Electrocyclisations Flashcards
What is an electrocyclication?
We’re rearraning the electrons that conjugated pi systems and connecting the ends of the pi system to form a cyclobutane ring
What is the thermal and photochemical product of the following electrocyclisation reaction of E,E-butadiene?
What is the following thermal and photochemical product of the following electrocyclisation reaction of E,Z,E-hexatriene
What is disrotatory motion
- When the terminal lobes have rotate in opposite directions in order to form a new σ-bond
What is conrotatory motion?
When the terminal lobes have to rotate in the same direction in order to form a new σ-bond
Woodward and Hoffmann noticed that the sterochemistry of the electrocyclisation could be predicted by examining….
The HOMO of the conjugated alkene starting material
Identify and draw the HOMO for butadiene
4π electrons
What type of motion is needed for thermal electrocyclisaton of butadiene?
Conrotatory - the terminal lobes of the conjugated system have opposite phases
Thermal electrocyclisation involves the interacting lobes being on the opposite faces of the diene
This is called a….
Antarafacial process
What is the overall name for this process
4πₐ conrotatory electrocyclisation
(a - antarafacial)
For the following photochemical electrocyclication of butadiene, can you drawn out the HOMO and the SUMO
The sterochemistry of the photochemical electrocyclisation can be predicted by
The SUMO of the conjugated alkene starting material
The terminal lobes of the conjugated system have the same phases and the photochemical electrocylication must proceed in a….
Disrotatory mode in order to form a new σ-bond
The interacting lobes are on the same face of the diene, meaning this is a
Suprafacial process
What is the overall name for this photochemical electrocyclisation?
4πₛ disrotatory electrocyclisation
(s - suprafacial)
what is a simplified way we can represent this butadiene 4π e- system
Fill out the following table for the Woodward-Hoffman rules for electrocyclisation reactions between 2-8π electrons
What is a numonic to remeber the Woodward-Hoffmann rule
- Thermal, Odd, Supra, Dis
- E.g. 6π electrons, there is an odd number of π-bonds, and disrotatry motion for the thermal reaction
- And derrive all other point in the table from this
Fill out the following Woodward-Hoffmann rules for electrocyclisation reactions for the general rule
What is the product of this photochemical electrocyclisation reaction
What is the product of this reverse electrocyclisation?
(Going from even, antarafacial, conrotatory TO thermal, odd, suprafacial, disrotatory)
What is the product of this reverse electrocyclisation
(Going from even, antarafacial, conrotatory, TO thermal, odd, suprafacial, disrotatory)
BUT steric hindrance of the mechanistically favoured product means it doesnt form
What is the product of this electrocyclisation
(going from even, suprafacial, disrotatory, TO odd, antarafacial, conrotatory)
keep in mind steric hinderance
What is the product of this electrocyclisation
(going from even, suprafacial, disrotatory, TO odd, antarafacial, conrotatory)
What is the product of this electrocyclisation
(going from odd, antarafacial, conrotatory TO even, suprafacial, disrotatory)
What is the product of this reverse electrocyclisation?
(goes from even, antarafacial, conrotatory TO odd suprafacial, disrotatory
How can we regenerate the aromaticity of the product of this reaction?
