1. HYDROGENATION HETEROGENEOUS CATALYSIS

2. 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) REVIEW OF HYDROGENATION Ch 14 Section 14.4 Hydrogenation of Alkynes

3. HETEROGENEOUS CATALYSTS Do not dissolve in the solvent. These catalysts are usually powder- ed metals or metal oxides. 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 They adsorb hydrogen on the surface. H H H H H H H H2H2 Pd, Pt, PtO 2, Ni.....

4. Hydrogenation of Alkenes + CCHH CC HH catalyst The catalyst is Pt, PtO 2, Pd, or Ni (in special cases Ru, Rh, Re) a syn addition reaction ( Sections 3.18, 4.13, 6.12, 6.13, 8.11 ) fine powder, very porous, suspended in solution

5. Hydrogenation of Alkynes With normal catalysts it is not possible to stop at the alkene by the addition of one mole of H 2. Alkenes are more reactive than the starting alkyne, and the second addition proceeds at a faster rate than the first. Alkynes react twice (two double bonds)

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

8. 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 PtO 2 (Adam’s Catalyst) Ni (Raney Nickel) Pd on BaSO 4 with quinoline (Lindlar Catalyst) Rh Ru etc., etc., etc. …. DESIGNER CATALYSTS

9. HOMOGENEOUS CATALYSIS HYDROGENATION

10. 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). Fairly Modern Hydrogen is transferred internally to the alkene by a rearrangement (insertion).

11. Rh Cl L L L L = (Ph) 3 P: WILKINSON’S CATALYST Homogeneous catalysts are soluble in organic solvents. Rh Cl L H L H Rh L H L H C=C H-C-C-H Rh L L H C-C-H Cl reductive elimination alkene pi complex I III H-H insertion H2H2

12. DISSOLVING METAL REDUCTIONS REDUCTIONS

13. Na + NH 3 (liq) Na + + (NH 3 ) n. _ solvated electrons The solution is royal blue SODIUM IN LIQUID AMMONIA. powerful reducing solution (electron donor) Fe or Fe 3+ NaNH 2 + H 2 catalyst SODIUM AMIDE IN LIQUID AMMONIA strong base SODIUM IN LIQUID AMMONIA

14. 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.

15. MECHANISM OF Na-LIQUID-NH 3 REDUCTIONS ( from (NH 3 ) n ). - anti addition electron transfer 1 electron transfer 2 +2e +2H + Electrons come from the complex and the hydrogens from the ammonia. :NH 2 - All intermediates prefer the trans geometry. radical- anion radical anion

16. OXYMERCURATION Markovnikoff

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

18. Oxymercuration of an Alkene Hg(OCOCH 3 ) 2 Hg 2+ + 2 CH 3 COO _ Mercuric Acetate electrophile BETTER METHOD Follows Markovnikoff rule

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

20. Step 2 anti opening Attack by Water

21. Reduction Step 3 reducing agent Hg 2+ + 2e - Hg 0 H: - H + + 2e - +

22. SODIUM BOROHYDRIDE NaBH 4 Na + HBH H H - DONOR OF HYDRIDE IONS ( H: - ) TO A SUITABLE ACCEPTOR BH 3 + H: - ( = H + + 2e - ) Formal charge is on boron, but H is more electronegative than boron (slightly): B 2.0 H 2.1 All 5 atoms share the charge.

23. EXAMPLE REACTION continued …..

24. REMOVAL OF THE MERCURY ( REDUCTION ) overall result follows Markovnikoff rule 1) Hg(OAc) 2 2) NaBH 4

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

26. A PICTURE OF BORON ( a “schizophrenic” element ) trivalent tetravalent trivalent backbonding Incomplete octet; it is an electrophile (wants electrons); nucleophiles will add; often stabilizes by dimerization or solvation by an unreactive electron-donor solvent (ethers). - Negatively-charged; has too many electrons; usually wants to expel either H: - or R: - to an acceptor. :Nu :.. 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. reactive “GOOD”

27. BH H H Unusual “three-center” bonding, two electrons bond three atoms. B H H B H H H H.... Preparation of Diborane B 2 H 6 is a dimer of BH 3 Mechanistically we can approximate B 2 H 6 by using the BH 3 structure. sp 2 empty 2p 122 o 97 o a gas

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

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

30. Hydroboration of an Alkene a trialkylboron compound ( or a trialkylborane ) 3 moles note abbreviation above bubble B 2 H 6 gas through the solution ““

31. anti-Markovnikoff concerted H added to the most-substituted carbon syn addition Stereochemistry of Hydroboration B2H6B2H6

32. HYDROBORATION - PROTONOLYSIS

33. Hydroboration - Protonolysis yields an alkane ( 3 moles ) trialkylborane acetic acid 3 3 3 3 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. Separate reactions: add diborane, when done, add acetic acid.

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

35. HYDROBORATION - OXIDATION

36. H2O2H2O2 OH - B(OH) 3 + trialkylborane alcohol ( 3 moles) basic hydrogen peroxide trialkoxyborane 3 3 3 acid H3O+H3O+ This work up step hydrolyzes the trialkoxyboron compound. Hydroboration - Oxidation Each of these arrows is a separate reaction - first add B 2 H 6, when complete add H 2 O 2 /NaOH, then finally hydrolyze with acid.

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

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

39. HYDROBORATION OF ALKYNES

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

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

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

43. REVIEW / OVERVIEW

44. Summary of Hydroboration Reactions Anti-Markovnikoff

45. OXYMERCURATION VS. HYDROBORATION-OXIDATION OXYMERCURATION HYDROBORATION-OXIDATION 1) Hg(OAc) 2 2) NaBH 4 1) B 2 H 6 2) H 2 O 2, NaOH Markovnikoff Additionanti-Markovnikoff Addition antisyn result similar to 3M H 2 SO 4 /H 2 O 3) H 3 O +

46. TWO REACTIONS THAT ARE SIMILAR, BUT DIFFERENT ! BUT DIFFERENT !

47. NH 2 - + Na + H. H 2 MORE SODIUM-LIQUID AMMONIA CHEMISTRY or Fe 3+ Fe (gas) Royal blue electron solution Clear sodium amide solution catalyst required for this step AMIDE ION Strong Base Reducing Solution x2 Strong Base

48. SODIUM IN LIQUID AMMONIA SODIUM AMIDE IN LIQUID AMMONIA TWO DIFFERENT REAGENTS ! Reducing Agent Strong Base = = NaNH 2 / NH 3 (l) Na / NH 3 (l)

49. APPLICATION

50. SYNTHESIS PROBLEM Show how to synthesize all four compounds from acetylene. Requires both Na / NH 3 ( liq ) and NaNH 2 / NH 3 ( liq ).