Pericyclic Reactions

Show how ionic reactions are pericyclic reactions

• electron pairs move

• clear sense of direction of electron pair movement

Show how radical reactions are pericyclic reactions

• electron moves one by one

Show how Diels-Alder Addition reactions are pericyclic reactions

Define pericyclic reactions

• concerted processes in which bond orbitals overlap in a continuous cycle in the transition state

What does concerted mean using a diagram

• all bond breaking and making happens simultaneously

What is a continuous cycle and show a diagram of SN2

• critical feature of pericyclic chemistry

What does Huckel theory do?

• simple way to predict MOs for conjugated systems

How would you approach to determine the phases of orbitals only

• for an alkene with n p-orbitals, draw n horizontal stacked on top of each other (MOs)

• each line, draw n p-orbitals with an increasing no of nodes starting from zero nodes

• shade p-orbitals - phase is inverted at each node

• fill orbitals with electrons - two per each MO

• lower orbital filled with electrons is HOMO, one above is LUMO

What is an approach to determine orbital coefficient

• draw a line with n+2 spaced dots for an alkene with p-orbitals eg. for butane draw 6 dots

• superimpose sine waves onto line

• height of p-orbitals on each dot equals the size of corresponding orbital coefficient

• phase inverted at each node

How do you approach the orbital coefficient for substituted polyalkenes

• replace an EWG with a CH2+ group and an EDG with a CH2- group

• determine the orbital coefficient as normal

• HOMO and LUMO coefficient for the substituted alkene are assumed to be average between the unsubstituted compound and the alkene fragment of compound with CH2+/CH2- groups

How do you approach the relative energy of MO orbitals

• longer conjugation length, smaller the HOMO-LUMO gap

• add electron to orbital with EDG makes easier to remove electron

• HOMO and other orbitals increase in energy

• removing electrons w EDG makes it easier to add new ones hence LUMO decreases in energy

What does the conservation of orbital symmetry mean?

• symmetry of the orbitals with respect to any symmetry operation of the molecule must be conserved in moving from the starting material to product

Show a diagram of an orbital correlation diagram

Show an orbital diagram of the orbital correlation diagram

• bonding orbital π2 correlates with σ3 which is unfavourable so the reaction is symmetry forbidden

How do we understand pericyclic reaction using the frontier molecular orbital approach - use an example

• draw a curly arrow mechanism and a 3D shape

• work out component will use HOMO and which use LUMO for overlao

• determine the phases of overlapping orbitals

• phases of orbitals forming new bonds should match

What is the KEY Woodward-Hoffman Rule?

• a thermal pericyclic reaction is symmetry allowed if (4q + 2) + (4r)a is an odd number

• q is an integer → components with 2,6,10 electrons

• 4r → components with 4,8,12 electrons

What is ()s and ()a

supra- and antarafacial components

What are all the different types of σ,π,ω for supra facial and antarafacial

How do you actually use the Woodward-Hoffman rule?

• draw curly arrow mechanism

• identify components

• draw a 3D shape of the reaction to show overlapping components

• label components as supra- or antarafacial and use Woodward Hoffman rule

Do an example of how Woodward-Hoffman rule works

When is a photochemical pericyclic reactions symmetry allowed? Is it different for thermal reactions?

• if (4q + 2)s + (4r)a is an even number which is opposite to thermal rules

Show how cycloaddition is a pericyclic reactions

Show how electrocyclic reactions is a pericyclic reactions

Show how sigmatropic rearrangements is a pericyclic reactions

Show how group transfer reactions is a pericyclic reactions

What is the nomenclature of cycloaddition?

• [m+n] where m and n are the number of electrons in which component

• Diels-Alder reaction is [4+2]-cycloaddition

Draw a pericyclic reaction using cubes

Show how Woodward-Hoffman treatment is in cycloadditions and diels-alder reactions

Show how frontier molecular orbital treatment is in cycloadditions and diels-alder reactions

Show an example of how Diels-Alder reactions are more difficult to recognise

Show the conformations in diels-alder reactions for a s-cis vs s-trans dienes

• secondary interactions at the back of molecule favourable

• if DA reaction is reversible in eqm the thermodynamic product (exo) will dominate

Show energetics and catalysis of DA reaction of normal and inverse electronic demand

• ‘normal’ DA reactions accelerated by EWG on dienophile and EDG on diene (provides HOMO)

• inverse DA reactions are rare. Require EDG on dienophile and EWG on diene

Show diagram of normal and inverse electronic demand DA reaction

Show a lewis acid catalysis reaction with HOMO and LUMO values

• as complexation with LA is reversible only catalytic around of LA needed

Show an example of substituent effect on DA reaction - regiochemistry

Show the difference between large and small overlaps for substituent effects on DA reactions

Draw a DA reaction on the sides of a cube. Why does OMe go down?

Show enantio- vs diasteroselectivity

What is a trick to induce enantioselectivity in a DA reaction?

• use a chiral lewis acid catalyst

What is photochemical cycloadditions in terms on DA reactions

[2+2] - thermally forbidden but allowed photochemically

Give an example of a cycloaddition - addition of 1,3-dipolar compounds

What is cheletropic reactions and show an example

• cycloadditions (and reverse reactions) with two σ-bonds formed/broken to the same atom

Draw a diagram to show conrotatory and disrotatory movement

• all electrocyclisation are allowed (either con- or disrotatory) - but they may be geometrically constrained

Show electrocyclisation in frontier MO treatment

• use HOMO for thermal reactions

• SOMO* for photochemical reactions

Show electrocyclisation in Woodward-Hoffman treatment

Show examples of stereoisomers in electrocyclisation

Show an example of the reversibility of electrocyclisation

• direction of reactions depend on relative rate which difficult to predict

• many rxn are reversible, equilibria rarely achieved

Give another example of electrocyclisation

Define classification in sigmatropic rearrangements

• migration of a sigma bond from one end of a π-system to the other

Give the rule for classification of sigmatropic rearrangements

[m,n]-sigmatropic rearrangement

Sigmatropic rearrangement: Show examples of Claisen, Cope and oxy-Cope rearrangements

How hydride shift in sigmatropic rearrangements