14.11 Alkane Synthesis Using Organocopper Reagents

1. 14.11Alkane Synthesis Using Organocopper Reagents

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

3. R Li R Cu Cu I How? the alkyllithium first reacts with the copper(I) halide Li+ I–

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

5. + + + R'X LiX R R' RCu R2CuLi Lithium diorganocuprates are used toform C—C bonds + + + R'X LiX Ar R' ArCu Ar2CuLi

6. 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%)

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

8. CH2CH2CH2CH3 Vinylic halides can be used + Br (CH3CH2CH2CH2)2CuLi diethyl ether (80%)

9. Aryl halides can be used + I (CH3CH2CH2CH2)2CuLi diethyl ether CH2CH2CH2CH3 (75%)

10. 14.12An Organozinc Reagent forCyclopropane Synthesis

11. 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

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

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

14. C C Stereospecific syn-addition CH3CH2 H H CH2CH3 CH2I2, Zn/Cu diethyl ether CH3CH2 H H CH2CH3

15. 14.13Carbenes and Carbenoids

16. •• 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.

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

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

19. Carbenes react with alkenesto give cyclopropanes CBr2 is an intermediate stereospecific syn addition Br KOC(CH3)3 + CHBr3 (CH3)3COH Br (75%)

20. 14.14Transition-Metal Organic Compounds

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

22. 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.

23. Example Ni has the electron configuration [Ar]4s23d8 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

24. Cr OC CO CO Example 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 p electrons to bind to Cr. (Benzene)tricarbonylchromium

25. Fe Example Ferrocene Fe2+has the electron configuration [Ar]3d6 Each cyclopentadienide anion contributes 6 p electrons Total 6 + 6 + 6 = 18 Organometallic compounds with cyclopentadienide ligands are called metallocenes.

26. 14.15Ziegler-Natta Catalysis of Alkene Polymerization The catalysts used in coordination polymerization are transition-metal organic compounds.

27. 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

28. 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

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

30. 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

31. 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

32. Ziegler-Natta Catalysts A typical Ziegler-Natta catalyst is a combination of TiCl4 and (CH3CH2)2AlCl, or TiCl3 and (CH3CH2)3Al. Many Ziegler-Natta catalyst combinations include a metallocene.

33. Mechanism of Coordination Polymerization + TiCl4 Al(CH2CH3)3 ClAl(CH2CH3)2 + CH3CH2TiCl3

34. H2C CH2 H2C CH2 Mechanism of Coordination Polymerization + TiCl4 Al(CH2CH3)3 ClAl(CH2CH3)2 + CH3CH2TiCl3 + CH3CH2TiCl3 CH3CH2TiCl3

35. H2C CH2 Mechanism of Coordination Polymerization CH3CH2TiCl3

36. H2C CH2 Mechanism of Coordination Polymerization TiCl3 CH3CH2CH2CH2 CH3CH2TiCl3

37. H2C CH2 H2C CH2 Mechanism of Coordination Polymerization TiCl3 CH3CH2CH2CH2 TiCl3 CH3CH2CH2CH2

38. CH3CH2CH2CH2CH2CH2 TiCl3 H2C CH2 Mechanism of Coordination Polymerization TiCl3 CH3CH2CH2CH2

39. CH3CH2CH2CH2CH2CH2 TiCl3 H2C CH2 Mechanism of Coordination Polymerization etc.