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Reduction Chem 115 Myers Reduction Chem 115 General References • Catalytic hydrogenation is used for the reduction of many organic functional groups. The reaction can be modified with respect to catalyst, hydrogen pressure, solvent, and temperature in order to Carey, F. A.; Sundberg, R. J. In Advanced Organic Chemistry Part B, Springer: New York, 2007, execute a desired reduction. p. 396–431. • A brief list of recommended reaction conditions for catalytic hydrogenations of selected functional Brown, H. C.; Ramachandran, P. V. In Reductions in Organic Synthesis: Recent Advances and groups is given below. Practical Applications, Abdel-Magid, A. F. Ed.; American Chemical Society: Washington DC, 1996, p. 1-30. Catalyst/Compound Substrate Product Catalyst Ratio (wt%) Pressure (atm) Ripin, D. H. B. Oxidation. In Practical Synthetic Organic Chemistry; Caron, S., Ed.; John Wiley & Sons: New Jersey, 2011. Alkene Alkane 5% Pd/C 5-10% 1-3 Reactivity Trends Alkyne Alkene 5% Pd(BaSO4) 2% + 2% quinoline 1 • Following are general guidelines concerning the reactivities of various reducing agents. Aldehyde Alcohol PtO2 2-4% 1 (Ketone) Substrates, Reduction Products Halide Alkane 5% Pd/C 1-15%, KOH 1 Iminium Ion Acid Halide Aldehyde Ester Amide Carboxylate Salt Nitrile Amine Raney Ni 3-30% 35-70 Hydride Donors Adapted from: Hudlicky, M. In Reductions in Organic Chemistry 2nd Ed., American Chemical LiAlH4 Amine Alcohol Alcohol Alcohol Amine Alcohol Society Monograph 188: Washington DC, 1996, p. 8. Summary of Reagents for Reductive Functional Group Interconversions: DIBAL – Alcohol Alcohol Alcohol or Amine or Alcohol Aldehyde Aldehyde O O R OH R H NaAlH(O-t-Bu)3 – Aldehyde Alcohol Alcohol Amine – R OR' R H (slow) (slow) ester aldehyde alcohol alkane AlH3 – Alcohol Alcohol Alcohol Amine Alcohol Diisobutylaluminum Hydride Sodium Borohydride (DIBAL) NaBH4 Amine – Alcohol – – – Luche Reduction Barton Deoxygenation (NaBH , CeCl ) Lithium Triethoxyaluminohydride 4 3 Reduction of Alkyl Tosylates ** (LTEAH) NaCNBH3 Amine – Alcohol – – – Ionic Hydrogenation (slow) (Et SiH, TFA) Diazene-Mediated Deoxygenation Reduction of Acid Chlorides, 3 Amides, and Nitriles Samarium Iodide Na(AcO)3BH Amine – Alcohol Alcohol Amine – (slow) (slow) (slow) O B2H6 – – Alcohol Alcohol Amine Alcohol O O (slow) (slow) R CH3 R H R OH R OH R H R OH Li(Et)3BH – Alcohol Alcohol Alcohol Alcohol – (tertiary amide) acid alcohol aldehyde alkane acid alkane (–1C) H2 (catalyst) Amine Alcohol Alcohol Alcohol Amine – Lithium Aluminum Hydride (LAH) Wolff–Kishner Reduction Barton Decarboxylation Lithium Borohydride Reduction of Tosylhydrazones LAB – – Alcohol Alcohol Alcohol – Borane Complexes Desulfurization with Raney ** !