Elsevier

Tetrahedron

Volume 61, Issue 28, 11 July 2005, Pages 6665-6691
Tetrahedron

Tetrahedron report number 723
Advances in singlet oxygen chemistry

https://doi.org/10.1016/j.tet.2005.04.017Get rights and content

Introduction

Oxygen is ubiquitous. It comprises nearly 50% of the Earth's crust and is an essential component in metabolic pathways in all higher organisms.1 It was first identified by Carl Wilhelm Scheele and by Joseph Priestly in the late 18th century. Its unpaired electronic structure and the possibility of a spin-paired electronic state was first predicted by Lewis in 1924. These seminal contributions, along with the Mulliken molecular orbital prediction of two low-lying oxygen excited states (1Δg and 1Σg+), and the demonstration by Katusky2, 3, 4 of a metastable intermediate species in photooxygenation reactions in the 1930s, laid the foundation for the following 50 years of singlet oxygen research. These years, between 1940 and 1990, were characterized by delineation of the physical5 and chemical6 pathways of singlet oxygen formation and deactivation. The physical studies, with the aid of technological advances that have taken advantage of the luminescence of both the 1Δg and 1Σg+ states, have demonstrated that only 1Δg (approximately 22.5 kcal/mol above the ground state triplet) has a sufficient lifetime to allow it to play a role in chemical reactions in solution. The chemical studies have identified the fundamental [2+2], [4+2], ene, and heteroatom oxidation reactions of 1Δg (referred to as singlet oxygen or 1O2 throughout this review) and have established their basic mechanistic details. In a review published in 2000 we outlined the efforts initiated in the 1990s to influence the regio- and stereoselectivity of singlet oxygen reactions.6 In this review we discuss the advances made in both mechanistic and synthetic aspects of the fundamental reactions discussed in our previous review.6 In addition, we have also expanded the discussion to include new developments in heteroatom and heterocyclic photooxygenations. We have made no attempt to be exhaustive in our treatment of the singlet oxygen literature. In particular, advances in the photophysical and biological aspects of singlet oxygen chemistry, although briefly mentioned, are not discussed in detail. Recent excellent reviews should be consulted for more information on these aspects of singlet oxygen chemistry.5, 7, 8

Section snippets

Ene, [2+2] and [4+2] cycloadditions

The ene, [2+2], and [4+2] reactions (Scheme 1) represent powerful protocols for the addition of molecular oxygen to organic substrates. The ene reaction generates allylic hydroperoxides which can be converted to synthetically valuable allylic alcohols (Scheme 1).9 The [2+2] cycloaddition is observed with electron rich alkenes, and with alkenes devoid of, or those containing only geometrically inaccessible, allylic hydrogens.10 The dioxetane products (Scheme 1) are often sensitive molecules that

Five-membered rings

A significant number of articles regarding the reactions of singlet oxygen with heterocyclic systems have been published. The definition of heterocyclic system itself embraces a very wide class of organic compounds and as a consequence it is difficult to classify and summarize all the reported reactions in an organized manner. In fact, at the beginning of the preparation of this manuscript no recent comprehensive review on this topic was available.129 Wasserman and Lipshutz in 1979130 published

Formation and reactions of singlet oxygen in heterogeneous media

Photooxygenations in heterogeneous media have attracted considerable attention because of their relevance to biological and environmental oxidations and to the expectation that constrained environments could enhance regio- and stereochemistry of these synthetically useful reactions.218

Conclusion and future prospects

Efforts to understand the impact of the oxidative destruction of natural and manmade materials will continue to drive interest in singlet oxygen chemistry. In addition, increased emphasis on environmental and economic concerns will also generate significant synthetic interest in this readily available and green reagent to make novel natural products such as the litseaverticillols.244 In particular, new methods to direct the stereo- and regiochemistry of singlet oxygen addition to organic and

Acknowledgements

We thank the National Science foundation and the Italian MIUR for their generous support of this research.

Edward L. Clennan was born in 1951 in St. Paul Minnesota. He received his BS degree in chemistry and mathematics from the University of Wisconsin at River Falls and his PhD from the University of Wisconsin at Madison. Prior to joining the faculty at the University of Wyoming he spent two years at Texas Christian University working as a postdoctoral fellow in the laboratory of Professor P. D. Bartlett. In addition, in recent years he spent a year at the National Science Foundation as a program

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    Edward L. Clennan was born in 1951 in St. Paul Minnesota. He received his BS degree in chemistry and mathematics from the University of Wisconsin at River Falls and his PhD from the University of Wisconsin at Madison. Prior to joining the faculty at the University of Wyoming he spent two years at Texas Christian University working as a postdoctoral fellow in the laboratory of Professor P. D. Bartlett. In addition, in recent years he spent a year at the National Science Foundation as a program officer and three years as Head of the Department of Chemistry at the University of Wyoming where he is currently a Professor of Chemistry. His research interests are in the area of oxidation chemistry in homogeneous and heterogeneous media. Current projects include the study of singlet oxygen reactions in zeolites and the development of new mechanistic tools to study organic reactions in homogeneous and heterogeneous media.

    Andrea Pace was born in 1970 in Palermo, Italy. He received his BS degree in chemistry from the University of Palermo. He spent two years at the Naval Academy of Livorno working as a teaching officer for the General Chemistry course. He worked in the laboratory of Professor R. Noto as a graduate student. In 1997 he became a member of the Faculty of Science at the University of Palermo as an assistant professor of Organic Chemistry joining the research group of Professor N. Vivona. In addition, in recent years he spent two years at the University of Wyoming working with Professor E. L. Clennan. His research interests are in the area of heterocyclic chemistry, photochemistry and fluorinated compounds. Current projects include the photooxidation of natural products, the synthesis of fluorinated macromolecules and the study of heterocyclic chemistry in zeolites.

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