New Chiral Phosphoramidite Complexes of Iron as Catalytic Precursors in the Oxidation of Activated Methylene Groups

New phosphoramidite complexes of iron were synthesized and structurally characterized. Reaction of the known chiral phosphoramidites (RO)2PNR’2 (R = binaphthyl, R’ = CH3, 1a; R = binaphthyl, R’ = benzyl, 1b) with [FeBr(Cp)(CO)2] afforded the title compounds [FeBr(Cp)(CO)(1a,b)] (4a,b) in 34 and 65 % isolated yields as mixtures of diastereomers, since both the metal and the ligand are stereogenic. Similarly, reaction of 1b with [Fe(Cp)I(CO)2] in the presence of catalytic [Fe(Cp)(CO)2]2 afforded [Fe(Cp)I(CO)(1b)] (5b) in 81% yield as a mixture of diastereomers. The molecular structures of 4a, 4b and 5 were determined, revealing a pseudo octahedral coordination geometry about the iron center. The new metal complexes are catalytically active in the oxidation of benzylic methylene groups to the corresponding ketones, utilizing t-BuOOH as oxidant (2 mol% catalyst, 36 h, room temperature, 31−80% yield).

The substrate 9,10-dihydroanthracene (0.100 g, 0.555 mmol) and the catalyst 2a (0.006 g, 0.011 mmol) were dissolved in pyridine (1.0 mL). The oxidant t-BuOOH (0.3 mL of 5.5M solution in decane, 1.664 mmol) was added and the brownish solution was shaken for 36 h at rt. The product anthraquinone precipitated from the reaction mixture and was isolated by filtration and washing with CH 2 Cl 2 (2 mL).
The filtrate also contained the product. The solvent was removed from the filtrate and the residue chromatographed on silica using CH 2 Cl 2 as eluent. The combined yield was 0.063 g (0.303 mmol, The substrate cinnamyl alcohol (0.100 g, 0.746 mmol) and the catalyst 4a (0.009 g, 0.015 mmol) were dissolved in pyridine-d 5 (1.0 mL). The oxidant t-BuOOH (0.41 mL of 5.5M solution in decane, 2.24 mmol) was added and the brownish solution was shaken for 42 h at rt and a 1 H-NMR of the reaction mixture was recorded. The NMR yield was determined by comparing the peak intensity of the CH 2 OH group of cinnamyl alcohol with the peak intensity of the -CHO group of cinnamaldehyde (31%). The 1 H NMR spectrum is given below.
The substrate phenylmethanol (0.100 g, 0.925 mmol) and the catalyst 4a (0.011 g, 0.019 mmol) were dissolved in pyridine-d 5 (1.0 mL). The oxidant t-BuOOH (0.50 mL of 5.5M solution in decane, 2.8 mmol) was added and the brownish solution was shaken for 42 h at rt and a 1 H-NMR of the reaction mixture was recorded. The NMR yield was determined by comparing peak intensity of the CH 2 OH group of phenylmethanol with the peak intensity of the -CHO group of benzaldehyde (47%). The 1 H NMR spectrum is given below.

X-Ray determination for Complex 4a
X-ray quality crystals of 4a were obtained by layering a CH 2 Cl 2 solution with hexanes, which was stored at -18 °C for one week.
Preliminary examination and X-ray data collection were performed using a Bruker Kappa Apex II single crystal X-Ray diffractometer equipped with an Oxford Cryostream LT device. Intensity data were collected by a combinations of ϖ and φ scans. Apex II, SAINT and SADABS software packages were used for data collection, integration and correction of systematic errors, respectively [2].
Crystal data and intensity data collection parameters are listed in Table S1. Structure solution and refinement were carried out using the SHELXTL-PLUS software package [3]. The structure was solved by direct methods and refined successfully in the space group P2 1 2 1 2 1 . Due to weak diffraction and poor quality data only the heavy atoms (Fe, Br, P, Cl) were refined with anisotropic displacement parameters. All other non-hydrogen atoms were refined with isotropic displacement parameters. The hydrogen atoms were treated using appropriate riding model (AFIX m3).