Sulfoxides, photochemistry

No mention is made of the potential for simple a cleavage of the sulfone as is observed in sulfoxide photochemistry, but aryl absorption is clearly involved in the naphthylmethyl cases as shown by emission and transient absorption studies [162]. Since the dibenzylsulfone studies were performed at 254 nm and dial-kylsulfones (unlike the sulfoxide analogs) are transparent in the near-UV region... 

The history of sulfoxide photochemistry dates back at least to the early 1960s, but this important hmctional group has received substantially less attention than some of the other chromophores whose chemistry was explored in those years [1,2], On the other hand, the sulfoxide s chiral namre has brought its thermal chemistry into greater exposure [3-5]. Much of that is due to the relative ease of preparation of optically pure samples and their utility as chiral auxiliaries, directing the stereochemistry of subsequent synthetic steps. Also, the sulfoxide is an intermediate oxidation state of sulfur, which can be made achiral or to have different reactivity by easily achievable oxidations and reductions. 

The primary photochemical process that is best established and probably most frequently invoked in sulfoxide photochemistry is homolytic C-S cleavage, or a-cleavage. Compared to the equivalent process in ketones, we shall see that the sulfoxide is, if anything, more susceptible. For instance, a-cleavage (under 1-photon conditions) is not observed for benzophenone, whereas diphenyl sulfoxide suffers this reaction, albeit with low quantum efficiency. 

This section is broken up into three parts 1.) benzylic and ally lie systems, 2.) aryl alkyl and diaryl sulfoxides, and 3.) extrusions. It turns out that the isolated materials from these reactions are only infrequently the primary photochemical products. Thus we shall also see photochemistry of a few other functional groups, notably the sulfenic ester/sultene and the sulfine (R2C=S=0). While secondary photochemistry can be a disadvantage in synthetic respects, the variety of rearrangements observed in sulfoxide photochemistry is all the richer for it. 

Sulfenic esters and sultenes are relatively stable thermodynamically, lying only several kcal/mol over the sulfoxides, all other things being equal [50]. However, in the laboratory, sulfenic esters are very difficult to handle without significant decomposition, in the absence of stabilizing substitutions [51], Their absorption spectra often extend to the red of the isomeric sulfoxides, which contributes further to difficulty in their isolation by way of sulfoxide photochemistry. Thus it is exciting that two independent laboratories isolated stable sultene derivatives in the 1980s, derived from sulfoxide photolysis. 

In sulfoxide photochemistry, products that appear to derive from ionic intermediates (to the exclusion of radical intermediates) are rare indeed. Even here, the yields Kropp found were low. Perhaps they were only found because Kropp s group sought them. Regardless, the mass balances of many reactions of sulfoxides are low, and such homolysis/electron-transfer mechanisms that simply yield unobserved compoimds cannot be ruled out. Results of photolysis of DMSO in water (vide infra) appear to follow just such a mechanism. Thus it should be something to consider for reactions in very polar media. 

Sulfoxide photochemistry is unlikely suddenly to catch fire as a field of interest due to intense synthetic interest in certain reactions. Nonetheless, we should keep a watchful eye for useful, as well as interesting, chemistry. The work of Paste in sulfenic esters clearly shows that a well-chosen substitution can tame otherwise difficult compoimds, and it would not be surprising to see other dramatic effects on sulfoxide photochemistry, which has been scoped out with simpler, relatively unsubstituted systems. The scope of rearrangements here is wide because of the flexible reactivity of some of the intermediates the clever soul among us should not hesitate to design transformations around the chemistry described herein. 

Chromium, hexacyano-, 3, 703, 777 hexaamminecobaltate coordination isomerism, 1, 183 ligand field photochemistry, 1, 398 photochemistry excited states, 1, 398 production, 3, 704 Chromium, hexafluoro-, 3, 927 Chromium, hexabalo-, 3, 889 Chromium, hexaiodo-, 3, 766 Chromium, hexakis(dimethyl sulfoxide)-photoanation, 1, 399 Chromium, u-oxalatodi-reduction... 

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. 

Majeti11 has studied the photochemistry of simple /I-ketosulfoxides, PhCOCH2SOCH3, and found cleavage of the sulfur-carbon bond, especially in polar solvents, and the Norrish Type II process to be the predominant pathways, leading to both 1,2-dibenzoylethane and methyl methanethiolsulfonate by radical dimerization, as well as acetophenone (equation 3). Nozaki and coworkers12 independently revealed similar results and reported in addition a pH-dependent distribution of products. Miyamoto and Nozaki13 have shown the incorporation of protic solvents into methyl styryl sulfoxide, by a polar addition mechanism. 

One of the major areas of interest in the photochemistry of cyclic sulfoxides has been photochemical extrusion (photodesulfurization). The general area of photoextrusion of small molecules has been comprehensively reviewed by Givens14. Although it is probably fair to point out that more photochemical studies of this type have been carried out for sulfones (q.v.) than for sulfoxides, there are now several examples in the literature of photochemical desulfinylation and these will be reviewed here. 

IV. DONOR-ACCEPTOR PROPERTIES OF SULFONES AND SULFOXIDES IN PHOTOCHEMISTRY... 

Using correspondence principles96 between photochemistry, electrochemistry and ordinary chemistry tells us that the redox properties of sulfones and sulfoxides could photochemically manifest themselves in two main directions. 

The second direction in which redox properties of sulfones and sulfoxides could manifest themselves in photochemistry is redox photosensitization108,110-114. In such a photosensitization the photosensitizer is transformed by light into a short-lived oxidant or reductant able to react with the substrate to be activated. Tazuke and Kitamura115 have discussed the parameters to play with when one... 

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