Catalytic Asymmetric α‑Iminol Rearrangement: New Chiral Platforms

A series
of 19 different asymmetric catalysts were screened in
an effort to identify the first chiral catalyst for the rearrangement
of α-hydroxy imines to α-amino ketones involving a 1,2-carbon
shift. Although aluminate complexes of VAPOL, VANOL, and 7,7′-tBu2VANOL were quite effective
catalysts giving up to 88% ee, the ne plus ultra catalyst for this
reaction was found to be a zirconium complex of VANOL which gives
97 to >99% ee for the majority of the substrates examined. An X-ray
diffraction study of the catalyst reveals that the zirconium exists
as a homoleptic complex with three VANOL ligands and two protonated N-methyl imidazoles.