Zircon, stereochemistry

Furthermore, if we consider the carbometalative ring expansion to produce the corresponding five-membered ring zirconacycle 86, the carbon-heteroatom bond of the sp3 metalated center Cx should isomerize to produce the most stable intermediate. Such isomerization could be due to an interaction between the heteroatom moiety XR and the zirconium atom [65], which would produce a weakness of the Cj-Zr bond and would facilitate the isomerization. Thus, whatever the stereochemistry of the starting material, a conformation is always possible in which Cj-SR is antiperiplanar to C2-C3 in 86 with a trans relationship between R and the ZrCp2 fragment. The elimination reaction, or decarbo-zirconation, occurs in a concerted way to give the E-vinyl zirconium 83. Unfortunately, neither the zirconacyclopentane nor the zirconacyclopropane have been trapped as intermediates. 

Zirconate, tetrakis(oxalato)-stereochemistry, 94 Zirconium gravimetry, 532... 

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