Racemic-meso interconversion for ansa-scandocene and ansa-yttrocene derivatives. Molecular structures of rac-{l_brace}Me{sub 2}Si[{eta}{sup 5}-C{sub 5}H{sub 2}-2,4-(CHMe{sub 2}){sub 2}]{sub 2}{r_brace}ScCl {center_dot} LiCl(THF){sub 2}, [meso-{l_brace}Me{sub 2}Si[{eta}{sup 5}-C{sub 5}H{sub 2}-2,4-(CHMe{sub 2}){sub 2}]{sub 2}{r_brace} Y({mu}{sub 2}-Cl)]{sub 2}, and meso-{l_brace}Me{sub 2}Si[{eta}{sup 5}-C{sub 5}H{sub 2}-2,4-(CHMe{sub 2}){sub 2}]{sub 2}{r_brace}Zr(NMe{sub 2}){sub 2}
- California Inst. of Tech., Pasadena, CA (United States). Arnold and Mabel Beckman Labs. of Chemical Synthesis
The dilithium bis(cyclopentadienide) species Li{sub 2}{l_brace}Me{sub 2}Si[C{sub 5}H{sub 2}-2,4-(CHMe{sub 2}){sub 2}]{sub 2}{r_brace} (Li{sub 2}Ip) reacts with ScCl{sub 3}(THF){sub 3} to afford rac-IpScCl{center_dot}LiCl(THF){sub 2} (1) and [meso-IpSc({mu}{sub 2}-Cl)]{sub 2} (2) and with YCl{sub 3}(THF){sub 3.5} to afford rac-IpYCl{center_dot}LiCl(THF){sub 2} (3) and [meso-IpY({mu}{sub 2}-Cl)]{sub 2} (4). Metalation with both scandium and yttrium chlorides yields the metallocene chlorides in approximately 3:1 racemic:meso ratios. Reaction of IpH{sub 2} with Zr(NMe{sub 2}){sub 4} yields exclusively meso-IpZr(NMe{sub 2}){sub 2} (9). Treatment of 1 or 2 with allylmagnesium bromide affords the allyl complexes rac-IpSc-({eta}{sup 3}-c{sub 3}H{sub 5}) (5) and meso-IpSc({eta}{sup 3}-C{sub 3}H{sub 5}) (6) and with crotylmagnesium chloride affords rac-IPSc({eta}{sup 3}-C{sub 3}H{sub 4}Me) (7) and meso-IpSc({eta}{sup 3}-C{sub 3}H{sub 4}Me) (8). Diastereomerically pure rac dichlorometalate compounds (1 or 3) or pure meso chloro dimers (2 or 4) undergo spontaneous isomerization upon dissolution in THF-d{sub s}, above 55 C, affording an equilibrium ratio of {approximately}2:1 racemic:meso isomers. While spontaneous isomerization of 5, 6, 7, or 8 is very slow at room temperature; the isomerizations are not accelerated by light. The proposed mechanism for racemic-meso isomerization involves heterolytic dissociation of one cyclopentadienide ligand from the metal, rotation around that Si-Cp{sup {minus}} bond, and recoordination on the opposite face, effecting net epimerization. X-ray diffraction studies have been performed on rac-IpScCl{center_dot}LiCl(THF){sub 2} (1), [meso-IpY({mu}{sub 2}-Cl)]{sub 2} (4), and meso-IpZr(NMe{sub 2}){sub 2} (9).
- Sponsoring Organization:
- USDOE, Washington, DC (United States); National Science Foundation, Washington, DC (United States)
- OSTI ID:
- 315975
- Journal Information:
- Organometallics, Vol. 17, Issue 23; Other Information: PBD: 9 Nov 1998
- Country of Publication:
- United States
- Language:
- English
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