The Effects of Triplet Sensitizers' Energies on the Photoreactivity of β,γ-Unsaturated Methyl Ketones

✍ Scribed by Diego Armesto; Maria J. Ortiz; Antonia R. Agarrabeitia; Noureddin El-Boulifi

• Publisher: John Wiley and Sons
• Year: 2005
• Tongue: English
• Weight: 89 KB
• Volume: 44
• Category: Article
• ISSN: 0044-8249
• DOI: 10.1002/anie.200502619

✦ Synopsis

The photochemistry of b,g-unsaturated ketones has been a topic of great interest to organic photochemists for many years. [1] The results of studies in this area show that these substances undergo a 1,3-acyl migration on direct irradiation and an oxa-di-p-methane (ODPM) rearrangement in the presence of triplet sensitizers. Both processes usually occur with high chemical and quantum efficiencies and high degrees of stereoselectivity. [1] As a result, the photochemical 1,3-acyl migration and ODPM rearrangement reactions have found important synthetic applications. [1] Despite the fact that a large majority of b,g-unsaturated ketones undergo tripletstate ODPM reactions, some acyclic compounds in this series, even those having substitution patterns that are known to favor this reaction, are unreactive. [1a] For example, it has been reported that the b,g-unsaturated ketones 1 a, [2] 1 b [3] 1 c, [4] and 1 d [2] do not undergo the ODPM rearrangement in the presence of acetophenone or 3-methoxyacetophenone. Instead, under these conditions 1 a and 1 b undergo alkene E/Z isomerization.

To gain a better understanding of this unusual behavior, we have carried out a more detailed study in which we have probed the photochemistry of enones 1 a-e by using a variety of other triplet sensitizers. The results arising from this effort reveal that these acyclic b,g-unsaturated ketones do indeed undergo the ODPM process when the triplet energies of the sensitizers used to promote the reaction are close to those of the triplet states of the alkene moieties, and that the ODPM reactivity of these substrates is intimately related to the features of the triplet formed in the energy-transfer processes.

In this study, solutions of b,g-unsaturated ketones 1 a-e containing sensitizers of varying triplet energies [5] (acetophenone: E T = 74 kcal mol À1 ; 3-methoxyacetophenone: E T = 71 kcal mol À1 , depending on the absorption properties of the enone;