DYNAMIC BINUCLEAR ENEDIOLATO-ZIRCONOCENE AND CATECHOLATO-ZIRCONOCENE COMPLEXES RELEVANT TO THE STRUCTURAL PROBLEM ASSOCIATED WITH SHARPLESS OXIDATION CATALYSTS

Dimethylzirconocene complexes [ZrMe2(C5H4R)2](R = H or CH3) were treated with catechol to give the (catecholato)zirconocene complexes. Low-temperature NMR spectra indicated dimeric metallatricyclic structures for these complexes and the corresponding (ethene-1,2-diolato)- and (but-2-ene-2,3-diolato)-zirconocenes. Dynamic temperature-dependent NMR spectroscopy revealed that all these complexes underwent a thermally induced degenerate rearrangement process, similar to the reorganization reaction of some Sharpless catalyst precursors, with Gibbs activation energies close to 50 kJ mol-1. A labelling experiment showed the intramolecular nature of this automerization. (4-tert-Butylcatecholato)zirconocene formed two diastereoisomeric dimers which did not interconvert on the NMR time-scale (DELTA-G double-ended-dagger inter > 90 kJ mol-1) although the intramolecular rearrangement was observed [DELTA-G double ended dagar intra (241 K) = 52 +/- 2 kJ mol-1]. The intermolecular equilibration process probably proceeding via monomeric (catecholato)zirconocene was monitored in the case of the bulky (4-tert-butyl-catecholato)bis(tert-butylcyclopentadienyl)zirconium system by dynamic NMR spectroscopy at high temperature [DELTA-G double ended dagar inter (364 K) = 78 +/- 5 kJ mol-1].