Cyclobutanone

Cyclobutanone is an organic compound with molecular formula (CH2)3CO. It is a four-membered cyclic ketone (cycloalkanone). It is a colorless volatile liquid at room temperature. Since cyclopropanone is highly sensitive, cyclobutanone is the smallest easily handled cyclic ketone.

Preparation
The Russian chemist Nikolai Kischner first prepared cyclobutanone in a low yield from cyclobutanecarboxylic acid. Kischner's process, involving several steps, is cumbersome and inefficient; more efficient, high-yielding syntheses have since been developed.

One strategy involves degradation of five-carbon building blocks. For example, the oxidative decarboxylation of cyclobutanecarboxylic acid was improved by the use of other reagents and methods.

A newer, more efficient preparation of cyclobutanone was found by P. Lipp and R. Köster in which a solution of diazomethane in diethyl ether is reacted with ketene. This reaction is based on a ring expansion of the cyclopropanone intermediate initially formed, wherein molecular nitrogen is split off:
 * Cyclobutane Synthesis V.1.svg

The reaction mechanism was confirmed by a reaction using 14C-labeled diazomethane.

Another synthesis of cyclobutanone involves lithium-catalyzed rearrangement of oxaspiropentane, which is formed by epoxidation of the easily accessible methylenecyclopropane:
 * Cyclobutanon Synthesis A V.1.svg

Cyclobutanone can also be prepared in a two step procedure by dialkylation of 1,3-dithiane with 1-bromo-3-chloropropane followed by deprotection to the ketone with mercuric chloride (HgCl2) and cadmium carbonate (CdCO3).

Cyclobutanones are the intermediates of the homo-Favorskii rearrangement, and can be isolated when nucleophiles are absent, as in the synthesis of kelsoene:

Reactions
At about 350 °C, cyclobutanone decomposes into ethylene and ketene. The activation energy for this [2+2] cycloelimination is 52 kcal/mol. The reverse reaction, the [2+2] cycloaddition of ketene and ethylene, has never been observed.


 * Cyclobutanone Decomposition V.2.svg