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Stereoselective Oxidation and Reduction Reactions

Stereoselective Oxidation and Reduction Reactions. Dr Simon Woodward School of Chemistry, University of Nottingham. Epoxidation Epoxide opening Dihydroxylation Aminohydroxylation Alcohol oxidation. Oxidation Reactions. “ Click Chemistry: Diverse Chemical Function

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Stereoselective Oxidation and Reduction Reactions

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  1. Stereoselective Oxidationand Reduction Reactions Dr Simon Woodward School of Chemistry, University of Nottingham

  2. Epoxidation Epoxide opening Dihydroxylation Aminohydroxylation Alcohol oxidation Oxidation Reactions “Click Chemistry: Diverse Chemical Function from a Few Good Reactions” Kolb, Finn, Sharpless, Angew. Chem. Int. Ed. 2001, 40, 2004.

  3. Sharpless Asymmetric Epoxidation of Allylic Alcohols Review: Katsuki and Martin, Org. React., 1996, 48, 1. •tBuOOH, Ti(OiPr)4, DET, CH2Cl2, 4Å MS Sharpless, J. Am. Chem. Soc., 1987, 109, 5765 . Mechanism: J. Am. Chem. Soc., 1991, 113, 106 and 113. • Hydroxamic acid ligands for Vanadium-catalysed asymmetric epoxidation of allylic alcohols: • Yamamoto, J. Am. Chem. Soc. 2000, 122, 10452.

  4. Chiral Mn(salen) Catalysts: Overview Review: Katsuki Coord. Chem.Rev. 1995, 140, 189. Stoichiometric co-oxidants: Usually aq. NaOCl, CH2Cl2 mCPBA / NMO (low temperature) Preparation of catalyst: Organic Syntheses, 1998, 75, 1. Polymer supported catalyst: e.g. Janda, J. Am. Chem. Soc., 2000, 122, 6929. Cis-Disubstituted alkenes: J. Am. Chem. Soc. 1991, 113, 7063. Trisubstituted alkenes: J. Org. Chem. 1994, 59, 4378. Tetrasubstituted alkenes: Tetrahedron Lett. 1995, 36, 5123. Cinnamate esters: Tetrahedron1994, 50, 4323. • Poor enantioselectivities for trans-disubstituted and terminal alkenes] • (but see Katsuki, Synlett, 2000, 1557) • Via radical intermediate, so stereospecificity with respect to alkene geometry sometimes eroded. • Can use to make trans-epoxides from cis-alkenes: Jacobsen, J. Am. Chem. Soc.1994, 116, 6937. • Asymmetric epoxidation of E-alkenes using Cr(salens): Gilheany, Org. Lett. 2001, 3, 663, and refs. therein

  5. Dioxiranes • Electrophilic oxidants, but successful for epoxidation of electron poor alkenes: e.g. Baumstark, J. Org. Chem., 1993, 58, 7615. Isolation of dioxiranes: neutral, anhydrous oxidants Preparation of dimethyldioxirane (DMDO) solutions: Adam, Chem. Ber., 1991, 124, 2377. More concentrated, “acetone free” solutions: Messeguer, Tetrahedron Lett., 1996, 37, 3585. In situ dioxirane formation Biphasic, CH2Cl2 / H2O: Denmark, J. Org. Chem., 1995, 60, 1391. Monophasic, CH3CN / H2O: Yang, J. Org. Chem., 1995, 60, 3887. In situ DMDO prep.: Shi, J. Org. Chem., 1998, 63, 6425. Trifluoroacetone + H2O2: Shi, J. Org. Chem., 2000, 65, 8808.

  6. Chiral Dioxiranes: Asymmetric Epoxidation of trans-Alkenes Shi, J. Am. Chem. Soc. 1997, 119, 11224. Review: Shi, Synthesis, 2000, 1979. • Preparation: 2 steps from D-fructose (enantiomer available in 5 steps from L-sorbose) • Excellent enantioselectivities for epoxidation of trisubstituted and trans-disubstituted alkenes • Poor ee for cis- and terminal alkenes • Ketone decomposes by Baeyer-Villiger reaction - cannot be recycled. High pH conditions required. Other substrate types:Conjugated dienes: J. Org. Chem. 1998, 63, 2948 Enynes: Tetrahedron Lett. 1998, 39, 4425. Modified catalyst for cis-alkenes: J. Am. Chem. Soc. 2000, 122, 11551. Terminal alkenes: Org. Lett., 2001, 3, 1929. Stable catalysts:Armstrong, Chem. Commun. 1998, 625; Tetrahedron: Asymmetry, 2000, 11, 2057. Shi, Org. Lett. 2001, 3, 715.

