From Asymmetric Hydrogenation to Hydrogen Generation. Dublin City University, 4t
Maze, Shannon, News Director has reference to this Academic Journal, PHwiki organized this Journal From Asymmetric Hydrogenation to Hydrogen Generation. Dublin City University, 4th February 2011 Professor Martin Wills Department of Chemistry Warwick University, UK. Homogeneous asymmetric hydrogenation – considerations: A homogeneous catalyst (e.g. Wilkinsons catalyst) promotes the reaction by bringing the reagents together: Catalytic hydrogenation is a powerful synthetic method usual substrates are C=C, C=O in addition to C=N bonds: This is often the weak point in the process- due to weak binding. Asymmetric hydrogenations often require a directing group on the substrate: Typical asymmetric hydrogenation catalysts: Rh-EtDuPHOS catalyst. Ru-BINAP catalyst. Rh-DiPAMP catalyst. Y is a co-ordinating group that helps the substrate bind on the catalyst.
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Hence the classic use of acylamino acrylates as substrates: b-amino acids a-amino acids Other directing groups include acids in addition to alcohols. Heller, D.; Holz, J.; Komarov, I.; Drexler, H.-J.; You, J.; Drauz, K.; Borner, A. Tetrahedron: Asymmetry, 2002, 13, 2735. Application of the directing group effect to a total synthesis of cytisine: Synthesis of cytisine using asymmetric C=C hydrogenation:
Synthesis of cytisine precursor in addition to asymmetric reduction: At 20 bar, 30C, 5 days, 100% conversion 93% e.e. (but full reduction of pyrrolidine ring). With thanks to Ian Lennon/Paul Moran (Dow) as long as the [Rh.DuPHOS] complex. Akira Shiibashi (Summer visitor from Prof Ikariyas group, 2007) Asymmetric catalysis of the reduction of unfunctionalised ketones by pressure hydrogenation: However – addition of a diamine to a Ru/diphosphine converts it to a more versatile ketone reduction catalyst: Rear view Front view (DPEN removed) [(Br-XuPHOS)2Ru(DPEN)] (S, S, SS is matched combination) Reviews: R. Noyori in addition to T. Okhuma, Angew. Chem., Int, Edn., 2001, 40, 40-73. R. Noyori, C. A. S in addition to oval, K. Muniz in addition to T. Ohkuma, Phil. Trans. R. Soc. A, 2005, 363, 901-12.
Relationship of enantioselectivity to structure. Mechanism of the ketone reduction: Organic Letters, 2004, 6, 4105-4107. J. Org. Chem., 2005, 70, 8079-8087 Dr Yingjian (Andy) Xu Origin of asymmetric induction:
Origin of asymmetric induction: Noyori et al., Angew. Chem., Int. Edn. 2001, 40, 2818-2821. Asymmetric transfer hydrogenation may also be used to reduce isolated ketones, e.g.: Amino alcohol in addition to monotosylated diamine lig in addition to s are most used as long as asymmetric transfer hydrogenation of ketones. Reviews; Tetrahedron: Asymmetry, 1999, 10, 2045-2061. Review by Noyori in addition to Hashiguchi; Acc. Chem. Rev., 1997, 30, 97-102. Monotosylated diamines first reported by Noyori, Ikariya et al.; J. Am. Chem. Soc., 1995, 117, 7562-63. Gladiali, S.; Alberico, E. Chem. Soc. Rev. 2006, 35, 226. The Noyori lig in addition to . Give the fastest reactions(2 -3 hours). Works with iPrOH, but not HCO2H. Reduce C=O but not C=N. Give slower reactions (>20 hours). Works with iPrOH, in addition to HCO2H. Reduce C=O in addition to C=N. i.e. more versatile overall.
