Chiral metallocenes: the synthesis and X-ray crystal structures of TiCl₂5:5-C₅Me₄SiMe₂C₅H₃R*) (R* = menthyl or neomenthyl) and related compounds

摘要

The syntheses of the chiral ansa-metallocene complexes TiCl2(eta5:eta5-C5Me4SiMe2C5H3R*) (R* = menthyl (4a) or neomenthyl (4b)) are reported; initially 4a was obtained as a 3:1 mixture of (R:S) diastereoisomers, which differ only in which face of the asymmetrically substituted cyclopentadienyl ring is bonded to the titanium (chiral descriptor shown). The major diastereomer 4aR was crystallized out optically pure from the initial reaction mixture, whereas the 4aS diastereoisomer was isolated after isomerizing a racemic mixture of 4a to a 1:3 mixture of (R:S) diastereoisomers using UV irradiation. The corresponding neomenthyl complex 4b was obtained as a 1.3:1 mixture of diastereoisomers that could not be separated. The optically pure 4aR was converted stereoselectively into the corresponding (R)-TiMe2(eta5:eta5-C5Me4SiMe2C5H3R*) (R* = menthyl). Syntheses of the related indenyl ligand system C5Me4SiMe2C9H7 (3) is reported but complexation to titanium proved to be problematic although ZrCl2(eta5:eta5-C5Me4SiMe2C9H6) (6) was isolated. The crystal structures of both 4aR and 4aS are reported and compared. Crystals of 4aR are orthorhombic, a = 27.857(11), b = 9.985(5), and c = 9.596(4) A, Z = 4, space group P212121 (D24, No. 19), and those of 4aS are monoclinic, a = 8.5810(10), b = 38.679(4), and c = 8.5842(10) A, beta = 113.001(2), Z = 4, space group P21 (C22, No. 4). The structures were solved by the Patterson method and 4aR was refined by blocked-cascade least-squares procedures to R = 0.0628 (Romega = 0.0503) for 902 reflections with |F|/sigma(|F|) 3.0, whereas 4aS was refined by full-matrix least-squares procedures to R = 0.0646 (wR2 = 0.1829) for 5734 reflections with |F|/sigma(|F|) 4.0. Both diastereomers of 4a catalyze hydrosilylation of ketones, but as expected from a comparison of the two crystal structures, the 4aR isomer is the more stereoselective catalyst, i.e., hydrosilylation of acetophenone followed by hydrolysis gives 82% enantiomeric excess (ee) of (S)-PhCH(Me)OH with 4aR whereas only 16% ee of (R)-PhCH(Me)OH with 4aS.Key words: titanium, metallocene, chiral, structure, catalyst.On a realise la synthese des complexes ansa-metallocenes chiraux TiCl2(eta5:eta5-C5Me4SiMe2C5H3R*) (R* = menthyle (4a); R* = neomenthyle (4b)). Originalement 4a a ete obtenu sous la forme d'un melange 3:1 des diastereomeres (R) et (S) qui ne different que par quelle face du noyau cyclopentadienyle substitue de facon asymetrique est liee au titane (on explicite le descripteur chiral). Le diastereomere principal, 4aR a ete retire optiquement pur par cristallisation du melange reactionnel initial; le diastereomere 4aS n'a ete isole qu'apres avoir isomerise un melange racemique de 4a en un melange 1:3 des diastereomeres (R) et (S) par irradiation UV. Le complexe neomenthylique correspondant 4b a ete obtenu sous la forme d'un melange 1:4:1 de diastereomeres qui n'a pas pu etre separe. Le produit 4aR a ete optiquement pur a ete transforme stereoselectivement en (R)-TiMe2(eta5:eta5-C5Me4SiMe2C5H3R*) (R* = menthyle) correspondant. On rapporte aussi la synthese du systeme de ligand indenyle apparente C5Me4SiMe2C9H7 (3), mais sa complexation avec le titane s'est averee etre problematique meme si on a pu isoler le ZrCl2(eta5:eta5-C5Me4SiMe2C9H6) (6). On a determine les structures cristallines des composes 4aR et 4aS et on en a fait une comparaison. Les cristaux de 4aR sont orthorhombiques, groupe d'espace P212121 (D24, No. 19), avec a = 27,857(11), b = 9,985(5) et c = 9,596(4) A et Z = 4 alors que ceux de 4aS sont monocliniques, groupe d'espace P21 (C22, No. 4), avec a = 8,5810(10), b = 38,679(4) et c = 8,5842(10) A, beta = 113,001(2) et Z = 4.