Thietane, photolysis

The photolysis of various substituted thiete dioxides under similar conditions resulted in the formation of the unsaturated ketones (255)264, most probably via a vinyl sulfene intermediate followed by a loss of sulfur monoxide as shown in equation 96. The same results were obtained in the thermolysis of 6e (R1 = R3 = Ph R2 = R4 = H)231, which further demonstrates that similar mechanisms are operative in thermolyses and pho-tolyses of thietane dioxides and thiete dioxides. 

Kaiser and Wulfers reported the first example of a thietane formation reaction involving photolysis of thiobenzophenone with olefins [27]. Since then a large number of systems have been reported. Thiobenzophenone and related compounds, xanthione and related compounds, the a,p-unsaturated thiones such as thiocoumarin, and adamantanethione have been most thoroughly examined. The wavelength dependence of these reactions has been systematically investigated in order to ascertain differences, if any, in the photochemistry of the thione S2 and T states. Scheme 1 shows one example of photocycloaddition of... 

Irradiation of a benzene solution of 3a gave tricyclic thietane 4a in 83% yield (Scheme 5 and Table 4, entry 1). The sohd-state photolysis also gave racemic 4a in 81% yield when the reaction conversion was 80% (entry 2). The solid-state reaction proceeded even at -78 °C (entry 3). 

A plausible mechanism for the formation of 4 is rationalized on the basis that photolysis of 3 results in [2-1-2] cyclization to thietane 4 and is subsequently followed by rearrangement to thiolactone 5 (Scheme 6). Ring opening of the initially formed thietane 4 leads to a zwitterion, which is facilitated by lone pair electrons of nitrogen and oxygen atoms, and nucleophilic reaction of the thiolate anion to carbonyl carbon gives 5. For the tricyclic thietane 4a, nucleophilic addition of the thiolate anion is difficult, and results in the formation of stable thietane 4a. 

Ultraviolet-visible spectroscopy has played only a minor role in the investigation of the thietanes and its counterparts. As expected, the colorless compounds absorb in the UV region below 300 nm. For photolysis experiments, many phenylated and alkylated thietanes and thietane oxides have been measured in methyl cyanide or methanol by Langendries and de Schryver. The spectra exhibit a maximum in the vicinity of 250-265 nm ( 12,000-25,000). 

Homolytic ring opening of thietanes also resulted from reaction with pentafluorophenyl radicals, which were generated by photolysis of C Fj in the presence of Mc3SnSnMe3. 

The photochemistry of thietanes involves entirely different pathways from those encountered in azetidines the low bond dissociation energy of the C—S bond seems to be mainly responsible. The direct photolysis of thietane vapor with 213.9-228,8- and 253.7-nm light leads to ethylene and propylene, cyclopropane, and thiocyclopropene. A white polymer appeared as a constant by-product. ... 

Sakamoto et al. reported an intramolecular [2+2] thietane formation in the solid state (Scheme 7). [26] Achiral A-(thiobenzoyl)methacrylamide 39 formed (E,Z)-conformation of the imide moiety, crystallized in a chiral space group Phhh, and the photolysis of single homochiral crystals at room temperature resulted in the formation of an optically active thietane-fused 3-lactam (40,75%) with 10% ee. The solid-state photoreaction proceeded even at -45°C to give higher ee value, 40% ee (conv. 30%, yield 70%). 

Similar absolute asymmetric synthesis was demonstrated in the solid-state photoreaction of A-(P,y-unsaturated carbonyl)thiocarbamate 41. [27] Achiral 0-methyl AT-(2.2-dmeth ibut-3-enoyl)-iV-phenylthiocarbarnate 41 crystallized in chiral space group P2i, and irradiation of these crystals gave optically active thiolactone in 10-31% ee. A plausible mechanism for the formation of 42 is rationalized on the basis that photolysis of 41 undergoes [2 + 2] cyclization to thietane and is subsequently followed by rearrangement to thiolactone 42. 

Steady-state photolysis (Ray-o-Net) of 7-methoxy-l, 1,3,3,7-pentamethyl-8-oxa-2-thia-5,6-diazaspiro[3,4]oct-5-ene 9 in cyclohexane at 300 nm followed by GC-MS analysis of the resulting mixture showed the formation of 3-isopropy-lidene-2,2-dimethylthiirane 20 as a product of the rearrangement of 2,2,4,4-tetramethyl-3-thietan-l-ylidene 6 (Scheme 4). The second product observed from the photolysis of 9 had strong peaks in the mass spectrum at m/z... 

