What is intramolecular cyclopropanation using α-diazo ketone?

α-diazo-ketone is a ketone with diazo part in its structure; two charged nitrogen atoms. α-diazo-ketone is one of more stable diazo-compounds due to the charge distributing effect of the carbonyl group. α-diazo-ketones have been used in a variety of applications, such as the preparation of biological compounds. As shown in the our mechanism, the diazo-ketone reacts in presence of metal catalyst, and undergoes an intramolecular cyclopropanation, producing a fully substituted cyclopropane.

During the intramolecular cyclopropanation, the carbon that is attached to the diazo (N2) group reacts with the metal catalyst. The metal ligand forms a double bond with the carbon, causing the N2 to leave the molecule. The N2 is highly electrophilic, hence it is a good leaving group; it forms a much more stable compound on its own, namely nitrogen gas. Part of the metal ligand that left the ligand in an earlier process (when the carbon attacked the metal catalyst) comes back and attacks the metal which is double-bonded to the carbon. Then the carbon attracts the electron from the metal and forms an anion. This anion then attacks the nearby double bond, which leads to the formation of a cyclopropane.