lecture 7. rearrangement involving carbocations qn: why is rearrangement quite common to...
TRANSCRIPT
Lecture 7
R R
TS
R
R R
TS
R R
TS
C R
Antibonding
Bonding
Orbital at migrating centre
Rearrangement Involving Carbocations
Qn: Why is rearrangement quite common to carbocations, and not in case of radicals or anions?
Formation of TS involves combination of bonding orbital (C-R)with the vacant orbital on the carbocation,
singly occupied orbital in case of radical and doubly occupied orbital in case of carbanion.This overlap generates
one bonding and one antibonding scenario.
In case of cation, both the electrons occupy the bonding orbital……hence rearrangement is facile
R3C
Carbocation
CR2
Carbene
YY
R CH2
CH2 RHC CH2
RR
R1 R
R
R1R
R1
CH2
RR
R R
RCH2
R
1)
2) - H
3)
Polymer
4) Product
R3C R3C
Electron Deficient Carbon Carbene
Reactions of Carbocations:
1) Combination with a nucleophile
2) Elimination of a proton
3) Addition to an unsaturated linkage
4) rearrangement
Me
Me
HO
Me
OH
Me
Me
Me
Me O
Me
H
Me
Me
HO
Me
OH
Me
Me
Me
HO
Me
OH2
Me
H H2O-Me
Me
HO
Me
Me
Me
HO
Me
Me
Me
Me Me
Me
MeHO
Me
Me
Me O
Me
Pinacol - Pinacolone Rearrangement ----
Pinacol Pinacolone
Involvs 1, 2 shift of Me :--
Pinacolone
Pinacol
Qn: Why does the methyl migrate to a tertiary cation?
Ans: To achieve greater stabilization by delocalization of charge through electron pair of oxygen atom.
R1
R2
HO
R3
OH
R4
R1
R2
HO
R3
R4
R1OCC
R2
R3
R4
R2OCC
R1
R3
R4
R1
R2
R3
OH
R4
R1
CCOR4R2
R3
R2 CCOR3
R4
R1
4 Possibilities
Ph
HO
Ph
Me
Me
OH
Ph
Ph
Me
Me
OH
Ph
HO
Ph
Me
Me
Ph
PhMe
O
Me
Example : --
OR
( Major )
( less stable )
For differently substituted pinacols
How to decide which will be major?
1) First decide which carbocation is more stable
2) Then consider the relative migratory aptitude of the groups that will undergo 1,2-shift.
HO OH2
HO
ph
HOPh
Ph
PhO
Migratory aptitude aryl>alkyl usually H>alkyl
Aryl has greater migratory aptitude because it assists in the departure of leaving group (anchimeric assistance)
Via bridged carbocation
Electron donating groups at the ortho or para position will enhance the migratory aptitude while
electron withdrawing groups will lower the aptitude for migration.
H3C
H3CHO
CH3
CH3Br
H3C
H3CHO
CH3
CH3
H3C
H3CHO
CH3
CH3NH2
H3C
H3CHO
CH3
CH3N2
H3C
H3CHO
CH3
CH3
Ag
AgBr
HNO2
Product.
Product
H
H3CHO
H3C
HO
CH3
H
HO
Stabilised by hyperconjugation
If Methyl migrates
Less Stable
In case of hydride vs alkyl migration
Hydride generally migrates because of the generation of a more stable carbocation
Other ways of carrying out pinacol-pinacolone type rearrangements
HO
R1
R3
R2
O
R1R3
R2
H
OH
NH2
HNO2
OH
NH2HNO2
??
??
R'
R
OH
R'
R
OH
OH OH
+
Stereochemistry of migration:
The migrating group retains configuration. That means migrating group is never totally detached
This is also proved by cross over experiments Home Assignment
LECTURE 8
NH2
Me
OH
HMeNaNO2
PhHCl
A =HMe
PhMe
OH
NH2
OH
PhMeHMe
NH2
OHMe
Ph
Me H
Ph
Me H
Me O
COMe
Ph
HMe
MeH
N2
A
Me
Me
Me
Nu
Ph
OHMe
Ph
OHMe
H
N2+
NH2
HNO2
H
H
?
Ans.
1 2
Nu (Ph)
2 2
(major product)
211
2
2
Qn: Write down the Fisher projection formula of the major product
Problems on Pinacol-pinacolone rearrangement
O
O
O
OH OHOH
OHO
Mg
Benzene
H
Convert
NH2
OH
N2
OH
H
OH
O
NaNO2
HCl
(major)
N2
OH
OH
NaNO2
HClOH
NH2
CHO
NH2
OH
34
56 1
1
23
4
5
6
1
2
3
4
5 6
( major)
2
R C
O
N
R C
O
NH2
R C
O
N
B
A
R C
O
NHOCOR
R
RC NHBr
O
R N C O
R C
O
N
R C
O
N A
R C
O
N OCOR
R C
O
N Br
R C
O
N
RNH2
R NHCO2H
RCON
RCON
"a"
B = H
"b"
B =
Acyl nitrene
- CO2R'OH
NHCOOR'Carbamate or Urethane
Hofmann :
Lossen :
Migration to electron deficient Nitrogen
Hofman, Curtius,
Lossen, Schmidt
ROCHN C
O
NHR
RCONHOH
NaOBr
RCOOH
H2O
R C N
R C
O
NH2
O
RCON
HN3
RCONHNH2
R CO
N N N
Curtius(Schimdt)
(1/2 eq)
HNO2
Problem
HO R2
N
R1
R2 C
O
NHR1
N
NH
O
H2O
R2 C
O
NHR1
N
R2
R1
NOH2
PPA
R1 R2
NOH2
H
NOH
R1
C
R2
N
OH
H
R1
C
R2
N
OH
Base[ NH(CH2)5CO]n
Nylon 6Caprolactam
Cyclohexanone Oxime
H2SO4
SOCl2
P2O5
BF3
Beckman Rearrangement
Anti group always migrates…no question of migratory aptitude!
Mechanism
R
O
R
O
R
H
R
R1 C
O
O O H
R C
OH
O R
R C
O
OR- H
R
O
OR
R
OH
R
HO R
R O O C
O
R1
Peracid
orPeroxide
O
X
X
O
OH
COOH
1. OH
2. H
Rearrangement Involving Carbanions
Favorskii Rearrangent
Migration to electron Deficient Oxygen
Bayer Villiger Oxidation
COOH
O
OH
OH
O
COOH
O
Cl
O
OH
COOH
Cl
COOH
OH
OH
OH
O
O
Cl
O
Cl
COOH
Cl
*
*
* *
*
*
*
*
+
*
*
*
*
*
*
*
*
*
*
** *
*
+
R1
R2
O
R4
R1
R2 CO2H
R3
R4
OH
R1 CO2H
R2CHR3R4
R1
R2
O
R4
R3
X
OHR1
R2
H
R3
R4
X
O
+
R3
Earlier mechanism
However
Application to acyclic systems
OH OH