Synthesis and important reactions of organoboranes were discussed.

1. Prepared By
Dr. Krishnaswamy. G
Faculty
DOS & R in Organic Chemistry
Tumkur University
Tumakuru
Organoboranes
or
Organo boron Compounds

2. Organoborane Chemistry deals with the chemistry of
organoboranes or organoboron compounds.
The C-B bond has low polarity because of small
electronegativity difference (C-2.55 and B-2.04).
Because of low electronegativity boron often forms electron
deficient compounds such as triorganoboranes.
Organoboranes acts as strong electrophile in organic chemistry
since boron is unable to gain a full octet of electrons.
Organoboron compounds are important reagents in organic
synthesis and the most important one is hydroboration.
Introduction

3. Synthesis of Organoboranes
(1) From Grignard Reagents
(2) From Borylation
Simple organoboranes such as triethylborane can be prepared from
trifluoroborane and ethyl Grignard reagent.
Metal catalysed C-H borylation produce an organoboron compound.
B
F
F
F
EtMgBr
B
Et
Et
Et
B(OMe)2 B(OH)2
Li
(MeO)3B

4. (3) From alkenes
Alkenes react with borane (BH3) and its derivatives to give
synthetically useful alkyl boranes.
R
BH3
R
BH2

5. The addition of borane to alkene is an electrophilic process and
it takes place through a concerted process in which the alkene
donates electron density to the borane and at the same time
hydogen shifts to carbon. This reaction is known as
hydroboration reaction.
B
H
H
H
C C
Concerted process
Lewis acid /
electrophile
Lewis base /
nucleophile
Hydroboration reaction

6. Hydroboration reaction is highly regioselective because of
combination of steric as well as electronic factors.
In hydroboration reaction boron bonds to less substituted
carbon via syn addition resulting in anti-Morkovnikov product.
This regiochemistry is just reverse of typical HX addition to the
alkene.

7. Syn additionB
H
H
H
C C
H
H
CH3
CH3
Less substituted carbon
+ -
Bond is polarized because of
electronegativity difference

8. Hydroboration reaction is very important reaction since boron
can eventually be replaced by hydroxy, carbonyl, amine and
halogen substituents with retention of configuration.
R
BH3
R
BH2
R
OH
R
CHO
R
NH2 / XHydroboration Reaction

9. Important organo borane reagents
B2H6
BH
disiamylborane
(bis-1, 2-dimethylborane)
B2H6
2
BH2
thexylborane
(1, 1, 2-trimethylpropylborane)
2,3-dimethylbut-2-ene
B2H6 HB
(1Z,5Z)-cycloocta-1,5-diene
9-BBN
(9-bicyclo[3.3.1]nonane)
2-methylbut-2-ene
Sia2BH
ThxBH2

10. BH
2
B2H6
(-) Ipc2BH
Diisopinocamphenyl borane
(+)-Pinene
O
BH
O
O
BH
O
Catechol borane Pinacol borane
4,4,5,5-tetramethyl-1,3,2-dioxaborolanebenzo[d][1,3,2]dioxaborole

11. Reactions of Organoboranes
Organoboranes
Isomerization
Oxidation
Carbonylation Cyanidation
Protonolysis
Here the focus is on the following reaction of organoboranes

12. Isomerization reaction
Some alkyl boranes rearrange at elevated temperature (160oC
and above) to form more stable isomer i.e. boron prefer to bond
with least substituted terminal carbon to minimize unfavourable
steric interaction.
C C C C C
B
R R
160o
C
C C C C C B
R
R

13. C
B
C
C
CH3H3C
CH3
CH3
CH3
CH3
H3C
H3C CH3
C
B
CH2
C
C
CH3
CH3
CH3
H3C
H3C CH3
CH3H3C
H
Terminal carbon
160o
C
Severe
steric
repulsion
Rearrangement is associated with the fact that hydroboration
reaction is thermally reversible at 160oC and above. This occurs
intramolecularly through three membered cyclic bridged TS
analogues to bridged carbocations.

14. C C C
H
C
H
CH2
B
R R
CH C C C C B
R
R
C C C C C
B
R
HR
C C C C C
B
R R
C C C C CH
B
R
H
R
C C C C CH
B
R
H
R
C C C C C
B
R
H
R
H H
H
H
H
H
H
H
H
HH
H H
H
H
H

16. Most widely used reaction of organo boranes is the oxidation to
alcohols is known as hydroboration-oxidation.
Alkaline hydrogen peroxide is the reagent usually employed to
effect the oxidation.
Oxidation reactions
R3B 3H2O2 NaOH 3ROH NaH2BO3 H2O
The trialkylborane is converted into trialkoxyborane by a series
of Boron to Oxygen migrations. Finally hydrolysis of alkoxy
boron bonds by aqueous alkaline solution generates alcohol.

17. H2O2 NaOH
3ROH NaH2BO3 H2O
HOO Na H2O
HOO R B
R
R
O
OH
R
B
R
OR
R
B
R
OR
R
B
R
R
HOO RO B
R
R
O
OH
RO
B
R
OR
RO
B
R
OR
HOO RO B
OR
R
O
OH
RO
B
OR
OR
RO
B
OR
OR
3H2O
NaOH
Mechanism

18. Other oxidants includes molecular oxygen, sodium
peroxycarbonate and amine oxides permit oxidation of
organoborane to alcohols.
The oxidation reaction proceeds with retention of configuration.
(1) B2H6
(2H2O2-NaOH
OH

19. Alkyne hydroboration-oxidation strategy does not usually yield
an alcohol as the resulting enol is tautomerized to form a ketone.
R R
BH3
RR
H B
H
R
R
R
H
R
3 H2O2
NaOH
RR
H OH
3
RR
O
3
Alkenylboranes

20. Protonolysis
Organoboranes are stable to mineral acids (HCl, H2SO4, etc..)
but are readily attacked by carboxylic acids. Boiling an alkyl
borane with a liquid acid results in the formation of a
hydrocarbons. The process is termed as protonolysis of
organoboranes.
B
R
CH3COOH

B
O
OH3C
R H

21. Mechanism
B R
HO
O
H3C
B
O
OH3C
R H
This method of reduction of double bond is a good alternative
to catalytic hydrogenation.

22. R R
BH3
RR
H B
H
R
R
R
H
R
3 CH3COOH
RR
H H
3
Alkenylboranes
Protonolysis of the alkenylborane can lead to alkenes in good
yield.

23. Carbonylation reaction
Carbonylation of organoboranes with carbon monoxide leads to
formation of Lewis acid-base complex which gives rise to range
of products which depends on reaction conditions.
Three possible migrations of groups may occur leading to
formation aldehydes and ketones as well as primary, secondary
and tertiary alcohols.
B
R
RR
C O B
R
R
R
C O
Lewis acid-base complex

24. Single migration of group followed by reduction with mild
reducing agent lithium trimethoxyaluminium hydride leads to
formation of mono migrated product.
This mono migrated product undergoes oxidation with alkaline
hydrogen peroxide to give aldehyde.
On the other hand treatment with LiAlH4 followed by oxidative
workup produces primary alcohols.
Synthesis of aldehyde and alcohols

25. B
R
RR
C O B
R
R
R
C O B
R
R
C
O
R
Li[AlH(OMe)3]
B
R
R
C
OAl(OMe)3
RH
Li
LiAlH4
H2O2 / NaOH
HO C
OAl(OMe)3
RH
Li
R
O
H RHO
Mono-migrated product
H2O2 / NaOH

26. If the carbonylation reaction is done in the presence of a small
amount of water at 100 °C, a second alkyl group migrates and
after oxidative workup produce corresponding ketone.
B
R
RR
C O B
R
R
R
C O B
R
R
C
O
R
H2O
BR
R
C
O
R
H2O2 /
NaOH
R
O
R
Mono-migrated product

OH
BHO
R
C
OH
RR
Second-migrated product
Synthesis of Ketones

27. The use of thexylborane (1,1,2-trimethylpropylborane) as the
hydroboratingagent permits
(a) the synthesis of mixed trialkyboranes which upon
carbonylation gives unsymmetrical ketones.
BH2
alkene-1
BH
alkene-1
alkene-2
B
alkene-1
alkene-2
Mixed trialkyl boranes
CO-H2O
H2O2-NaOH
alkene-2
alkene-1
O

28. (b) cyclic hydroboration of dienes followed by carbonylation
yields cyclic ketones.
BH2
B
CO-H2O
H2O2-NaOH
O
Due to low migratory aptitude of thexyl moiety in
carbonylation and it serves as an anchor group

29. Synthesis of tertiary alcohols
Carbonylation of trialkylboranes in the presence of ethylene
glycol results in migration of all three alkyl group from Boron to
carbonyl Carbon to give, after oxidation, the corresponding tert-
alcohols.
B
R
RR
C O
HO
OH
H2O2 /
NaOH
HO C
R
R
R

30. B
R
RR
C O B
R
R
R
C O B
R
R
C
O
R
BR
R
C
O
R
H2O2 /
NaOH
Mono-migrated product

O
BO
R
C
OH
RR
Second-migrated product
HO
OH
HO
OH
BO
R
C
OH
RR
O
HO
OH
B
O
C
R
R
R
O
Third-migrated product
HO C
R
R
R

32. Cyanidation reaction
Since nitrile anion is isoelectronic with carbonyl and reacts with
trialkyl boranes.
Reaction of nitrile with trialkyl borane is known as cyanidation
reaction. The intermediate cyanoborates are thermally stable
hence to induce 1, 2-migration with electrophiles such as
benzoyl chloride or trifluoroacetic anhydride.
Cyanidation is useful alternative route to prepare ketones and
trialkylmethanols.
Formation of ketones and trialkylmethanols occurs under milder
condition.

33. B
R
RR
C N
H2O2 /
NaOH
O
C
RR
Ph Cl
O
F3C O
O
CF3
O HO C
R
R
R
Formation of ketones and trialkylmethanols depends on the
stoichiometry of the electrophile.
Excess of TFAA results in the formation of trialkylmethanol after
oxidation.

34. B
R
RR
C N B
R
R
R
C N
B
R
R
C
N
R
H2O2 /
NaOH
H2O2 /
NaOH
O
C
RR
F3C O
O
CF3
O
HO C
R
R
R
B
R
R
R C N
O
CF3
CF3
O
B
R
C
N
R
CF3
O
R
F3C O
O
CF3
O
B
R
C
N
R
CF3
O
R
O
O
CF3
CF3
O
B
C
NR
CF3
O
R
O
O
CF3
CF3
O
R