Mechanism of the Reaction of Alkynes with a “Constrained Geometry” Zirconaaziridine. PMe₃ Dissociates More Rapidly from the Constrained Geometry Complex than from its Cp₂ Analogue

The “constrained geometry” (cg) zirconaaziridines Me4C5SiMe2N(tBu)Zr-(η2)-[N(Ph)CH(Ph)](PMe2R) (R = Me, Ph) have been synthesized, and the R = Ph derivative has been structurally characterized by X-ray crystallography. Treatment of these zirconaaziridines with unsaturated electrophiles such as diphenylacetylene results in insertion. Kinetic data for these irreversible reactions indicate that PMe3 dissociation must occur prior to insertion and that PMe3 dissociation from the cg zirconaaziridine 4a is faster (k1 = 0.204(7) s−1) than from the Cp2 zirconaaziridine 5a (k1′ = 0.0013(1) s−1). The reaction of 4a with diphenylacetylene is several orders of magnitude faster than the reaction of 5a with diphenylacetylene.