1. Part 2(i): Electrocyclic Reactions
Second Year Organic Chemistry Course
CHM3A2
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.

2. Electrocyclic Reactions
CHM3A2
– Introduction to FMOs –
– Learning Objectives Part 2(i) –
Electrocyclic Reactions
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.

3. 6p-Electron Systems RS
Meso RR
Enantiomers SS

4. 4p-Electron Systems RR
Enantiomers SS RS
Meso

5. HOMOs of Polyenes
A new s-bond is forming at the termini of each of the polyene systems.
Thus, it is clear that the p-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.

7. Disrotatory Motion: Dark/Dark
In-phase
Meso

8. Disrotatory Motion: Light/Light
In-phase
Meso

9. Conrotatory Motion: Dark/Dark
In-phase RS
Enantiomer

10. Conrotatory Motion: Light/Light
In-phase SR
Enantiomer

11. 4n+2 p Electron Electrocyclic Reactions
1, 3, 5-Hexatriene
6 p AOS 6 p
MOs
Electrons y
3 = HOMO
two nodes (7/3) y 3 – H O M
Disrotatory

12. y 3 – H O M M e M e Dark/Dark Or Light/Light DISROTATORY DISROTATORY
Meso
RR and SS (enantiomers)

13. 4n p Electron Electrocyclic Reactions

15. 4n p Electron Electrocyclic Reactions
Butadiene y 2 – H O M
4 p AOS 4 p
MOs
Electrons y
2 = HOMO
one node (5/2)
Conrotatory

16. y 2 – H O M Dark/Dark Or Light/Light CONROTATORY CONROTATORY
RR and SS (enantiomers) see next slide
Meso

17. Two alternative and equivalent modes of conrotatory in-phase overlap
Enantiomer Formation y 2 – H O M
Two alternative and equivalent modes of conrotatory in-phase overlap
CONROTATORY RR S
A Pair of Enantiomers

19. Coping with Ring Opening Reactions
1. Draw out the p-HOMO of the product without the substituents
2 HOMO

20. 2. Draw out the MO of the Starting material
Same Phase
Bonding: Must be in phase!
2 HOMO

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

22. Product stereochemistry
4. Decide how the substituents move
CONROTATORY
2 HOMO

23. Rules for Electrocyclic Reactions
_______________________________________________________Number of -Electrons Thermal Photochemical
(CHM3A2)
4n CONrotatory DISrotatory
4n DISrotatory CONrotatory
_______________________________________________________
Photochemical reactions will be dealt with in the third year course (CHM3A2), where the first electronically excited stated state becomes the HOMO.

24. Electrocyclic Reactions
CHM3A2
– Introduction to FMOs –
– Summary Sheet Part 2(i) –
Electrocyclic Reactions
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 6p 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.

25. Exercise 1: 4n+2 p Electrocylic Systems
The triene 1 undergoes a thermal electrocyclic cyclisation. Using FMOs identify all the products. 1

26. Answer 1: 4n+2 p Electrocylic Systems
The triene 1 undergoes a thermal electrocyclic cyclisation. Using FMOs identify all the products. 1 y 3 – H O M
DISROTATORY R S
Dark R S
Light
Superimposable Mirror Images
Exactly the same compound
MESO Compound

27. Exercise 2: 4n+2 p 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 stereoisomers formed from each reaction and the stereochemical relationship of the products between the pair of reactions 1 2

28. Answer 2: 4n+2 p 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
DISROTATORY y 3 – H O M
DISROTATORY R S
Dark R S
Light R
Dark S
Light
Enantiomers
Diasteroisomers
Enantiomers

29. Exercise 3: 4n p Electrocylic Systems
The cyclobutadiene derivative undergoes an stereospecific electrocyclic ring opening reaction to afford a single product. Utilise FMOs to identify the product. 1

30. Answer 3: 4n p 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 y
2 HOMO

31. Exercise 4: A Cascade Electrocylic System
Use FMOs to predict the stereochemical outcomes in the reaction scheme below.

32. Answer 4: A Cascade Electrocylic System
Use FMOs to predict the stereochemical outcomes in the reaction scheme below. y 4
(3 nodes 9/4)
of 1, 3, 5, 7-octatetraene y 3
(2 nodes)
of 1, 3, 5-hexatriene M e H M e
4n - CONROTATORY
(4n + 2) - DISROTATORY M e H H M e
Dark/Dark
Dark/Dark M e M e H H M e
Light/Light
Light/Light

33. 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.

34. Product. Least sterically hindered
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.
The arrow pushing mechanism reveals that the reaction involves the ring closure of two 1,3,5-hexatriene systems. Thus, need to consider y3 HOMO of 1, 3, 5-hexatriene.
Light
Light H
Thermodynamic
Product. Least sterically hindered
Disrotatory
of both
triene systems
Light
Dark H

35. Exercise 6: Complex Electrocyclic Reaction
Cyclooctatetraene undergoes an electrocyclic ring closure forming only the cis-isomer as depicted right. Rationalise this result using FMOs.

36. Answer 6: Complex Electrocyclic Reaction
Cyclooctatetraene undergoes an electrocyclic ring closure forming only the cis-isomer as depicted right. Rationalise this result using FMOs.
4p Electron Process H H
CONROTATORY y 2
HOMO
Butadiene
6p Electron Process H y 3
HOMO
1,3,5-Hexatriene H
DISROTATORY
Thus, the reaction must proceed by a 6 p electron process, despite the 4 p 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 6p electron aromatic transition state is lower in energy than the 4p electron anti-aromatic transition state.

37. Exercise: 4n p Electrons Electrocyclic Reactions
Using FMOs rationalise why the two diastereoisomers have such different reactivities.

38. Answer: 4n p Electrons Electrocyclic Reactions
Using FMOs rationalise why the two diastereoisomers have such different reactivities.