A stereoselective reaction of Sn(NMe2)4 with the silyl-bridged bis(cyclopentadienyl) derivative 1 generates the novel spiro compound 2, in which two C2-symmetric six-membered rings of opposite configuration are joined at a tin center: One ring is R,R-, and the other S,S-configured. Subsequent reaction with two equivalents of ZrCl4 affords, by stereoselective Sn/Zr exchange, exclusively the C2-symmetric isomer of the ansa-zirconocene rac-3.
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Hüttenhofer et al. (1998) studied this question.