14.11 Alkane Synthesis Using Organocopper Reagents

1. 14.11Alkane Synthesis Using Organocopper Reagents

2. 2RLi + CuX R2CuLi + LiX [customary solvents are diethyl ether and tetrahydrofuran (THF)] Lithium Dialkylcuprates Lithium dialkylcuprates are useful synthetic reagents. They are prepared from alkyllithiums and a copper(I) halide.

3. R Li R Cu Cu I then a second molecule of the alkyllithium reacts with the alkylcopper species formed in the first step. R Li+ R Li R Cu – R Cu How? The alkyllithium first reacts with the copper(I) halide, Li+ I–

4. + + + R'X LiX R R' RCu R2CuLi Lithium Diorganocuprates are Used toForm C—C Bonds + + + R'X LiX Ar R' ArCu Ar2CuLi

5. Example: Lithium Dimethylcuprate + (CH3)2CuLi CH3(CH2)8CH2I Primary alkyl halides work best (secondary and tertiary alkyl halides undergo elimination). diethyl ether CH3(CH2)8CH2CH3 (90%)

6. Example: Lithium Diphenylcuprate + (C6H5)2CuLi CH3(CH2)6CH2I diethyl ether CH3(CH2)6CH2C6H5 (99%)

7. CH2CH2CH2CH3 Vinylic Halides Can be Used + Br (CH3CH2CH2CH2)2CuLi diethyl ether (80%)

8. Aryl Halides Can be Used + I (CH3CH2CH2CH2)2CuLi diethyl ether CH2CH2CH2CH3 (75%)

9. 14.12An Organozinc Reagent forCyclopropane Synthesis

10. Iodomethylzinc Iodide Formed by reaction of diiodomethane withzinc that has been coated with copper(called zinc-copper couple). Reacts with alkenes to form cyclopropanes. Reaction with alkenes is called theSimmons-Smith reaction. Cu CH2I2 + Zn ICH2ZnI

11. Example CH2CH3 CH2CH3 CH2I2, Zn/Cu H2C C CH3 diethyl ether CH3 (79%)

12. CH3CH2 CH2CH3 C C H H CH3CH2 CH2CH3 H H Stereospecific syn-Addition CH2I2, Zn/Cu diethyl ether

13. C C CH2I2, Zn/Cu diethyl ether CH3CH2 H H CH2CH3 Stereospecific syn-Addition CH3CH2 H H CH2CH3

14. 14.13Carbenes and Carbenoids

15. •• C Br Br dibromocarbene Carbene Name to give to species that contains adivalent carbon (carbon with two bondsand six electrons). Carbenes are very reactive; normally cannot be isolated and stored. Are intermediates in certain reactions.

16. – •• OC(CH3)3 C • • •• Br •• – + OC(CH3)3 H Br C • • •• Br Generation of Dibromocarbene Br + Br H Br

17. Br – Br C • • Br Generation of Dibromocarbene •• C – + Br Br Br

18. Carbenes React with Alkenesto Give Cyclopropanes CBr2 is an intermediate stereospecific syn addition Br KOC(CH3)3 + CHBr3 (CH3)3COH Br (75%)

19. 14.14Transition-Metal Organometallic Compounds

20. Introduction Many organometallic compounds derivedfrom transition metals have useful properties. Typical transition metals are iron, nickel,chromium, platinum, and rhodium.

21. 18-Electron Rule The number of ligands attached to a metalwill be such that the sum of the electronsbrought by the ligands plus the valenceelectrons of the metal equals 18. When the electron-count is less than 18, metal is said to be coordinatively unsaturatedand can take on additional ligands. 18-Electron rule is to transition metals asthe octet rule is to second-row elements.

22. Example Ni is in Group 10, so: Ni has 10 valence electrons Each CO uses 2 electrons to bond to Ni 4 CO contribute 8 valence electrons 10 + 8 = 18 CO OC CO Ni CO Nickel carbonyl

23. Cr OC CO CO (Benzene)tricarbonylchromium Cr has the electron configuration [Ar]4s23d4. Cr has 6 valence electrons. Each CO uses 2 electrons to bond to Cr. 3 CO contribute 6 valence electrons. Benzene uses its 6  electrons to bind to Cr.

24. Fe Ferrocene Fe2+is in Group 8, but is in the +2 oxidation state, so it contributes 8-2 = 6 electrons. Each cyclopentadienide anion contributes 6  electrons. Total 6 + 6 + 6 = 18 Organometallic compounds with cyclopentadienide ligands are called metallocenes.

