Of A-4-thiazoline-2-thione

Thiazolium derivatives unsubstituted at the 2-position (35) are potentially interesting precursors of A-4-thiazoline-2-thiones and A-4-thiazoline-2-ones. Compound 35 in basic medium undergoes proton abstraction leading to the very active nucleophilic species 36a and 36b (Scheme 16) (43-46). Special interest has been focused upon the reactivity of 36a and 36b because they are considered as the reactive species of the thiamine action in some biochemical reaction, and as catalysts for several condensation reactions (47-50). 

A bathochromic shift of about 5 nm results for the 320-nm band when a methyl substituent is introduced either in the 4- or 5-posiiion, The reverse is observed when the methyl is attached to nitrogen (56). Solvent effects on this 320-nm band suggest that in the first excited state A-4-thiazoline-2-thione is less basic than in the ground state (61). Ultraviolet spectra of a large series of A-4-thiazoline-2-thiones have been reported (60. 73). 

The auto-association of A-4-thiazoline-2-thione is clearly indicated b the hypsOchromic shift (5 nm) of the 315-nm band when the spectrum is first recorded at 50°C and then at —25°C (10 M in cyclohexane). In the same temperature range the spectrum of 3-methyl-A-4-thiazoline-2-thione remains unchanged (61). 

Ultraviolet spectra of A-4-thiazoline-2-thione in 10 A HCl show that protonation occurs on the exocyclic sulfur rather than on the cyclic nitrogen or on the enamine-like Co-position (56). 

A recent report (62). using UNDO approximations, describes and interprets the phoioelectronic spectra of A-4-thiazoline-2-thione and other thiocarbonyl heterocycles. The results are given in Table VIl-3. The major feature is the clean separation between the two highest MOs and the others. The highest MO of tt symmetry 17.74 eV) is essentially localized on the dithiocarbamic part of the structure. The second one (8.12 eV) is highly localized on the exocyclic sulfur atom. This peculiaritv... 

Infrared and Raman spectra of A-4-thiazoline-2-thione and of isotopi-cally labeled derivatives (56. 59) were interpretated completely. (Table VII-41. 

Auto-association of A-4-thiazoline-2-thione and 4-alkyl derivatives has been deduced from infrared spectra of diluted solutions in carbon tetrachloride (58. 77). Results are interpretated (77) in terms of an equilibrium between monomer and cyclic dimer. The association constants are strongly dependent on the electronic and steric effects of the alkyl substituents in the 4- and 5-positions, respectively. This behavior is well shown if one compares the results for the unsubstituted compound (K - 1200 M" ,). 4-methyl-A-4-thiazoline-2-thione K = 2200 M ). and 5-methyl-4-r-butyl-A-4-thiazoline-2-thione K=120 M ) (58). 

Physical properties of A-4-thiazoline-2-one and derivatives have received less attention than those of A-4-thiazoline-2-thiones. For the protomeric equilibrium, data obtained by infrared spectroscopy favors fbrm 51a in chloroform (55, 96, 887) and in the solid state (36. 97. 98) (Scheme 23). The same structural preference is suggested by the ultraviolet spectroscopy studies of Sheinker (98), despite the fact that previous studie.s in methanol (36) suggested the presence of both 51a and... 

Reactivity of A-4-thiazoline-2-thiones and derivatives involves four main possibilities nucleophilic reactivity of exocyclic sulfur atom or ring nitrogen, electrophilic reactivity of carbon 2 and electrophilic substitution on carbon 5. 

TABLE VII-10a SOLVENT EFFECTS ON THE DIAZOMETHANE METHYLA-TION OF A-4-THIAZOLINE-2-THIONE AT 25°C ... 

Ambident reactivity of A-4-thiazoline-2-thione has been discussed (101) in terms of the Hard and Soft Acids and Bases classification (199) and the Klopman-Hudson approach (200). 

The most useful synthetic method involves the reaction of A-4-thiazoline-2-thione with the appropriate alkylating agent (see Section I.l.C). An example is given Scheme 54. 

Because of their use in the rubber industry various sulfenamido thiazoles (131) have been prepared. They are obtained in good yields through the oxidation of A-4-thiazoline-2-thiones (130) in aqueous alkaline solution in the presence of an amine or ammonia (Scheme 66) <123, 166, 255, 286, 308, 309). Other oxidizing agents have been proposed (54, 148. 310-313) such as iodine (152), chlorine, or hydrogen peroxide. Disulfides can also be used as starting materials (3141. 

Disulfides (136) are formed in good yields by oxidation of A-4-thiazoline-2-thiones (135) and derivatives in basic media Scheme 69 (7, 130, 146-151, 317-319). 

Metal salts of A-4-thiazoline-2-thione are used in the rubber industry Zn salts (123, 152), Pb and Mg salts (54). Cd salts (151, 324), Cu salts (325), in photographic processes (146). and in analysis (328). Zn, Ni, Co and Cd salts are used as germicides (329). Despite their wide range of application, little is known about their physical and chemical properties. 

TABLE VII-1. COMPARISON OF TYPICAL PHYSICOCHEMICAL PROPERTIES OF A-4-THIAZOLINE-2-THIONE (Ii WITH THOSE OF 2-(METHYLTHIO)THIAZOLE (II) AND 3-METHYL-A-4-THIAZO-LINE-2-THIONE (111) ... 

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