SYNTHESIS OF PURE RACEMIC ISOMERS OF ANSA-TITANOCENE DICHLORIDES - CONFORMATIONAL PREFERENCES ARE DETERMINED BY SUBSTITUTION PATTERNS ON THE CYCLOPENTADIENYL RINGS

A series of [1,2-ethylene-1,1'-bis(4-alkyl-2-methylcyclopentadienyl)]titanium dichlorides (3a-c) have been prepared by a new route wherein the presence of the 2-methyl substituent on the cyclopentadiene rings enables the stereoselective preparation of racemic isomers. Racemic ethylene-1,1'-bis(2-methyl-4-tert-butylcyclopentadienyl)titanium dichloride (3c) was characterized by an X-ray structural determination: triclinic, space group P1BAR, a = 9.910 (1) angstrom, b = 14.511 (1) angstrom, c = 15.141 (2) angstrom, alpha = 83.81 (1)-degrees, beta = 88.62 (1)-degrees, gamma = 81.27 (1)-degrees, V = 2139.5 (4) angstrom 3, Z = 4, final R = 0.0361, R(w) = 0.0573 based on 5944 reflections with F > 6.0-sigma-(F). Two independent molecules of compound 3c are present in the asymmetric unit; the conformations adopted by the ansa ligand in these two molecules are similar, however. The structural features that influence the solid-state conformations of this class of compounds are discussed. Variable-temperature NMR studies reveal that both compound 3c and its normethyl analogue 3c (R' = H) are fluxional at low temperature; the line-broadening behavior is different, however, for the two compounds. The possible origins for these differences are discussed.