1.3- Dithioles special

There are few examples of the preparations of heterocyclic compounds containing two or more heteroatoms which involve cyclization with formation of a bond between two heteroatoms. The best known instances of this type of reaction, all of which are [6 + 0] reactions, are the preparations of benzocinnolines as outlined in equations (l)-(4). A similar type of approach to that outlined in equation (4) has been used for the direct preparation of the di-N-oxide (2) from the dioxime (1 equation 5). The naphthotriazine betaine (4) is obtained as one of the products of the thermal decomposition of the azidoazo compound (3 equation 6). 1,2-Dithiins and their dibenzo derivatives have been prepared by oxidation of appropriate dithiols and related starting materials as outlined in equation (7). All of these reactions are, however, somewhat specialized and there has been essentially no systematic study of the preparation of six-membered heterocycles via formation of a bond between two heteroatoms. 

Certain classes of ligand deserve special mention as their Mo02+ complexes have received particular attention. The 1,1-dithiolate ligands, especially A,A-disubstituted dithiocarbamates, have been very thoroughly studied.131-133 Evidence for the existence of the dithiophosphate complex Mo02[S2P(OEt)2]2 in solution has been obtained.95 This and a number of other dithiolate complexes may be unstable toward internal redox in which the S ligand serves as the reductant to produce a lower valent Mo—S complex. 

Transition metal complexes of unsaturated 1,2-dithiolates (metal dithiolenes) have attracted much attention because of their interesting structural and redox properties.169 Molybdenum dithiolene complexes have featured prominently170 in these studies and have special significance following the suggestion171,172 that the molybdenum-containing cofactor of the oxomolybdoen-zymes (Section 36.6.7) incorporates a molybdenum complex of an unsymmetrically substituted alkene-1,2-dithiolate. 

Transition metal complexes (1) of 1,2-dicyanoethylene-1,2-dithiol (maleonitrile), mnt, have received special study since this ligand can stabilize transition metals in a variety of oxidation states and the high electron affinity of the terminal cyanide groups aids the delocalization of charge within the complex, thus reducing potential coulombic repulsions. 

A special derivatization reaction is required for lewisite 1, which is so reactive that it cannot be determined by GC/MS in low quantities (e.g. below 10 ng per injection). It has been known for a long time that lewisite 1 reacts with compounds having an a, P-dithiol structure, such as 2,3-dimercaptopropanol-l (British-Anti-Lewisite (BAL) also used for medical treatment). The derivatization reaction can be performed at an analytical level and several examples have been described (29). The reaction product of lewisite 1 with 3,4-dimercaptotoluene, 2-(2-chlorovinyl)-5-methyl-l,3,2-benzodithiarsole (see (1)), is a useful derivative for GC/MS analysis. Its mass spectrum is simple with molecular ion peaks at m/z 290/292 and the base peak at m/z 229 due to the loss of the 2-chlorovinyl group (30). 

The preparation of the 1,3-dithiolylium perchlorate (229) follows a more special route (80MI43202). Thus, treatment of the dithiocarbamate (226) with perchloric acid at 50 °C gives an 86% yield of the perchlorate (227) which then can be reduced to the 1,3-dithiole (228). Upon further action with perchloric acid in acetone at room temperature, (228) is converted into the 1,3-dithiolylium perchlorate (229). Also a bis( 1,3-dithiolylium) salt (231) is obtained by treating the diester (230) with cold concentrated sulfuric acid (77TL4607). Furthermore, compounds of structure (233) which contain two 1,3-dithiolylium nuclei in the same molecule, can also be prepared by acidic cyclodehydration of the corresponding diester (232). These substituted salts (233) can be further converted-in excellent yields into the salts (234), which are unsubstituted at the 2-position. The explosive perchlorate (235) can be prepared in a similar manner (80AHC(27)151). 

