Mechanistic investigation of cycloreversion/cycloaddition reactions between zirconocene metallacycle complexes and unsaturated organic substrates

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(Pk)=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 Me-3-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)) (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-Pr-2)N(i-Pr-2)) (9b) with alkyne Sa resulted in the formation of the six-membered zirconacycles 10a,b, respectively, upon heating at 75 degreesC. The products 10a,b are generated from the overall insertion of alkyne 5a into the nitrogen-carbon bond of the zirconium-containing diazacyclobutane. Couples 10a has been characterized lay 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 zirconcycle 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 ia 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 iia 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 Ga 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.