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Group 4 ureate complexes : synthesis, reactivity, and catalytic carbon-element bond formation

University of British Columbia

The synthesis, characterization, and reactivity patterns of three new classes of ureate- supported group 4 compounds are described, constituting the first comprehensive examination of the use of these electron-rich ligands. Four ureates with varying steric and electronic properties are included here: two mono(ureate)s and two tethered bis(ureate)s. Synthesis of dichlorobis(ureato) titanium and zirconium derivatives can be accomplished using several methods, with direct protonolysis between the urea proligand and M(NMe₂)₂Cl₂ precursors giving the highest yield. Solid-state and solution-phase characterization indicates the distal dialkylamino substituent on the ureate donates electron density into the chelate. While use of non-tethered ligands results in zirconium complexes that are fluxional in solution, tethered bis(ureato) ligands support well-defined species. These complexes retain neutral ligands, which are not easily removed. Ureate- supported zirconium dialkyl complexes can also be prepared by protonolysis between a urea and tetraalkyl zirconium compounds. The electron-rich nature of the ureate ligands allows the isolation of coordinatively unsaturated dialkyl complexes. Ureate-supported bis(amido) compounds of titanium and zirconium have been developed as precatalysts for hydroamination. A comprehensive structural comparison between related amidate and ureate complexes reveals that the ureate ligands bind tighter to their metal centers than amidates. As a consequence of this, and the electron-rich nature of the ureate ligands, amidate precatalysts are generally more effective for intramolecular hydroamination of alkenes than ureate precatalysts. In contrast, precatalysts with tethered ureate ligands are more effective than analogous amidates. The most active system identified through catalytic screening exhibits broad substrate scope and functional group tolerance. Most importantly, this is the first group 4 system that is highly effective with both primary and secondary amines. Mechanistic investigations have revealed that catalysis with the tethered bis(ureate) precatalyst does not proceed through an imido-mediated [2+2] cycloaddition-type mechanism. Instead, the key bond forming step is proposed to occur through concerted insertion of the alkene into a Zr–N bond and protonation of the terminal alkene carbon by a coordinated amine ligand. This proposal is supported by stoichiometric and kinetic investigations, which indicate that a proton-source accelerates alkene insertion, and a primary kinetic isotope effect when using an N-deuterated aminoalkene.

Text

2011-11-01

Attribution-NonCommercial-NoDerivs 3.0 Unported

http://hdl.handle.net/2429/28867

Chemistry

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/3.0/