3.2.2 Lombardo Reaction
The combination of Zn/CH2Br2 /TiCl4 can convert carbonyl into methylene group (Scheme 4).
Scheme 4
Mechanism
The reaction is believed to proceed through a dimetalated intermediate carbonyl group (Scheme 5).
Scheme 5
Examples
3.2.3 Simmons-Smith Reaction
This reaction affords the cyclopropanation of alkenes and the reactive species is iodomethylzinc iodide (Scheme 6).
Scheme 6
The reaction is sensitive to steric hindrance, adding from the less-hindered face. In case the substrates have neighboring hydroxyl group, the reaction is accelerated and the cyclopropanation takes place syn to the hydroxyl group (Scheme 7).
Scheme 7
Many modifications of the procedure have been made to allow the use of less reactive methylene group donors like chloroiodomethane. Such methods utilize the use of Lewis acids like TiCl 4 or organic reagents like acetyl chloride or trimethylsilyl chloride (Scheme 8).
Scheme 8