  7. Oxidation of silyl enol ethers Shi, Tetrahedron Lett. 1998, 39, 7819: Other methods for asymmetric oxidation of silyl enol ethers: Chiral Mn(salens): Thornton, Chem. Commun. 1992, 172; Adam, Tetrahedron Lett. 1996, 37, 6531, and refs. therein. Chiral oxaziridines: Review: Davis, Chem. Rev., 1992, 92, 919. Asymmetric dihydroxylation: J. Org. Chem. 1992, 57, 5067.

  8. Hydrolytic Kinetic Resolution Jacobsen, Science1997, 277, 936. Acc. Chem. Res. 2000, 33, 421. • Catalyst can be recycled (AcOH, air) • Easily-synthesised oligomeric Co(salen) catalysts are highly active for epoxide opening • by water, alcohols and phenols: J. Am. Chem. Soc. 2001, 123, 2687.

  9. Asymmetric Epoxidation of Electron-Deficient Alkenes Review: M.J. Porter and J. Skidmore, Chem. Commun., 2000, 1215. • Polyleucine, H2O2, base: e.g. Tetrahedron Lett.,2001, 42, 3741. • Reviews: Tetrahedron: Asymmetry1997, 8, 3163; 1998, 9, 1457. • Catalytic Mg peroxides (tBuOOH, cat. Bu2Mg, cat. diethyl tartrate): R1, R2=Ph • Jackson, Angew. Chem., Int. Ed. Engl. 1997, 36, 410. • Chiral phase-transfer catalysts (R2 can be alkyl): Lygo, Tetrahedron,1999, 55, 6289; • Tetrahedron Lett.2001, 42, 1343. • Lanthanide catalysis (BINOL, La(OiPr)3 or Yb(OiPr)3, 4Å MS, tBuOOH): • R1=Ph, iPr or Me; R2=Ph, iPr, Ph(CH2)2 or Me. • La-BINOL-Ph3AsO -mechanistic studies: J. Am. Chem. Soc., 2001, 123, 2725. • Chiral hydroperoxides, KOH, CH3CN: Adam, J. Am. Chem. Soc., 2000, 122, 5654. • Stoichiometric zinc alkylperoxides (O2, Et2Zn, R*OH): R1=Ph or tBu, R2=alkyl or aryl • Enders, Angew. Chem. Int. Ed. Engl. 1996, 35, 1725; Liebigs Ann. Chem. 1997, 1101 • Chiral dioxiranes: e.g. Tetrahedron: Asymmetry, 2001, 12, 1113.

  10. Alkene Dihydroxylation • Catalytic systems: • NMO / acetone / H2O (Upjohn procedure): Tetrahedron Lett. 1976, 23, 1973. • Cat. Me3NO•2H2O, CH2Cl2: Poli, Tetrahedron Lett. 1989, 30, 7385. • K3Fe(CN)6, K2CO3, tBuOH / H2O: Minato, Yamamoto, Tsuji, J. Org. Chem. 1990, 55, 766. • NMO, PhB(OH)2, CH2Cl2: Narasaka, Chem. Lett. 1988, 1721. • - Diol trapped as boronate ester - useful if diol is unstable or highly water soluble • Selenoxides as co-oxidants: Krief, Synlett, 2001, 501. • H2O2, cat. flavin, cat. N-methylmorpholine: Backvall, J. Am. Chem. Soc.1999, 121, 10424; • J. Am. Chem. Soc.2001, 123, 1365. • H2O2, cat. V(O)(acac)2, NMM, acetone/water: Backvall, Tetrahedron Lett., 2001, 42, 2569. • O2, K2[OsO2(OH)4], tBuOH / H2O: • Beller, Angew. Chem. Int. Ed. 1999, 38, 3026; J. Am. Chem. Soc. 2000, 122, 10289. • Wirth, Angew. Chem. Int. Ed. 2000, 39, 334. • Fe-catalysed asymmetric dihydroxylation: Que, J. Am. Chem. Soc. 2001, 123, 6722.