Proposed mechanism of transfer hydrogenation Note- use of HCO2H in place of iPrOH reduces problem of reversibility Proposed mechanism of transfer hydrogenation Note- use of HCO2H in place of iPrOH reduces problem of reversibility or HCO2H or CO2 Proposed mechanism of transfer hydrogenation X-=O ESI-MS evidence of intermediates; Chem. Commun, 2000, 99-100. With Jenny Kenny And Prof Albert Heck (Utrecht). Note- use of HCO2H in place of iPrOH reduces problem of reversibility or HCO2H or CO2 X=NTs, X-rays of Intermediates: Noyori et al., Angew. Chem., Int. Edn. 1997, 36, 285-288.
Control of asymmetric reduction: Importance of aryl ring(results as long as cis-aminoindanol/Ru(II)). Tethered lig in addition to s as long as transfer hydrogenations; Tethered lig in addition to s h6-aryl cannot rotate; substitutents can be position to control reactions. Dissociation of lig in addition to should be reduced due to multipoint binding. Could change basis of stereocontrol from electronic to steric: Target tethered lig in addition to s based on TsDPEN: Tethered lig in addition to 1 Tethered lig in addition to 2 (reverse tethered) Gave improved results compared to the untethered catalyst. Gave similar results to the untethered. Jerome Hannedouche Aidan Hayes David Morris With Jerome Hannedouche, J. Am. Chem. Soc. 2004, 126, 986-987.
Synthesis of reverse-tethered diamine catalysts. With Aidan Hayes, J. Am. Chem. Soc. 2005, 127, 7318-9. (S,S)-enantiomer Tethered catalyst is highly active 0.01 mol% is effective: At 0.01 mol% level, acetophenone is reduced in 96% e.e. in addition to 98% yield after ca. 84 hours! t = 100 min Rate studies; Acetophenone reduction at 40oC. t = 0 Dimer precursor
Further catalyst derivatives prepared: Kinetic tests using further derivatives: effect of substituents. 4 Me (96% e.e.) 3,5 di Me (93% e.e.) Untethered Fung Kei (Kathy) Cheung Adriana Lorente 4C (97% e.e.) 2C (92% e.e.) 5C (94% e.e.) 3C (96% e.e.) All give same (R) product enantiomer. Results with arene-ring substituted catalysts confirms that the CH-p interaction operates through methyl groups: Di-meta-Mesub- stituted arene reduces rate, but e.e. remains high (93%), suggesting stabilising effect is retained: Electron donating groups on the arene slightly lower the rate of hydride regeneration, but not reduction: d+ d+ d+ d+ d- d- d- d- d-
Kinetic discussion The 4C tethered complex is fast because of the high rate of hydride regeneration, in addition to ketone reduction! F. K. (K.) Cheung, M. Graham, D. J. Fox et al. Org. Lett. 2007, 9, 4659-4662. Dr David Fox M. Wills, D. J. Fox et al, Dalton Trans. 2010, 39, 1395-1402. 1H NMR of hydride during reduction follows predicted trend. Two isomers Are visible. Diastereo- isomers One isomer must be much more reactive! 3C tether hydride 1H NMR with 700MHz NMR allows %RuH to be followed. Following reduction of acetophenone using 4C tether at S/C=200. [ketone]=1.62M
Alternative ether linked lig in addition to s: F. K. (Kathy) Cheung, Changxue Lin, Mark A. Graham, Martin Wills, Organometallics, 2007, 26, 5346-51 Other uses of TsDPEN in addition to its derivatives: simple iridium(III) ketone pressure hydrogenation catalysts. With Eduardo Martins in addition to David Morris Tetrahedron Lett. 2008, 50, 688-692, Tetrahedron, 2009, 65, 5782-5786. Analogy of pyridone directing effect to that established in acylamino acrylates: Heller, D.; Holz, J.; Komarov, I.; Drexler, H.-J.; You, J.; Drauz, K.; Borner, A. Tetrahedron: Asymmetry, 2002, 13, 2735. Akira Shiibashi (Summer visitor from Prof Ikariyas group, 2007)
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