Andrew L. Gott

Catalytic alkene cyclohydroamination via an imido mechanism

Chiral-at-metal half-sandwich diamide complexes catalyse enantioselective cyclohydroamination of aminoalkenes at unexpectedly high rates given their high coordination number and steric bulk; substantial evidence is presented which argues against the established sigma-bond insertion process and is strongly indicative of an imido [2+2] cycloaddition mechanism.

Mechanism of Catalytic Cyclohydroamination by Zirconium Salicyloxazoline Complexes

The mechanism of hydroamination/cyclization of primary aminoalkenes by catalysts based on Cp*LZr(NMe(2))(2) (L = κ(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 (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(double dagger) = -13 to -23 cal/K·mol) and substantial overall bond disruption (ΔH(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.

Synthesis of mono-amino-functionalised ferrocene, ferrocene salts and ferrocenium saltsElectronic supplementary information (ESI) available: Selected interatomic distances (?) and selected angles between interatomic vectors () with estimated standard deviations in parentheses for compound 5. See http://www.rsc.org/suppdata/dt/b3/b315359m/

A number of mono-amino-functionalised ferrocenes, ferrocene salts and a mono-amino-functionalised ferrocenium salt have been synthesised and characterised. This represents a novel method of accessing these important classes of molecules. In the cases of some of the ferrocene salts, there are some interesting structural features in the solid state. These include N-H...O and N-H...F hydrogen bonds.