Second Year Organic Chemistry CourseCHM3A2

Frontier Molecular Orbitals and Pericyclic Reactions

Part 2(i):

Electrocyclic Reactions

An electrocyclic reaction involves the formation of a -bond between the termini of a linear conjugated -system by two of the -electrons - or the reverse reaction.

MeMe

HOMO

MeMe

HOMOn n

DISROTATORY CONROTATORY

– Learning Objectives Part 2(i) –

Electrocyclic Reactions

CHM3A2– Introduction to FMOs –

After completing PART 2(i) of this course you should have an understanding of, and be able to demonstrate, the

following terms, ideas and methods.

(i) An electrocyclic reaction involves the formation of a -bond between the termini of a linear

conjugated -system by two of the -electrons - or the reverse reaction.

(ii) Electrocyclic reactions are stereospecific. The stereospecificity being afforded by the disrotatory

or conrotatory nature of the bond making/breaking process

(iii) 4-electron systems are conrotatory when thermally promoted, (and disrotatory when

photochemically promoted - CHM3A2).

(iv) 6-electron systems are disrotatory when thermally promoted (and conrotatory when

photochemically promoted - CHM3A2).

(v) The disrotatory or conrotatory process involved in the bond making/breaking process is controlled

by the HOMO (thermal reaction) or SOMO (photochemical reaction - CHM3A2) of the linear

conjugated -system which either is the starting material or product.

Meso

MeMe MeMe

6-Electron Systems

Me

Me

MeMe

MeMe

MeMe

MeMe

SS

RR

RS

Enantiomers

SS

RR

RS

MeMe MeMe

MeMe MeMe MeMe

MeMe

MeMe

Enantiomers

4-Electron Systems

Meso

HOMOs of Polyenes

A new -bond is forming at the termini of each of the polyene systems.

Thus, it is clear that the -system of the polyene systems must be interacting in

some fashion.

Analysis of the polyenes has shown that by considering the HOMOs, and

rotating the termini of them to overlap them in an in-phase fashion produces the

correct stereochemical outcome.

The termini of the orbitals can be rotated in two manners referred to as:

Conrotatory,

Disrotatory.

Disrotatory Motion: Dark/Dark

MeMe

n

MeMe

n

MeMe

n

In-phase

Meso

Disrotatory Motion: Light/Light

MeMe

n

MeMe

n

MeMe

n

In-phase

Meso

Conrotatory Motion: Dark/Dark

MeMe

n

MeMe

n

MeMe

n

In-phase

RSEnantiomer

Conrotatory Motion: Light/Light

MeMe

n

MeMe

n

MeMe

n

In-phase

SREnantiomer

4n+2 Electron Electrocyclic Reactions

Disrotatory

3 – HOMO1, 3, 5-Hexatriene

6 p AOS

6 MOs

6 Electrons

3 = HOMO

two nodes (7/3)

RR and SS (enantiomers)

MeMe Me

Me

3 – HOMO

Meso

DISROTATORY DISROTATORY

Dark/DarkOr

Light/Light

Butadiene

4n Electron Electrocyclic Reactions

4n Electron Electrocyclic Reactions

Conrotatory

2 – HOMOButadiene

4 p AOS

4 MOs

4 Electrons

2 = HOMO

one node (5/2)

Dark/DarkOr

Light/Light

RR and SS (enantiomers) see next slide Meso

2 – HOMO

CONROTATORY CONROTATORY

Enantiomer Formation

Two alternative and equivalent modes of conrotatory in-phase

overlap

2 – HOMO

CONROTATORY CONROTATORY

RR SS

A Pair of Enantiomers

Coping with Ring Opening Reactions

1. Draw out the -HOMO of the product without the substituents

2 HOMO

2. Draw out the MO of the Starting material

2 HOMOBonding: Must be in phase!

Same Phase

3. Open the C-C bond two afford the HOMO of the product

2 HOMOCONROTATORY

4. Decide how the substituents moveProduct

stereochemistry

2 HOMOCONROTATORY

_______________________________________________________Number of -Electrons Thermal Photochemical

(CHM3A2)

4n CONrotatory DISrotatory4n + 2 DISrotatory CONrotatory_______________________________________________________

Rules for Electrocyclic Reactions

Photochemical reactions will be dealt with in the third year course (CHM3A2), where the first electronically excited stated state becomes the HOMO.

– Summary Sheet Part 2(i) –

Electrocyclic Reactions

CHM3A2– Introduction to FMOs –

An electrocyclic reaction involves the formation of a -bond between the terminals of a linear conjugated -system

by two of the -electrons – or the reverse process.

Electrocyclic reactions are either 'allowed' or 'forbidden' – and they are stereospecific, occurring by either a so-

called conrotatory or disrotatory motion.

Electrocyclic reactions can be brought about by heat (CHM2C3B), by ultraviolet irradiation (CHM3A2), and

sometimes by the use of metal catalysts (CHM3A2). They are nearly always stereospecific. In many cases,

detection of their stereospecificity depends on distinguishing chemically similar stereoisomers - a problem which

has been overcome mainly by the development of spectroscopic methods of structure determination, especially

NMR spectroscopy. Thus, the recognition that stereospecific electrocyclic reactions form a coherent group

extends only over the last quarter of a century. Nowadays, the group includes some important synthetic reactions

as well as some of the most clear cut examples of the successful predictive power of orbital symmetry theory.

