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POLARITY-REVERSAL CATALYSIS OF HYDROGEN-ATOM ABSTRACTION REACTIONS

POLARITY-REVERSAL CATALYSIS OF HYDROGEN-ATOM ABSTRACTION REACTIONS. Minh Tu Ha Laboratory of Synthesis of Natural Products, Institute of Chemical Sciences and Engineering École polytechnique fédérale de Lausanne, CH-1015 Lausanne, Switzerland. GROUP SEMINAR Nov 19 th 2015. INTRODUCTION.

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POLARITY-REVERSAL CATALYSIS OF HYDROGEN-ATOM ABSTRACTION REACTIONS

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  1. POLARITY-REVERSAL CATALYSIS OF HYDROGEN-ATOM ABSTRACTION REACTIONS Minh Tu Ha Laboratory of Synthesis of Natural Products, Institute of Chemical Sciences and Engineering École polytechnique fédérale de Lausanne, CH-1015 Lausanne, Switzerland GROUP SEMINAR Nov 19th2015

  2. INTRODUCTION Review: Polarity-reversal catalysis (PRC) of hydrogen-atom abstraction reactions: Concepts and applications in organic chemistry Chem. Soc. Rev., 1999, 28, 25-35 • Prof. Brian Robert • BSc degree from university College London in 1965 • PhD degree from UCL in 1968 with Prof. Alwyn Davies • Turner and Newall Research Fellow at UCL in 1970 • Worked with Prof. John Perkins at Chelsea College in 1971 • Returned to UCL in 1972 as a lecturer • Reader 1982 – 1999 • Emeritus Professor 2003- • Research interests • Free radical reactions Prof. Brian Roberts

  3. INTRODUCTION What is polar effect? Transition state of hydrogen-atom abstraction “The activation energy of hydrogen-atom abstraction would be decreased as the properties of the attacking (A) and departing (B) radicals become more mutually conductive to the participation of charge-transfer structures of type 1c and 1d” 1c >> 1d A: electrophilic radical B: nucleophilic radical 1d >> 1c A: nucleophilic radical B: electrophilic radical Electrophilic radical: HIGH ionization energy and HIGH electron affinity Nucleophilic radical: LOW ionization energy and LOW electron affinity

  4. INTRODUCTION What is polar effect? Transition state of hydrogen-atom transfer (HAT) For hydrogen-atom abstraction, polarities of participating radicals should be matched:

  5. INTRODUCTION What is polarity-reversal catalyst (PRC)? Notes: It is not necessary both steps are exothermic

  6. HYDRIDIC PRC Amino/Phospho-Boranes – strong nucleophilic radicals as hydridic PRC BDE 92-105 80-90 kcal.mol-1 Stability stable (RT) stable Activity Amino-borane and borane chemistry Properties H-abstraction H-donor For reactions of NHC-boranes: Curran et al, JACS2013, 135, 10484

  7. HYDRIDIC PRC Amino/Phospho-Boranes as hydridic PRC Roberts et al, J. Chem. Soc., Perkin Trans. 2, 1994, 2411 Roberts et al, J. Chem. Soc., Perkin Trans. 2, 1990, 1663 Roberts et al, J. Chem. Res. (S), 1988, 264

  8. HYDRIDIC PRC Amino/Phospho-Boranes as hydridic PRC Alkoxyl radical is required to complete the catalytic cycles Roberts et al, J. Chem. Soc., Perkin Trans. 1, 1993, 891

  9. HYDRIDIC PRC Chiral hydridic PRC: Kinetic resolution Roberts et al, J. Chem. Soc., Chem. Commun., 1991, 150 Roberts et al, J. Chem. Soc., Perkin Trans. 1, 1994, 1033

  10. PROTIC PRC Thiol – electrophilic radical as protic PRC Org. Biomol. Chem., 2010, 8, 3609–3617 • Thiol radicals are easily formed • H-atom abstracting ability is not good Very first example of protic PRC A. Waters et al., Nature1952, 170, 212

  11. PROTIC PRC HYDROACYLATION OF ALKENES • With EWG, polar effect is favourable, high yields • Not compatible to CH2=CH-EDG (~ <20%) • Comparative process: H-atom abstraction between acyl radical and tin hydride D. L. Boger et al., J. Org. Chem. , 1989, 54 , 1777; J. Am. Chem. Soc. , 1990, 112 and J. Am. Chem. Soc. , 1990, 112 , 4008 Roberts et al, J. Chem. Soc., Perkin Trans. 1, 1998, 67

  12. PROTIC PRC DEHALOGENATION, DEOXYGENATION AND DESULFURISATION • Silanes and NHC-Borane - Hydrogen donors • Polar effect is unfavorable • BED Bu3Sn-H ~ 74 kcal.mol-1 • Practical and ecological reason Roberts et al, J. Chem. Soc., Perkin Trans. 1, 1993, 103 Roberts et al, J. Chem. Soc., Tetrahedron Lett. 2001 , 763

  13. PROTIC PRC DEHALOGENATION, DEOXYGENATION AND DESULFURISATION • NHC-Borane - Hydrogen donor Electron negativity: 2.72 eV Strong nucleophilic radical (PhS radical: 5.33 ev) Curran et al, JACS2012, 134, 5669

  14. PROTIC PRC HYDROSILYLATION OF ALKENES ENATIOSELECTIVE VERSION Roberts et al, J. Chem. Soc., Tetrahedron Lett., 1995, 36, 2875 Roberts et al, J. Chem. Soc., Perkin Trans. 1, 1998 , 2881

  15. PROTIC PRC REDUCTIVE CARBOXYALKYLATION Roberts et al, Chem. Commun., 1998, 1413

  16. PROTIC PRC OTHER TRANSFORMATIONS 87 kcal.mol-1 96 kcal.mol-1 McMilan, Nature., 2015, 525, 87

  17. PROTIC PRC OTHER TRANSFORMATIONS Studer et al, ACIE 2008, 47, 779 Studer et al, ACIE 2005, 44, 4914 –4917 Studer et al, JACS. 2007 , 129 , 4498 - 4503 • Thiol as protic PRC • Broad scope • More efficient • Thiol as protic PRC • Broad scope • Hight yields • H-atom abstraction step (propagation) is unfavoured • Limited scope • Thiol cat. was used in few examples

  18. CONCLUSION • Well-developed based on the principle of polar effect • Provided alternative solution to unfavorable radical transformation PERSPECTIVES For borane PRC • More transformations need to investigated • NHC-borane as PRC • Chiral Amino-borane or NHC-borane • Aminyl radical in Amino-borane For Thiol • Enatioselective versions need to be developed

  19. Thank you for your attention ! QUESTION ?

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