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 (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.