Chiral metallocenes: the synthesis and X-ray crystal structures of TiCl[sub 2] η[sup 5] :η[sup 5] -C[sub 5] Me[sub 4] SiMe[sub 2] C[sub 5] H[sub 3] R[sup *] ) (R[sup * ] = menthyl or neomenthyl) and related compounds

Beagley, Paul; Davies, Philip; Adams, Harry; White, Colin
May 2001
Canadian Journal of Chemistry;May2001, Vol. 79 Issue 5/6, p731
Academic Journal
The syntheses of the chiral ansa-metallocene complexes TiCl[sub 2] (η[sup 5] :η[sup 5] -C[sub 5] Me[sub 4] SiMe[sub 2] C[sub 5] H[sub 3] R[sup *] ) (R[sup *] = 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 4a[sub R] was crystallized out optically pure from the initial reaction mixture, whereas the 4a[sub S] 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 4a[sub R] was converted stereoselectively into the corresponding (R)-TiMe[sub 2] (η[sup 5] :η[sup 5] -C[sub 5] Me[sub 4] SiMe[sub 2] C[sub 5] H[sub 3] R[sup *] ) (R[sup *] = menthyl). Syntheses of the related indenyl ligand system C[sub 5] Me[sub 4] SiMe[sub 2] C[sub 9] H[sub 7] (3) is reported but complexation to titanium proved to be problematic although ZrCl[sub 2] (η[sup 5] :η[sup 5] -C[sub 5] Me[sub 4] SiMe[sub 2] C[sub 9] H[sub 6] ) (6) was isolated. The crystal structures of both 4a[sub R] and 4a[sub S] are reported and compared. Crystals of 4a[sub R] are orthorhombic, a = 27.857(11), b = 9.985(5), and c = 9.596(4) Å, Z = 4, space group P2[sub 1] 2[sub 1] 21 (D[sub 2] [sup 4] , No. 19), and those of 4a[sub S] are monoclinic, a = 8.5810(10), b = 38.679(4), and c = 8.5842(10) Å, β = 113.001(2)°, Z = 4, space group P2[sub 1 ] (C[sub 2] [sup 2] , No. 4). The structures were solved by the Patterson method and 4a[sub R] was refined by blocked-cascade least-squares procedures to R = 0.0628 (R[sub ω] = 0.0503) for 902 reflections with |F|/σ(|F|) 3.0, whereas 4a[sub S] was refined by full-matrix least-squares procedures to R = 0.0646 (wR[sub 2] = 0.1829) for 5734 reflections with |F|/σ(|F|) 4.0. Both diastereomers of 4a catalyze hydrosilylation of ketones, but as expected from a comparison of the two crystal structures, the 4a[sub R] 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 4a[sub R] whereas only 16% ee of (R)-PhCH(Me)OH with 4a[sub S] .Key words: titanium, metallocene, chiral, structure, catalyst.On a réalisé la synthèse des complexes ansa-métallocènes chiraux TiCl[sub 2] (η[sup 5] :η[sup 5] -C[sub 5] Me[sub 4] SiMe[sub 2] C[sub 5] H[sub 3] R[sup *] ) (R[sup *] = menthyle (4a); R[sup *] = néomenthyle (4b)). Originalement 4a a été obtenu sous la forme d'un mélange 3:1 des diastéréomères (R) et (S) qui ne diffèrent que par quelle face du noyau cyclopentadiényle substitué de façon asymétrique est liée au titane (on explicite le descripteur chiral). Le diastéréomère principal, 4a[sub R] a été retiré optiquement pur par cristallisation du mélange réactionnel initial; le diastéréomère 4a[sub S] n'a été isolé qu'après avoir isomérisé un mélange racémique de 4a en un mélange 1:3 des diastéréomères (R) et (S) par irradiation UV. Le complexe néomenthylique correspondant 4b a été obtenu sous la forme d'un mélange 1:4:1 de diastéréomères qui n'a pas pu être séparé. Le produit 4a[sub R] a été optiquement pur a été transformé stéréosélectivement en (R)-TiMe[sub 2] (η[sup 5] :η[sup 5] -C[sub 5] Me[sub 4] SiMe[sub 2] C[sub 5] H[sub 3] R[sup *] ) (R[sup *] = menthyle) correspondant. On rapporte aussi la synthèse du système de ligand indényle apparenté C[sub 5] Me[sub 4] SiMe[sub 2] C[sub 9] H[sub 7] (3), mais sa complexation avec le titane s'est avérée être problématique même si on a pu isoler le ZrCl[sub 2] (η[sup 5] :η[sup 5] -C[sub 5] Me[sub 4] SiMe[sub 2] C[sub 9] H[sub 6] ) (6). On a déterminé les structures cristallines des composés 4a[sub R] et 4a[sub S] et on en a fait une comparaison. Les cristaux de 4a[sub R] sont orthorhombiques, groupe d'espace P2[sub 1] 2[sub 1] 2[sub 1] (D[sub 2] [sup 4] , No. 19), avec a = 27,857(11), b = 9,985(5) et c = 9,596(4) Å et Z = 4 alors que ceux de 4a[sub S] sont monocliniques, groupe d'espace P2[sub 1] (C[sub 2] [sup 2] , No. 4), avec a = 8,5810(10), b = 38,679(4) et c = 8,5842(10) Å, β = 113,001(2)° et Z = 4. Les structures ont été résolues par la méthode de Patterson et celle de 4a[sub R] a été affinée par des méthodes de moindres carrés à cascade bloquée jusqu'à des valeurs de R = 0,0628 (R[sub ω] = 0,0503) pour 902 réflexions avec |F|/σ|F| 3,0 alors que celle de 4a[sub S] a été affinée par la méthode des moindres carrés (matrice complète) jusqu'à des valeurs de R = 0,0646 (wR[sub 2] = 0,1829) pour 5734 réflexions avec |F|/σ|F| 4,0. Les deux diastéréomères de 4a catalyzent l'hydrosilylation des cétones mais, comme on peut s'y attendre sur la base d'une comparaison des deux structures cristallines, l'isomères 4a[sub R] est plus stéréosélectif; par exemple l'hydrosilylation de l'acétophénone, suivie d'une hydrolyse, conduit à un mélange avec 82% d'ee du (S)-PhCH(Me)OH avec 4a[sub R] alors que l'on n'obtient que 16% d'ee de (R)-PhCH(Me)OH avec 4a[sub S] .Mots clés : titane, métallocène, chiral, structure, catalyseur.[Traduit par la Rédaction]


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