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)Zr=N t Bu is used as the catalyst precursor in the metathesis reaction between PhCH=NPh and p-TolCH=N-p-Tol, a 1:1:1:1 equilibrium mixture with the two mixed imines p-TolCH=NPh and PhCH=N-p-Tol is generated in C 6 D 6 at 105 °C. The catalyst was still active after 20 days with an estimated 847 turnovers (t 1/2 170 m; TON = 1.77 h −1 ). When the azametallacyclobutene Cp 2 Zr(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 (t 1/2 170 m; TON = 4.3 h −1 ). An extensive kinetic and equilibrium analysis of the metallacyclobutene-catalyzed metathesis of PhCH=N-p-Tol and p-F-C 6 H 4 CH=N-p-F-C 6 H 4 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′ Zr=NAr" (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 TolCH=NPr and PhCH=NMe 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.

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The development of new reactions employing imidozirconium complexes or their derivatives as catalysts is an elusive goal in organometallic chemistry. To date, the imidozirconocene-catalyzed hydroamination of alkynes and allenes to yield new enamines (and, on tautomerization, imines) is the lone example of such a process. [1,2] We recently reported that aldehydes and electron-deficient imines insert into the carbon-zirconium bond of azazirconacyclobutene 1 to afford new six-membered ring metallacycles 2 and 3 (see Scheme 1). On heating, these expanded zirconacycles undergo a retro-[4+2] cycloaddition to afford a,b-unsaturated imine 4 and oxozirconium complex 5 or electron-poor imidozirconocene dimer 6, both of which are unreactive (Scheme 1; EWG = electron-withdrawing group, Cp = C 5 H 5 ). [3,4] We reasoned that insertion of an aldimine 7 with N-substitution identical to that of the nitrogen group in the metallacyclobutene, followed by subsequent retro-cycloaddition, would not only afford a,b-unsaturated imine 4 but would also generate imidozirconocene complex 8 previously used to prepare the starting azazirconacyclobutene (see Scheme 2). [5] Carrying out the reaction in the presence of the necessary alkyne would regenerate the starting metallacycle and close the catalytic cycle (Scheme 2).[6] This reaction is deemed a carboamination, because it results in the overall cleavage of an imine C = N bond and addition of the resulting C and N fragments across an alkyne, forming a new carbon-carbon double bond and a new ketimine carbon-nitrogen double bond. [7][8][9] Herein, we present the development of a novel, high-yielding imidozirconocene-catalyzed carboamination reaction that also represents the best method for preparing the highly arylated a,b-unsaturated imine co-products.The most frequently studied imidozirconium complexes bear sterically bulky groups on the nitrogen atom to prevent competitive dimerization of the imido compound.[5] As such, N-2,6-dimethylphenyl-and N-tert-butyl-substituted azazirconacyclobutenes 1 and 9 (see Scheme 3) were explored in the desired insertion chemistry. Unfortunately, even at high concentrations of substrate and temperatures up to 165 8C, neither of these zirconium compounds was an effective catalyst as no imine insertion products were observed. In the case of metallacycle 1 and imine 10, it appears that the combined steric bulk of the zirconacycle and imine Nsubstituents prohibits insertion; with metallacycle 9 and Ntert-butyl imine 11, we began to observe CÀD bond activation of the [D 6 ]benzene solvent. Presumably, this pathway arises from [2+2]-cycloreversion to afford the free imidozirconocene complex 12, which had been shown to activate benzene CÀH bonds at a minimum temperature of 75 8C (Scheme 3).

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Treatment of the diazametallacycle Cp 2 Zr(N(t-Bu)C=N(SiMe 3 )N(SiMe 3 )) (4a) with diphenylacetylene resulted in the formation of the azametallacyclobutene Cp 2 Zr(N(t-Bu)C(Ph)=C (Ph)) (6a) and Me 3 SiN=C=NSiMe 3 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 Cp 2 Zr=N-t-Bu (7a) and Me 3 -SiN=C=NSiMe 3 . 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 Cp 2 Zr(N(2,6-Me 2 Ph)C=N(SiMe 3 )N(SiMe 3 )) (8a) and Cp 2 Zr-(N(2,6-Me 2 Ph)C=N(t-Bu)N(t-Bu)) (8b) with diphenylacetylene (5a) to give the corresponding azametallacyclobutene complex Cp 2 Zr(N(2,6-Me 2 Ph)C(Ph)=C(Ph)) (6c) and extruded carbodiimides (Me 3 SiN=C=NSiMe 3 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 Cp 2 Zr(N(2,6-i-Pr 2 Ph)C=N (Cyc)N(Cyc)) (9a) and Cp 2 Zr-(N(2,6-i-Pr 2 Ph)C=N(i-Pr 2 )N(i-Pr 2 )) (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 Cp 2 Zr(N(2,6-i-Pr 2 Ph)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 Me 3 SiN=C=NSiMe 3 . Kinetic studies were carried out on the exchange reaction between 4a and (m-Tol)N=C=N(m-Tol) to form 4e and Me 3 SiN=C=NSiMe 3 . 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 6a and Me 3 SiN=C=NSiMe 3 . These findings provide additional support for a dissociative mechanistic pathway in the exchange reactions, since the rate constant in the formal [2 + 2] retrocycloaddition † This paper is dedic...