Mechanisms of Sn-to-Zr cyclopentadienyl transfer in the formation of Me2Si-bridged zirconocenes from sila-stanna-tetrahydro-s-indacenes

The stereochemistry of Sn-to-Zr transmetalation was studied by reacting ZrCl4 with that isomer of meso-BnMeSi(3-tBuC5H3)2SnMe2 which has the benzyl group in axial position. Exchange of SnMe2 against ZrCl2 generates both isomers of the CS-symmetric ansa-zirconocene meso-BnMeSi(3-tBuC5H3)2ZrCl2, but not the C1-symmetric, rac-like isomer. The major product is formed under inversion at both Sn-bound C atoms by consecutive ‘back-side’ attacks of the Zr electrophile, while the minor product appears to be formed, under retention at both Sn-bound C atoms, by a concerted ‘front-side’ attack of ZrCl4.