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Dehalogenations and other reductive cleavages

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(Bis(dimethylamino)carbazole) was used as photocatalyst in the reduction of unactivated aryl chlorides and alkyl chlorides in the presence of CHD as hydrogen atom donor at room temperature. The catalytic system can also be applied to the coupling of aryl chlorides with electron-rich arene and heteroarenes to affect C-C bond-forming reactions.
R. Matsubara, T. Yabuta, U. M. Idros, M. Hayashi, F. Ema, Y. Kobori, K. Sakata, J. Org. Chem., 2018, 83, 9381-9390.

Under irradiation of 407 nm LEDs using sodium formate as reductant and thiol as hydrogen atom transfer agent, a variety of (hetero)aryl chlorides, bromides, and iodides can be reduced to the corresponding (hetero)arenes. The key intermediates, aryl radicals, can be trapped by either hydrogen, phosphite, or borates. The same conditions can be used for the deprotection of sulfonamides.
S.-D. Wang, B. Yang, H. Zhang, J.-P. Qu, Y.-B. Kang, Org. Lett., 2023, 25, 816-820.

A combined visible light and base system initiates a radical-mediated hydrogenation of aryl fluorides, chlorides, bromides, and iodides to the corresponding (hetero)arenes with excellent yields under mild conditions. This simple and efficient transition-metal-free hydrogenation tolerates various functional groups.
T.-H. Ding, J.-P. Qu, Y.-B. Kang, Org. Lett., 2020, 22, 3084-3088.

Visible-light irradiation enables a Ni-catalyzed photodehalogenation of a broad scope of aryl bromides that utilizes tetrahydrofuran as hydrogen source. The protocol obviates the need for exogeneous amine reductants or photocatalysts and is characterized by its simplicity.
B. Higginson, J. Sanjosé-Orduna, Y. Gu, R. Martin, Synlett, 2021, 32, 1633-1636.

Nanoparticles formed from PdCl2 in the presence of tetramethyldisiloxane (TMDS) on water enable a mild and environmentally attractive dehalogenation of functionalized aryl halides. The reaction medium with the catalyst can be recycled.
A. Bhattacharjya, P. Klumphu, B. H. Lipshutz, Org. Lett., 2015, 17, 1122-1125.

A transition-metal- and catalyst-free hydrogenation of a wide range of aryl halides is promoted by bases using either aldehydes or alcohols as hydrogen source. One equivalent of benzaldehyde affords an equal yield as that of 0.5 equiv of benzyl alcohol. The kinetic study reveals that the initial rate of PhCHO is nearly 4 times faster than that of BnOH. Radical trapping experiments indicate the radical nature of this reaction.
H.-X. Zheng, X.-H. Shan, J.-P. Qu, Y.-B. Kang, Org. Lett., 2017, 19, 5114-5117.

Exposure of (functionalized) aryl chlorides to catalytic quantities of nickel-on-charcoal in the presence of stoichiometric amounts of Me2NH · BH3/K2CO3 in refluxing acetonitrile leads to high yields of the dehalogenated arenes.
B. H. Lipshutz, T. Tomioka, K. Sato, Synlett, 2001, 970-973.

Air- and moisture-stable (NHC)Pd(allyl)Cl complexes have been used in Suzuki-Miyaura cross-coupling, catalytic dehalogenation of aryl halides, and aryl amination. A general system involving (IPr)Pd(allyl)Cl as catalyst and NaOtBu as base has proven to be highly active for all reactions. Microwave-assisted processes have afforded yields similar to those of the conventionally heated analogous reactions.
O. Navarro, H. Kaur, P. Mahjoor, S. P. Nolan, J. Org. Chem., 2004, 69, 3173-3180.

Bromo and chloro substituents serve as excellent blocking groups on aromatic rings. The halo group can be removed by catalytic hydrogenation under neutral conditions. As expected, bromides are reduced more quickly than chlorides and the reaction requires the use of less catalyst. Bromides can be selectively reduced in the presence of nitro, chloro, cyano, keto, or carboxylic acid groups.
A. Ramanathan, L. S. Jimenez, Synthesis, 2010, 217-220.

Aryl halides are reduced into the corresponding arenes in high yields, using 2-propanol as reductant and solvent, cesium carbonate as base, and di-tert-butyl peroxide (or di-tert-butyl hyponitrite) as radical initiator. This simple system reduces various aryl bromides and iodides through a SET mechanism with high functional-group tolerance.
R. Ueno, T. Shimizu, E. Shirakawa, Synlett, 2016, 27, 747-744.

