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

Abstract

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 fluorenyllithium with (−)-menthyltosylate, and the complex (+)-(pseudo-neomenthylfluorenyl)Co(COD) (6) was synthesized by the metathetic reaction of the corresponding lithium salt with (PPh3)3CoCl and COD, in which an inversion of the cyclohexane ring of the terpene moiety was observed upon complexation. The structures of the compounds 1, 2, 3, 4, 5, and 6 were determined by single-crystal X-ray diffractometry. Structural features of the complexes are also discussed on the basis of 1H, 13C NMR, and MS spectra.