Synthesis and Coordination
Chemistry of an Enantiomerically Pure Pentadienyl Ligand
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Abstract
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(η<sup>5</sup>-Pdl*)<sub>2</sub>] (M = Ti, V, Cr, Fe) and mono(pentadienyl)
complexes [(η<sup>5</sup>-Cp′)Fe(η<sup>5</sup>-Pdl*)]
(Cp′ = 1,2,4-(Me<sub>3</sub>C)<sub>3</sub>C<sub>5</sub>H<sub>2</sub>), [(η<sup>7</sup>-C<sub>7</sub>H<sub>7</sub>)Zr(η<sup>5</sup>-Pdl*)], and [(η<sup>4</sup>-COD)Ir(η<sup>5</sup>-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(η<sup>5</sup>-Pdl*)<sub>2</sub>] (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 Fe<sup>2+</sup> center
