Cleavage of Carbonyl Compounds

The oxacarbene in (2.1) is usually trapped by water or an alcohol to give hydroxy- or alkoxy tetrahydrofuran derivatives. This sequence has been connected with the synthesis of prostaglandines (2.3) 206). 

The ketene in (2.2) can be trapped by Water or alcohol to give carboxylic acids or esters. Such reactions have already been applied more than seventy years ago in the formation of substituted hexanoic acids (2.4)207). 

The unsaturated aldehyde in (2.2) very often undergoes intramolecular photocycloaddition with formation of an oxetane (cf, chapter 4.3.6). The 

Such a secondary photoreaction can be avoided by trapping the unsaturated aldehyde, e.g. as a hemiacetal. This principle is demonstrated in the synthesis of a furanoid sesquiterpene (2.6)210). 

In the presence of proper substituents the primarily formed diradical rearranges to a more stable one, as by a ring opening reaction. In (2.7) medium ring lactones are synthesized by such a sequence 2U). 

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Photochemical elimination reactions include all those photoinduced reactions resulting in the loss of one or more fragments from the excited molecule. Loss of carbon monoxide from type I or a-cleavage of carbonyl compounds has been previously considered in Chapter 3. Other types of photoeliminations, to be discussed here, include loss of molecular nitrogen from azo, diazo, and azido compounds, loss of nitric oxide from organic nitrites, and loss of sulfur dioxide and other miscellaneous species. 

McLafferty rearrangement. Electron-impact-induced cleavage of carbonyl compounds having a hydrogen in the y-position, to an enolic fragment and an olefin. 

All experimental observations with respect to these reactions, including the dependence on neither orbital character nor spin multiplicity of the excited state (which is in striking contrast to a-cleavage of carbonyl compounds) and the influence of substituents of the aryl moiety on the reaction rate, could be rationalized by correlation diagrams that were obtained on the basis of semiempirical MO calculations [105]. /1-Cleavage of phenoxyketones as well as photo-Fries and photo-Claisen rearrangements were characterized to be no photodissociations. 

Thermal decomposition of dihydroperoxides results in initial homolysis of an oxygen—oxygen bond foUowed by carbon—oxygen and carbon—carbon bond cleavages to yield mixtures of carbonyl compounds (ketones, aldehydes), esters, carboxyHc acids, hydrocarbons, and hydrogen peroxide. 

As is clear from the preceding examples, there are a variety of overall reactions that can be initiated by photolysis of ketones. The course of photochemical reactions of ketones is veiy dependent on the structure of the reactant. Despite the variety of overall processes that can be observed, the number of individual steps involved is limited. For ketones, the most important are inter- and intramolecular hydrogen abstraction, cleavage a to the carbonyl group, and substituent migration to the -carbon atom of a,/S-unsaturated ketones. Reexamination of the mechanisms illustrated in this section will reveal that most of the reactions of carbonyl compounds that have been described involve combinations of these fundamental processes. The final products usually result from rebonding of reactive intermediates generated by these steps. 

Reductive cleavage of phenylhydrazones of carbonyl compounds provides a route to amines. The reduction is carried out conveniently in ethanol containing ammonia over palladium-on-carbon. Ammonia is used to minimize formation of secondary amines, derived by addition of the initially formed amine to the starting material (160). Alternatively, a two-phase system of benzene, cyclohexane, toluene, or dioxane and aqueous hydrochloric acid can be used. 

Reactions involving type I cleavage are especially prominent in gas-phase photolyses of carbonyl compounds. With molecules containing y-hydrogens type II cleavage is also commonly observed[Pg.379]

When considered as a part of the photochemistry of carbonyl compounds, irradiations of esters constitute a minor component. The more frequent photolyses of other carbonyl compounds, in particular ketones, is not surprising, as, even though parallels exist between ester and ketone photochemistry (for example, both experience a-cleavage and hydrogen abstraction-reactions), esters require radiation of higher energy for reaction, and typically produce more-complex mixtures of products. In addition to their similarity to other carbonyl compounds in their reactivity, esters also experience reactions that are uniquely their own. 

The loss of alkenes from molecular ions of carbonyl compounds has early been noted. [23,82] Soon, a mechanism involving y-H shift and P-cleavage has been proposed and studied in detail. [24-26,83,84] Strictly speaking, the term McLafferty rearrangement only describes an alkene loss from molecular ions of satu-... 

Lead tetraacetate is a selective oxidizing agent causing oxidative cleavage of polyhydroxy compounds. It cleaves compounds that have hydroxyl groups on adjacent carbon atoms, breaking the carbon-carbon bonds to form carbonyl compounds, such as aldehydes, ketones or acids. The reaction is carried out in organic solvents. A typical example is as follows ... 

Many distonic radical ions are involved in unimolecular dissociation reactions. For example, the McLafferty rearrangement of carbonyl compounds proceeds by C—C bond cleavage of distonic ions. ° Alternatively, bond-cleavage reactions of cyclic molecular ions may generate distonic ions (see above).For example, C—C bond cleavage of cyclic ketone molecular ions (e.g., 61 " ) generates 62 ". ... 

NORRISH TYPE I CLEAVAGE REACTION OF CARBONYL COMPOUNDS 215... 

A second common photochemical reaction of carbonyl compounds is the cleavage of the bond adjacent to the carbonyl group ... 

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