hydroboration ppt

7/27/2019 hydroboration ppt

1/34

WWU -- Chem istry

How Do You Hydrate a Double

Bond?

CH CH2R R CH CH3

OH

H2O

H+

The prob lem w ith th is approach is that yields

are low, and carbo cat ion rearrangements cancompl icate the results .

We need a better app roach !


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Oxymercuration of an Alkene

Hg(OCOCH3)2

H2O

NaBH4

CH CH2

R R CH CH2

Hg(OCOCH3)

OH

R CH CH2

H

OH

Yields are high, there are norearrangements, and the con dit ions

are m ilder.

The sodium borohy dr ide reduces the

mercury to Hg0.

NOTE:The prod uct is the same one that

wou ld be ob tained i f you added w ater

across the double bond accord ing to

Markovnikovs Rule!


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Mechanism of Oxymercuration

slow

+ CH3COO_

+

Hg OCOCH3

OCOCH3

H HHg

OCOCH3


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Step 2...

..

..

- H+

::

or

..

..

H

O

H

H

Hg

O

H

H

OCOCH3

H

H

OHg

H

OCOCH3

Hg

H H

OCOCH3

+


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Approach f rom

the bot tom is

blocked.

This is the

only opening

for the

nucleophi le.


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Reduction

::

or

..

..

NaBH4

::

..

..

or + Hg

(ppt)

H

H

O

H

H

H

Hg

O

H

H

OCOCH3

H

H

OHg

H

OCOCH3

H

H

OH

H


7/34WWU -- Chem istry

slow

Hg(OCOCH3)

H2O

25 C

..:

+

- H+

+

CH3

H CH3

O

H

CH3

CH3

OH

Hg OCOCH3

H

HgH

CH3

H

CH3

OCOCH3

HgH

CH3

H

CH3

OCOCH3

H O

H



NaBH4

H

CH3

CH3

OH

Hg OCOCH3

H

H

CH3

CH3

OHH

H

This is equ ivalent to Markovniko v addi t ion ofwater acros s th e double bond.

Note that the -OH and the -H are anti to one

another.



but what if you want the

isomeric alcohol?

CH CH2R R CH2 CH2 OH

how???



Hydroboration of an Alkene

3

BR CH CH2

R CH2

CH2

BH3

a trialkylborane



Borane

H

B

H H

This is not s table under ordinary

con di t ions , so we have to generate it w i th in

the react ion -- in situ



Preparation of Diborane

3 NaBH4 + 4 BF3ether

2 B2H6 + 3 NaBF4

2B

H

H

H

HB

H

H H

BH H

Diborane



BH3+

2 3

B

a trialkylborane

R CH CH2

R CH CH2

R CH CH2

B H

H

H

R CH CH2

BH2

H

R CH CH2

B H

CH2

H

CH R

H

R CH CH2

H

B H

R CH CH2

R CH CH2

H

Notice the anti-Markovnikov

or ientation o f addi t ion!

Regioselect ive



Stereochemistry of

Hydroboration

CH3

H

CH3

H

H BH2H BH2



We can prepare a w ide variety of organobo ranes

by th is method.

Bu t now : we made an organoborane -- what can

we do w i th it?

Bo ron has a strong aff in i ty for oxygen -- i t formsa very strong B -O bond. When we react the

organoborane w ith oxy gen-conta in ing

com pounds, we can obtain new organic prod ucts .

For example: the react ion o f an organoborane

w ith a carbox yl ic acid yie lds a reduct ionprodu ct .



Hydroboration - Protonolysis

+

B

3

3

+

(CH3COO)3B

R CH2

CH2

CH3 C OH

O

R CH2

CH3

3



Examples of Hydroboration-Protonolysis

CH3 CH2 CH2 CH CH2 CH3 CH2 CH2 CH2 CH33

1) BH3

2) CH3COOH3

1) BH3

2) CH3COOH3 3



Mechanism of Protonolysis

R CH2 CH2 B

O

C

O CH3

H

R CH2 CH2

H

B

O

CO CH3

+



Stereochemistry of Hydroboration-Protonolysis

BH3

CH3COOH

CH3

CH3

CH3

B

H

CH3

CH3

H

H

CH3Not ice that the twohydrogens are syn

to one another!


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DO YOU REMEMBER OUR ORIGINAL

QUESTION? HOW DO YOU DO THIS?

CH CH2R R CH2 CH2 OH

how???

Another var iat ion on the decomposi t ion of

organoboranes w i th oxygen-conta in ing com pounds is

to perform the oxidat ion w ith an alkal ine solu t ion of

hydrogen peroxide.

The react io n, in th is c ase, is an oxidat ion, and the

pr inc ipal prod ucts are alcohols.


