3-Alkylidene-l,2-dithioles

As mentioned above (Section 4.31.2.1), 3-alkylidene-l,2-dithioles (3d) may possess considerable ionic character consistent with contributions from structures (62), and condense with suitable carbonyl or other compounds. Condensation reactions of 3-aIkyI-l,2-dithiol-ylium salts (105) (66ZC321, 64CI(L)461, 80AHC(27)15l> are readily explained in terms of initial proton transfer from the salt to the substrate, as in the formation of salts of type... 

For the protonated species, various limiting forms may be considered, and their contributions to the resonance hybrid have been discussed in a previous review (Ref. 1 p. 54). The protonation equilibrium depends on the nature of X. When X is O, S, or Se, the protonated species exists in noticeable amounts only in strong acids such as concentrated sulfuric acid. On the other hand, when X is NR or CRR, the protonation is easy and the formation of the 3-imino- or 3-alkylidene-l,2-dithiole needs the action of a base. The protonation of 3-alkylidene-l,2-dithioles is discussed in the earlier review (Ref. 1 p. 57). 

Owing to their positive charge, 1,2-dithiolium cations are, of course, almost unreactive to electrophiles. On the other hand, their conjugate bases, such as 3-alkylidene-l,2-dithioles, react easily with electrophilic reagents such as aromatic aldehydes, dimethylformamide, and nitrous acid. Some of these reactions are useful for the synthesis of partially bonded compounds and have been described, for papers prior to 1970, in a previous review (Ref 2 pp. 181, 187, 193). 

If 3(5) substituents are present, 3-alkylidene-l,2-dithioles (2d) can be formed under the conditions of mass spectroscopy. Otherwise carbenes are formed which can dimerize to form bis-l,2-dithiol-3-ylidenes (9) <82SUL9>. 

An example of the formation of 1,2-dithiolium salts by protonation of an alkylidene-l,2-dithiole is the decarboxylation of 1,2-dithiol-3-ylidene malonic acids (14) in the presence of perchloric acid. In this particular case, a strongly acidic reaction medium is necessary for obtaining satisfactory yields because the corresponding 3-aryl-5-methylene-l,2-dithiole is unstable and undergoes an autocondensation reaction leading to an a-(thiopyran-2-ylidene)thioketone (cf. Ref. 1 p. 83). 

Dithiolylium bromides (4 X = Br) on thermolysis in the ion source of a mass spectrometer give stable l,2-dithiol-3-yl radicals (18). These subsequently form ions corresponding to the molecular ion, which in turn may either lose hydrogen atoms to form 3-alkylidene-... 

The cycloaddition of alkynes with the tributylphosphine-carbondisulfide adduct 131 results in the in situ formation of the ylides 132 which react with aldehydes to give the novel 2-arylidene or 2-alkylidene-l,3-dithioles 133 (Scheme 36) [132]. Concerning ylides C-substituted by sulfur we can also mention a publication on the behavior of various keto-stabilized ylides towards acyclic and cyclic a s-disulfides allowing the synthesis of substituted thiazoles, thiols, and dithiols [133]. 

The reaction of trialkyl- and triaryl-phosphines with 1,3-benzodithiolylium salts leads to formation of phosphonium salts which are deprotonated by treatment with n-butyllithium to produce (69). The similar reaction of trialkyl phosphites in the presence of sodium iodide yields dialkyl phosphonates which can be deprotonated to (70). Both (69) and (70) can react further with ketones to give the 2-alkylidene-l,3-dithiole derivatives (71) (80AHC(27)151>. The ylide (72) has also been prepared (80H(l4)27l>. 

Some 2-alkylidene-l,3-dithioles are converted to 1,3-dithiolium salts by protonation with perchloric acid. These salts tend to lose a proton, and weak bases are sufficient to displace the equilibrium completely toward the neutral species. This is given as proof of the low resonance energy of the dithiolium cation (Eq. 34). ... 

Either 193 or 194 can react with carbonyl compounds R COR to give the 2-alkylidene-l,3-dithioles 195. ... 

E. C. Taylor and R. Doetzer, Model studies directed toward the molybdenum cofactor 2-alkylidene- and 2-(phenylimino)-l,3-dithioles from acetylenes,/ Org. Chem., 1991, 56,1816-1822. 

An intramolecular Friedel-Crafts alkylation of the 2-alkylidene branched 1,3-dithiole (159) leads to l,l-bis(organothio)tetralines (160) (Equation (28)) <78BCJ2674>. 

---