13-Dipolar cycloaddition 13-Dithioles

Analogously, the mesoionic jV-methyl thiazol-5-ones and l,3-dithiol-4-ones afforded A-methyl-4-pyridones and thiapyran-4-ones when reacting with diphenyl cyclopropenone and its thione261. Benzonitrile oxide apparently gives a 1,3-dipolar cycloaddition to the C=0 group of diphenyl cyclopropenone rationalizing the formation of triphenyl-l,3-oxazin-6-one 41626i ... 

The meso-ionic l,3-dithiol-4-ones (134) participate - in 1,3-dipolar cycloaddition reactions giving adducts of the general type 136. They show a remarkable degree of reactivity toward simple alkenes including tetramethylethylene, cyclopentene, norbomene, and norbor-nadiene as well as toward the more reactive 1,3-dipolarophilic olefins dimethyl maleate, dimethyl fumarate, methyl cinnamate, diben-zoylethylene, A -phenylmaleimide, and acenaphthylene. Alkynes such as dimethyl acetylenedicarboxylate also add to meso-ionic 1,3-dithiol-4-ones (134), but the intermediate cycloadducts are not isolable they eliminate carbonyl sulfide and yield thiophenes (137) directly. - ... 

The anion derived from the 1,3-dithiole 367 reacts with thioxanthone to give the 9-(l,3-dithiol-2-ylidene)thiox-anthene derivative which is a 7t-electron donor system. The introduction of a benzaldehyde unit at the 4-position of the dithiole ring was achieved by iodination and a Pd-mediated cross-coupling with 4-formylbenzeneboronic acid. The aldehyde group allowed attachment to C(,o via a 1,3-dipolar cycloaddition of an azomethine ylide (Scheme 87) <2005JMC1232>. 

The fluorine containing 7t-exTTF 550 was obtained in the 1,3-dipolar cycloaddition of DMAD to dithiocarboxylic moiety of the dithiocrotonate 549 and subsequent reaction of the resulting intermediate ylide with the starting 549 as a Michael acceptor in an addition-elimination process followed by ethyl bromide elimination. Formation of the second 1,3-dithiole moiety in 550 was connected with the consecutive cycloaddition followed by ethyl bromide elimination (Equation 71) <2003CEJ4324>. 

Monocyclic and some bicyclic l,2-dithiole-3-thiones are known that readily undergo 1,3-dipolar cycloaddition reactions to give cyclic 1,3-dithiole and spiro[l,3]dithiole systems. Due to a great variety of applications, the chemistry of this class of compounds still remains a subject of active research. 

No systematic studies of the reaction mechanism have been undertaken, but it is suggested that the 1,3-dithiole derivative is formed directly by a 1,3-dipolar cycloaddition where the thiocarbonyl serves as the negative end of the dipole. The formation of l,6,6a/l4-trithiapentalenes may start with a 1,2-addition of the alkyne to the thiocarbonyl group, followed by a rearrangement to give 136. 

The meso-ionic 1,3-dithiolium 4-olate (283 R = SMe) exists in equilibrium with the dimer (284 R = SMe). Complete regiospecificity is observed in the 1,3-dipolar cycloaddition of styrene to the diphenyl-derivative (283 R = Ph), which gives only compound (285 R = H,R = Ph) in contrast, methyl acrylate yields a mixture of (285 R = C02Me, R = H) and the isomeric adduct (285 R = H, R = C02Me)/ Irradiation of the diphenyl-compound (283 R = Ph) yields a mixture of 4,5-diphenyl-l,2-dithiole-3-thione (286) via the bridged intermediate shown), sulphur, diphenylacetylene, tetraphenylthiophen, and the latter s precursor, the dithiin (287). The dithiin is thought to arise from the dimeric compound (284 R = Ph) by loss of carbon oxysulphide. ... 

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