Sulfur photochemical elimination

Photochemical elimination reactions include all those photoinduced reactions resulting in the loss of one or more fragments from the excited molecule. Loss of carbon monoxide from type I or a-cleavage of carbonyl compounds has been previously considered in Chapter 3. Other types of photoeliminations, to be discussed here, include loss of molecular nitrogen from azo, diazo, and azido compounds, loss of nitric oxide from organic nitrites, and loss of sulfur dioxide and other miscellaneous species. 

Valence Isomerization of the 2-Thiabicyclo[3.2.0]heptadiene Moiety In principle, a valence isomerization of thiabicyclo[3.2.0]heptadiene skeleton would lead to a thiepin ring system. Wynberg et al. 23) reported that the photochemical adduct (28) from benzo[6]thiophene and dimethyl acetylenedicarboxylate was not thermally stable. When heated in diglyme, it loses sulfur to give dimethyl 1,2-naphthalenedicarboxylate. This reaction presumably proceeds via ring opening of 28 to 2,3-dimethoxycarbonylbenzo[6]thiepin (29) which readily eliminates sulfur. This synthetic route was successfully applied to the reaction of electron-deficient acetylenes with enamines of 2,3-dihydrobenzo[fe]thiophen-3-ones in which the enamine moiety constitutes part of a thiophene system. When 3-pyrrolidin-l-yl-benzo[6]thiophene (30) was allowed to react with dimethyl acetylenedicarboxylate... 

In simple terms, the global sulfur cycle has two components. One is biochemical involving the conversion of sulfate to sulfide and the formation of DMS the other is atmospheric photochemical oxidation of DMS to sulfur oxyacids. DMS is formed mainly in the oceans by microorganisms and to a lesser extent in plants. About 38M0 Tg year-1 of DMS are released to the atmosphere from the oceans. The major precursor for DMS formation is the sulfonium salt, dimethylsulfoniopropionate, (CH3)2 S+ CH2 CH2 COOH, DMSP. DMSP lyase enzymes catalyze an elimination of acrylic acid from DMSP (Equation 12) with the release of DMS ... 

An example of the photodecomposition of a dithiin to give a stable dithiet, with elimination of ethylene, has been reported.206 A different reaction is preferred in the dithiins (261), which on irradiation, are converted to benzene derivatives (262) via the 1,4-dithiocins (263).207 The cyclic trimer of thio-acetophenone on irradiation in cyclohexane yields thioacetophenone.208 An initial carbon-sulfur bond homolysis is also responsible for the conversion, on irradiation in methanol, of a 3-cephem to two thiazoles,209 whereas a 1,2-alkyl migration followed by a further photochemically induced carbon-sulfur bond homolysis has been proposed to account for the photorearrangement of the sulfonium ylides (264) to the pyrimido-l,4-benzo[h]thiazepines (265).210 The novel photorearrangemenl of a 1,3-thiazine to a cyclopro-pathiazolidine has also been rationalized in terms of an initial carbon-sulfur... 

A variety of examples of photochemically induced elimination of sulfur dioxide from sulfur heterocycles have been reported, with particular attention to cyclic sulfones. Thietan-3-one 1,1-dioxide (525), however, undergoes fragmentation on irradiation in acetonitrile to ketene (526) and to the sulfene (527), the latter identified by infrared spectroscopy and as the methanol... 

Although the experimental evidence suggests that the observed photogeneration of hydrogen may not involve homogeneous metal dithiolene photocatalysts, a theoretical study by Alvarez and Hoffmann addressed possible mechanisms for hydrogen elimination from ds square-planar bis(dithiolene) complexes (77). Concerted elimination of H2 from protonated sulfur atoms in the complex was proposed to be a thermally forbidden but photochemically allowed pathway, and protonation of a metal hydrido complex was also considered, as shown in Scheme 3. 

