Zirconium-Mediated Metathesis of Imines: A Study of the Scope, Longevity, and Mechanism of a Complicated Catalytic System

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Abstract
By kinetically stabilizing imidozirconocene complexes through the use of a sterically demanding ligand, or by generating a more thermodynamically stable resting state with addition of diphenylacetylene, we have developed transition metal-catalyzed imine metathesis reactions that are mechanistically analogous to olefin metathesis reactions catalyzed by metal carbene complexes. When 5 mol % of Cp*Cp(THF)ZrNtBu is used as the catalyst precursor in the metathesis reaction between PhCHNPh and p-TolCHN-p-Tol, a 1:1:1:1 equilibrium mixture with the two mixed imines p-TolCHNPh and PhCHN-p-Tol is generated in C6D6 at 105 °C. The catalyst was still active after 20 days with an estimated 847 turnovers (t1/2 170 m; TON = 1.77 h-1). When the azametallacyclobutene Cp2Zr(N(Tol)C(Ph)C(Ph)) is used as the catalyst precursor under similar reaction conditions, a total of 410 turnovers are obtained after 4 days (t1/2 170 m; TON = 4.3 h-1). An extensive kinetic and equilibrium analysis of the metallacyclobutene-catalyzed metathesis of PhCHN-p-Tol and p-F-C6H4CHN-p-F-C6H4 was carried out by monitoring the concentrations of imines and observable metal-containing intermediates over time. Numerical integration methods were used to fit these data to a detailed mechanism involving coordinatively unsaturated (16-electron) imido complexes as critical intermediates. Examination of the scope of reaction between different organic imines revealed characteristic selectivity that appears to be unique to the zirconium-mediated system. Several zirconocene complexes that could generate the catalytically active “CpCp‘ZrNAr” (Cp‘ = Cp or Cp*) species in situ were found to be effective agents in the metathetical exchange between different N-aryl imines. N-Alkyl aldimines were found to be completely unreactive toward metathesis with N-aryl aldimines, and metathesis reactions involving the two N-alkyl imines TolCHNPr and PhCHNMe gave slow or erratic results, depending on the catalyst used. Metathesis was observed between N-aryl ketimines and N-aryl aldimines, but for N-aryl ketimine substrates, the catalyst resting state consists of zirconocene enamido complexes, generated by the formal C−H activation of the α position of the ketimine substrates.