Syntheses of Chiral Nonracemic Half-Sandwich Cobalt Complexes with Menthyl-Derived Cyclopentadienyl, Indenyl, and Fluorenyl Ligands

Several optically active (CpR)Co(L) (L = 1,5-cyclooctadiene, norbornadiene) complexes were prepared. The complex (−)-(menthylCp)Co(COD) (1) was synthesized by the direct treatment of (−)-menthylcyclopentadiene with highly reactive “atomic” cobalt in the presence of COD. The other examples, (+)-pR-(1-neomenthylindenyl)Co(COD) (2) and (+)-pR-(1-neomenthylindenyl)Co(NBD) (3) were prepared by deprotonation of (−)-3-neomenthylindene with BuLi and the metathetic reaction of the corresponding lithium salt with tris(triphenylphosphine)cobalt(I) chloride, followed by replacement of PPh3 with COD or NBD, respectively. The diastereoselectivity of the complexation was as high as 74.5% de, and the major diastereomers were separated chromatographically. Optically pure complex (−)-pS-(1-neomenthylindenyl)Co(COD) (4) was prepared similarly, starting from (+)-3-neomenthylindene, and the diastereomeric excess of the major diastereomer was 74.4% de. The (+)-neomenthylfluorene 5 was prepared by the alkylation of fluorenyllit...