Synthesis and Characterization of N<i>-</i>Donor-Functionalized Enantiomerically Pure Pentadienyl Ligands Derived from (1<i>R</i>)‑(−)-Myrtenal

Abstract

A series of enantiomerically pure −SiMe₂NR₂ (R = Me, Et) substituted pentadienyl ligands were prepared starting from the natural product (1<i>R</i>)-(−)-myrtenal. Deprotonation with a Schlosser superbase yields the corresponding potassium salts, which were characterized by various spectroscopic techniques. In solution these neutral N-donor-substituted pentadienyl systems predominantly adopt a <i>U</i> conformation, but in two cases the rare <i>S</i> conformation was also observed as a minor component in solution. Addition of 18-crown-6 allowed the molecular structures of two of these potassium pentadienyls to be determined by X-ray diffraction. Interestingly, η⁵ and κ<i>N</i> coordination of the pentadienyl system to the [K­(18-crown-6)]⁺ cation was observed. Furthermore, these ligand systems also coordinate to transition metals and form an open titanocene, open vanadocenes, open chromocenes, and half-open trozircenes with [TiCl₃(thf)₃], [VCl₃(thf)₃], CrCl₂, and [(η⁷-C₇H₇)­ZrCl­(tmeda)], respectively. These complexes were characterized by elemental analyses and various spectroscopic techniques. However, no coordination of the pendant −SiMe₂NR₂ group to the metal centers was observed. In addition, significant steric crowding in these open metallocenes prevents the formation of isolable CO or PMe₃ adducts. This was further corroborated by EPR studies on an open vandadocene, which showed that no adduct formation occurs at ambient temperature in solution, but a weak PMe₃ adduct was detected at 26 K