HỮU cơ NC THUYẾT TRÌNH NHÓM

[1,3] Sigmatropic Rearrangements - Suprafacial [1,3] hydrogen shift under thermal condition is forbidden.. [1,3] Sigmatropic Rearrangements- It has been observed that in a [1,3] thermal

Phản ứng chuyển vị б

(Sigmatropic rearrangement)

Nhóm thực hiện: Nhóm 3

Nguyễn Thị Kiều Oanh

Nguyễn Xuân Phong

Nguyễn Thị Thắm

Trần Thị Ngân Thoa

Nguyễn Thị Kim Thoa

Phạm Quang Thống

Phan Vũ Thuyền

Nguyễn Đinh Ngọc Trâm

Lương Văn Tri

Nguyễn Thị Mai Trinh

Nguyễn Hữu Trọng

5 major categories :

I Generalization :

II Suprafacial and Antarafacial processes

1 [1,3] Sigmatropic Rearrangements: H migration

2 [1,3] Sigmatropic Rearrangements: C migration

1 [1,5] Sigmatropic Rearrangements: H migration

2 [1,5] Sigmatropic Rearrangements: C migration

Phản ứng chuyển vị б

Sigmatropic Rearrangements

I Generalization :

Phản ứng chuyển vị б

Sigmatropic Rearrangements

• Migration of a s-bond across a conjugated p-system

• [i,j] shift when the s-bond migrates across i atoms of one system and j of another

II Suprafacial/Antarafacial

- Suprafacial migration: Group moves across same face

- Antarafacial migration: Group moves from one face to the other

1 [1,3] Sigmatropic Rearrangements: H migration

III [1,3] Sigmatropic Rearrangements

- Suprafacial [1,3] hydrogen shift under thermal condition is

forbidden Antarafacial [1,3] hydrogen shift is allowed only by the principles of orbital symmetry The transition state is a highly contorted species and the reaction is forbidden because of the

steric inhibition involved in such a process

1 [1,3] Sigmatropic Rearrangements: H migration

- Photochemical HOMO ( Ψ3) of this radical has same sign on the terminal lobes (m symmetry), which permits suprafacial [1,3]

hydrogen shift

1 [1,3] Sigmatropic Rearrangements: H migration

III [1,3] Sigmatropic Rearrangements

- It has been observed that in a [1,3] thermal suprafacial

process, symmetry is conserved only if the migrating

carbon has opposite lobes

2 FMO Analysis of [1,3] Sigmatropic Rearrangements of Alkyl Group

2 FMO Analysis of [1,3] Sigmatropic Rearrangements of Alkyl Group

- However, the [1,3] thermal suprafacial process that involves the same lobe gives the product with retention of configuration in the migrating group, but the process is not symmetry allowed On the other hand, symmetryallowed [1,3] thermal

antarafacial process with retention is geometrically forbidden and antarafacial

process with inversion is symmetry forbidden

IV [1,5] Sigmatropic Rearrangements

1 [1,5] Sigmatropic Rearrangements : H migration

- The [1,5] sigmatropic shift of hydrogen or deuterium atom occurs via a suprafacial pathway under thermal conditions

- The ground state electronic configuration of pentadienyl radical is Ψ12

Ψ 22 Ψ31

- [1,5] suprafacial migration will be a thermally allowed process

- Under photochemical conditions [1,5] suprafacial migration is no

longer possible and the shift has to be an antarafacial process

X

X

X+

IV [1,5] Sigmatropic Rearrangements

Example:

IV [1,5] Sigmatropic Rearrangements

2 [1,5] Sigmatropic Rearrangements : C migration

During [1,5] thermal suprafacial process, symmetry can be

conserved only if the migrating carbon involves the same lobe In other words, if the migrating carbon atom was originally bonded via its positive lobe, it will have to use its positive lobe to form the new C-C bond The entire process results in the retention of configuration in the migrating group However, if migrating

carbon uses the opposite lobe, it may give the product with

inversion of configuration, but the process becomes symmetry forbidden On the other hand, [1,5] thermal antarafacial process with retention is symmetry forbidden and antarafacial process

with inversion is symmetry allowed

X

X

X+

IV [1,5] Sigmatropic Rearrangements

2 [1,5] Sigmatropic Rearrangements : C migration

IV [1,5] Sigmatropic Rearrangements

1 The Cope Rearrangement

- The thermal [3,3] sigmatropic rearrangement of 1,5- dienes is called the Cope rearrangement

V [3,3] Sigmatropic Rearrangements

- In the Cope rearrangement, the migrating group is an allyl radical An analysis of the symmetry of the orbitals involved shows that [3,3] sigmatropic rearrangements are thermally allowed and photochemically forbidden

1 The Cope Rearrangement

 - Stereochemistry of the Cope Rearrangement: Boat

versus Chair Transition State

V [3,3] Sigmatropic Rearrangements

- In [3,3] sigmatropic shifts, a chair-like transition state is

energetically preferred to aboat-like structure Rearrangement of the meso diene through such a transition state then would give the cis - trans isomer

1 The Cope Rearrangement

V [3,3] Sigmatropic Rearrangements

- Stereochemistry of the Cope Rearrangement: Boat versus

Chair Transition State

- The case of the rearrangement of the racemic mixture the

trans-trans

isomer is the major product

2 The Claisen Rearrangement

- The Claisen rearrangement is a [3,3] sigmatropic

rearrangement in which an allyl vinyl ether is converted thermally to an unsaturated carbonyl compound

V [3,3] Sigmatropic Rearrangements

The Claisen rearrangement of allyl vinyl ether

2 The Claisen Rearrangement

The aromatic Claisen rearrangement involves the thermal transformation of a prop-2-enyl phenyl ether (1) into a

2-(prop-2-enyl)-cyclohexa-3,5-dienone (2)

V [3,3] Sigmatropic Rearrangements