Photooxygenations of Sulfur Compounds

Authored by: Edward L. Clennan

CRC Handbook of Organic Photochemistry and Photobiology

Print publication date:  March  2012
Online publication date:  March  2012

Print ISBN: 9781439899335
eBook ISBN: 9781466561250
Adobe ISBN:

10.1201/b12252-33

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Abstract

Molecular oxygen (O2) was independently discovered by Joseph Priestly and Carl Wilhelm Scheele in the late eighteenth century [1]. It can react at sulfur and with nearly every other element under a variety of conditions to produce oxides [2]. The first photooxygenations (photochemically initiated incorporation of oxygen into molecules) were reported as early as 1867 [3], well before the electronic character of oxygen was understood. It was not until 1924 that G. N. Lewis attributed the paramagnetism of oxygen to a structure with unpaired electrons [4,5]. This discovery was rapidly followed in 1928 by R. S. Mulliken’s suggestion that the electronic configuration of oxygen, [ ( σ 1 s ) 2 ( σ 1 s * ) 2 ( σ 2 s ) 2 ( σ 2 s * ) 2 ( σ 2 p ) 2 ( π 2 p ) 4 ( π 2 p * ) 2 ] , leads to three states in order of increasing energy, Σ g − 3 , Δ 1 g , and Σ g + 1 since the ≠ 2 ⁢ p * orbital is only half filled [6,7]. Finally, it was not until the early 1930s that Kautsky and coworkers [8,9] reported that the metastable 1Δ g state of oxygen was involved in many of these photooxygenations. It is now well appreciated that photooxygenations can involve both the ground-state triplet, Σ g − 3 , and the metastable 1Δ g oxygen. On the other hand, the higher energy Σ g + 1 state at 37 kcal/mol above the Σ g − 3 ground state is vibrationally and quantitatively deactivated to the lower energy 1Δ g state (22.5 kcal/mol) and does not have a sufficient lifetime to chemically react with organic substrates [10].

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