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

Several optically active (Cp R )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 PPh 3 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 (PPh 3 ) 3 CoCl 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 1 H, 13 C NMR, and MS spectra.