Chem 634 Fall 2013 Ketone, Imine, and Related Reductions Prof. Donald Watson " Assistant Professor" " " Reducant Cheat Sheet C=X reductants comment/electophile iminium ion acid chloride aldehyde/ketone LiAlH4 (LAH) Very strong, low solubility in tol amine ROH ROH NaAlH2(OCH2CH2OMe)2 (Red-Al) Very strong, soluble amine –– ROH LiAlH(OEt)3 Weaker than LAH –– –– –– NaAlH(OtBu)3 Weaker yet amine slow to ROH –– NaBH4 Moderate amine ROH ROH LiBH4 More reactive than Na verison amine –– ROH NaBH(OAc)3 Weaker than NaBH4, more selective amine –– slow to ROH NaCNBH3 even more so amine –– slow to ROH LiBHEt3 (Super-Hydride) Very strong reductant –– ROH ROH (iBu)2AlH (DIBAL or DIBAL-H) electrophilic –– ROH ROH BH3•L (L = THF or DMS) or B2H6 electrophilic –– –– ROH H2/ cat hydrogenation amine ROH or aldehyde ROH –– = not product or not commonly used combination n/r = no reaction, ROH = alcohol, RHO = aldehyde ester amide carboxylate ROH amine ROH ROH amine ROH alcohol RHO (3° amide) –– slow to ROH slow to amine –– slow to ROH n/r –– ROH –– slow to ROH n/r slow to ROH –– n/r n/r –– ROH RHO (3° amide) –– ROH or RHO amine or RHO ROH slow to ROH slow to amine ROH (fast) ROH amine –– nitrile amine amine RHO –– –– –– –– –– likely red. RHO –– amine Adapted from Carey and Sunburg, 5th Ed. A Closer Look At One Series H Al LiAlH4 H small, inorganic... not very soluable upto 4 "H–" H H H H NaAlH2(OCH2CH2OMe)2 "Red-Al" + AlH H O HO Me Al- MeO O H H OMe O more organic soluable, but a little less reactive H H LiAlH(OEt)3 + AlH H AlHO Me Me O H O Me O weaker than LAH, but only 1 "H—" Me H H NaAlH(OtBu)3 + AlH H H Me Me Me HO Me Me Me O Me Me Me O Me O Me Me Al- steric bulk makes it weake yet, only 1 "H—" General Mechanism (Nucleophilic) w/ LAH Li H Li O R H Al H H O R R w/ NaBH4 Na R O R AlH3 OH H H R R H R Na BH3 H O R BH3 H R OH H R H R metal ion activates carbonyl four coordinate Al and B are nucleophilic anions Examples of Nuc. Red. (Not Inclusive) O NMe2 LAH NMe2 HO Me LiBH4 MeO2C CO2H Me OH HO Me HO CO2H O N Me Bu Me O LiAlH(OEt)3 then HCl TFA 99% de H Bu Me 99%ee Refer to chart. Electrophilic Reductants Me O R Carbonyl activated by Lewis Acid Me R O R R H R Al Me H O R R H Al Me Three coordinate Al and B are electrophilic Al activated by Lewis base Me H R Al H+ Me H Me OH R R H + Me Me Me Note: NOT "H" from Al-H that is the reductant! Partial Reductions O O OMe O N Me Me DIBAL -78 °C BOC H O N Me Me BOC note proximal Lewis Base O R OMe N Me Weinreb amide DIBAL R O AlH OMe R N Me good way to make aldehydes H O R H Reduction of Nitriles O DIBAL R C N R H BH3 R O BH3 O OH R H B H O H O R H + HOBH2 Note: hydroboration competes when alkenes present! Hydrogenations Adam's cat. 2% PtO2 H2 O Pr H R OH R NH2 Raney Ni R C N O R H2 O Pd/ BaSO4 Cl R poisoned cat. (Rosemond Reduction) H Reductive Amination Recall: Direct alkylation of amines often leads to completive overalkylation. R NH2 R'X R' R NH base + R' R N R' + R' R N X R' R' Solution: Reductive Amination Aldehyde or ketone O Recall R R'NH2 amine: 1° or 2° H +H - H2O H R NaHBR3 N R' H iminium ion Red: NaBH4 cheap Na(CN)BH3 or NaHB(OAc)3 more selective R' H N R Reductive Amidation O MeNH2 R O Cl R LAH N H Me harsh! R N H Me Meerwein - Ponndorf - Verley (MPV) Reduction OH O R R R' H Me Me R' Al(OiPr)3 OR O Al OR OH Me R R' Me R Me O O + R' H Me Me Me O O OR Al OR