Dithiolones

The mesoionic 1,3-dithiolone (328), being a cyclic thiocarbonylylide, reacts as a 1,3-dipole with N- phenylmaleimide to give a mixture of adducts (329) and (330). Dehydrogenation of the adduct mixture with palladium eliminates COS to give the thieno[3,4-c]pyrrole derivative (331 Scheme 113) (78CB3029). 

The mesoionic l,3-dithiol-4-ones of type (2) are best represented as resonance hybrids of several limiting formulae (2a)-(2e), of which (2a) and (2b) are presumably most representative both formulae are, for example, in good agreement with the direction of the dipole moments. According to the IUPAC nomenclature rules, the mesoionic 1,3-dithiolones (2) should be named as l,3-dithiolylium-4-olates (B-79MI43202). 

The mesoionic 1,3-dithiolones of type (2) show substituent-dependent IR carbonyl stretching vibrations between 1612 and 1558 cm-1 (Table 7). These low frequencies are characteristic for this class of compound and are in agreement with the mesoionic structure. Most of the mesoionic 1,3-dithiolones containing p-substituted phenyl groups show split carbonyl absorption bands, presumably as the result of Fermi resonances (76CB740). 

This section is divided into reactions of fully unsaturated rings like 1,3-dithiolylium ions, mesoionic 1,3-dithiolones, l,3-dithiol-2-ones and l,3-dithiole-2-thiones, and reactions of the saturated 1,3-dithiolanes. As a consequence of the positive charge in 1,3-dithiolylium ions, the main reactions of this class of compounds consist of nucleophilic attack at the 2-position, whereas the mesoionic 1,3-dithiolones undergo cycloaddition reactions. Reactivity of the benzo ring in benzo-l,3-dithiole and related systems has not been studied to any extent. 

A similar reaction has been observed for the 1,3-dithiolone (18) which leads to formation of the tetrathiooxalate (19). The latter is in equilibrium with its [4 + 2] dimer (20) in solution (80CB1898). 

Crystalline dimers of type (27) have been synthesized under thermal conditions for the first time from phenylacetic acid derivatives (25) (79CB1650). The structure of (27 R = Me) was established by X-ray analysis (78TL671). The dimer (27 R = Me) is stable in solution, whereas the analogous alkylthio-substituted dimers are in a temperature and solvent dependent equilibrium with the monomeric 1,3-dithiolone (26). The monomers (26), generated from the dimers (27), display the same propensity towards [3 + 2] cycloaddition as do the mesoionic dithiolones which are generated in situ from the precursors (25). A photochemical dimerization of (9) has also been described (80CL717) (see Section 4.32.3.1.8(iii)). 

Since 1,3-dithiolylium ions are positively charged, attack of electrophiles at the ring carbons is very rare. One example, where the mesoionic 1,3-dithiolone is acylated at the 5-position in situ during its preparation, occurs in the preparation of (29) from its precursor (28) (78CB2021). 

For example, the mesoionic 1,3-dithiolones (2) combine across the 2,5-position at 90-130 °C with symmetrically substituted alkynes as dipolarophiles with formation of non-isolable primary adducts of type (85). The latter fragment in a retro Diels-Adler type... 

With unsymmetrically substituted mesoionic 1,3-dithiolones (2) and alkynes, the cycloaddition reactions normally yield the two possible isomeric thiophene derivatives. Thus methyl propiolate reacts with (2a) to produce an 81 19 ratio of the two possible isomeric thiophenes (88) and (89) in 90% yield, whereas the same reaction with the isomeric 1,3-dithiolone (2b) yields the thiophene derivatives in a 30 70 ratio (99%) (78CB2028). A further example is provided by the reaction of phenylacetylene with (2a) which proceeds with formation of the thiophene isomers (90) and (91) in an 89 11 ratio however, in the case of (2b) and the same substrate, the product isomer ratio is completely reversed (11 89) (78CB2028). 

These results clearly indicate that, at least in these cases, the orientation of the dipolarophile during the primary attack at the 2,5-positions of the 1,3-dithiolone is approxi-... 

An interesting example of an intramolecular cycloaddition reaction has been observed in the case of the mesoionic 1,3-dithiolone (97) which contains a non-activated alkyne in the same molecule. Thus, on heating (97) at 40-45 or 100 °C, the tricyclic thiophene derivative (99) is obtained via the non-isolable primary adduct (98) (81LA347). 

A reversion of the addition direction has been observed in the case of electron rich alkenes such as enol ethers and enamines. For example, ethyl vinyl ether reacts with the mesoionic compound (9) to produce the cycloadduct (108) in 58% yield. Other examples are the reactions of mesoionic 1,3-dithiolones with cyclohexyl vinyl ether, cyclopenten-l-yl ethyl ether, ethyl isobuten-l-yl ether and /V-(isobuten-l-yl)morpholine. The observed regioselectivities have been also qualitatively discussed on the basis of MO perturbation theory (79LA360). 

Cyclic alkenes also combine with mesoionic 1,3-dithiolones to produce isolable primary cycloadducts. Thus substituted cyclopropenes (109) react with l,3-dithiolylium-4-olates (2) at 100 °C with formation of the isomeric free exo tricyclic compound (110), whereas cyclopropene itself reacts at room temperature to yield the corresponding 1 1 cycloadduct (78CB3037). 

The mesoionic 1,3-dithiolones of type (2) also react with acenaphthylene or with cyclopen-tene to produce the endo adducts (111) or (112), respectively. On the other hand, the... 