  11. Directed Dihydroxylations “Kishi rule” - dihydroxylation occurs anti- to oxygen functionality. Review: Cha, Chem. Rev. 1995, 95, 1761. Hydroxyl-directed dihydroxylation: Donohoe, Tetrahedron Lett. 1997, 38, 5027. • Bidentate ligand required • Dihydroxylation directed by trichloroacetamides: Donohoe, J. Org. Chem. 1999, 64, 2980. • Catalytic directed dihydroxylation of cyclic trichloroacetamides: Donohoe, Tetrahedron Lett.2000, 41, 4701. • Syn-selective dihydroxylation of acyclic allylic alcohols: Donohoe, Tetrahedron Lett.1999, 40, 6881.

  12. Sharpless Asymmetric Dihydroxylation Review: Sharpless, Chem. Rev. 1994, 94, 2483. DHQD series DHQD= dihydroquinidine DHQ= dihydroquinine "pseudoenantiomers" DHQ series

  13. Improved ligands: • Pyrimidine (PYR) spacer for sterically congested / terminal alkenes: J.Org. Chem. 1993, 58, 3785. • Anthraquinone (AQN) spacer gives better results for almost all alkenes having only aliphatic substituents: • Angew. Chem. Int. Ed. Engl.1996, 35, 448. Sharpless AD: Recent Developments Mechanism: • Comparison of theoretical and experimental kinetic isotope effects supports [3+2]-mechanism • Sharpless, Houk et al. J. Am. Chem. Soc. 1997, 119, 9907. Origins of asymmetric induction: Sharpless: J. Am. Chem. Soc. 1997, 119, 1840. Corey (“enzyme like” binding pocket): J. Am. Chem. Soc. 1996, 118, 319; 11038. Polymer supported chiral ligands: Review: Synlett, 1999, 1181. Crudden, Org. Lett. 2001, 3, 2325. Bolm, Synlett, 2001, 93 (AQN-ligands). Polymer supported Os-catalyst: Kobayashi, J. Am. Chem. Soc. 1999, 121, 11229. Org. Lett.2001, 3, 2649. Importance of pH control: improved rates for internal olefins at pH 12 (no MeSO2NH2); higher ee for terminal olefins at pH 10: Beller, Tetrahedron Lett. 2000, 41, 8083.

  14. Review: O’Brien, Angew. Chem., Int. Ed. Engl. 1999, 38, 326. Aminohydroxylation • DHQD-ligand series generally provide opposite enantiomer. • Effect of substrate structure on regioselectivity: Janda, Chem. Eur. J. 1999, 5, 1565. Cinnamates: • Aryl esters (and AQN-ligands) give opposite regioselectivity! Panek, Org. Lett. 1999, 1, 1949. • Can be run at higher concentration in presence of acetamide to suppress diol formation: • Wuts, Org. Lett. 2000, 2, 2667. Styrenes, Aryl alkenes (X=Ts, CBz, Boc, Teoc): • Altering ligand spacer, solvent can reverse regioselectivity without decreasing ee!

  15. Recent Developments in Asymmetric Aminohydroxylation • Amino-substituted heterocycles as nitrogen sources: Sharpless, Angew. Chem. Int. Ed. Engl.1999, 38, 1080. • Adenine derivatives as N-source: Sharpless, Tetrahedron Lett. 1998, 39, 7669. • N-bromo-N-lithio salts of primary carboxamides as N-source: Sharpless, Org. Lett. 2000, 2, 2221. • Unsaturated phosphonates as substrates: Sharpless, J. Org. Chem., 1999, 64, 8379.

  16. Pd-Catalysed Oxidative Kinetic Resolution of Secondary Alcohols with O2 Sigman, J. Am. Chem. Soc. 2001, 123, 7475. Stoltz, J. Am. Chem. Soc. 2001, 123, 7725.

