Chapter 12, Problem 12.30P

Interpretation Introduction

Interpretation:

The alkyne, which, when treated with diazomethane and irradiated with UV light, will produce the given compound, is to be drawn. A full mechanism for the reaction is to be drawn.

Concept introduction:

A cyclopropyl ring can be introduced in a product by treating the corresponding alkene or alkyne with a carbene. A carbene is a reactive species with two groups, mostly hydrogens or halogens, bonded to a carbon that also has a lone pair of electrons. Since the carbon lacks an octet of electrons, it acts as an electrophile. At the same time, the lone pair on it also gives it a nucleophilic character. Carbenes react with alkenes and alkynes to replace the $\text{π}$ bond with two $\text{C}-\text{C σ}$ bonds and form a cyclopropyl ring.

A carbene is a highly reactive, transient species and must be generated in situ. A common method of generating a carbene is to irradiate diazomethane with UV light. The $\text{C}-\text{N}$ bond breaks heterolytically to produce the carbene and a molecule of nitrogen.