6.10 acid-catalyzed hydration of alkenes - columbia university · ethylene ch 2 =ch 2 1.0 propene...
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6.106.10AcidAcid--Catalyzed Hydration of AlkenesCatalyzed Hydration of Alkenes
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HH——OHOHCC CC ++ OHOHCC CCHH
AcidAcid--Catalyzed Hydration of AlkenesCatalyzed Hydration of Alkenes
reaction is acid catalyzed; typical hydration reaction is acid catalyzed; typical hydration medium is 50% Hmedium is 50% H22SOSO44--50% H50% H22OO
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(90%)(90%)
50% H50% H22SOSO44
50% H50% H22OO
HH33CC
HH33CC CHCH33
HH
CC CC
OHOH
CC CHCH22CHCH33CHCH33
CHCH33
Follows Markovnikov's RuleFollows Markovnikov's Rule
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50% H50% H22SOSO44
50% H50% H22OO
Follows Markovnikov's RuleFollows Markovnikov's Rule
(80%)(80%)
OHOH
CHCH33
CHCH22
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+ H+ H22OOHH++
involves a carbocation intermediateinvolves a carbocation intermediate
is the reverse of acidis the reverse of acid--catalyzed dehydrationcatalyzed dehydrationof alcohols to alkenesof alcohols to alkenes
HH33CC
HH33CC
CC CHCH22
OHOH
CC CHCH33CHCH33
CHCH33
MechanismMechanism
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Step (1) Protonation of double bondStep (1) Protonation of double bond
slowslow
++
HH33CC
HH33CC
CC CHCH22 OO++
HH
HH
HH
::
++HH33CC
HH33CC
CC CHCH33
HH
++ OO
HH
::::
MechanismMechanism
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Step (2) Capture of carbocation by waterStep (2) Capture of carbocation by water
fastfast
++HH33CC
HH33CC
CC CHCH33
HH
++ OO
HH
::::
++
HH
OO
HH
::CC
CHCH33
CHCH33
CHCH33
MechanismMechanism
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Step (3) Deprotonation of oxonium ionStep (3) Deprotonation of oxonium ion
fastfast
++
HH
OO
HH
::CC
CHCH33
CHCH33
CHCH33 OO
HH
::::
HH
OO++
HH
HH
HH
::++
++
HH
OO ::CC
CHCH33
CHCH33
CHCH33
....
MechanismMechanism
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ethyleneethylene CHCH22=CH=CH22 1.01.0
propenepropene CHCH33CH=CHCH=CH22 1.6 x 101.6 x 1066
22--methylpropene methylpropene (CH(CH33))22C=CHC=CH22 2.5 x 102.5 x 101111
The more stable the carbocation, the fasterThe more stable the carbocation, the fasterit is formed, and the faster the reaction rate.it is formed, and the faster the reaction rate.
AcidAcid--catalyzed hydrationcatalyzed hydration
Relative RatesRelative Rates
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In an equilibrium process, the same intermediates In an equilibrium process, the same intermediates and transition states are encountered in the and transition states are encountered in the forward direction and the reverse, but in the forward direction and the reverse, but in the opposite order. opposite order.
+ H+ H22OOHH++
HH33CC
HH33CC
CC CHCH22
OHOH
CC CHCH33CHCH33
CHCH33
Principle of Microscopic ReversibilityPrinciple of Microscopic Reversibility
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6.116.11HydroborationHydroboration--Oxidation of AlkenesOxidation of Alkenes
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Suppose you wanted to prepare 1Suppose you wanted to prepare 1--decanol decanol from 1from 1--decene?decene?
SynthesisSynthesis
OHOH
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Suppose you wanted to prepare 1Suppose you wanted to prepare 1--decanol decanol from 1from 1--decene?decene?
Needed: a method for hydration of Needed: a method for hydration of alkenes with a regioselectivity opposite to alkenes with a regioselectivity opposite to Markovnikov's rule.Markovnikov's rule.
