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HYDROGENATION

HETEROGENEOUS CATALYSIS. HYDROGENATION. REVIEW OF HYDROGENATION. CATALYTIC HYDROGENATION . IS COVERED IN SEVERAL PLACES IN THE TEXT,. PRINCIPALLY:. Ch3 Section 3.18 Hydrogenation of Alkenes. Ch4 Section 4.13 Relative Stabilities of Alkenes.

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HYDROGENATION

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  1. HETEROGENEOUS CATALYSIS HYDROGENATION

  2. REVIEW OF HYDROGENATION CATALYTIC HYDROGENATION IS COVERED IN SEVERAL PLACES IN THE TEXT, PRINCIPALLY: Ch3 Section 3.18 Hydrogenation of Alkenes Ch4 Section 4.13 Relative Stabilities of Alkenes Ch6 Section 6.12 Heat of Hydrogenation Section 6.13 Resonance Energy of Benzene Ch8 Section 8.11 Hydrogenation of the Double Bond Pages 723-725 (Addition to Triple Bonds) Ch 14 Section 14.4 Hydrogenation of Alkynes

  3. HETEROGENEOUS CATALYSTS Do not dissolve in the solvent. Traditional type The catalyst is suspended by con- stantly stirring the solution (this is a two phase, or heterogenous, system). Hydrogen is bubbled through the solution H2 These catalysts are usually powder- ed metals or metal oxides. Pd, Pt, PtO2, Ni . . . . . They adsorb hydrogen on the surface. H H H H H H H

  4. Hydrogenation of Alkenes ( Sections 3.18, 4.13, 6.12, 6.13, 8.11 ) catalyst + C C H H C C H H fine powder, very porous, suspended in solution a syn addition reaction The catalyst is Pt, PtO2, Pd, or Ni (in special cases Ru, Rh, Re)

  5. Hydrogenation of Alkynes Alkynes react twice (two double bonds) Alkenes are more reactive than the starting alkyne, and the second addition proceeds at a faster rate than the first. With normal catalysts it is not possible to stop at the alkene by the addition of one mole of H2 .

  6. Lindlar Catalyst syn addition Lindlar is a special catalyst that allows the hydrogenation of an alkyne to stop after one mole of hydrogen is added. Most amines, and compounds containing sulfur, reduce the activity of catalysts or “poison” them. quinoline

  7. DESIGNER CATALYSTS It is not uncommon that either specific or specially designed catalysts are used to control the results in hydrogenation reactions. For this reason a very large variety of catalysts are available to the organic chemist. Pd Pd on Carbon Pt PtO2 (Adam’s Catalyst) Ni (Raney Nickel) Pd on BaSO4 with quinoline (Lindlar Catalyst) Rh Ru etc., etc., etc. ….

  8. HOMOGENEOUS CATALYSIS HYDROGENATION

  9. HOMOGENEOUS CATALYSTS Dissolve in the solvent to make a homogeneous (one phase) solution. These catalysts are usually a transition metal complex that can bind both hydrogen and the alkene reactant as ligands (many are based on Rh or Ru complexes). Hydrogen is transferred internally to the alkene by a rearrangement (insertion). Fairly Modern

  10. L L Rh L Cl insertion WILKINSON’S CATALYST Homogeneous catalysts are soluble in organic solvents. H L H III H-H I Rh H2 L Cl L = (Ph)3P: C=C H H L C-C-H L reductive elimination H Rh Rh L L Cl Cl H-C-C-H C=C alkene pi complex

  11. DISSOLVING METAL REDUCTIONS

  12. _ . Na + NH3 (liq) Na+ + (NH3)n Fe or Fe3+ catalyst SODIUM AMIDE IN LIQUID AMMONIA NaNH2 + H2 strong base SODIUM IN LIQUID AMMONIA SODIUM IN LIQUID AMMONIA solvated electrons . powerful reducing solution (electron donor) The solution is royal blue

  13. Dissolving Metal Reduction anti This reaction proceeds with anti addition (trans compound). Catalytic reduction proceeds with syn addition, hence we have a choice of methods.

  14. MECHANISM OF Na-LIQUID-NH3 REDUCTIONS electron transfer 1 . - All intermediates prefer the trans geometry. ( from (NH3)n ) +2H+ +2e radical- anion anion anti addition radical :NH2- electron transfer 2 Electrons come from the complex and the hydrogens from the ammonia.

  15. OXYMERCURATION Markovnikoff

  16. ACID-CATALYZED HYDRATION OF AN ALKENE Rearrangements possible. + .. : .. + Chapter 8 : : : + .. alcohol : Yields by this method are not always good; not stereoselective. Works best if the product is a tertiary alcohol.

  17. BETTER METHOD Oxymercuration of an Alkene Mercuric Acetate Hg(OCOCH3)2 _ Hg2+ + 2 CH3COO electrophile Follows Markovnikoff rule

  18. Mechanism of Oxymercuration bridged ion (2+) Formation of Bridged Ion Step 1

  19. anti opening Attack by Water Step 2

  20. H:- H+ + 2e- Hg2+ + 2e- Hg0 + reducing agent Reduction Step 3

  21. SODIUM BOROHYDRIDE NaBH4 Formal charge is on boron, but H is more electronegative than boron (slightly): H - B 2.0 H 2.1 Na+ H B H All 5 atoms share the charge. H DONOR OF HYDRIDE IONS ( H:- ) TO A SUITABLE ACCEPTOR BH3 + H:- ( = H+ + 2e- )

