Yoder, Jeffrey C. and Day, Michael W. and Bercaw, John E. (1998) Racemic−Meso Interconversion for ansa-Scandocene and ansa-Yttrocene Derivatives. Molecular Structures of rac-{Me_2Si[η^5-C_5H_2-2,4-(CHMe_2)_2]_2}ScCl·LiCl(THF)_2, [meso-{Me_2Si[η^5-C_5H_2-2,4-(CHMe_2)_2]_2}Y(μ_2-Cl)]_2, and meso-{Me_2Si[η^5-C_5H_2-2,4-(CHMe_2)_2]_2}Zr(NMe_2)_2. Organometallics, 17 (23). pp. 4946-4958. ISSN 0276-7333. doi:10.1021/om980349j. https://resolver.caltech.edu/CaltechAUTHORS:20180518-154959101
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Abstract
The dilithium bis(cyclopentadienide) species Li_2{Me_2Si[C_5H_2-2,4-(CHMe_2)_2]_2} (Li_2Ip) reacts with ScCl_3(THF)_3 to afford rac-IpScCl·LiCl(THF)_2 (1) and [meso-IpSc(μ_2-Cl)]_2 (2) and with YCl_3(THF)_(3.5) to afford rac-IpYCl·LiCl(THF)_2 (3) and [meso-IpY(μ_2-Cl)]_2 (4). Metalation with both scandium and yttrium chlorides yields the metallocene chlorides in approximately 3:1 racemic:meso ratios. Reaction of IpH_2 with Zr(NMe_2)_4 yields exclusively meso-IpZr(NMe_2)_2 (9). Treatment of 1 or 2 with allylmagnesium bromide affords the allyl complexes rac-IpSc(η^3-C_3H_5) (5) and meso-IpSc(η^3-C_3H_5) (6) and with crotylmagnesium chloride affords rac-IpSc(η^3-C_3H_4Me) (7) and meso-IpSc(η^3-C_3H_4Me) (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_8 with reversion back to a 3:1 racemic:meso ratio. Isomerization of 5 and 6 is observed in THF-d_8, above 55 °C, affording an equilibrium ratio of ∼2:1 racemic:meso isomers. While spontaneous isomerization of 5, 6, 7, or 8 is very slow at room temperature, various salts and Grignard reagents promote isomerization even 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- bond, and recoordination on the opposite face, effecting net epimerization. X-ray diffraction studies have been performed on rac-IpScCl·LiCl(THF)_2 (1), [meso-IpY(μ_2-Cl)]_2 (4), and meso-IpZr(NMe_2)_2 (9).
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| Alternate Title: | Racemic−Meso Interconversion for ansa-Scandocene and ansa-Yttrocene Derivatives. Molecular Structures of rac-{Me2Si[η5-C5H2-2,4-(CHMe2)2]2}ScCl·LiCl(THF)2, [meso-{Me2Si[η5-C5H2-2,4-(CHMe2)2]2}Y(μ2-Cl)]2, and meso-{Me2Si[η5-C5H2-2,4-(CHMe2)2]2}Zr(NMe2)2 | |||||||||
| Additional Information: | © 1998 American Chemical Society. Received May 5, 1998. The work has been supported by the Department of Energy (Grant No. DE-FG03-88ER13431) and by Exxon Chemicals America. J.C.Y. wishes to thank the National Science Foundation for a Graduate Research Fellowship. | |||||||||
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| Issue or Number: | 23 | |||||||||
| DOI: | 10.1021/om980349j | |||||||||
| Record Number: | CaltechAUTHORS:20180518-154959101 | |||||||||
| Persistent URL: | https://resolver.caltech.edu/CaltechAUTHORS:20180518-154959101 | |||||||||
| Official Citation: | Racemic−Meso Interconversion for ansa-Scandocene and ansa-Yttrocene Derivatives. Molecular Structures of rac-{Me2Si[η5-C5H2-2,4-(CHMe2)2]2}ScCl·LiCl(THF)2, [meso-{Me2Si[η5-C5H2-2,4-(CHMe2)2]2}Y(μ2-Cl)]2, and meso-{Me2Si[η5-C5H2-2,4-(CHMe2)2]2}Zr(NMe2)2 Jeffrey C. Yoder, Michael W. Day, and John E. Bercaw Organometallics 1998 17 (23), 4946-4958 DOI: 10.1021/om980349j | |||||||||
| Usage Policy: | No commercial reproduction, distribution, display or performance rights in this work are provided. | |||||||||
| ID Code: | 86474 | |||||||||
| Collection: | CaltechAUTHORS | |||||||||
| Deposited By: | George Porter | |||||||||
| Deposited On: | 21 May 2018 15:23 | |||||||||
| Last Modified: | 15 Nov 2021 20:39 |
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