Thio- and Seleno-amides

A seemingly straightforward method to prepare aromatic selenoamides (451) is to react the corresponding nitrile (450) with elemental selenium in aqueous THF containing Et N under 5 atm of carbon monoxide.Presumably the key reagent is hydrogen selenide formed in a water-gas shift reaction the method is unfortunately much less efficient when applied to aliphatic nitriles. 

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This indicates that there is a greater contribution from canonical structures of the form shown in (3) this is in accord with results obtained from i.r. and u.v. studies. Another corresponding trend is observed in the anilides and thioanilides, where introduction of sulphur in place of oxygen in the anilides should cause a smaller downfield shift than observed between the amides and the corresponding thioamides, since there will already be some delocalization of charge present through structures shown in (3). The value for selenobenzamide, only 3 8p.p.m. downfield from thiobenzamide, perhaps reflects the similarity of bond order 5 and n electron densities between the thio- and seleno-amides. A summary of the chemical shifts in these systems is shown in Table VII. 

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