Indium (low valent)
Low-valent metal compounds are commonly used as reducing agents. With its most stable oxidation state as 3+, indium is used in the 2+ oxidation state as an one-electron reducing agent. Indium hydrides such as InCl2H, generated from readily available Et3SiH or NaBH4 and InCl3, offer mild conditions and low toxicity, and are therefore promising alternatives to Bu3SnH.
An Et3SiH-promoted diastereoselective reductive aldol reaction has been developed using InBr3 as a catalyst. This three-component reaction afforded only silyl aldolates as products without any side reactions.
I. Shibata, H. Kato, T. Ishida, M. Yasuda, A. Baba, Angew. Chem. Int. Ed., 2004, 43, 711-714.
Oxidative addition of In/InCl3 to enones proceeds exclusively in aqueous media and leads to water-tolerant, ketone-type indium homoenolates. The synthetic utility of the indium homoenolates was demonstrated through the synthesis of 1,4-dicarbonyl compounds via palladium-catalyzed coupling with acid chlorides.
Z.-L. Shen, K. K. K. Goh, H.-L. Cheong, C. H. A. Wong, Y.-C. Lai, Y.-S. Yang, T.-P. Loh, J. Am. Chem. Soc., 2010, 132, 15852-15855.
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.
A direct reduction of alcohols to the corresponding alkanes using chlorodiphenylsilane as hydride source in the presence of a catalytic amount of InCl3 showed high chemoselectivity for benzylic alcohols, secondary alcohols and tertiary alcohols while not reducing primary alcohols and functional groups that are readily reduced by standard methods such as esters, chloro, bromo, and nitro groups.
M. Yasuda, Y. Onishi, M. Ueba, T. Miyai, A. Baba, J. Org. Chem., 2001, 7741-7744.
Indium tri(isopropoxide)-catalyzed Meerwein-Ponndorf-Verley reduction of aliphatic and aromatic aldehydes in 2-propanol gave selectively the corresponding primary alcohols in good to excellent yields at room temperature. The reaction tolerates a wide range of functional groups including alkene, ether, ketone, ester, nitrile, and nitro.
J. Lee, T. Ryu, S. Park, P. H. Lee, J. Org. Chem., 2012, 77, 4821-4825.
The combination of 1,1,3,3-tetramethyldisiloxane (TMDS) and trimethylbromosilane (Me3SiBr) enabled a direct bromination of carboxylic acids in the presence of indium bromide (InBr3) as catalyst. The reducing system was tolerant to several functional groups and produced the corresponding alkyl bromides in very good yields.
T. Moriya, S. Yoneda, K. Kawana, R. Ikeda, T. Konakahara, N. Sakai, Org. Lett., 2012, 14, 4842-4845.
A novel one-pot procedure for a directly reductive conversion of esters to the corresponding ethers by Et3SiH in the presence of a catalytic amount of InBr3 is described. This simple catalytic system appeared to be remarkably tolerant to several functional groups.
N. Sakai, T. Moriya, T. Konakahara, J. Org. Chem., 2007, 72, 5920-5922.
Indium hydride (Cl2InH) was generated by the transmetalation of InCl3 with Et3SiH. In the previously reported system (NaBH4-InCl3), the coexistent borane can cause side reactions. The use of Et3SiH instead of NaBH4 affords effective hydroindation of alkynes. N. Hayashi, I. Shibata, A. Baba, Org. Lett., 2004, 6, 4981-4983.
Dichloroindium hydride (Cl2InH) generated in situ from the combination of a catalytic amount of indium(III) chloride and sodium borohydride in acetonitrile reduces activated vic-dibromides to the corresponding (E)-alkenes in excellent yields.
B. C. Ranu, A. Das, A. Hajira, Synthesis, 2003, 1012-1014.
Simple and mild indium- and zinc-mediated dehalogenation reactions of vicinal dihalides in an aqueous solvent enable the synthesis of various allenylmethyl aryl ethers and monosubstituted allenes in very good yields.
M.-H. Lin, W.-S. Tsai, L.-Z. Lin, S.-F. Hung, T.-H. Chuang, Y.-J. Su, J. Org. Chem., 2011, 76, 8518-8523.
A novel, convenient and stereoselective synthesis of trisubstituted E-alkenones has been achieved by InCl3-mediated chemoselective reduction of Baylis-Hillman adducts with NaBH4 as reductant.
B. Das, J. Banerjee, N. Chowdhury, A. Majhi, H. Holla, Synlett, 2006, 1879-1882.
The use of Et3SiH leads to fewer side reactions in the intramolecular cyclization of enynes when compared to the NaBH4-InCl3 system reported previously.
N. Hayashi, I. Shibata, A. Baba, Org. Lett., 2004, 6, 4981-4983.
An indium(III) hydroxide-catalyzed reaction of carbonyls and chlorodimethylsilane afforded the corresponding deoxygenative chlorination products. Ester, nitro, cyano, or halogen groups were not affected during the reaction course. Typical Lewis acids such as TiCl4, AlCl3, and BF3ˇOEt2 showed no catalytic activity. The reaction mechanism is discussed.
Y. Onishi, D. Ogawa, M. Yasuda, A. Baba, J. Am. Chem. Soc., 2002, 124, 13690-13691.
Organic azides are easily and chemoselectively reduced to the corresponding amines by reaction with dichloroindium hydride under very mild conditions. γ-Azidonitriles give pyrrolidin-2-imines in an outstanding cyclization.
L. Benati, G. Bencivenni, R. Leardini, D. Nanni, M. Minozzi, P. Spagnolo, R. Scialpi, G. Zanardi, Org. Lett., 2006,8, 2499-2502.
A three-component reaction of nitro compounds, carbonyl compounds, and phosphites in the presence of indium in dilute aqueous HCl enables a high-yielding synthesis of α-amino phosphonates at room temperature. This one-pot conversion consists of reduction of nitro compounds to amines, formation of imines, and hydrophosphonylation.
B. Das, G. Satyalakshmi, K. Suneel, K. Damodar, J. Org. Chem., 2009, 74, 8400-8402.
An efficient synthetic method provides tri- and tetra-substituted allenes by the reaction of allylindium reagents with 3°-propargyl alcohols. Allylindium reagents are generated in situ from indium and allyl bromides.
K. Lee, P. H. Lee, Org. Lett., 2008, 10, 2441-2444.
Reactions of various carbonyl compounds with organoindium reagent in situ generated from indium and 1-bromopent-4-en-2-yne derivatives gives functionalized vinyl allenols in good yields. Treatment of vinyl allenols with gold catalyst, dienophile, or indium trihalide produced functionalized dihydrofuran, cyclohexene, or 2-halo-1,3-diene derivatives in very good yields.
J. Park, S. Hong, P. H. Lee, Org. Lett., 2008, 10, 5067-5070.
Pd-catalyzed cross-coupling reactions of electron-rich and electron-poor aryl iodides with organoindium reagents generated in situ from indium and ethyl 4-bromo-2-alkynoates produced selectively ethyl 2-aryl-2,3-alkadienoates in good yield.
P. H. Lee, J. Mo, D. Kang, D. Eom, C. Park, C.-H. Lee, Y M. Jung, H. Hwang, J. Org. Chem., 2011, 76, 312-315.
Sulfoxides and aza-aromatic N-oxides were deoxygenated using a system of indium and pivaloyl chloride at room temperature to give the corresponding sulfides and aza-aromatics in high yields.
E. S. Park, S. Hwan, Lee, J. H. Lee, H. J. Rhee, C. M. Yoon, Synthesis, 2005, 3499-3501.