  22. EXAMPLE REACTION continued …..

  23. REMOVAL OF THE MERCURY ( REDUCTION ) 1) Hg(OAc)2 2) NaBH4 overall result follows Markovnikoff rule

  24. Hydroboration- protonolysis HYDROBORATION Hydroboration- oxidation Anti-Markovnikoff

  25. A PICTURE OF BORON ( a “schizophrenic” element ) Incomplete octet; it is an electrophile (wants electrons); nucleophiles will add; often stabilizes by dimerization or solvation by an unreactive electron-donor solvent (ethers). trivalent reactive :Nu Negatively-charged; has too many electrons; usually wants to expel either H:- or R:- to an acceptor. tetravalent - reactive Boron is most stable when it is trivalent and has attached elements with unshared pairs ( esp. oxygen ) that can stabilize it through back- bonding resonance. Borates and boric acid are especially stable. .. trivalent backbonding : “GOOD”

  26. . . H H H B B . . H H H Preparation of Diborane a gas B2H6 is a dimer of BH3 97o 122o Unusual “three-center” bonding, two electrons bond three atoms. sp2 H H B H empty 2p Mechanistically we can approximate B2H6 by using the BH3 structure.

  27. concerted DIBORANE ADDS TO ALKENES ANTI-MARKOVNIKOFF substituent stabilizes transient carbocation center R R hydride transfer electrophile sp2 empty 2p orbital R Since there are 3 B-H bonds, addition can occur three times (3 moles of alkene) syn addition anti-Markovnikoff Boron adds to the carbon with the most hydrogens (!) which is the reverse of the Markovnikoff Rule.

  28. d+ R - d Concerted versus Carbocation Intermediate R R + - Carbocation intermediate would allow rotation; the reaction would not be stereospecific, giving both syn + anti addition. Concerted reaction, no intermediate; syn addition

  29. Hydroboration of an Alkene bubble B2H6 gas through the solution 3 moles “ “ note abbreviation above a trialkylboron compound ( or a trialkylborane )

  30. concerted Stereochemistry of Hydroboration B2H6 syn addition H added to the most-substituted carbon anti-Markovnikoff

  31. HYDROBORATION - PROTONOLYSIS

  32. Hydroboration - Protonolysis trialkylborane 3 3 3 acetic acid 3 Separate reactions: add diborane, when done, add acetic acid. yields an alkane ( 3 moles ) This gives the same result as catalytic reduction - why do this? Typically yields are high 90-100% - a big advantage. This reaction will work in some cases where catalytic methods won’t.

  33. .. .. .. .. PROTONOLYSIS MECHANISM - + .. : : ‘ the alkyl group moves with its electrons ‘ - : : : : ‘ : ‘ makes an alkane .. + .. 6-ring concerted notice that the trivalent boron has resonance oxygen attached After the first one, each of the other R groups reacts as well.

  34. HYDROBORATION - OXIDATION

  35. Hydroboration - Oxidation trialkylborane 3 3 basic hydrogen peroxide OH- H2O2 acid H3O+ 3 alcohol ( 3 moles) + trialkoxyborane B(OH)3 This work up step hydrolyzes the trialkoxyboron compound. Each of these arrows is a separate reaction - first add B2H6, when complete add H2O2/NaOH, then finally hydrolyze with acid.

  36. .. .. .. .. .. .. .. .. .. .. .. .. OXIDATION MECHANISM - - .. : : H-O-O-H + OH O-O-H + H-O-H .. cast-off - - .. : : internal SN2 adds into empty orbital hydroperoxide ion migration and cleavage of O-O (R group maintains configuration) 2 more times - trialkoxyborane :

  37. Stereochemistry of Hydroboration-Oxidation syn addition cis (work up) H3O+ retains cis stereochemistry syn

  38. HYDROBORATION OF ALKYNES

  39. Only one pi bond reacts Hydroboration-Protonolysis of Alkynes A second reaction sequence is required to react the second pi bond.

  40. R ' R BH R ' R C C C C 3 H B Only one pi bond reacts / OH H O 2 2 R ' R ‘ ‘ C C H O H Hydroboration - Oxidation of Alkynes 3 + _ H3O+ 3 immediate tautomerism Ketone Enol unfavored tautomer !

  41. .. : H .. + .. .. : + H TAUTOMERS Tautomers are in equilibrium and differ in the position a proton. enol form keto form ketone or aldehyde Most enols have high energy and rapidly convert to the lower energy keto form which predominates in the equilibrium. enol lost H keto gained H

  42. REVIEW / OVERVIEW

  43. Summary of Hydroboration Reactions Anti-Markovnikoff

  44. OXYMERCURATION VS. HYDROBORATION-OXIDATION result similar to 3M H2SO4/H2O OXYMERCURATION HYDROBORATION-OXIDATION 1) Hg(OAc)2 1) B2H6 2) NaBH4 2) H2O2, NaOH 3) H3O+ anti syn Markovnikoff Addition anti-Markovnikoff Addition

  45. TWO REACTIONS THAT ARE SIMILAR, BUT DIFFERENT !

  46. Reducing Solution - . + N a N H H + 2 MORE SODIUM-LIQUID AMMONIA CHEMISTRY Royal blue electron solution Fe or Fe3+ catalyst required for this step AMIDE ION Strong Base x2 Clear sodium amide solution Strong Base (gas) H 2

  47. TWO DIFFERENT REAGENTS ! SODIUM IN LIQUID AMMONIA = Reducing Agent Na / NH3(l) Strong Base = SODIUM AMIDE IN LIQUID AMMONIA NaNH2 / NH3(l)

  48. APPLICATION

  49. SYNTHESIS PROBLEM Show how to synthesize all four compounds from acetylene. Requires both Na / NH3(liq) and NaNH2 / NH3(liq).

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