RACEMIC-MESO INTERCONVERSION FOR ANSA-SCANDOCENE AND ANSA-YTTROCENE DERIVATIVES - MOLECULAR-STRUCTURES OF 2-2,4-(CHME2)(2)](2))SCCL-CENTER-DOT-LICL(THF)(2), 2SI[ETA(5)-C5H2-2,4-(CHME2)(2)](2))Y(MU-CL-2)](2), AND -(ME2SI[ETA(5)-C5H2-2,4-(CHME2)(2)](2))ZR(NME2)(2)

The dilithium bis(cyclopentadienide) species Li-2{Me2Si[C5H2-2,4-(CHMe 2)(2)](2)} (Li(2)Ip) reacts with ScCl3(THF)(3) to afford rac-IpYCl . L iCl(THF)(2) (I) and [meso-IpSc(mu(2)-Cl)](2) (2) and with YCl3(THF)(3. 5) to afford rac-IpYCl . LiCl(THF)(2) (3) and [meso-IpY(mu(2)-Cl)](2) (4). Met;alation with both scandium and yttrium chlorides yields the m etallocene chlorides in approximately 3:1 racemic:meso ratios. Reactio n of IpH(2) with Zr(NMe2)(4) yields exclusively meso-IpZr(NMe2)(2) (9) . Treatment of 1 or 2 with allylmagnesium bromide affords the allyl co mplexes rac-IpSc(eta(3)-C3H5) (5) and meso-IpSc(eta(3)-C3H5) (6) and w ith crotylmagnesium chloride affords rac-IpSc(eta(3)-C3H4Me) (7) and m eso-IpSc(eta(3)-C3H4Me) (8). Diastereomerically pure rac dichlorometal ate compounds (1 or 3) or pure meso chloro dimers (2 or 4) undergo spo ntaneous isomerization upon dissolution in THF-d(8) with reversion bac k to a 3:1 racemic:meso ratio. Isomerization of 5 and 6 is observed in THF-d(8), above 55 degrees C, affording an equilibrium ratio of simil ar to 2:1 racemic:meso isomers. While spontaneous isomerization of 5, 6, 7, or 8 is very slow at room temperature, various salts and Grignar d reagents promote isomerization even at; room temperature; the isomer izations are not accelerated by light. The proposed mechanism far race mic-meso isomerization involves heterolytic dissociation of one cyclop entadienide ligand from the metal, rotation around that Si-Cp- band, a nd recoordination on tl le opposite face, effecting net epimerization. X-ray diffraction studies have been performed on rac-IpScCl . LiCl(TH F)(2) (1), [meso-IpY(mu(2)-Cl)](2) (4), and meso-IpZr(NMe2)(2) (9).