I. Mechanism of insertion of alkynes into a constrained geometry zirconaaziridine. II. Synthesis and reactivity of titanaaziridines
We developed syntheses of the "constrained geometry" (cg) zirconaaziridines Me4C5SiMe2N(tBu)Zr-(2 2)-[N(Ph)CH(Ph)](PMe2R) (R = Me, Ph) by analogy with the well-known Cp2-ligated zirconaaziridines Cp2Zr-(η 2)[N(Ph)CH(Ph)](PR3). cgZr-(η2)-[N(Ph)CH(Ph)](PMe 2Ph) has been structurally characterized by X-ray crystallography. Comparison of its solid-state structure with that of its Cp2 analogue Cp 2Zr-(η2)-[N(Ph)CH(Ph)](PMe2Ph) reveals that the Zr–P bond length in the cg complex is significantly longer than in the Cp2 complex.
Treatment of these zirconaaziridines with unsaturated electrophiles such as diphenylacetylene results in insertion. The product of diphenylacetylene insertion into Me4C5SiMe2N(tBu)Zr-(η 2)-[N(Ph)CH(Ph)](PMe3) has been characterized by X-ray crystallography. The reaction of Me4C5SiMe2N( tBu)Zr-(η2)-[N(Ph)CH(Ph)](PMe2R) with diphenylacetylene is several orders of magnitude faster than the reaction of Cp2Zr-(η2)-[N(Ph)CH(Ph)](PMe2R) with diphenylacetylene. Kinetic data for these irreversible reactions indicate that PMe3 dissociation must occur prior to insertion. We have proposed a rate law for the insertion reactions and used kinetic simulations along with the integrated form of the rate law to generate good fits to the experimental data.
We have measured the PMe3 dissociation rate constants for both Me4C5SiMe2N(tBu)Zr-(η 2)-[N(Ph)CH(Ph)](PMe3) and Cp2Zr-(η 2)[N(Ph)CH(Ph)](PMe3), providing quantitative ligand binding data for directly analogous cg and Cp2 complexes. PMe3 dissociation from Me4C5SiMe2N(tBu)Zr-(η 2)-[N(Ph)CH(Ph)](PMe3) is faster (k 1 = 0.204(7) s-1) than from Cp2Zr-(η 2)-[N(Ph)CH(Ph)](PMe3) (k1' = 0.0013(1) s-1).
Early transition metal η2-imine complexes (metallaaziridines) are well-documented as reaction intermediates for the coupling of organic molecules to form a variety of desirable functionalized products. The structures of zirconaaziridines are well-understood, as are their reactions with unsaturated electrophiles. There are few examples of titanaaziridines in the literature that have been either isolated or structurally characterized, though they are implicated as intermediates in many synthetic transformations. The synthesis of new titanaaziridines would fill a gap in the structural data for early transition-metal imine complexes, as well as potentially reveal new and useful modes of reactivity.
We have synthesized the titanaaziridines (CpR1)2 Ti-(η2)-[N(R2)CH(R 3)](PMe3) (R1= H, Me) from the reactions of (CpR1)2Ti(PMe3)2 with imines. Preliminary studies of their reactivity towards unsaturated organic compounds suggest that alkynes and cyclic carbonates displace the imine, while aldehydes insert into the Ti–C or Ti–N bonds.
0490: Organic chemistry