Synthesis of Thioindigo

Thioindigo 1 is obtained in 80% yield when 2,3-dibromothiochromone 5-oxide is heated with 2 equivalents of sodium acetate in refluxing acetic acid for 15 minutes. No thioindigo is formed if 2,3-dibromothiochromone is used as starting material. 

Give a mechanism to account for the formation of thioindigo from the 5-oxide. 

An Unexpected Reaction During Studies of Amine Oxide 

The tetrahydroquinoline derivative 1 was treated with MCPBA in methylene chloride at room temperature in the expectation that the product would be the tricyclic compound 2. Despite a number of attempts, however, 2 was not formed. The product, obtained in 30% yield, was shown to be 3. 

Explain on a mechanistic basis why 2 was the expected product from the oxidation of 1, and give a mechanism for the formation of 3. 

---

Although there is one route [3] which describes the direct synthesis of the tetrachlorothioindigo pigment by oxidation of 3-hydroxy-4,7-dichlorothionaphthenone with oxygen in an aqueous alkaline medium, this is somewhat of an exception. In most cases, it is necessary to modify the crude thioindigo derivative by appropriate aftertreatment in order to develop the desired pigment properties. 

The synthesis of the red dye thioindigo (Friedlander), and its technically important derivatives, proceeds in a corresponding way. The starting material is thiosalicylic acid ... 

The object of Lesser and TVeiss in carrying out -work on these compounds was to obtain a selenium compound analogous to thioindigo, and the following deals with the intermediates used in the preparation of this body and condensation products of oxyselenonaphthene. The complete synthesis of selenindigo is shown by the scheme on opposite page. ... 

In addition, sulfur monochloride (37) will react with p-chloroarylamines, like (45) in the presence of sodium hydroxide to give the sodium thiophenolate (46) this is the Herz reaction (Scheme 31) used in the synthesis of o-aminothiophenols, which are useful in the manufacture of thioindigo dyes (see Chapter 11, p. 221). 

The symmetrical thioindigo dyes have the structure (355), where R = Cl, Br, Me, NH2, OMe, OEt or SEt located at the 4-, 5-, 6- or 7-position of the benzo[6 jthiophene parent compound, or combinations of two or three of these substituents. The original Friedlander synthesis involved mild oxidation of a 3-hydroxybenzo[6 jthiophene, but numerous methods, many patented, have been used. The 2,3 -bithioindigos, also known as thioindirubins (356), by analogy with the indigo isomer, indirubin, are also dyes of some interest. 

L. Dinescu, K.E. Maly, R.P. Lemieux, Design of photonic liquid crystal materials synthesis and evaluation of new chiral thioindigo dopants designed to photomodulate the spontaneous polarization of ferroelectric liquid crystals. J. Mater. Chem. 9, 1679-1686 (1999)... 

Thioindigo and derived dyes (Figure 2.36). In thioindigo the —NH— of indigo is replaced by —S—, a typical synthesis being that of Friedlander (Figure 2.35). 

---