Trithiapentalenes

Diaza-l,6,6aA -trithiapentalene synthesis, 6, 591 Diazatrithiapentalenes synthesis, 6, 914 Diazecine synthesis, 7, 730 Diazepam... 

The moleeular geometry and eleetronie strueture of 3,4-diaza-l,6,6aiC -trithia-pentalene 85a has been studied and eompared with the nitrogen-free l,6,6aiC -trithiapentalene [98PS35]. Whereas Hartree-Foek ealeulations prediet 85b and 85c to be valenee isomers, DFT and MP2 ealeulations prediet the minimum to be of C2u symmetry eorresponding to 85a (Seheme 56). 

There has long been interest in the structure of l,6-dioxa-6aA4-thiapentalene 174 and l,6,6aA4-trithiapentalene 175, for which the resonance forms 176 and 177 (with further delocalization of charges) can be included (see Figure 9). 

After many incorrect assignments, structures 178 and 179 were eventually proposed on the basis of the IR spectrum of the former and on an X-ray structure for the latter. Static structure 178 is correct, but 179 showed equal S-S distances. Also, the H NMR spectrum of 179 showed equivalent methyls and equivalent methines. Moreover, the two S-S distances in the parent l,6,6aA4-trithiapentalene 175 are equal, and the SSS angle is 178°. It was therefore proposed that these molecules are symmetric. 

The next debate in the literature was whether these molecules have C2v or Cs symmetry. The nuclear motion of a C2v symmetric structure would be described by a single-well potential (see Figure 10). The alternative is a rapid interconversion of two valence tautomers, each of Cs symmetry. This would occur via the C2v structure as transition state (see Figure 11). In this case the motion of the central sulfur would be described by a double-well potential, and dioxathiapentalene and trithiapentalene would be misnomers for (3//-l,2-oxathiol-3-ylidene)acetaldehyde 180 and (3/7-1,2-dithiol-3-ylidene)thioacetaldehyde 181. One advantage of C2v symmetry is aromatic stabilization from the 1071 electrons <2001CRV1247>. The alternative Cs symmetry has the advantage of avoiding a hypervalent sulfur. 

In summary, this NMR method of isotopic perturbation has resulted in an unequivocal answer to the long-standing question of the structure l,6-dioxa-6aA4-thiapentalene 174 and 1,6,6aA4-trithiapentalene 175. Over the coming years it is envisaged that this powerful method will be utilized on other triheterapentalene systems in order to resolve similar debates about their structure. 

Pederson and co-workers investigated the thermal behavior of 1,6,6aA4-trithiapentalene and some methyl-substituted derivatives using flash vacuum pyrolysis (FVP). The main products of the fragmentation, following loss of CS and/or CH2=C=S, were shown to be thiophene-3-thiones, or the thiol tautomers <1997J(P2)1261>. 

Table 25 Synthesis of 3,4-bridged 1,6,6aA4-trithiapentalenes 188 (Scheme 22)...

Thermal [2+3] cycloaddition reactions of 4jt-electron component 3-benzylidene-5-phenyl-3/7-l,2-dithiole 189 with 2Jt-electron component heterocumulenes 190 provided a novel route to 2-(substituted amino)-trithiapentalenes 191a and 2-(substituted amino)-dithia-l-selenapentalenes 191b (see Equation 49 and Table 26) <1997HAG233>. 

Treatment of ketodienamines 490 with phosphorus pentasulfide or Lawesson s reagent furnished bridged trithiapentalenes 491 (Scheme 39) <2001SL1129>. 

S/S insertion is also part of the reaction scheme when carbenes (or carbenoids) interact with l,6,6aX,4-trithiapentalenes 377 (Scheme 38) 352). The origin of the 4-diphenylmethylene-thiopyran 378 resulting from the reaction at higher catalyst concentration has not been elucidated, however. 

Bicyclic systems such as trithiapentalene 61 or thia-aza-pentalenes 62 (Scheme 30) show significant charge separation. Alternative representations for the above systems include nonclassical structures 61b,c and 62c with S(IV). Density functional calculations showed that the stability and aromaticity of bis-... 

A nice example of a degenerate rearrangement has been reported for the complex pentacarbonyltungsten(0)-thioaldehyde-l,2-dithiol 177 (R = H) or the thioketone-1,2-dithiol complex 177 (R = CH3). These compounds (obtained by the reaction of silver nitrate with tetraethylammonium pentacar-bonyliodotungstate(O) in the presence of l,6,6 z -trithiapentalenes) appeared to exist as an equilibrium mixture of the two isomers 177/178 (Scheme IV.70) [83JCS(CC)289]. The symmetrically substituted 1,2-dithiol (R = CH3, R = H) displays fluxional behavior as appears by NMR spectroscopy... 

Graubaum et al. studied the reaction of 5-aminothiatriazole (10) with chloroformates in the presence of base and isolated 1,2,4-thiadiazoles (104 R = OR) <89M997, 90JPR208). In the reaction between 5-aminothiatriazole and chlorothioformates in the presence of base the main products were trithiapentalenes (116) (Scheme 21). However, in the absence of base the intermediary 5-alkoxy-and 5-aryloxy-3-imino-A -1,2,4-dithiazoline (115) hydrochlorides are isolated. These reactions are analogous to those of (10) and acid chlorides (Section 4.19.6.1.2). 

Alkylamino and 5-arylaminothiatriazoles were reported to be more reactive towards isothiocyanates than 5-aminothiatriazole giving rise to 5-thiourea-l,2,4-thiadiazolin-3-thiones (128) in 50-84% yield. Formation of trithiapentalenes was not observed in these cases <89JPR115>. 

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