Mesoionic dithiolylium-4-olates

The mesoionic dithiolylium-4-olates 43 provide examples of both types of cycloadduct. All but one of the derivatives studied react with o-chloranil to give the [4+2] cycloadducts 44 (14—96% yield) the exception is the 2-methyl-5-phenyl derivative 43 (R =Ph, R =Me), which gives the [4+3] cycloadduct 45 (59% yield) (81ZNB609). 

Scheme 59 [4 + 3] Cycloadduct from mesoionic dithiolylium-4-olates and ortho-chloranilA.

X-Ray structure determinations for 1,2-dithiolylium salts (4) (80MI43100) showed S—S bond distances of 2.02 A. For 1,2-dithiolylium iodides (4 X = I), charge transfer between the cation and iodide was found. 3,5-Diamino-l,2-dithiolylium ions (8) and derivatives showed long S—S bonds ( 2.07 A), indicating substantial iminium character, i.e. from structures of type (8a). The mesoionic l,2-dithiolylium-4-olates (7) have typical dithiolylium bond distances (78AJC297) and are best represented as shown. 

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). 

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 compounds 2 (also named l,3-dithiolylium-4-olates or l,3-dithiolium-4-olates) and 3 (also named 1,3-dithiolylium-4-thiolates or 1,3-dithiolium-4-thiolates) are described by canonical structures 2,3a-e, of which those with an oxyanion or a thioxyanion 2a and 2b, are presumably most representative. 

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