Reduction of Ketone to Alkanes Classical Conditions: All Very Harsh! Wolfe-Kishner H2N NH2 , NaOH O R R' H H R R' Clemmenson Reduction Zn (Hg) O HCl H H R R' R R' EtS SEt Raney Ni H H R R' H2 R R' Also: Tosyl Hydrazone Reduction O R H2N NHTs R' N -H2O R Ts NH R' tosyl hydrazone N R Ts NH R' B HN NaBH3CN HX R N Me - HO R H NHTs H R' BH4 R NH R' -N2 H N SO (tol) 2 N H H H R R' R S O sulfinic acid Last step involves radical intermediates – high energy Be Aware O R H2NNHTs R' R then NaBH3CN N N H H R' R allylic transposition R' Diasteroselective Reductions (and Additions) O Me "H-" H Me Me Me OH vs Me Me small OH Me H Me Me axial equat LAH 92 8 NaBH4 80 20 7 93 Me Me very large 3 "L-Selectride" Me Me Me BHLi O > 4 kcal/mol Me MeMe H H favored H H O Model for Small Reducing Agents: Torsional Strain O Me Me Me H H O H H H H HO H H vs H H δ− H torsional motion results in eclipsing interaction H H H axial H H O H OB δ− H O No torsional strain H equatorial Favored for small nucs. OB blue arrow = atomic motion of oxygen atom as carbon goes from sp2 to sp3 Model for Large Reductants R3BH Me Me Me O vs Me O Me Me R3BH Need to know how the nucleophiles approach carbonyl. Burgi-Dunitz Angle FMO's of C O π* π C O C O 110° Approach of Nucleophiles C O Nuc110° Model for Large Reductants: Developing Diaxial Interactions R3BH O Me vs Me Me Me Me R3B Me Me Me H H H O Me R3BH O Me vs O Me Me R3B H favored developing 1,3 diaxial interactions OH Me H Me Me major Steric Interactions Can Override Me Me O Me Me H Me Me Me vs OH Me Me NaBH4 17 42 83 58 L-select. 0.2 99.8 LAH all favor equatorial attack Me Me Me Me O H > 4 kcal/mol Me Me O OH H Similar for Carbon Nucleophiles Me O Me Me Nuc Me OH Me Me Nuc = H C C Li EtMgBr iPrMgBr tBuMgBr 88 53 18 0 vs OH Me Nuc Me Me 12 47 82 100 Acyclic Stereocontrol in Additions to Carbonyls O RL H R HO Nuc RL RM Nuc R RM HO or RL Nuc R RM More complex as there sigma bond rotation can occur! O RL R H RM Consider case where three substituents at alpha carbon all differ in size. Felkin - Ahn model O RL R RM Assumption: Will add away from largest substituent. Limits problem to two conformations that much be considered. (Note you MUST get stereochemistry correct in Newman projections.) RL RL R O RM H vs O R RM H Note these not lowest energy conformers, but most reactive conformers. Felkin – Ahn Model RL RL R O RM H vs O R RM H RL Think about FMO’s. RL R O RM H vs R O RM Nuc— And Burgi-Dunitz Angle H Nuc— disfavored favored Least sterically demanding approach! RL HO RL Nuc R RM RL R OH RM Nuc H R HO RM H Nuc HO RL Nuc R RM favored Felkin – Ahn Model Example: O Ph H H Me HO 0 °C Ph Me LAH 74 : 26 L-sel. 99 : 1 Ph H Me Me Me Ph O H Me H HO Me Me H H Note: Ratio (dr) implies favored diastereomer (shown) vs unfavored (not shown). It is very common to only show one product and assume the reader understands the chemistry well enough to predict the other product. Yamamoto JACS, 1998, 110, 4475 Similar for Carbon Nucleophiles O Ph MeLi Me Me –78 °C HO Ph Me Me Me Ohno, JACS 1988, 110, 4826 Cram Chelation Control O BOMO R Me H R Me (98 : 2) Not Felkin! Me Me R R Me –10 °C Me O RO BOMO O BOM = M HO LAH M O O R R H M O O R Lewis base chelates metal with carbonyl R H HO RO H Nuc Nuc • Lewis Basic groups make good chelators. • Examples: BnO, MeO, BOMO, MOMO, NR2, etc. Overman TL, 1982, 2355 