Sulfur photochemical behavior

The photochemistry of sulfoxides and sulfones, which was first comprehensively reviewed in 19691, continues to be an area of active research interest. In this early review some 30 to 40 primary publications on the photochemistry of sulfoxides and sulfones were described. Since that date, interest in this field has continued at a steady, rather than accelerated, pace but further reviews of the general area of photochemistry of organic sulfur compounds have appeared2,3. The present review will focus on the main areas of interest for both sulfoxides and sulfones which, in spite of their apparent similarity, exhibit quite different photochemical behavior. 

See the Section Photochemical Behavior in the review on solid sulfur allotropes by R. Steudel, B. Eckert, Top. Curr. Chem. 2003, in print. 

Recently the first reported stable thioaldehyde was described, having the structure shown in equation 91. In comparing its photochemical behavior (using a medium-pressure mercury lamp at 5 °C) with its thermal behavior, it was found that two reactions occurred. These involved (a) a 1,3-silyl shift from carbon to sulfur forming a silylthioenol ether and (b) extrusion of sulfur yielding a trisilylalkene139. 

Griesbeck and co-workers further investigated PET reactions of sulfur-containing proteinogenic and non-proteinogenic amino acids and discovered an unusual solvent dependence of the product composition. Even more remarkable was the photochemical behavior of phthaloyl L-methionine 18 irradiation in acetone gave the tetracyclic lactone 20 in 72% yield (Scheme 6). In this particular case, PET oxidation at sulfur proceeds faster than a-decarboxylation vide infra). 

Mislow and co-workers (258) and Hammond (259) have shown that optically active diaryl sulfoxides, which are configurationally stable in the dark at 200°C, lose their optical activity after 1 hr at room temperature on irradiation with ultraviolet light. Similarly, an easy conversion of the trans isomer of thianthrene-5,10-oxide 206a into the thermodynamically more stable cis isomer takes place upon irradiation in dioxane for 2 hr. However, the behavior of a-naphthylethyl p-tolyl sulfoxide under comparable irradiation conditions is different, namely, it is completely decomposed after 4 min. These differences are not surprising because the photochemical racemization of diaryl sulfoxides occurs by way of the pyramidal inversion mechanism whereas decomposition of the latter sulfoxide occurs via a radical mechanism with the cleavage of the sulfur-carbon bond. It is interesting to note that photoracemization may be a zero-order process in which the rate depends only on the intensity of the radiation and on the quantum yield. 

As mentioned above, a 1 1 stoichiometric relationship between the photooxidized donor, P700 and the reduced terminal acceptors has not yet been established for the PS-I reaction center. As previously noted, the total amount of the recognized terminal acceptors reduced at 15 K is, on average, approximately 74% of the P700 photooxidized. Even more intriguing, in the reconstituted PS-I complexes from either the Cys-14->Asp or Cys-51->Asp mutant PsaC protein, the extent of photoreduction of each intact iron-sulfur cluster at 15 K remained nearly the same as in the wild-type preparation. The presence of the other cluster that was made photochemically inactive by site-directed mutagenesis apparently had no effect on the behavior of the unaltered cluster. 

Behavior of iron-sulfur proteins in particles after phylloquinone removal. According to the electron-carrier sequence presented above, removal of A] from the chain is expected to block forward electron transfer to succeeding carriers. This area of research, however, has been somewhat controversial. For instance, Itoh et a/initially found that, contrary to expectation, in particles whose phylloquinone was nearly quantitatively extracted, FeS-A/B could still be photochemically reduced at both 10 K and at room temperature. These authors also demonstrated photoaccumulation ofFeS-X . Meanwhile, Mansfield, Hubbard, Nugent and Evans performed ether extraction on PS-I particles prepared from pea chloro-plasts and obtained a similar correlation between phylloquinone extraction and the development of iron-sulfur-protein EPR signals, showing that the iron-sulfur proteins were retained. However, these authors found that the iron sulfur centers could only be reduced chemically, not photochemically. Furthermore, they also reported that addition of synthetic vitamin Kj to the extracted particles did not restore electron transfer to the iron-sulfur proteins. The authors explained that ether not only extracted phylloquinone and pigment molecules but also lipids, whose removal could possibly have affected the structure of the reaction center and consequently electron-transfer behavior. 

Describe the origins and behavior of sulfur oxides and nitrogen oxides as air pollutants, including the generation of add rain and photochemical smog. (Section 18.2)... 

This way of rationalization is substantiated by the photolytic behavior of, -unsaturated aldehydes (478) (Scheme 162). Only in one of the five cases studied was 479 observed this result is in accordance with the difficult intramolecular addition of alkoxyl radicals in the Cy6/Cy7 case. Since thiyl cyclizations appear efficient even in the Cy6/Cy7 case one might expect to observe easy photocyclizations of unsaturated thiones also in this case the easy photochemical synthesis of quinolines from A -(o-styryl)thioamides appears to be an excellent illustration of this view. Here, the sulfur analog of 479 would be an intermediate. To complete the analogy, one may expect the converse P-scission reaction to be efficient with radicals that readily open such as oxyranylalkyl ones (see Section VIII.2) in this way an interesting macrolide synthesis was described by Carlson. Photolysis of epoxycyclanone (480)... 

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