Constrained geometry aminooxazolinate ligands giving chiral zirconium guanidinates; catalytic cyclohydroamination

Abstract

DFT calculations indicate that contrary to a prior report, N-heterocycle-augmented constrained geometry ligands e.g.Cp′-SiMe₂-NR (Cp′ = Me₄C₅, R = 2-pyridine) should be capable of binding both atoms of the diazaallyl fragment at zirconium. This was confirmed by a molecular structure of a previously reported complex. Similar R = 2-oxazoline complexes were also shown to be feasible, although an additional N,O binding mode was accessible. The proligands HCp′-SiMe₂-NHR (R = chiral non-racemic 2-oxazoline) were readily synthesised in high yield via base mediated reaction of 2-aminooxazolines and Cp′-SiMe₂Cl. Subsequent reaction with Zr(NMe₂)₄ gave, rather than the desired complexes, configurationally stable chiral-at-zirconum guanidinate/alkoxide chelate products; the aminooxazolinate units had undergone ring-opening and migratory insertion of –NMe₂. Trends in the level diastereoselection follow the steric demand of the oxazoline substituent, with the larger groups (^tBu, ⁱPr) giving single diastereomers. The modest performance of these guanidinate compounds in enantioslective catalytic cyclohydroamination of aminoalkenes follows the expected trends for metal accessibility in a σ-amido insertative mechanism.