Photoextrusion

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. 

Small (three- and four-membered) cyclic sulfoxides, or strained bicyclic sulfoxides, appear to be particularly good candidates for the photoextrusion process. For example,... 

The course of the photolysis of a number of cyclic sulfoxides, however, has been shown not to involve simple photoextrusion processes. In fact, the work of Schultz and Schlessinger19,20 and Still and coworkers21 has shown the existence of a novel desulfurization pathway leading to cyclic ethers or to carbonyl compounds by formal loss of the sulfur atom only, by certain cyclic sulfoxides. 

Several studies of the application of the photoextrusion of S02 have been conducted in substituted analogs of the parent compounds above. For example, the photolysis of 2-phenylthietane 1,1-dioxides (34), in dichloromethane or methanol, at 254 nm affords the expected phenylcyclopropanes in almost quantitative yields68. The 2-phenyl substituent... 

Kobayashi and Mutai75 have recently reported an interesting rearrangement of the 1,4-dithiin sulfone (53) to the thiophenes (54) and (55) (equation 12). While 54 presumably arises as a result of simple photoextrusion, the rearranged thiophene (55) is postulated to arise via the valence isomer (56), followed by cyclization to the thiophene, concomitant with, or preceded by, loss of S02. Some support for the intermediacy of the thioketone (56) was revealed by the isolation of the pyrrole (57), when the photolysis was conducted in n-butylamine. Compound 57 presumably arises by cyclization of the iV-butylimine analog of 56 initially formed. 

The photoextrusion of sulphur dioxide to form cyclophanes or other novel aromatic molecules has been reviewed and studied by Givens208-210, while the photodecomposition of aromatic sulphones to form products of radical coupling reactions has recently also received attention211. 

The initial step in a photochemical extrusion reaction is again cleavage of a bond to give a pair of radicals or a diradical, but now a second bond is broken immediately after the first one, thus leading to the extrusion of a small molecule as CO, C02, N2 or S02. As in a recent review on photoextrusion of small molecules 248) only such reactions will be outlined wherein the small molecule that is to be expelled is initially part of a ring, and where the product formed is again a cyclic system. 

Aliphatic azo compounds usually undergo photoextrusion of nitrogen (cf. chapter 2.3.3). Nevertheless 1,2-diazetidines are formed efficiently when there is a favourable interaction between the pi-systems of the N—N and the C-C bonds (4.85)499). 

A photoextrusion of a nitrogen molecule from a partially saturated tetrazolo[l,5- ]pyridine derivatives has been described by Quast et al. <1998EJ0317> (Scheme 11). The starting bicyclic compound 39 when irradiated at low temperature (at -60 °C) afforded annulated iminoaziridine 40 as a mixture of (E)- and (Z)-isomers. These two geometric isomers equilibrated at higher temperature (20 °C). Upon heating of the mixture of ( )-40 and (Z)-40, a thermal cycloreversion took place with methyl isocyanide elimination to afford the dihydropyrrole 41. 

Photoextrusion of Molecular Nitrogen from Annulated 5-Alkylidene-4,5-dihydro-1H-... 

Three types of photoextrusion reactions have been identified in the irradiation of aryl-substituted 1,3,2-dioxathiolane 2-oxides [5 1-1-2 4-2] cycloelimination to produce a carbonyl compound, a carbene and sulfur dioxide extrusion of sulfur dioxide accompanied by a pinacol-like rearrangement to yield an aldehyde or ketone and extrusion of sulfur trioxide to give an alkene <72JOC2589>. Sensitization and quenching experiments indicate that a singlet state is responsible for the cycloelimination reaction, whereas the rearrangement and sulfur trioxide extrusion reactions arise from a triplet state <82JCR(S)175>. 

GENERATION OF SILYLENES BY THERMALLY INDUCED -ELIMINATION AND PHOTOEXTRUSION FROM OLIGOSILANES... 

Photoextrusion of nitrogen (70JA6218) and carbon monoxide (71JCS(C)2920> has been used to prepare 1,2-diazetidines (Scheme 123). Photoisomerization has alternately been reported to give fused derivatives of 1,2-diazetidines (Scheme 124) (68CC686,69JA2818). 

Bond-order changes upon photoexcitation obtained from Sandorfy SCF-CI calculations have been employed to predict the ease of photoextrusion of a silylene from polysilane chains or rings236. Smaller bond-order changes with longer polysilane chains suggested that they are less efficient silylene sources. Similar conclusions were reached for branched and cyclic polysilanes. This approach focuses on a to a excitation without participation by Si 4s orbitals in descriptions of excited states237. 

---