Some examples of photochemical cycloaddition and electrocyclic reactions

The photochemistry of carbonyl compounds has been extensively studied both in solution and in the gas phase. It is not surprising that there are major differences between the two phases. In the gas phase, the energy transferred by excitation cannot be lost rapidly by collisions, whereas in the liquid phase, the energy is rapidly transferred to the solvent or to other components of the solutions. Solution photochemistry will be emphasized here, since most organic chemists interested in either mechanistic studies or preparative photochemistry have studied this aspect of the problem. 

An alternative excited state available to carbonyl compounds involves promotion of a bonding n--electron to the antibonding ir -orbital. This is called a tt-tt transition, and is most likely to occur when the ketone group is conjugated with an extensive 7r-bonding system. 

It is not possible to draw unambiguous Lewis structures of excited states of the sort that are so useful for depicting ground-state chemistry. Instead, it is common to asterisk the normal carbonyl structure and provide information about the multiplicity of the excited state if it is available  

Many aromatic ketones react by hydrogen abstraction from solvent or some other a hydrogen atom by the oxygen of the carbonyl group or with cleavage of the carbon-carbon bond adjacent to the carbonyl group  

The hydrogen atom abstraction can be either intramolecular or intermolecular. Many aromatic ketones undergo hydrogen abstraction from solvent or some other hydrogen donor, followed by coupling of the resulting a-hydroxy radicals. Such reactions go best in solvents that have easily abstractable hydrogens  

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Scheme 13.1. Some Examples of Photochemical Cycloaddition and Electrocyclic Reactions...

Scheme 13.1 lists some example of photochemical cycloaddition and electrocyclic reactions of the type that are consistent with the predictions of orbital symmetry considerations. We will discuss other examples in Section 13.4. 

Some examples of photochemical cycloaddition and electrocyclic reactions... 

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