1. Chemistry 125: Lecture 49 February 10, 2010 Electrophilic Addition to Alkenes with Nucleophilic Participation This For copyright notice see final page of this file

2. Section 9.7 (380-384) Acid-Catalyzed Hydration Section 10.3 (421-423) Oxymercuration / Reduction

3. Larger than H, Hg bridges near the middle, preventing cation rearrangement. Subsequent reduction by BH 4 - completes Markovnikov hydration without rearrangement.

4. Section 10.2b (414-421) Addition of Cl 2, Br 2 (more bridging)

5. electrophile LUMO (  *) Ethylene HOMO (  ) bonding antibonding + - HOMO (p) HOMO-2 (where the  electrons went) Chlorine Two Cl-C Bonds? “oxidizing agent” removes e - from alkene poor overlap

6. LUMO (  *) nucleophile HOMO (  *) Ethylene + - HOMO-1 Chlorine :

7. LUMO - HOMO-1 HOMO + -

8. LUMO + - H2CH2C CH 2 :Cl Cl Cl - H2CH2C CH 2 Cl + H2CH2C CH 2 Cl “Electrophilic” Addition of Cl 2 to an alkene is both electrophilic and nucleophilic simultaneously.

9. Section 10.2b (417-420) Halohydrins H2CH2C CH 2 :Cl Cl Cl - H2CH2C CH 2 Cl + H-O H H2CH2C CH 2 Cl O-H H + + H : :

10. Bromonium Regiochemistry p. 420 How do we know the ion is bridged? “Halohydrin” shows Markovnikov Regiochemistry (like oxymercuration)

11. 49:48-54:24 Section 7.9C (308-311) Stereochemistry

12. Bromonium Stereochemistry p. 419 H 2 O attacks  * C-Br from backside. (Unbridged C + would have been attacked from both sides.) Anti Addition

13. Other “Simultaneous” Reagents Cl 2 C: (Carbene) R 2 BH (Hydroboration) CH 2 I 2 Zn/Cu (Carbenoid) O 3 (Ozonolysis) H-metal (Catalytic Hydrogenation) R-metal (Metathesis, Polymerization) RC (Epoxidation) OOHOOH O

14. Jack Hine, Ph.D. (1950) “about as earthy a research project as could be appropriate for a postdoctoral”  -Elimination Section 10.4d (431-2) Carbenes X 2 C:X3CX3C X 3 C HO-C(CH 3 ) 3 K+K+ ( X = Cl,Br ) X Reaction of CH n Cl 4-n with HO - CHCl 3 CH 2 Cl 2 CHCl 3 CCl 4 fastslowv.slowfast Reaction of CH n Cl 4-n with PhS - fast slowv. slow base nucleophile

15. C C H H H H LUMO HOMO C Cl Most reactions in this lecture use analogous LUMOs and HOMOs to mix with the  HOMO and  * LUMO of H 2 C=CH 2, respectively, so as to form two bonds simultaneously. But both pairs are orthogonal! + +

16. Transition State Motion for F 2 C: + H 2 C=CH 2 shrinking rotating stretching

17. LUMO F 2 C: H 2 C=CH 2 Transition State F 2 C: + H 2 C=CH 2 HOMO LUMO

18. How do you know both bonds form at once?

20. Sections 9.10-9.11 (390-401) Hydroboration Oxidation

21. Transition State Motion for BH 3 + H 2 C=CHCH 3 shrinking stretching rotating

22. HOMO Transition State BH 3 + propene BH 3 C3H6C3H6 HOMO LUMO + 3 LUMO

23. Rearrangement after Oxidation of BR 3 CH 3 R2BR2B O HO

24. Subsequent CH 3 BH 2 -O-OH rearrangement is like S N 2 Cl -  * C-Br CH 3 R2BR2B O HO  * O-O  B-C

25. Subsequent CH 3 BH 2 -O-OH rearrangement is like S N 2

26. Transition State HOMOs very similar Transition State LUMOs very similar

27. Why do Hydroboration/Oxidation if it just adds H and OH to C=C? Product is syn (H,OH from same face of C=C) and anti-Markovnikov (less substituted at C-OH) ! (Contrast with acid-catalyzed hydration)

28. Simmons-Smith “Carbenoid” Metal R-X Metal + R-X single-electron transfer (SET) e Metal + R X Metal R-M X +

29. The next three slides suggest a plausible, but incorrect, two-step mechanism for addition of ICH 2 ZnI to H 2 C=CH 2

30. ClZnCH 3 Model for I-Zn-CH 2 I 4s Zn LUMO 4p Zn LUMO + 1 bent for transition state LUMO` 4sp n Zn HOMO

31. Model for I-Zn-CH 2 I LUMO`  Zn-C HOMO

32. Model for I-Zn-CH 2 I ClZnCH 3 CH 2 ClZn CH 3 IZn CH 2 I HOMO  Zn-C LUMO  C-I ZnI 2 CH 2 “S N 2” If it were the diiodide instead of the model… But these two transition states were just guessed, not calculated quantum mechanically…

33. End of Lecture 49 Feb. 10, 2010 Copyright © J. M. McBride 2010. Some rights reserved. Except for cited third-party materials, and those used by visiting speakers, all content is licensed under a Creative Commons License (Attribution-NonCommercial-ShareAlike 3.0).Creative Commons License (Attribution-NonCommercial-ShareAlike 3.0) Use of this content constitutes your acceptance of the noted license and the terms and conditions of use. Materials from Wikimedia Commons are denoted by the symbol. Third party materials may be subject to additional intellectual property notices, information, or restrictions. The following attribution may be used when reusing material that is not identified as third-party content: J. M. McBride, Chem 125. License: Creative Commons BY-NC-SA 3.0