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Mechanism of Catalytic Cyclohydroamination by Zirconium Salicyloxazoline Complexes

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journal contribution
posted on 03.11.2010, 00:00 by Laura E. N. Allan, Guy J. Clarkson, David J. Fox, Andrew L. Gott, Peter Scott
The mechanism of hydroamination/cyclization of primary aminoalkenes by catalysts based on Cp*LZr(NMe2)2 (L = κ2-salicyloxazoline) is investigated in a range of kinetic, stoichiometric, and structural studies. The rate law is found to be d[substrate]/dt = k[catalyst]1[substrate]0 for all catalysts and aminoalkenes studied. The overall rate is similar for formation of five- and six-membered rings, and a substantial KIE (kH/kD) is observed, indicating the involvement of N−H bond-breaking in a rate-determining step (RDS) which is not ring-closure. Remarkably, the reaction proceeds at the same rate in THF as it does in toluene, but added non-cyclizable amine slows the reaction, indicating that while the metal is not acting as a Lewis acid in the RDS, the activated substrate is involved. Also in contrast to other catalysts, increasing steric bulk improves the rate, and the origins of this are investigated by X-ray crystallography. Thermodynamic parameters extracted from eight independent kinetic studies indicate moderate ordering (ΔS = −13 to −23 cal/K·mol) and substantial overall bond disruption (ΔH = 17 to 21 kcal/mol) in the rate-determining transition state. Secondary amines are unreactive, as is a catalyst with a single aminolyzable site, thus excluding an amido mechanism. A catalytic cycle involving rate-determining formation of a reactive imido species is proposed. Stoichiometric steps in the process are shown to be feasible and have appropriate rates by synthetic and in situ NMR spectroscopic studies. The fate of the catalyst in the absence of excess amine (at the end of the catalytic reaction) is conversion to a metallacyclic species arising from CH activation of a peripheral substituent.