Carbon—nitrogen-sulfur/selenium rings

Metallaheterocycles with the metal atom capable of supporting a carbon-metal double bond, and with oxygen, nitrogen, sulfur, or selenium as the second heteroatom are the only structures capable of full conjugation. These compounds show furan-like aromaticity with the heteroatom /i-electrons participating. No examples of this type of ring system were uncovered, undoubtedly due to the stability of the metalla-carbon double bonds required for their formation. 

Furthermore, the strongly metallic character of selenium weakens the C-Se bond and thus favors reactions involving opening of the ring. The basicity of the three heterocycles is approximately in the same order, the nitrogen atom of selenazole and thiazole possessing much the same properties as the heteroatom of pyridine. Of the two carbon atoms ortho to nitrogen, that is, the 2-carbon and the 4-carbon, only the one in the 2-position is fairly active as a result of its interaction with selenium or sulfur. The 4- and 5-positions of thiazole and selenazole are more susceptible to electrophilic substitution than the 3- and 5-positions of pyridine. This is particularly true of the 5-position of selenazole. Thus it can be said that the 2- and 5-positions of the selenazoles and thiazoles... 

The primary site of attack in the reactions of the 1,2,5-thiadiazoles and selenadiazoles with lithium alkyls or Grignard reagents is usually at the ring sulfur or selenium (Scheme 2, path A) but attack on nitrogen (path B) or the ring carbon (path C) also occurs. yV-Phenyl-o-phenylenediamine is produced in the reaction of (1) and (2) with phenyllithium, probably via path B (55JCS1468). The driving force for this mode of addition could depend on the stable benzenoid character of the addition intermediate. 

---