Stereoselective propene insertion reactions of rac-(EBI)Zr(eta(2)-pyridyl)(+) complexes

Stereoselective propene insertion reactions of chiral zirconocene species of the type rac-(EBI)Zr(eta(2)-pyridyl)(+) (EBI = ethylenebis(indenyl)) containing substituted eta(2)-pyridyl ligands are described. The reaction of rac-(EBI)ZrMe2 with B(C6F5)(3) followed by addition of 1 equiv of the appropriate pyridine yields [rac-(EBI)Zr(eta(2)-3-R-3-5-R-5-6-R-6-pyrid-2-yl][MeB(C6F5)(3)] pyridyl complexes (5a-f; a, R-3 = R-5 = H, R-6 = Me; b, R-3 = R-5 = H, R-6 = Ph; c, R-3 = R-5 = R-6 = H; d, R-3 = Me, R-5 = H, R-6 = Me; e, R-3 = H, R-5 = R-6 = Me; f, R-3 = R-5 = Me, R-6 = H). At 23 degrees C, 5a-f react with propene to yield the 1,2-insertion products [rac-(EBI)Zr{eta(2)(C,N)-CH2CHMe(pyrid-2-yl)}][MeB(C6F5)(3)], for which two diastereomers (6/6') that differ in the configuration of the metallacycle beta-carbon are possible. Propene insertion of 5a-f is under kinetic control at 23 degrees C and is highly stereoselective when the pyridyl ring contains a 6-substituent (R-6 not equal H; 6a, de = 80%; 6b, de > 96%; 6d, de > 96%; 6e, de = 80%). For these cases, the major diastereomer is that in which the metallacycle P-Me group points toward the EBI C-6 ring (configuration S,S,S (R,R,R), where the entries denote the configurations of the EBI bridgehead carbons and the metallacycle beta-carbon, respectively). The diastereoselectivity is low in cases where R-6 = H (6c', de = 20%; 6f, de = 10%). Propene insertion of 5a-f is reversible at 80 degrees C, and the kinetic 6/6' mixtures evolve to the thermodynamic mixtures at this temperature. Ed isomerizes to a mixture of 6d, 6d', and the 2,1-insertion product [rac-(EBI)Zr{eta(2)(C,N)-CHMeCH2(pyrid-2-yl)}][MeB(C6F5)(3)] (6 '') at 80 degrees C. Molecular modeling calculations for model rac-(EBI)Zr(eta(2)-pyridyl)(propene)(+) species suggest that steric interactions between the pyridyl R-6 substituent and the EBI C-6 ring cause a tipping of the pyridyl ligand, which influences the pyridyl/propene steric contacts and the facial selectivity of propene binding. The modeling calculations also suggest that the more stable propene adduct diastereomer leads to the kinetic insertion product.