Electrophilic Addition Reactions

1. Electrophilic Addition Reactions Part 2 Susan Morante All diagrams in this module are drawn using ISIS Draw software or are taken by permission from the textbook for this course by Hornback.

2. Hydroboration • “anti-Markovnikov” addition • provides a source of hard to get alcohols • why do we need another way of hydrating alkenes?

3. Hydroboration

4. Hydroboration • BH3 has a reversal of the usual polarity for hydrogen •  Usually exists as a dimer (diborane) 

5. Hydroboration • Also exists as a complex with THF

6. Hydroboration • mechanism

8. Hydroboration of alkynes • internal alkyne -C-CC-C- • Works in the same way as alkenes • Intermediate is an enol (see oxymercuration) • Final product is a ketone

9. Hydroboration of alkynes

10. Hydroboration of alkynes • Problem: In example above another product is also possible. Draw it. Is this second product more or less likely, or equally as likely as 5-methyl-2-hexanone?

11. Hydroboration of alkynes • terminal alkyne -C-CC-H • Vinylborane produced is very reactive • Reacts with a second molecule of borane • When treated this gives a gem diol • (gem diol = geminal diol = 1,1-diol)

12. Hydroboration of alkynes

13. Hydroboration of alkynes • to convert alkyne to aldehyde (instead of diol) need to use a sterically hindered borane • i.e. BHR2 with R as a large bulky group as in diisoamylborane

14. Hydroboration of alkynes • made by reaction of BH3 with 2 moles of 2-methyl-2-butene • adding a group like this to alkyne stops a second borane from adding to the vinyl borane and leads to the aldehyde

15. Hydroboration of alkynes

16. Addition of Carbene • What is Carbene? • C with 2 bonds, 1 pair nonbonding e- • sp2 hybridized • unstable • transient intermediate • simplest carbene :CH2 methylene

17. Addition of Carbene

18. Addition of Carbene • Reacts in a similar fashion to Br and Hg

19. Addition of Carbene • How is carbene formed: •  method 1: most commonly from diazomethane •  species is so reactive that the alkene must be present before the carbene is formed.

22. Addition of Carbene • Method 2: formed from chloroform and bromoform • by 1,1-elimination in the presence of a strong base • again the alkene must be present in solution to allow the next reaction to occur • reaction is stereoselective

25. Addition of Carbene • Problem: E-2-butene in reaction with CHBr3 and tert-BuOK gives 68% yield of a racemic mixture of trans-1,1-dibromo-2,3,-dimethylcyclopropane isomers. Show the reaction including stereochemistry.

26. Addition of Carbene • Method 3: use a carbenoid to achieve the same result. • Gives stereospecific products • Simmons - Smith reagent

27. Addition of Carbene • mechanism

29. Epoxidation • Use percarboxylic acid reagents • R group does not affect reaction

30. Epoxidation

31. Epoxidation

32. Hydroxylation of Alkenes • Colour change of KMnO4 (purple) to MnO2 (brown) is a lab test to distinguish alkenes • OsO4 oxidises three times better than KMnO4

33. Hydroxylation of Alkenes • Partial Mechanism

34. Hydroxylation of Alkynes Gives 1,2-dicarbonyl compounds unless the triple bond is terminal in which case a carboxylic acid is obtained Usually uses osmium tetroxide (because it is a more powerful oxidising agent than potassium permanganate) Additional reagents are KClO3 with water and diethyl ether as the solvent system

35. Ozonolysis of alkenes • A cleavage reaction • O3 formed in upper atmosphere, during lightning flashes, or by machine • Bubble gas stream into a low temperature solution of alkene which is dissolved in an inert solvent • Reactive intermediate is explosive

36. Ozonolysis of Alkenes Often water is added to destroy the reactive intermediate and convert to carbonyl and H2O2 Then Zn dust and acetic acid is added to destroy H2O2 This is known as a reductive workup and produces aldehydes and ketones An oxidative workup results in the production of ketones and carboxylic acids

38. Question 15 The ozonolysis of 2,4-dimethyl-2-pentene with oxidative workup will produce: A) B) C) D)

39. Question 16 The ozonolysis of 2,4-dimethyl-2-pentene with reductive workup will produce: A) B) C) D)

40. Ozonolysis of Alkynes • Always produces carboxylic acids

41. Question 17 What product is formed when 2-butyne is subjected to ozonolysis? A) B) C) D)

42. Addition of Hydrogen (Hydrogenation) • Alkenes

43. Addition of Hydrogen (Hydrogenation) • Usual catalysts are Ni, Pd or Pt (as PtO2) • because of solid catalyst both H's add to the same side of the =

45. Addition of Hydrogen (Hydrogenation) • reaction of alkynes with H2 in the presence of Ni, Pd or Pt gives alkanes

46. Addition of Hydrogen (Hydrogenation) • to obtain alkene must use Lindlar catalyst (a special Pd catalyst)

47. Question 18 Which one of the following terms best applies to the hydrogenation of an alkene in the presence of finely divided platinum? (in ethanol as solvent) A) anti addition B) concerted reaction C) heterogeneous catalysis D) endothermic reaction

48. Question 19 How would you accomplish the following conversion? A) NaNH2 B) H2, Lindlar Pd

49. Question 20 What is the structure of Compound Y in the following synthetic sequence? A) pentane B) cis-2-pentene C) trans-2-pentene D) 2-pentyne

50. Question 21 Which would be the best sequence of reactions to use in order to prepare cis-3-nonene from 1-butyne? A) 1. NaNH2 in NH3; 2. 1-bromopentane; 3. H2, Lindlar Pd B) 1. H2, Lindlar Pd; 2. NaNH2 in NH3; 3. 1- bromopentane