Paternò–Büchi reaction

Paternò–Büchi reaction is a photochemical reaction that involves the cycloaddition of a carbonyl compound to an alkene (or alkyne) to form an oxetane. This reaction is named after the Italian chemist Emanuele Paternò and the Swiss chemist George Büchi who independently discovered this reaction. The Paternò–Büchi reaction is a valuable tool in organic synthesis for the construction of four-membered cyclic ethers, which are useful intermediates in the synthesis of a variety of complex organic molecules.

Mechanism

The Paternò–Büchi reaction proceeds through a photoexcitation step where the carbonyl compound absorbs light, typically UV, to reach an excited singlet state. This excited state can undergo intersystem crossing to form a triplet state, which is more reactive towards the alkene. The excited carbonyl compound and the alkene undergo a [2+2] cycloaddition to form an oxetane. The reaction mechanism involves the formation of a diradical intermediate, which quickly collapses to give the oxetane product.

Substrates

The choice of substrates is crucial for the success of the Paternò–Büchi reaction. The carbonyl compound can be an aldehyde, ketone, or even a quinone, while the alkene can be mono-, di-, or trisubstituted. The reaction's efficiency and selectivity can be significantly influenced by the nature of the substituents on both the carbonyl compound and the alkene.

Applications

The Paternò–Büchi reaction has been employed in the synthesis of natural products, pharmaceuticals, and complex organic molecules. Its ability to construct four-membered rings efficiently makes it a valuable tool in the synthesis of cyclic ethers, which are present in many biologically active compounds.

Limitations

One of the limitations of the Paternò–Büchi reaction is its sensitivity to the steric and electronic nature of the substrates. The reaction may also compete with other photochemical processes, such as photoisomerization or photodissociation, which can lower the yield of the desired oxetane product. Additionally, the reaction often requires UV light for activation, which can cause the decomposition of sensitive substrates.

Recent Advances

Recent advances in the Paternò–Büchi reaction include the development of new photocatalysts and the use of visible light, which can improve the reaction's efficiency and substrate scope. Additionally, asymmetric versions of the Paternò–Büchi reaction have been developed, allowing for the enantioselective synthesis of oxetanes.

See Also

• Photochemistry
• Cycloaddition
• Organic synthesis
• Cyclic ether