-alkoxy esters are reduced to the corresponding alcohols. Nickel via 1,3-dithiane (BH3•L) – indicates no reaction or no productive reaction (alcohols are deprotonated in many instances, Clemmensen Reduction e.g.) Mark G. Charest, Fan Liu 1 Myers Reduction Chem 115 O R OH TESO O CH3 TESO O CH3 R OH O LiAlH4, ether O Acid Alcohol CH3O N CH3O N H –78 °C H (CH3)2N OTES N (CH3)2N OTES N Lithium Aluminum Hydride (LAH): LiAlH4 CO2CH3 CH2OH • LAH is a powerful and rather nonselective hydride-transfer reagent that readily reduces 72% carboxylic acids, esters, lactones, anhydrides, amides and nitriles to the corresponding alcohols or amines. In addition, aldehydes, ketones, epoxides, alkyl halides, and many other functional groups are reduced readily by LAH. Evans, D. A.; Gage, J. R.; Leighton, J. L. J. Am. Chem. Soc. 1992, 114, 9434-9453. • LAH is commercially available as a dry, grey solid or as a solution in a variety of organic solvents (e.g., ethyl ether). Both the solid and solution forms of LAH are highly flammable and should be stored protected from moisture. O • Several work-up procedures for LAH reductions are available that avoid the difficulties of Ph Ph separating by-products of the reduction and minimize the possibility of ignition of liberated H2. OH LiAlH4, THF OH In the Fieser work-up, following reduction with n grams of LAH, careful successive dropwise N OEt N addition of n mL of water, n mL of 15% NaOH solution, and 3n mL of water provides a H 0 ! 65 ºC H H granular inorganic precipitate that is easy to rinse and filter. For moisture-sensitive substrates, ethyl acetate can be added to consume any excess LAH and the reduction O product, ethanol, is unlikely to interfere with product isolation. 8.93 g 98% • Although, in theory, one equivalent of LAH provides four equivalents of hydride, an excess of the reagent is typically used. Becker, C. W.; Dembofsky, B. T.; Hall, J. E.; Jacobs, R. T.; Pivonka, D. E.; Ohnmacht, C. J. Paquette, L. A. In Handbook of Reagents for Organic Synthesis: Oxidizing and Reducing Reagents, Burke, S. D.; Danheiser, R. L., Eds., John Wiley and Sons: New York, 1999, p. 199-204. Synthesis 2005, 2549-2561. Fieser, L. F.; Fieser, M. Reagents for Organic Synthesis 1967, 581-595. H H • Examples O O HO O O CH3 LiAlH4 O HO N N CH3 ether H3C H3C 89-95% CH O LiAlH4 CH3O CH3 CH3 3 H H O THF O H H 70% OH O Heathcock, C. H.; Ruggeri, R. B.; McClure, K. F. J. Org. Chem. 1992, 57, 2585-2599. (+)-codeine White, J. D.; Hrnciar, P.; Stappenbeck, F. J. Org. Chem. 1999, 64, 7871-7884. O H3C H3C HOCH OCH LiAlH4 OH CH3O2C 2 3 O C(CH3)3 OH CH3O2C HOCH2 OCH3 THF, 0 ºC OH H3C H3C H O H TIPSO O >99% TIPSO LiAlH4, THF reflux 102 g H H OH H3C CO2H 72% H3C Yamaguchi, J.; Seiple, I.; Young, I. S.; O'Malley, D. P.; Maue, M.; Baran, P. S. Angew. Chem., Int. Bergner, E. J.; Helmchen, G. J. Org. Chem. 2000, 65, 5072-5074. Ed. Engl. 2008, 47, 3578–3580. Mark G. Charest, Fan Liu 2 Myers Reduction Chem 115 Lithium Borohydride: LiBH4 Borane Complexes: BH3•L • Lithium borohydride is commonly used for the selective reduction of esters and lactones to the • Borane is commonly used for the reduction of carboxylic acids in the presence of esters, corresponding alcohols in the presence of carboxylic acids, tertiary amides, and nitriles. lactones, amides, halides and other functional groups. In addition, borane rapidly reduces Aldehydes, ketones, epoxides, and several other functional groups can also be reduced by aldehydes, ketones, and alkenes. lithium borohydride. • Borane is commercially