0) Intro to Pericyclic Reactions

First part of 301H Lecture 1

What is a pericyclic reaction?

A reaction that proceeds via a concerted process, with a cyclic movement of e^-s and is brought about via either thermal or photochemical activation.

What are the three main attributes of pericyclic reactions?

1) Concerted reactions - single step process

2) Reaction proceeds via a cyclic transition state

3) There’s cyclic movement of electrons.

Explain the first attribute of pericyclic reactions

Concerted rxns - single step process

• Involves (several) simultaneous bond making/breaking processes

Explain the second attribute of pericyclic reactions

Rxn proceeds via cyclic transition state

• NO INTERMEDIATE - only a defined transition state for the process

  • (→ no detectable intermediates e.g. by NMR)

• ΔS* is large and -ve

  • Indicates ordering between reactant(s) and TS structure (well-organised TS) - a preferred/specific rxn orientation

Explain the third attribute of pericyclic reactions

There is a cyclic movement of electrons.

• Shown using curly arrows in mechanisms.

• (Clockwise or anti-clockwise)

What are the 5 classes of pericyclic reactions?

1) Electrocyclic reactions

2) Cycloaddition reactions

3) Sigmatropic rearrangements

4) Cheletropic reactions

5) Group transfer reactions

Which type of pericyclic reaction is the Diels-Alder reaction?

Cycloaddition reaction!

Show the concerted process, TS and correct phasing of orbitals for this reaction:

Which pericyclic reaction class do CC rearrangements (via hydride shifts) belong to?

Give one example.

Sigmatropic rearrangements!

e.g. in Friedel-Crafts alkylation of aromatics

What is the Wagner-Meerwein rearrangement?

1,2-migration of a hydride to an adjacent CC centre (but can also involve an alkyl or aryl migration).

Redraw this 1º CC from the Wagner-Meerwein rearrangement to show the orbital hyper-conjugation and the TS.

Which theoretical model is used to predict pericyclic reactions?

Frontier Molecular Orbital Theory (FMO)

What does FMO consider?

Considers the nature and extent of HOMO/LUMO interactions.

Show an overview of FMO for butadiene.