THIOCYANATES AND ISOTHIOCYANATES

N=C=S st as 2200-2050 Very strong, generally doublet, Fermi resonance 

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Aliphatic methyl ketones (CH3C(0)CH3) Simple aromatic nitro compounds (NO+CO) Straight-chain mercaptans Thiocyanates and isothiocyanates... 

Figure 4.10 Selected ion-monitoring chromatograms of the thiocyanate and isothiocyanate derivatives of methyl, ethyl and isopropyl mesylates (final concentrations 0.25 pg/ml). Reproduced from [60] by permission of the Royal Society of Chemistry.

In an article that collates information not extensively treated before, Z. J. Witczak (West Lafayette) describes the synthesis, chemistry, and preparative applications of monosaccharide thiocyanates and isothiocyanates the thiocyanate anion is an ambident nucleophile of great synthetic versatility in approaches to nucleoside analogs and to thio and deoxy sugars. 

As the bonding arrangement of cyanate and isocyanate groups to tellurium is not known (see p. 617), diorgano tellurium halide cyanates, isocyanates, thiocyanates, and isothiocyanates are treated together on the basis of formal tellurium-nitrogen bonding. 

The Marckwald synthesis116 employed the reaction of a-amino-ketones with cyanates, thiocyanates, and isothiocyanates to yield 3 -imidazol-2-ones or AH-imidazole-2-thiones which are readily converted into imidazoles. The chief limitation of this method, which has been discussed adequately in earlier reviews,1-3 is in the synthesis of the a-aminocarbonyl compounds. The most convenient procedure is by reduction with sodium amalgam of a-amino acids.117 Among recent applications of the method118 119 is the synthesis118 of 4,5-... 

Reactions between a-aminocarbonyl compounds (or their acetals) and cyanates, thiocyanates and isothiocyanates are accomplished in the presence of acid, usually at a pH around 4. Table 4.1.1 lists some examples. Acetals are usually much more stable than a-aminoaldehydes, and they become the substrates of choice in many instances, being hydrolysed in situ [21]. 

In the ionization mechanism exemplified by benzhydryl thiocyanate, the reaction is strictly first order over a wide concentration range. The rate of isomerization increases with increasing solvent polarity Tracer and stereochemical evidence indicates that this involves an internal ion-pair mechanism. Isomerization is faster than isotopic exchange and so it was concluded that the former process occurs via an intimate ion pair which was shown to collapse to thiocyanate and isothiocyanate in the ratio of 5 1. In the case of optically active 4-chlorobenzhydryl thiocyanate in acetonitrile, racemization occurs at a rate comparable with isomerization. With a given solvent the structural effect acts essentially on the energy term and for a given substrate, the solvent effect acts essentially on the entropy term. 

Only preliminary details have been reported of the reversible photoisomerization of benzyl thiocyanate and isothiocyanate ... 

We shall give an example of a compound of type (ii) which is both a thiocyanate and isothiocyanate Ni(NH3)3(SCN)2 may represent a fourth class in which there are both bridging and singly attached NCS groups in the same crystal. In (i) and (iii) there is no distinction between thiocyanate and isothiocyanate. 

Miiller and Wilhelms applied the Fischer glycosyl thiocyanate method to the preparation of 6-deoxy-6-thiocyano-D-glucose and further explored the synthesis of l-D-glucosyl-2-thiourea. These workers have examined the question of the isomerization of thiocyanates and isothiocyanates, and have made some observations which should prove exceedingly useful. 

The reagent converts primary alcohols into thiocyanates, RSCN, in yields of about 60-957o. Tertiary alcohols are converted into isothiocyanates, RNCS, and secondary alcohols are converted into mixtures of thiocyanates and isothiocyanates. 

The deamination of 4-chlorobenzhydrylamine in the presence of thiocyanate ion (29) gives the thiocyanate and isothiocyanate 3.8), in addition to... 

Reactions of Thiocyanates and Isothiocyanates.—Although these functional groups are structiurally dissimilar from many points of view, a thiocyanate behaving as a sulphenyl cyanide, while an isothiocyanate shows the reactions expected of it on the basis of its heterocumulene structure, there are several examples now of isomerization reactions — S—C=N —N=C=S, partly justifying a unified treatment of their reactions. 

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