Constrained geometry aminooxazolinate ligands giving chiral zirconium guanidinates; catalytic cyclohydroamination.
DFT calculations indicate that contrary to a prior report, N-heterocycle-augmented constrained geometry ligands e.g. Cp'-SiMe(2)-NR (Cp' = Me(4)C(5), 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'-SiMe(2)-NHR (R = chiral non-racemic 2-oxazoline) were readily synthesised in high yield via base mediated reaction of 2-aminooxazolines and Cp'-SiMe(2)Cl. Subsequent reaction with Zr(NMe(2))(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 -NMe(2). Trends in the level diastereoselection follow the steric demand of the oxazoline substituent, with the larger groups ((t)Bu, (i)Pr) 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 sigma-amido insertative mechanism.
Gott
Andrew L
AL
Department of Chemistry, University of Warwick, Gibbet Hill Road, Coventry, UK CV4 7AL.
Clarkson
Guy J
GJ
Deeth
Robert J
RJ
Hammond
Max L
ML
Morton
Colin
C
Scott
Peter
P