  17. Reduction Reactions Asymmetric reduction of alkenes, ketones, imines……. • Hydrogenation • Transfer hydrogenation • Boranes • Hydride reagents • Hydrosilylation

  18. Monsanto synthesis of L-DOPA

  19. Rh(I)-BINAP Complexes Noyori, J. Am. Chem. Soc. 1980, 102, 7932. Ru BINAP complexes are more general; work for e.g. simple acrylic acids. Mechanistically distinct: Tetrahedron Lett. 1990, 31, 7189.

  20. Rh(I)-diphosphole Complexes Review: Burk, Acc. Chem. Res. 2000, 33, 363 • sense of enantioselectivity independent of acrylamide geometry

  21. Monodentate ligands MonoPhos: deVries, Feringa, J. Am. Chem. Soc. 2000, 122, 11539. Ligand A: Reetz, Angew. Chem. Int. Ed. 2000, 39, 3889. Review: Angew. Chem. Int. Ed. 2001, 40, 1197.

  22. Asymmetric Hydrogenation of Functionalised Ketones Ru BINAP: J. Am. Chem. Soc. 1987, 109, 5856; J. Am. Chem. Soc. 1988, 110, 629. Ru BPE: J. Am. Chem. Soc. 1995, 115, 4423. Review: Ager, Tetrahedron Asymm. 1997, 8, 3327.

  23. Noyori, J. Am. Chem. Soc. 2000, 122, 6510. Catalytic asymmetric hydrogenation of aminoketones • Mixed Ru bisphosphine/diamine complexes afford much improved turnover numbers: • Basic conditions allow dynamic kinetic resolution: Also for reduction of unfunctionalised ketones. Review: Noyori, Angew. Chem., Int. Ed., 2001, 40, 40.

  24. Asymmetric Transfer Hydrogenation of Ketones Reduction with the aid of a hydrogen donor in the presence of a catalyst Reviews: Wills, Tetrahedron Asymmetry, 1999, 10, 2045; Noyori, Acc. Chem. Res. 1997, 30, 97. Aminoalcohols as ligands: Arylalkylketones with electron rich aryl groups and benzocycloalkanones suffer from reversibility and give lower ee…but aminoalcohols are generally not compatible with the irreversible hydride donor formic acid

  25. Monotosylated diamines: formic acid-tolerant ligands for transfer hydrogenation - monotosylated diamines give slightly less reactive catalysts than the amino alcohols but have proven to be a more useful ligand system for ruthenium based transfer hydrogenations since compatibility with the formic acid system allows efficient reduction even in readily reversible systems:

  26. Ketone Reduction Catalyzed by Oxazaborolidines Review: Angew Chem. Int. Ed. 1998, 37, 1986

  27. Polymer-supported amino alcohol and in situ generation of borane Angew. Chem. Int. Ed., 2001, 40,1109

  28. Asymmetric reduction of ketones: stoichiometric aluminium hydrides Noyori, J. Am. Chem. Soc., 1984, 106, 6709, 6717

  29. A catalytic binapthyl-substituted hydride source based on hard-soft principles Woodward, Angew. Chem., Int. Ed. Engl., 1999, 38, 335; Chem. Eur. J.2000, 6, 3586. - replacing 'hard' aluminium by 'soft' gallium, the intermediate 'hard' alkoxide can be transferred to the 'hard' borane stoichiometric co-reductant, allowing catalytic turnover:

  30. Hydrosilylation of imines/ketones: an alternative to hydrogenative reduction Organometallics, 1991, 10, 560;Tetrahedron:Asymm., 1991, 2, 919

  31. Asymmetric hydrosilylation of ketones and imines using cheap siloxanes PMHS = poly(methylhydrosiloxane) J. Am. Chem. Soc., 1999, 121, 5640 (ketone); Angew. Chem., Int. Ed. Engl., 1998, 37, 1103, Org. Lett., 2000, 2, 713 (imine)

  32. Catalytic asymmetric hydrosilylation of enones/enoates Buchwald, J. Am. Chem. Soc., 1999, 121, 9473; J. Am. Chem. Soc., 2000, 122, 6797

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