Asymmetric alkene epoxidation with chromium oxo salen complexes. A systematic study of salen ligand substituents.
[structure: see text]. In the stoichiometric asymmetric epoxidation of E-beta-methylstyrene with cationic chromium-salen oxo complexes, enantioselectivity is increased by halo-substitution at the 3,3'- and 6,6'-positions and decreased at the 4,4'- and 5,5'-positions on the salen rings. Addition of triphenylphosphine oxide significantly increases selection with 3,3'- or 5,5'-substituents but not with 4,4'- or 6,6'-substituents. Use of nitrate counterion is beneficial in most cases. The results are discussed with respect to the mode of stereoselection in metal-salen epoxidations.
O'Mahony
C P
CP
Chemistry Department and Conway Institute of Biomolecular and Biomedical Sciences, University College Dublin, Belfield, Dublin 4, Ireland.
McGarrigle
E M
EM
Renehan
M F
MF
Ryan
K M
KM
Kerrigan
N J
NJ
Bousquet
C
C
Gilheany
D G
DG