkyne hydrogen signals do not fully coalesce, even at 105 °C. Related to this is the behavior of 10 and 11 during HPLC analysis; 10 elutes as a single peak, but 11 elutes as two overlapping peaks. The overlapping peaks and the high isomer interconversion temperature of 11 as compared to 10 are attributed to the presence of additional intramolecular hydrogen bonding interactions between the two ends of the peptide in 11.