Conversion of metallacyclic zirconoxycarbene complexes yielding conventional Fischer-type carbene complexes or metal-free organic products

Article abstract of DOI:10.1021/om00156a040

Reaction of (butadiene)zirconocene with hexacarbonyltungsten yields the metallacyclic (π-allyl)-zirconoxycarbene complex Cp₂ZrOC[=W(CO)₅]C₄H₆ (2). Subsequent coupling with pinacolone gives the nine-membered metallacycle trans-Cp₂ZrOC[=W(CO)₅]CH₂CH=CHCH ₂CMe(CMe₃)O (3). Both compounds 2 and 3 show a very low Fischer carbene complex reactivity. Hydrolytic removal of the zirconocene moiety transforms the rather unreactive zirconoxycarbene complexes into conventional heteroatom-stabilized transition-metal carbene complexes. Thus, treatment of 3 with water in tetrahydrofuran generates a hydroxycarbene tungsten complex in situ, which is efficiently trapped by added excess diazomethane to yield the enol ether H₂C=C(OCH₃)CH₂CH=CHCH₂CMe(CMe ₃)OH as the major reaction product. Similarly, controlled hydrolysis of 3 in the presence of pyridine N-oxide gives HOOCCH₂CH= CHCH₂CMe(CMe₃)OH. The reaction of 3 with tetra-n-butylammonium fluoride trihydrate in tetrahydrofuran cleaves the metallacycle to give a zirconium-free acylmetallate anion (Bu₄N⁺ counterion) which is readily O-alkylated by using Meerwein's reagent to produce H₅C₂OC[=W(CO)₅]CH₂CH= CHCH₂CMe(CMe₃)OH (11, ca. 70% overall yield). Similarly, the zirconoxycarbene complex 2 yields H₅C₂OC[=W(CO)₅]CH₂CH=CHCH ₃ upon treatment with Bu₄NF(H₂O)₃ followed by alkylation with (H₅C₂)³OBF₄.