The mass spectral fragmentation patterns of a variety of fused and spiro thietanes and thietes were presented in CHEC(1984) and CHEC-II(1996). Only more recent data are included here. The mass spectrometry (MS) spectra of thietanes 10 and 11 show molecular ion peaks at m/z 330 and 270, respectively, corresponding to the molecular weights of thiobarbiturates used for their preparation by photolysis in acetonitrile <2003H(59)303>. The structures of different derivatives (substituted in the thietane ring by CH3, Ph, or OC2H5 groups) of 5,7,7,9-tetramethyl-l-thia-... 

The photolysis of N(3- or 4-alkenyl)mono- or di-thioglutarimides seems to proceed in several steps involving initial intramolecular thietane formation between thiocarbonyl and N-substituted alkene. Photochemical cleavage of both C-S and C-C bonds of a thietane resulted in the formation of the corresponding indolizines or quinolizines (Equation 3) <2000H(53)2781>. 

Thermolysis of 4-benzoyl-3-phenyl-l,2-dithiolane 2,2-dioxide at 230° in a sealed tube gives 3-benzoyl-2-phenylthietane in 55% yield. ° Photolysis of 1,2-dithiolanes gives low yields of thietanes. ... 

Good yields of thietanes have been obtained by photolysis of a thiocarbonyl compound and an alkene. The thione is excited initially to one of two singlet states, Si (n -> 7T ) or Si (ir Thietane formation via the state proceeds through... 

Photolysis of the bicyclic enone 57 gives a modest yield of thietane thermolysis of 5-cyano-2,2,4,4-tetraphenyl-l,3-dithiane gives 3-cyano-2,2-diphenyl-thietane by loss of thiobenzophenone. Photolysis of 3,3,6,6-tetramethyl-S-acetoxy-l-thiacycloheptan-4-one gives a small amount of 3,3-dimethyl-2-acetoxy-thietane which also can be obtained in 30% yield by treatment of 3,3-dimethyl-thietane with lead tetraacetate in pyridine. ... 

Cyclization of /3-mercaptoaldehydes has been reported to give 2-hydroxy-thietanes (which are thioacetals) in 16-84% yield.They are said to be useful as flavoring substances. However, cyclization of 4-mercaptopropene gave only 0.1% of 2-methylthietane. The major product was thiolane. Photolysis of the a-diazoketone of 2,2,5,5-tetramethylthiolane-3,4-dione in methanol gave a 28% yield of 2,2,4,4-tetramethyl-3-carbomethoxythietane. " Several dispirothietanes have been obtained from the bis-hydrazone of 3-thietanone. ... 

Thietanes are photochemically unstable and should be protected from light if they are to be stored for any length of time. Short-lived (hot) biradical intermediates, for example, 98a, appear to be formed and can undergo a variety of reactions, as shown for thietane. " Mercury-sensitized photolysis gives triplet biradicals that are longer lived than the singlet biradicals formed on direct excita-tion. " Some cyclopropane product is produced in the sensitized photolysis. The second excited singlet state of thietane decomposes either to ethylene and thio-formaldehyde via a 1,4-biradical or to elemental sulfur and cyclopropane. ... 

Polymerizations also have been effected with heat ° and Photolysis of thietane in the presence of catalytic amounts of diaryliodonium salts gives a 14% yield of polymer. Poly(hexafluorothietane) is obtained on photolysis of the monomer for four days. 6-Thiabicyclo[3.1.1]heptane is said to give positive indications for free-radical formation in acrylonitrile. ... 

Photolysis of thietane sulfones (e.g., 169 ) also yields cyclopropanes. The need for a chromophore and a 2-substituent to stabilize an intermediate diradical has been emphasized. The sulfone 170 undergoes an unusual transformation. ... 

The first diradical intermediate is the same as that involved in the photolysis and thermolysis of thietane l,l-dioxides ° and interconversion of the y-sultine and thietane dioxide often accompanies cyclopropane formation. In flash-vacuum pyrolysis, formation of the alkene may compete with the desired cyclopropane-forming reaction. Examples of this reaction are summarized in Table 11. 

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