This chapter deals with the synthesis, reactivity, and characterization of five-membered heterocycles containing two ring sulfur atoms (1,3-dithiole derivatives) and is a review of the literature in the period 1995-2006. Previous reviews covered the literature till 1982 (CHEC(1984)) <1984CHEC(6)813> and 1995 (CHEC-II(1996)) <1996CHEC-11(3)607>. Among 1,3-dithiole compounds, 1,3-dithiolylium ions 1, mesoionic l,3-dithiol-4-ones 2, mesoionic 1,3-dithiole-4-thiones 3, 1,3-dithioles 4, 1,3-dithiolanes 5, and the tetrathiafulv alene (TTF) system 6 as a special class of compounds are discussed. 7l-Extended tetrathiafulvalenes (7t-exTTFs) containing more than one conjugated multiple bond between two 1,3-dithiole moieties are also discussed, in conjunction with 6. 

In this section, the reactivity of carbon- and heteroatom-containing substituents attached directly or indirectly to the heterocyclic ring of 1,3-dithiole derivatives is discussed. Here, chemical manipulations on these systems exclude ring carbon and sulfur atoms, which are the subject of Section 4.12.6. Special 1,3-dithioles called Jt-extended TTFs, containing two or more 1,3-dithiole moieties linked together via =C-(C=C) -C= or similar conjugated spacers, are discussed with typical TTFs in Section 4.12.11, although formally they constitute an example of functionalization of the 1,3-dithiole by 2rt-extended substituents, terminated by another 1,3-dithiole moiety. 

Some approaches toward the synthesis and chemical modification of TTF and related donors will be mentioned only very briefly and the reader is recommended to consult more specialized reviews [213]. A common method of TTF synthesis by formation of the central exocyclic double bond starts with 1,3-dithiol-2-thione precursors 150, which are subjected to trialkyl-phosphite-induced coupling (scheme 36a) [214]. Mixed coupling products of the general... 

The adducts formed by mercury(Il) acetate and l,2-dithiole-3-thiones are of particular interest because they are often used for converting 1,2-dithiole-3-thiones into l,2-dithiol-3-ones. The difference in the behavior of mercury(II) halides and acetate toward dithiolethiones suggests that a special mechanism operates in the latter case, probably a concerted cyclic reaction as indicated in Scheme 10. The 3-acetoxy-l,2-dithioliumcation thus obtained is rapidly converted to the corresponding dithiolone by reaction of the acetate anion or water. 

A special type of nucleophilic attack on 1,2-dithiolium compounds is the pyridine-catalyzed rearrangement of 3-[(l,2-dithiol-3-ylidene)methyl]-l,2-... 

The preparation of thioacetals involves treatment of the carbonyl substrate with a dithiol in the presence of an acid catalyst, usually TsOH or Bp3 OEt2. Since thioacetals are quite stable toward hydrolysis, there is no special need to remove the H2O formed during the reaction. Also, since it is more difficult to equilibrate thioacetals than acetals via protonation, double bond migration in thioacetalization of enones is usually not observed. 

Many derivatives of 1,2- and 1,3-dithiols have been used as protecting groups, but those discussed are the most common. Greene and Wuts discuss other methods for protection of ketones and aldehydes, including cyanohydrins, hydrazones, oximes, oxazolidines, and imidazolidines. 9 Most of these are rather specialized and will not be discussed in this general presentation. 

An efficient conversion of cyclobutanone to a 7-lactone ring is part of a total synthesis of prostaglandins Among the various products obtained by reaction of acetylenes with carbon disulfide are those of special interest which originate by the addition of third components to intermediate 1,3-dithiolium carbenes to form 2-subst. 1,3-dithioles... 

Dibenzylthiobutadienes 59 undergo reductive debenzylation under the action of sodium metal in liquid ammonia at — 70°C with the formation of sodium dithiolate (60) which is instantly oxidized either with air oxygen or with special oxidants (iodine, iron chloride) to 1,2-dithiines 55. l,4-Di(/-butylthio)buta-1,3-dienes (61) afford 1,2-dithiins 55 by treatment with (o-nitrophenyl)sulfenylchloride via disulfide 62. 1,2-Dithiins 55 can be transformed to the corresponding thiophenes 63 [see also (92MI1)]. The formation of 63 is also observed upon the fragmentation of compounds 55 in the mass spectra (principal peak) (96T12677). 

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