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(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 Me3SiN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 obtai