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Page 1: Reaksi Penataan Ulang
Page 2: Reaksi Penataan Ulang
Page 3: Reaksi Penataan Ulang

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

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H

CH3 CH3

H

NH2HONO

HOAc

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H3C

CH3

CH3

CH

CH2

HCl

CH3

H3C

Cl CH3

CH3

H

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The order of migration is

R3C > R2CH > Ar > RCH2 > CH3 > H

The product ratio may also depend on the acid used.

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H3C

OH

CH3

OH

CH3

CH3

H+

H3C C

O CH3

CH3

CH3

OH OH

OMe OMe

H+

C

O

OMe

OMe

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H3C

OH

H

OH

H

H+

OHOH

H3C

CH3

H+

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isocyanate

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• 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

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isocyanate

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C

OH2N

OH- , Br2

H2O

NH2

H3CH2C C

HC

CH3

O

NH2

H3CH2C C

HNH2

CH3

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N

H3C

OH H3C C

O

HN

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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

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• 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

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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

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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

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[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

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42

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1 2 3

1 2 3 4 5

1 2 3 4 5 6 7

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[ 1,3 ]

[ 1,5 ]

[ 1,7 ]

Suprafacial migration of H

forbidden

allowed

forbidden

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[ 1,3 ]

[ 1,5 ]

[ 1,7 ]

Suprafacial migration of R

allowed with inversionof configuration

allowed with retentionof configuration

allowed with inversionof configuration

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Orbital Picture of a Suprafacial [1,5]-H Shift

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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

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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

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• Both involve reorganization of an odd number of electron pairs (two bonds and one bond)

• Both react by suprafacial pathways

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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

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H

D

OAc

H* H

H

OAc

D*

[1,3] sigmatropic rearrangement of carbon requires inversion of configuration about a chiral center: