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Iron (low valent)

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Fe(0) is cost-effective, environmentally friendly alternative to Cr(II) for the olefination of carbonyls by activated polyhalides. Fe(0) proved compatible with a wide range of functionality, such as unprotected phenol, aryl nitro, carboxylic acid, and alkyl nitrile.
J. R. Falck, R. Bejot, D. K. Barma, A. Bandyopadhyay, S. Joseph, C. Mioskowski, J. Org. Chem., 2006, 71, 8178-8182.

An operationally simple and very efficient Reformatsky reaction of aldehydes has been carried out in THF in the presence of low valent iron or copper which were prepared in situ employing a bimetal redox strategy through reduction of Fe(III) or Cu(II) salts with magnesium.
A. Chattopadhyay, A. Kr. Dubey, J. Org. Chem., 2007, 72, 9357-9359.

An efficient, economical, and practical Reformatsky reaction of α-halo esters with carbonyl compounds is mediated by cheap iron(0) powder. The reactions proceeded effectively in the presence of a catalytic amount of iodine to afford synthetically useful β-hydroxyl carbonyl compounds in good yields.
X.-Y. Liu, X.-R. Li, C. Zhang X.-Q. Chu, W. Rao, T.-P. Loh, Z.-L. Shen, Org. Lett., 2019, 21, 5873-5878.

An iron complex containing electronically coupled acidic and hydridic hydrogens catalyzes the hydrogenation of ketones under mild conditions and shows high chemoselectivity for aldehydes, ketones, and imines. Isolated carbon double and triple bonds, aryl halides, nitrates, epoxides, and ester functions are unaffected by the hydrogenation conditions.
C. P. Casey, H. Guan, J. Am. Chem. Soc., 2007, 129, 5816-5817.

For highly stereoselective reductions of a large number of five- and six-membered cyclic ketones to the most thermodynamically stable alcohols, ketones are treated with lithium dispersion and either FeCl24H2O or CuCl22H2O in THF at room temperature. This protocol is more convenient and efficient than those commonly reported for similar reductions.
N. Kennedy, T. Cohen, J. Org. Chem., 2015, 80, 8134-8141.

A facile and practical method for the synthesis of N-acetyl α-arylenamides from the corresponding ketoximes with ferrous acetate as the reducing reagent offers mild reaction conditions, simple purification procedures, and high yields for a variety of N-acetyl enamides.
W. Tang, A. Capacci, M. Sarvestani, X. Wei, N. K. Yee, C. H. Senanayake, J. Org. Chem., 2009, 74, 9528-9530.

An intermolecular reductive Schiff base formation from nitroarenes and benzaldehydes to yield diarylimines is carried out in the presence of iron powder and dilute acid. This process tolerates various functional groups and often proceeds quantitatively with no need for purification.
A. L. Korich, T. S. Hughes, Synlett, 2007, 2602-2604.

The very inexpensive carbonyl iron powder (CIP), a highly active commercial grade of iron powder, enables an especially mild, safe, efficient, and environmentally responsible reduction of aromatic and heteroaromatic nitro groups in water. These reductions are conducted in a recyclable aqueous reaction medium in the presence of nanomicelles composed of TPGS-750-M.
N. R. Lee, A. A. Bikovtseva, M. Cortes-Clerget, F. Gallou, B. H. Lipshutz, Org. Lett., 2017, 19, 6518-6521.

An efficient Fe/CaCl2 system enables the reduction of nitroarenes and reductive cleavage of azo compounds by catalytic transfer hydrogenation in the presence of sensitive functional groups including halides, carbonyl, aldehyde, acetyl, nitrile, and ester substituents with excellent yields. The simple experimental procedure and easy purification make the protocol advantageous.
S. Chandrappa, T. Vinaya, T. Ramakrishnappa, K. S. Rangappa, Synlett, 2010, 3019-3022.

S. Chandrappa, T. Vinaya, T. Ramakrishnappa, K. S. Rangappa, Synlett, 2010, 3019-3022.

Reduction of o-nitroarylcarbaldehydes o-aminoarylcarbaldehydes with iron in the presence of a catalytic amount of aqueous hydrochloric acid followed by in situ condensation of the resulting amines with ketones or aldehydes (Friedlaender quinoline synthesis) gives mono- or disubstituted quinolines, respectively, in very good yields.
A.-H. Li, D. J. Beard, H. Coate, A. Honda, M. Kadablbajoo, A. Kleinberg, R. Laufer, K. M. Mulvihill, A. Nigro, P. Rastogi, M. W. Siu, A. G. Steinig, T. Wang, D. Werner, A. P. Crew, M. J. Mulvihill, Synthesis, 2010, 1629-1632.

A tandem condensation of a cyanoimidate with an amine followed by reductive cyclization in an iron-HCl system enables an efficient route to N4-substituted 2,4-diaminoquinazolines. An additional N-alkylation can produce two fused heterocycles in a one-pot procedure.
P. Yin, N. Liu, Y.-X. Deng, Y. Chen, Y. Deng, L. He, J. Org. Chem., 2012, 77, 2649-2658.

Quinolines can be synthesized from Δ2-isoxazolines under reductive conditions. The reductive cyclization to quinolines is achieved in the presence of iron or sodium dithionite under metal-free conditions.
P. Kamath, R. C. Viner, S. C. Smith, M. Lal, Synlett, 2017, 28, 1341-1345.