**5.4 Rearrangement of carbenes**

Since carbenes are electron-deficient intermediates with an empty p-orbital, an atom or group on the neighboring carbon migrates to the electron-deficient center and undergoes easy rearrangement with simultaneous formation of a new C=C bond. Usually this rearrangement is called 1,2-shift, and it usually involves the migration of a hydrogen atom, since the order of migrating ability is: H> > aryl>alkyl. This hydrogen shift is also considered as the intramolecular addition of the carbene to the adjacent C-H bond.

The best known carbene rearrangement is the rearrangement of diazoketones to ketenes. In this reaction, called Wolff rearrangement, α-diazoketo compounds primarily form α-ketocarbene by removing nitrogen gas thermally, photochemically, or in the presence of various metals. When the -R group electrons attached to the carbonyl group migrate to the carbene center, the carbene electrons form a double bond with the carbonyl carbon and ketene is formed. Later, ketenes react with water and turn into carboxylic acids [48]. According to the experiments, the Wolff rearrangement proceeds according to a concerted mechanism in which the migration occurs at the same time as the leaving group and the leaving nitrogen group and the migrating alkyl group should be in antiperiplanar conformation as shown in **Figure 22**.

**Figure 22.** *Wolff rearrangement.*

When Wolff rearrangement is applied to cyclic systems, a ring reduction reaction occurs if the carbene located at the α-position to the carbonyl group is formed in a ring (**Figure 23**).

#### **Figure 23.**

*Ring reduction reaction in the cyclic system.*

If the carbene is attached to the cyclic structure as a substituent, then a ring expansion reaction occurs (**Figure 24**).

**Figure 24.** *Ring expansion reaction in the cyclic system.*
