Transcript Document
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 termini of a linear conjugated
-system by two of the
-bond between the
-electrons - or the reverse reaction.
HOMO n HOMO n Me Me DISROTATORY Me Me CONROTATORY
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
conjugated
involves the formation of a -system by two of the
-bond between the termini of a linear
-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
photochemically promoted - CHM3A2).
when thermally promoted, (and disrotatory when (iv)
6
-electron
systems are
disrotatory
photochemically promoted - CHM3A2).
when thermally promoted (and conrotatory when (v) The
disrotatory
or
conrotatory
process involved in the bond making/breaking process is
controlled
by the
HOMO conjugated
(thermal reaction) or
-system SOMO
(photochemical reaction - CHM3A2) of the which either is the starting material or product.
linear
6
-Electron Systems Me Me Me Me Me Me Me Me Me Meso Me RS Me Me Enantiomers SS Me Me RR
4
-Electron Systems Me Me Me Me Me Me Me Me Me Me Enantiomers RR SS Me Me Me Meso Me RS
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 In-phase n n Me Me Me Me n Me Me Meso
Disrotatory Motion: Light/Light In-phase n n Me Me Me Me n Me Meso Me
Conrotatory Motion: Dark/Dark In-phase n n Me Me Me Me n Me Me RS Enantiomer
Conrotatory Motion: Light/Light In-phase n n Me Me Me Me n Me Me SR Enantiomer
4n+2
Electron Electrocyclic Reactions 1, 3, 5-Hexatriene
y
3 – HOMO 6 p AOS 6
MOs 6
Electrons
y
3 = HOMO two nodes (7/3) Disrotatory
Me Me
y
3 – HOMO Dark/Dark Or Light/Light DISROTATORY Meso Me Me DISROTATORY RR and SS (enantiomers)
4n
Electron Electrocyclic Reactions Butadiene
4n
Electron Electrocyclic Reactions Butadiene
y
2 – HOMO 4 p AOS 4
MOs 4
Electrons
y
2 = HOMO one node (5/2) Conrotatory
y
2 – HOMO Dark/Dark Or Light/Light CONROTATORY RR and SS (enantiomers) see next slide CONROTATORY Meso
Enantiomer Formation
y
2 – HOMO Two alternative and equivalent modes of conrotatory in-phase overlap RR CONROTATORY A Pair of Enantiomers SS CONROTATORY
Coping with Ring Opening Reactions 1. Draw out the
-HOMO of the product without the substituents
y
2 HOMO
2. Draw out the MO of the Starting material Same Phase
y
2 HOMO Bonding: Must be in phase!
3. Open the C-C bond two afford the HOMO of the product CONROTATORY
y
2 HOMO
4. Decide how the substituents move Product stereochemistry CONROTATORY
y
2 HOMO
Rules for Electrocyclic Reactions _______________________________________________________ Number of
-Electrons Thermal Photochemical (CHM3A2) 4n CONrotatory DISrotatory 4n + 2 DISrotatory CONrotatory _______________________________________________________ Photochemical reactions will be dealt with in the third year course (CHM3A2), where the first electronically excited stated state becomes the HOMO.
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 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
y
3 – HOMO DISROTATORY S R Superimposable Mirror Images Exactly the same compound MESO Compound Dark Dark R S Light Light
Exercise 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 stereoisomers formed from each reaction and the stereochemical relationship of the products between the pair of reactions 1 2
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
y
3 – HOMO S R Enantiomers R S Dark Dark DISROTATORY Light Light Diasteroisomers DISROTATORY R R Enantiomers S S Dark Dark Light Light
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
y
2 HOMO
Exercise 4: A Cascade Electrocylic System Use FMOs to predict the stereochemical outcomes in the reaction scheme below.
Me Me H Me Me H H Me Me H
Answer 4: A Cascade Electrocylic System Use FMOs to predict the stereochemical outcomes in the reaction scheme below.
Me Me
y4
(3 nodes 9/4) of 1, 3, 5, 7-octatetraene H Me Me H
y 3
(2 nodes) of 1, 3, 5-hexatriene H Me Me H Me Me 4n - CONROTATORY H H Me Me (4n + 2) - DISROTATORY Dark/Dark H Me Me H Dark/Dark H Me Me H Me Me Light/Light H Me Me H Light/Light H Me Me H
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
y
3 HOMO of 1, 3, 5-hexatriene.
Light Light H Light Light H H H H H Disrotatory of both triene systems H Light Light H H Dark Dark H Thermodynamic Product. Least sterically hindered H H
Exercise 6: Complex Electrocyclic Reaction Cyclooctatetraene undergoes an electrocyclic ring closure forming only the cis-isomer as depicted right.
Rationalise this result using FMOs.
H 0% H H 100% H
Answer 6: Complex Electrocyclic Reaction Cyclooctatetraene undergoes an electrocyclic ring closure forming only the cis-isomer as depicted right.
Rationalise this result using FMOs.
4
Electron Process H 0% H H 100%
y
2 HOMO Butadiene 6
Electron Process H H CONROTATORY H H
y
3 HOMO 1,3,5-Hexatriene DISROTATORY H H H H Thus, the reaction must proceed by a 6
electron 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
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 CON H H H H H H H HH H H CON H H GEOMETRICALLY IMPOSSIBLE: Hydrogen placed inside a six-membered ring H H