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is a broad class of organic reactions where the carbon skeleton of skeleton of a molecule is rearranged to give a structural isomer of the original molecule Often a substituent moves from one atom to another atom in the same molecule.
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adanya atom C yang kekurangan elektron (karbonium) dan adanya atom N atau O yang memiliki pasangan elektron bebas dalam satu molekul
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terbentuknya senyawa antara ( intermediet) dari produk yang diharapkan yang berenergi tinggi. Energi dapat diturunkan dengan adanya pergeseran ( shift) dari substituen/gugus/atom substituen/gugus/atom tetangganya sehingga menghasilkan senyawa antara yang stabil
Zat antara yang diharapkan adalah karbokation 2o, tetapi energi karbokation ini sangat besar
Energi tersebut dapat diturunkan bila gugus tetangga metil CH 3 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
Wagner-Meerwein Pinakol-pinakolon Hofmann Beckman Curtius-Schmidt Baeyer-Villiger Cope Claisen
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Rearrangement of the carbon skeleton via carbonium ions
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dapat terjadi pada reaksi substitusi SN1, reaksi eliminasi E1 dan reaksi adisi elektrofil yang produk zat antaranya merupakan suatu karbokation
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Synthetic importance especially in the chemistry of terpenes and related compounds
:
phenyl > tert-butyl > ethyl > methyl
Produk Substitusi 1
Produk Substitusi 2
Produk Substitusi 3
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Rearrangement of vicinal diols Menghasilkan senyawa keton Can be viewed as a special case of the Wagner Meerwein rearrangement Limited synthetic importance; although it can be a useful alternative to the standard methods for synthesis of aldehydes and ketones –
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The order of migration is R 3C > R 2CH > Ar > RCH2 > CH3 > H The product ratio may also depend on the acid used.
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primary amines as product from amides can be considered as a removal of the carbonyl group from an amide initiated by generating an electron-sextet configuration at nitrogen
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
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Rearrangement of oximes to give N-substituted amides concentrated sulfuric acid, hydrochloric acid, liquid sulfur dioxide, thionyl chloride, phosphorus pentachloride, zinc oxide and even silica gel can be used as reagents A side reaction can give rise to the formation of nitriles The reaction conditions often are quite drastic : concentrated sulfuric acid at 120 °C The required oxime can be easily prepared from the respective aldehyde or ketone and hydroxylamine
= Baeyer – Villiger Oxidation
Oxidation of ketones to carboxylic esters
Reagents: hydrogen peroxyde or a peracid
Modern variants are the enzyme and the transitionSynthetically metal catalyzed very useful which allowing reaction for an oxidation under mild (synthesis of conditions in natural good yields, products) with one stereoisomer being formed predominantly
An approximate order of migration: R 3C > R 2CH > 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
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acylphenols from phenyl esters
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need a Lewis acid as catalyst
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a rearrangement reaction to yield ortho- and parasubstitutions
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This rearrangement reaction is an important method for the synthesis of hydroxyaryl ketones
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As catalysts : aluminum halides, zinc chloride, titanium tetrachloride, boron trifluoride and trifluoromethanesulfonic acid
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ELEKTROSIKLISASI
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PENATAAN ULANG SIGMATROPIK
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REAKSI CHELETROPIK
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REAKSI TRANSFER GUGUS/ATOM
Migration of an atom or group with its sigma bond within a conjugated π framework. G | C (C=C)n —
(C=C)n
G | C
—
G
G
[ 1,3 ]
C C C
C C C
G
[ 1,5 ]
C C C C C
G C C C C C
[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
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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
Suprafacial migration of H [ 1,3 ]
[ 1,5 ]
forbidden
allowed
[ 1,7 ] forbidden
Suprafacial migration of R [ 1,3 ]
[ 1,5 ]
allowed with inversion of configuration
allowed with retention of configuration
[ 1,7 ] allowed with inversion of configuration
A [1,5] shift involves the shift of 1 substituent (-H, -R or -Ar) down 5 atoms of a π system
A [1,5] sigmatropic rearrangement involves three electron pairs (two bonds and one s bond)
1,5shift These reactions are predicted to proceed suprafacially, via a Huckel-topology transition state.
Hydrogen has been shown to shift in both cyclic and open chain systems at temperatures at or above 200˚C.
Orbital Picture of a Suprafacial [1,5]-H Shift
[1,7] sigmatropic shifts are predicted by the Woodward-Hoffmann rules to proceed in an antarafacial fashion
Antarafacial [1,7] shifts are observed in the conversion of lumisterol to vitamin D and in Walk reactions of bicyclic nonatrienes
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The Claisen rearrangement is a powerful carboncarbon 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.
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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-1,5hexadiene heated to 300°C yields 1,5heptadiene
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Both involve reorganization of an odd number of electron pairs (two bonds and one s bond) Both react by suprafacial pathways
Ring Strain can be employed to drive the Cope process:
H 5-20 °C
Brown Chem. Commun. 1973, 319
H
H 120 °C
Vogel Annalen 1958, 615 , 1
H
H 60 °C H
Reese Chem. Commun. 1970, 1519 equilibrium stongly favors this isomer