SynthesisSynthesis
OHOH
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TwoTwo--step reaction sequence called hydroborationstep reaction sequence called hydroboration--oxidation converts alkenes to alcohols with a oxidation converts alkenes to alcohols with a regiochemistry opposite to Markovnikov's rule.regiochemistry opposite to Markovnikov's rule.
1. hydroboration1. hydroboration
2. oxidation2. oxidation
SynthesisSynthesis
OHOH
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++ HH——BHBH22CC CC HH BHBH22CC CC
Hydroboration can be viewed as the addition ofHydroboration can be viewed as the addition ofborane (BHborane (BH33) to the double bond. But BH) to the double bond. But BH33 is is not the reagent actually used.not the reagent actually used.
Hydroboration StepHydroboration Step
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++ HH——BHBH22CC CC HH BHBH22CC CC
Hydroboration reagents:Hydroboration reagents:
HH22BB
HH
HH
BHBH22
Diborane (BDiborane (B22HH66))normally used in an normally used in an etherether-- like solventlike solventcalled "diglyme"called "diglyme"
Hydroboration StepHydroboration Step
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++ HH——BHBH22CC CC HH BHBH22CC CC
Hydroboration reagents:Hydroboration reagents:
BoraneBorane--tetrahydrofurantetrahydrofurancomplex (Hcomplex (H33BB--THF)THF)
Hydroboration StepHydroboration Step
+ OO
BHBH33–
••••
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HH22OO22, HO, HO––
HH BHBH22CC CC HH OHOHCC CC
Organoborane formed in the hydroborationOrganoborane formed in the hydroborationstep is oxidized with hydrogen peroxide.step is oxidized with hydrogen peroxide.
Oxidation StepOxidation Step
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1. B1. B22HH66, diglyme, diglyme
2. H2. H22OO22, HO, HO––
(93%)(93%)
ExampleExample
OHOH
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(98%)(98%)
HH33CC
HH33CC
CHCH33
HH
CC CC1. H1. H33BB--THFTHF
2. H2. H22OO22, HO, HO––
HH
CC CCCHCH33
CHCH33
CHCH33
HH OHOH
ExampleExample
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hydration of alkeneshydration of alkenes
regioselectivity opposite to Markovnikov's rule regioselectivity opposite to Markovnikov's rule
no rearrangementno rearrangement
stereospecific stereospecific synsyn additionaddition
Features of HydroborationFeatures of Hydroboration--OxidationOxidation
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1. B1. B22HH66, diglyme, diglyme
2. H2. H22OO22, HO, HO––
(82%)(82%)
ExampleExample
OHOH
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6.126.12Stereochemistry of HydroborationStereochemistry of Hydroboration--OxidationOxidation
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hydration of alkeneshydration of alkenes
regioselectivity opposite to Markovnikov's rule regioselectivity opposite to Markovnikov's rule
no rearrangementno rearrangement
stereospecific stereospecific synsyn additionaddition
Features of HydroborationFeatures of Hydroboration--OxidationOxidation
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H and OH become attached to same H and OH become attached to same face of double bondface of double bond
1. B1. B22HH66
2. H2. H22OO22, NaOH, NaOH
only product is only product is transtrans--22--methylcyclopentanolmethylcyclopentanol(86%) yield(86%) yield
synsyn--AdditionAddition
CHCH33
HH
HH
CHCH33
HH
HOHO
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6.136.13Mechanism of HydroborationMechanism of Hydroboration--OxidationOxidation
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syn addition of syn addition of HH andandB B to double bondto double bond
B B adds to less adds to less substituted carbonsubstituted carbon
11--Methylcyclopentene + BHMethylcyclopentene + BH33
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Organoborane intermediateOrganoborane intermediate
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OOH replaces B on H replaces B on same sidesame side
Add hydrogen peroxideAdd hydrogen peroxide
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transtrans--22--MethylcyclopentanolMethylcyclopentanol