Question

Show how you would accomplish each of the following synthetic conversions.

a) Trans-but-2-ene$\to$trans-1,2-dimethylcyclopropane

b) Cyclopentene$\to$

c) Cyclohexanol$\to$

Short answer

(a)

(b)

(c)

Step-by-step solution

Addition of Carbene to Alkenes

Carbene insertion reaction is inserting carbene into electron rich pi$\left(\text{C}=\text{C}\right)$ bond double bond of an alkene. This reaction is a concerted reaction that occurs in a single step only.

Conversion of a

Trans-2-butene on reaction with methylene iodide, ${\text{CH}}_2{\text{I}}_2$ and zinc-copper couple, $\mathrm{Zn}-\mathrm{Cu}$ gives trans-cyclopropane as the product..This reaction is known as Simmons-Smith reaction. This reaction is stereospecific, which means trans-alkene will convert to trans-cyclopropane and cis-alkene will convert to cis-cyclopropane.

Conversion of b

Sodium hydroxide, on reaction with methylene bromide produces carbene :${\text{CBr}}_2$, which is singlet carbine. This will then undergo insertion reaction at the double bond of cyclopentene and form bromocyclopropane. In the process, one bromide ion acts as a leaving group.

Conversion of c

Cyclohexanol, on reaction with sulphuric acid followed by heating undergoes dehydration, that is, loss of water molecules. It produces cyclohexene as the product. Then, on further reaction with trichloromethane, ${\text{CHCl}}_3$and a base such as sodium hydroxide, $\text{NaOH}$, a singlet carbene is produced, that is, ${\text{CCl}}_2$, which undergoes insertion reaction at the double bond and forms chlorocyclopropane as the product with the loss of one chloride ion which acts as a leaving group.