columbia大学有机化学课件06_10_13 html

1、6.10Acid-Catalyzed Hydration of Alkenes+CHOHCHOHCCreaction is acid catalyzed; typical hydration medium is 50% H2SO4-50% H2OAcid-Catalyzed Hydration of AlkenesCH3H3CH50% H2SO4CCH2CH3CCH3C50% H O2H3CCH3OH(90%)Follows Markovnikovs RuleCH350% H2SO450% H2O2OH(80%)CHFollows Markovnikovs Ruleinvolves a car

2、bocation intermediateis the reverse of acid-catalyzed dehydration of alcohols to alkenesCH3H3CH+H2OCH3CCH3CCH2H3COHMechanismStep (1)H3CProtonation of double bondH+O :HCCH2H3CHslowHH3C+ C:O :+CH3H3CHMechanismStep (2)Capture of carbocation by waterHH3C+ C:O :+CH3H3CHfastCH3H+O :CH3CHCH3MechanismStep (

3、3)Deprotonation of oxonium ionCH3H+O :CH3C:O :HHCH3fastHCH3.O :+O :HCCH3HHCH3HMechanismAcid-catalyzed hydrationethylene propene2-methylpropeneCH2=CH2 CH3CH=CH2(CH3)2C=CH21.01.6 x 1062.5 x 1011The more stable the carbocation, the fasterit is formed, and the faster the reaction rate.Relative RatesCH3H

4、3CH+CH3+H2OCCH3CCH2H3COHIn an equilibrium process, the same intermediates and transition states are encountered in the forward direction and the reverse, but in theopposite order.Principle of Microscopic Reversibility6.11Hydroboration-Oxidation of AlkenesSuppose you wanted to prepare 1-decanolfrom 1

5、-decene?OHSynthesisSuppose you wanted to prepare 1-decanolfrom 1-decene?Needed:a method for hydration ofalkenes with a regioselectivity opposite toMarkovnikovs rule.OHSynthesisTwo-step reaction sequence called hydroboration- oxidation converts alkenes to alcohols with aregiochemistry opposite to Mar

6、kovnikovs rule.1. hydroboration2. oxidationOHSynthesis+HBH2HBH2CCCCHydroboration can be viewed as the addition ofborane (BH3) to the double bond. not the reagent actually used.But BH3 isHydroboration Step+HBH2HBH2CCCCHydroboration reagents:HDiborane (B2H6) normally used in anether- like solvent call

7、ed diglymeH2BBH2HHydroboration Step+HBH2HBH2CCCCHydroboration reagents:Borane-tetrahydrofurancomplex (H B-THF)O3BH3+ Hydroboration StepH2O2, HOHCBH2CHCOHCOrganoborane formed in the hydroborationstep is oxidized with hydrogen peroxide.Oxidation Step1. B2H6, diglyme2. H2O2, HOOH(93%)ExampleHOHH3CCH31.

8、 H3B-THF2. H2O2, HOCHCHCCHCC33CH3H3CH(98%)Examplehydration of alkenesregioselectivity opposite to Markovnikovs rule no rearrangementstereospecific syn additionFeatures of Hydroboration-OxidationOH1. B2H6, diglyme2. H2O2, HO(82%)Example6.12Stereochemistry of Hydroboration-Oxidationhydration of alkene

9、sregioselectivity opposite to Markovnikovs rule no rearrangementstereospecific syn additionFeatures of Hydroboration-OxidationH and OH become attached to sameface of double bondH1. B2H62. H2O2, NaOHCH3CH3HOHHonly product is trans-2-methylcyclopentanol(86%) yieldsyn-Addition6.13Mechanism of Hydroboration-Oxidation1-Methylcyclopentene + BH3syn addition of H andB t