GENERAL SYNTHETIC ROUTES TO CHIRAL, ETHYLENE-BRIDGED ANSA-TITANOCENE DICHLORIDES

Two synthetic routes for the preparation of chiral (l,2-ethylenebis (η5-3-alkylcyclopentedienyl))titanium dichlorides (3) are described wherein the substituent, R, can be a primary, secondary, or tertiary alkyl group. Compounds 3a-d and 4a-d are prepared from TiCl3-3THF and the corresponding l,2-ethylenebis(3-alkyl-1,3-cyclopentadienes) (8a-d; R = Me, Et, iPr, tBu) in 80-85% yieldS. C.ompounds 8a and 8b (R = Me, Et) can be prepared in five steps and in 44 and 24% overall yield, respectively, from 1,2-ethylene-bis(l,3-cyclopentadiene) (5). Compounds 8c and 8d (R = iPr, tBu) can be prepared in two steps and in >90% overall yield from compound 5. A mixture of rac- and meso-titanocene dichlorides, 3 and 4, respectively, is produced on reaction of the dilithium salts of compounds 8 with TiCl3 (the ratio of rac:meso varies between 1:1.3 and 1:2.0). Isomers 3d and 4d may be separated by HPLC or, more conveniently, by flash chromatogaphy on silica gel. Compound 3d crystallizes in two distinct modifications, both of which were characterized by X-ray crystallography: modification 1, space group P21/c, Z = 4, a = 11.537 (4) Å, b = 13.436 (6) Å, c = 13.007 (4) Å, β = 98.21 (3)°, V = 1995 (1) Å3, R = 0.085 and Rw = 0.096 on the basis of 2270 reflections with I> 3σ(I); modification 2, space group P421c, Z = 8, a = 14.443 (1) Å, c = 19.249 (3) Å, V = 4010 (1) Å3, R = 0.0346 and Rw = 0.0355 on the basis of 1976 reflections with I > 3σ(I). The conformations adopted by the ansa ligand in the two modifications are markedly different; it appears that the energy difference between the two conformers is probably low and might be influenced by crystal-packing effects. © 1990, American Chemical Society. All rights reserved.