Chapter 8 Alkenes: Reactions

Transcription

1 Chamras Glendale Community College Organic Chemistry 105 Chapter 8 Alkenes: Reactions Name Type 1. alo-hydrogenation (Ionic & Radical) Electrophilic Addition 2. ydration (Acid-Catalyzed) Electrophilic Addition 3. Oxymercuration-Demercuration Electrophilic Addition 4. Alkoxymercuration-Demercuration Electrophilic Addition 5. ydroboration-oxidation Electrophilic Addition 6. ydrogenation Catalytic Addition 7. Cyclopropanation Carbene Addition 8. alogenation Oxidative Addition 9. Vicinal alohydrin Formation Oxidative Addition 10. Epoxidation Oxidative Addition 11. Anti ydroxylation Oxidative Addition 12. Syn ydroxylation Oxidative Addition 13. Oxidative Cleavage by Ozonolysis Oxidative Cleavage 14. Oxidative Cleavage by Potassium Permanganate Oxidative Cleavage The Common Theme in Addition Reactions of Alkenes First step: The C=C π-bond act as a source of available electrons, attacks electron-deficient species. Y X 1

2 ydro-alogenation Reaction (Addition of hydrogen halides) + X X Rearrangement? Variety of solvents are used: C 6 6, C 5 12, C 3 COO, CCl 3, etc Reactivity of alides: The Reason: F << Cl < Br < I Regiochemistry of addition: (Reaction is Regioselective) 2

3 Example: Br + Br Acetic Acid Mechanism: Br + Br intermediate Regioselectivity of the first addition Facial selectivity of the second addition Markovnikov s Rule: 3

4 Markovnikov Addition vs. Anti-Markovnikov Addition: Example: Write the mechanism & predict the product: (Possibility for rearrangement?) + Cl 4

5 ***Special Case for ydro-alogenation of Alkenes*** In Presence of PEROXIDES, when the alkene is hydrobrominated, the reaction yields the opposite regiochemical outcome: Example: + Br 3 C O O C 3 Br Mechanism: Regiochemistry of hydrohalogenation reactions of alkenes: (Reaction is Regioselective) 5

6 ydration (Acid-Catalyzed) O O DILUTE (50%) sulfuric acid is required for this reaction. Rearrangement? Regiochemistry of addition: (Reaction is Regioselective) Mechanistic Example: + 2 O + 6

7 ydration by Oxymercuration-Demercuration g(oac) 2 (aq) O goac NaB 4 O Rearrangement? Regiochemistry of addition: (Reaction is Regioselective) 7

8 Mechanistic Example: 8

9 Alkoxymercuration-Demercuration g(oac) 2 RO RO goac NaB 4 RO Rearrangement? Regiochemistry of addition: (Reaction is Regioselective) Mechanistic Example: 9

10 ydroboration-oxidation R B 3. TF R 3 B R 3 R 2 O 2 O 3 R O Rearrangement Regiochemistry of addition: (Reaction is Regioselective) Stereochemistry of addition: (Reaction is stereospecific) Mechanistic Example: 1. ydroboration: 10

11 Stereochemistry of ydroboration: (A relevant issue for cyclic alkenes) 2. Oxidation: Exercise: Propose reaction conditions that would achieve the following transformations: O O 11

12 ydrogenation R 1 R 3 R 1 R 3 2, Cat. R 2 R 4 R 2 R 4 General Remarks: Two C- Bonds are formed. (Very Strong) Very exothermic: Δ rxn < 0 Catalysts: Pd, Pt, Rh, or Ni: all Solid metals (heterogeneous catalysis) Stereochemistry of addition: Syn Addition: Addition of both s takes place on TE SAME FACE of the double bond. (Reaction is Stereospecific) C C Reaction happens in one step. Catalyst 12

13 Cyclopropanation (AKA: Simmons-Smith Reaction) Addition Type: Carbene Addition + C Problems with direct use of carbenes: 1. Diazomethane is VERY EXPLOSIVE 2. Diazomethane is VERY TOXIC 3. UNDESIRED SIDE PRODUCTS usually result Simmons-Smith Reaction utilizes a CARBENOID to achieve the same transformation. Simmons-Smith reagent: C 2 I 2 C 2 I 2 Zn, CuCl Example: 13

14 alogenation X 2 + X X Reaction is stepwise Rearrangement: Stereochemistry of addition: (Reaction is Stereospecific) Intermediate: Mechanistic Example: 14

15 alogenation of asymmetric alkenes: Br 2 Br 2 15

16 Stereochemistry of alogenation: Br 2 Br 2 16

17 Vicinal alohydrin Formation + X 2 2 O X + O + X Reaction is stepwise Rearrangement Stereochemistry of addition: (Reaction is Stereospecific) Regiochemistry of Addition: (Reaction is Regioselective) Intermediate: Mechanistic Example: 17

18 Epoxidation O O O + + R O O R O Mechanism: Rearrangement: Stereochemistry of addition: (Reaction is Stereospecific) Mechanistic Example: MCPBA: 18

19 Anti ydroxylation Acid-Catalyzed Epoxide Ring Opening O 3 O + O O Mechanism: Rearrangement: Stereochemistry of addition: (Reaction is Stereospecific) Mechanistic Example: In case of asymmetric alkenes: MCPBA C 2 Cl 2 19

20 Syn ydroxylation 1. KMnO 4 2. O (aq) OR 1. OsO O 2 O O Rearrangement: Stereochemistry of addition: (Reaction is Stereospecific) Mechanistic Example: 20

21 Oxidative Cleavage by Ozonolysis R 1 R 3 2. Zn, 2 O R 1 R O 3, C 2 Cl 2 OR O + O R 2 R 4 2. (C 3 ) 2 S R 2 R 4 Reaction is stepwise Oxidative Cleavage by Potassium Permanganate R 1 R 3 R 1 + KMnO 4 (Conc.) O + O R 3 R 2 eat R 2 O 21