available as a complex with tetrahydrofuran (THF) or dimethysulfide in • The reactivity of lithium borohydride is dependent on the reaction medium and follows the solution. In addition, though highly flammable, gaseous diborane (B H ) is available. order: ether > THF > 2-propanol. This is attributed to the availability of the lithium counterion 2 6 for coordination to the substrate, promoting reduction. • The borane-dimethylsulfide complex exhibits improved stability and solubility compared to the • Lithium borohydride is commercially available in solid form and as solutions in many organic borane-THF complex. solvents (e.g., THF). Both are inflammable and should be stored protected from moisture. • Competing hydroboration of carbon-carbon double bonds can limit the usefulness of borane- THF as a reducing agent. Nystrom, R. F.; Chaikin, S. W.; Brown, W. G. J. Am. Chem. Soc. 1949, 71, 3245-3246. Lane, C. F. Chem. Rev. 1976, 76, 773-799. Banfi, L.; Narisano, E.; Riva, R. In Handbook of Reagents for Organic Synthesis: Oxidizing and Reducing Reagents, Burke, S. D.; Danheiser, R. L., Eds., John Wiley and Sons: New York, Brown, H. C.; Stocky, T. P. J. Am. Chem. Soc. 1977, 99, 8218-8226. 1999, p. 209-212. • Examples • Examples O 1. BH3•THF, 0 °C O O O H CH3 2. dihydropyran, THF H CH3 F TsOH, 0 °C O2N Br CO2H Br CH2OTHP O CO2CH3 H LiBH , CH OH 86% N OTBS 4 3 N THF, Et O, 0 °C H 2 Corey, E. J.; Sachdev, H. S. J. Org. Chem. 1975, 40, 579-581. O H C CH 3 3 SO CH SO CH 83% 2 3 2 3 O HO F HO BH •THF, 0 °C O2N OH 3 O EtO2C EtO2C H THF, 98% Laïb, T.; Zhu, J. Synlett. 2000, 1363-1365. N OTBS N H O 500 g Br Br H3C CH3 Lobben, P. C.; Leung, S. S.-W.; Tummala, S. Org. Process Res. Dev. 2004, 8, 1072–1075. • The combination of boron trifluoride etherate and sodium borohydride has been used to generate diborane in situ. O LiBH4 THF, i-PrOH CO2H CH2OH H3C OEt H3C OH 15 ºC, 100% CO2H CO2H NaBH4, BF3•Et2O 450 g THF, 15 °C HN SO 2 95% HN SO2 Hu, B.; Prashad, M.; Har, D.; Prasad, K.; Repic, O.; Blacklock, T. J. Org. Process Rev. Dev. 2007, 11, 90–93. Miller, R. A.; Humphrey, G. R.; Lieberman, D. R.; Ceglia, S. S.; Kennedy, D. J.; Grabowski, E. J. J.; Reider, P. J. J. Org. Chem. 2000, 65, 1399-1406. Brown, H. C.; Tierney, P. A. J. Am. Chem. Soc. 1980, 80, 1552–1558. Mark G. Charest, Fan Liu 3 Myers Reduction Chem 115 O O R OR' R H O OMOM H Ester Aldehyde H C N O 3 TMS O MOMO Diisobutylaluminum Hydride (DIBAL): i-Bu2AlH CH3 DIBAL, THF OMOM H3C CH3 CH • At low temperatures, DIBAL reduces esters to the corresponding aldehydes, and lactones to O 3OAc OAc O O –100 ! –78 °C lactols. • Typically, toluene is used as the reaction solvent, but other solvents have also been employed, including dichloromethane. CH3 CH3 CH3 CH3 CO2CH3 O O Miller, A. E. G.; Biss, J. W.; Schwartzman, L. H. J. Org. Chem. 1959, 24, 627-630. Zakharkin, L. I.; Khorlina, I. M. Tetrahedron Lett. 1962, 3, 619-620. • Examples O OMOM H CO CH CHO H C N O 2 3 3 TMS O DIBAL, toluene O N –78 °C N O Boc Boc MOMO CH3 H3C H3C O OMOM H3C CH3 CH3 CH3 OMOM H CH 76% H C N O O 3OAc OAc O O 3 TMS O Garner, P.; Park, J.
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