Helical Oxidovanadium(IV) Salen‐Type Complexes: Synthesis, Characterisation and Catalytic Behaviour

Abstract

Abstract Chiral [VO(salen)] complexes ( R , R )‐ 7 and ( R , R )‐ 8 were synthesised by ligand replacement from [VO(acac) 2 ] with the resolved (1 R ,2 R )‐cyclohexyl ligands 3,3′‐[(1 R ,2 R )‐1,2‐cyclohexane‐diylbis(nitrilomethylidyne)]bis‐4‐phenanthrenol [( R , R ) ‐3 ] and 3,3′‐[(1 R ,2 R )‐1,2‐cyclohexanediylbis(nitrilomethylidyne)]bisbenz[ a ]anthracen‐1‐ol [( R , R )‐ 4 ], respectively. Similarly, complexes ( R )‐ 9 and ( R )‐ 10 were synthesised using the ( R )‐binaphthyl ligands 3,3′‐[(1 R )‐(1,1′‐binaphthalene)‐2,2′‐diylbis(nitrilomethylidyne)]bis‐4‐phenanthrenol [( R )‐ 5 ] and 3,3′‐[(1 R )‐(1,1′‐binaphthalene)‐2,2′‐diylbis(nitrilomethylidyne)]bis‐benz[ a ]anthracen‐1‐ol [( R )‐ 6 ], respectively. These complexes were characterised by IR spectroscopy, ESI‐TOF‐MS, electronic absorption spectroscopy and circular dichroism (CD). Single crystal X‐ray analysis of ( R , R )‐ 7 and ( R , R )‐ 8 revealed distorted square pyramidal geometries and a 1:1 ratio of diastereomeric M and P helical conformers. Solution CD studies in THF combined with DFT calculations indicate that the M conformer dominates in solution for ( R , R )‐ 7 . Preliminary catalytic oxidation of thioanisole with H 2 O 2 and 1 % [VO(salen)] showed good selectivity for sulfoxides over sulfones but low enantioselectivities (8 to 33 %) which are solvent dependent but insensitive to the catalyst used.