Thiocarboxylates, 0-alkyl amines

Nucleophilic attack of ammonia or of a primary or secondary amine on an O-alkyl thiocarboxylate (2) provides a formally straightforward approach to thioamides and a number of examples have been reported (equation l). - However, some limitations should be noted. Thus, there is a tendency of esters (2) to rearrange to their 5-alkyl isomers on heating (cf. Volume 6, Chapter 2.5) and these yield amides with amines rather than thioamides. Besides, excess primary amine will lead to amidine formation, or the tetrahedral intermediate of the substitution reaction may break down to an imidate rather than a thioamide (cf. Volume 6, Chapter 2.7). These unwanted side reactions are favoured in polar, protic solvents such as ethanol. In contrast, THF has proven to be particularly useful in the synthesis of tertiary thioamides according to equation (1). For improved reactivity in the preparation of V-aryl derivatives and milder reaction conditions, it is advantageous to employ the amine in the form of its Mg salt. ... 

Dithiocarboxylates, R CSSR give a faster reaction with amines than G-alkyl thiocarboxylates (2 cf. Section 2.4.2. ). As shown in equation (8), this pronounced reactivity may lead to an undesired in situ reaction with the liberated ammonia to give a primary thioamide, when the ester is generated by thiolysis of a thioimidate. ... 

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