Reetz, Acc. Chem. Res. 1993, 26, 462 Polar Felkin-Ahn A values: O tBuPh 2SiO LAH R Me –10 °C OH H tBuPh SiO 2 R Me 95 : 2 Me = 1.7 kcal/mol OSiMe3 = 0.74 kcal/mol H R' C H H Not Chelate! Note: Silyl ethers are not good Lewis Bases (nO -> σ*Si-C) R R Si R R' O trans?!? H H O Me Arguement: Low lying OSiR3 OSiR3 H H Nuc σ*C-OSiR3 O stablizes developing Me σ Nuc–C Nuc Proposed TS σ*C-OSiR3 At TS: σ Nuc–C σ*C-OSiR3 Occurs with highly electronegative alpha substituents that can not chelate, such as OSiR3, Cl, F, etc. Summary of Felkin-Like Models alpha stereocenter model steric only Felkin-­‐Ahn chela2ng Cram electronega2ve, but non-­‐chela2ng Polar Felkin 1,3 Control OH O Syn Reduction: R R' Et then workup Et -MeOH R' Et Et H B O R' OH OH R B OH O R Et2BOMe NaBH4 O R O R' R R O O O R' R' Et B Et cyclohexene conformational analysis H R R H H H H H R Narasaka Chem Lett, 1990, 1415 Model for Diastereoselectivity "H " H R R' H Et B O O R Et Et B Et R O O O O R' R Et O R' R' "born into" chair Et B R Et H "H " OH OH H R Et H+ B O H Et B Et H chair-like TS favored H R' O O R' axial addition R H H twist-boat-like TS O R' O Et B OH OH H+ Et R H "born into" twist-boat disfavored As carbonyl carbon rehybridizes from sp2 to sp3 it “moves” to meet incoming nucleophile. Conformational analysis of the TS’s based upon this motion predict selectivity. R' 1,3 Control NaHB(OAc)3 OH O Anti Reduction: R OH OH R' R AcO NaHB(OAc)3 OH O R vs 1,3 diaxial interaction worse with R than O R' R' OAc Na B O H O R H OAc O R O B OAc H R favored then H+ H OAc R R O B OAc H O disfavored then H+ R' H OH R OH H R anti HR R HO OH H syn Evans JACS, 1988, 110, 3560 Enantioselective Reductions and Additions to C=X Me Me B From: H Me Alpine - hydride (Aldrich) chiral borohydride Me Me Me α−pinene (single enantiomer) Midland Chem Rev. 1989, 89, 1553 DIP-Cl O Me Me OH DIP-Cl R BCl R DIP-Cl Me > 98% ee Cl Me B Me H H Rs Me 2 R O RL Akin to a chiral DIBAL. HC Brown (Nobel 1979) Brown, TL, 1991, 32, 6691 Corey - Bakhi – Shibata (CBS) Catalyst cat. 1, BH3 O Ph H Me Ph N B Ph O Me H H B H H OH Ph Me H Ph O N Ph O B H2B Me H R Ph Ph O N B 1 Me H Ph N R' H2B H Ph O B Me O R R' Note primarily electronic differentiation between R & R’. Corey (Nobel 1990) Corey JACS, 1987, 109, 7925 Functionalized Ketone Hydrogenation Using Ruthenium Catalysts Hydrogenation of α-ketoesters: OH O OMe O Me OMe RuCl2(S-BINAP) 100 atm H2 Me O 93% ee • Note: This is NOT a simple isolated ketone. PPh2 PPh2 Noyori (Nobel 2001) Noyori β-Ketoester Hydrogenation O Me O O OH O [RuCl2(R-BINAP)]2 OMe Me 100 atm H2 O OMe OH O OMe [NH2Me2][{RuCl[(S)-SEGPHOS]}2(m-Cl)3] OMe 30 atm H2 98% ee O PPh2 PPh2 O O PPh2 PPh2 O BINAP Noyori (Nobel 2001) SEGPHOS Noyori JACS 1987, 109, 5856. Substrate Acts As A Chelate Ligand Cl P * Ru P Solv Solv Cl –H+ Cl H+ P * H2 +H2 Ru P O Solv R Solv OMe H Cl Cl P * Ru P OH P Solv * Solv H Cl O P OMe Ru P H * R O * P Ru OMe O O * H R OMe O O R Substituted β-Ketoester Hydrogenation O OH O O OMe NH2•HCl [RuCl2(R-BINAP)]2(DMF)n OMe NH2•HCl 30 atm H2 96% ee, 95% de O OH O O OMe [RuCl2(R-BINAP)]2(C6H6) OMe 100 atm H2 93% ee, 98% de • Why does this work so well? O O R OH O OMe R' R OMe R' Dynamic Kinetic Resolution Other Related Chelating Substrates β-Aminoketones: OH [RuCl2(S-BINAP)](cymene) O NMe2 Me 105 atm H2 Me NMe2 99.4% ee β-Diketones: O Me O OH OH [RuCl2(R-BINAP)]2(C6H6) Me 100 atm H2 Me Me >99% ee and de Note: These substrates doubly reduced to the trans diol. Dialkyl Zinc Additions Me Me NMe2 O Ph H aldehydes only Me OH Me ZnEt2 HO Ph Et H 98%, 99%ee Me Me N Me Zn Et Me O O H Zn Et H • Many chiral amino alcohols work. • Specific to R2Zn. Ph Et Noyori JACS 1986, 108, 6071 Review: Evans Science 1988, 240, 420 Cyanohydrin Formation cat. O P Ph2 O Cl Al O Ph2 P O O + Ph H TMSO CN TMSCN Ph H 97%, 97% ee allyl, vinyl, alkyl Lewis Base Activation Bifunctional Catalyst: P O Al Lewis Acid Activation SiMe3 CN H O R Shibasaki JACS, 1999, 121, 2641 Asymmetric Alkyne Addition OH O H H cat Zn(OTf)2, Et3N 96%ee cat. Ph Me HO NMe2 Carreira JACS 2001, 123, 9687 Chiral Amine Synthesis Using Ellman Auxiliary O O O R H2N H S N tBu CuSO4 R S O O tBu H2N H S = H2N tBu S tBu O O N R S H R'MgBr HN tBu R' = Ph, Me, Et, iPr, etc R S tBu R' H NH2 R R' >90% dr RS Me S N RL M R' O Me Me Ellman ACR, 2003, 35, 984 Catalytic Asymmetric Additions to Imines N Ph Ts ArB(OH)2 3% Rh(acac)(coe)2 K3PO4 Ph 20% Ph2P HN Ph Ts Ar 94% 95% ee N PPh2 Similar reactions with nucleophiles bases on Zn, Sn, Ti, etc Ellman, ACIE, 2008, 47, 5623 Reductions of α,β-Unsaturated Systems R OR DIBAL R OH O R R' O cis/ trans NaBH4 R CeCl3 Luche Reduction (w/o CeCl3 1,4 reduction competative) R' OH Catalytic Hydrogenation R R' O cis or trans H2 Pd/C R R' O Note: hydrogenation of trans and/or electron deficient alkenes can be slow Dissolving Metal Reductions O Na or Li NH3 O -33 °C then H+ Mech O M(0) O M O M NH3 -NH2– O M(0) H O M H H2O/ H O H H Stryker Reagent O O [PPh3 CuH] H then H H also acyclic O O "H–" [Cu] H Stryker JACS, 1988, 110, 291 Asymmetric Stryker Reaction O n cat. CuCl cat. (ptol)-Binap R n = 1-3 PHMS = PHMS O R H up to 98%ee n Me O Si O H N Limited to cyclic enones for good ee. Buchwald JACS, 2000, 122, 6787 Conjugate Addition of Cuperates O R2CuLi O R Review see C&SB 8.1/ 8.7 Asymmetric Version O Et2Zn cat. Cu(OTf)2/ L O Et >98% ee L= Ph O P N O Ph Me Me Feringa ACIE 1997, 36, 2620 Asymmetric Addition of Boronates R R' O Ar B(OH)2 cat. Rh(I)/BINAP R R' Ar O also vinyl Hayashi Chem Rev, 2003, 103, 2829 Radical Reductions Barton McCombie Deoxygenation AIBN = S R OH R O Bu3SnH AIBN R' NC R H Me (thioxoester) CN N N Me Me Me Azobisisobutyronitrile Typical Conditions: R OH S NaH, CS2, MeI R O SMe S Cl R OPh S OH R O Barton: Nobel 1969 OPh Barton Perkin Trans I, 1975, 1574 Barton McCombie Mechanism Radical Initiation: NC Me N N Me CN CN N N Me Me Me Me CN + Me Me 2 Me Note: AIBN breaks down in two steps, not one. Bu3Sn H + Me2CCN Bu3Sn + Me2CHCN Hydrogen atom abstraction CN Me + N2 Baron Decarbonylation S N O O OH R OH Std coupling reagent R O Bu3SnH N R H AIBN S Mech: O R O O N R S Bu3Sn O R N S SnBu3 + O C O + N S SnBu3 Barton Chem Commun. 1983, 939 Radical Dehalogenation Bu3SnH AIBN R R' X R R' H I > Br > Cl 3° > 2° >> 1° (radical stability) Also SmI2 can be used here (Kagan JACS, 1980, 102, 2693) Barton McCombie Mechanism Radical Chain: R R R Bu3Sn Bu3Sn + Bu3Sn O R' S R' + O R R S Bu3Sn R' O R' O S S R3Sn R H + Bu3Sn H R' O R Bu3Sn R R' + O S S R3Sn Misc. Alkyl Halide Reduction and Related OTs H LiEt3BH R X X = I > Br + H2 Pd/ C 80% 20% SN2 E2 R H SmI2 Reduction of Ketones SmI2 O HO H H2O CO2Et CO2Et SmI2 H O H2O HO SmI2 O H + Sm(III) CO2Et + Sm(III) Chem Rev 1992, 92, 29