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

A series of enantiomerically pure −SiMe2NR2 (R = Me, Et) substituted pentadienyl ligands were prepared starting from the natural product (1R)-(−)-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 U conformation, but in two cases the rare S 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 κN 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.