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

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)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.

A case of acute myocardial infarction treated with a new thrombectomy system

“…Detailed kinetic analysis7, 36 established that the catalysis proceeds by a mechanism analogous to that observed in the stoichiometric system35—cycloreversion of azazirconacyclobutenes 48 and 49 to generate base‐free imidozirconocene species was identified as the turnover‐limiting step. This finding is consistent with the observation of 48 and 49 as the predominant Zr‐containing species during the catalysis.…”

“…Tert ‐butyl and TBDMS (TBDMS = (Me 3 C)Me 2 Si) groups are also sufficiently bulky to suppress imido dimerization. Monomeric terminal imido complexes with less‐hindered p ‐tolylimido substituents are isolated using sterically demanding ancillary ligands, as illustrated by Cp*CpZrN( p ‐tolyl)(THF)7 ( 7a · THF ) (Cp* = η 5 ‐C 5 Me 5 ) and (ebthi)ZrN( p ‐tolyl)(THF)8 ( 8a · THF ) (ebthi = ethylenebis(tetrahydroindenyl)) (Fig. 2).…”

“…However, catalyst lifetime was severely limited by formation of catalytically inactive bis(μ‐arylimido) dimers 32. Addition of an equimolar (to the imido) amount of diphenylacetylene to the catalytic reaction mixture resulted in a marked decrease in the rate of catalysis but also completely suppressed the dimerization side reaction by trapping the phenyl‐ and p ‐tolylimido complexes generated during the catalysis (Scheme ) 7, 36. Accordingly, analysis of the catalytic reaction mixture by 1 H NMR spectroscopy revealed that the major Zr‐containing species are N ‐phenyl‐ and N ‐ p ‐tolylazazirconacyclobutene complexes 48 and 49 .…”

Dynamic imine chemistry in metal–organic polyhedra

“…Similar to imine formation, transimination also proceeds to the formation of tetrahedral intermediate (aminal) that subsequently decomposes to give a new imine and amine. [10][11][12][13][14][15][16] Recently, Stefano and Ciaccia critically discussed the mechanism operating in imine chemistry in organic solvents and unambiguously explained the mechanistic aspects of hydrolysis, transimination, and metathesis reactions. 3 Transimination may be homotransimination (both amines are aliphatic or aromatic) and heterotransimination (one is aliphatic and other is aromatic) and influence by Brönsted and Lewis acids, [4][5][6] for instance, Sc III triflate salts catalyse the exchange reactions between sterically hindered imines, derived from 9anthracenecarboxaldehyde, and several amines in chloroform.…”

Interaction of 2-(2′,6′-dialkylphenylazo)-4-methylphenols with iridium. C–H activation and migration of alkyl group

“…Although an ortho methyl group in ligand L 1 was observed to undergo interesting elimination [53], migration [51] and oxidative migration [50], such a metal-carbon bond formation via methyl C-H bond activation of ligand L 1 was not experienced. It is also noteworthy that methyl C-H activation is less common than phenyl C-H activation [54][55][56][57][58][59][60][61][62]. This interesting reactivity of L 1 has prompted us to scrutinize reactions of two related 2-(2 0 ,6 0 -dialkylphenylazo)-4-methylphenols (L 2 and L 3 ) with iridium.…”