A simple and efficient Ru(II)-catalyzed transfer hydro-dehalogenation using 2-propanol as the hydride source is applicable for various aromatic halides and α-haloesters and amides. The potential synthetic application of this method was demonstrated by efficient gram-scale transformation with catalyst loading as low as 0.5 mol %.
T. You, Z. Wang, J. Chen, Y. Xia, J. Org. Chem., 2017, 82, 1340-1346.

A silicon nanoarray palladium catalyst mediates a hydrogenolysis of iodoarenes under microwave irradiation in the presence of triethanolamine as the sacrificial reductant. The reductive deiodination proceeded under an aerobic atmosphere affording the corresponding hydrogen-substituted arenes in high yields. No reaction occurred in the absence of microwaves.
Y. Matsukawa, Y. M. A. Yamada, Synlett, 2022, 33, 777-780.

Nickel-on-graphite is a very inexpensive, heterogeneous catalyst for the chemoselective reduction of aryl tosylates and mesylates. The catalyst can be used under conventional heating conditions or microwave irradiation and is recyclable without loss of activity.
B. H. Lipshutz, B. A. Frieman, T. Butler, V. Kogan, Angew. Chem. Int. Ed., 2006, 45, 800-803.

An [IrCl(cod)]2/2,6-bis(benzimidazol-2'-yl)pyridine complex catalyzes a reduction of alkyl chlorides using triethylsilane. Primary, secondary, tertiary, and benzylic C-Cl bonds are effectively dehalogenated. The tridentate N-ligand can be easily prepared in one step from commercially available reagents.
T. Fukuyama, Y. Hamada, I. Ryu, Synthesis, 2021, 53, 3404-3408.

The aqueous stability of deuterated calcium/sodium hypophosphites as nontoxic, cost-effective, and environmentally benign deuteration reagents enables deuteration of a broad range alkyl and aryl iodides with ample isotopic incorporation in aqueous (H2O) solution using 2,2′-azobis(2-methylpropionamidine) dihydrochloride as radical initiator.
Z. Song, J. Zeng, T. Li, X. Zhao, J. Fang, L. Meng, Q. Wan, Org. Lett., 2020, 22, 1736-1741.

An operationally simple, tin-free reductive dehalogenation system allows the reduction of activated C-X bonds in good yields with excellent functional-group tolerance and chemoselectivity over aryl and vinyl C-X bonds in the presence of the well-known visible-light-activated photoredox catalyst Ru(bpy)3Cl2 in combination with iPr2NEt and HCO2H or Hantzsch ester as the hydrogen atom donor.
J. M. R. Narayanam, J. W. Tucker, C. R. J. Stephenson, J. Am. Chem. Soc., 2009, 131, 8756-8757.

(Me3Si)3SiH was successfully used in various radical-based transformations in water. The system comprising substrate, silane, and initiator (ACCN) mixed in aqueous medium at 100°C worked well for both hydrophilic and hydrophobic substrates. In case of water-soluble water-soluble substrates, an amphiphilic thiol was also needed.
A. Postigo, S. Kopsov, C. Ferreri, C. Chatgilialoglu, Org. Lett., 2007, 9, 5159-5162.

The In(OAc)3-catalyzed reaction of bromo- and iodoalkanes with PhSiH3 in THF at 70 C gave dehalogenated alkanes in good to high yields in the presence of Et3B and air. 2,6-lutidine as additive enabled an efficient reduction of simple and functionalized iodoalkanes in EtOH. GaCl3 was found to be an effective catalyst for the reduction of haloalkanes with poly(methylhydrosiloxane).
K. Miura, M. Tomita, Y. Yamada, A. Hosomi, J. Org. Chem., 2007, 72, 787-792.

Nanopalladium particles supported on a amphiphilic polystyrene-poly(ethylene glycol) resin catalyzed hydrogenation of olefins and hydrodechlorination of chloroarenes under aqueous conditions.
R. Nakao, H. Rhee, Y. Uozumi, Org. Lett., 2005, 7, 163-165.

Several ureates formed by treatment of the corresponding ureas with n-BuLi activate SmI2 to a substantial extent toward dehalogenations of alkyl and aryl halides including substrates of low reactivity such as aryl fluorides.
C. E. McDonald, J. D. Ramsey, C. C. McAtee, J. R. Mauck, E. M. Hale, J. A. Cumens, J. Org. Chem., 2016, 81, 5903-5914.

