Synthesis and Coordination Chemistry of Pentadienyl Ligands Derived from (1<i>R</i>)‑(−)-Myrtenal

With the natural product (1<i>R</i>)-(−)-myrtenal as the starting material, a series of chiral pentadienes (Pdl*) such as dimethylnopadiene (<b>2a</b>), methylphenylnopadiene (<b>2b</b>), and methylnopadiene (<b>2c</b>) have been prepared by Wittig reactions. Deprotonation with the Schlosser base gives the corresponding potassium pentadienides <b>3a-K</b>–<b>3c-K</b>, 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 [(η⁷-C₇H₇)­Zr­(η⁵-Pdl*)] and their PMe₃ and <i>t</i>BuNC 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

Synthesis and Coordination Chemistry of Pentadienyl Ligands Derived from (1<i>R</i>)‑(−)-Myrtenal

With the natural product (1<i>R</i>)-(−)-myrtenal as the starting material, a series of chiral pentadienes (Pdl*) such as dimethylnopadiene (<b>2a</b>), methylphenylnopadiene (<b>2b</b>), and methylnopadiene (<b>2c</b>) have been prepared by Wittig reactions. Deprotonation with the Schlosser base gives the corresponding potassium pentadienides <b>3a-K</b>–<b>3c-K</b>, 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 [(η⁷-C₇H₇)­Zr­(η⁵-Pdl*)] and their PMe₃ and <i>t</i>BuNC 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

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

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

Synthesis and Coordination Chemistry of an Enantiomerically Pure Pentadienyl Ligand

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 (1<i>R</i>)-(−)-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