Photooxygenation of Naphthalene

Abstract

The photooxygenation of naphthalene to the corresponding endoperoxide (EPO) under various conditions is described. Substantial conversion is only observed at À 10 °C and after more than two days, indicating that the [4 + 2] cycloaddition of singlet oxygen to this acene proceeds much more slowly than corresponding reactions of substituted naphthalenes, a rate constant of k = 5.4 � 0.3 M À 1 s À 1 was determined by competition kinetics. Another problem is the thermal lability and photochemical cleavage of the naphthalene EPO. We investigated the mechanism of this radical process depending on the light source and sensitizer in comparison to known cyclohexadiene EPO. Thus, bisepoxides and keto epoxides are formed after homolysis of the OÀ O bond by irradiation with sodium lamps or blue LEDs and subsequent cyclization. This process is accelerated by the sensitizers methylene blue and 9,10-dicyanoanthracene, indicating an electron transfer mechanism. Finally, the cleavage of the peroxidic bond is inhibited with red LEDs, and photooxygenation under such conditions affords 20 % EPO. Thus, we could demonstrate that contrary to literature statements singlet oxygen does indeed react with naphthalene.

Bis-borylated arylisoquinoline-derived dyes with a central aromatic core: towards efficient fluorescent singlet-oxygen photosensitizers

“…The marked photoreactivity of linear acenes towards 1 O 2 is commonly ascribed to the formation of endoperoxides by means of a [4 + 2] cycloaddition. 32,36,67–70 Analogous chemical processes are expected for the naphthalene- and anthracene-derived 1 and 2 , respectively. Similar pyrene oxidation products by the action of 1 O 2 are not described.…”

Aromatic Endoperoxides†

“…Many endoperoxides are not accessible via the [4 + 2] cycloaddition route. For example, benzene has not been reported to react (2) and naphthalene reacts extremely slow (44) with singlet oxygen, and pentacene forms only one of the three possible endoperoxides.…”

“…As a benchmark for the following discussion, it is important to recognize that the chemical rate constant for endoperoxide formation, k r , in the unsubstituted acenes increases with the addition of each benzene ring from naphthalene (5.4 m −1 s −1 at −10°C) (44), to anthracene (1.5 × 10 5 m −1 s −1 ), tetracene (1.2 × 10 7 m −1 s −1 ) and pentacene (4.2 × 10 9 m −1 s −1 ) at room temperature (65). The main factors that have been used to describe the regiochemical preferences of endoperoxide formation in this acene series and in their substituted homologs have been: (1) HOMO electron density in the acene (10,11); (2) loss or gain of aromaticity in the transition states (66); (3) the distortion energies of the transition states (62); and (4) steric effects (2).…”

“…Benzene endoperoxide 27 (48) is very unstable and undergoes cycloreversion with a rate constant an order of magnitude larger at −30°C than naphthalene endoperoxide, 28 , (49) at 25°C. 1,4‐Dimethylnaphthalene endoperoxide, 67 , and naphthalene endoperoxide, 28 , have nearly identical half‐lives for cycloreversion (Scheme 11); however, the use of 28 as a singlet oxygen source is impractical because of its very slow rate of formation (44). Steric effects play important roles in the cycloreversion rates of 66 (76) and 69 (92).…”

“…A reduction is Scheme 11. Rate constants and half-lives for thermolysis of endoperoxides (44,48,49,50,76,81,92,93).…”