Multinuclear NMR study of the reactive intermediates in enantioselective epoxidations of allylic alcohols catalyzed by a vanadium complex derived from a planar-chiral hydroxamic acid

Abstract

Structure and reactivity of vanadium(V) complexes formed in situ in the catalytic system VO(Oalkyl)₃/hydroxamic acid/tert-butylhydroperoxide (TBHP) have been examined by means of ⁵¹V, ¹³C and ¹⁷O NMR spectroscopy. For the first time, reactive vanadium(V) alkylperoxo intermediates in vanadium/hydroxamic acid epoxidations were observed and spectroscopically characterized. With a planar-chiral [2.2]paracyclophane-derived hydroxamate as ligand two diastereomeric alkylperoxo vanadium(V) complexes were formed in a 3∶1 ratio. They differ in the relative positioning of the VO group and the planar-chiral aromatic part. Upon addition of geraniol as substrate, these complexes disappear in a parallel manner, and geraniol epoxide is formed. Probably, the existence of these two diastereomeric complexes and their comparable reactivity account for the observed enantioselectivity level (≤71% ee) and hence sets a fundamental limitation for the use of such ligands in this asymmetric catalysis.