Reductions - REDUCTIONS Carey & Sundberg Chapter 5...

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REDUCTIONS 30 Carey & Sundberg Chapter 5 problems: 1a,b,c,d,f,h,j; 2; 3a-g, n,o; 4b,j,k,l; 9; 11; Smith: Chapter 4 March: Chapter 19 Reductions 1. Hydrogenation 2. Boron Reagents 3. Aluminium Reagents 4. Tin Hydrides 5. Silanes 6. Dissolving Metal Reductions Hydrogenations Heterogeneous Catalytic Hydrogenation Transition metals absorbed onto a solid support metal: Pd, Pt, Ni, Rh support: Carbon, alumina, silica solvent: EtOH, EtOAc, Et 2 O, hexanes, etc. - Reduction of olefins & acetylenes to saturated hydrocarbons. - Sensitive to steric effects and choice of solvent - Polar functional groups, i.e. hydroxyls, can sometimes direct the delivery of H 2 . - Cis addition of H 2 . R 2 R 1 R 1 R 2 H R 2 H R 2 R 1 R 1 H 2 , Pd/C - Catalyst can be "poisoned" - Directed heterogeneous hydrogenation O O MeO OH H 2 , Pd/C O O MeO OH H (86 : 14) O O MeO CO 2 Me O O MeO CO 2 M 2 H 2 , Pd/C H Lindlar Catalyst ( Pd/ BaSO 4 / quinoline)- partially poisoned to reduce activity; will only reduce the most reactive functional groups. acetylenes + H 2 , Pd/BaSO 4/ quinoline cis olefins (Lindlar Reduction) Acid Chlorides + H 2 , Pd/BaSO 4 Aldehydes (Rosemund Reduction) Org. Rxn. 1948 , 4 , 362 R SiMe 3 R SiMe 3 O CO 2 Me O CO 2 Me H 2 , Pd/BaSO 4 , Quinoline TL 1976 , 1539 H 2 , Pd/BaSO 4 , pyridine JOC 1982 , 47 , 4254
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REDUCTIONS 31 HO H H H HO OH HO OH HO OH OH H 2 , Lindlar Catalyst CH 2 Cl 2 : MeOH: Quinoline (90:9.5:0.5) 6 π e - disrotatory 8 π e - conrotatory JACS 1982 , 104 , 5555 Ease of Reduction: (taken from H.O. House Modern Synthetic Reactions , 2nd edition) R NO 2 Ar O R R' R CHO R COCl R CH CH R' R R' O R C N R R R OR' O N O R' R' R CO 2 - Na + R CHO R NH 2 R R CH 2 -OH R' R CH 2 CH 2 R' R R' HO H Ar CH 3 HO R R CH 2 NH 2 R CH 2 -OH HO R' R CH 2 N R' R' + + no reaction requires high Raney Nickel Desulfuriztion , Reviews: Org. Rxn. 1962 , 12 , 356; Chem. Rev. 1962 , 62 , 347. R O R R S R (CH 2 ) n S R H R H Raney Nickel
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REDUCTIONS 32 H S S HO O O O H HO O O O JOC 1987 , 52 , 3346 EtOH (74%) Raney Nickel Homogeneous Catalytic Hydrogenation - catalyst is soluble in the reaction medium - catalyst not "poisoned" by sulfur - very sensitive to steric effects - terminal olefins faster than internal; cis olefins faster than trans R R R R R R R R R R R R R R >> > > > > - (Ph 3 P) 3 RhCl (Wilkinson's Catalyst); [R 3 P Ir(COD)py] + PF 6 - (Crabtree's Catalyst) OH OH JOC 1992 , 57 , 2767 C 6 H 6 (92%) (Ph 3 P) 3 RhCl, H 2 Directed Hydrogenation Review: Angew. Chem. Int. Ed. Engl. 1987 , 26 , 190 - Diasterocontrolled hydrogenation of allylic alcohols directed by the -OH group MeO O - K + (Ph 3 P) 3 RhCl, H 2 O Rh H PPh 3 PPh 3 H MeO O - K + H Rh + P P Ph Ph Ph Ph Ir + P(C 6 H 11 ) 3 N (Crabtree's Ctalyst) JACS 1983 , 105 , 1072 Brown's Catalyst PF 6 - BF 4 - Regioselective Hydrogenation- allylic and homoallylic alcohols are hydrogenated faster than isolated double bonds OH MeO 2 C OH MeO 2 C
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REDUCTIONS 33 mechanism: L M L H 2 lose + L M S S L + OH L M O H L + H
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This note was uploaded on 03/11/2012 for the course CHEM 233 taught by Professor Staff during the Fall '10 term at Vanderbilt.

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Reductions - REDUCTIONS Carey & Sundberg Chapter 5...

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