Synthesis of cyclopropanes

Question 1

To form cyclopropanes you must use…

Answer 1

• irreversible cyclisation
• Too much ring strain

Question 2

What are two limitations of irreversible cyclisations

Answer 2

• Specific starting material is required
• Must have an EWG (scope of molecules which can be synthesised is limited)

Question 3

Describe some features of a singlet carbene

Answer 3

• a singluar pair of electrons in an sp² orbital
• empty p-orbitals
• can act as either a nucleophile or an electrophile
• favoursed if R = halogen

Question 4

Describe some features of a triplet carbene

Answer 4

• 1e- in the sp² orbital
• 1e- in the p-orbital
• Unpaired electron hence as similar to radicals
• Favoured if R = alkyl

Question 5

Carbenes will react with ……. to form cyclopropane

Answer 5

akenes
(benefit because really common functional group)

Question 6

How are singlet carbenes formed?

Answer 6

• Generated via α-elimination

Question 7

How is the following singlet carbene generated?

Answer 7

Question 8

How do Singlet Carbenes typically react with alkenes?

Answer 8

• React in a concerted fashion with alkenes (both new C-C bonds formed simultaneously), so alkene geometry is retained in the cyclopropanes product
• Are electrophilic, and thus react preferentially with electron-rich alkenes

Question 9

How is cyclopropane formed via a singlet carbene

Answer 9

• Sterochemistry is conserved within the product

Question 10

How can triplet carbenes be generated?

Answer 10

• Can be generated via decomposition of diazo compounds
• Using light or heat

Question 11

How can cyclopropane by synthesised via triplet carbenes

Answer 11

• Because this reaction isn’t concerted (stepwise), it is non spectrospecific - form two different isomers
• This is slower due to having to wait for one of the electrons to revert it’s spin

Question 12

Carbenoids are used more frequently in labs as they are considered safer
What are they?

Answer 12

• Are metal-bound species that exhibits carbene-like reactivity
• Has a nucleophilic side (metal) and an electrophilic side
• Are typically more stable, and often more selective, than free carbenes

Question 13

The reaction below is the Simmons-Smith cyclopropanation
It requires the formation of a carbenoid
What is the mechanism for the following reaction?

Answer 13

• Zn is nucleophilic side, I is electrophilic side
• Concerted syn addition (bonds broken and formed at same time), so alkene geometry is conserved in the product