In the case of 6 systems, the thermal ring closure of 1,3,5-hexatrienes to conjugated cyclohexadienes is

stereospecific - and disrotatory - as the theory predicts. Ring closure of 1,3, 5-hexatrienes is a relative facile

process relative to butadiene ring closure which generates a highly strained butadiene derivatives.

In the case of 4 systems, the thermal ring opening of cyclobutenes to butadienes is stereospecific - and

conrotatory - as the theory predicts. In most cases, the ring opening goes to completion and there are very few

examples of the reverse process, the thermal cyclisation of butadienes. Fused cyclobutenes, however, are

thermally rather stable, especially those in which the second ring is five- or six-membered.

Exercise 1: 4n+2 Electrocylic Systems

The triene 1 undergoes a thermal electrocyclic cyclisation. Using FMOs identify all the products.

1

Answer 1: 4n+2 Electrocylic Systems

The triene 1 undergoes a thermal electrocyclic cyclisation. Using FMOs identify all the products.

1

Superimposable Mirror Images

Exactly the same compound

MESO Compound

RS

DarkDark

R S

LightLight

DISROTATORY

3 – HOMO

Exercise 2: 4n+2 Electrocylic Systems

1 2

The two diastereoismeric trienes 1 and 2 undergo thermal electrocyclic cyclisation reactions each affording a pair of disubstituted conjugated cyclic dienes. Identify all four products by constructing the transition state geometries, and state the stereochemical relationships that exist between the pairs of stereoisomers formed from each reaction and the stereochemical relationship of the products between the pair of reactions

Answer 2: 4n+2 Electrocylic Systems

The two diastereoismeric trienes 1 and 2 undergo thermal electrocyclic cyclisation reactions each affording a pair of disubstituted conjugated cyclic dienes. Identify all four products by constructing the transition state geometries, and state the stereochemical relationships that exist between the pairs of stereoisimers formed from each reaction and the stereochemical relationship of the products between the pair of reactions

1 2

3 – HOMO

Enantiomers EnantiomersDiasteroisomersRS

DarkDark

R R

DarkDark

R S

LightLight

S S

LightLight

DISROTATORY DISROTATORY

Exercise 3: 4n Electrocylic Systems

The cyclobutadiene derivative undergoes an stereospecific electrocyclic ring opening reaction to afford a single product. Utilise FMOs to identify the product.

1

Answer 3: 4n Electrocylic Systems

The cyclobutadiene derivative undergoes an stereospecific electrocyclic ring opening reaction to afford a single product. Utilise FMOs to identify the product.

1

CONROTATORY 2 HOMO

Exercise 4: A Cascade Electrocylic System

MeMe

HH

MeMeMe Me

HH

Use FMOs to predict the stereochemical outcomes in the reaction scheme below.

Answer 4: A Cascade Electrocylic System

Use FMOs to predict the stereochemical outcomes in the reaction scheme below.

MeMe

HH

MeMeMe Me

HH

MeMe

(3 nodes 9/4)of 1, 3, 5, 7-octatetraene

4n - CONROTATORY

Me Me

HH

(2 nodes)

of 1, 3, 5-hexatriene

(4n + 2) - DISROTATORY

MeMe

HH

MeMeMe Me

HH

Me Me

HHHH

MeMe

Dark/Dark Dark/Dark

Light/Light Light/Light

Exercise 5: Tandem Electrocyclic Reaction

Use FMOs to predict the stereochemical outcomes in the reaction scheme right. In principle, there are two possible products. Which will be formed in highest yield. Justify your answer.

H

H

H

H

Answer 5: Tandem Electrocyclic Reaction

Use FMOs to predict the stereochemical outcomes in the reaction scheme right. In principle, there are two possible products. Which will be formed in highest yield. Justify your answer.

H

H

H

H

The arrow pushing mechanism reveals that the reaction involves the ring closure of two 1,3,5-hexatriene systems. Thus, need to consider 3 HOMO of 1, 3, 5-hexatriene.

HH

HH

Thermodynamic Product. Least

sterically hinderedDisrotatoryof both

triene systems

LightLight

H

H

H

H

H

H

H

H

LightLight

LightLight

DarkDark

Exercise 6: Complex Electrocyclic Reaction

Cyclooctatetraene undergoes an

electrocyclic ring closure forming only

the cis-isomer as depicted right.

Rationalise this result using FMOs.

HH H H

0% 100%

Answer 6: Complex Electrocyclic Reaction

Cyclooctatetraene undergoes an

electrocyclic ring closure forming only

the cis-isomer as depicted right.

Rationalise this result using FMOs.

HH H H

0% 100%4 Electron Process

6 Electron Process

Thus, the reaction must proceed by a 6electron process, despite the 4 electron process being

possible by FMO theory. Reasons for formation of cis-isomer are possibly two-fold: (i) cis-isomer is

the thermodynamically more stable product, and/or (ii) the aromatic 6 electron aromatic transition

state is lower in energy than the 4 electron anti-aromatic transition state.

H H

CONROTATORY

HH

2

HOMO

Butadiene

HH

H H

DISROTATORY

3

HOMO

1,3,5-Hexatriene

Exercise: 4n Electrons Electrocyclic Reactions

Using FMOs rationalise why the two

diastereoisomers have such different reactivities.

H H

H H

Answer: 4n Electrons Electrocyclic Reactions

Using FMOs rationalise why the two

diastereoisomers have such different reactivities.

H H

H H

H HH

H

H H HH

CON

H H

H H H

HH

H

GEOMETRICALLY IMPOSSIBLE: Hydrogen placed

inside a six-membered ring

CON