Transcript
Zat antara yang diharapkan adalah karbokation 2o, tetapi energi karbokation ini sangat besar
Energi tersebut dapat diturunkan bila gugus tetangga metil CH3 pindah (migrasi) membentuk karbokation 3o yang lebih stabil
Pada umumnya produk penataan ulang lebih banyak daripada produk yang diharapkan karena energi zat antara hasil penataan ulang lebih kecil dibandingkan dengan zat antara senyawa sebelumnya
H
CH3 CH3
H
NH2HONO
HOAc
H3C
CH3
CH3
CH
CH2
HCl
CH3
H3C
Cl CH3
CH3
H
The order of migration is
R3C > R2CH > Ar > RCH2 > CH3 > H
The product ratio may also depend on the acid used.
H3C
OH
CH3
OH
CH3
CH3
H+
H3C C
O CH3
CH3
CH3
OH OH
OMe OMe
H+
C
O
OMe
OMe
H3C
OH
H
OH
H
H+
OHOH
H3C
CH3
H+
isocyanate
• Generally yields are good.
• R can be alkyl or aryl.
• Modern variants of the Hofmann rearrangement use lead tetraacetate or iodosobenzene instead of hypobromite
• The reaction is closely related to the Curtius rearrangement
isocyanate
C
OH2N
OH- , Br2
H2O
NH2
H3CH2C C
HC
CH3
O
NH2
H3CH2C C
HNH2
CH3
N
H3C
OH H3C C
O
HN
An approximate order of migration: R3C > R2CH > Ar > RCH2 > CH3
Thus the Baeyer–Villiger oxidation of unsymmetrical ketones is regioselective
On the other hand aldehydes usually react with migration of the hydrogen to yield the carboxylic acid
• acylphenols from phenyl esters• need a Lewis acid as catalyst• a rearrangement reaction to yield
ortho- and para-substitutions • This rearrangement reaction is an
important method for the synthesis of hydroxyaryl ketones
• As catalysts : aluminum halides, zinc chloride, titanium tetrachloride, boron trifluoride and trifluoromethanesulfonic acid
Migration of an atom or group with its sigma bond within a conjugated π
framework.
G G | |
C—(C=C)n (C=C)n—C
G
C C C C C C
G[ 1,3 ]
G
C C C C C C[ 1,5 ]
C C C C
G
G = H, R
[5,5] Shifts The Woodward-Hoffman rules predict that [5,5]
sigmatropic shifts would proceed suprafacially These reactions are rarer than [3,3]
sigmatropic shifts, but this is mainly a function of the fact that molecules that can undergo [5,5] shifts are rare than molecules that can undergo [3,3] shifts
• Migration of a group across the same face of the system is a suprafacial rearrangement
• Migration of a group from one face of the system to the other face is called an antarafacial rearrangement
42
1 2 3
1 2 3 4 5
1 2 3 4 5 6 7
[ 1,3 ]
[ 1,5 ]
[ 1,7 ]
Suprafacial migration of H
forbidden
allowed
forbidden
[ 1,3 ]
[ 1,5 ]
[ 1,7 ]
Suprafacial migration of R
allowed with inversionof configuration
allowed with retentionof configuration
allowed with inversionof configuration
Orbital Picture of a Suprafacial [1,5]-H Shift
CH3
CD2
[1,5]-H
CH3
H
H
[1,5] shif t
25oC
H3C H
[1,5]
CHD2
CH2
CH3
H3C H
+
CH3
H
H
Claisen rearrangement• The Claisen
rearrangement is a powerful carbon-carbon bond-forming chemical reaction
• Discovered by Rainer Ludwig Claisen.
• The heating of an allyl vinyl ether will initiate a [3,3]-sigmatropic rearrangement to give a γ,δ-unsaturated carbonyl.
Cope rearrangement
• The Cope rearrangement is an organic reaction involving the [3,3]-sigmatropic rearrangement of 1,5-dienes
• It was developed by Arthur C. Cope.
• For example 3-methyl-1,5-hexadiene heated to 300°C yields 1,5-heptadiene
• Both involve reorganization of an odd number of electron pairs (two bonds and one bond)
• Both react by suprafacial pathways
equilibrium stongly favors this isomer
Reese Chem. Commun. 1970, 151960 °C
120 °C Vogel Annalen 1958, 615, 1
Brown Chem. Commun. 1973, 3195-20 °C
Ring Strain can be employed to drive the Cope process:
H
H
H
H
H
H
H
D
OAc
H* H
H
OAc
D*
[1,3] sigmatropic rearrangement of carbon requires inversion of configuration about a chiral center:
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