In an analogous reaction of (2) with norbomadiene, the tetracyclic adduct (116) is formed which on heating undergoes a retro Diels-Alder reaction with formation of the thiophene derivative (117). Further cycloaddition reactions of mesoionic 1,3-dithiolones have been carried out with cyclopentadiene, 1,3-cyclohexadiene and 1,5-cyclooctadiene (78CB3037). 

Some hetero double bond systems have been shown to enter [3 + 2] cycloaddition reactions with the mesoionic 1,3-dithiolones. Thus, the mesoionic 1,3-dithiolones (2) react with formaldehyde, prepared in situ by depolymerization of paraformaldehyde, with regiospecific formation of the 2-oxa-6,7-dithiabicyclo[2.2.1]heptanone derivatives (131). The corresponding reaction of (2) with the N=N double bond of dimethyl azodicarboxylate proceeds via cycloaddition yielding (132), and a similar reaction takes place between (2) and 4-phenyl-l,2,4-triazoline-3,5-dione (78CB3171). 

Furthermore, the mesoionic 1,3-dithiolone (9) reacts with the C=N double bond of ethyl 3-phenyl-2//-2-azirinecarboxylate (133 R = C02Et) at 100 °C with regio- and stereospecific formation of the exo adduct (134 R = C02Et 92%). The second isomer of type (135) has not been observed (78CB3171). On the other hand, in boiling xylene the reaction of (9) with 2,3-diphenylazirine (133 R = Ph) forms the two isomeric adducts (134 R = Ph) and (135 R = Ph) in 4% and 41% yield, respectively (76JOC1724). 

Only a few examples of a Diels-Alder type reaction of the valence tautomeric ketene of mesoionic 1,3-dithiolones are known. Thus the reactions of differently substituted mesoionic compounds (2) with o-chloranil give the unusual adducts (142), derived via [4 + 2] cycloaddition to the non-detectable valence tautomeric ketene (141) which is in equilibrium with (2). The reaction rate depends upon the nature of the substituents, for example substituents with —I and/or —M effects lower the reaction rates (81ZN(B)609). 

An interesting route to the mesoionic 1,3-dithiolones of type (2) is offered by the anhydrocyclization of a-(thioacylthio)- or a-(thiocarbamoylthio)carboxylic acids with trifluoroacetic anhydride or a mixture of acetic anhydride and triethylamine (1 1) at 0-10 °C (Table 12) (81LA347, 78CB2021, 76CB740, 76JOC1724). These easy anhydrocyclizations... 

It has been shown (76JOC1724) that a previously described compound (65BCJ596), prepared by treatment of thiobenzoylthioglycollic acid (264) with acetic anhydride in the presence of boron trifluoride, is indeed the acylated 1,3-dithiolone (265) and not the claimed 2-phenyl-l,3-dithiolylium-4-olate. The latter can be prepared from thiobenzoylthioglycollic acid (264) and acetic anhydride-triethylamine (76JOC1724) or acetic anhydride-dicyclohexyl-ethylamine (76CB740). 

Another method of preparing mesoionic 1,3-dithiolones involves the reaction of dithiocar-boxylic acids with bielectrophiles. Thus treatment of aromatic dithiocarboxylic acids with a-bromoacyl chlorides in benzene in the presence of triethylamine results in the formation of the mesoionic compounds (327) (77JOC1633). 

Cycloaddition of mesoionic 1,3-dithiolones 5 to cyclopropene at 100 °C leads to exo-adducts... 

The attack of electrophiles at the ring carbons of 1,3-dithiolylium ions is seldom observed. Thus, upon reaction of (24) with TFAA the intermediary mesoionic 1,3-dithiolone (25) is acylated in the 5-position to afford (26) (Scheme 6) <78CB202l>. 

The observed regioselectivities during these cycloadditions have been explained in terms of qualitative frontier molecular orbital perturbation theory <78CB2028>. An intramolecular cycloaddition has been observed for the 1,3-dithiolone (111) to afford the tricyclic thiophene derivative (112) (Equation (13)) <81LA347>. 

Cyclic alkenes afforded, upon reaction with mesoionic 1,3-dithiolones (117), the primary cycloadducts. Thus, the cyclopropenes (118) gave the exo tricyclic compounds (119) ( uation (16))... 

Similar reactions have been reported for acenaphthylenes, cyclopentenes, p-benzoquinone, nor-bornadiene, cyclopentadiene, 1,3-cyclohexadiene, and 1,5-cyclooctadiene <78CB3037>. The reaction of dimethyl-7-oxabicyclo[2.2. l]hepta-2,5-diene-2,3-dicarboxylate (120) with the 1,3-dithiolone (113) proceeds with the formation of the cycloadduct (121) (Scheme 28) pyrolysis or photolysis of this adduct (121) resulted in a double fragmentation to give the thiophene (122) and the furan (123) <75CC840>. 

Further examples including extrusion and ring expansion reactions <75JCS(Pi)632> as well as intramolecular [3 -I- 2] cycloadditions have been reported <81LA347>. In addition, the mesoionic 1,3-dithiolones (124) react with formaldehyde regiospecifically in a [3 -i- 2] cycloaddition reaction with formation of the 2-oxa-6,7-dithiabicyclo[2.2.1]heptanone derivative (125) (Equation (17)) <78CB3l7l>. Furthermore, upon reaction with the N=N bond of dimethyl azodicarboxylate, (126) is obtained (Equation (18)) <78CB3171>. 

A [4 + 4] cycloaddition reaction has been used for trapping an in situ formed 1,3-dithiolone. Thus, treatment of the thioester (136) with dec in the presence of o-chloranil results in the intermediate formation of the 1,3-dithiolone (137) which is trapped by the o-chloranil to form (138) (Scheme (31)) <79CB1650>. 

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