L,6,6aA4-Trithiapentalenes

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. 

Diaza-l,6,6aA4-trithiapentalene synthesis, 6, 591 Diazatrithiapentalenes synthesis, 6, 914 Diazecine synthesis, 7, 730 Diazepam... 

Dithiolylium cations, 3-(2-alkylthiovinyl)-l,6,6aA4-trithiapentalenes from, 6, 1065... 

Likewise, thiobenzoyl chloride gives 3,4-diaza-l,6,6aA4-trithiapentalene (59) in low yield (11%), which may also be prepared from the above diazapentalene and P4Si0 (81BSB89). 

Benzo and other fused derivatives are also known. In addition, the 1,2-dithiole ring is an integral part of the structure of the l,6,6aA4-trithiapentalenes (5) covered in Chapter 4.38. Naming in this chapter will conform mostly to that used in Chemical Abstracts except for the dithiolylium cation (4) (IUPAC Rule C-83) and also for the trithiapentalenes. 

Alkyl groups in dithioles (3a, b) possess acidic protons and condense with aldehydes to form 5-vinyldithioles (109). This type of reactivity has been widely used in the preparation of some l,6,6aA4-trithiapentalenes (5) or precursors (7iAHC(i3)i6i) by reaction with carbon disulfide (Scheme 17). 

Since 1958, the structure of l,6,6aA4-trithiapentalenes and of related compounds has been the subject of much study and discussion. The essential question was whether such a compound is a mixture of valence isomers in rapid equilibrium (equation 1) or is best represented by a symmetrical formula (1) or by resonance forms (equation 2). In fact equation (2) or formula (1) are equivalent and imply a C2v symmetry, while equation (1) implies a Cs type of structure. 

More precise numerical data, such as heats of combustion, are not available but theoretical methods (see Section 4.38.2.1) suggest that l,6,6aA4-trithiapentalenes exhibit a naphthalene-like 7r-electron delocalization. It should be noted, however, that the resonance... 

Interest in the bonding pattern of l,6,6aA4-trithiapentalenes has led to the study of various related structures. The simplest consists of the presence of more than one trithiapen-talene system in the same compound. Some examples of this type are known. 

A large quantity of work has been devoted in recent years to the nature of the bonding in l,6,6aA4-trithiapentalenes and related compounds. The electron distribution in these molecules has been a subject of controversy but, at least for l,6,6aA4-trithiapentalenes themselves, it is now generally assumed that such a distribution involves a delocalized IO77-- electron system similar to that of naphthalene. 

CNDO/2 calculations have been carried out on l,6,6aA4-trithiapentalene and on its 2-and 3-methyl and 2- and 3-phenyl derivatives which are consistent with experimental results. A 2-methyl group and a twisted 2-phenyl group cause a lengthening of the bond between the sulfur atoms 1 and 6a. A 4-phenyl group affects this S—S bond to a smaller degree (72CC222). 

Interatomic distance is one of the most important features of a chemical bond and the particular problem raised by l,6,6aA4-trithiapentalenes prompted numerous structural studies on these compounds and their analogues. 

The first X-ray structure determination of a l,6,6aA4-trithiapentalene dates back to 1958 and since that time the number of such studies shows the interest raised by this type of compound. Some data have been collected before (71AHC(13)161,p. 212) and Tables 1 and 2 contain more recent results together with the most significant previous ones. 

Table 2 Partial Bonding in Compounds Related to l,6,6aA4-Trithiapentalenes...

On the other hand, dipole moments of l-oxa-6,6aA4-dithiapentalenes are significantly larger than for the corresponding l,6,6aA4-trithiapentalene, a fact indicating that the S-O interaction is weaker than the corresponding S-S interaction. 

Photoelectron spectroscopy using electrons expelled by He (584 A) or Mg-K X-rays (9.869 A) has been applied to l,6,6aA4-trithiapentalenes and to l-oxa-6,6aA4-dithia, l,6-dithia-6aA4-selena and l,6,6aA4-triselena analogues. 

Mercury (II) acetate reacts with l,6,6aA4-trithiapentalenes or their 3-aza derivatives to give the corresponding l-oxa-6,6aA4-dithiapentalenes or l-oxa-6,6aA4-dithia-3-azapen-talenes <71AHC(l3)16l p. 179,199). This reaction proceeds first by acetoxymercuration of a sulfur atom, followed by hydrolysis of the intermediate dithiolylium cation as indicated in Scheme 1. 

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