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Chapter 8

Chapter 8. Alkenes and Alkynes II Addition Reactions Part 2. 8.6 Alcohols From Alkenes through Oxymercuration-Demeruration: Markovnikov Addition. Two steps mechanism that is useful to avoid rearrangement Both reaction take place very rapidly at RT or below.

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Chapter 8

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  1. Chapter 8 Alkenes and Alkynes II Addition Reactions Part 2

  2. 8.6 Alcohols From Alkenes through Oxymercuration-Demeruration: Markovnikov Addition • Two steps mechanism that is useful to avoid rearrangement • Both reaction take place very rapidly at RT or below

  3. Regioselectivity of Oxymercuration-Demercuration • The orientation of the addition of the elements of water, H– and –OH, is in accordance with Markovnikov’s rule

  4. Examples

  5. Mechanism of Oxymercuration

  6. Mechanism

  7. Mechanism

  8. example • Starting with an appropriate alkene, show all steps in the synthesis of 2-methyl-2-propanol

  9. Example • Consider the following reaction • Outline a likely mechanism for the solvomercuration step of this ether synthesis • Show how you would use solvomercuration-demercuration to prepare tert-butyl methyl ether • Why would one use Hg(OCCF3)2 instead of Hg(Oac)2

  10. Answer for c • The electron-withdrawing fluorine atoms in mercurc trifluoroacetate enhance the electrophilicity of the cation. Experiments have demonstrated that for the preparation of tertiary alcohols in satisfactory yields, the trifluoroacetate must be used rather than the acetate

  11. 8.7 Alcohols from Alkenes through Hydrocarboration-Oxidation:Anti-Markovnikov Syn Hydration • Addition of water is indirect and two reactions are involved • 1. addition of a boron atom and hydrogen to C= C • Hydrolysis of the alkylborane intermediate to an alcohol and boric acid

  12. 8.8 Hydroboration: Synthesis of Alkylboranes • Preparation

  13. Mechanism • In each addition step, the boron atom becomes attached to the less subsituted carbon atom of the double bond • A hydrogen atom is transferred from the boron atom to the other carbon atom of the double bond

  14. Con’t • As this transition state is approached, electrons shift in the direction of the boron and away from the more substituted carbon atom of the double bond • The more substituted carbon bears an electron-releasing alkyl group, it is better able to accommodate this positive charge

  15. Mechanism

  16. Example • Starting with an appropriate alken, show the synthesis of tributyl borane

  17. 8.9 Oxidation and Hydrolysis of Alkylboranes • These reactions are occurred in the same vessel by the addition of hydrogen peroxide in an aqueous base

  18. mechanism

  19. mechanism

  20. Example • Starting with the appropriate alkene, show how you could use hydroboration-oxidation to prepare each of the alcohol • 1-pentanol • 2-methyl-1-pentenol

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