With the natural product (1R)-(−)-myrtenal as the starting material, a series of chiral pentadienes (Pdl*) such as dimethylnopadiene (2a), methylphenylnopadiene (2b), and methylnopadiene (2c) have been prepared by Wittig reactions. Deprotonation with the Schlosser base gives the corresponding potassium pentadienides 3a-K−3c-K, whose structures were investigated by NMR spectroscopy and X-ray diffraction studies. In all cases a "U" conformation was observed. Furthermore, the coordination chemistry and electronic properties of these new pentadienyl systems were explored in several half-open trozircene complexes [(η 7 -C 7 H 7 )Zr(η 5 -Pdl*)] and their PMe 3 and tBuNC adducts. Density functional theory (DFT) computations are consistent with the experimentally observed face selectivity upon metal coordination: namely, that the metal coordinates exclusively from the sterically less encumbered side.

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.

Open-metallocenes of the heavy alkaline-earth metals [(η(5)-Pdl')2M(thf)n] (M = Ca (1), Sr (2), n = 1; M = Ba (3), n = 2; Pdl' = 2,4-tBu2C5H5) are readily prepared by salt-metathesis between MI2 and KPdl' and characterized by NMR spectroscopy and X-ray diffraction studies.

The coordination chemistry of the enantiomerically pure dimethylnopadienyl ligand (Pdl*) with early to late transition metals is presented. Dimethylnopadiene is prepared by a Wittig reaction from (1R)-(−)-myrtenal, which is readily available from the chiral pool. Deprotonation of dimethylnopadiene with a Schlosser base gives K­(Pdl*), which is a good starting material for the preparation of the early- to late-transition-metal open metallocenes [M­(η5-Pdl*)2] (M = Ti, V, Cr, Fe) and mono­(pentadienyl) complexes [(η5-Cp′)­Fe­(η5-Pdl*)] (Cp′ = 1,2,4-(Me3C)3C5H2), [(η7-C7H7)­Zr­(η5-Pdl*)], and [(η4-COD)­Ir­(η5-Pdl*)]. These complexes have been fully characterized by several spectroscopic techniques, elemental analysis, and X-ray crystallography. In all of these cases the Pdl* ligand exhibits excellent face selectivity upon metal coordination, because it coordinates exclusively from the sterically less hindered site of the bicyclic ligand framework. Within the series of open metallocenes [M­(η5-Pdl*)2] (M = Ti, V, Cr, Fe) the open ferrocene is the least thermally stable molecule and degrades to iron metal in solution. This instability is attributed to the severe steric demand of this ligand system in combination with the relatively small Fe2+ center.

The synthesis of C2 symmetric enantiomerically pure open Ca and Sr metallocenes, [(η(5)-pdl*)2Ca(thf)] (1) and [(η(5)-pdl*)2Sr(thf)2] (2) (pdl* = dimethylnopadienyl) is described and these complexes were fully characterized. The solid state structures confirm that the pdl* ligands coordinate exclusively with the less sterically demanding site to the Ca and Sr atoms. These complexes are active catalysts for the controlled ring opening polymerization (ROP) of rac-lactide to give heterotactically enriched polylactides (PL) with narrow polydispersities (PDI = 1.29-1.31) and without adding further activators.