25. 14.15Homogeneous Catalytic Hydrogenation Wilkinson’s Catalyst

26. Wilkinson’s Catalyst Ni, Pt, Pd, and Rh can act as a heterogeneous catalyst in the hydrogenation of alkenes. However, tris(triphenylphosphine)rhodium chloride was found to be soluble in organic solvents. This catalyst was developed by Sir Geoffrey Wilkinson, who received a Nobel Prize in 1973.

27. Mechanism of Homogeneous Hydrogenation Steps 1 and 2: Catalyst is converted to the active form. This is the activeform of the catalyst.

28. Mechanism of Homogeneous Hydrogenation Step 3: Alkene bonds to rhodium through  electrons.

29. Mechanism of Homogeneous Hydrogenation Step 4: Rhodium-alkene complex rearranges.

30. Mechanism of Homogeneous Hydrogenation Step 5: Hydride migrates from Rh to carbon.

31. Mechanism of Homogeneous Hydrogenation Step 6: Active form of the catalyst is regenerated.

32. 14.16Olefin Metathesis

33. Olefin Metathesis In crossed-olefin metathesis, one alkene is converted to a mixture of two new alkenes. The reaction is reversible, and regardless of whether we start with propene or a 1:1 mixture of ethylene and 2-butene, the same mixture is obtained.

34. Olefin Metathesis The reaction is generally catalyzed a transition metal complex. Typically Ru, W, or Mo are used. Shown below is Grubbs’ catalyst.

35. Ring-Opening Metathesis Ring-opening metathesis is used as a method of polymerization. Usually, it is applied most often when ring opening creates a relief of strain, as in some bicyclic alkenes.

36. 14.17Ziegler-Natta Catalysis of Alkene Polymerization The catalysts used in coordination polymerization are transition-metal organic compounds.

37. n H2C CH2 CH3CH2(CH2CH2)n-2CH CH2 Ethylene Oligomerization Triethylaluminum catalyzes the formation of alkenes from ethylene. These compounds are called ethyleneoligomers and the process is called oligomerization. Al(CH2CH3)3

38. n H2C CH2 CH3CH2(CH2CH2)n-2CH CH2 Karl Ziegler (1950) Ziegler found that oligomerization was affected differently by different transition metals. Some gave oligomers with 6-18 carbons, others gave polyethylene. Al(CH2CH3)3

39. n H2C CHCH3 Giulio Natta Natta found that polymerization of propene under Ziegler's conditions gave mainly isotactic polypropylene. This discovery made it possible to produce polypropylene having useful properties. Al(CH2CH3)3 polypropylene

40. n H2C CH2 CH3CH2(CH2CH2)n-2CH CH2 Karl Ziegler (1950) The ethylene oligomers formed under Ziegler's conditions are called linear -olefins and have become important industrial chemicals. Al(CH2CH3)3

41. n H2C CH2 CH3CH2(CH2CH2)n-2CH CH2 Karl Ziegler (1950) The polyethylene formed under Ziegler's conditions is called high-density polyethylene and has, in many ways, more desirable properties than the polyethylene formed by free-radical polymerization. Al(CH2CH3)3

42. Cl Zr Cl Ziegler-Natta Catalysts Early Ziegler-Natta catalyst were a combination of TiCl4 and (CH3CH2)2AlCl, or TiCl3 and (CH3CH2)3Al. Currently used Ziegler-Natta catalyst combinations include a metallocene such as bis(cyclopentadienyl)zirconium dichloride.

43. Ziegler-Natta Catalysts Early Ziegler-Natta catalyst were a combination of TiCl4 and (CH3CH2)2AlCl, or TiCl3 and (CH3CH2)3Al. Currently used Ziegler-Natta catalyst combinations include a metallocene such as bis(cyclopentadienyl)zirconium dichloride.

44. O—Al—O—Al CH3 CH3 n Ziegler-Natta Catalysts The metallocene is used in combination with a promoter such as methyl alumoxane (MAO).

45. Cl Cl Zr Zr Cl CH3 – Cl– + Zr CH3 Mechanism of Coordination Polymerization MAO This is the activeform of the catalyst.

46. CH3 H2C CH2 + Zr CH2 + Zr H2C CH2 CH2 CH3 + Zr CH3 Mechanism of Coordination Polymerization

47. CH2CH2CH3 + Zr CH2 H2C CH2 H2C + Zr CH2 CH2 CH3 + Zr CH2CH2CH2CH2CH3 Mechanism of Coordination Polymerization

48. H2C CH2 + Zr CH2CH2CH2CH2CH3 Mechanism of Coordination Polymerization etc.