The combination of HMPA and SmBr2 in THF is a powerful reductant that is capable of reducing ketimines and alkyl chlorides at room temperature. The structure of this reductant has not been established.
B. W. Knettle, R. A. Flowers, II, Org. Lett., 2001, 3, 2321-2324.

A highly efficient and catalytic organotin reagent on soluble support has been developed for free radical reactions. A variety of alkyl halides (1°, 2°, 3°, aryl) underwent radical reductions in good isolated yields using only 0.01-0.2 equiv of the polymer catalyst.
E. J. Enholm, J. P. Schulte, Org. Lett., 1999, 1, 1275-1277.

A stannane, which is simple to prepare, was successfully used in standard radical reactions as replacement of Bu3SnH and Ph3SnH. The tin-containing byproducts are removed by mild hydrolysis and extraction with aqueous NaHCO3. The performance of this new reagent was tested for reactions involving halides, selenides, Barton-McCombie deoxygenation and enyne cyclization.
D. L. J. Clive, J. Wang, J. Org. Chem., 2002, 67, 1192-1198.

Trifluoromethyl arenes were reduced with lithium aluminum hydride to give toluene derivatives in good yields in the presence of 5 mol % of niobium(V) chloride. Stepwise, partial reduction of a bis(trifluoromethyl) arene was also demonstrated.
K. Fuchibe, Y. Ohshima, K. Mitomi, T. Akiyama, Org. Lett., 2007, 9, 1497-1499.

Indium hydride generated from readily available Et3SiH and InCl3 offers mild conditions and low toxicity, and is therefore a promising alternative to Bu3SnH.
N. Hayashi, I. Shibata, A. Baba, Org. Lett., 2004, 6, 4981-4983.

N. Hayashi, I. Shibata, A. Baba, Org. Lett., 2004, 6, 4981-4983.

A mild, convenient, fast, and nonreductive deiodination is reported for ortho-iodo-hydroxylated arenes using pyridine, triethylamine, and N-methylmorpholine in the presence of water. The regioselectivity is discussed.
R. S. Talekar, G. S. Chen, S.-Y. Lai, J.-W. Chern, J. Org. Chem., 2005, 70, 8590-8593.

An easily accessible copper(I)/N-heterocyclic carbene (NHC) complex enables a regioselective allylic reduction of allylic bromides with (TMSO)2Si(Me)H as hydride source. The reaction provides aryl- and alkyl-substituted branched α-olefins in good yields, which are valuable building blocks for synthesis.
T. N. T. Nguyen, N. O. Thiel, F. Pape, J. F. Teichert, Org. Lett., 2016, 18, 2455-2458.

Zinc-promoted reduction of 2-(bromomethyl)alkenoates derived from Baylis-Hillman adducts gave (E)-2-methylacrylates in good yield and high stereoselectivity. Synthesis of the male ant pheromone (E)-2,4-dimethyl-2-hexenoic acid was performed using this simple methodology.
L. Fernandes, A. J. Bortoluzzi, M. M. Sa, Tetrahedron, 2004, 60, 9983-9989.

The use of SmI2 in the reductive elimination of 1,2-acetoxy sulfones (Julia-Lythgoe olefination) and the reductive cleavage of vinyl sulfones is reported.
G. E. Keck, K. A. Savin, M. Weglarz, J. Org. Chem., 1995, 60, 3194-3204.

Catalyst-free debrominations and dechlorinations of aromatic α-haloketones under visible light irradiation in the presence of Hantzsch ester provide products in excellent yield and tolerate a broad spectrum of substrates.
Z. Lu, Y.-Q. Yang, Synthesis, 2019, 51, 508-515.

NaBH4-TMEDA as hydride source and catalytic PdCl2(dppf) in THF is an efficient system for the hydrodehalogenation of bromo(chloro)-heteropentalenes with one or two heteroatoms, while Pd(OAc)2/PPh3 is able to reduce reactive haloheteropentalenes, and PdCl2(tbpf) allows the removal of the 2-chlorine from a thiophene ring. The reaction conditions tolerate various functional groups and allow highly chemo- and regioselective reactions.
G. Chelucci, S. Baldino, A. Ruiu, J. Org. Chem., 2012, 77, 9921-9925.

Treatment of o-bromonitrobenzenes with various vinyl Grignard reagents gives 7-bromoindoles in good yields, using the o-bromine atom to direct the cyclization. A subsequent reduction using a heteroaryl radical methodology gives 7-unsubstituted indoles in nearly quantitative yields.
A. Dobbs, J. Org. Chem., 2001, 66, 638-641.