An excellent utilisation of frustrated energy between sterically crowded lewis pairs for the development of a metal free alternative method for catalytic hydrogenation.

1. Frustrated Lewis Pairs
M. K. Sahoo

2. B F
F
F
N H
H
H
N H
H
H
BF
F
F
Lewis Acid
(Electron Pair Acceptor)
Lewis Base
(Electron Pair Donor)
Lewis Acid-Base
Adduct
 G. N. Lewis, FRS, 1923: Valence & Structure of Atoms & Molecules
Lewis Acid Base Concept

3. Frustrated Lewis Pairs (FLP)
Combination of sterically hindered Lewis donors and lewis acceptors
B P

4. • 1923 : Lewis
• Lewis Base : an electron pair donor
• Lewis Acid : an electron pair acceptor
• 1942 : Brown
• 1959 : Wittig and Benz
• Ancestor of the Frustrated Lewis Pair
• 1966 : Tochtermann
• « Antagonistisches Paar »
F
Br
Mg
PPh3
BPh3
PPh3
BPh3
BF3 BM e3
No ReactionNN BF3
CPh3Na
BPh3
Ph3C
BPh3
NanNa Ph3BCPh3
CPh3Na
BPh3
Historical Background

5. Prof D. W. Stephan: Father of FLP
 First non-transition metal system that can reversibly activate & liberate hydrogen

6. 1. Creation of a polar and frustrated environment
BR3R3P H H
2. Polarization of H2
3. Activation of H2
Why would an FLP activate H2?
Facts on H2 bond
0.74 Å
432 kJ/mol

7. Proposed Concerted Mechanism

8. Mechanism of H2 Activation by FLPs
1. 2.

9. Electron Transfer Model

10. Grimme et.al.; Angew. Chem. Int. Ed. 2010, 49, 1402
Electric Field Model

11. F=0
F=0.09
F=0.04
F=0.06
F=0.08
F=0.10

12. Papai et.al; J. Am. Chem. Soc. 2013, 135, 4425-4437
Electric Field along H-H Axis
For six FLPs

14. Phosphines Carbenes
Imines Amines
Boranes
Linked Frustrated Lewis Pairs
Non-Linked Frustrated Lewis Pairs

15.  Sufficient Acidity & Basicity is key for Activation
Factors Responsible for Activation
3

16. Decreased Lewis Acidity & Basicity Allow Reversible Activation
(C6H5)3B (o-C6H4Me)3P(p-C6F4H)3B
Reversible Activation

17.  2,6-Lutidine exibhits Classical and FLP behaviour
 Dialkylethers, THF, Dioxane also shows dual behaviour
Classical Vs FLP: Orthogonal???

18. Hydrogenation: Additon of molecular hydrogen (H2) to another molecule
 Cleanest Reducing Agent, 100% Atom Economy
 Food, Petrochemicals, Agricultural, Pharmaceutical Industries
X Y
H 2
X Y
H H
Application: Catalytic Hydrogenation

19. HYDROGENATION HISTORY
Paul Sabatier, 1897, trace nickel (as a catalyst) to
facilitate addition of H2 to organic compounds.
(Nobel Prize 1912)
Sir Geoffrey Wilkinson FRS, 1966, “one of the fathers
of organometallic Chemistry” discovered homogeneous
Rh catalyst for hydrogenation
(Nobel Prize 1973)
Ryōji Noyori FRS,1994, introduction Ru catalyst for
Highly selective asymmetric hydrogenations
(Nobel Prize 2001)

21. ( Current Technologies)
Transition metal free Hydrogenation

22. 1. Catalytic Hydrogenation of Imines
X Y
H2
XY
HH
FLP FLP[H...H]

23. Mechanism of Catalytic reduction

24. Catalytic Reduction: B(C6F5)3 Protected Imines

25. 2. Imines as Basic FLP Partner
Ph2C

26. Mechanism

27. 3. Hydrogenation of Nitriles

28. 4. Hydrogenation Of N-Heterocycles

29. 5. Hydrogenation of Carbonyl Compounds
R R'
O
R R'
OHCat:B(C6F5)35-20mol%,
H2(5Bar)
T(80-1000C),t(06-120hrs)
Dioxane
99%
60%
0%
99%
80%
14%
84%
99%
97%
82%
78%
0%
75%
O B(C6F5)3 O B(C6F5)3
ClassicalFLP
H2
O H
n
H-B(C6F5)3

30. Mechanism
Ashley et. al.; J. Am. Chem. Soc. 2014, 136, 15813-15816
A.
nBu4N H-B(C6F5)3
Ketone No Reaction
nBu4N H-B(C6F5)3
Ketone Sec Alcohol
B(C6F5)3
nBu4N H-B(C6F5)3
Aldehyde
Primary Alcohol
B.
OH
n
H-B(C6F5)3

31. 6. Hydrogenation by FLPs
(other functional Groups)
FLP
Silylenolethers

32. P
C6H6
C6F5C6H6
P
C6H6
C6F5C6F5
P
C6H3Cl2
C6H3Cl2Cl2C6H3
P
1-Napthyl
1-NapthylNapthyl-1
7. Hydrogenation of Nonpolar Substrates
(Alkenes)

33. Olefin Nucleophilicity (N): 1.1 to 4.4

34. 8. Hydrogenation of Nonpolar Substrates
(Alkynes cis-Alkenes)

35. 9. Assymmetric hydrogenation by Chiral FLPs

36. 10. Conversion of CO2 to Fuels

37. FLP HYDROGENATIONS
unprecedented paradigm for H2 activation and use
Advantages
1. Avoids cost of precious metal catalysts
2. Avoid cost of separation of toxic metal residues
3. Greener catalysts as uses readily accessible abundant elements
Goals
1. Enhance catalyst activity
2. Broaden range of applications
3. Develop selective FLP catalysts

38. Hydrogen storage: 0.25 wt% far below 6-9 wt% for practical use
Small Molecule Activation

39. Questions