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The Art of Heterogenous Catalytic Hydrogenation Part 2 Applications.

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Presentation on theme: "The Art of Heterogenous Catalytic Hydrogenation Part 2 Applications."— Presentation transcript:

1 The Art of Heterogenous Catalytic Hydrogenation Part 2 Applications

2 Topics to be covered Applications of Heterogenous Catalytic Reductions Simple Reductions Differential reductions hydrogenolysisEquipment Tour of the High Pressure Lab

3 Recommended Books: Heterogenous Catalysis for the Synthetic Chemist Robert L Augustine (1996) Good for theory, kinetics, applications & Equipment Practical Catalytic Hydrogenation, Techniques and Applications Morris Freifelder 1971 Alchemic secrets of success

4 Recommended References Catalytic Hydrogenation over Platinum Metals Catalytic Hydrogenation over Platinum Metals P. N. Rylander 1967 P. N. Rylander 1967

5 Factors That Impact Reduction Choices Functional group reduced Local structure Presence of other reducible groups Products that act as inhibitors/poisons Desirability of hydrogenolysis as one of the actions Equipment limitations

6 Olefins Under mild conditions, ease of reduction can be correlated inversely with degree of substitution (except when conjugated) RHC=CH2, RHC=CHR > R 2 C=CHR > R 2 C=CR 2 Many different catalysts reduce double bonds. The key to differentiating reduction of double bonds is monitoring equivalents hydrogen consumed.

7 Olefins continued Bond migrations prior to reduction are common and may result in scrambling of nearby stereochemistry (Requires H 2 !) Bond migrations prior to reduction are common and may result in scrambling of nearby stereochemistry (Requires H 2 !) Certain groups act as directors Certain groups act as directors

8 Bond Migration: More with Ni, Pd, less with Pt

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10 Access to catalyst surface influences stereochemistry

11 Catalyst approach: OH blocks Pd but favors Ni

12 Hydrogen Addition is from the Least Hindered Side

13 Selective Reduction of Polyenes Pd and Ni often cause bond migration Pd and Ni often cause bond migration Greatly influenced by local structure Greatly influenced by local structure Conjugated di- and polyenes give mixtures except in special cases Conjugated di- and polyenes give mixtures except in special cases

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15 Catalyst Addition is in Equilibrium

16 Effect of Solvent and Pressure on Stereochemistry SolventPercent cis Product SolventPercent cis Product Low H2 PressHigh H2 Press Low H2 PressHigh H2 Press n-Hexane7748 n-Hexane7748 DMF8675 DMF8675 tert-Butyl Alcohol9148 tert-Butyl Alcohol9148 Ethanol7848 Ethanol7848 0.3 N HCl/Ethanol9180 0.3 N HCl/Ethanol9180 0.3 N NaOH5050 0.3 N NaOH5050

17 Alkyne Reduction Usual catalysts: Lindlars (Pd/CaCO 3 ) Pd/BaSO4, Nickel boride, Cu and Co. Usual catalysts: Lindlars (Pd/CaCO 3 ) Pd/BaSO4, Nickel boride, Cu and Co. Selectivity for cis reduction: Pd >Rh >Pt > Ru> Ir Selectivity for cis reduction: Pd >Rh >Pt > Ru> Ir Quinoline commonly used as a modifier. Quinoline commonly used as a modifier.

18 Reduction of Alkynes: a Game of Relative Rate

19 Alkyne Reduction

20 Aromatic Reduction Catalyst Activity: Rh > Ru > Pt > Ni > Pd > Co Catalyst Activity: Rh > Ru > Pt > Ni > Pd > Co Ru minimizes C-O and C-N hydrogenolysis. Ru minimizes C-O and C-N hydrogenolysis. C-Halide bonds do not survive aromatic reductions C-Halide bonds do not survive aromatic reductions Correct choice of conditions allows other functionalities to survive Correct choice of conditions allows other functionalities to survive

21 Aromatic Reduction

22 Phenols to Cyclohexanones: thin film on catalyst modifies products

23 Ring Differentiation in Aromatic Reduction

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26 Other Aromatic Reductions

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28 Heterocyclic Reductions

29 Some Functional Group Reductions: faster than Aromatic

30 Reductive Amination

31 Takes advantage of relative ease of imine reduction. Takes advantage of relative ease of imine reduction. Takes advantage of equilibrium between imine and ketone in presence of an amine. Takes advantage of equilibrium between imine and ketone in presence of an amine. Some aldehydes produce significant byproducts of diamine and polymers. Some aldehydes produce significant byproducts of diamine and polymers. Use of one eq. acid improves yield of primary amine Use of one eq. acid improves yield of primary amine

32 Reductive Amination Raney Nickel is the catalyst of choice Raney Nickel is the catalyst of choice Palladium, Rhodium and Platinum do not perform as well as RaNi Palladium, Rhodium and Platinum do not perform as well as RaNi Ruthenium on carbon has been used successfully Ruthenium on carbon has been used successfully Use of 1 eq. ammonium acetate or HOAc significantly improves results Use of 1 eq. ammonium acetate or HOAc significantly improves results Aromatic Halides have been reported to survive conditions (using Rhodium) Aromatic Halides have been reported to survive conditions (using Rhodium) Can be done on sensitive aromatics, like furan. Can be done on sensitive aromatics, like furan.

33 Reductive Amination

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35 Hydrogenolysis Reductive cleavage of sigma bonds: Reductive cleavage of sigma bonds: C-C, C-N, C-O, C-S and others C-C, C-N, C-O, C-S and others Choice of catalyst, structure of substrate, and solvent greatly influence whether double bond reduction continues on to hydrogenolysis. Choice of catalyst, structure of substrate, and solvent greatly influence whether double bond reduction continues on to hydrogenolysis.

36 Carbon-Carbon Hydrogenolysis

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38 Halogen Weakens Opposite bond

39 C-O Hydrogenolysis Generally benzyl alcohols, ethers and esters Generally benzyl alcohols, ethers and esters Often facilitated by acid Often facilitated by acid Frequently occurs in competition with aromatic ring reduction Frequently occurs in competition with aromatic ring reduction Palladium favors hydrogenolysis while platinum favors ring reduction. Palladium favors hydrogenolysis while platinum favors ring reduction.

40 C-O Hydrogenolysis

41 Contrasting Pt with Pd

42 C-O Hydrogenolysis

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44 Carbonyl Hydrogenolysis

45 C-N Hydrogenolysis

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47 Parr Shaker Demo and HP Lab Tour

48 Hydrogenolysis: Carbon-Carbon


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