Enantiopure Pyridine Bis(oxazoline) “Pybox” and Bis(oxazoline) “Box” Iron Dialkyl Complexes: Comparison to Bis(imino)pyridine Compounds and Application to Catalytic Hydrosilylation of Ketones

Abstract

The chemistry of enantiopure pyridine bis(oxazoline) iron compounds has been explored. The electronic properties of this ligand class have been evaluated relative to aryl- and alkyl-substituted bis(imino)pyridines using cyclic voltammetry and CO stretching frequencies of iron dicarbonyl compounds. Attempts to prepare a pyridine bis(oxazoline) iron dinitrogen complex by sodium amalgam reduction of the corresponding dichloride precursor yielded a catalytically inactive S = 1 bis(chelate) iron complex. Dialkyl derivatives of the general formula ( R Pybox)Fe(CH 2 SiMe 3 ) 2 were prepared either by pyridine displacement from (py) 2 Fe(CH 2 SiMe 3 ) 2 or by direct dialkylation of the appropriate iron dichloride. Two examples of the S = 2 complexes were characterized by single-crystal X-ray diffraction. Each enantiopure Pybox iron dialkyl was evaluated in the catalytic hydrosilylation of various ketones. Efficient turnover was observed at 23 °C with 0.3% of the iron precatalyst using PhSiH 3 as the stoichiometric reductant. For comparison, related bis(oxazoline) iron dialkyl compounds were also evaluated and found to be equally effective at 1.0 mol % iron loadings. While low to modest enantioselectivities were observed with both classes of neutral iron dialkyl compounds, high levels of asymmetric induction were obtained for a hindered acetophenone and α-tetralone upon activation of the iron derivative with B(C 6 F 5 ) 3 .