[60308-01-2] · C8HFe2NaO8 · Sodium Octacarbonylhydridodiferrate · (MW 359.78)
(reduction of a,b-unsaturated carbonyl compounds)
Physical Data: X-ray structure of the HFe2(CO)8- ion as the PPN+ salt has been reported.1
Solubility: sol THF.
Form Supplied in: prepared as a solution in THF; not commercially available.
Analysis of Reagent Purity: IR (THF) 1987 (s), 1940 (s), 1880 (s), 1852 (s), 1770 (m) and 1730 cm-1 (w); 1H NMR (THF, C6D6) d -8.47 (32 °C).
Preparative Method: Na2Fe2(CO)8 (0.764 g, 2.00 mmol)2 is taken into a test tube equipped with a magnetic stir bar. With a pipet, 20 mL of dry THF is added; 0.112 mL of HOAc (2.00 mmol) is then added and the test tube is sealed with a serum stopper, removed from the dry box, and immediately placed in an ice bath. The sample is centrifuged for 5-10 min. This procedure gives about 12-13 mL of a yellow-brown homogeneous 0.10 M NaHFe2(CO)8 solution.
Handling, Storage, and Precautions: must be prepared and transferred under inert gas (Ar or N2) to exclude oxygen and moisture; solutions in THF slowly decompose at ambient temperature; the solutions are stable for a few days in a -22 °C freezer.
Added Carbon Monoxide cleaves the metal-metal bond in NaHFe2(CO)8 and forms the products shown in eq 1. The addition of other ligands such as Triphenylphosphine to NaHFe2(CO)8 in THF also results in iron-iron bond cleavage. The reaction gives significant amounts of NaHFe(CO)4, Fe(CO)3(Ph3P)2, Fe(CO)5, and some Fe(CO)4Ph3P, but not NaHFe(CO)3Ph3P. The reaction stoichiometry, precise to ±15%, is shown in eq 2.
The addition of Acetyl Chloride to NaHFe2(CO)8 in THF gives 18% of acetoxymethylenediiron heptacarbonyl and 20-25% acetaldehyde (eq 3).3
A solution of NaHFe2(CO)8 and HOAc in THF reduces the alkenic bond in a,b-unsaturated carbonyl compounds in high yield.2 The double bonds in unsaturated esters, ketones, aldehydes, nitriles, amides, and lactones are all reduced without concurrent carbonyl reductions, demonstrating the mild conditions of these reductions. Carvone is reduced without isomerization or concurrent reduction of the isopropenyl group (eq 4). This is in contrast to Wilkinson's catalyst4 (Chlorotris(triphenylphosphine)rhodium(I)), which preferentially hydrogenates the isopropenyl group in carvone.
D1,9-2-Octalone is reduced to 2-decalone with a cis:trans ratio of 4:1 (eq 5). The selectivity is better than that of Noyori's reduction5 based on Pentacarbonyliron and NaOH in methanol, which yields products with a cis:trans ratio of 1:2. A bulky substrate such as testosterone is also reduced with a cis:trans ratio of 10:1 (eq 6). However, the yield of this reaction is poor.
Except for exceptionally active substrates like Dimethyl Maleate and mesityl oxide, 2 or more equiv of NaHFe2(CO)8 are required to reduce 1 equiv of substrate. Compared to acetic acid, the weak, insoluble acid NH4Cl increases yields, while the stronger acid Trichloroacetic Acid decreases yields. This reagent is a better reducing agent for the alkenic bond in a,b-unsaturated carbonyl compounds than Sodium Borohydride, which is limited primarily to double bonds activated by two carbonyls.6 The reagent also has the advantage of greater selectivity and mildness over heterogeneous hydrogenation.7
M. Mahmun Hossain & Anjan K. Saha
University of Wisconsin-Milwaukee, WI, USA