Replacing Chloride by Alkoxide: Cp2Zr(H)OR, Searching for Alternatives to Schwartz's Reagent

Abstract

Attempts to synthesize zirconocene hydrido alkoxide species, Cp2Zr(H)OR, via several synthetic pathways consistently failed to yield the targeted complexes, which seem prone to undergo disproportionation into Cp2ZrH2 and a bisalkoxide complex. Reaction of Cp2ZrH2 with HOR, Me3SiOR, or Ph2CO yielded the corresponding bisalkoxide complexes Cp2Zr(OR)2, of which Cp2Zr{OC(H)Ph2}2 was characterized by an X-ray diffraction study. Insertion of vinyl ethers into Cp2ZrH2 yielded Cp2Zr(OR)2 and Cp2Zr(Et)OR. Attempted hydrogenation of Cp2Zr(Me)OR or reaction of Cp2Zr(BH4)OR with NEt3 did not yield the targeted complexes. Cp2Zr(H)OC6H5 and Cp2Zr(H)OC6F5 could be obtained, however, as transient species by β-H elimination from Cp2Zr(tBu)OR, ligand exchange between Cp2ZrH2 and Cp2Zr(OC6F5)2, or protonation in the presence of olefin to yield the corresponding hydrozirconation products. In hydrozirconation reactions of styrene both species are highly regioselective. The alkoxide substituent failed to inhibit β-H eliminatio...