Isothiocyanates dithiocarbamic acid ester

Even though the most widely used preparation is still the reaction of alkali isothiocyanates or alkyl and aryl isothiocyanates with a-amino carboxylic acid derivatives, the reaction of dithiocarbamic acid esters (CCX) with a-amino carboxylic acids (CCXI) has certain advantages especially in the synthesis of heterocyclically substituted imidazolidin-4-one-2-thiones (221). 

It appears quite probable that this reaction occurs by the same mechanism as the reaction of nitrous acid with thiosemicarbazides and of sodium azide with isothiocyanates. The primary reaction product is a thiocarbamoyl azide (CCXIII) to which, however, the cyclic structure of a 5-amino-l,2,3,4-thiatriazole (CCXIV) is attributed. In alkaline medium these compounds rearrange to the corresponding 1 -substituted-Zla-tetrazoline-5-thiones (CCXII) 236, 237). A survey of these compounds, prepared from the dithiocarbamic acid esters, is given in Table 33. 

Without additional reagents Isothiocyanates from dithiocarbamic acid esters... 

In a weakly alkaline solution, isothiocyanate reacts with the a-amino group of cysteine (Figure 2.64). The reaction of isothiocyanates with cystine leads to the cleavage of disulfide bond (-S-S-) with the formation of cysteine sulfenic acid and a dithiocarbamic acid ester. Dehydration of this ester yields 5-amino-l,3-thiazine-4-one-3-thione derivative and elimination of hydrogen sulfide produces a cyclic 2-thiazoline-4-carboxylic acid derivative (Figure 2.65). Reaction of cystine with thiocyanates (R-S-C=N), formed by spontaneous isomerisation of isothiocyanates, produces disulfide Cys-S-S-R and -thiocyanoalanine, which cyclises to 2-thiazoline derivative (Figure 2.66). 

Pyridine sodium hydrogen carbonate Dithiocarbamic acid esters from isothiocyanates and mercaptans C,HiN/NaHCO, N C S NHC(S)SR... 

These products can also be obtained from the esters of dithiocarbamic acid and sodium azide 21S). The two reagents are dissolved in a suitable solvent, usually a water-ethanol mixture, and heated until no more alkane thiol is split off. The thione is isolated by acidifying the sodium salt. This method of preparation avoids the use of the difficult to prepare isothiocyanates. 

Aromatic amines are too weak to undergo the above reactions they do add to carbon disulfide but the resulting dithiocarbamic acids cannot form salts the resulting unstable free acids decompose at once with evolution of hydrogen sulfide and formation of isothiocyanates which add the excess of amine and thus give thioureas as final products 93,94 a small amount of sulfur catalyses the formation of the thioureas.95 Treatment of salts of the dithiocarbamic acids obtained in this way with chloroformic esters in chloroform at 0° leads to loss of COS and alcohol and formation of isothiocyanates in high yield.96... 

The reaction of isothiocyanates with cysteine gives (depending on pH) two types of primary products. In weakly acidic solutions (pH 5-6), isothiocyanate reacts exclusively with the thiol group of cysteine, producing N-substituted esters of dithiocarbamic acids and heterocyclic products. The same products are formed from... 

Despite the chemical diversity of the several hundred structures representing herbicidal activity, most reactions of herbicides fall within only a limited number of mechanistic types oxidation, reduction, nucleophilic displacements (such as hydrolysis), eliminations, and additions. "Herbicides", after all, are more-or-less ordinary chemicals, and their principal transformations in the environment are fundamentally no different from those in laboratory glassware. Figure 2 illustrates three typical examples which have received their share of classical laboratory study—the alkaline hydrolysis of a carboxylic ester (in this case, an ester of 2,4-dichlorophenoxyacetic acid, IX), the cycloaddition of an alcohol to an olefin (as in the acetylene, VI), and the 3-elimination of a dithiocarbamate which provides the usual synthetic route to an isothiocyanate (conversion of an N.N-dimethylcarbamic acid salt, XI, to methyl isothiocyanate). Allow the starting materials herbicidal action (which they have), give them names such as "2,4-D ester" or "pronamide" or "Vapam", and let soil form the walls of an outdoor reaction kettle the reactions and products remain the same. 

Preparation of Isothiocyanates.—A conventional synthetic method is illustrated in the preparation of AT-(isothiocyanato-acyl)-amino-acids, e.g. S C N (CH2) C0 NHCHR C02H, starting from the peptide trimethyl-silyl ester treatment with CSg and an alkyl chloroformate gives an alkoxycarbonyl dithiocarbamate which readily cyclo-eliminates COS and alkanol. Acylation of an amino-acid trimethylsilyl ester with SCN--(CH2) C0 C1 provides an alternative route cyclization of an a-isothio-cyanato-acid (158) to a 2-thio-oxazolidone (159) occurs readily in solution, ... 

---