Synthesis and reactivity of a cyclopentadienyl-indenyl ligand ring-coupled by a chiral bridge derived from ethyl (S)-(−) lactate

Abstract

In order to prepare a new unsymmetrical chiral ligand, C₉H₇CH(CH₃)CH₂C₅H₅, with an indenyl moiety connected to a cyclopentadienyl unit by a chiral ethylene bridge, we reacted the optically active tosylate 4, derived from ethyl (S)-(−) lactate, with LiCp. The only product resulting from this reaction was an optically active spirocyclopropane 6 obtained with a high diastereoselectivity (81.8% de). We also observed that, when the reduction of the ethyl indene lactate 2 was realized with an excess of LiAlH₄ in Et₂O under reflux, spirocyclopropane 6 was obtained in high yield with the same diastereoselectivity. When tosylate 4 was reacted with MgCp₂, in place of LiCp, ligand 5 was obtained in good yield as a mixture of two double-bond isomers of the Cp unit. Ligand 5 was fully structurally characterized after conversion into transition monometallic complexes such as C₉H₇CH(CH₃)CH₂η⁵-C₅H₄Mo(CO)₃Me 7 and C₉H₇CH(CH₃)CH₂η⁵-C₅H₄Rh(COD) 9, in which the indene moiety was kept intact due to the difference in reactivity of the indenyl moiety with respect to the cyclopentadienyl unit. An attempt to prepare a heterobimetallic complex from the sodium indenide salt of 7 and [RhCl(COD)]₂, afforded a rhodium cyclopentadienyl complex 9, resulting from a metal exchange reaction with loss of the molybdenum part initially coordinated to the Cp unit, and conservation of the optical purity. An homobimetallic rhodium complex 10 could be prepared by deprotonation of ligand 5 with TlOEt, followed by quenching with [RhCl(COD)]₂. The bis-rhodium complex 10 is obtained as a mixture of two diastereoisomers 10_aaa and 10_bb with respect to the planar chirality of the indenyl ring system, with a good diastereoisomeric excess de = 70%. The structures of both complexes 9 and major (pS)-diastereoisomer 10_aaa were determined by single crystal X-ray diffraction.