Allan, Laura E. N., Clarkson, Guy J., Fox, David J., Gott, Andrew L. and Scott, Peter (2010) Mechanism of catalytic cyclohydroamination by zirconium salicyloxazoline complexes. Journal of the American Chemical Society, Vol.132 (No.43). pp. 15308-15320. doi:10.1021/ja106588m

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Abstract

The mechanism of hydroamination/cyclization of primary aminoalkenes by catalysts based on Cp*LZr(NMe2)(2) = kappa(2)-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 (k(H)/k(D)) is observed, indicating the involvement of N-H bond-breaking in a rate-determining step (ADS) 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 ADS, 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 (Delta S double dagger = -13 to -23 cal/K.mol) and substantial overall bond disruption (Delta H double dagger = 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.

Item Type:	Journal Article
Subjects:	Q Science > QD Chemistry
Divisions:	Faculty of Science > Chemistry
Library of Congress Subject Headings (LCSH):	Catalysis, Zirconium, Ring formation (Chemistry), Amination
Journal or Publication Title:	Journal of the American Chemical Society
Publisher:	American Chemical Society
ISSN:	0002-7863
Official Date:	3 November 2010
Dates:	Date Event 3 November 2010 Published
Volume:	Vol.132
Number:	No.43
Number of Pages:	13
Page Range:	pp. 15308-15320
DOI:	10.1021/ja106588m
Status:	Peer Reviewed
Publication Status:	Published
Funder:	Engineering and Physical Sciences Research Council (EPSRC)

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