Inactivation of mycotoxins. When removal or elimination of mycotoxins is not possible, mycotoxins can be inactivated by (a) physical methods such as thermal inactivation, photochemical or gamma irradiation (b) chemical methods such as treatment of commodities with acids, alkalis, aldehydes, oxidizing agents, and gases like chlorine, sulfur dioxide, ozone and ammonia [201] and (c) biological methods such as fermentations or enzymatic digestions that cause the breakdown of mycotoxins [202]. 

In the late 1980s, the reaction of organic azides and 1,3-thiazol-5-thiones (Scheme 10) to provide good yields of l,3-thiazole-5-imines (i.e., (58)) was interpreted mechanistically in terms of successive eliminations from the initially formed adduct of N2 and S via the thermally unstable thiaziridine (57) <88HCA1673>. Analogously, the photochemical reaction between aryl azides and /V-sulfinyl-anilines has been shown to yield mainly azoaromatics (59) (Scheme 11). The experimental results (including the formation of sulfur and sulfur dioxide from the presumably extruded sulfur monoxide) were interpreted in terms of the intermediacy of thiadiaziridine-1 -oxide (13). 

All of these carbenes are reactive intermediates that must be generated from the appropriate precursors in the presence of the alkene (or arene) which is to be cyclopropanated. The following methods of carbene-transfer reactions to C-C double bonds will be discussed path a. from a-halo-a-metal (or alkylmetal) compounds by a-elimination path b. from iodine or sulfur ylides by thermal, photochemical or transition metal catalyzed decomposition ... 

The various methods of generating o-quinone methides,4-5 including the thermal or (Lewis) acid-catalyzed elimination of a phenol Mannich base,149 150-160-161163 the thermal or (Lewis) acid-catalyzed dehydration of an o-hydroxybenzyl alcohol (ether),147-149-151-153-156-157-162-163-165-168 171-175-178-183 the thermal 1,5-hydride shift of an o-hydroxy styrene,171-173 175 178-183 the thermal dissociation of the corresponding spirochromane dimer,158 163-164,166 oxidation of substituted o-alkylphenols,152-170 and the thermal or photochemical-promoted cheletropic extrusion154-155 159 of carbon monoxide, carbon dioxide, or sulfur dioxide (Scheme 7-III), as well as their subsequent in situ participation in regiospecific, intermolecular [4 + 2] cycloadditions with simple olefins and acetylenes,147 149-151 152 153159 162-164... 

The comparison of sulfur ylides to diazo compotmds raises the question of a-elimination to carbenes. Early reports of Franzen i ), Johnson and Swain claimed the observation of thermal decompositions to carbenes although one of these has subsequently been disproved. On the other hand, several photochemical reactions have been reported in which carbenes are the most likely intermediates. Corey and Chaykovsky discovered an Amdt-Eistert t3q>e process when p-keto oxysulfonium ylides are irradiated. ) By analogy to the Wolff rearrangement, the key step in the Amdt-Eistert... 

An alkyne may also be used in place of the diene component. With the cyclic sulfone complex 11.174a, this leads to an interesting synthesis of cyclooctatetraenes as the initial cycloadduct 111.76 is photochemically unstable and readily loses SO2 (Scheme 11.59). The adduct may be isolated if uranium glass filters are employed to exclude the high-energy wavelengths. Subsequent photolysis with the more transparent vycor filters causes chelotropic elimination of sulfur dioxide to give the tetraene 11.177. 

The general synthetic methods for [2.2] CPs have been already estabhshed. Most of them employ the ring contraction reactions of the 2,11-diheteroatom-substituted [3.3]CPs. Photochemical ehmination of sulfur or selenium atoms from [3.3] CP-2,11-disulfide or diselenide and flash vacuum pyrolysis of sulfur dioxide obtained by the oxidation of the corresponding sulfides are the most general methods. 2,11-Diaza[3.3]CPs are converted to the corresponding [2.2]CPs via their nitroso derivatives by reductive elimination of nitrogen. " Photoextrusion reactions of COj from cychc diesters can also be appHed to the synthesis of [2.2]paracyclophanes (PCPs)"- and [2.2]heterophanes, as described below. "... 

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