Mechanistic Investigation of Cycloreversion/Cycloaddition Reactions between Zirconocene Metallacycle Complexes and Unsaturated Organic Substrates

摘要

Treatment of the diazametallacycle Cp2Zr(N(t-Bu)C=N(SiMe3)N(SiMe3)) (>4a) with diphenylacetylene resulted in the formation of the azametallacyclobutene Cp2Zr(N(t-Bu)C(Ph)=C(Ph)) (>6a) and Me3SiN=C=NSiMe3 in high yield. A kinetic study using UV–vis spectroscopy was carried out on the transformation. Saturation kinetic behavior was observed for the system, which is supportive of a mechanism that involves a reversible formal [2 + 2] retrocycloaddition of >4a to generate the transient imido species Cp2Zr=N-t-Bu (>7a) and Me3-SiN=C=NSiMe3. Trapping >7a with diphenylacetylene in an overall [2 + 2] cycloaddition reaction affords zirconacycle >6a. The study of cycloreversion/cycloaddition reactions between diazametallacycle complexes and diphenylacetylene was extended to other zirconocene systems. Detailed kinetic studies were performed for the exchange reactions between the diazametallacycle complexes Cp2Zr(N(2,6-Me2Ph)C=N(SiMe3)N(SiMe3)) (>8a) and Cp2Zr-(N(2,6-Me2Ph)C=N(t-Bu)N(t-Bu)) (>8b) with diphenylacetylene (>5a) to give the corresponding azametallacyclobutene complex Cp2Zr(N(2,6-Me₂Ph)C(Ph)=C(Ph)) (>6c) and extruded carbodiimides (Me₃SiN=C=NSiMe₃ for >8a and (t-Bu)N=C=N(t-Bu) for >8b). For both systems, the reactions were found to be first order in metallacycle and zero order in alkyne. Treatment of the diazametallacycle complexes Cp₂Zr(N(2,6-i-Pr₂Ph)C=N(Cyc)N(Cyc)) (>9a) and Cp₂Zr-(N(2,6-i-Pr₂Ph)C=N(i-Pr₂)N(i-Pr₂)) (>9b) with alkyne >5a resulted in the formation of the six-membered zirconacycles >10a,>b, respectively, upon heating at 75 °C. The products >10a,>b are generated from the overall insertion of alkyne >5a into the nitrogen–carbon bond of the zirconium-containing diazacyclobutane. Complex >10a has been characterized by an X-ray crystallographic study. When the azacyclobutene Cp₂Zr(N(2,6-i-Pr₂Ph)C(Ph)=C(Ph)) (>6e) was treated with CycN=C=NCyc or (i-Pr)N=C=N(i-Pr), the same six-membered zirconacycle complexes >10a,>b were obtained. Kinetic analysis of the reaction of >6e and (i-Pr)N=C=N(i-Pr) to yield >10b supports an associative process wherein alkyne >5a directly inserts into the zirconium–carbon bond of >6e. The diazametallacycle complex >4a underwent a stoichiometric metathetical exchange with symmetrical carbodiimides RN=C=NR (R = p-Tol, m-Tol, i-Pr, Cyc) to generate new cyclic zirconocene complexes and Me₃SiN=C=NSiMe₃. Kinetic studies were carried out on the exchange reaction between >4a and (m-Tol)N=C=N(m-Tol) to form >4e and Me₃SiN=C=NSiMe₃. The experimental rate data obtained are consistent with a dissociative mechanism. Additionally, the saturation rate constant derived for this system from the data is the same (within experimental error) as the saturation rate constant obtained from the kinetic study of >4a and diphenylacetylene to form >6a and Me₃SiN=C=NSiMe₃. These findings provide additional support for a dissociative mechanistic pathway in the exchange reactions, since the rate constant in the formal [2 + 2] retrocycloaddition reaction to generate imidozirconocene species Cp₂Zr=N-t-Bu (>7a) and Me₃SiN=C=NSiMe₃ should be the same for both reactions.