21/34

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Hydroboration - Oxidation

3

B

+

3 H2O2

OH

_ 3

+

B(OH)3

R CH2

CH2 R CH2 CH2 OH

The produc t is an alcohol , bu t in th is case it

is the regio isomer of the one obtained by

hydrat ion o f an alkene or by oxymercurat ion.


22/34

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)(3

B + OOH

_

_

+ OH

__

OH

_-

OOH

OOH

_

_

OH-

_

( )3

B

H2

O

OH_ 3

+ B( )3OH

R CH2

CH2

B O

CH2

CH2

O H

CH2

R

CH2R

R CH2

CH2

B

CH2

CH2

R

O

CH2CH2R

R CH2

CH2

B

CH2

CH2

R

O O H

OCH2CH2R

CH2

CH2

R

R CH2

CH2

B O

OCH2

CH2

R

R CH2

CH2

O R CH2

CH2

OH

a trialkoxyborane

R CH2

CH2

Each addit ion is a synaddi t ion


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Stereochemistry of Hydroboration-Oxidation

BH3

H2O2/OH

_

CH3

CH3

CH3

B

H

CH3

CH3

OH

H

CH3

The produc t is formed w i th

syns tereospecif ic i ty.

cis-1,2-Dimethylcyclopentanol


24/34

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Methods of Hydrating an Alkene

R CH CH2 R CH CH3

OH

H2O + H2SO4: follows Markovnikovs Rule

Hg(OCOCH3)2, H2O, fol lowed by NaBH4: follows Markovnikovs Rule

R CH CH2 R CH2 CH2 OH

B2H6, fol lowed by H2O2and OH-: gives an apparent anti-

Markovnikov product

NOTE: Each of these react io ns is regioselect ive!


25/34

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If you go back and analyze the mechanism of

hydroborat ion , you w i l l f ind that the react ion

actual ly doesfo l low an expandedMarkovnikovs Rule.

The Lewis acid (the species that add s f irs t)

does indeed add to th e carbon w ith the greater

num ber of hydrogens.

The dif ference here is that the Lew is acid isboron. Hyd rogen, in this react ion, is act in g as

a base(nu cleop hile)!


26/34

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Summary of Hydroboration

ReactionsBH3

CH3COOH H2O2/OH

_

C C

R H

H H

C C

H B

H

H

R HH H

C C

H H

R HH H

C C

H OH

R HH HEach of these

addi t ions is

stereosp ecif ical ly a

synaddit ion .


27/34

WWU -- Chem istry

The hydroborat ion react ion s w ere discovered by Prof .

Herbert C. B row n (Purd ue Universi ty - emeri tus)

Browns achievements, focused principally on his

discovery of hydrob orat ion, have contr ibuted to the

wel l-being o f humankind -- thu s, he earned th e Nobel

Prize in Chemist ry in 1979.

Hyd roborat ion , and the reagents that have been

developed from these basic m ethod s, have been veryusefu l in the synthesis of important organic

compound s, includ ing pharmaceut ica ls.


28/34

WWU -- Chem istry

Hydrob orat ion h as also been used to develop new

chemical reagents . Am ong these are:

Super Hydride (lithium triethylborohydride) -- a very

powerfu l reducing agent

Selectride (lithium tri-sec-buty lbo rohyd ride) -- a very

stereoselect ive reducing agent. Reduc t ions with

Selectr ide give a predom inance of one enantiomer

9-BBN (9-bo rabicy clo [3.3.1]nonane) -- a very eff ic ient

hyd robo rat ion reagent.


29/34

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Preparation of 9-BBN

B

H

+ BH3


30/34

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Advantages of 9-BBN

CH CH2B2H6

CH2 CH2 B

Yield: 80%

CH CH29-BBN

BCH2CH2

Yield: 98.5% !!!

For each of theseorganoboranes, the next step

would be oxidation with basic

hydrogen peroxide to yield the

corresponding alcohol.


31/34

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Hydroboration-Protonolysis

of Alkynes

This reaction yields exactly the

same product that would be

obtained from hydrogenation by

the Lindlar method.

To obtain the trans-alkene,

reduce the alkyne with sodium

in liquid ammonia.

C CR R' C C

R

H

R'

C C

R

H

R'

H

3 + BH3

3

B

3

CH3COOH

c is


32/34

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Hydroboration - Oxidation

of Alkynes

C CR R' C C

R

H

R'

C C

R

H

R'

OH

3 + BH3

3

B

3

H2O2/OH

_


33/34

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But.

TAUTOMERISM !!!

C C

R

H OH

H

R C C H

OH

H

enol keto


34/34

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Aldehydesare formed by the hydrob orat ion -

oxidation of alkynes,

Whereas,

Hydrat ion of an alkynew i th H2O, H2SO4, and HgSO4

produces ketones.

hydroboration ppt

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